Extracyclic stereocontrolled alkylation of (1R,5S)-4-ethyl-6, 6-dimethyl-3-(phenylsulfonyl)bicyclo/3.1.1~hept-3-en-2-one: a highly stereocontrolled synthesis of (-)-kanshone A

A high quantity of (1R, 5S)-4-Ethyl-6,6-dimethyl-3-(phenysulfonyl)bicyclo(3.1.1)hept-3-en-2-one is derived from (+)-nopinone. This complex is alkylated using reactive alkyl bromides which reveal the participation of allyl bromide. Gamma-alkylated derivatives are obtained in adequate quantities from the complex in an extracyclic stereocontrolled method. 4,4-disubstituted nopinoes are obtained when enone is added to carbon nucleophiles preceding desulfonylation. Enol acetates are obtained with the cleavage of the cyclobutane ring of nopinoes, with no loss of optical purity. 4,4-disubstituted nopinones and cyclobutanes can be formed by chemically converting gamma-alkylated products.