Beckmann and cyclization reactions of δ-oxo-α,β- unsaturated ketoximes obtained from pyrylium salts and hydroxylamine. Formation of 2-aryl(or alkyl)amino-4,6- disubstituted pyrylium salts

Article abstract of DOI:10.1016/S0040-4020(99)00958-8

The reaction of 2,4,6-trisubstituted pyrylium salts 1 with hydroxylamine gave regio- and stereo-selectively 1,3,5- trisubstituted 2-cis-pentene-1,5-dione 1-oximes 4. On cyclization, 3,5,5-trisubstituted 2-isoxazolines 6 and 2,4,6- trisubstituted pyridine 1-oxides 5 were obtained, originating in the anti/syn stereoisomers of oxime 4, respectively. Beckmann reaction of keto-ketoximes 4 with thionyl chloride unexpectedly gave 2-aryl (or alkyl)amino-4,6-disubstituted pyrylium salts 7, the first example of rearrangement/cyclization involving carbonylic oxygen as terminator. Crystallographic data are provided for (Z)-N-t-butyl-3,6,6-trimethyl-2-heptenecarboxamide 13b.