Intramolecular Diels–Alder reactions of N-alkenyl-2(1H)-pyrazinones: generation of a novel type of cis-1,7-naphthyridine

The intramolecular Diels–Alder reaction of three 1-(ω-alkenyl)-2(1H)-pyrazinones was investigated. Cycloaddition was shown to proceed for 1-(4-pentenyl)- and 1-(5-hexenyl)-2(1H)-pyrazinone 5a and 5b to give the corresponding tricyclic hydrolysed adducts 7a and 7b, but not for the 1-(3-butenyl) derivative 5c. In contrast to 7b, the more strained 7a underwent selective acidic solvolysis of the tertiary lactam group to give methyl cis-8-oxo-6-phenyldecahydro[1,7]naphthyridine-6-carboxylate 8. An NMR analysis of 8 is presented.