CONFORMATIONAL-ANALYSIS OF METHYLSULFINYL DERIVATIVES OF FURAN AND THIOPHENE BY EMPLOYING NUCLEAR MAGNETIC-RELAXATION AND LANTHANIDE INDUCED SHIFTS

Relaxation parameters such as nuclear Overhauser effect (NOE) and spin-lattice relaxation times (T1) were applied to the conformational analysis of methylsulphinyl derivatives of furan and thiophene. Correlation times for molecular tumbling tau-c, and for methyl reorientation, tau-i, were obtained. The tau-i values for the different molecules examined are nearly the same, the tau-c are twice as large in derivatives that also contain an iodine substituent in the heterocyclic ring. From non-selective, selective and bi-selective T1 measurements and NOE values, relative to H-1 nuclei, and a best-fit procedure, the most probable conformational intervals for the methylsulphinyl group were located. Owing to the pyramidal structure of sulphoxides, the preferred conformers could not be fixed unambiguously. From the Lanthanide Induced Shift (LIS) technique on H-1 and C-13 nuclei, intervals of preferred orientations of the S-O bond with respect to the heterocyclic ring were found. Reliable conclusions about the conformational behaviour of these molecules have been drawn by matching the two sets of results. Relaxation parameters turn out to be useful for conformational analysis of small molecules when employed as a complementary tool with other experimental approaches.