... [4] Simple N, O and S heterocyclic esters: [a] I. Felner- Caboga and K. Schenker, German Patent 2,100,422 (1971); Chem. ... [5] Fused-ring N heterocycles: [a] CJ Blankley, European Patent 58,567 (1982); Chem. ... [17] WC Stille, M. Kahn and A. Mitra, J. Org. Chem., 43, 2923 (1978). ...