CONFORMATIONAL STRUCTURE AND DYNAMICS OF ARYLMETHOXYACETATES - DNMR SPECTROSCOPY AND AROMATIC SHIELDING EFFECT

The ground state conformational geometry and energy of esters of (R)- and (S)-arylmethoxyacetic acids were evaluated from low temperature H-1 and C-13 NMR spectra and by means of MM, AM1, and aromatic shielding effect calculations. In solution, the title compounds are constituted by two rotamers in equilibrium. Rotamer ap has the C-alpha-O and C=O bonds antiperiplanar and rotamer sp (lower in energy by 0.4-1.1 kcal/mol) has these bonds synperiplanar. In both the ap and the sp the aryl ring is ca. perpendicular to the C=O bond and coplanar with the CalphaH bond. Their relative populations and its temperature dependence evaluated by DNMR spectra are in agreement with the stereoselectivity reported for Diels-Alder reactions of dienyl-AMAA esters and support the ''perpendicular model'' proposed to account for that stereoselectivity. For conformations in which the aryl ring shields the alcohol moiety of the ester, calculated ring current shielding increments compare well with experimental shift values obtained from low-temperature NMR spectra and agree with the conformational model used in the Mislow-Dale-Mosher-Trost method for chiral recognition by NMR.