1, 3-Thlazolioin-4-ones. Synthesis and X-Ray Structure Clarification of the Structure of the Products Derived from the Condensation of Thioureas and Acetylene Dicarboxylc Acid or Its Esters

The reaction of thioureus or N-Substituted Thioureas with dimethyl acetylene dicarboxylate is an extremely simple reaction which proceeds rapidly and quantitatively. The resultant heterocyclic products have been the subject of numerous publication, 2–8 the variety of which demonstrates the truly international interest in the field. Various structures have ben proposed and received support: the 5-membered ring systems 1, 3-thiazolidines (3)2–4 and 2-thiohydantoius (e.g.12)5 and the 6-membered 1,3-thiazines (5)6–8. The most convincing chemical and spectroscopic evidence has been reported by loan and ifa 6 in support of the 1,3-thiazine system. This work has since been challenged by ourselves 9 and others4. Indeed the recent work by Cameron et al 4, who have carried out an X-Ray structure analysis on Product (1)(Derived from the reaction of N-thiocarbomaoyl-piperidine and dimethyl actylane dicarboxylate), fully justifies our reservation.