Synthetic Investigation on Isothiazolo[5,4-b]- and [4,5-c]pyridines

Different approaches to the title ring systems were examined, based on the cleavage of the N–O bond in the isoxazolopyridine-4-thiols (3) and (4) which afforded the hydroxy derivatives (6) and (7), respectively, in moderate to good yields. 3-Methylisothiazolo-[5,4-b]- and -[4,5-c]-pyridine (11) and (18), as well as several functionalized derivatives, were easily obtained from these key intermediates through the corresponding 4,6-dichloroisothiazolopyridines (10) and (17)