Factors Affecting Conformation of ( R,R )-Tartaric Acid Ester, Amide and Nitrile Derivatives. X-Ray Diffraction, Circular Dichroism, Nuclear Magnetic Resonance and Ab Initio Studies

Abstract

Derivatives 2a–15a of (R,R)-tartaric acid (1a) with all combinations of methyl ester, amide, N-methylamide and N,N-dimethylamide groups, as well as the corresponding O,O′-dibenzoyl derivatives 1b–15b and nitriles 16–18 have been synthesized. Their conformations have been studied by the NMR and CD methods in solution as well as by X-ray diffraction in the crystalline state. The preference for planar. T conformation of the four carbon chain is observed under conditions restricting the α-hydroxyacid, ester or amide group to be nearly planar, this conformation being stabilized by intramolecular hydrogen bonds of the S(5) motif and the electrostatic CO/C(β)H and CN/C(β)H coplanar bond interactions. The C=O/C(α)-O bond system tends to be either synplanar (ester, acid), or antiplanar (ester, primary and secondary amide). Ab initio calculations allowed to demonstrate that for the isolated molecules of diamides 10a and 15a there is strong preference for gauche G+(a,a) conformers, the driving force being the formation of the hydrogen bonded six-membered cycles of the S(6) motif joining the OH and C=O groups from two different halves of the molecule. The results compare favourably with the experimental values derived from NMR spectra of 15a in nonpolar solvent. In the absence of intramolecular hydrogen bonding the N,N-dimethylamide group is better accomodated in a gauche G− conformer. This releases the nonbonded interaction due to the amide methyl group anti to the carbonyl group. © 1997 Elsevier Science Ltd.