Structural analysis of pentacyclic triterpenes from the gum resin of Boswellia serrata by NMR spectroscopy

3alpha-Acetyl-beta-boswellic acid (1), 3alpha-acetyl-alpha-boswellic acid (2), 3alpha-acetyl-9,11-dehydro-beta-boswellic acid (3), 3alpha-acetyl-9,11-dehydro-alpha-boswellic acid (4) and 3alpha-acetyl-11-keto-beta-boswellic acid (5) were isolated from the gum resin of Boswellia serrata. 1D and 2D NMR (COSY45, HMQC, HMBC, ROESY) spectra at 500 MHz were used for shift assignments and structure verification. All boswellic acids investigated share the cis conformation at ring D/E and the 3alpha orientation of the acetyl ester group. Owing to high-order spectra, NMR could not determine the exact conformation of H-20/H-30 of the beta-boswellic acids. 3alpha-Acetyl-beta-boswellic acid methyl ester W) was synthesized for experiments with a shift reagent, Eu(fod)(3), that enhanced the resolution considerably. The oxygen atoms of the 3alpha-acetyl group form the apparent complex binding site for the shift reagent. Copyright (C) 2003 John Wiley Sons, Ltd.