OXIDATIVE AND THERMAL ROUTES TO NOVEL ISOXAZOLOPYRIDINES

The novel bicyclic compounds isoxazolo[3,4-b]pyridine (1), 3-phenyliso xazolo[3,4-b]pyridine (2) and 3-phenylisoxazolo[3,4-c]pyridine (3) hav e been synthesized by oxidative cyclization of the amines 2-aminopyrid ine-3-carbaldehyde (4), (2-amino-3-pyridyl)phenylmethanone (5) and (3- amino-4-pyridyl)phenylmethanone (6). The oxidant was bis(acetato-O)phe nyliodine. 3-Phenylisoxazolo[3,4-c]pyridine (3) was also obtained by t hermolysis of 3-azido-4-benzoylpyridine (7). Comparison of the rate of thermolysis of this azide with that of 3-azidopyridine identified a n eighbouring group effect (from the benzoyl group) which is about four times as large as in a corresponding 2-azidobenzophenone. COSY and HET COR techniques have been used to assign the H-1 and C-13 n.m.r. spectr a of isoxazolo[3,4-b]pyridine (1).