Toward the Total Synthesis of Alpkinidine: Michael Addition to Isoquinolinetrione CE Ring-System Synthons

Strategies toward the total synthesis of the marine pyrroloacridine alkaloid alpkinidine have been explored, focusing on linking quinonoid CE ring-system synthons with the A ring, followed by condensation to form the B and D rings. The key Michael addition of the ester enolate derived from ethyl o-nitrophenylacetate to 2-methylisoquinoline-1,5,8(2H)-trione proceeded with the wrong regiochemistry. This issue was addressed by incorporating the D-ring nitrogen at an earlier stage, affording advanced intermediates possessing the complete carbon skeleton of alpkinidine. However, attempts to close the D and B rings were unsuccessful. The novel isoquinolinetriones reported here, and the general strategy of connecting CE- and A-ring synthons through Michael additions, may be useful in the synthesis of other pyrrolo- and pyridoacridines, in particular the anticancer lead neoamphimedine and analogues.


■ INTRODUCTION
In the preceding paper in this issue, we reported concise syntheses of 6-halo and 6,7-dichloroisoquinolinetriones matching the CE ring system of the marine secondary metabolite alpkinidine (10) and unsuccessful efforts to elaborate these to the natural product (Scheme 1). We previously described an efficient route to the model pentacyclic pyrroloacridine 5 by Michael substitution of 2,3-dichloronaphthoquinone (1) with the anion derived from ethyl o-nitrophenylacetate (2) to give 3, D-ring formation through nucleophilic lactamization with methylamine, and reductive cyclization of 4 to close the B ring. 1 Despite this precedent, attempts to effect an analogous Michael substitution of dichloroisoquinolinetrione 6 were unsuccessful. 2 Formation of the key carbon−carbon bond ultimately comprising the BD-ring fusion was achieved in the analogous reaction of monobromide 8; however, we were not able to progress adduct 9 toward alpkinidine (10).
Herein, we report a continuation of our efforts toward the total synthesis of alpkinidine, specifically the use Michael addition (as opposed to substitution) to form the key carbon− carbon bond highlighted in red in Scheme 1.

■ RESULTS AND DISCUSSION
At the outset, attempts to use Michael addition reactions to couple 1,4-naphthoquinone (11) and o-nitrophenylacetonitrile (12) had been unsuccessful, producing what appeared to be oligomeric products instead of the expected hydroquinone 13 or quinone 14 adducts (Scheme 2), 1 hence our prior focus on Michael substitution of haloquinones 6 and 8 (Scheme 1). 2 In an effort to resurrect the general strategy of connecting the CE-and A-ring systems of potential precursors to alpkinidine, through the reaction of carbanions with quinone electrophiles, 2 the reaction of ethyl o-nitrophenylacetate (2) with isoquinolinetrione 15 2 was investigated and indeed provided a hydroquinone adduct in modest yield (Scheme 3). Unfortunately, heteronuclear multiple bond correlation (HMBC) spectroscopy revealed the product to be the undesired regioisomer 16. In an attempt to elaborate 16 through to the isomer of alpkinidine with an inverted E-ring 19, the hydroquinone was oxidized, and quinone 17 was treated with methylamine in air, but the expected lactam 18 was not detected in the complex mixture of products formed.
The successful Michael addition to give 16 was encouraging and inspired efforts to achieve the opposite regioselectivity required for the synthesis of alpkinidine (10). Isoquinoline-1,5,8-(2H)-trione (30) had not been reported, but its hydroxypyridine tautomer 31 was assumed to be more stable by virtue of a strong intramolecular hydrogen bond (Scheme 4). It was hypothesized that this hydrogen bond might promote the desired regiochemistry by withdrawing electron density from the peri-carbonyl, thereby increasing electrophilicity at C6, and potentially provide anchimeric assistance for the Michael addition reaction, that is, through general acid catalysis under equilibrium deprotonation conditions. N-Methylation of the E-ring nitrogen could then follow at a later stage en route to alpkinidine. Hence, we set out to prepare 31.
