Design and Synthesis of New α-hydroxy β-fluoro/β-trifluoromethyl and Unsaturated Phosphonates from Carbohydrate-Derived Building Blocks via Pudovik and Horner–Wadsworth–Emmons Reactions

Herein, we present the application of fluorinated carbohydrate-derived building blocks for α-hydroxy β-fluoro/β-trifluoromethyl and unsaturated phosphonates synthesis. Pudovik and Horner–Wadsworth–Emmons reactions were applied to achieve this goal. The proposed pathway of the key reactions is supported by the experimental results, as well as quantum chemical calculations. The structure of the products was established by spectroscopic (1D, 2D NMR) and spectrometric (MS) techniques. Based on our data received, we claim that the progress of the Pudovik and HWE reactions is significantly influenced by the acidic protons present in the molecules as assessed by pKa values of the reagent.


Introduction
The synthesis of biologically significant compounds is based mainly on the ability to introduce an appropriate group of atoms into a target molecule [1,2]. An interesting approach relevant to this case may be introduction of fluorine containing groups into organic molecules. The relatively small size and electron properties of fluorine gave it a versatile use as a bioisostere and hence it is commonly applied as a substitute for a hydrogen atom. In addition, it is also an excellent mimetic of a carbonyl group, for example in peptide chains [3][4][5][6]. Nature has not endowed us with numerous fluorinated organic derivatives [7,8]. For this reason, synthesis in this area is the only way to obtain such molecules. Due to its remarkable chemical and physical properties, fluorine possessing compounds play an important role in biologically active molecules. This is particularly important in the aspect of obtaining new drugs [9][10][11][12][13][14][15]. Phosphonic acid derivatives are also among the known and used drugs (Fosfomycin 1, Figure 1). These derivatives have found application as analogues of phosphates commonly occurring in living organisms and involved in important biological processes (component of cell membranes-Phosphatidylcholine 2, Figure 1) [16,17]. Replacement of a bridging oxygen atom (R-O-P) in a phosphate group by a methylene group (-CH 2 -) can have significant biological effects. This modification may affect greater stability because carbon-phosphorus bond cannot be hydrolyzed by the enzyme that cleaves O-phosphate esters. For this reason, these isosteric analogues can be better substrates due to their similarity in size and shape to natural phosphates [18]. In the literature, there are many examples of various properties and biological activities of phosphonic acid derivatives containing a hydroxyl group [19,20] and/or fluorine atom in α position to a phosphonate group [21][22][23][24][25][26]. These compounds are also isosteric analogues of natural products [27] and enzyme inhibitors ( Figure 2) [19,28]. There are also examples of phosphonate derivatives having a fluorine atom in a β position to a phosphorus atom [24,29,30] and fluorinated or nonfluorinated unsaturated phosphonates [31][32][33][34][35][36][37][38].  [19].
An efficient approach towards the synthesis of fluoro-containing compounds is through modular assembly using fluorinated building blocks. In the recent study we investigated application of the Claisen rearrangement reaction for construction of fluorine-containing scaffolds [46]. Carbohydrates were used as starting compounds, which are a very valuable source of building blocks, as well as chiral auxiliaries [47][48][49][50][51][52]. Specific intent of our present research effort is using early obtained carbohydrate-derived building blocks to receive new α-hydroxy β-fluoro/β-trifluoromethyl and unsaturated phosphonates via Pudovik and Horner-Wadsworth-Emmons reactions. The modifiable functional groups of our newly synthesized compounds give access for their further transformations. They can be considered feasible tools for more complex molecule design. An efficient approach towards the synthesis of fluoro-containing compounds is through modular assembly using fluorinated building blocks. In the recent study we investigated application of the Claisen rearrangement reaction for construction of fluorine-containing scaffolds [46]. Carbohydrates were used as starting compounds, which are a very valuable source of building blocks, as well as chiral auxiliaries [47][48][49][50][51][52]. Specific intent of our present research effort is using early obtained carbohydrate-derived building blocks to receive new α-hydroxy β-fluoro/β-trifluoromethyl and unsaturated phosphonates via Pudovik and Horner-Wadsworth-Emmons reactions. The modifiable functional groups of our newly synthesized compounds give access for their further transformations. They can be considered feasible tools for more complex molecule design.
