Syntheses with isoxazoles. Part II. Rearrangement of isoxazolo[2,3-a]-pyridinium salts into 5,6-dihydro-4H-furo[3,2-b]pyridin-2-ones

Abstract

Brief treatment of the tetrahydro-4-oxoisoxazolo [2,3-a]pyridinium salts (1) and (12) with boiling acetic anhydride gave the 5,6-dihydro-4H-furo[3,2-b]pyridin-2-ones (4) and (13). The structure of compound (4) has been proved by complete hydrogenation to piperidin-2-ylacetic acid, and by X-ray diffraction. The 2-methylisoxazolo-[2,3-a]pyridinium salt (11) did not undergo the rearrangement; this suggests a keten intermediate, formed by removal of the hydrogen atom at position 2. The 5-bromo-derivative (22) gave no furopyridine; cleavage of either the pyridine or the isoxazole ring occurred.