Intramolecular 1,4-dipolar cycloadditions utilizing heteroaromatic betaines

Abstract

anhydro-2-(2-Allyloxyphenyl)-3,5-diphenyl-4-hydroxy-6-oxo-1,3-thiazinium hydroxide and the corresponding ethynyl derivative underwent ready thermal 1,4-dipolar cycloaddition to give a 1 : 1-cycloadduct (whose structure was determined by X-ray crystallography) in a highly regio- and stereo-selective cycloaddition and 1,3-diphenyl-2H-benzo[h]pyrano[4,3-b]pyrid-2(1H)-one, respectively; no cycloaddition was observed, however, on increasing the alkenyl chain by one carbon atom, rearrangement of the thiazinium nucleus to a substituted quinol-4-one being the preferred reaction pathway.