Enantiodivergent syntheses of cycloheptenone intermediates for guaiane sesquiterpenes

The syntheses of enantiomeric 6-isopropenyl-3-methyl-2-cycloheptenones 16 a nd 22 have been effected starting from (R)-(-)-carvone. In the synthesis of 16, (R)-(-)-carvone was reduced and the resulting dihydrocarvone transform ed regioselectively into silyl enol ethers. Cyclopropanation with dibromoca rbene and in situ rearrangement gave an alpha -bromo-cycloheptenone which w as reduced to the (R)-(+)-cycloheptenone 16. In the synthesis of 22, (R)-(- )-carvone was cyclopropanated with a sulfur ylide, followed by reduction wi th LAlH4 and acid-catalyzed cyclopropylcarbinyl rearrangement to afford a c ycloheptenol. Oxidation and double bond conjugation led to the (S)-(-)-cycl oheptenone 22 in a partially racemized form. Four cycloheptenones have tree n obtained and are suitable intermediates for the enantiodivergent synthese s of guaiane sesquiterpenes. (C) 2000 Elsevier Science Ltd. All rights rese rved.