Tryptophan methyl ester HCl
Potassium Bicarbonate (ACS grade, 99.7%; Mallinckrodt)
1,4-Dibromobutane (99%, Aldrich)
Acetonitrile (ACS grade, 99.9%; Fisher)

Tryptophan methyl ester HCl (1.159 g, 3.783 mmol, 1 eq.) was suspended in acetonitrile (100 mL). Potassium bicarbonate (1.790 g, 17.87 mmol, 4 eq.) was added, followed by 1,4-dibromobutane (0.82 mL, 6.87 mmol, 1.5 eq). The mixture was then heated at reflux for 4 days after which the solvent was removed under reduced pressure. The residue was partitioned between EtOAc (100 mL) and water (100 mL). The organic phase was separated and the aqueous phase was extracted with EtOAc (2 × 40 mL). The combined organic phases were dried over magnesium sulfate and the solvent was removed under reduced pressure, leaving a brown oil. The crude product was purified by column chromatography (1:10–1:1 ethyl acetate:hexane on basic Al₂O₃), giving a yellow oil (0.970 g, 78%).

The level of dilution used in this experiment is necessary to prevent crosslinking of multiple tryptophan methyl ester molecules. If the volume of acetonitrile used is reduced the yield of product decreases.

R_f = 0.31 (Al₂O₃, 1:2 hexane-ethyl acetate)
¹H NMR (500 MHz, CD₂Cl₂): δ 8.137 (s, br., 1H), 7.582 (d, J = 7.9, 1H), 7.359 (dt, J = 8.0, 0.9, 1H), 7.160 (ddd, J = 8.3, 7.0, 1.2, 1H), 7.088 (ddd, J = 8.0, 7.0, 0.9, 1H), 7.047 (d, br., J = 2.2, 1H), 3.572–3.544 (m, 1H), 3.524 (s, 3H), 3.240–3.158 (m, 2H), 2.783–2.681 (m, 4H), 1.816–1.761 (m, 4H)
¹³C NMR (126 MHz, CD₂Cl₂) δ 173.06, 136.03, 127.32, 122.50, 121.82, 119.21, 118.55, 111.47, 111.08, 67.93, 51.30, 50.94, 27.28, 23.36
IR (KBr, cm^-1) 3407, 3056, 2952, 2876, 2840, 1732, 1620, 1457, 1356, 1435, 1355, 1341, 1293, 1211, 1168, 1068, 1010, 741 cm^-1
MS (EI, m/z) [M]⁺ calculated for C₁₆H₂₀N₂O₂⁺ 272.1519, found 272.1525