The present invention relates to a novel class of xcex1-aminoboronic acids, which are useful as intermediates in synthetic processes for boronic acid inhibitors of the serine proteases, leukocyte elastase, pancreatic elastase, cathepsin G, and chymotrypsin. More specifically, the xcex1-aminoboronic acids are useful as intermediates for the synthesis of Hepatitis C Virus (HCV) protease inhibitors. This invention also generally relates to novel methods for the preparation of xcex1-aminoboronic acids.
Inhibitors of HCV protease have been prepared by replacing the scissile bond cleaved in peptide substrates with electrophillic groups. Of the various electrophilic groups examined, boronic acids have a distinct advantage. The concept of using boronic acids as serine protease inhibitors was introduced in the early 70""s Antonov et al. FEBS Lett 7, 23 (1970); Koelhler and Lienhard Biochemistry 10, 2477-2483 (1971). An xcex1-aminoboronic acids, Ac-boroPhe-OH, was first prepared by Matteson J. Am. Chem. Soc. 103, 5241-5242 (1981). This compound inhibits chymotrypsin with a Ki of 2.1 xcexcM. Kettner and Shenvi J. Biol. Chem. 259, 15106-15114 (1984) were able to couple xcex1-aminoboronic acids to peptides and were able to show that they were very effective inhibitors of the serine proteases, leukocyte elastase, pancreatic elastase, cathepsin G, and chymotrypsin. Their specificity was highly dependent on the nature of the side chain of the xcex1-aminoboronic acid. These initial compounds are described in U.S. Pat. No. 4,499,082 (1985). U.S. Pat. No. 4,499,082 also discloses the use of side chain protected amino acids and equivalent amino acids known to those skilled in the art. More recent patents cover peptide boronic acids containing basic side chains. U.S. Pat. No. 5,187,157 discloses boronic acid inhibitors specially designed as inhibitors of trypsin-like serine proteases. U.S. Pat. No. 5,658,885 (1997) discloses boronic acid inhibitors containing the following side chains: C1-C12-alkyl substituted with xe2x80x94NHC(NH)H, xe2x80x94ONHR6, or xe2x80x94ONHC(NH)NHR6 as well as phenylalanine analog substituted with a cyano group. Other boronic acid inhibitors containing basic sidechain are disclosed in Fevig et al U.S. Pat. No. 5,462,964 (1995), Dominguez et al. U.S. Pat. No. 5,639,739 (1997) and Amparo et al. U.S. Pat. No. 5,698,538 (1997). Additionally, boronic acid inhibitors of Hepatitis C Virus (HCV) protease are disclosed in Kettner et al PCT Publication WO 01/02424 (Jan. 11, 2001).
Schemes 1A, 1B and 1C outline the different approaches that have been used in the synthesis of xcex1-aminoboronic acids containing a variety of sidechains. These include compounds where R is alkyl, aryl, and alkylaryl containing various degrees of unsaturation.
In Scheme 1A, a Grignard reagent or other suitable nucleophile is added to a trialkyl boronate to give a substituted dialkyl boronate. Transesterification with a suitable diol protecting group gives the boronate ester 2. 2 is shown protected as the pinanediol ester, however, pinacol or C2 symmetrical diol, such as (R,R)2,3-butandiol, and (R,R)dicyclohexaneethanediol can also be used effectively. The xcex1-chloroalkyl intermediate 3 is obtained by the nucleophilic addition of the anion of methylene chloride to the boronic acid ester. Nucleophilic additions to boronates are generally performed under harsh conditions and sub-zero temperatures. 3 is treated with the lithium salt of hexamethyldisilazane to give the bis-silane protected amine 4. Compound 4 is treated with either anhydrous HCl or trifluoroacetic acid to give the amine 5 as a hydrochloride salt or trifluoroacetate salt. For example of Scheme 1A see U.S. Pat. No. 4,499,082 (1985). 
Scheme 1B shows the introduction of an alkyl sidechain as an olefin (see Matteson et al. Organometallics 3, 1284-1288, 1984 and U.S. Pat. No. 5,187,157, 1993). Hydroboration with catacholborane give the alkyl boronate. After transesterification with pinanediol, compound 2 is obtained. For example hydroboration of 3-bromopropene provide a 3-bromopropyl sidechain intermediate. These reactions are amenable for nucleophilic additions but not for electrophilic additions. 
Scheme 1C shows the preparation of xcex1,xcex1-dichloromethyl boronate ester 7, which then allows the nucleophilic addition of a sidechain to give 3 (see Kinder et al J. Med. Chem. 28, 1917-1925, 1985). The presence of acidic protons in reagents diminishes their ability to undergo nucleophilic addition reactions. 
Although, the procedures in Schemes 1A, 1B and 1C allow the synthesis of a number of xcex1-aminoboronic acids, there are limitations on this chemistry for the preparation of xcex1-aminoboronic acids of the present invention. First, a stable nucleophile or olefin must be available for generation of 2. Second, the sidechain of 2 must be stable to the harsh reaction conditions (highly basic and sub-zero temperatures) required to convert 2 to 3. These reaction conditions are not amenable to hydrocarbons containing electrophilic centers.
None of the above-cited references provide methods for introducing boroaminoacid sidechains as electrophiles. The present invention provides a novel procedure as shown herein that allows the synthesis of xcex1-aminoboronic acids with primary or secondary amino groups required for the preparation of aminoboronic acids peptides with versatile sidechains. The present invention provides synthesis of novel xcex1-aminoboronic acid by introducing sidechains as electrophiles such as 2,2-difluoro-1-bromoethane, 3,3,3-trifluoro-1-bromopropane, and 2-bromoacetate esters. Similarly, sidechains can be introduced as olefins, wherein the harsh conditions such as treatment with the anion of methylene chloride are avoided. The sequence of reactions of the present invention has made it possible to prepare structurally diverse xcex1-aminoboronic acids. In addition to the specific compounds demonstrated in the present invention, higher order acrylates or alkyl halides can be used to give more complex sidechains. This is particularly valuable for the preparation of compounds with sidechains containing sensitive groups such as ketones, phosphonates and sulfonamides.
The present invention concerns novel processes for the preparation of xcex1-aminoboronic acids which are useful as HCV protease inhibitors.
There is provided by this invention a process for preparation of a compound of Formula (V): 
wherein:
R2 is xe2x80x94CH2(CH2)mW, xe2x80x94CH2(Cxe2x95x90O)R5, xe2x80x94CH2CH2(Cxe2x95x90O)R5, xe2x80x94CHR4(CR4aR3)mW, xe2x80x94CHR4a(Cxe2x95x90O)R5, xe2x80x94CHR4CHR4a(Cxe2x95x90O)R5, xe2x80x94CHR4a(Pxe2x95x90O)(R6)2, xe2x80x94CHR4CHR4a(Pxe2x95x90O) (OR6)2, xe2x80x94CHR4aSO2NH2xe2x80x94CHR4CHR4aSO2NH2, xe2x80x94CHR4aSO3R6, xe2x80x94CHR4CHR4aSO3R6; 
R3 is H, F, Cl or Br;
m is 0-4;
W is xe2x80x94CH2F, xe2x80x94CHF2, xe2x80x94CF3, xe2x80x94CH2Cl, xe2x80x94CHCl2, or xe2x80x94CCl3;
R4 and R4a are independently H or C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-;
R5 is C1-C6 alkyl, aryl, aryl-C1-C6 alkyl-, xe2x80x94OR6, xe2x80x94NH2, xe2x80x94N(R6)2, or xe2x80x94NHR6;
R6 is C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-; and
OY1 and OY2 are independently selected from:
b) C1-C8 alkoxy, and
when taken together with B, OY1 and OY2 form:
c) a cyclic boronic ester where said cyclic boronic ester contains from 2 to 20 carbon atoms, and, optionally, 1, 2, or 3 heteroatoms which can be N, S, or O;
xe2x80x83said process comprising:
(1) (addition) contacting a compound of Formula (I): 
xe2x80x83wherein Ar is aryl;
with a hindered base followed by addition of a hydrocarbon containing an electrophilic center selected from:
Lxe2x80x94CH2(CH2)mW,
Lxe2x80x94CH2(Cxe2x95x90O)R5,
CH2xe2x95x90CH2(Cxe2x95x90O)R5,
Lxe2x80x94CHR4(CR4aR3)mW,
Lxe2x80x94CHR4a(Cxe2x95x90O)R5,
CHR4xe2x95x90CHR4a(Cxe2x95x90O)R5,
Lxe2x80x94CHR4a(Pxe2x95x90O)(OR6)2,
CHR4xe2x95x90CHR4a(Pxe2x95x90O)(OR6)2,
Lxe2x80x94CHR4aSO2NH2,
CHR4xe2x95x90CHR4aSO2NH2,
Lxe2x80x94CHR4aSO3R6,
CHR4xe2x95x90CHR4aSO3R6; 
xe2x80x83to form a compound of Formula (II): 
xe2x80x83wherein L is a leaving group;
(2) (alkylation) contacting the compound of Formula (II) with an alkylating agent to form a compound of Formula is (III): 
xe2x80x83wherein R8 is C1-C6 alkyl;
(3) (halogenation) contacting the compound of Formula (III) with a metal halide Mxe2x80x94X to form a compound of Formula (IV): 
xe2x80x83wherein X is halogen; M is an alkali metal or an alkaline-earth metal; and
(4) (amination) aminating the compound of Formula (IV) by either
(i) contacting the compound of Formula (IV) with NaN3 followed by addition of a hydrogenation agent to form the compound of Formula (V); or
(ii) contacting the compound of Formula (IV) wherein R2 is xe2x80x94CH2(CH2)mW or xe2x80x94CHR4(CR4aR3)mW; with lithium hexamethyldisilazane followed addition of a strong acid; to form the compound of Formula (V);
xe2x80x83or alternatively, Step (2) may be followed by
(5) (direct amination) aminating the compound of Formula (III) by either
(i) contacting the compound of Formula (III) with NaN3 followed by addition of a hydrogenation agent to form the compound of Formula (V); or
(ii) contacting the compound of Formula (III) wherein R2 is xe2x80x94CH2(CH2)mW or xe2x80x94CHR4(CR4aR3)mW; with lithium hexamethyldisilazane followed addition of a strong acid; to form the compound of Formula (V).
