Synthesis 2012; 44(21): 3337-3352
DOI: 10.1055/s-0032-1316790
paper
© Georg Thieme Verlag Stuttgart · New York

Reactions of Keto–Enol Tautomers of 2-Thiazolyl-, 2-Oxazolyl-, 2-Benz­oxazolyl-, or 2-Benzothiazolyl-1-phenylethenols with α,β-Alkynyl Esters: Syntheses of Highly Functionalized Fused-Ring Heterocycles

Hondamuni I. De Silva*
Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA   Fax: +1(662)3251618   Email: cpittman@chemistry.msstate.edu   Email: hicdesilva@gmail.com
,
William P. Henry
Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA   Fax: +1(662)3251618   Email: cpittman@chemistry.msstate.edu   Email: hicdesilva@gmail.com
,
Charles U. Pittman Jr*
Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA   Fax: +1(662)3251618   Email: cpittman@chemistry.msstate.edu   Email: hicdesilva@gmail.com
› Author Affiliations
Further Information

Publication History

Received: 03 July 2012

Accepted after revision: 10 September 2012

Publication Date:
09 October 2012 (online)


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.

Supporting Information

 
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