Amidation of 2,5-diacetoxybenzoic acid (20) 7 with commercial aminoacetal 21 gave 22, along with the partially deacetylated product 23 (Scheme 4). The mixture was treated with concentrated H 2 SO 4 but, while the analogous N-methylamide cyclized under these conditions, 2 in this case the isoquinolone 26 could not be detected in the complex mixture of products formed. In an attempt to avoid phenol protecting groups, methyl 2,5-dihydroxybenzoate (24) was heated with neat amino acetal 21, providing the amide 25 in acceptable yield. Annulation in neat sulfuric acid was successful in this instance but provided the hydroquinone 26 in poor yield. The Pomeranz−Fritsch-like cyclization was more efficient with the dimethyl ether 28, derived from 2,5dimethoxybenzoic acid (27), and smooth Prey demethylation 8 of 29 was achieved. However, attempts to oxidize the hydroquinone 26 to the quinone 30/31 gave complex mixtures of products, and thus, this line of investigation was abandoned.
The regiochemical issues described above (and in Scheme 3) could potentially be solved by incorporation of the 7aminomethyl group at an earlier stage of the synthesis (Scheme 5). Michael addition of ethyl o-nitrophenylacetate (2) to a suitably protected aminoquinone 32 would give hydroquinone 33, which upon deprotection of the amino group, would be expected to cyclize to give lactam 34. Reduction of the nitro group followed by aerial oxidation would then allow cyclodehydration to give alpkinidine (10).
Serendipity played a role in allowing us to explore the general strategy set out in Scheme 5. During an attempt to oxidatively demethylate 35, 2 Ag 2 O was mistakenly used instead of AgO (Scheme 6). Unsurprisingly, no quinone 15 was isolated from this reaction, but the electron-rich ring system was sufficiently reactive to undergo nitration with the nitric acid. Two regioisomers were isolated, with X-ray crystallography confirming the identities as the 7-nitroisoquinolone 36 and 4-nitroquinoline 37 isoquinolones. A similar yield of the major regioisomer 36 was obtained when the reaction was repeated in the absence of the silver(I) oxide. Surprisingly, when this reaction was repeated several years later, the ratio of 36:37 was inverted (see yields in brackets), as confirmed by comparison of the 1 H NMR spectra of the crude product from each reaction. The only possible explanation we could posit for the change in product ratio is differing water content of the nitric acid/acetone. The variable outcomes for this reaction led us to devise alternative routes to 7nitrogenated isoquinolones; the first is set out in Scheme 7. Formylation of p-methoxyphenol (38), 9 or peri demethylation of 2,5-dimethoxybenzaldehyde 10 (see the Experimental Section), gave salicylaldehyde 39, which was regioselectively nitrated providing 40. 11 O-Methylation was followed by oxidation of 41 to the benzoic acid 42, which was converted to the acid chloride and amidated with secondary amine 43. 2 Unsurprisingly, cyclization of benzamide 44 in sulfuric acid was not as facile as for the des-nitro analogue, 2 and a temperature of 100°C was required to achieve an acceptable yield of the isoquinolone 36.  Smooth reduction of 36 to the aniline 45 was effected with Fe or SnCl 2 , and followed by derivatization, affording the acetanilide 46 or carbamates 47 and 48 (Scheme 8). The carbamates were also N-methylated providing 49 and 50. It became apparent that these compounds could be accessed more efficiently from 35 2 (Scheme 8). Thus, selective demethylation peri to the carbonyl provided phenol 51, which underwent regioselective nitration affording 52, as confirmed by 2D NMR experiments. Reduction to the aniline 53 was followed by conversion to methyl carbamate 54; the use of pyridine as solvent/base avoided O-carboxymethylation, which was a competing side reaction with the stronger base triethylamine. Finally, N,O-dimethylation afforded 49.
An attempted oxidative demethylation of 45 (Scheme 9) gave an intractable mixture, in line with previous observations from reactions of dimethoxyanilines with CAN. 12 In any case, it was expected that the amino group in 55 would be so electron-donating as to preclude a subsequent Michael addition at C6, hence the protection with electron-withdrawing groups. The oxidative demethylation of the secondary acetamide 46 and carbamates 47 and 48 proceeded as expected to the give corresponding quinones 56−58, respectively. In surprising contrast, the analogous reactions of the tertiary carbamates 49 and 50 failed to give the expected quinones 59 and 60. In the case of the methyl carbamate 49, dinitration was instead observed, affording 61 in moderate yield. Related nitrations of electron-rich aromatic compounds with CAN have been reported. 13−16 No reaction of 49 was observed with phenyliodine(III)-bis(trifluoroacetate) (PIFA). 17 When the t-butyl carbamate 50 was treated with CAN, cleavage of the Boc group to give aniline 62 was the only reaction observed. CAN-mediated cleavage of Boc groups is well known; 18 however, a sensible explanation for the difference in reactivity between the secondary (47/48) and tertiary (49/50) carbamates escapes us.
