Nuclear magnetic resonance assignment strategy for pentacyclic triterpenes, using lup-20(29)-ene from Pilotrichella flexilis as model system, combining spectrally filtered proton-to-carbon schemes and DFT–GIAO approach

Abstract

The present work comprises a method to obtain full proton-to-carbon nuclear magnetic resonance chemical shift assignment of a C30H50 lup-20(29)-ene, for the first time obtained from the Mexican native mosses Pilotrichella flexilis, wherein said method consists in a combination of the following NMR schemes: 1D-13C (DEPT-135), 2D-{1H–13C} HMBC with a spectral filter for promoting only weak-c.a. 2 Hz-long-range scalar couplings, 2D-{1H–1H} EXSY with long mixing times to favour only weak H–H dipolar correlations and ultra-high resolution one- and two-dimensional 1H instant homodecoupling Psyche pure shift. Full set of assigned resonances were compared against the theoretical isotropic chemical shifts computed with a gauge invariant atomic orbital–density functional theory with self consistent reaction field calculation, retrieving accurate agreements, despite the intrinsic severe signal overlap that these C30 hydrocarbon triterpenes experimentally present. Therefore, a 3D-structure supported by experimental NMR data of this type of important metabolite precursor in plants can be proposed.