Conformational stability of 5-substituted orotic acid derivatives analyzed by measuring C-13 NMR chemical shifts and applying linear free energy relationships

Abstract

Conformational stability of various 5-substituted orotic acid derivatives was studied by applying linear free energy relationships (LFER) to the C-13 NMR chemical shifts. The correlation analysis for the substituent-induced chemical shifts (SCS) with inductive (rI), and various resonance (rR) parameters were carried out through Single Substituent Parameter (SSP) and Dual Substituent Parameter (DSP) methods, and multiple regression analysis. Good Hammett correlations for all carbons were obtained, while electrophilic substituent constants better fitted for C2 carbon with electron-donor substituents. Conformational analysis of various derivatives using RB3LYP/6-311 ++G (3df,3dp) DFT method, together with C-13 NMR data suggests that most of the substituted orotic acid derivatives exist in planar conformation, except nitro and alkyl substituted derivatives. Internal rotation of carboxylic group showed significant impact on the extent of conjugative interaction making syn conformation more stable in all the derivatives studied. Further, of all 5-substituted orotic acid derivatives, diketo form proved to be the most stable form compared to zwitterionic and enol tautomeric forms. Optimized geometries and transmission effects of particular substituent through well-defined p-resonance units suggest that these units behave as isolated as well as conjugated fragments, depending on the type of substituent.