A Study on the Base–Catalyzed Reverse Vinylogous Aldol Reaction of (4aβ5β)-44a5678-Hexahydro-5-hydroxy-14a-dimethylnaphthalen-2(3H)-one under Robinson Annulation Conditions

摘要

We have proposed a pathway of the base–catalyzed reverse vinylogous aldol reaction of (−)-(4aβ,5β)-4,4a,5,6,7,8-hexahydro-5-hydroxy-1,4a-dimethylnaphthalen-2(3H)-one ((−)->8) under Robinson annulation conditions. For confirmation, 4-(2,6-dimethyl-3-oxocyclohex-1-enyl)butanal (>11) and 4-(2,6-dimethyl-5-oxocyclohex-1-enyl)butanal (>12), both of which potentially produce enolate >I, were synthesized regioselectively. Unexpectedly, >11 gave a complex mixture including only a trace amount of (±)-8 (less than 5% yield) under these basic conditions. To the contrary, >12 cleanly afforded (±)->8 in 66% yield. This result provides evidence for our proposed mechanism of the above reaction.