Enolethers. XXI . Synthesis of 5,5′‐disubstituted 4,4′‐bipyrimidines

1,6‐Dialkoxy‐3,4‐diones 3 are easily accessible by acylation of enol ethers 1 with oxalyl chloride and subsequent elimination of hydrogen chloride using triethylamine. The open‐chain 2,5‐dimethyl derivative 3b is converted with amidines 4a‐c and S ‐methylisothiourea (4d) , respectively, to give 2,2′‐disubstituted 5,5′‐dimethyl‐4,4′‐bipyrimidines 5a‐d . The dihydrofuran and dihydropyran derivatives 3c and 3d , however, react with benzamidine (4c) in dimethylformamide only in the presence of calcium hydride as condensation agent yielding 5,5′‐bis(2‐hydroxyethyl)‐ and 5,5′‐bis(3‐hydroxypropyl)‐2,2′‐diphenyl‐4,4′‐bipyrimidine 6a and b.