A new base‐induced ring transformation of pyrido[2,3‐ d ]pyrimidines

8‐Substituted 5,8‐dihydro‐5‐oxopyrido[2,3‐ d ]pyrimidine‐6‐carboxylates ( 3 ) rearranged to 8‐substituted 7,8‐dihydro‐5‐hydroxy‐7‐oxopyrido[2,3‐ d ]pyrimidine‐6‐carboxaldehydes ( 5 ) when treated with sodium ethoxide in an aprotic polar solvent at room temperature. The 6‐cyano analogue ( 18 ) also underwent ring transformation under the same mild conditions giving 7‐amino‐8‐ethyl‐5,8‐dihydro‐5‐oxopyrido[2,3‐ d ]pyrimidine‐6‐carboxaldehyde ( 21 ). However, the ring transformations of the pyrido[2,3‐ d ]pyrimidine bearing no N 8 ‐substituent ( 12 ), ethyl 1‐ethyl‐1,4‐dihydro‐4‐oxo‐1,8‐naphthyridine‐ ( 14 ) and ‐quinoline‐3‐carboxylates ( 16 ) failed to occur. A mechanism is discussed.