5‐Imino‐Δ 3 ‐1,2,4‐thiadiazoline derivatives with a linear N‐S…O grouping. Synthesis and crystal structures

A series of carbonyl derivatives of 5‐imino‐Δ 3 ‐1,2,4‐thiadiazolines has been prepared and shown by X‐ray analysis of a selected example to have a trithiapentalene‐like structure. When diphenylketene was used as acylating reagent, the primary product ( 14 ), obtained at room temperature, rearranged on heating in a polar solvent into a Δ 2 ‐thiazolin‐4‐one ( 15 ). The structure of 15 has been confirmed by X‐ray analysis, and shown to be a zwitterion. Rearranged products ( i.e. , 16 and 20‐22 ) were also obtained when 3 was reacted with dimethyl acetylenedicarboxylate or methanesulfonyl chloride, whereas tosyl chloride gave normal tosylated derivatives ( 17‐19 ). The latter have also a nearly linear N‐S…O arrangement, but the interaction between S and O is weak.