Two-Step Synthetic Approach to 6-Substituted Pyrido[2,3-d]pyrimidine(1H,3H)-2,4-diones from 6-Amino-, 6-Alkylamino-, and 6-Arylamino-1,3-dimethyluracils*

The Mannich reaction of 7-aryl-5,6-dihydropyrido[2,3-d]pyrimidines 3, easily accessible by condensation of 6-amino-1,3-dimethyluracil (1) with Mannich bases 2a – c, gives rise to a mixture of 7-aryl-6-(N,N-dimethylaminomethyl)pyrido[2,3-d]pyrimidines 6 and 7 as well as 1,2-bis(7-arylpyrido[2,3-d]pyrimidin-6-yl)ethane 13 the ratio of which depends on the reaction conditions and the amine used. 6-Alkylamino-1,3-dimethyluracils 15 – 18 were converted to the corresponding 5-(3-oxo-3-phenylpropyl)uracils 19 – 22 by condensation with the Mannich base 2a. Ring closure of 19 – 22 was performed by Vilsmeier formylation to afford the 8-alkyland 7,8-diaryl-5,8dihydropyrido[2,3-d]pyrimidine-6-carbaldehydes 9 – 12 via the corresponding iminium salts 27 – 30.