1,3-Thiazolidine-dicarboxylates from Thioketones and Thermally Generated Azomethine Ylides

The reaction of 9H-fluorene-9-thione (1) with the cis- and trans-isomers of dimethyl 1-(4-methoxyphenyl)-aziridine-2,3-dicarboxylate (cis- and trans-2, resp.) in xylene at 110° yielded exclusively the spirocyclic cycloadduct with trans- and cis-configurations, respectively (trans- and cis-3, resp.; Scheme 1). Analogously, less- reactive thioketones, e.g., thiobenzophenone (5), and cis-2 reacted stereoselectively to give the corresponding trans-1,3-thiazolidine-2,4-dicarboxylate (e.g., trans-8; Scheme 2). On the other hand, the reaction of 5 and trans-2 proceeded in a nonstereoselective course to provide a mixture of trans- and cis-substituted cycloadducts. This result can be explained by an isomerization of the intermediate azomethine ylide. Dimethyl 1,3-thiazolidine-2,2-dicarboxylates 14 and 15 were formed in the thermal reaction of dimethyl aziridine-2,2-dicarboxylate 11 with aromatic thioketones (Scheme3). On treatment of 14 and 15 with Raney-Ni in refluxing EtOH, a desulfurization and ring-contraction led to the formation of azetidine-2,2-dicarboxylates 17 and 18, respectively (Scheme 4)