With appropriately nitrogenated quinones in hand, the key Michael addition step was investigated (Scheme 10). The reaction of 2 with acetamide 56 gave a complex mixture of products, but encouragingly, Michael additions to the methyl (57) and t-butyl (58) carbamates were successful; the initially formed hydroquinone adducts presumably oxidized during workup to the isolated quinones 64 and 65, respectively. A single attempt to deprotect 64 failed to give the expected Scheme 6. Serendipitous Regioselective Nitration of 35 a lactam 67, producing a complex mixture instead. More effort was devoted to the deprotection/cyclization of 65. Surprisingly, the t-butyl carbamate 65 was resistant to deprotection with TFA at rt and gave complex mixtures with BF 3 ·OEt 2 19 or H 2 SO 4 in dioxane. 20 Treatment with 5 M hydrochloric acid under reflux, or ethanolic HCl, gave a major product that could not be conclusively identified. The 1 H NMR spectrum of this product is consistent with the ortho-quinone methide 66 or its geometric isomer, although definitive structural elucidation was precluded by material availability, and hence, this assignment is tentative. Unfortunately, attempts to cyclize this product to give lactam 67, or a tautomer, were unsuccessful.

■ CONCLUSIONS
Michael additions of ethyl o-nitrophenylacetate (2) to quinonoid electrophiles have been investigated as a means to connect fragments comprising the CE ring system and A ring of the marine pyrroloacridine alkaloid alpkinidine (10). While the reaction of the anion derived from 2 with isoquinolinetrione 15 proceeded with the wrong regiochemistry, the successful coupling of the two fragments and oxidation to give  To direct the regiochemistry of the key Michael addition, 7nitrogentated isoquionlinetriones were targeted, and several efficient syntheses were developed (Scheme 11B). These isoquinolones and the general concept of Michael additions to such synthons may be of value in the synthesis of analogues of the closely related marine alkaloid neoamphimedine, which has a near-identical ABCE ring system to alpkinidine and promising anticancer potential. 3,21−23 Proof of concept was achieved in that 2 underwent Michael addition to isquinolinetriones 57/58 and aerial oxidation to afford adducts 64/65, containing the complete carbon skeleton of alpkinidine. However, attempts to close rings B and D were unsuccessful. The key to bringing the approach outlined herein to fruition probably lies in finding the right order of redox and deprotection reactions on an advanced intermediate like 64/ 65, an objective that will be pursued in the future.
Method C (36 exclusively): Ice-cold conc. H 2 SO 4 (10 mL) was added dropwise to neat 44 (5.37 g, 15.1 mmol) with stirring at 0°C. After the addition was complete, the solution was warmed to rt, then heated to 100°C. After 1 h, the solution was cooled and carefully neutralized with ice-cold sat. aq. NaHCO 3 until effervescing ceased. The aqueous phase was extracted with EtOAc (3 × 30 mL), and the extract was dried and evaporated. The residue was purified by flash column chromatography. Elution with 3:2 EtOAc/hexanes yielded isoquinolone 36 as a yellow solid (2.59 g, 65%), identical to the material described above.
