1H NMR and X-ray conformational analyses of (+)-corydalic acid methyl ester, a 6,7-secoberbine alkaloid

Abstract

Conformational analysis of (+)-corydalic acid methyl ester (1) by ¹H NMR data indicated that 1 in chloroform solution at room temperature exists in a conformational equilibrium. The NOEs in the NOESY spectra of 1 and the temperature dependence of the NMR spectral pattern suggested that rotation of the ring A moiety around the C(14)–C(15) bond is obstructed by two neighbouring methyl groups on the N(7) and C(13) positions. The structure of 1 was determined to be methyl (6R-trans)-6-(6,7,8,9-tetrahydro-6,8-dimethyl-1,3-dioxolo[4,5-h]isoquinolin-7-yl)-1,3-benzodioxole-5-acetate by X-ray crystal structure analysis. The crystal conformer of 1 agrees well with one of the two stable conformers derived from NMR analysis and empirical energy calculations. The function of 6,7-secoberbine type alkaloids for the biosynthetic pathway from protoberberine type into the hexahydrobenzo[c]phenanthridine type is discussed in relation to their conformational features.