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Solar photo-thermochemical syntheses of 4-bromo-2,5-substituted oxazoles from N-arylethylamides

Dinda, Milan, Samanta, Supravat, Eringathodi, Suresh, Ghosh, Pushpito K.
RSC advances 2014 v.4 no.24 pp. 12252-12256
Lewis bases, bromination, carbon-hydrogen bond activation, chemical bonding, cold treatment, hydrobromic acid, oxazoles, thiazoles
Solar photo-thermochemical C(sp³)–H bromination, conducted efficiently in a specially designed reactor, was reported recently. In the present study, the more complex formation of 4-bromo-2,5-substituted oxazoles from N-arylethylamides using this approach was achieved. The one-pot syntheses were carried out with N-bromosuccinimide–dichloroethane over 6 h (10.00 am to 4.00 pm) on sunny days. The isolated yields were in the range 42–82%. Benzylic bromination, followed by O–C bond formation through intramolecular nucleophilic substitution, and a second benzylic bromination followed by HBr elimination, gave the oxazole ring. A third bromination of the ring yielded the final product. The feasibility of synthesizing thiazole derivatives using a similar approach was also demonstrated. During the course of the reactions, succinimide and HBr were co-generated in the aqueous phase. Treatment with NaBrO₃ and additional acid returned the reagent in 57% isolated yield upon chilling. The overall methodology was greener as a result.