Studies on pyrimidine-annelated heterocycles: synthesis of novel pyrazolo[3′,4′:4,5]pyrido[2,3-d]pyrimidines by intramolecular 1,3-dipolar cycloadditions

Abstract

Suitably functionalised 1,3-substituted 6-chloro-1,2,3,4-tetrahydro-2,4-dioxopyrimidine-5-carbaldehydes 3a–1 cyclise intramolecularly to yield novel furo- and thieno-[2″,3″:4′,5′]pyrazolo[3′,4′:4,5]pyrido[2,3-d]pyrimidines 8a–1 in fair to good yields together with the pyrazolopyrimidines 7 as side products. Remarkably, this synthesis not only left the pyrimidine nucleus unaffected but also gave no dimer formation.