Regioselectivity of Alkoxyisobenzofuran-Aryne Cycloadditions

Article abstract of DOI:10.1021/jo00381a016

The cycloaddition reactions of some unsymmetrical arynes with 1-ethoxy-3-(trimethylsilyl)isobenzofuran and a naphtho<1,2-c>furan analogue were examined for prospective regioselectivity.The arynes, generated by lithium tetramethylpiperidide induced dehydrohalogenation of the appropriate haloaromatics, were 3-bromo, 3-chloro, 3-methoxy, and 3-methylbenzyne, 3,4-pyridyne, and 1,2-naphthalyne.Regioselectivities ranged from nil (50/50 isomer ratio with 3,4-pyridyne) to modest (ca. 80/20).The products are bridgehead trimethylsilylated ketals, which undergo a novel acid-catalyzed rearrangement to 9-alkoxy-10-<(trimethylsilyl)oxy>anthracenes.These position-differentiated anthracenediol analogues are thought to be formed by ring opening, followed by Brook rearrangement.Isomeric ketal pairs were found to react at different rates, and this selective decomposition was used to isolate one of the cycloadduct isomers from the reaction of 3-bromobenzyne.Lithium-bromine exchange followed by methylation was used to determine its structure, and this information in turn was used to clarify the mechanism of the acid-catalyzed reaction.