Some novel annulated pyrido[2,3-d]pyrimidines 6 and 7 were synthesized stereoselectively by intramolecular hetero Diels-Alder reactions involving 1-oxa-1,3-butadienes.
Deb, M. L., & Bhuyan, P. J. (2010). Synthesis of some novel annulated pyrido[2,3-d] pyrimidines via stereoselective intramolecular hetero Diels-Alder reactions of 1-oxa-1,3-butadienes. Beilstein Journal of Organic Chemistry, 6. https://doi.org/10.3762/bjoc.6.11