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

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
PPh3-catalyzed reaction of allenoate H2C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.