Diastereoselective manipulations of bicyclo[m.1.0]alkane derivatives. 6. Stereocontrolled synthesis of tricyclo[m.n.0.0]alkenones

Enolates derived from bicyclo[m.1.0]alkan-2-ones possessing 5-, 6-, and 7-membered rings were sequentially alkylated with iodomethane and with precursors to 2-oxopropyl or 3-oxobutyl substituents. High diastereoselectivities were observed. Product yields for more active electrophiles were generally good to very good and were fair for less active electrophiles. Following unmasking of the 2-oxopropyl or 3-oxobutyl substituents, ring closure and dehydration under basic conditions provided the corresponding tricyclic γ,δ-cyclopropyl-α,β-enones. Reversal of the order of alkylation switched the configuration of the angular methyl substituent relative to the stereogenic cyclopropane in the tricyclo[m.n.0.0]alkenone product.