A cooperative catalysis-enabled (4 + 3) cycloaddition
of 2-indolylmethanols
with ortho-naphthoquinone methides (o-NQMs), which were in situ-generated from enynones,
has been established in the presence of silver/Brønsted acid
cocatalysts. In the reaction pathway, the key o-NQM
intermediates were formed through Ag(I)-catalyzed cyclization of enynones,
while the indole-based carbocation intermediates were generated via
Brønsted acid-catalyzed dehydration of 2-indolylmethanols. By
this approach, a wide range of seven-membered cyclohepta[b]indoles were synthesized in good yields with high efficiency under
mild reaction conditions, which serves as a useful strategy toward
constructing indole-fused seven-membered rings. Moreover, the catalytic
asymmetric version of this (4 + 3) cycloaddition has been realized
under the cooperative catalysis of Ag(I) with chiral phosphoric acid,
which offered chiral cyclohepta[b]indole with a good
enantioselectivity (75% ee). This work not only represents the first
cooperative catalysis-enabled (4 + 3) cycloaddition of 2-indolylmethanols
but also provides a good example for o-NQM-involved
cycloadditions, which will contribute to the chemistry of 2-indolylmethanols
and enrich the research contents of cooperative catalysis.