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.