Asymmetric Hydrovinylation of Vinylindoles. A Facile Route to Cyclopenta[g]indole Natural Products (+)-cis-Trikentrin A and (+)-cis-Trikentrin B

Vinylindoles undergo Ni(II)-catalyzed asymmetric hydrovinylation under very mild conditions (-78 degrees C, 1 atm ethylene, 4 mol% catalyst) to give the corresponding 2-but-3-enyl derivatives in excellent yields and enantioselectivities. Hydroboration of the alkene and oxidation to an acid, followed by Friedel-Crafts annulation, gives an indole-annulated cyclopentanone that is a suitable precursor for the syntheses of cis-trikentrins and all known herbindoles. For example, the cyclopentanone from 4-ethyl-7-vinylindole is converted into (+)-cis-trikentin A in four steps (Wittig reaction, alkene isomerization, diastereoselective hydrogenation, and nitrogen deprotection). The previous synthesis of this molecule from (S)-(-)-malic acid involved more than 20 steps and a preparative HPLC separation of diastereomeric intermediates.