Allyl-vinyl ethers 12, as well as scaffolds bearing fluorine atoms resulting from Claisen rearrangement 18 were used as excellent precursors for the synthesis of fluorinated phosphonates (Scheme 2). The former 12, depending on the olefin employed, can be yielded as a single Z isomer or a Z/E mixture, and readily transformed to a carbonyl compound 13 by reacting with periodic acid. The aldehyde 13 after evaporation of solvent was pure enough be subjected to subsequent reactions. For this purpose, tetraethyl methylenebisphosphonate was converted into appropriate anion with NaH and proceeded with pure Z isomer of 13 to HWE protocol (Scheme 3). The reaction was carried out with 1.75 eq of both tetraethyl methylenebisphosphonate and NaH in THF, for 15 h at room temperature. Surprisingly, the reaction did not progress according to our assumption. On the basis of 1 H, 13 C NMR spectra of the obtained product 14, it can be concluded that in the initial stage of the process, under basic conditions, the substrate 13 isomerized to intermediate compound int-A, and afterwards the expected HWE reaction took place. In the following Scheme 3, a pathway of the discussed transformation is proposed. The observed 1,3-shift in the first step of the overall process is related to the presence of an acidic proton in the α position to the carbonyl group of fluorinated β,γ-unsaturated aldehyde 13, which can be abstracted under reaction conditions. In the literature, a few examples of this type of isomerization of carbonyl compounds can be found, some of them occurring even with very weak bases. [53][54][55] However, to the best of our knowledge, this is the first known example of such a 1,3-shift involving β,γ-unsaturated aldehydes. As a result of HWE reaction, between α,β-unsaturated aldehyde int-A and nucleophilic derivative of tetraethyl methylenebisphosphonate, an α,β-unsaturated phosphonate with a fluorinated allyl group 14 was synthesized. In spectra of purified product 14, two sets of signals in the ratio 1:0.2 were observed. It is caused by the presence of two diene conformers: "s-cis" 14 and "s-trans" 14. Geometry of the double bond located directly to the phosphorus atom was determined based on the NMR analysis, and is consistent with the expectations of the Allyl-vinyl ethers 12, as well as scaffolds bearing fluorine atoms resulting from Claisen rearrangement 18 were used as excellent precursors for the synthesis of fluorinated phosphonates (Scheme 2). The former 12, depending on the olefin employed, can be yielded as a single Z isomer or a Z/E mixture, and readily transformed to a carbonyl compound 13 by reacting with periodic acid. The aldehyde 13 after evaporation of solvent was pure enough be subjected to subsequent reactions. For this purpose, tetraethyl methylenebisphosphonate was converted into appropriate anion with NaH and proceeded with pure Z isomer of 13 to HWE protocol (Scheme 3). The reaction was carried out with 1.75 eq of both tetraethyl methylenebisphosphonate and NaH in THF, for 15 h at room temperature. Surprisingly, the reaction did not progress according to our assumption. On the basis of 1 H, 13 C NMR spectra of the obtained product 14, it can be purified product 14, two sets of signals in the ratio 1:0.2 were observed. It is caused by the presence of two diene conformers: "s-cis" 14′ and "s-trans" 14. Geometry of the double bond located directly to the phosphorus atom was determined based on the NMR analysis, and is consistent with the expectations of the HWE outcome where the E isomer should be formed as the main product. [56] A coupling constant value was crucial for the analysis-it was found to be 17 Hz, which confirmed the trans relationship of the protons across the double bond.