In a first embodiment, the present invention provides a novel process for the preparation of compounds of formula (V): 
wherein:
R2 is xe2x80x94CH2(CH2)mW, xe2x80x94CH2(Cxe2x95x90O)R5, xe2x80x94CH2CH2(Cxe2x95x90O)R5, xe2x80x94CHR4(CR4aR3)mW, xe2x80x94CHR4a(Cxe2x95x90O)R5, xe2x80x94CHR4CHR4a(Cxe2x95x90O)R5, xe2x80x94CHR4a(Pxe2x95x90O) (OR6)2, xe2x80x94CHR4CHR4a(Pxe2x95x90O) (OR6)2, xe2x80x94CHR4aSO2NH2, xe2x80x94CHR4CHR4aSO2NH2, xe2x80x94CHR4aSO3R6, xe2x80x94CHR4CHR4aSO3R6; 
R3 is H, F, Cl or Br;
m is 0-4;
W is xe2x80x94CH2F, xe2x80x94CHF2, xe2x80x94CF3, xe2x80x94CH2Cl, xe2x80x94CHC12, or xe2x80x94CCl3;
R4 and R4a are independently H or C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-;
R5 is C1-C6 alkyl, aryl, aryl-C1-C6 alkyl-, xe2x80x94OR6, xe2x80x94NH2, xe2x80x94N(R6)2, or xe2x80x94NHR6;
R6 is C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-; and
OY1 and OY2 are independently selected from:
b) C1-C8 alkoxy, and
when taken together with B, OY1 and OY2 form:
c) a cyclic boronic ester where said cyclic boronic ester contains from 2 to 20 carbon atoms, and, optionally, 1, 2, or 3 heteroatoms which can be N, S, or O;
xe2x80x83said process comprising:
(1) contacting a compound of Formula (I): 
xe2x80x83wherein Ar is aryl;
with a hindered base followed by addition of a hydrocarbon containing an electrophilic center selected from:
Lxe2x80x94CH2(CH2)mW,
Lxe2x80x94CH2(Cxe2x95x90O)R5,
CH2xe2x95x90CH2(Cxe2x95x90O)R5,
Lxe2x80x94CHR4(CR4aR3)mW,
Lxe2x80x94CHR4a(Cxe2x95x90O)R5,
CHR4xe2x95x90CHR4a(Cxe2x95x90O)R5,
Lxe2x80x94CHR4a(Pxe2x95x90O)(ORg6)2,
CHR4xe2x95x90CHR4a(Pxe2x95x90O)(OR6)2,
L- CHR4aSO2NH2,
CHR4xe2x95x90CHR4aSO2NH2,
Lxe2x80x94CHR4aSO3R6,
CHR4xe2x95x90CHR4aSO3R6; 
xe2x80x83to form a compound of Formula (II): 
xe2x80x83wherein L is a leaving group;
(2) contacting the compound of Formula (II) with an alkylating agent to form a compound of Formula (III): 
xe2x80x83wherein R8 is C1-C6 alkyl;
(3) contacting the compound of Formula (III) with a metal halide Mxe2x80x94X to form a compound of Formula (IV): 
xe2x80x83wherein X is halogen; M is an alkali metal or an alkaline-earth metal; and
(4) aminating the compound of Formula (IV) by either
(i) contacting the compound of Formula (IV) with NaN3 followed by addition of a hydrogenation agent to form the compound of Formula (V); or
(ii) contacting the compound of Formula (IV) wherein R2 is xe2x80x94CH2(CH2)mW or xe2x80x94CHR4(CR4aR3)mW; with lithium hexamethyldisilazane followed addition of a strong acid; to form the compound of Formula (V);
xe2x80x83or alternatively, Step (2) may be followed by
(5) aminating the compound of Formula (III) by either
(i) contacting the compound of Formula (III) with NaN3 followed by addition of a hydrogenation agent to form the compound of Formula (V); or
(ii) contacting the compound of Formula (III) wherein R2 is xe2x80x94CH2(CH2)mW or xe2x80x94CHR4(CR4aR3)mW; with lithium hexamethyldisilazane followed addition of a strong acid; to form the compound of Formula (V).
In another embodiment the present invention provides a process for the preparation of a compound of Formula (V) wherein:
R2 is xe2x80x94CH2(CH2)mW, xe2x80x94CH2(Cxe2x95x90O)R5, or xe2x80x94CH2CH2(Cxe2x95x90O)R5;
m is 0-2;
W is xe2x80x94CH2F, xe2x80x94CHF2, xe2x80x94CF3, xe2x80x94CH2Cl, xe2x80x94CHC2, or xe2x80x94CCl3;
R5 is C1-C6 alkyl, aryl, aryl-C1-C6 alkyl-, xe2x80x94OR6, xe2x80x94NH2, xe2x80x94N(R6)2, or xe2x80x94NHR6;
R6 is C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-; and
OY1 and OY2 are taken together with B to form a cyclic boronic ester where said cyclic boronic ester is formed from the group: pinanediol, pinacol, 1,2-ethanediol, 1,3-propanediol, 1,2-propanediol, 2,3-butanediol, 1,2-diisopropylethanediol, 5,6-decanediol, 1,2-dicyclohexylethanediol, diethanolamine, and 1,2-diphenyl-1,2-ethanediol;
xe2x80x83said process comprising:
(1) contacting a compound of Formula (I): 
with a hindered base followed by a hydrocarbon containing an electrophilic center selected from:
Lxe2x80x94CH2(CH2)mW,
Lxe2x80x94CH2(Cxe2x95x90O)R5, and
CH2xe2x95x90CH2(Cxe2x95x90O)R5;
xe2x80x83to form a compound of Formula (II): 
xe2x80x83wherein L is a leaving group selected from:
I, Br, Cl, methylsulfonyloxy, p-toluylsulfonyloxy and trifluoromethylsulfonyloxy;
(2) contacting the compound of Formula (II) with an alkylating agent to form a compound of Formula (III): 
xe2x80x83wherein R8 is C1-C6 alkyl;
(3) contacting the compound of Formula (III) with a metal halide Mxe2x80x94X to form a compound of Formula (IV): 
xe2x80x83wherein X is halogen; M is a alkali metal or an alkaline-earth metal; and
(4) aminating the compound of Formula (IV) by either
(i) contacting the compound of Formula (IV) with NaN3 followed by addition of a hydrogenation agent to form the compound of Formula (V); or
(ii) contacting the compound of Formula (IV) wherein R2 is xe2x80x94CH2(CH2)mW; with lithium hexamethyldisilazane followed addition of a strong acid; to form the compound of Formula (V);
xe2x80x83or alternatively, Step (2) may be followed by
(5) aminating the compound of Formula (III) by either
(i) contacting the compound of Formula (III) with NaN3 followed by addition of a hydrogenation agent to form the compound of Formula (V); or
(ii) contacting the compound of Formula (III) wherein R2 is xe2x80x94CH2(CH2)mW; with lithium hexamethyldisilazane followed addition of a strong acid; to form the compound of Formula (V).
In another alternative embodiment, the present invention provides provide a process for the preparation of a compound of Formula (V) wherein:
R2 is xe2x80x94CH2(CH2)mW, xe2x80x94CH2(Cxe2x95x90O)OR6, or xe2x80x94CH2CH2(Cxe2x95x90O)OR6;
m is 0-2;
W is xe2x80x94CH2F, xe2x80x94CHF2, xe2x80x94CF3, xe2x80x94CH2Cl, xe2x80x94CHC12, or xe2x80x94CCl3;
R6 is C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-; and
OY1 and OY2 are taken together with B to form a cyclic boronic ester where said cyclic boronic ester is formed from the group: pinanediol, pinacol, 1,2-ethanediol, 1,3-propanediol, 1,2-propanediol, 2,3-butanediol, 1,2-diisopropylethanediol, 5,6-decanediol, 1,2-dicyclohexylethanediol, diethanolamine, and 1,2-diphenyl-1,2-ethanediol;
xe2x80x83said process comprising:
(1) contacting a compound of Formula (I): 
xe2x80x83with a hindered base selected from:
lithium diisopropylamide, lithium 2,2,6,6-tetramethylpiperidine, lithium hexamethyldisilazane and n-butyllithium;
followed by a hydrocarbon containing an electrophilic center selected from:
Lxe2x80x94CH2(CH2)mW,
Lxe2x80x94CH2(Cxe2x95x90O)OR6, and
CH2xe2x95x90CH2(Cxe2x95x90O)OR6;
xe2x80x83to form a compound of Formula (II): 
xe2x80x83wherein L is a leaving group selected from:
I, Br, Cl, methylsulfonyloxy, p-toluylsulfonyloxy and trifluoromethylsulfonyloxy;
(2) contacting the compound of Formula (II) with an alkylating agent selected from:
C1-C6 alkyl halides, trimethyloxonium tetrafluoroborate, dimethylsulfate and methyltriflate;
xe2x80x83to form a compound of Formula (III): 
wherein R8 is C1-C6 alkyl;
(3) contacting the compound of Formula (III) with a metal halide Mxe2x80x94X selected from sodium iodide, lithium bromide and potassium iodide;
xe2x80x83to form a compound of Formula (IV): 
xe2x80x83wherein X is I or Br; and
(4) aminating the compound of Formula (IV) by either
(i) contacting the compound of Formula (IV) with NaN3 followed by contacting a hydrogenation agent selected from:
H2/Pdxe2x80x94C; and
SnCl2 in methanol;
xe2x80x83to form the compound of Formula (V); or
(ii) contacting the compound of Formula (IV) wherein R2 is xe2x80x94CH2(CH2)mW lithium hexamethyldisilazane followed by contacting a strong acid selected from:
anhydrous HCl; and
trifluoroacetic acid;
xe2x80x83to form the compound of Formula (V).
