π-Annulation reactions of 4-thiazolidinone enaminones in the synthesis of fused bi- and tri-cyclic compounds

The α-C position in 4-oxo thiazolidinones functions as one of the three possible nucleophilic sites in these molecules. We used the inherent reactivity of α-C of the exocyclic double bond (a so called push pull system) to obtain bicyclic fused thiazolidinones via π-annulation cyclization. Appropriate reaction conditions to selectively activate this position and secondary nitrogen towards N,π-annulation were found. Furthermore, the intramolecular vinylogous iminium ion 6-exo-trig cyclization was used to access fused bicyclic precursors for the π-annulation in order to obtain the novel tricyclic structures stereoselectively. [Display omitted]