Deciphering composition and connectivity of a natural product with the assistance of MS and 2D NMR

Abstract

A complementary application of three analytical techniques, viz. multidimensional nuclear magnetic resonance spectroscopy (NMR), mass spectrometry (MS), and single-crystal X-ray diffractometry was required to identify and refine two natural products isolated from Millettia versicolor and solvent of crystallization. The two compounds, namely 3-(2H-1,3-benzodioxol-5-yl)-6-methoxy-8,8-dimethyl-4H,8H-pyrano[2,3-h]chromen-4-one, or durmillone, (I), and (2E)-1-(4-{[(2E)-3,7-dimethylocta-2,6-dien-1-yl]oxy}-2-hydroxyphenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one, (II), could not be separated by routine column chromatography and cocrystallized in a 2:1 ratio with 0.13 molecules of ethanol solvent. Compound (II) and ethanol could not be initially identified by single-crystal X-ray analysis due to complex disorder in the aliphatic chain region of (II). Mass spectrometry ensured that (II) represented only one species disordered over several positions in the solid state, rather than several species cohabitating on the same crystallographic site. The atomic identification and connectivity in (II) were established by several 2D (two-dimensional) NMR techniques, which in turn relied on a knowledge of its exact mass. The derived connectivity was then used in the single-crystal analysis to model the disorder of the aliphatic chain in (II) over three positions and allowed identification of a partially occupied ethanol solvent molecule that was disordered over an inversion center. The disordered moieties were refined with restraints and constraints.