One-carbon homologation of unsymmetrical ketones through magnesium β-oxido carbenoid rearrangement and trapping the enolate intermediates with electrophiles

A procedure for one-carbon homologation of unsymmetrical ketones, including one-carbon ring-expansion of 2-substituted cyclohexanones, through magnesium β-oxido carbenoid rearrangement as the key reaction is described. Addition of the α-sulfinyl carbanion of 1-chloroethyl p-tolyl sulfoxide to an unsymmetrical ketone gave two diastereomers as adducts in good yields. The adducts were treated with a base to give alkoxides, which were treated with a Grignard reagent to give magnesium β-oxido carbenoids. The β-oxido carbenoid rearrangement then took place to afford one-carbon homologated ketones having a methyl group at the α-position. Remarkable specificity or selectivity was observed in the rearrangement. The magnesium enolate intermediates of this reaction were found to be able to get trapped with several electrophiles to give one-carbon homologated α,α-disubstituted ketones. Origin of the specificity and selectivity is also discussed.