Organocatalytic Synthesis of Benzo[b]thiepines

Significance

Reported is the synthesis of benzo[b]thiepines from ortho-thiosalicylaldehydes and cyclopropanealdehydes under organocatalytic conditions. Lower loadings of catalyst could be used, but at the expense of yield. The proposed mechanism proceeds via iminium ion formation with the cyclopropanealdehyde forming a reactive intermediate which undergoes sequential ring opening by thiophenol nucleophilic attack followed by iminium ion driven ring closure. Whilst this mechanism is entirely reasonable and has precedence (S. Danishefsky Acc. Chem. Res. 1979, 12, 66), no supporting evidence is provided.