Vinylphosphonium salt mediated simple synthesis of 7-oxo-7H-pyrido[1,2,3-cd]perimidine derivatives. Dynamic NMR spectroscopic study of prototropic tautomerism in ethyl 1H-perimidine-2-carboxylate

The reactive 1:1 intermediate produced in the reaction between dimethyl acetylenedicarboxylate and triphenylphosphine was trapped by ethyl 1H-perimidine-2-carboxylate to yield 5-ethyl 9-methyl 7-oxo-7H-pyrido[1,2,3-cd]perimidine-5,9-dicarboxylate and 5-ethyl 9-methyl 1-[3-methoxy-1-(methoxycarbonyl)-3-oxo-1-propenyl]-7-oxo-7H-pyrido[1,2,3-cd]perimidine-5,9-dicarboxylate in nearly 7:1 ratio and overall good yields. The first of these compounds is quantitatively converted to ethyl 10-[(2-ethoxy-2-oxoacetyl)amino]-2-hydroxybenzo[h]quinoline-4-carboxylate by refluxing in water-saturated chloroform, while the later compound remained unchanged under the same reaction conditions. The free-energy barrier (ΔG≠) for prototropic tautomerism in ethyl 1H-perimidine-2-carboxylate is determined by dynamic 1H NMR studies to be 50.6 kJ mol−1. The methylene protons of 5-ethyl 9-methyl 7-oxo-7H-pyrido[1,2,3-cd]perimidine-5,9-dicarboxylate are diastereotopic as a result of peri interaction of carboethoxy group with the adjacent carbonyl bond. The free-energy barrier for conformational racemization of this compond is 56.3 kJ mol−1.