On the conformational preferences of the dehydrochlorination of α-chlorosulfoxides

Several acyclic α-chlorosulfoxides have been shown to undergo a γ-dehydrochlorination upon treatment with LDA. The proposed immediate products of γ-dehydrochlorination, thiirane-S-oxides, are unstable under the basic conditions and react further with the LDA; the isolated products are usually E-alkenes and (or) E-vinyl sulfoxides. Some of the proposed intermediate thiirane-S-oxides, compounds 6, 7, 8, and 18, were synthesized independently and treated with one equivalent of LDA in order to mimic the second step of the overall dehydrochlorination/ring opening sequence. The products obtained from the reactions of compounds 6 and 18 compared favourably with those products which were believed to arise from certain conformations of α-chlorosulfoxides 1kB and 1e, respectively. The addition of one equivalent of LDA to 1kA afforded a mixture containing thiirane-S-oxide 8, which is proposed as the immediate product of γ-dehydrochlorination of 1kA. The configurations of 1kB and 1hA were both shown to be threo by X-ray crystallographic studies. Those conformations which are preferred for the dehydrochlorination possess a geometry where the sulfinyl oxygen is anti to any of the substituents of the ring carbons.