Synthesis of Thiochromen-4-one-annelated Heterocycles: Regioselective Cyclization of 3-Hydroxy-2-cyclohex-2′-enylthiochromen-4-one

Abstract

2-Cyclohex-2′-enyl-3-hydroxythiochromen-4-one (4) was synthesized in 80% yield by the thermal [3,3] sigmatropic rearrangement of 3-cyclohex-2′-enyloxythiochromen-4-one (3). Treatment of 4 with palladium (bisbenzonitrile) chloride in refluxing benzene afforded linearly cyclized product, tetrahydrobenzofuro[3,2-b]thiochromen-6-one (5) in 98% yield which on dehydrogenation with palladized charcoal gave benzofuro[3,2-b]thiochromen-6-one (6). Substrate 4 on reaction with hexamethylenetetramine hydrotribromide gave a mixture of three products, linearly cyclized heterocycle 10 (35%), bicyclic heterocyclic 8 (15%), and dibromide 9 (20%). The same reaction when conducted with pyridine hydrotribromide furnished only the dibromide 9 which was then cyclized in acetone–potassium carbonate to give the bicyclic compound 8 in 85% yield. The dibromide 9 was cyclized by heating with palladized charcoal in diphenyl ether to give a mixture of linearly cyclized product 5 (7%) and bicyclic product 8 (63%). Substrate 4 was also cyclized in cold conc. H₂SO₄ to give a bicyclic product 11 in 95% yield.

Acknowledgments

We thank the CSIR, New Delhi for financial assistance and University of Kalyani for providing laboratory facilities.