Radical-Cation-Initiated Diels-Alder Di- and Trimerization of 3,4-Dimethoxypropenylbenzene: Stereochemical Correlation with the Lignans Galbulin and Isogalbulin

Article abstract of DOI:10.1021/ja00187a029

Diels-Alder reactions of methylisoeugenol (3,4-dimethoxypropenylbenzene, 1) initiated by the radical cation tris(p-bromophenyl)aminium hexachloroantimonate (BAHA) lead to the naphthalene regioisomers 7 and 8 as well as the trimers 9-15.The radical-cation origins of these products have been established through detailed regio- and stereochemical studies which indicate that the initial Diels-Alder step proceeds with (1) a lack of regiospecificity and (2) a preference for the endo rather than the exo reaction configuration.The number of regio- and stereoisomers is substantially reduced in the BAHA-initiated rearrangement of the cyclobutane 16, which affords only the naphthalene 7 and the trimers 11-13.While the initial Diels-Alder step provides the same stereochemistry that is found in the lignans galbulin (3) and isogalbulin (5), the formation of the regioproducts (8, 14, and 15) and the involvement of a third unit of 1 to form the trimers (9-15) preclude any straightforward correlation of radical-cation Diels-Alder reactions with the biosynthesis of the aryltetralin lignans.