Scheme 3. Synthesis of 14.
In order to study the reactivity and diastereoselectivity of the reaction with another nucleophilic agent, this time a mixture of Z/E isomers of aldehyde 13 was subjected to the Pudovik reaction (Scheme 4). This time the reaction was carried out under very mild conditions, without a solvent, and in the presence of stoichiometric amounts of diethyl phosphite, and triethylamine at room temperature. The reaction, as described previously, proceeded in an alkaline environment. In the first step, the double bond rearrangement was observed, and then followed by the actual nucleophilic attack. The Pudovik reaction was highly stereoselective, such a great selectivity can be explained by the Felkin-Ahn model presented in Figure 3. In order to study the reactivity and diastereoselectivity of the reaction with another nucleophilic agent, this time a mixture of Z/E isomers of aldehyde 13 was subjected to the Pudovik reaction (Scheme 4). This time the reaction was carried out under very mild conditions, without a solvent, and in the presence of stoichiometric amounts of diethyl phosphite, and triethylamine at room temperature. The reaction, as described previously, proceeded in an alkaline environment. In the first step, the double bond rearrangement was observed, and then followed by the actual nucleophilic attack. The Pudovik reaction was highly stereoselective, such a great selectivity can be explained by the Felkin-Ahn model presented in Figure 3. Having results of both reactions, it can be concluded that the progress of those transformations is strongly dependent on the presence of an acidic proton in the α position of the carbonyl group in 13. In the present work we show a second class of molecules that we functionalizedthese are the previously obtained by Claisen rearrangement α-fluoro-α-trifluoromethylγ,δ-unsaturated 18a/18a′ (1:0.65 ratio) and α-trifluoromethyl-γ,δ-unsaturated 18b/18b′ (1:1 ratio) alcohols [46]. These compounds were oxidized with quantitative yields to the corresponding aldehydes 19a/19a′ and 19b/19b′ using Dess-Martin reagent. The reactions were carried out under mild conditions at room temperature. Complete disappearance of the substrates and selective formation of new products was observed after only 5 min. Having results of both reactions, it can be concluded that the pro transformations is strongly dependent on the presence of an acidic proton in of the carbonyl group in 13. In the present work we show a second class of molecules that we f these are the previously obtained by Claisen rearrangement α-fluoro-α-tri γ,δ-unsaturated 18a/18a′ (1:0.65 ratio) and α-trifluoromethyl-γ,δ-unsaturate ratio) alcohols [46]. These compounds were oxidized with quantitative corresponding aldehydes 19a/19a′ and 19b/19b′ using Dess-Martin reagent were carried out under mild conditions at room temperature. Complete dis In the present work we show a second class of molecules that we functionalizedthese are the previously obtained by Claisen rearrangement α-fluoro-α-trifluoromethyl-γ,δ-unsaturated 18a/18a (1:0.65 ratio) and α-trifluoromethyl-γ,δ-unsaturated 18b/18b (1:1 ratio) alcohols [46]. These compounds were oxidized with quantitative yields to the corresponding aldehydes 19a/19a and 19b/19b using Dess-Martin reagent. The reactions were carried out under mild conditions at room temperature. Complete disappearance of the substrates and selective formation of new products was observed after only 5 min. Aldehydes 19a/19a and 19b/19b were isolated by simple extraction, and were pure enough to be subjected to further transformations. Both fluorinated carbonyl compounds 19a/19a , 19b/19b were subjected to HWE and Pudovik reactions under the same conditions as described above as well.
The HWE reaction carried out on α-fluoro-α-trifluoromethyl-γ,δ-unsaturated 19a/19a derivatives proceeded as expected and yielded E olefins 20a/20a as the main products (Scheme 5). In this case the coupling constant between the protons located at the newly produced double bond was also 17 Hz. Unfortunately, an analogous reaction with αtrifluoromethyl-γ,δ-unsaturated 19b/19b aldehydes provided only traces of the expected HWE products 20b/20b . These compounds were characterized on the basis of 1 H, 19 F, 31 P and HRMS spectra. 20b/20b occurred to be unstable during prolonged NMR experiments ( 13 C, 2D NMR). Perhaps, the initial abstraction of acidic proton occurs, and then highly electrophilic and reactive structures are formed. Aldehydes 19a/19a′ and 19b/19b′ were also subjected to the Pudovik reaction (Scheme 6). When the derivatives containing an α-fluoro-α-trifluoromethyl-γ,δunsaturated moiety 19a/19a′ were used as substrates, the corresponding hydroxy phosphonates 21a/21a′ were obtained with good yield. A very high diastereoselectivity of the process was observed this time. The reaction led to only one new stereogenic center on a carbon atom bonded to a phosphorus atom. The reaction with the second aldehydes 19b/19b′, resulted in a slight reduction in the yield of the products 21b/21b′. This may also be related to the presence of an acidic proton located adjacent to the -CF3 group.
In order to explain and investigate the unexpected reactions of aldehydes 13 and 19b/19b′, we decided to perform quantum mechanical calculations to determine their pKa values.