In an alternative embodiment, the present invention provides a process for the preparation of a compound of Formula (V) wherein:
R2 is xe2x80x94CH2(CH2)mW, xe2x80x94CH2(Cxe2x95x90O)OR6, or xe2x80x94CH2CH2(Cxe2x95x90O)OR6;
m is 0-2;
W is xe2x80x94CH2F, xe2x80x94CHF2, xe2x80x94CF3, xe2x80x94CH2Cl, xe2x80x94CHCl2, or xe2x80x94CCl3;
R6 is C1-C6 alkyl, aryl, or aryl-C1-6 alkyl-; and
OY1 and OY2 are taken together with B to form a cyclic boronic ester where said cyclic boronic ester is formed from the group: pinanediol and pinacol, 1,2-ethanediol, 1,3-propanediol, 1,2-propanediol, 2,3-butanediol, 1,2-diisopropylethanediol, 5,6-decanediol, 1,2-dicyclohexylethanediol, diethanolamine, and 1,2-diphenyl-1,2-ethanediol;
xe2x80x83said process comprising:
(1) contacting a compound of Formula (I): 
xe2x80x83with a hindered base selected from:
lithium diisopropylamide, lithium 2,2,6,6-tetramethylpiperidine and lithium hexamethyldisilazane;
followed by a hydrocarbon containing an electrophilic center selected from:
Lxe2x80x94CH2(CH2)mW,
Lxe2x80x94CH2(Cxe2x95x90O)OR6, and
CH2xe2x95x90CH2(Cxe2x95x90O)OR6;
xe2x80x83to form a compound of Formula (II): 
xe2x80x83wherein L is I or Br;
(2) contacting the compound of Formula (II) with an alkylating agent selected from:
methyl iodide, ethyl iodide and trimethyloxonium tetrafluoroborate;
xe2x80x83to form a compound of Formula (III): 
xe2x80x83wherein R8 is methyl or ethyl;
(3) contacting the compound of Formula (III) with sodium iodide to form a compound of Formula (IV): 
xe2x80x83wherein X is I; and
(4) aminating the compound of Formula (IV) by either
(i) contacting the compound of Formula (IV) with NaN3 followed by addition of H2/Pdxe2x80x94C to form the compound of Formula (V); or
(ii) contacting the compound of Formula (IV) when R2 is xe2x80x94CH2(CH2)mW; with lithium hexamethyldisilazane followed addition of a strong acid selected from:
anhydrous HCl; and
trifluoroacetic acid;
xe2x80x83to form the compound of Formula (V).
In another alternative embodiment, the present invention provides a process for the preparation of a compound of Formula (V-1): 
wherein OY1 and OY2 are taken together with B to form a cyclic boronic ester where said cyclic boronic ester is formed from the group: pinanediol, pinacol, 1,2-ethanediol, 1,3-propanediol, 1,2-propanediol, 2,3-butanediol, 1,2-diisopropylethanediol, 5,6-decanediol, 1,2-dicyclohexylethanediol, diethanolamine, and 1,2-diphenyl-1,2-ethanediol;
xe2x80x83said process comprising:
(1) contacting a compound of Formula (I): 
xe2x80x83with a hindered base followed by Lxe2x80x94CH2CHF2;
xe2x80x83to form a compound of Formula (II-1): 
xe2x80x83wherein L is a leaving group;
(2) contacting the compound of Formula (II-1) with an alkylating agent to form a compound of Formula (III-1): 
xe2x80x83wherein R8 is C1-C6 alkyl;
(3) contacting the compound of Formula (III-1) with a metal halide Mxe2x80x94X to form a compound of Formula (IV-1): 
xe2x80x83wherein X is halogen; M is a alkali metal or an alkaline-earth metal; and
(4) aminating the compound of Formula (IV-1) by either
(i) contacting the compound of Formula (IV-1) with NaN3 followed by addition of a hydrogenation agent to form the compound of Formula (V-1); or
(ii) contacting the compound of Formula (IV-1) with lithium hexamethyldisilazane followed addition of a strong acid; to form the compound of Formula (V-1);
xe2x80x83or alternatively, Step (2) may be followed by
(5) aminating the compound of Formula (III-1) by either
(i) contacting the compound of Formula (III-1) with NaN3 followed by addition of a hydrogenation agent to form the compound of Formula (V-1); or
(ii) contacting the compound of Formula (III-1) with lithium hexamethyldisilazane followed addition of a strong acid; to form the compound of Formula (V-1).
In another alternative embodiment, the present invention provides a process for the preparation of a compound of Formula (V-1) wherein:
OY1 and OY2 are taken together with B to form a cyclic boronic ester where said cyclic boronic ester is formed from the group: pinanediol and pinacol;
xe2x80x83said process comprising:
(1) contacting a compound of Formula (I): 
xe2x80x83with a hindered base selected from:
lithium diisopropylamide, lithium 2,2,6,6-tetramethylpiperidine, lithium hexamethyldisilazane and n-butyllithium;
followed by Lxe2x80x94CH2CHF2;
xe2x80x83to form a compound of Formula (II-1): 
xe2x80x83wherein L is a leaving group selected from:
I, Br, Cl, methylsulfonyloxy, p-toluylsulfonyloxy and trifluoromethylsulfonyloxy;
(2) contacting the compound of Formula (II-1) with an alkylating agent selected from:
C1-C6 alkyl halides, trimethyloxonium tetrafluoroborate, dimethylsulfate and methyltriflate;
xe2x80x83to form a compound of Formula (III-1): 
xe2x80x83wherein R8 is C1xe2x80x94C6 alkyl;
(3) contacting the compound of Formula (III-1) with a metal halide Mxe2x80x94X selected from sodium iodide, lithium bromide and potassium iodide;
xe2x80x83to form a compound of Formula (IV-1): 
xe2x80x83wherein X is I or Br; and
(4) aminating the compound of Formula (IV-1) by either
(i) contacting the compound of Formula (IV-1) with NaN3 followed by addition of a hydrogenation agent selected from:
H2/Pdxe2x80x94C; and
SnCl2 in methanol;
xe2x80x83to form the compound of Formula (V-1); or
(ii) contacting the compound of Formula (IV-1) with lithium hexamethyldisilazane followed addition of a strong acid a strong acid selected from:
anhydrous HCl; and
trifluoroacetic acid;
xe2x80x83to form the compound of Formula (V-1).
In another alternative embodiment, the process for the preparation of a compound of Formula (V-1) comprises:
(1) contacting a compound of Formula (I): 
xe2x80x83with a hindered base selected from:
lithium diisopropylamide, lithium 2,2,6,6-tetramethylpiperidine and lithium hexamethyldisilazane;
followed by Lxe2x80x94CH2CHF2;
xe2x80x83to form a compound of Formula (II-1): 
xe2x80x83wherein L is I or Br;
(2) contacting the compound of Formula (II-1) with an alkylating agent selected from:
methyl iodide, ethyl iodide and trimethyloxonium tetrafluoroborate;
xe2x80x83to form a compound of Formula (III-1): 
xe2x80x83wherein R8 is methyl or ethyl;
(3) contacting the compound of Formula (III-1) with sodium iodide to form a compound of Formula (IV-1): 
xe2x80x83wherein X is I; and
(4) aminating the compound of Formula (IV-1) by either
(i) contacting the compound of Formula (IV-1) with NaN3 followed by addition of H2/Pdxe2x80x94C to form the compound of Formula (V-1); or
(ii) contacting the compound of Formula (IV-1) with lithium hexamethyldisilazane followed addition of a strong acid a strong acid selected from:
anhydrous HCl; and
trifluoroacetic acid;
xe2x80x83to form the compound of Formula (V-1).
In another alternative embodiment, the present invention provides a process for the preparation of a compound of Formula (V-2): 
xe2x80x83wherein
OY1 and OY2 are taken together with B to form a cyclic boronic ester where said cyclic boronic ester is formed from the group: pinanediol, pinacol, 1,2-ethanediol, 1,3-propanediol, 1,2-propanediol, 2,3-butanediol, 1,2-diisopropylethanediol, 5,6-decanediol, 1,2-dicyclohexylethanediol, diethanolamine, and 1,2-diphenyl -1,2-ethanediol;
xe2x80x83said process comprising:
(1) contacting a compound of Formula (I): 
xe2x80x83with a hindered base followed by Lxe2x80x94CH2CH2CF3;
xe2x80x83to form a compound of Formula (II-2): 
xe2x80x83wherein L is a leaving group;
(2) contacting the compound of Formula (II-2) with an alkylating agent to form a compound of Formula (III-2): 
xe2x80x83wherein R8 is C1-C6 alkyl;
(3) contacting the compound of Formula (III-2) with a metal halide Mxe2x80x94X to form a compound of Formula (IV-2): 
xe2x80x83wherein X is halogen; M is a alkali metal or an alkaline-earth metal; and
(4) aminating the compound of Formula (IV-2) by either
(ii) contacting the compound of Formula (IV-2) with NaN3 followed by addition of a hydrogenation agent to form the compound of Formula (V-2); or
(ii) contacting the compound of Formula (IV-2) with lithium hexamethyldisilazane followed addition of a strong acid; to form the compound of Formula (V-2);
xe2x80x83or alternatively, Step (2) may be followed by
(5) aminating the compound of Formula (III-2) by either
(i) contacting the compound of Formula (III-2) with NaN3 followed by addition of a hydrogenation agent to form the compound of Formula (V-2); or
(ii) contacting the compound of Formula (III-2) with lithium hexamethyldisilazane followed addition of a strong acid; to form the compound of Formula (V-2).
