Studies on uracils: Synthesis of novel uracil analogues via 1,5- and 1,6-intramolecular cycloaddition reactions

6-(Tertiary amino)uracils 1 react with dimethyl acetylenedicarboxylate (DMA D) to afford 5.6-dihydropyrrolo[2.3-d]pyrimidines 3a and 3b and the tricycl ic analogues 3c-f via 1,5-electrocyclisation in excellent yields, whereas s uitably functionalized uracil derivatives 5 undergo intramolecular 1,6-cycl oaddition reactions to afford 5.6.7.8-tetahydropyrido[2.3-d] pyrimidines 6a and 6b and the tricyclic analogues 6c-h in high yields.