A cautionary note on the use of commercial (R)-MTPA-Cl and (S)-MTPA-Cl in determination of absolute configuration by Mosher ester analysis

While the absolute configuration of a Mosher ester derivative (MTPAOR) is identical with that of the Mosher acid (MTPAOH) precursor, it is opposite that of the Mosher acid chloride (MTPA-Cl). Since (R)-MTPA-Cl and (S)-MTPA-Cl are now commercially available, incorrect conclusions may be drawn in deriving the absolute configuration, if this fact is overlooked. The absolute configuration of (−)-vasicinone (1) derived by Mosher ester analysis has been revised as 3S. Mosher ester analysis of (−)-vasicine (2) confirmed a 3S configuration for this alkloid