Piperazinedione formation from esters of dipeptides containing glycine, alanine, and sarcosine: the kinetics in aqueous solution

Abstract

Piperazinedione formation from the methyl esters of glycylglycine, glycyl-L-alanine, L-alanylglycine, glycyl-sarcosine, and sarcosylglycine and from the ethyl ester of glycylglycine in aqueous solution, pH 7·3–8·5, and 25·0° was studied. It was found to be a self-catalysed reaction and other amines also served as catalysts. Some concomitant ester hydrolysis occurred. The kinetics were analysed by regression analysis and the data fitted equation (i). Values of k_hyd′, {k_s+k_OH[OH⁺]}, k_gb, and k_gb′ were obtained. Glycylsarcosine methyl ester d[ester, total]/dt=[ester, free base]{k_s+k_gb‘[amine, free base]+k_OH[OH^–]}+[ester, free base]²k_gb+[ester, total]k_hyd’(i) cyclised most rapidly (t_½ca. 5 min), by more than an order of magnitude, and this was attributed to the much higher content of cis-isomer. The methyl esters of sarcosylglycine and glycylalanine cyclised considerably faster than the glycylglycine ester; sarcosylglycine methyl ester showed evidence of steric hindrance in its smaller self-catalytic (k_gb) rate constant. Alanylglycine methyl ester cyclised the slowest and had a markedly low value of k_gb. Saponification rates were similar to those of the esters of N-acylamino-acids. The mechanism of the cyclisation process, together with the effects of N- and C-methyl substituents on the reaction at the ester carbonyl carbon and on the ring opening of the piperazinediones are discussed.