Scheme 5. Synthesis of 20.
Aldehydes 19a/19a and 19b/19b were also subjected to the Pudovik reaction (Scheme 6). When the derivatives containing an α-fluoro-α-trifluoromethyl-γ,δ-unsaturated moiety 19a/19a were used as substrates, the corresponding hydroxy phosphonates 21a/21a were obtained with good yield. A very high diastereoselectivity of the process was observed this time. The reaction led to only one new stereogenic center on a carbon atom bonded to a phosphorus atom. The reaction with the second aldehydes 19b/19b , resulted in a slight reduction in the yield of the products 21b/21b . This may also be related to the presence of an acidic proton located adjacent to the -CF 3 group. the process was observed this time. The reaction led to only one new stereogenic center on a carbon atom bonded to a phosphorus atom. The reaction with the second aldehydes 19b/19b′, resulted in a slight reduction in the yield of the products 21b/21b′. This may also be related to the presence of an acidic proton located adjacent to the -CF3 group.
In order to explain and investigate the unexpected reactions of aldehydes 13 and 19b/19b′, we decided to perform quantum mechanical calculations to determine their pKa values. Scheme 6. Synthesis of 21.

DFT Calculations
The aim of the calculations was to obtain calculated value of pKa to estimate the acid strength of 2-((2,3,3,3-tetrafluoroprop-1-en-1-yl)oxy)but-3-enal 13 and α-trifluoromethylγ,δ-unsaturated aldehyde 19b in comparison to other organic acids whose pKa were measured in aqueous solution. Energy calculations were performed within DFT framework at M062X/6-31+G(d) level of theory [57,58]. This method and basis set was used by Zeng et al. [59] for similar calculations which resulted in calculated values of pKa. We also conducted frequency analysis [60] to access thermochemistry data needed, and to verify that the geometry optimization led to the potential energy minimum. Calculations were carried out in vacuo and aqueous solution with use of the Polarizable Scheme 6. Synthesis of 21.
In order to explain and investigate the unexpected reactions of aldehydes 13 and 19b/19b , we decided to perform quantum mechanical calculations to determine their pK a values.
DFT Calculations The aim of the calculations was to obtain calculated value of pK a to estimate the acid strength of 2-((2,3,3,3-tetrafluoroprop-1-en-1-yl)oxy)but-3-enal 13 and α-trifluoromethylγ,δ-unsaturated aldehyde 19b in comparison to other organic acids whose pK a were measured in aqueous solution. Energy calculations were performed within DFT framework at M062X/6-31+G(d) level of theory [57,58]. This method and basis set was used by Zeng et al. [59] for similar calculations which resulted in calculated values of pK a . We also conducted frequency analysis [60] to access thermochemistry data needed, and to verify that the geometry optimization led to the potential energy minimum. Calculations were carried out in vacuo and aqueous solution with use of the Polarizable Continuum Model (PCM) [61]. All quantum mechanics calculations were performed with the GAUSSIAN 09 [62] available within Pl-Grid infrastructure. To calculate pK a we used thermodynamic cycle depicted in Figure 4 below. Continuum Model (PCM) [61]. All quantum mechanics calculations were performed with the GAUSSIAN 09 [62] available within Pl-Grid infrastructure. To calculate pKa we used thermodynamic cycle depicted in Figure 4 below. Such a type of thermodynamic cycle was recommended in papers by Liptak et al. and by Zeng et al. [59,63]. We applied methodology presented in those articles in our work. To calculate pKa we used equation (Equation (1) Parameters A and B were determined on the basis of known pKa values for five chemical compounds: formic acid, trifluoroacetic acid, dimethyl malonate, acetone and chloroacetone. These compounds were also optimized in vacuo and water solution, and on the basis of information gathered from frequency analyses we were able to calculate pKa values for each structure. Calculated and known experimental values of pKa of formic acid, trifluoroacetic acid, dimethyl malonate, acetone and chloroacetone are gathered in Table 1 below. Such a type of thermodynamic cycle was recommended in papers by Liptak et al. and by Zeng et al. [59,63]. We applied methodology presented in those articles in our work. To calculate pK a we used equation (Equation (1)) proposed by Zeng et al. [59] included below.
where: ∆G aq -solution phase free energy R-gas constant T-temperature A & B-linear parameters Parameters A and B were determined on the basis of known pK a values for five chemical compounds: formic acid, trifluoroacetic acid, dimethyl malonate, acetone and chloroacetone. These compounds were also optimized in vacuo and water solution, and on the basis of information gathered from frequency analyses we were able to calculate pK a values for each structure. Calculated and known experimental values of pK a of formic acid, trifluoroacetic acid, dimethyl malonate, acetone and chloroacetone are gathered in Table 1 below. As shown in Table 1 values of calculated pK a differed slightly from experimentally measured values of pK a (values of empirical parameters A and B needed in Equation (1). were 0.3541 and −0.3387 for parameters A and B, respectively). The calculated values of pK a for 2-((2,3,3,3-tetrafluoroprop-1-en-1-yl)oxy)but-3-enal 13 and α-trifluoromethylγ,δ-unsaturated aldehyde 19b were 13.3 and 15.1, respectively. Obtained pK a values suggest that acidity of 13 is similar to acidity of dimethyl malonate (experimental pK a value 13.5), while acidity of 19b is greater than the acidity of chloroacetone (experimental pK a value 16.5). Examples of use of, weak bases such as K 2 CO 3 , Et 3 N, and NaH to abstract acidic protons from dimethyl malonate or chloroacetone can be found in the literature. [65][66][67][68] Thus, the obtained calculated results of pK a values of 13 and 19b correlate with experimental observations, in which under the reaction conditions the abstraction of acidic proton can be observed in the first step of the reaction.

