New Strategy for Determining the Absolute Stereochemistry of Mono-Functional Natural Products by the CD Exciton Chirality Method

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1. We have developed a new strategy for determining the absolute configuration of mono-fucntional compounds by the CD exciton chirality method. This method is powerful for the absolute configurational studies of chiral compounds including natural products.2. We have recently developed two "powerful chiral acids" for enantioresolution of various alcohols by HPLC on silica gel and also for determination of their absolute configurations by X-ray crystallography and/or by the 1H NMR anisotropy method. The first is the chiral dichlorophthalic acid (-)-1, which was reacted with racemic alcohol to give a diastereomeric mixture of esters. The mixture of esters obtained was separated by HPLC on silica gel, and two separated esters were crystallized respectively to afford single crystals suitable for X-ray analysis. The absolute configuration of the alcohol part was determined by X-ray crystallography of the esters, and enantiopure alcohols were easily recovered by solvolysis with K_2CO_3 in methanol.3. The second powerful chiral acid is 2-methoxy-2-(1-naphthyl)propionic acid (MαNP acid, (S)-(+)-2 or R-(-)-2) for determining the absolute configuration of chiral alcohols by the 1H NMR anisotropy method.The enantiopure MαNP acid (S)-(+)-2 was allowed to react with racemic alcohols yielding diastereomeric mixtures of esters, which were easily separated by HPLC on silica gel. The NMR signals of diastereomeric esters obtained were fully assigned by the various two-dimensional methods leading to the calculation of Δδ values (Δδ = δ(R,X)-δ(S,X)). By applying the MαNP sector rule, the absolute configuration X was determined. Separated esters were hydrolyzed with KOH/EtOH to recover enantiopure alcohols.4. Using MαNP acid, (S)-(+)-2 and its deuterium labeled enantiomer R-(-)-2-d_6, we have developed a new diastereomer method for determining enantiomeric excess (% ee) of chiral alcohols by MS spectrometry.