Fig. 4

Reagents and conditions: (a) (i) AcOH, aldehydes, reflux, 3 h; (ii) K₂Cr₂O₇, AcOH, 100 °C, 20 min; (b) 4-bromobutanol, Ph₃P, DIAD, THF, 0 °C-r.t, 16 h; (c) NaH, dry DMF, 60–80 °C, 48 h. Numbers in brackets denote; (1) L-tryptophan, (2a-d) β-carboline intermediates, (3a-d) corresponding phenols, (4a-d) tail synthons, (5a-g) target β-carboline analogs.