CONFORMATIONAL DISTRIBUTION OF BACLOFEN ANALOGS BY H-1 AND C-13 NMR ANALYSIS AND AB-INITIO HF MO STO-3G OR STO-3G-ASTERISK CALCULATIONS

The conformations of 3-(substituted furan-2-yl) and 3-(substituted thi en-2-yl)-gamma-aminobutyric acid 1-9 in solution (D2O) are estimated f rom high-resolution (300 MHz) H-1 NMR coupling data. Conformations and populations of conformers are calculated by means of a modified Karpl us-like relationship for the vicinal coupling constants. The results a re compared with X-ray crystallographic investigations (torsion angles ) and ab initio HF MO ST-3G or STO-3G calculations. H-1 NMR spectral analysis shows how 1-9 in solution retain the preferred g(-) conformat ion around the C3-C4 bond, as found in the solid state, while a partia l rotation is set up around the C2-C3 bond: the conformations about C2 -C3 are all highly populated in solution. The C-13 spin-lattice relaxa tion times are also discussed.