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111. Ring Closure Reactions of Pyrido[2,3-d]pyrimidines to Pyrano[2',3':4,5]- and Oxazolo[5',4':4,5]pyrido[2,3-d]pyrimidines
Dang V. T. and W. Stadlbauer, in press (4/08)

The cyclocondensation of 5-hydroxy-pyrido[2,3-d]pyrimidines 1 with malonates gives pyrano[2',3':4,5]pyrido[2,3-d]pyrimidines 2. Nitration of 1 and reduction with zinc in the presence of carboxylic acids/anhydrides gave 2-alkyloxazolo[5',4':4,5]pyrido[2,3-d]pyrimidines 4, which were ring-opened to 6-aminopyrido[2,3-d]pyrimidines 5, 6 and 7. Cyclization of 6-aminopyrido[2,3-d]pyrimidines 6 with benzoylchlorides 8 gave 2-aryloxazolo[5',4':4,5]pyrido[2,3-d]pyrimidines 9. Reaction conditions for the cyclization have been studied by differential scanning calorimetry (DSC).

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