Influence of ring size on the outcome of sulfide contraction reactions with thiolactams. Isolation of bicyclic ketene S,N-acetals and thioisomunchnones

Reaction between diethyl bromomalonate and 3-phenylpyrrolidine-2-thione yie lded a vinylogous urethane by the Eschenmoser sulfide contraction. However, with 3-substituted piperidine-2-thiones, sulfur was retained in the produc ts, and a range of bicyclic heterocycles. including bicyclic ketene S,N-ace tals (2-alkylidene-1,3-thiazolidin-4-ones) and stable thioisomunchnones, wa s isolated. A novel dimeric ketene SN-acetal was characterised by X-ray cry stallography.