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

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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).