UNEXPECTED FORMATION OF A 5-TRIFLUOROMETHYLOXAZOLE FROM A 1,2-DIBENZAMIDOALKENE

An unusual cyclisation of Z-1,2,4-tris(benzamido)butene with trifluoroacetic anhydride gives Z-4- [1,3-bis(benzamido)prop-1-enyl]-2-phenyl-5-trifluoromethyloxazole (3). The structure and stereochemistry were confirmed by H-1, C-13 and F-19 NMR, together with H-1-H-1 COSY, H-1-H-1 NOESY and H-1-->F-19 and F-19 --> H-1 heteronuclear NOE experiments. Trifluoroacetylation at the less sterically hindered N-acyl-enamine position, with subsequent cyclisation and 1,4-elimination is proposed.