The 2-acylcycloalkanones 1a-g and 3a-c, possessing a polyfluoroalkyl group. react with hydroxylamine regio- and stereoselectively to yield 4,5-dihydroisoxazol-5-ols 2a-g and 4a-c, respectively i.e., products of N-addition to the oxo group at the cycloalkane ring (Schemes 1 and 2). The products 2 and 4 can be dehydrated under drastic conditions only (Schemes 3 and 4). The structure of one of the 4,5-dihydroisoxazol-5-ols was confirmed by X-ray crystal-structure analysis.
Polyfluorinated 1,3-diketones containing carbocycles react with aldehydes under Lewis-acid catalysis and also with dimethylformamide-dimetylacetal without catalysis, yielding exclusively (E)-configured condensation products involving the methylene group of the carbocycle. Novel trifluoromethylated chromenes are prepared from 2-trifluoroacetylcycloalkanones and salicylaldehyde. The structures of two new compounds, 2-(E)-benzylidene-6-trifluoroacetylcyclohexanone and 4-trifluoroacetyl-2,3-dihydro-1H-xantene, are confirmed by X-ray structure analysis.