ether gave pure la (30 mg, 15%): mp 188-189 "C; NMR7 (acetone-d6 + DzO) d 3.04 (s, 3, CH3), 4.38 (d of q, 1, H3), 4.78 (d, 1, H4), 6.08 (d of d, 1, Hz), 7.25-8.25 (m, 7, H1 and aromatic), 8.73 trans-3,4-Dihydroxy-anti-1,2-epoxy-1,2,3,4-tetrahydro-7methylbenz[a]anthracene (2). A solution of la (23 mg, 0.083 mmol) and rn-chloroperbenzoic acid (230 mg) in 25 mL of dry THF was stirred under Nz for 2 h at room temperature. The solution was diluted with ether, washed twice with 10% aqueous NaOH solution and water, and dried. Evaporation of the solvent (avoiding heating), followed by trituration with ether, gave 2 (13 mg, 57%) as a white solid: mp 183-185 "C; NMR ( a~e t 0 n e -d~ 4.88 (d, 1, H4), 5.40 (d, 1, H1), 7.38-8.41 (m, 6, aromatic), 8.85 (s, trans -1,2-Dihydroxy-an ti -3,4-epoxy-1,2,3,4-tetrahydro-7methylbenz[a]anthraoene (4a). Epoxidation of 3a (23 mg, 0.08 mmol) was carried out by the procedure employed for the analogous reactions of la. Evaporation of the solvent (avoiding heating) gave crude 4a (20 mg, 74%) which showed one major peak (80%) on LC on a Ihpont Si1 column (THF-heptane, 4555). The major peak was collected and identified as 3a: NMR (acetone-d6 + The reactions of 1,2,6-trimethyl-3,5-bis(methoxycarbonyl)-l,4-dihydropyridine (2a), l-methyl-3,5-bis(methoxycarbonyl 1-1,4-dihydropyridine (2b), and l-benzyl-3-acetamido-1,4-dihydropyridine (1) with various sulfonium salts have been investigated. On thermal (60 "C) activation 2a,b react with, for example, methylphenacylphenylsulfonium tetrafluoroborate (3b) to give the respective pyridinium salts, acetophenone, and phenyl methyl sulfide. There is some competing isomerization of the 1,4-dihydropyridines to their (nonreactive) 1,2-dihydro isomers, this being catalyzed by the pyridinium salt formed on reduction of the sulfonium salt. Phenacyl-, acetonyl-, and in some cases benzylsulfonium salts can be reduced. The group to which hydride transfer takes place should be electron deficient. These reductions can be initiated at rwm temperature with visible light (at rmm temperature pyridinium :salt induced isomerization of the 1,4-dihydropyridine to its 1,2 isomer occurs only over a period of several days:i. The effect of visible light can be enhanced greatly by adding small amounts of dyes to the reaction mixtures. Eosin sodium salt, tetraphenylporphine, and R~"(2,2'-bpy)~Cl~ are all capable of increasing the rate of reduction; the last dye is by far the most effective. The results of mechanistic investigations are consistent with the hypothesis that the light-induced reductions are one-electron transfer reactions. In some cases there appears to be a separate thermal mechanism not sensitive to either light or sensitizers. Some sulfonium salts react with dihydropyridines to give the pyridinium salt and sulfide in less than quantitative yield and no reduction products from the sulfonium salt. As determined for the case of the reaction of bromomalonitrile with 1, and presumably also for other reactions, alkylation of an enamine carbon in the dihydropyridine is ...
