Crystalline State Conformation of 2-Methoxy-2-(1-naphthyl)propionic Acid Ester

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Abstract
The enantioresolution method using ( S )-(+)-2-methoxy-2-(1-naphthyl)propionic acid (M f NP acid 1 ) was applied to 1,2,3,4-tetrahydro-4-phenanthrenol ( 4 ). Racemic alcohol ( - )- 4 was esterified with ( S )-(+)- 1 , and a diastereomeric mixture of esters formed was separated by HPLC on silica gel giving the first-eluted ester 5a and the second-eluted one 5b . Ester 5a was reduced with LiAlH 4 yielding enantiopure alcohol (+)- 4 . A single crystal of ester 5a was subjected to X-ray crystallography to determine its absolute configuration as ( S , S ), and therefore to assign the S absolute configuration to (+)- 4 . The most striking finding in the X-ray analysis is that the methoxyl oxygen atom is syn to the ester carbonyl oxygen even in the crystalline state. This is just the preferred conformation of M f NP acid esters assumed in the 1 H NMR anisotropy method. The S absolute configuration of (+)- 4 was also confirmed by the CD and 1 H NMR anisotropy methods.

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