2-Hydroxy-5-methoxybenzoic acid (39). Method A: 9 Anhydrous THF (250 mL) was added to anhydrous MgCl 2 (15.35 g, 161.2 mmol) and paraformaldehyde (7.26 g, 242 mmol) under a positive pressure of N 2 . NEt 3 (22.47 mL, 161.2 mmol) was added dropwise to the stirred suspension, and after 10 min, 4-methoxyphenol (38) (10.00 g, 80.55 mmol) was added, resulting in an opaque, light-green mixture. The reaction mixture was heated under gentle reflux whereupon it rapidly turned orange/yellow color. After 6 h, the reaction mixture was cooled to rt and rinsed with ether (150 mL) into a separatory funnel. The organic phase was washed with 1 M HCl (3 × 150 mL), water (3 × 150 mL), and brine (150 mL), dried, and evaporated to leave a pale-yellow oil, which was subjected to flash column chromatography. Elution with 2:23 EtOAc/hexanes gave the benzaldehyde 39 as a pale-yellow oil (12.15 g, 99%). 1 26 Method B: 10 Anhydrous AlCl 3 (12.04 g, 90.32 mmol) was added to a stirred solution of 2,5-dimethoxybenzaldehyde (10.00 g, 60.12 mmol) in anhydrous DCM (200 mL) at 0°C under N 2 . The reaction mixture was allowed to slowly warm to rt, and stirring was continued for 5 h. When TLC indicated that the reaction was complete, the mixture was diluted with ice/water (300 mL) and the DCM layer was separated. The aqueous phase was extracted with EtOAc (2 × 100 mL). The aqueous phase was then acidified with concentrated HCl and reextracted with EtOAc (2 × 100 mL). The combined organic phase was washed with brine (200 mL), dried, and evaporated to give a brown oil that was subjected to flash column chromatography. Elution with 8:92 EtOAc/hexanes gave the phenol 39 as a colorless oil (7.84 g, 86%), spectroscopically identical with the material described above.

181−183°C
]. 1 30 N-(2,2-Diethoxyethyl)-2,5-dimethoxy-N-methyl-3-nitrobenzamide (44). A solution of 2,5-dimethoxy-3-nitrobenzoic acid (42) (7.59 g, 33.4 mmol) and SOCl 2 (10.67 mL, 147.1 mmol) in DCM (10 mL) was heated under reflux and moisture guard for 2 h before the solvent and excess SOCl 2 were evaporated under a stream of N 2 . The residue was cooled to 0°C, and a solution of pyridine (10.66 mL, 132.3 mmol) in DCM (7 mL) was added dropwise, followed by dropwise addition of a solution of amine 43 2 (7.38 g, 50.2 mmol) in DCM (7 mL). The mixture was stirred at rt for 3 h, then diluted with H 2 O (50 mL), and extracted with DCM (3 × 20 mL). The extract was washed with 10% aq. CuSO 4 (4 × 20 mL), sat. aq. NaHCO 3 (20 mL), and brine and dried and evaporated to give the amide 44 as a reddish oil (9.50 g, 80%) sufficiently pure for the next step.  (30 mL), and the mixture was heated under reflux for 1.5 h. The reaction mixture was cooled to rt, then poured into ice (150 g), and the resulting suspension was made alkaline by an addition of sat. aq. NaHCO 3 . The aqueous phase was extracted with DCM (3 × 25 mL), and the extract was washed with brine (25 mL) and evaporated. The residue was then acidified with 1 M HCl and extracted with DCM (2 × 25 mL). The aqueous layer was made alkaline by addition of solid NaHCO 3 . The excess solid was filtered, and the filtrate was extracted with DCM (3 × 20 mL). The combined organic phase was washed with brine (20 mL) and evaporated to afford the aniline 45 as a yellow oil (1.00 g, 85%), spectroscopically identical to the material described above. 0.28 mmol) and NEt 3 (0.12 mL, 0.86 mmol) in DCM (10 mL) at 0°C. The solution was allowed to warm to rt, and stirring was continued for 3 h; then, the reaction was diluted with H 2 O (20 mL) and extracted with DCM (3 × 20 mL). The extract was dried and evaporated, and the residue was subjected to flash chromatography. Elution with EtOAc gave carbamate 47 as a pale-yellow solid (54 mg, 66%), mp 160−163°C. R f (EtOAc) 0.3. IR (ATR) ν max cm −1 : 3423 (NH), 1736 (O COMe), 1650 (OC1). 1  Method B: MeI (0.11 mL, 1.77 mmol) and NaH 60% dispersion in oil (72 mg, 1.8 mmol) were added to a stirred solution of 54 (123 mg, 0.44 mmol) in anhydrous THF (1 mL) under N 2 . Stirring was continued overnight, then the reaction mixture was poured into ice/water (30 mL) and extracted with DCM (3 × 10 mL). The extract was washed with brine, dried, and evaporated, and the residue was subjected to flash chromatography. Elution with 1:19 MeOH/DCM gave tertiary carbamate 49 as a white solid (106 mg, 78%), identical with the material described above.