Divergence in the reactivity between amine- and phosphine-catalyzed cycloaddition reactions of allenoates with enynals: One-pot gold-catalyzed synthesis of trisubstituted benzofurans from the [3 + 2] cycloadduct via 1,2-alkyl migration and dehydrogenation

Abstract

Regioselective synthesis of functionalized dihydropyran derivatives by DABCO-catalyzed [2 + 4] cycloaddition of allenoates with enynals or enynones has been developed. Phosphine-catalyzed [3 + 2] cycloaddition of allenoates with enynals provides 1,1-alkyne (aldehyde)-substituted cyclopentenes wherein enynals act as electrophiles. These alkyne-tethered cyclopentenes upon [Au]/[Ag] catalysis lead to substituted benzofurans via 1,2-alkyl migration and dehydrogenation (aromatization). One-pot reaction of allenoates with enynals using sequential phosphine and gold catalysis is also reported. The cyclopentene obtained from the PPh < inf > 3 < /inf > -catalyzed reaction of allenoate H < inf > 2 < /inf > C=C=CH(COO-t-Bu) with enynal undergoes decarboxylation under the [Au]/[Ag] catalysis and forms a carboxylate-free benzofuran. The structures of key products are confirmed by single-crystal X-ray analysis.