Conformational studies on 5-formyl-5,6,7,8-tetrahydrofolic acid (folinic acid) using 1H and 13C nuclear magnetic resonance measurements: two interconverting conformations

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Abstract
The 1H and 13C n.m.r. spectra of the (±)-L-diastereoisomers of folinic acid have been recorded. No spectral differences between the two diastereoisomers were detected but a mixture of two slowly interconverting conformations of unequal populations was shown to be present. The relative populations of the two forms changed with temperature and the thermodynamic parameters for the equilibrium have been obtained. Transfer of saturation experiments have been used to show that the two forms are slowly interconverting. The most likely explanation for the difference between the two forms is deduced as due to hindered rotation of the formyl group about the C–N bond. The 9-methylene side-chain exists in a well defined conformation with the methylene protons both in gauche-positions with respect to the 6-H proton.