A rhodium(III)-catalyzed controllable [4 + 1] and [4
+ 2] annulation
of N-aryl pyrazolones with maleimides as C1 and C2
synthon has been explored for the synthesis of spiro[pyrazolo[1,2-a]indazole-pyrrolidines] and fused pyrazolopyrrolo cinnolines.
The product selectivity was achieved through time-dependent annulation.
The [4 + 1] annulation reaction involves sequential Rh(III)-catalyzed
C–H alkenylation of N-aryl pyrazolone, followed
by an intramolecular spirocyclization via aza-Michael-type addition
to afford spiro[pyrazolo[1,2-a]indazole-pyrrolidine].
However, prolonged reaction time converts in situ formed spiro[pyrazolo[1,2-a]indazole-pyrrolidine]
into fused pyrazolopyrrolocinnoline. This unique product formation
switch proceeds via strain-driven ring expansion through a 1,2-shift
of the C–C bond.
Rh(III)-catalyzed cascade C-H activation and cyclization of 2-aryl benzimidazoles with maleimides for the synthesis of benzimidazole-fused isoquinolines and benzimidazole spiro isoindoles is reported. The switchable selectivity in the formation of...
A Rh(III)-catalyzed [4 + 1] cyclization of 2-arylbenzimidazoles
with alkynoates through C–H activation/ortho-alkenylation/intramolecular
annulation cascade to obtain benzimidazole-fused isoindoles is reported.
The reaction of the Rh catalyst and internal alkyne ester provides
benzo[4,5]imidazo[2,1-a]isoindole acetate exclusively.
Conversely, internal alkyne amide participates in the annulation process
in the presence of a Ru catalyst to provide benzo[4,5]imidazo[2,1-a]isoindole acetamide. The alkyne acts as a C1 synthon and
undergoes [4 + 1] cyclization rather than traditional [4 + 2] annulation.
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