A synthetic strategy using Witkop's pyrroloindole for (-)-debromoflustramine B, (+)-ent-debromoflustramine B and (+)-ent-debromoflustramide B

While prenylation of (-)-Witkop's pyrroloindole (2), secured from L-tryptophan under standard N-alkylation conditions, led to a ca. 1: 1 diastereoisomeric mixture of two C(3a)-alkylated indolenines 3 and 4, use of phase-transfer conditions altered this to ca. 1: 2. Reduction followed by N-prenylation of the resulting secondary amines gave C, N-dialkylated products. The derived separable diastereoisomeric (-)-and (+)-Barton esters 19a and 19b were then converted into (-)-debromoflustramine B and (+)-ent-debromoflustramine B, respectively. A novel reaction involving oxygen and the carbanion derived from Barton ester 19b led to (+)-ent-debromoflustramide B. Treatment of N(8)-prenylated Witkop's pyrroloindole 5 with Lewis acid (BF(3)center dot Et(2)O) uncovered a new clean intramolecular cyclisation involving the prenyl unit. (C) 2007 Elsevier Ltd. All rights reserved.