pi-Annulation reactions of 4-thiazolidinone enaminones in the synthesis of fused bi- and tri-cyclic compounds
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The a-C position in 4-oxo thiazolidinones functions as one of the three possible nucleophilic sites in these molecules. We used the inherent reactivity of alpha-C of the exocyclic double bond (a so called push pull system) to obtain bicyclic fused thiazolidinones via pi-annulation cyclization. Appropriate reaction conditions to selectively activate this position and secondary nitrogen towards N, pi-annulation were found. Furthermore, the intramolecular vinylogous iminium ion 6-exo-trig cyclization was used to access fused bicyclic precursors for the pi-annulation in order to obtain the novel tricyclic structures stereoselectively. (c) 2012 Elsevier Ltd. All rights reserved.