cis-(9S,10S)-Methyl 1-propyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylate

The title compound, C16H20N2O2, was synthesized from (S)-tryptophan methyl ester hydrochloride and butyraldehyde. The absolute configuration 9S,10S was assigned on the basis of the unchanging chirality of the C9 centre. The NH group of the indole ring is involved in intermolecular N—H⋯O hydrogen bonding, while the NH group of the six-membered ring is not. This latter ring has a half-chair conformation.

The title compound, C 16 H 20 N 2 O 2 , was synthesized from (S)tryptophan methyl ester hydrochloride and butyraldehyde. The absolute configuration 9S,10S was assigned on the basis of the unchanging chirality of the C9 centre. The NH group of the indole ring is involved in intermolecular N-HÁ Á ÁO hydrogen bonding, while the NH group of the six-membered ring is not. This latter ring has a half-chair conformation.
MH is grateful to the Alexander von Humboldt Foundation, Germany, and AF thanks the Swedish Research Council for financial assistance.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: BI2260).
containing the tetrahydro-β-carboline nucleus. We report here the crystal structure of the title compound.

Experimental
The title compound was prepared by condensation of (S)-tryptophan methyl ester hydrochloride with an aldehyde under polar protic conditions (Greenstein & Winiz, 1961;Snyder et al., 1948). (S)-tryptophan methyl ester hydrochloride (1.5 g, 0.0059 mol) and butyraldehyde (1.50 ml, 0.0059 mol) were dissolved in methanol/water solution (50 ml, 75/25%, v/v). The mixture was refluxed for 48 h, cooled and the solvent evaporated under vacuum. The residue was dissolved in 14% ammonium hydroxide, extracted with chloroform and dried over anhydrous sodium sulfate. The solvent was removed under reduced pressure to yield an oily residue which was subjected to column chromatography. The purified oil was crystallized from benzene/pet. ether (b.p. 373-393 K) to give the title compound: yield 1.2 g, 74.8%, m.p. 413 K, R f 0.84 methanol / chloroform (3:7).
[a] D 28 -132.8 (c = 0.00348, acetonitrile). The product after purification was subjected to different spectroscopic techniques. This data together with the result of elemental analysis confirmed the formation of a pure stereoisomer.

Refinement
In the absence of significant anomalous scattering effects, Friedel pairs were merged prior to refinement. All C-bonded H atoms were placed at calculated positions. The two N-bonded H atoms were located from the Fourier map and were refined with the restraint N-H = 0.89 (1) Å. The isotropic displacement parameters of the H atoms were fixed at U iso (H) = 1.2U eq (C/N) (1.5U eq (C) for the methyl groups). Fig. 1. The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level for non-H atoms.