Chalcones (I) reacted with malanonitrile and ammonium acetate yielded 2-amino-3-cyano-4,6-disubstituted pyridines (II) in excellent yield. 4-Amino-5,7-disubstituted pyrido [2,3-d]pyrimidine-2(1H)-thiones (III), 4-amino-5,7-disubstituted pyrido[2,3-d]pyrimidines (IV) and 4-imino-3,5,7-trisubstituted pyrido[2,3-d]pyrimidin-2(1H)-ones (V) have been synthesized by the condensation of compound (II) with thiourea, formamide and arylisocynate respectively. The ribofuranosides viz. 4-amino-5,7-disubstituted-1- [2',3',5'-tri-o-benzoyl- β , D-ribofuranosyl]pyrido[2,3-d]pyrimidine-2-(1H)-thiones (VI) and 4-imino-3,5,7- trisubstituted-1- [2',3',5'-tri-o-benzoyl-β , D-ribofuranosyl] pyrido [2,3-d] pyrimidine-2(1H)-one (VII) were synthesized by converting compounds III and V to trimethylsilyl derivative in situ by reacting them with hexamethyldisilazane to give corresponding ribofuranosides with β-D-ribofuranose-1-acetate-2,3,5-tribenzoate. Compounds III-V and their ribofuranosides have been screened for antimicrobial and antifungal evaluations.