Abstract
2-Methyl-1,3-thiazole, 2,4-dimethyl-1,3-thiazole, 2,4,5-trimethyl-1,3-thiazole, 2,4,5-trimethyl-1,3-oxazole,
2-methyl-1,3-benzoxazole and 2-methyl-1,3-benzothiazole were each treated with benzoyl
chloride in acetonitrile containing triethylamine to give the corresponding (Z )-2-(heterocyclic)-1-phenylvinyl benzoates. Base hydrolysis of these vinyl benzoates
gave the corresponding 2-(heterocyclic)-1-phenylethenols, which exist in both keto
and enol tautomeric forms. These tautomers were used as starting materials for the
syntheses of fused-ring heterocycles. The reactivity of the keto–enol tautomers depends
on the nature of the heteroatom and the substituents that are present on the ring.
Each tautomeric pair reacts with dimethyl acetylenedicarboxylate (DMAD) in methanol
to give the 5,6-ring-fused 8-benzoyl-5-oxo-5H -thiazolo- and 8-benzoyl-5-oxo-5H -oxazolopyridinecarboxylate, 4-benzoyl-1-oxo-1H -pyrido[2,1-b ]benzoxazolecarboxylate and 4-benzoyl-1-oxo-1H -pyrido[2,1-b ]benzothiazolecarboxylate derivatives. Two novel 5,7-ring-fused compounds, tetramethyl
9-benzoyl-2,3-dimethyl-5,6-dihydrothiazolo[3,2-a ]azepine-5,6,7,8-tetracarboxylate and its oxazole analogue, were also obtained when
the tautomers formed from 2,4,5-trimethyl-1,3-thiazole and 2,4,5-trimethyl-1,3-oxazole,
respectively, were treated with DMAD. Reactions of the tautomers with methyl propiolate
did not, however, give satisfactory results.
Key words tautomerism - heterocycles - polycycles - Michael additions - cyclocondensations
