Unusual ring expansion observed during the Dakin–West reaction of tetrahydroisoquinoline-1-carboxylic acids using trifluoroacetic anhydride: an expedient synthesis of 3-benzazepine derivatives bearing a trifluoromethyl group

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The reaction of N-acyl-1,2,3,4-tetrahydroisoquinoline-1-carboxylic acids with trifluoroacetic anhydride proceeds through mesoionic 1,3-oxazol-5-one intermediates followed by ring expansion to form 2-trifluoromethyltetrahydro-3-benzazepinone derivatives in good yields.