Unexpected Formation of Dimethylthioketene Cycloadducts in the Reaction of 1,3-Diphenylaziridine-2,2-dicarboxylate with Cyclobutanethione Derivatives

Abstract

The reaction of 2,2,4,4-tetramethyl-3-thioxocyclobutanone (1) with cis-1-alkyl-2,3-diphenylaziridines 5 in boiling toluene yielded the expected trans-configured spirocyclic 1,3-thiazolidines 6 (Scheme 1). Analogously, dimethyl trans-1-(4-methoxyphenyl)aziridine-2,3-dicarboxylate (trans-7) reacted with 1 and the corresponding dithione 2, respectively, to give spirocyclic 1,3-thiazolidine-2,4-dicarboxylates 8 (Scheme 2). However, mixtures of cis- and trans-derivatives were obtained in these cases. Unexpectedly, the reaction of 1 with dimethyl 1,3- diphenylaziridine-2,2-dicarboxylate (11) led to a mixture of the cycloadduct 13 and 5-(isopropylidene)-4-phenyl-1,3-thiazolidine-2,2-dicarboxylate (14), a formal cycloadduct of azomethine ylide 12 with dimethylthioketene (Scheme 3). The regioisomeric adduct 16 was obtained from the reaction between 2 and 11. The structures of 6b, cis-8a, cis-8b, 10, and 16 have been established by X-ray crystallography.