Conclusions
In conclusion, starting from conveniently obtained via Claisen rearrangement fluorinated scaffolds 18 or allyl-vinyl ethers 12, new α-hydroxy β-fluoro/β-trifluoromethyl and unsaturated phosphonates have been achieved. The key reactions proceeded under mild conditions with good yields. The unexpected progress of the reactions has been correlated with quantum mechanical calculations and indicates connection with pK a values of the molecules 13 and 19b/19b . To the best of our knowledge of the ever-growing interest in fluorinated phosphonates as enzyme inhibitors, the compounds obtained in this research can serve as material for future synthetic studies. These compounds have been also prepared as examples to illustrate the synthetic potential of fluorinated derivatives 12 and 18.

General Methods
1 H NMR, 13 C NMR, 19 F NMR, 31 P NMR and 2D-NMR spectra were performed on Bruker ASCEND 400 (400 MHz) and Bruker ASCEND 600 (600 MHz) spectrometers. Chemical shifts of 1 H NMR were expressed in parts per million downfield from TMS as an internal standard (δ = 0) in CDCl 3 . Chemical shifts of 13 C NMR were expressed in parts per million downfield and upfield from CDCl 3 as an internal standard (δ = 77.0). Chemical shifts of 19 F NMR were expressed in parts per million upfield from CFCl 3 as an internal standard (δ = 0) in CDCl 3 . 31 P NMR chemical shifts were expressed in parts per million in CDCl 3 . High-resolution mass spectra were recorded by electron spray (MS-ESI) technique using QToF Impact HD Bruker spectrometer. Reagent grade chemicals were used and solvents were dried by refluxing with sodium metal-benzophenone (THF) and distilled under an argon atmosphere. All moisture sensitive reactions were carried out under an argon atmosphere using oven-dried glassware. Reaction temperatures below 0 • C were obtained using a bath cooling (dry ice/iso-propanol). Thin-layer chromatography (TLC) was performed on Merck Kieselgel 60-F254 with EtOAc/hexane as developing systems. Visualization of the reactions products was achieved using UV light (254 nm) and a standard procedure (solution of phosphomolybdenic acid or solution of potassium permanganate). Merck Kieselgel 60 (230-400 mesh) was used for column chromatography (more detailed data in Supplementary Materials).

General Procedure of Periodic Acid Oxidation Synthesis of 13
A solution of 12/12 (1 equiv.) and H 5 IO 6 (2 equiv.) in EtOAc (5 mL) was stirred for 4.5 h. The mixture was filtered, and the filtrate was evaporated to give the crude product as a creamy oil. The crude product was used for the next step [69].

General procedure of Dess-Martin oxidation Synthesis of 19
The Dess-Martin periodinane (1.2 equiv.) was added to a stirred solution of 18 (1 equiv.) in 5 mL of CH 2 Cl 2 at RT. After 5 min the solution was diluted with CH 2 Cl 2 and a mixture of sat. NaHCO 3 and aq. Na 2 S 2 O 3 (1:1) 5 mL. After stirring for 5 min the phases were separated. Organic phases were dried (Na 2 SO 4 ) and concentrated to give oils which were used without further purification [70].

General Procedure of HWE Reaction Synthesis of 14 and 20
Tetraethyl methylenediphosphonate (1.75 equiv.) was added dropwise to a mixture of sodium hydride (60% dispersion in oil, 1.75 equiv.) in 2 mL of dry THF at 0 • C under argon. The reaction mixture was stirred for 10 min at 0 • C and then stirred at room temperature for 20 min. The reaction mixture was cooled to 0 • C, a solution of aldehyde 13/13 (1.75 equiv.) in 1 mL of dry THF was added, and the resulting solution was stirred at room temperature for 15 h. The reaction mixture was concentrated, and the residue was purified using column chromatography (n-hexane/ethyl acetate 90:10 → 50:40) [71].
Diethyl   TEA (1.0 equiv.) was added under an argon atmosphere to a mixture of stirred solution of diethyl phosphite (1.0 equiv.) and an aldehyde 13 or 19 (1.0 equiv.). The reaction mixture was stirred at room temperature overnight, then was diluted with 20 mL of water and extracted with ethyl acetate (3 × 15 mL). The organic layers were washed with NaCl sat. , dried over MgSO 4 or Na 2 SO 4 , filtrated and concentrated under reduced pressure. The residue was purified using column chromatography (chloroform/methanol 100:0 → 100:0.5) [26].