Synthesis and structural study of 3,4-dihydro-2(1H)-pyridones and isoxazolo[5,4-b]pyridin-6(7H)-ones

A series of isoxazolo[5,4-b]pyridin-6(7H)-ones (8) have been prepared with a high stereochemical control from the novel 3,4-dihydro-2(1H)-pyridones (7) by reaction with hydroxylamine hydrochloride and subsequent 5-endo-trig cyclization. A structural study by X-ray analyses and theoretical calculations (AM1) of both heterocyclic systems (7 and 8) shows a favoured conformer with the aryl group on C4 in a pseudoaxial position. The same favoured conformation was found in solution according to NOE experiments and comparison of theoretical and experimental coupling constants.