In another alternative embodiment, the present invention provides a process for the preparation of a compound of Formula (V-2) wherein:
OY1 and OY2 are taken together with B to form a cyclic boronic ester where said cyclic boronic ester is formed from the group: pinanediol and pinacol;
xe2x80x83said process comprising:
(1) contacting a compound of Formula (I): 
xe2x80x83with a hindered base selected from:
lithium diisopropylamide, lithium 2,2,6,6-tetramethylpiperidine, lithium hexamethyldisilazane and n-butyllithium;
followed by Lxe2x80x94CH2CH2CF3;
xe2x80x83to form a compound of Formula (II-2): 
xe2x80x83L is a leaving group selected from:
I, Br, Cl, methylsulfonyloxy, p-toluylsulfonyloxy and trifluoromethylsulfonyloxy;
(2) contacting the compound of Formula (II-2) with an alkylating agent selected from:
C1-C6 alkyl halides, trimethyloxonium tetrafluoroborate, dimethylsulfate and methyltriflate;
xe2x80x83to form a compound of Formula (III-2): 
xe2x80x83wherein R8 is C1-C6 alkyl;
(3) contacting the compound of Formula (III-2) with a metal halide Mxe2x80x94X selected from sodium iodide, lithium bromide and potassium iodide;
xe2x80x83to form a compound of Formula (IV-2): 
xe2x80x83wherein X is I or Br; and
(4) aminating the compound of Formula (IV-2) by either
(i) contacting the compound of Formula (IV-2) with NaN3 followed by addition of a hydrogenation agent selected from:
H2/Pdxe2x80x94C; and
SnCl2 in methanol;
xe2x80x83to form the compound of Formula (V-2); or
(ii) contacting the compound of Formula (IV-2) with lithium hexamethyldisilazane followed addition of a strong acid a strong acid selected from:
anhydrous HCl; and
trifluoroacetic acid;
xe2x80x83to form the compound of Formula (V-2).
In another alternative embodiment, the process for the preparation of a compound of Formula (V-2) comprises:
(1) contacting a compound of Formula (I): 
xe2x80x83with a hindered base selected from:
lithium diisopropylamide, lithium 2,2,6,6-tetramethylpiperidine and lithium hexamethyldisilazane;
followed by Lxe2x80x94CH2CH2CF3;
xe2x80x83to form a compound of Formula (II-2): 
xe2x80x83wherein L is I or Br;
(2) contacting the compound of Formula (II-2) with an alkylating agent selected from:
methyl iodide, ethyl iodide and trimethyloxonium tetrafluoroborate;
xe2x80x83to form a compound of Formula (III-2): 
xe2x80x83wherein R8 is methyl or ethyl;
(3) contacting the compound of Formula (III-2) with sodium iodide to form a compound of Formula (IV-2): 
xe2x80x83wherein X is I; and
(4) aminating the compound of Formula (IV-2) by either
(i) contacting the compound of Formula (IV-2) with NaN3 followed by addition of H2/Pdxe2x80x94C to form the compound of Formula (V-2); or
(ii) contacting the compound of Formula (IV-2) with lithium hexamethyldisilazane followed addition of a strong acid a strong acid selected from:
anhydrous HCl; and
trifluoroacetic acid;
xe2x80x83to form the compound of Formula (V-2).
In another alternative embodiment, the present invention provides a process for the preparation of a compound of Formula (V-3): 
wherein
R6 is Me or t-Bu; and
OY1 and OY2 are taken together with B to form a cyclic boronic ester where said cyclic boronic ester is formed from the group: pinanediol, pinacol, 1,2-ethanediol, 1,3-propanediol, 1,2-propanediol, 2,3-butanediol, 1,2-diisopropylethanediol, 5,6-decanediol, 1,2-dicyclohexylethanediol, diethanolamine, and 1,2-diphenyl-1,2-ethanediol;
xe2x80x83said process comprising:
(1) contacting a compound of Formula (I): 
xe2x80x83with a hindered base followed by Lxe2x80x94CH2C(xe2x95x90O)OR6;
xe2x80x83to form a compound of Formula (II-3): 
xe2x80x83wherein L is a leaving group;
(2) contacting the compound of Formula (II-3) with an alkylating agent to form a compound of Formula (III-3): 
xe2x80x83wherein R8 is C1-C6 alkyl;
(3) contacting the compound of Formula (III-3) with a metal halide Mxe2x80x94X to form a compound of Formula (IV-3): 
xe2x80x83wherein X is halogen; M is a alkali metal or an alkaline-earth metal; and
(4) aminating the compound of Formula (IV-3) by contacting the compound of Formula (IV-3) with NaN3 followed by addition of a hydrogenation agent to form the compound of Formula (V-3); or
xe2x80x83or alternatively, Step (2) may be followed by
(5) aminating the compound of Formula (III-3) by contacting the compound of Formula (III-3) with NaN3 followed by addition of a hydrogenation agent to form the compound of Formula (V-3).
In another alternative embodiment, the present invention provides a process for the preparation of a compound of Formula (V-3) wherein:
R6 is Me or t-Bu; and
OY1 and OY2 are taken together with B to form a cyclic boronic ester where said cyclic boronic ester is formed from the group: pinanediol and pinacol;
xe2x80x83said process comprising:
(1) contacting a compound of Formula (I): 
xe2x80x83with a hindered base selected from:
lithium diisopropylamide, lithium 2,2,6,6-tetramethylpiperidine, lithium hexamethyldisilazane and n-butyllithium;
followed by Lxe2x80x94CH2C(xe2x95x90O)OR6;
xe2x80x83to form a compound of Formula (II-3): 
xe2x80x83wherein L is a leaving group selected from:
I, Br, Cl, methylsulfonyloxy, p-toluylsulfonyloxy and trifluoromethylsulfonyloxy;
(2) contacting the compound of Formula (II-3) with an alkylating agent selected from:
C1-C6 alkyl halides, trimethyloxonium tetrafluoroborate, dimethylsulfate and methyltriflate;
xe2x80x83to form a compound of Formula (III-3): 
xe2x80x83wherein R8 is C1-C6 alkyl;
(3) contacting the compound of Formula (III-3) with a metal halide Mxe2x80x94X selected from sodium iodide, lithium bromide and potassium iodide;
xe2x80x83to form a compound of Formula (IV-3): 
xe2x80x83wherein X is I or Br; and
(4) aminating the compound of Formula (IV-3) by contacting the compound of Formula (IV-3) with NaN3 followed by addition of a hydrogenation agent selected from:
H2/Pdxe2x80x94C; and
SnCl2 in methanol;
xe2x80x83to form the compound of Formula (V-3).
In another alternative embodiment, the process for the preparation of a compound of Formula (V-3) comprises:
(1) contacting a compound of Formula (I): 
xe2x80x83with a hindered base selected from:
lithium diisopropylamide, lithium 2,2,6,6-tetramethylpiperidine and lithium hexamethyldisilazane;
followed by Lxe2x80x94CH2C(xe2x95x90O)OR6;
xe2x80x83to form a compound of Formula (II-3): 
xe2x80x83wherein L is I or Br;
(2) contacting the compound of Formula (II-3) with an alkylating agent selected from:
methyl iodide, ethyl iodide and trimethyloxonium tetrafluoroborate;
xe2x80x83to form a compound of Formula (III-3): 
xe2x80x83wherein R8 is methyl or ethyl;
(3) contacting the compound of Formula (III-3) with sodium iodide to form a compound of Formula (IV-3): 
xe2x80x83wherein X is I; and
(4) aminating the compound of Formula (IV-3) by contacting the compound of Formula (IV-3) with NaN3 followed by addition of H2/Pdxe2x80x94C to form the compound of Formula (V-3).
In another alternative embodiment, the present invention provides a process for the preparation of a compound of Formula (V-4): 
wherein
OY1 and OY2 are taken together with B to form a cyclic boronic ester where said cyclic boronic ester is formed from the group: pinanediol, pinacol, 1,2-ethanediol, 1,3-propanediol, 1,2-propanediol, 2,3-butanediol, 1,2-diisopropylethanediol, 5,6-decanediol, 1,2-dicyclohexylethanediol, diethanolamine, and 1,2-diphenyl-1,2-ethanediol;
xe2x80x83said process comprising:
(1) contacting a compound of Formula (I): 
xe2x80x83with a hindered base followed CH2xe2x95x90CH(Cxe2x95x90O)OMe;
xe2x80x83to form a compound of Formula (II-4): 
(2) contacting the compound of Formula (II-4) with an alkylating agent to form a compound of Formula (III-4): 
xe2x80x83wherein R8 is C1-C6 alkyl;
(3) contacting the compound of Formula (III-4) with a metal halide Mxe2x80x94X to form a compound of Formula (IV-4): 
xe2x80x83wherein X is halogen; M is a alkali metal or an alkaline-earth metal; and
(4) aminating the compound of Formula (IV-4) by contacting the compound of Formula (IV-4) with NaN3 followed by addition of a hydrogenation agent to form the compound of Formula (V-4); or
xe2x80x83or alternatively, Step (2) may be followed by
(5) aminating the compound of Formula (III-4) by contacting the compound of Formula (III-4) with NaN3 followed by addition of a hydrogenation agent to form the compound of Formula (V-4).
