SYNTHESIS OF 8-METHYL-1,2,3,4-TETRAHYDROPYRIDO-[3,4-D]PYRIMIDINE-2,4-DIONES

A new method is described for the preparation of 8-methyl-1,2,3,4-tetrahydropyrido [3,4-d]pyrimidine-2,4-diones 4, and 4-imino-8-methyl-1,2,3,4-tetrahydropyrido [3,4-d]pyrimidine-2-ones, 3, starting from 4-methyl-5-oxazolylureas 1 and substituted olefins. A plausible mechanism is proposed with a Diels-Alder adduct from the reagents as an intermediate that aromatizes by losing water to yield compounds 3. The latter give the title compounds 4 after hydrolysis.