Determination of Absolute Configuration of Natural Products by X-ray Diffraction: A Novel Approach of Incorporating Heavy-Atom-Containing Solvent Molecules into the Single Crystals and Refinement of Flack Parameter

In this study, the method of determining the absolute configurations of 14b-hydroxy-3β-O-(L-thevetosyl)-5β-card-20(22)-enolide (I) which was isolated from the air-dried fruits of Cerbera odollam and 3α-feruloyl-taraxerol (II) which was isolated from the air-dried fruits of Bruguiera cylindrica will be presented. The novel method [1] of incorporating heavy-atom-containing solvent molecules into the single crystal structure of the compounds and refinement of the Flack parameter [2] was used to find the absolute configurations of these natural products. Compounds (I) has a cardenolide skeleton whereas compound (II) has a taraxerol skeleton. The present X-ray study shows the absolute L-form of (I) and the absolute α-form of (II). We have clearly demonstrated for the first time a novel approach which is an extremely useful and easy method to determine the absolute stereochemistry of natural products by X-ray diffraction. The importance of determining the absolute stereochemistry of natural products is emphasised in this work.