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

The reaction of 2,4,6-trisubstituted pyrylium salts 1 with hydroxylamine ga ve regio- and stereo-selectively 1,3,5-trisubstituted 2-cis-pentene-1,5-dio ne 1-oximes 4. On cyclization, 3,5,5-trisubstituted 2-isosazolines 6 and 2, 4,6-trisubstituted pyridine 1-oxides 5 were obtained, originating in the an ti/syn stereoisomers of oxime 4, respectively. Beckmann reaction of keto-ke toximes 4 with thionyl chloride unexpectedly gave 2-aryl (or alkyl)amino-4, 6-disubstituted pyrylium salts 7, the first example of rearrangement/cycliz ation involving carbonylic oxygen as terminator. Crystallographic data are provided for (Z)-N-t-butyl-3,6,6-trimethyl-2-heptenecarboxamide 13b. (C) 19 99 Published by Elsevier Science Ltd. All rights reserved.