In another alternative embodiment, the present invention provides a process for the preparation of a compound of Formula (V-4) wherein:
OY1 and OY2 are taken together with B to form a cyclic boronic ester where said cyclic boronic ester is formed from the group: pinanediol and pinacol;
xe2x80x83said process comprising:
(1) contacting a compound of Formula (I): 
xe2x80x83with a hindered base selected from:
lithium diisopropylamide, lithium 2,2,6,6-tetramethylpiperidine, lithium hexamethyldisilazane and n-butyllithium;
followed by by CH2xe2x95x90CH(xe2x95x90O)OMe;
xe2x80x83to form a compound of Formula (II-4): 
(2) contacting the compound of Formula (II-4) with an alkylating agent selected from:
C1-C6 alkyl halides, trimethyloxonium tetrafluoroborate, dimethylsulfate and methyltriflate;
xe2x80x83to form a compound of Formula (III-4): 
xe2x80x83wherein R8 is C1-C6 alkyl;
(3) contacting the compound of Formula (III-4) with a metal halide Mxe2x80x94X selected from sodium iodide, lithium bromide and potassium iodide;
xe2x80x83to form a compound of Formula (IV-4): 
xe2x80x83wherein X is I or Br; and
(4) aminating the compound of Formula (IV-4) by contacting the compound of Formula (IV-4) with NaN3 followed by addition of a hydrogenation agent selected from:
H2/Pdxe2x80x94C; and
SnCl2 in methanol;
xe2x80x83to form the compound of Formula (V-4).
In another alternative embodiment, the process for the preparation of a compound of Formula (V-4) comprises:
(1) contacting a compound of Formula (I): 
xe2x80x83with a hindered base selected from:
lithium diisopropylamide, lithium 2,2,6,6-tetramethylpiperidine and lithium hexamethyldisilazane;
followed by CH2xe2x95x90CH(xe2x95x90O)OMe;
L is I or Br;
xe2x80x83to form a compound of Formula (II-4): 
(2) contacting the compound of Formula (II-4) with an alkylating agent selected from:
methyl iodide, ethyl iodide and trimethyloxonium tetrafluoroborate;
xe2x80x83to form a compound of Formula (III-4): 
xe2x80x83wherein R8 is methyl or ethyl;
(3) contacting the compound of Formula (III-4) with sodium iodide to form a compound of Formula (IV-4): 
xe2x80x83wherein X is I; and
(4) aminating the compound of Formula (IV-4) by contacting the compound of Formula (IV-4) with NaN3 followed by addition of H2/Pdxe2x80x94C to form the compound of Formula (V-4).
The present invention also provides compounds of Formula (V-a): 
wherein:
R1 is H or C1-C6 alkyl;
R2 is xe2x80x94CH2(CH2)mW, xe2x80x94CH2(Cxe2x95x90O)R5, xe2x80x94CH2CH2(Cxe2x95x90O)R5, xe2x80x94CHR4(CR4aR3)mW, xe2x80x94CHR4a(Cxe2x95x90O)R5, xe2x80x94CHR4CHR4a(Cxe2x95x90O)R5, xe2x80x94CHR4a(Pxe2x95x90O)(OR6)2, xe2x80x94CHR4CHR4a(Pxe2x95x90O)(OR6)2, xe2x80x94CHR4aSO2NH2, xe2x80x94CHR4CHR4aSO2NH2, xe2x80x94CHR4aSO3R6, xe2x80x94CHR4CHR4aSO3R6; 
R3 is H, F, Cl or Br;
m is 0-4;
W is xe2x80x94CH2F, xe2x80x94CHF2, xe2x80x94CF3, xe2x80x94CHCl2, or xe2x80x94CCl3;
R4 and R4a are independently H or C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-;
R5 is C1-C6 alkyl, aryl, aryl-C1-C6 alkyl-, xe2x80x94OR6, xe2x80x94NH2, xe2x80x94N(R6)2, or xe2x80x94NHR6;
R6 is H, C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-; and
OY1 and OY2 are independently selected from:
b) C1-C8 alkoxy, and
when taken together with B, OY1 and OY2 form:
c) a cyclic boronic ester where said cyclic boronic ester contains from 2 to 20 carbon atoms, and, optionally, 1, 2, or 3 heteroatoms which can be N, S, or O.
In another embodiment the present invention provides compounds of Formula (V-a) wherein:
R1 is H or C1-C6 alkyl;
R2 is xe2x80x94CH2CHF2, xe2x80x94CH2CH2CF3, xe2x80x94CH2(Cxe2x95x90O)R5, or xe2x80x94CH2CH2xe2x80x94(Cxe2x95x90O)R5;
R5 is C1-C6 alkyl, aryl, aryl-C1-C6 alkyl-, xe2x80x94OR6, or xe2x80x94N(R6)2;
R6 is H, C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-; and
OY1 and OY2 are independently selected from:
a) xe2x80x94OH,
b) C1-C8 alkoxy, and
xe2x80x83when taken together with B, OY1 and OY2 form:
c) a cyclic boronic ester where said cyclic boronic ester contains from 2 to 14 carbon atoms, and, optionally, 1, 2, or 3 heteroatoms which can be N, S, or O.
In another alternative embodiment, the present invention provides compounds of Formula (V-a) wherein:
R1 is H;
R2 is xe2x80x94CH2CHF2, xe2x80x94CH2CH2CF3, xe2x80x94CH2(Cxe2x95x90O)OR6, or xe2x80x94CH2CH2xe2x80x94(Cxe2x95x90O)OR6;
R6 is H or C1-C6 alkyl; and
OY1 and OY2 are taken together with B to form a cyclic boronic ester where said cyclic boronic ester is formed from the group: pinanediol, pinacol, 1,2-ethanediol, 1,3-propanediol, 1,2-propanediol, 2,3-butanediol, 1,2-diisopropylethanediol, 5,6-decanediol, 1,2-dicyclohexylethanediol, diethanolamine, and 1,2-diphenyl-1,2-ethanediol.
In another alternative embodiment, the present invention provides a compound selected from:
1-amino-3,3-difluoropropyl boronate pinacol ester;
1-amino-4,4,4-trifluorobutyl boronate pinanediol ester;
1-amino-2-t-butoxycarbonylethane-1-boronate pinanediol ester;
1-amino-2-methoxycarbonylethane-1-boronate pinanediol ester;
1-amino-2-hydroxycarbonylethane-1-boronate pinanediol ester;
1-amino-3-methoxycarbonyl-propane-1-boronate pinanediol ester; and
1-amino-3-hydroxycarbonyl-propane-1-boronate pinanediol ester.
The following terms and abbreviations are used herein and defined as follows. The abbreviation:
xe2x80x9cTHFxe2x80x9d as used herein means tetrahydrofuran,
xe2x80x9cTHFxe2x80x9d as used herein means tetrahydrofuran,
xe2x80x9cDMSOxe2x80x9d as used herein means dimethylsulfoxide,
xe2x80x9cDMFxe2x80x9d as used herein means N,N-dimethylformamide,
xe2x80x9cBuLixe2x80x9d as used herein means butyllithium,
xe2x80x9cNaHxe2x80x9d as used herein means sodium hydride,
xe2x80x9cLDAxe2x80x9d as used herein means lithium diisopropylamide,
xe2x80x9cHMPAxe2x80x9d as used herein means hexamethyphosphoric triamide,
xe2x80x9cTMEDAxe2x80x9d as used herein means N,N,Nxe2x80x2,Nxe2x80x2,-tetramethylethylenediamine.
The reactions of the synthetic methods claimed herein are carried out in suitable solvents which may be readily selected by one of skill in the art of organic synthesis, said suitable solvents generally being any solvent which is substantially nonreactive with the starting materials (reactants), the intermediates, or products at the temperatures at which the reactions are carried out, i.e., temperatures which may range from the solvent""s freezing temperature to the solvent""s boiling temperature. A given reaction may be carried out in one solvent or a mixture of more than one solvent. Depending on the particular reaction step, suitable solvents for a particular reaction step may be selected.
Suitable ether solvents include: tetrahydrofuran, diethyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, or t-butyl methyl ether.
Suitable protic solvents may include, by way of example and without limitation, water, methanol, ethanol, 2-nitroethanol, 2-fluoroethanol, 2,2,2-trifluoroethanol, ethylene glycol, 1-propanol, 2-propanol, 2-methoxyethanol, 1-butanol, 2-butanol, i-butyl alcohol, t-butyl alcohol, 2-ethoxyethanol, diethylene glycol, 1-, 2-, or 3-pentanol, neo-pentyl alcohol, t-pentyl alcohol, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, cyclohexanol, benzyl alcohol, phenol, or glycerol.
Suitable aprotic solvents may include, by way of example and without limitation, tetrahydrofuran (THF), dimethylformamide (DMF), dimethylacetamide (DMAC), 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU), 1,3-dimethyl-2-imidazolidinone (DMI), N-methylpyrrolidinone (NMP), formamide, N-methylacetamide, N-methylformamide, acetonitrile, dimethyl sulfoxide, propionitrile, ethyl formate, methyl acetate, hexachloroacetone, acetone, ethyl methyl ketone, ethyl acetate, sulfolane, N,N-dimethylpropionamide, tetramethylurea, nitromethane, nitrobenzene, or hexamethylphosphoramide.
