| Abstract
| - The highly enantiocontrolled total syntheses of natural (+)-cheimonophyllon E (5) and (+)-cheimonophyllal (6), biologically intriguing oxygenated bisabolane-type sesquiterpenoids, have beencompleted. The present synthetic strategy featured the use of an asymmetric aldol-type reactionfor preparing in the first synthetic step an optically active 6-C-substituted 3-methyl-2-cyclohexenonederivative. Thus, a Mukaiyama aldol reaction of 1-methyl-3-silyloxy-1,3-cyclohexadiene 31 withα,β-unsaturated aldehyde 11 in the presence of a chiral (acyloxy)borane (CAB)-type Yamamotocatalyst 33 proceeded with high levels of both diastereo- and enantioselectivities. The predominantaldol adduct, syn-9, was transformed into γ,δ-epoxy allylic alcohol 8 by a nine-step sequence,including the substrate-controlled 1,2-reduction of enone, syn-12, also the epoxidation of allylicalcohol 15. Epoxy-alcohol 8 underwent 5-exo-cyclization in a high regioselective manner under acidicconditions to produce a bicyclic key intermediate (+)-7, which was eventually efficiently convertedto (+)-cheimonophyllon E (5) or (+)-cheimonophyllal (6).
|
