Stereospecific ring-contraction of 7,7-dichlorobicyclo[3.2.0]hept-2-en-6-ols with base

Abstract

The epimeric alcohols, 7,7-dichlorobicyclo[3.2.0]hept-2-en-6-endo-and 6-exo-ol (2) and (3) when treated with aqueous base undergo Stereospecific ring contraction with loss of hydrogen chloride to give respectively 6-chloro-bicyclo[3.1.0]hex-2-en-6-exo- and 6-endo-carbaldehyde (5) and (6): the latter exists largely as its valence tautomer. The reaction involves a bent cyclobutane ring with the reacting hydroxy- and chloro-groups disposed trans and diequatorially, and it has been extended to the epimeric 6-methyl derivatives (19) and (20) and a 6-nitro-methyl analogue (24). The betaine (27) formed by addition of stable phosphoranes to 7,7-dichlorobicyclo-[3.2.0]hept-2-en-6-one (1) does not give ring contracted products, but instead the 6-alkoxycarbonylmethylene-bicyclic compound (28) by a normal Wittig reaction. Some spectroscopic properties of the starting alcohols are discussed.