Nuclear magnetic resonance and X-ray stereochemical studies on 1-benzyltetrahydroisoquinolinium salts

The relative stereochemistry at positions 1 and 2 of N,N-dialkyl-1-benzyl- 1,2,3,4-tetrahydroisoquinolinium salts has been assigned via a combination of ¹H and ¹³C n.m.r. studies and confirmed by single-crystal X-ray analysis of trans-N-3-hydroxypropyl-5′-methoxylaudanosinium perchlorate [trans-2j)] and cis-N-3-hydroxypropyl-5′-methoxylaudanosinium iodide [cis-(2h)] monohydrate. Quaternization of either laudanosine or 5′-methoxylaudanosine with an alkyl iodide leads to major isomers which possess a trans-relationship between the entering N-alkyl group and the 1-benzyl substituent; these isomers characteristically have the less shielded 1-H and N-methyl protons in deuteriochloroform. Crystals of trans-(2j) and cis-(2h) monohydrate both belong to the monoclinic system. Space group P21/c, with a = 8.123(4), b = 20.380(8), c = 16.710(6) Å, β = 102.14(5)°, Z = 4 for the former, and a = 11.475(5), b = 17.749(6), c = 14.278(5) Å, β = 113.51(5)°, Z = 4, for the latter compounds. Full-matrix least-squares refinement of atomic positional and thermal parameters converged at R = 0.082 [2 215 reflections] for (trans-(2j) and 0.077 [2 987 reflections] for (cis-(2h) monohydrate. The stereochemistry of the protonation products arising from these tetrahydroisoquinoline bases is also discussed.

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Author Andrew T. McPhail Chemistry