Synthesis of lignan lactones by conjugate addition of thioacetal carbanions to butenolide

Abstract

The conjugate addition of carbanions dervied from arylbis(phenylthio)methanes to butenolide followed by trapping of the enolate anion so generated by a benzyl halide affords adducts (8) containing the basic lignan skeleton. Desulphurisation of these adducts by Raney nickel affords a short efficient synthesis of trans-dibenzylbutyrolactones and this route has been used to prepare enterolactone (9c) and an antitumour lignan (9b), derived from Bursera schlechtendalii. Treatment of the adducts (8) with heavy-metal salts induces cyclisation leading to arylnaphthalene lactones, including retrojusticidin B (13).