CONFORMATIONAL STUDIES BY DYNAMIC NMR .51. GEAR EFFECT IN SUBSTITUTEDPYRIDONES

Two conformational isomers have been MMR detected at -30 degrees in 2- substituted pyridones having an isopropyl group bonded to the Nitrogen atom. Their existence is due to the restricted rotation of the N-isop ropyl group influenced by the ''gear effect'' of the flanking 2-alkyl groups (Me, Et, Pr-i). The interconversion barrier for this process (s imilar to 15 kcal/mol) was measured by total line shape analysis and i s independent of the size of the flanking alkyl groups. Molecular Mech anics calculations and NOE difference experiments at -70 degrees allow the unambigous identification of the structures corresponding to the gear conformers.