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\u2013GIAO approach

Jose Enrique Herbert-Pucheta,Lino Reyes,Cinthia Mejía-Lara,Holber Zuleta-Prada,Benito Reyes-Trejo

doi:10.1186/s13765-019-0435-3

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.

Highlights

Triterpenoids belong to a very important and abundant class of naturally occurring compounds, withHerbert‐Pucheta et al Appl Biol Chem (2019) 62:28 structural backbone and therapeutically properties, pentacyclic triterpenes (PTs) can be classified within three major categories: oleananes, ursanes and lupanes types
As part of our ongoing research to identify bioactive substances from native mosses [18], we have recently isolated and identified for the first time a pentacyclic triterpene lup-20(29)-ene coming from Pilotrichella flexilis, which is a common species broadly distributed along Mexico [19], and barely one phytochemical study of an oversea species has been reported until now [20]
Density functional theory (DFT) procedure for energy minimization and orbital description was performed with the hybrid functional B3LYP [29, 30]

Summary

Introduction

Triterpenoids belong to a very important and abundant class of naturally occurring compounds, withHerbert‐Pucheta et al Appl Biol Chem (2019) 62:28 structural backbone and therapeutically properties, PTs can be classified within three major categories: oleananes, ursanes and lupanes types. Advantages of the use of this set of NMR schemes for signal assignment are evaluated by agreements with respect calculated 1H and 13C chemical shifts of lup-20(29)-ene

Results

Conclusion