Synthesis of thiopyrano[4″,3″:4′,5′]pyrido[3′,2′:4,5]furo[3,2-d]pyrimidines

Reactions of the 6-hydroxy-thiopyrano[3,4-c]pyridine-5-carbonitrile derivative 1 with α-halo-carbonyl compounds gave the ortho-substituted intermediates 2a–c which were converted into furo[2,3-b]thiopyrano[4,3-d]pyridines 3a–c by fusion of a furan moiety under basic conditions. Further cyclization of 3a–c led to a fusion of a pyrimidine ring, yielding the tetracyclic products 6, 7 and 8. In addition, condensation of 6 with various aromatic aldehydes afforded the corresponding imines 9a,b. Mannich reaction of 7 gave products 10a,b.