Heterocycles with a bridge nitrogen atom

Abstract

Mesoionic heterocycles as a rule are unstable. The atypical stable mesoionic 2-oxo derivatives of oxazolo[3,2a]pyridine (especially their 3-acyl-substituted derivatives [2, 3], easily obtained by cyclization of (2-oxo-l-pyridyl)acetic acid on treamaent with acid halides) have been studied in the most detail. A characteristic feature of this subclass of mesoionic systems is the occurrence of reactions of facile opening of the oxazole ring when treated with N- and O-containing nucleophiles. Thus on boiling in water, 2-oxo-3-acyloxazolo[3,2-a]pyridines are c6nverted to N-(~-oxoalkyl)-2-pyridones. Obviously the sequence of the reactions of formation and hydrolytic cleavage of the indicated mesoionie structures allows us to consider them as stable cyclic intermediates in the conversion of hetarylacetic acid to hetarylketone (a variation of the Dakin-West reaction to the amino acid series [4]). In an earlier report [5], the high lability of mesoionic oxazolopyridines relative to nucleophiles served as the basis for the hypothesis that previously unknown recyclizations might be possible for this class. Based on this hypothesis and using the GREH computer program [6], we did a combinatorial sort of promising but as yet unstudied recyclizations, and a number of predictions have already been confirmed experimentally [1]. One of the computer predictions (Scheme (8) in [5]) allowed us to expect a previously unknown "one-pot" recyclization of mesoionic 2-oxo-3-acyloxazolo[3,2-a]pyridines to oxazolo[3,2a]pyridinium cations through intermediate formation of N-(fl-oxoalkyl)-2-pyridone. In this report, this reaction has been actually observed experimentally. We found that when water is added to a solution of 2-oxo-3-benzoyloxazolo[3,2-a]pyridine (I) in cone. H2SOa and then the mixture is treated with HC104 or HBF4 solution, the 2-phenyloxazolo[3,2-a]pyridinium salt (1I) is formed in good yield.