Structures of two dimers formed from oroselol with acids

Abstract

The unidentified, high melting substance previously encountered during investigations of naturally occurring furocoumarins, is shown to be 4-methyl-2,4-bis-(2-oxofuro[2,3-h][1]benzopyran-8-yl)pent-1-ene (XI). It is accompanied by the isomeric substance, 4-methyl-2,4-bis-(2-oxofuro[2,3-h][1]benzopyran-8-yl)pent-2-ene (XII). The formation of (XI) and (XII) from oroselol {8-(1-hydroxy-1-methylethyl)furo[2,3-h][1]benzopyran-2-one}(VII) can be explained as occurring via the carbonium ion (XVI) and its subsequent condensation with oroselone (8-isopropenylfuro[2,3-h][1]benzopyran-2-one)(XIV). This postulated mechanism is supported by the formation in high yield of 8-(αα-dimethylbenzyl)-(VIII), 8-(p-methoxy-αα-dimethylbenzyl)-(IX), and 8-(p-ethoxy-αα-dimethylbenzyl)furo[2,3-h][1]benzopyran-2-one (X) from (VII) in the presence of Lewis acids.