Configurational assignments of conformationally restricted bis-monoterpene hydroquinones: utility in exploration of endangered plants

Abstract

Background: Endangered plant species are an important resource for new chemistry. Lindera melissifolia is native to the Southeastern U.S. and scarcely populates the edges of lakes and ponds. Quantum mechanics (QM) used in combination with NMR/ECD is a powerful tool for the assignment of absolute configuration in lieu of X-ray crystallography.

Methods: The EtOAc extract of L. melissifolia was subject to chromatographic analysis by VLC and HPLC. Spin-spin coupling constant (SSCC) were calculated using DFT at the MPW1PW91/6-31G(d,p) level for all staggered rotamers. ECD calculations employed Amber* force fields followed by PM6 semi-empirical optimizations. Hetero- and homo-nuclear coupling constants were extracted from 1D (1)H, E.COSY and HETLOC experiments.

Results: Two meroterpenoids, melissifolianes A (1) and B (2) were purified and their 2-D structures elucidated using NMR and HRESIMS. The relative configuration of 1 was established using the combination of NOE-based distance restraints and the comparisons of experimental and calculated SSCCs. The comparison of calculated and experimental ECD assigned the absolute configuration of 1. The relative configuration of a racemic mixture, melissifoliane B (2) was established utilizing J-based analysis combined with QM and NMR techniques.Conclusion Our study of the Lindera melissifolia metabolome exemplifies how new chemistry remains undiscovered among the numerous endangered plant species and demonstrates how analysis by ECD and NMR combined with various QM calculations is a sensible approach to support the stereochemical assignment of molecules with conformationally restricted conformations.

General significance: QM-NMR/ECD combined approaches are of utility for unambiguous assignment of 3-D structures, especially with limited plant material and when a molecule is conformationally restricted. Conservation of an endangered plant species can be supported through identification of its new chemistry and utilization of that chemistry for commercial purposes.

Fig. 1

Structure of melissifolianes A (1) and B (2).

Fig. 2

(A) Key HMBC (arrows) and COSY (bold lines) and (B) Key NOEs (dotted lines) of melissifoliane A (1).

Fig. 3

Diastereomers I and II, the representative structures of melissifoliane A (1) with different C-1′ configurations along with the Newman projections demonstrating all possible staggered rotamers associated with C-1′–C-2′ bond.

Fig. 4

Overlaid experimental and average calculated ECD spectra of melissifoliane A (1) utilizing the two different basis sets (see Supplementary data 2 for the details).

Fig. 5

DFT mPW1PW91/6-31G(d,p) geometry and energy-optimized gauche⁺ conformer of melissifoliane B (2) diastereomer I in Fig. S1-22. Dotted lines indicate observed NOE. The use of a distance constraint obtained through the calibrated integration of NOE to H-3 of the hydroquinone ring orients the plane of the ring. The optimization for all the staggered rotamers of melissifoliane A (1) were achieved in the same manner.

Fig. 6

The major conformers suggested by QM calculation (C-1′–C-2′, left) (see Table S1-1 and Fig. S1-21) and J-based analysis (C-3′–C-2′, right). δ_H(H) and δ_H′(L) indicate 2.26 and 2.19, respectively. ^aMPW1PW91/6-31G(d,p) calculated J values. ^bExperimental J values observed from HETLOC and 1-D ¹H-NMR spectrum. TAD: see Table 2. The geometry of melissifoliane B (2) was optimized as shown in Fig. 5. SSCCs of each C-1′–C-2′ staggered rotamer were calculated in a manner identical to that completed for melissifoliane A (1).