A convenient synthesis of azolo-fused 2H-[1] benzopyrans

Abstract

The synthesis of substituted 7,7-diphenyl-7H-pyrano[3,2-e]benzazoles 2a-f and 7,7-diphenyl-7H-pyrano[2,3-g]benzazoles 4a-d is described. Thus, suitable titanium(IV) phenolates reacted with β-phenylcinnamaldehyde in refluxing aprotic non-polar solvents. Electrocyclisation of o-quinone allides generated in situ gives regiospecifically the title compounds. Stoichiometric amounts of heterocyclic phenol, titanium tetraethoxide and the carbonyl compound, have been found to give better results. In this series, substitution on the α position leads to the formation of the regioisomer. The method could be also extended to the formation of 8,8-diphenyl-8H-pyrano[2,3-e]benzazole 8. A side reaction between the desired pyran and a second molecule of heterocyclic phenol was observed. This condensation product 9 was isolated and characterized in the case of the imidazole derivative, and a mechanism for its formation is proposed.