Mosher ester analysis for the determination of absolute configuration of stereogenic (chiral) carbinol carbons

This protocol details the most commonly used nuclear magnetic resonance (NMR)-based method for deducing the configuration of otherwise unknown stereogenic, secondary carbinol (alcohol) centers ((RRCHOH)-R-1-C-2 (or the analogous amines where OH is replaced by NH2)). This 'Mosher ester analysis' relies on the fact that the protons in diastereomeric alpha-methoxy-alpha-trifluoromethylphenylacetic acid (MTPA) esters (i.e., those derived from conjugation of the carbinol under interrogation with MTPA) display different arrays of chemical shifts (delta s) in their H-1 NMR spectra. The protocol consists of the following: (i) preparation of each of the diastereomeric S- and R-MTPA esters and (ii) comparative (DdSR) analysis of the H-1 NMR spectral data of these two esters. By analyzing the sign of the difference in chemical shifts for a number of analogous pairs of protons (the set of DdSR values) in the diastereomeric esters (or amides), the absolute configuration of the original carbinol (or amino) stereocenter can be reliably deduced. A typical Mosher ester analysis requires approximately 4-6 h of active effort over a 1- to 2-d period.