Effect of oxygen substituents on two- and three-bond carbon-proton spin-spin coupling constants

Abstract

Carbon–proton coupling constants of 12 iridoid glucosides were determined from NMR data. Proton–proton vicinal couplings, quantitative NOE and molecular modelling were used to determine the conformational behaviour of the compounds. The aglycone moiety of all the compounds included in the study were shown to be conformationally rigid. Comparison of the experimental carbon–proton couplings with the calculated geometries resulted in the quantification of the dependence of 2,3J(13C,1H) on the orientation of oxygen substituent attached to coupling pathway (ψ). This can be extended to other electronegative substituents. The effect was studied by considering the product of Si, the substituent constant at a given position, and a cosine type function of ψ, the dihedral angle between the considered oxygen substituent and the atoms of the coupling pathway. In taking into account the influence of —OR substituents at β- and γ-carbons on vicinal coupling constants (13Cα—Cβ—Cγ—1H fragment), we found that the Sicosϕ cosψ term can be applied successfully, where ϕ is the dihedral angle between coupled atoms. Copyright © 2000 John Wiley & Sons, Ltd.