Transition-metal mediated asymmetric synthesis. Part 3. Preparation and stereospecific alkylation of unsymmetrically substituted tricarbonyl(cyclohexadienyl)iron(1+) salts: an organometallic approach to the synthesis of carvone, cryptomerion, and bilobanone

Abstract

Tricarbonyl(η⁵-3-methoxy-2-methylcyclohexa-2,4-dienyl)iron(1+)hexafluorophosphate(1–) has been prepared by a novel method based on thallium(III) oxidation. The opposite substitution pattern is obtained by hydride abstraction. Alkylation by bis(1-methylethenyl)cadmium, to introduce an olefinic side-chain, has been shown to be stereospecific for the α-face, and removal of the metal affords the expected enones. (±)-Carvone and -sylvecarvone have been prepared by this means. The results indicate the importance of regiocontrol of alkylation if chiral salts are to be employed in synthesis.