Methoxy-a-trifluoromethylphenylacetic acid MTPA

Enantiomeric purity is often determined by derivatization with an optically pure chiral agent. For alcohols and amines, a-methoxy-a-trifluoromethylphenylacetic acid (MTPA) and a-cyano-a-fluoropheny lace tic acid (CFPA) [45] work well. 

We carried out another synthesis of (2R,6S,8R)-8Sr, as shown in Figure 4.37, by employing (.S )-malic acid as the starting material.67 In this synthesis, the enantiomeric purity of the intermediate C was rigorously proved to be 100% ee as checked by the H-NMR analysis of its a-methoxy-a-trifluoromethylphenylacetate (MTPA ester). Subsequently, C was converted to (2R,6.S ,8R)-88, whose rotation value was [a]o23 +60.1 (pentane). As a byproduct of this synthetic study, (2R,6R,8.S )-2,8-dimclhyl-l,7-dioxaspiro[5.5]undecane (F ) was synthesized from D, which was obtained by isomerization of E. This isomerization was made possible by the fact that the crystalline 3,5-dinitrobenzoate of D precipitated, while that of E remained oily. 

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