STRONG DIMERIZATION OF UREIDOPYRIMIDONES VIA QUADRUPLE HYDROGEN-BONDING

6-Methyl-2-butylureidopyrimidone dimerizes via four hydrogen bonds in the solid state as well as in CHCl3 solution via a donor-donor-accepto r-acceptor (DDAA) array of hydrogen bonding sites in the 4[1H]-pyrimid inone tautomer. An intramolecular hydrogen bond from the pyrimidine NH group to the urea oxygen atom preorganizes the molecules for dimeriza tion. The dimerization constant of the dimer in CHCl3 exceeds 10(6) M- 1. In CHCl3 containing DMSO, the dimer is in equilibrium with the mono meric 6[1H]-pyrimidinone tautomer. In 6-phenyl-2-butylureidopyrimidone , the 4[1H]-pyrimidinone tautomer coexists with the pyrimidin-4-ol for m, which dimerizes with similar high dimerization constants via four h ydrogen bonds in a DADA array. The latter tautomer predominates in der ivatives with electronegative 6-substituents, like 6-nitrophenyl- and 6-trifluoromethyl-2-butylureidopyrimidone. Due to its simple preparati on and high dimerization constant, the ureidopyrimidone functionality is a useful building block far supramolecular chemistry.