Suitable basic solvents include: 2-, 3-, or 4-picoline, pyrrole, pyrrolidine, morpholine, pyridine, or piperidine.
Suitable hydrocarbon solvents include: benzene, cyclohexane, pentane, hexane, toluene, cycloheptane, methylcyclohexane, heptane, ethylbenzene, m-, o-, or p-xylene, octane, indane, nonane, or naphthalene. As used herein, the term xe2x80x9chydrocarbon containing an electrophilic centerxe2x80x9d or xe2x80x9celectrophilexe2x80x9d refers to any electrophilic group which selectively reacts with a phenylthio boronate of Formula (I) to form a compound of Formula (II). Examples of hydrocarbons containing an electrophilic center include, but are not limited to, These include alkyl halides containing electrophiles, sulfonate esters containing electrophiles and olefins containing electrophiles.
Alkyl halides containing electrophiles, sulfonate esters containing electrophiles can be expressed as a general formula L-R2 selected from: Lxe2x80x94CH2(CHR3)mW, Lxe2x80x94CH2(Cxe2x95x90O)R5, Lxe2x80x94CHR4(CR4aR3)mW, Lxe2x80x94CHR4a(Cxe2x95x90O)R5, Lxe2x80x94CHR4a(Pxe2x95x90O)(OR6)2, Lxe2x80x94CHR4aSO2NH2, Lxe2x80x94CHR4aSO3R6, 
wherein L is a leaving group, R3 is H, F, Cl, Br or C1-C6 alkyl; m is 0-4; W is xe2x80x94CH2F, xe2x80x94CHF2, xe2x80x94CF3, xe2x80x94CH2Cl, xe2x80x94CHCl2, or xe2x80x94CCl3; R4 and R4a are independently H or C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-; R5 is C1-C6 alkyl, aryl, aryl-C1-C6 alkyl-, xe2x80x94OR6, xe2x80x94NH2, xe2x80x94N(R6)2, or xe2x80x94NHR6; R6 is C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-.
As used herein, the term xe2x80x9cleaving groupxe2x80x9d refers to any group that departs from a molecule in a substitution reaction by breakage of a bond. Examples of leaving groups include, but are not limited to, a halo group (such as chloro, bromo or iodo), a sulfonate ester group (such as methylsulfonyloxy, p-toluylsulfonyloxy or trifluoromethylsulfonyloxy).
An olefin containing an electrophilic center is selected from: CH2xe2x95x90CH2(Cxe2x95x90O)R5, CHR4xe2x95x90CHR4a(Cxe2x95x90O)R5, CHR4xe2x95x90CHR4a(Pxe2x95x90O)(OR6)2, CHR4xe2x95x90CHR4aSO2NH2, and CHR4xe2x95x90CHR4aSO3R6; wherein R4 and R4a are independently H or C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-; R5 is C1-C6 alkyl, aryl, aryl-C1-C6 alkyl-, xe2x80x94OR6, xe2x80x94NH2, xe2x80x94N(R6)2, or xe2x80x94NHR6; R6 is C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-.
As used herein, the term xe2x80x9cbasexe2x80x9d refers to any agent that reacts with a phenylthio boronate of Formula (I) followed by eletrophilic addition of a hydrocarbon containing an electrophilic center to form a compound of Formula (II) in Scheme 2. Preferred bases for the electrophilic addition are hindered bases. Examples of hindered bases include, but are not limited to, LDA (lithium diisopropylamide), lithium 2,2,6,6-tetramethylpiperidine, lithium hexamethyldisilazane and n-butyllithium. Metal counterions other than lithium such as sodium and potassium can be used.
As used herein, the term xe2x80x9calkylating agentxe2x80x9d refers to any agent that alkylates the sulfur of a compound of Formula (II) to form the corresponding sulfonium salt (III). Examples of alkylating agents include, but are not limited to, C1-C6 alkyl halides, trimethyloxonium tetrafluoroborate, dimethylsulfate and methyltriflate. Examples of C1-C6 alkyl halides include, but are not limited to, methyl iodide, ethyl iodide, methyl bromide, and ethyl bromide. Other alkylating agents will be apparent to those of skill in the art, once armed with the present disclosure.
As used herein, the term xe2x80x9cmetal halidexe2x80x9d refers to any halide salt of a alkali metal or an alkaline-earth metal, for example, Na, K, Li, Mg and Ca, that can effect the formation of a compound of Formula (IV). Examples of metal halides include, but are not limited to, sodium iodide, lithium bromide and potassium iodide.
As used herein, the term xe2x80x9chydrogenation agentxe2x80x9d refers to any agent or any combination of agents that catalyzes reduction reaction of the corresponding azide of a compound of Formula (IV) to form a compound of Formula (V). Examples of hydrogenation agents include, but are not limited to, a combination of hydrogen with palladium-carbon, stannous chloride in methanol, or alkali metal sulfides such as sodium sulfide and potassium sulfide.
As used herein, the term xe2x80x9cstrong acidxe2x80x9d refers to an acid which essentially reacts completely with water to give a hydronium ion. Examples of strong acids include, but are not limited to, hydrochloric acid, trifluoroacetic acid, sulfonic acid, nitric acid, and sulfuric acid.
xe2x80x9cAlkylxe2x80x9d or xe2x80x9calkylenexe2x80x9d as used herein is intended to include both branched and straight chain saturated aliphatic hyrdocarbon groups having one to twelve carbon atoms; for example, xe2x80x9cC1-C6 alkylxe2x80x9d denotes alkyl having 1 to 6 carbon atoms. Examples of alkyl include, but are not limited to, methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, sec-butyl, t-butyl, n-pentyl, n-hexyl, 2-methylbutyl, 2-methylpentyl, 2-ethylbutyl, 3-methylpentyl, and 4-methylpentyl. xe2x80x9cHaloxe2x80x9d or xe2x80x9chalogenxe2x80x9d as used herein refers to fluoro, chloro, bromo and iodo. xe2x80x9cCounterionxe2x80x9d is used to represent a small, negatively charged species such as chloride, bromide, hydroxide, acetate, sulfate, and the like. xe2x80x9cMetal counterionxe2x80x9d is used to represent metal ion species.
xe2x80x9cHaloalkylxe2x80x9d is intended to include both branched and straight-chain saturated aliphatic hydrocarbon groups having the specified number of carbon atoms, substituted with 1 or more halogen (for example xe2x80x94CvFw where v=1 to 3 and w=1 to (2v+1)). Examples of haloalkyl include, but are not limited to, difluoromethyl, trifluoromethyl, dichloromethyl, trichloromethyl, pentafluoroethyl, pentachloroethyl, 2,2-difluroethyl, 2,2,2-trifluoroethyl, 2,2-dichloroethyl, 2,2,2-trichloroethyl, heptafluoropropyl, and heptachloropropyl. Examples of haloalkyl also include xe2x80x9cfluoroalkylxe2x80x9d which is intended to include both branched and straight-chain saturated aliphatic hydrocarbon groups having the specified number of carbon atoms, substituted with 1 or more fluorine atoms.
As used herein, the term xe2x80x9carylxe2x80x9d, or xe2x80x9caromatic residuexe2x80x9d, is intended to mean an aromatic moiety containing 6, 10 or 14 carbon atoms, preferably 6 or 10 carbon atoms. A particularly preferred aryl moiety is the phenyl group. The aryl group is optionally substituted by 0-5 substituents independently selected from halogen, C1-C3 alkyl, and an electron-withdrawing group.
As used herein, the term xe2x80x9celectron wihdrawing groupxe2x80x9d has its normal meaning as chemical functionality which electronically or inductively causes the withdrawal of electron density from the moiety to which the electron withdrawing groups is attached. Representative electron withdrawing groups include, but are not limited to, nitro groups, halogens, cyano, carboxyl groups and substituted carboxy groups such as ester groups and amido groups. Other electron withdrawing groups will be apparent to those of skill in the art, once armed with the present disclosure.
As used herein, the term xe2x80x9caryl-C1-C6 alkyl-xe2x80x9d refers to a C1-C6 alkyl group which bears an aryl group, for example, benzyl group.
As used herein, xe2x80x9ccyclic boronic esterxe2x80x9d is intended to mean a stable cyclic boronic moiety of general formula xe2x80x94B(OR)(OR) wherein the two R substituents taken together contain from 2 to 20 carbon atoms, and optionally, 1, 2, or 3 heteroatoms which can be N, S, or O. Cyclic boronic esters are well known in the art. Examples of cyclic boronic ester include, but are not limited to, pinanediol boronic ester, pinacol boronic ester, 1,2-ethanediol boronic ester, 1,3-propanediol boronic ester, 1,2-propanediol boronic ester, 2,3-butanediol boronic ester, 1,2-diisopropylethanediol boronic ester, 5,6-decanediol boronic ester, 1,2-dicyclohexylethanediol boronic ester, diethanolamine boronic ester, and 1,2-diphenyl-1,2-ethanediol boronic ester.
The compounds herein described may have asymmetric centers. All chiral, diastereomeric, and racemic forms are included in the present invention. It will be appreciated that certain compounds of the present invention contain an asymmetrically substituted carbon atom, and may be isolated in optically active or racemic forms. It is well known in the art how to prepare optically active forms, such as by resolution of racemic forms or by synthesis, from optically active starting materials. All chiral, diastereomeric, racemic forms and all geometric isomeric forms of a structure are intended, unless the specific stereochemistry or isomer form is specifically indicated.
