Conformational analysis of MNCB (MBNC) esters and amides: Promising chiralreagents for stereoselective applications

Calculations (MM, AM1/PM3, ab initio) and DNMR experiments indicate that 2- (2'-methoxy-1'-naphthyl)-3,5-dichlorobenzoic acid (MNCB) and 2'-methoxy-1,1 '-binaphthyl-2-carboxylic acid (MBNC) esters and amides exist in solution a s two conformers in equilibrium: one of them (sp) has synperiplanar Ar-1-Ar -2 and C=O bonds, and another one (op) has antiperiplanar bonds. Due to the close populations of the forms only a part of large shielding ef fects attributed to the ap form is transformed into observed high field shi fts in NMR experiments. Thus, the low population of the "ap" conformer and the low selectivity of the aryl ring anisotropic influence on the alcohol ( amine) moiety are limitations of the efficiency of MNCB (MBNC) in their use as reagents for absolute stereochemistry determination of alcohols and ami nes by NMR. The way to increase the efficiency of this type of reagent has been revealed on the basis of these findings. (C) 1999 Elsevier Science Ltd . All rights reserved.