Synthesis of sophorol, violanone, lonchocarpan, claussequinone, philenopteran, leiocalycin, and some other natural isoflavonoids by the oxidative rearrangement of chalcones with thallium(III) nitrate

Abstract

Isoflavones are conveniently prepared by the oxidative rearrangement of 2′-hydroxy- or 2′-acetoxy-chalcones with thallium(III) nitrate in methanol into 1-(2-hydroxyphenyl)-3,3-dimethoxy-2-phenylpropan-1-ones [e.g.(46)] followed by cyclisation, and the following natural isoflavonoids were synthesised in this way: the isoflavanones sophorol (49) and violanone (68), the isoflavans vestitol (57), duratin (58), mucronulatol (59), laxifloran (60), and Ionchocarpan (61), the isoflavan quinones mucroquinone (66) and claussequinone (67), the pterocarpans philenopteran (69), leiocalycin (54), and 2-hydroxy-3-methoxy-8,9-methylenedioxy-6a,11a-dihydropterocarpan (56).

Acid catalysed cyclisation of 1-(2-hydroxy-4-methoxyphenyl)-2-(2-hydroxy-4,5-methylenedioxyphenyl)-3,3-dimethoxypropan-1-one (48) gave no isoflavone but the two tetracyclic compounds, (53)(a derivative of 2-methoxypterocarp-6-ene) and (52)(a benzofuro[2,3-b][1]benzopyran).

Claussequinone (67) was rapidly formed by autoxidation of its hydroquinone precursor (64), suggesting that oxidation may also have occurred during the isolation of the metabolite.