Combinations of substituents and/or variables are permissible only if such combinations result in stable compounds. By stable compound or stable structure it is meant herein a compound that is sufficiently robust to survive isolation to a useful degree of purity from a reaction mixture.
The term xe2x80x9csubstitutedxe2x80x9d, as used herein, means that one or more hydrogen on the designated atom is replaced with a selection from the indicated group, provided that the designated atom""s normal valency is not exceeded, and that the substitution results in a stable compound.
The methods of the present invention, by way of example and without limitation, may by further understood by reference to Schemes 2-4. Schemes 2-4 detail the general synthetic methods for the preparation of xcex1-aminoboronic acids.
It is to be understood that one skilled in the art of organic synthesis could follow the methods described or exemplified herein to prepare homologs of compounds of Formula (II) through (V), by appropriately choosing a hydrocarbon containing an electrophilic center. 
It is the object of the present invention to provide a novel procedure for the preparation of xcex1-aminoboronic acids, which are useful as intermediates in the synthesis of peptide bronic acids which are highly effective inhibitors of the serine proteases, leukocyte elastase, pancreatic elastase, cathepsin G, and chymotrypsin. More specifically, the xcex1-aminoboronic acids are useful as intermediates for the synthesis of Hepatitis C Virus (HCV) protease inhibitors.
Step 1: Addition: Preparation of Compound of Formula (II)
This step comprises reacting a compound of Formula (I) with a base followed by a hydrocarbon containing an electrophilic center to form a compound of Formula (II). By way of general guidance, one equivalent of compound (I) is treated with one equivalent or about 5-20% excess of a base in a suitable solvent at a reduced temperature (about xe2x88x9220 to about 0xc2x0 C.) to form the salt of compound (I). Optionally but preferably, salt formation is accompanied by the formation of a precipitant. The mixture is optionally stirred for 15 minutes to 5 hr at about xe2x88x9210 to about 0xc2x0 C., but not limited to this range. Suitable temperature range can be determined by one skilled in the art by following the methods described herein. Preferably, the mixture is stirred for 30 minutes to 4 hrs at about 0xc2x0 C. The desired electrophile is dissolved in a suitable inert solvent and added to the salt of compound (I) dropwise at about xe2x88x9220 to about 10xc2x0 C., preferably, about xe2x88x9210 to about 0xc2x0 C. Preferred ratio of the desired electrophile to the salt is 2-5 equivalents of electrophile vs. salt of compound (I). However, much large excess of the electrophile can be used. The mixture is slowly allowed to come to room temperature and to stir overnight.
Preferred bases for Step (1) are hindered bases include LDA (lithium diisopropylamide), lithium 2,2,6,6-tetramethylpiperidine, lithium hexamethyldisilazane and n-butyllithium. More preferred hindered bases are LDA (lithium diisopropylamide), lithium 2,2,6,6-tetramethylpiperidine, lithium hexamethyldisilazane. Metal counterions other than lithium such as sodium and potassium can be used.
Preferred suitable solvents for Step (1) include THF, dimethoxyethane, N,N-dimethylformamide, dimethylsulfoxide, N-methylpyrrolidine, hexamethylphosphoric triamide and hexane. A more perferred suitable solvent is THF. Reagent which binds the metal cation such as HMPA (hexamethyphosphoric triamide), TMEDA (tetramethylethylene diamine), and crown ethers can be added.
The R2 substituent in Scheme 2 is introduced as an electrophile. These include alkyl halides containing electrophiles, sulfonate esters containing electrophiles and olefins containing electrophiles, wherein the olefin is limited to compounds that are good Michael acceptors.
The R2 substituent can be introduced from a hydrocarbon containing an electrophilic center L-R2 selected from: Lxe2x80x94CH2(CHR3)mW, Lxe2x80x94CH2(Cxe2x95x90O)R5, Lxe2x80x94CHR4(CR4aR3)mW, Lxe2x80x94CHR4a(Cxe2x95x90O)R5, Lxe2x80x94CHR4a(Pxe2x95x90O)(OR6)2, Lxe2x80x94CHR4aSO2NH2, Lxe2x80x94CHR4aSO3R6, 
wherein L is a leaving group, R3 is H, F, Cl, Br or C1-C6 alkyl; m is 0-4; W is xe2x80x94CH2F, xe2x80x94CHF2, xe2x80x94CF3, xe2x80x94CH2Cl, xe2x80x94CHCl2, or xe2x80x94CCl3; R4 and R4a are independently H or C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-; R5 is C1-C6 alkyl, aryl, aryl-C1-C6 alkyl-, xe2x80x94OR6, xe2x80x94NH2, xe2x80x94N(R6)2, or xe2x80x94NHR6; R6 is C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-. Preferred Lxe2x80x94R2 for the Step (1) are Lxe2x80x94CH2(CH2)mW, Lxe2x80x94CH2(Cxe2x95x90O)R5. Preferred leaving groups are I, Br, Cl, methylsulfonyloxy, p-toluylsulfonyloxy or trifluoromethylsulfonyloxy. More preferred leaving groups are I or Br. Preferred W is xe2x80x94CHF2 or xe2x80x94CF3. Preferred m is 0, 1 or 2. Preferred R5 is xe2x80x94OR6. More preferred R5 is xe2x80x94OR-Me and xe2x80x94O-tBu.
The R2 substituent can also be introduced from an olefin containing an electrophilic center selected from: CH2xe2x95x90CH2(Cxe2x95x90O)R5, CHR4xe2x95x90CHR4a(Cxe2x95x90O)R5, CHR4xe2x95x90CHR4a(Pxe2x95x90O)(OR6)2, CHR4xe2x95x90CHR4aSO2NH2, and CHR4xe2x95x90CHR4aSO3R6; wherein R4 and R4a are independently H or C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-; R5 is C1-C6 alkyl, aryl, aryl-C1-C6 alkyl-, xe2x80x94OR6, xe2x80x94NH2, xe2x80x94N(R6)2, or xe2x80x94NHR6; R6 is C1-C6 alkyl, aryl, or aryl-C1-C6 alkyl-. Preferred olefin containing an electrophilic center is CH2xe2x95x90CH2(Cxe2x95x90O)R5. Preferred R5 is xe2x80x94OR6. More preferred R5 is xe2x80x94O-Me and xe2x80x94O-tBu. More preferred R5 is xe2x80x94O-Me and xe2x80x94O-tBu.
Optionally compounds containing a carbonyl can be introduced protected with known protecting groups (Greene and Wuts xe2x80x9cProtective Groups in Organic Synthesisxe2x80x9d 3d edition, John Wiley and Sons, Inc, New York, 1999). 2-Bromomethyl-1,3-dioxolane can be readily applied as Lxe2x80x94R2.
It is understood that one skilled in the art can determine the preferred reaction of Step (1) as dependent on temperature, solvent, hindered base, and electrophile.
Step 2: Alkylation: Preparation of Compound of Formula (III)
This step comprises reacting a compound of Formula (II) with an alkylating agent to form a compound of Formula (III). By way of general guidance, one equivalent of compound (II) is treated with one equivalent to excess of an alkylating agent in a suitable solvent to form the corresponding sulfonium salt (III). Preferably, the ratio of compound (II) to alkylating agent is 1:20. The reaction is preferred at reflux, but may be run at room temperature. Suitable temperature range is dependent on the solvent used, can be determined by one skilled in the art by following the methods described herein. Reaction times range from 3 hrs to 5 days, preferably, 3-8 hrs.
Preferred alkylating agents for Step (2) include C1-C6 alkyl halides, trimethyloxonium tetrafluoroborate, dimethylsulfate and methyltriflate. More preferred alkylating agents are methyl iodide, ethyl iodide or trimethyloxonium tetrafluoroborate.
Preferred suitable solvents for Step (2) include acetonitrile, N,N-dimethylformamide, dimethylsulfoxide, N-methylpyrrdidine, hexamethylphosphoric triamide, THF, and dimethoxyethane. A more preferred suitable solvent is acetonitrile.
Step 3: Halogenation: Preparation of Compound of Formula (IV)
This step comprises reacting a compound of Formula (III) with a metal halide to form a compound of Formula (IV). By way of general guidance, one equivalent of compound (III) is treated with 1 to 10 equivalent of an metal halide in a suitable solvent to form the xcex1-halide (IV) for 1-8 hrs, preferably 6 hrs. The preferred ratio of compound (III) to metal halide is 1:2. The reaction is preferred at reflux, but may be run at room temperature. Suitable temperature range is dependent on the solvent used, can be determined by one skilled in the art by following the methods described herein. Preferred metal halides for Step (3) are sodium iodide, lithium bromide or potassium iodide. A more preferred metal halide is sodium iodide.
Preferred suitable solvents for Step (3) include acetonitrile, N,N-dimethylformamide, dimethylsulfoxide, N-methylpyrrdidine, hexamethylphosphoric triamide, THF, and dimethoxyethane. A more preferred suitable solvent is acetonitrile.
Alternatively, the corresponding sulfonium salt (III) can be generated in situ and the xcex1-halide formed by treatment with an alkyl halide in the presence of a metal halide.
Step 4: Amination: Preparation of Compound of Formula (V).
