Regioselective synthesis of N-substituted dihydropyrimidin-2(1H) or (3H)-one

Alkoxycarbonylation of dihydropyrimidin-2-one 1 with alkyl chloroformate in the presence of NaH occurred regioselectively at position-3 to provide N-substituted dihydropyrimidin-2(1H)-one 2, but the reaction of 1 with trichloromethyl chloroformate (phosgene dimer) and aminoalcohol in the presence of NaH and Et₃N afforded regioselectively N-substituted dihydropyrimidin-2(3H)-one 3 or compound 2. In this case, regioselectivity was dependent upon the substitution pattern of the phenyl ring. The reaction of 1 with phosgene dimer and ordinary alcohol in the base gave regioselectively compound 3 in contrast to the result of alkyl chloroformate method.