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

Abstract

Calculations (MM, AM1/PM3, ab initio) and DNMR experiments indicate that 2-(2'methoxy-1'-naphthyl)-3,5-dichinrobenzoic acid (MNCB) and 2'-methoxy- 1,1'-binaphthyl-2-carboxylic acid (MBNC) esters and amides exist in solution as two conformers in equilibrium: one of them (sp) has synperiplanar Ar1- Ar2 and C=O bonds, and another one (ap) has antiperiplanar bonds. Due to the close populations of the forms only apart of large shielding effects attributed to the ap form is transformed into observed high field shifts 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 stereochemisity determination of alcohols and amines by NMR. The way to increase the efficiency of this type of reagent has been revealed on the basis of these findings.