This step comprises converting a compound of Formula (IV) to a compound of Formula (V). Two different methods are feasible for Step (4). By way of general guidance, the first method comprises reacting one equivalent of compound (IV) with 1 equivalent to excess of lithium hexamethyldisilazane followed by treatment with a strong acid to form the xcex1-aminoboronic acid ester as an amine salt (V). The preferred ratio of compound (IV) to lithium hexamethyldisilazane is 1:1. Compound (IV) is treated with lithium hexamethyldisilazane for 3-8 hrs, at about xe2x88x92100 to xe2x88x9220xc2x0 C., preferably xe2x88x9278xc2x0 C. The treatment with an excess at least 3 equivalents of a strong acid is performed at about xe2x88x92100 to 0xc2x0 C. Preferred strong acids for Step (4) are anhydrous HCl or trifluoroacetic acid.
The second method is particularly feasible for preparing compounds which have acidic protons like the xcex1 and xcex2 carboxylate esters leading to H-boroAsp(OMe)-pinanediol and H-boroGlu(OMe)-pinanediol. By way of general guidance, compound (IV) is treated with sodium azide followed by reduction with a hydrogenation agent. Preferred methods of reducing the azide are catalytic hydrogenation in the presence of an acid or reduction with stannous chloride in methanol. Preferred catalytic hydrogenation is contacting the azide compound of (IV) with hydrogen and palladium-carbon. The preferred method of synthesis of the azide is to react compound (IV) with a sodium azide in N,Nxe2x80x2-dimethylformamide for several hours at 60-80xc2x0 C. Alternately, the azide can be prepared using phase transfer conditions in which methylene chloride, water, tetrabutylammonium bromide, and sodium azide are used (Matteson et al. J. Am. Chem. Soc. 108 810-819, 1986).
Step 5: Direct Amination: Preparation of Compound of Formula (V) Direct from Compound of Formula (III)
As an alternative to formation of the iodide, sulfonium salt (III) can be reacted directly with lithium hexmethyldisilazane or sodium azide to provide intermediates leading to the xcex1-amino compound as discussed in the the above description.
The value of the reaction protocol in Scheme 2 is exemplified by the synthesis of an xcex1-aminoboronic acid with a 2,2-difluoroethyl sidechain shown in Scheme 3. 
1-Chloromethyl boronate pinacol ester 8 is readily prepared by literature procedures, Sadhu and Matteson Organometallics 4 1687-1689, 1985. Compound 8 can be readily converted to the corresponding iodide by treatment with sodium iodide. Either alkyl halide readily reacts with thiolphenol in the presence of a tertiary amine to give the corresponding thiol ether 9. Alternately, the thiol ether can be prepared by reacting the lithium salt of thioanisole with a trialkyl boronate as described by Matteson and Arne Organometallic 1 280-288, 1982. Transesterification with pinacol gives compound 9.
1-Bromo-2,2-difluoroethane was reacted with the anion of 9 to give 10 where R2 is 2,2-difluoroethyl. The desired product, 1-amino-3,3-difluoropropane boronate 13 was obtained by treating 11 with methyl iodide in the presence of iodide ion followed by lithium hexamethyldisilazane and HCl.
In the present invention, the conversion of 10 to 12 is optimized to obtain the desired xcex1-iodo derivative in quantitative yield. The phenylthio boronic ester 10 is refluxed in acetonitrile under anhydrous conditions for as little as 3 hours and the xcex1-iodo boronic ester is obtained without the formation of elimination products.
The isolation and characterization of the alkylated(sulfonium)boronic ester 11 is a novel reaction. The displacement of the sulfonium ion boronates with nucleophiles is also a novel procedure.
The utility and versatility of the chemistry outlined in Scheme 2 is further demonstrated in the preparation of H-boroAsp(OtBu)-pinanediol and H-boroGlu(OMe)-pinanediol. For the preparation of the former (R2xe2x95x90xe2x80x94CH2xe2x80x94COxe2x80x94OtBu), 9 is allowed to react with t-butyl bromoacetate to give an intermediate with a carboxylmethyl sidechain. Following the steps in Scheme 2, the iodide 12 is obtained. 12 is converted to the amine by treatment with sodium azide followed by catalytic hydrogenation to give 13, H-boroAsp(OtBu)-C10H16.HCl. This compound is readily incorporated into peptides and the sidechain protecting group is removed with anhydrous HCl to give peptide-boroAsp-C10H16. Similarly, H-boroAsp(OMe)-C10H16 can be synthesized from methyl bromoacetate and the final product is obtained by treating the sidechain methyl ester with potassium trimethylsilanolate (Laganis and Chenard Tetrahedron Letters 25, 5831-5834, 1984). H-boroGlu(OMe)-C10H16 is also readily prepared by a similar series of reactions (Scheme 4). 
After treatment of 9 with base to generate the anion at the xcex1-position, a Michael acceptor (in this case methyl acrylate) is added to give 14 where the sidechain is xe2x80x94CH2xe2x80x94CH2xe2x80x94C(O)OMe. Note that the desired product 10 was not obtained when Brxe2x80x94CH2xe2x80x94CH2xe2x80x94COOtBu was allowed to react with the anion of 9 in Scheme 3. Apparently, this is due to the acidity of the protons alpha to the carboxylate resulting in elimination products. 14 is converted to H-boroGlu(OMe)-C10H16 17 which is readily incorporated into peptides. The sidechain methyl ester is cleaved with potassium trimethylsilanolate. Both boroAsp-C10H16 and boroGlu-C10H16 peptides can be converted to the corresponding boroAsn-C10H16 and -boroGln-C10H16 by treating the sidechain carboxylate with ammonia.
In the preparation of H-boroGlu(OtBu)-C10H16, 10 (or 14 in Scheme 4) was not obtained when the anion of 9 was allowed to react with t-butyl 3-bromopropionate due to the formation of elimination products. However, as shown in Scheme 4 the anion of 9 readily adds to Michael acceptors (in this case methyl acrylate). The sequence of reactions shown in Schemes 3 and 4 has made it possible to prepare much more structurally diverse xcex1-aminoboronic acids. In addition to the specific compounds we have prepared, higher order acrylates or alkyl halides can be used to give more complex sidechains. This is particularly valuable for the preparation of compounds with sidechains containing sensitive groups such as ketones, phosphonates and sulfonamides.
The preferred boronic acid protecting group for the syntheses in Schemes 3 and 4 is pinacol. However, after the conversion of either 12 or 16 to the amine, the pinacol ester is readily converted by methods known to one skilled in the art to the pinanediol ester that is more compatible with further synthetic steps.
The xe2x80x94CH2-CH2xe2x80x94COOtBu substituted boronic ester is treated with anydrous trifluoroacetic acid in dichloromethane (1:1) for 1 hour at room temperature to obtain the sidechain free acid. Both the boroAsp and boroGlu free acids can be obtained in this manner.
The xe2x80x94CH2xe2x80x94(CH2)mxe2x80x94COOMe substituted xcex1-amino boronic ester is first acylated or coupled to a peptide to protect the free amine. It is then treated with potassium trimethylsilanolate (5 equivalents) in dichloromethane for 6 hours at room temperature to obtain the sidechain free acid. Both the boroAsp and boroGlu free acids can be obtained in this manner.
Compounds of the present invention can effectively be used as inhibitors of protease which do not require an extended peptide sequence such as the aminopeptidases described in Shenvi U.S. Pat. No. 4,537,773, 1985. The new xcex1-aminoboronic acids can be readily incorporated into peptides using methods known to those skilled in the art (Stewart and Young xe2x80x9cSolid Phase Peptide Synthesisxe2x80x9d Pierce Chemical Co., Rockford Ill., 1984) and used as inhibitors of endopeptidases such as hepatitis C protease. Compounds containing a carboxylate ester sidechain can be further modified by removing the ester protecting groups to yield peptide-boroAsp-pinanediol and peptide-boroGlu-pinanediol for example. Peptide-aminoboronic acid esters containing a free sidechain carboxylate can be coupled with ammonia or primary or secondary amine using standard coupling procedures know in peptide chemistry, Stewart and Young, 1984. Compounds can be tested in biological systems as inhibitors of proteases as free boronic acid or as pinanediol or pinacol esters. All three give comparable results due to the rapid hydrolysis of pinanediol and pinacol esters to the free boronic acid, Kettner and Shenvi J. Biol. Chem. 259, 15106-15114, 1984. However, methods exist which allow the preparation of the free boronic acid, Kettner U.S. Pat. No. 5,384,410 (1995). The boronic acid ester is suspended in 1.0 mM aqueous HCl and is allowed to react with an excess of phenyl boronic acid added in an equal volume of ether. The product is readily separated from phenyl boronic acid and phenyl boronic acid pinanediol ester by extracting with ether. The free boronic acid is obtained by lyophilizing the aqueous phase. Pinanediol esters are also readily removed by treating with anhydrous boron trichloride in methylene chloride as described by Kinder et al., J. Med. Chem. 28, 1917-1925 (1985). The boronic acid ester is treated with a 2-3 fold excess of BCl3 for 5 min at xe2x88x9278xc2x0 C. and the mixture is allowed to stir 15 min in a 0xc2x0 ice bath. Excess BCl3 is hydrolyzed by the slow addition of water. Less structurally rigid boronic acid esters such as pinacol esters can be prepared by transesterification with diethanolamine and by hydrolyzing the diethanolamine ester with aqueous acid (Kettner and Shenvi J. Biol. Chem. 259, 15106-15114, 1984). Free boronic acid can be converted to the difluoroborane (xe2x80x94BF2) using a modification of the procedure of Kinder et al., J. Med. Chem. 28, 1917-1925 (1985). The boronic acid is treated with a 5-fold molar excess of 0.50 N aqueous hydrofluoric acid at room temperature. Excess hydrofluoric acid and water are removed by lyophilization to give the desire product.
The following examples are meant to be illustrative of the present invention. These examples are presented to exemplify the invention and are not to be construed as limiting the invention""s scope.