The thio-adduct facilitated, enzymatic kinetic resolution of 4-hydroxycyclopentenone and 4-hydroxycyclohexenone

The addition of 3,4-dimethoxybenzyl thiol 8, as a benzyl thiol surrogate, to racemic 4-hydroxycyclopent-2-enone 2 and 4-hydroxycyclohex-2-enone 15 gave the corresponding cis-adducts (+/-)-3-(3,4-dimethoxybenzylthio)-4-hydroxycyclopentanone 4b and (+/-)-3-(3,4-dimethoxybenzylthio)-4-hydroxycyclohexanone 16 with good diastereocontrol. In both cases, subsequent treatment with vinyl acetate, in the presence of a lipase enabled enantiomer resolution. Thus, (+)-16 and the acetate of its enantiomer, (-)-(1R,2S)-2-(3,4-dimethoxybenzylthio)-4-oxocyclohexyl acetate, (-)-17 were isolated in 98% enantiomeric excess. Based on the 1,4-dioxygenation pattern, (-)-17 can be used to prepare both enantiomers of 4-(tert-butyldimethylsilyloxy)cyclohex-2-enone 19. Firstly, saponification, with a sub-stoichiometric amount of NaOMe, followed by a one-pot silyl ether formation-sulfide elimination sequence gave (+)-19. Then using the same starting material a 6-step sequence, featuring a diastereoselective NaBH4 reduction and a Cope-type sulfoxide elimination, gave (-)-19.