Reaction of pyrylium salts with nucleophiles. Part 25. Formation of pyridine-1-oxides, 2-isoxazolines and 1-pyrazoline-1-oxides

Article abstract of DOI:10.1016/S0040-4020(98)00530-4

The reaction of tri- and tetrasubstituted pyrylium salts with hydroxylamine affords acyclic keto-ketoximes (to be described in a separate paper) which cyclize yielding pyridine-1-oxides and/or 2-isoxazolines. Both these classes of compounds, with many possible applications, can thus be obtained simply and in good yield. The regioselectivity of 2-isoxazoline formation from unsymetrically substituted pyrylium salts is discussed. An unprecedented minor product formed from 2,6-di-t-butyl-4-methylpyrylium perchlorate and two molecules of hydroxylamine is the corresponding 1- pyrazoline-1-oxide. Mechanistic rationalization of all products are provided.