Barriers to enantiomerization of 1-(o-ARYL) barbituric and -2-thiobarbituric acid derivaties

Abstract

In this study, 5,5-dimethyl-1-(0-aryl)barbituricI, 1-(0-ary1)barbituric and 1-(0-aryi)- 2-thiobarbituric acids have been synthesized by the reaction of the o-arylureas or the oaryl- 2-thioureas with the diethylmalonate or 1,l-diethyl-2,2-dimethyl malonate in the presence of sodiumethoxide, 5,5-dimethyl-1-(o-ary1)-2-thiobarbituric acids have been synthesized by the reaction of the o-aryl-2-thioureas with the dimethylmalonic acid in the presence of acetylchloride. The studied barbituric and 2-thiobarbituric acids are chiral due to nonplanar ground states. The aim of this study is to separate the enantiomers of the barbituric and 2-thiobarbituric acid derivatives by liquid chromatography on an optically active sorbent and subsequent racemization to obtain the activation energies for the racemization process. The chiralities of the compounds have been proved by 'H and 13c NMR spectroscopy. The enantiomers have been separated by liquid chromatography on chiral sorbents, triacetylcellulose or cellulose tris-(3,s-dimethylphenyl) carbamate. The optical activities of the separated enantiomers have been detected by CD spectrometer or polarimeter. Upon thermal racemization the activation barriers of the 5,5-dimethyl-1-(0- ary1)barbituric and -2-thiobarbituric acids have been determined by using liquid chromatography. The activation barrier of the 1-(0-toly1)barbituric acid has been determined by using dynamic NMR spectroscopy. The racemization mechanisms have been discussed with reference to the determined barriers. In addition, the keto-en01 tautomerization of the 1-(0-ary1)barbituric and 1-(0-ary1)-2-thiobarbituric acids have been investigated in different solvents by using 'H and I3c NhXR spectroscopy.