Conformational analysis.: 50.: C-methyl-1,2,3,4-tetrahydroisoquinolines

Conformational equilibria in 1-, 3-, and 4-methyl-1,2,3,4-tetrahydroisoquinolines (THIQs) and the diastereomeric pairs of their 1,3- and 1,4-dimethyl homologs have been determined by measurement of H-3/H-4(trans) coupling constants and have been confirmed by molecular mechanics [MMP2(85]) calculations. The experimental -Delta G degrees values (a --> e) for the monomethyl compounds (computed values in parentheses) in kcal mol(-1) are Me-1, 0.56 (0.46); Me-3, 1.63 (1.53); and Me-4, -0.32 (-0.22). Agreement of experimental and calculated values is very goad as is the additivity of values for the dimethyl compounds (Table 1). Values for the corresponding hydrochlorides are Me-1, 0.19 (-0.34); Me-3, 1.15 (1.46); and Me-4, 0.35 (0.10) kcal mol(-1). The less than satisfactory agreement of experimental with computed data here is probably due to neglect of solvation. The very small or negative Delta G degrees values for Me-1 and Me-4 were ascribed not only to the pseudoaxial (rather than axial) nature of Me(ax) and the absence of a syn-axial hydrogen on the side of the benzene ring but also to a peri interaction with H(8) and H(5), respectively, destabilizing equatorial methyl at positions 1 and 4. This was confirmed by comparing computed conformational energy values with values at corresponding positions in Delta(3,4)-tetrahydropyridines (THPs). While Delta G degrees in the two series is the same for Me-3 (THIQ numbering), that for Me-1 and Me-4 is considerably smaller in the THIQ than in the THP series which latter is devoid of peri hydrogens.