[Weitz' aminium salt initiated electron transfer reactions and application to the synthesis of natural products].

A stable cation radical Weitz' aminium salt, tris(4-bromophenyl)aminium hexachloroantimonate (BAHA) initiated electron transfer has been found to efficiently promote a great variety of reactions on electron-rich substrates: e.g. the cation radical pericyclic reactions, the chain-induced cation radical oxygenation of strained olefines and dienes, and several other intriguing reactions on a great variety of electron-rich substrate. The discovery of the Weitz' aminium salt catalysed Diels-Alder reaction has stimulated interest in cation radical chemistry and facilitated the development of a wide range of cation radical pericyclic chemistry. Applications of the method to the synthesis of natural products utilizing BAHA are also presented. Reaction of a lignan precursor, cinnamyl alcohols with BAHA in tetrahydrofuran (THF) gave a furofuran lignan, (+/-)-sesamin in one step. Podophyllum lignans, (+/-)-isopodophyllotoxin and (+/-)-isopicropodophyllin were synthesized by a biomimetic procedure from the doubly unsaturated esters by means of the BAHA induced cation-radical cycloaddition reaction. A new general synthesis for (+/-)-dibenzocyclooctadiene lignans was established utilizing novel synthetic method for quinone derivatives by oxidation with BAHA. Reactions of phenol derivatives using BAHA gave the corresponding quinone derivatives, which may be useful synthon for obtaining quassinoids such as quassin.