SEMIEMPIRICAL CONFORMATIONAL ANALYSIS OF(R,R) - TARTARIC ACID, ITS DIMETHYL DIESTER,DIAMIDE AND N, N, N', N'-TETRAMETHYL DIAMIDE.AB-INITIO CALCULATIONS OF SOME MODEL COMPOUNDS
Rychlewski Jacek, Rychlewska Urszula
(R,R)-tartaric acid (AC), its dimethyl diester (ME), diainide (AM) and N,N,N',N'-tetramethyl diamide (TMA) as well as their model compounds, namely hydroxyacetic acid, itsmethyl ester and amide, have been studied in order to find general conformationalpreferences among (RR)-tartaric acid derivatives. A rotation around all rotable bonds havebeen scanned systematically using semiempirical methods for AC, ME, AM and TMA andab-initio calculations for the model compounds have been carried out. In the case of AC andME we found a tendency towards the extended conformation, which is in good agreementwith available experimental data. For AM and TMA the results of semiempirical calculationsare contradictory to each other. Conformations similar to those observed in the crystalstructure were predicted by MNDO in the case of AM (the T conformer) and by PM3 in thecase of TMA (the G" conformer). Energetically preferred conformational isomers arestabilized by intramolecular hydrogen bonds and the electrostatic CO/C*βH coplanar bondinteractions. In T and G" rotamers, intramolecular hydrogen bonds leading to the formationof five-membered rings prevail, while in G+ conformers, hydrogen bonded six-memberedrings dominate.