Abstract
A synthesis of chiral dimethylcyclopropane derivatives, useful for the synthesis of casbane, lathyrane, and ingenane-type diterpenoids, from easily available (+)-3-carene is described. The silyl enol ether (8) was derived from (-)-cis-4-caranone (7b), which was obtained from (+)-3-carene (6). Ozonolysis of 8 gave 9a and 10. The right half segment (11b) for a synthesis of crotonitenone (3) was formed from 9a in three steps. The epoxide (12) was isomerized to a mixture of the allylic alcohols (13a) and (14a), which was transformed to the ketone (20) in five steps. Methylation of 20 followed by phosphorylation and reduction gave (+)-cis-4-caranone (23) in about 40% overall yield from 12. The methylester (+)-(32b) and its enantiomer (-)-(34b) were synthesized from (+)-6. Ozonolysis of a mixture of the silyl enol ether (30) and (31) followed by methylation with diazomethane gave 32a, which was hydrolyzed to give the desired (+)-32b. The enantiomer (-)-34b was derived from 32a in five steps.
