Comparison of the NMR coupling paths in leucodrin and conocarpin by low-temperature X-ray structure analysis

Abstract

Clear ⁵J_3-H,9-H couplings have been observed for four representatives of the leucodrin series of compounds, and also for their diastereoisomer, conocarpin. The relative degree of co-planarity of the six atoms involved in these couplings is discussed in terms of the detailed geometry of the leucodrin and conocarpin molecules. The importance of favourable transoid dihedral (torsion) angles along the coupling pathway, rather than co-planarity of the nuclei along the chain, is demonstrated. The solid–state ¹³C NMR characteristics of leucodrin reflect the restricted rotation of the aromatic ring in the molecular packing pattern of the crystal lattice. The crystal structures of the diastereoisomers leucodrin and conocarpin, C₁₅H₁₆O₈, have been determined by single-crystal X-ray analysis, the former at both room temperature and liquid nitrogen temperature, the latter only at room temperature. The experimentally determined low-temperature hydrogen positions serve as a starting point from which to rationalize the observed five-bond NMR coupling constants in terms of the torsion angles along the chain.