Synthesis of 2-Phenyl-4,5-Substituted Oxazoles by Copper-Catalyzed Intramolecular Cyclization of Functionalized Enamides
2012
Vijay Kumar, S. | Saraiah, B. | Misra, N. C. | Ila, H.
An efficient two-step synthesis of 2-phenyl-4,5-substituted oxazoles involving intramolecular copper-catalyzed cyclization of highly functionalized novel β-(methylthio)enamides as the key step has been reported. These enamides are obtained by nucleophilic ring-opening of newly synthesized 4-[(methylthio)hetero(aryl)methylene]-2-phenyl-5-oxazolone precursors by alkoxides, amines, amino acid esters and aryl/alkyl Grignard reagents, thus leading to the introduction of an ester, N-substituted carboxamide or acyl functionalities at 4-position of the product oxazoles. Synthesis of two naturally occurring 2,5-diaryloxazoles, i.e., texamine and uguenenazole, via two-step hydrolysis–decarboxylation of the corresponding 2,5-diaryloxazole-4-carboxylates has also been described. Similarly, three of the serine-derived oxazole-4-carboxamides were elaborated to novel trisubstituted 4,2′-bisoxazoles through DAST/DBU-mediated cyclodehydration–dehydrohalogenation sequence. The present protocol is complementary and an improvement to our previously reported silver carbonate-induced cyclization of β-bis(methylthio)enamides to 2-phenyl-5-(methylthio)-4-substituted oxazoles.
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