A highly
efficacious photoinduced reversible complexation-mediated
polymerization (photo-RCMP) system using amino alcohol N-oxide as
the catalyst has been developed, allowing access to controlled high-molecular-weight
polymers of different methacrylates (M
n = 497–815 kg/mol, Đ < 1.5), which
is very challenging for those known catalytic systems of photo-RCMPs
and most organocatalytic photoinduced controlled radical polymerization
as it demands higher controllability of catalysts. The monomers can
be converted into the polymers nearly quantitatively under simulated
or natural sunlight irradiation. Strong hydrogen bond effects are
observed in this high-molecular-weight photo-RCMP, in terms of both
monomer conversion and molecular weight controllability. The insights
of the effects are revealed as the intramolecular hydrogen bond of
the N-oxide catalyst modifies the electron density of N-oxide, and
thereby facilitates the regeneration of the catalyst (activator) and
release of I2 as a predominant effective regulator of radical
concentration, by the studies of X-ray analysis, control experiments,
density functional theory calculation, ultraviolet–vis absorption
spectroscopy, etc.
Both cis- and trans- tetracyclic
spiroindolines are the core of many important biologically active
indole alkaloids, but the divergent synthesis of these important motifs
is largely hampered by the limited stereoselectivity control. A facile
stereoinversion protocol is reported here in Michael addition-initiated
tandem Mannich cyclizations for constructing tetracyclic spiroindolines,
providing an easy access to two diastereoisomeric cores of monoterpene
indole alkaloids with high selectivity. The mechanistic studies including
in situ NMR experiments, control experiments, and DFT calculations
reveal that the reaction undergoes a unique retro-Mannich/re-Mannich
rearrangement including a C–C bond cleavage that is very rare
for a saturated six-membered carbocycle. Insights into the stereoinversion
process have been uncovered, and the major effects were determined
to be the electronic properties of N-protecting groups
of the indole with the aid of Lewis acid catalysts. By understanding
these insights, the stereoselectivity switching strategy is also smoothly
applied from enamine substrates to vinyl ether substrates, which are
enriched greatly for the divergent synthesis and stereocontrol of
monoterpene indole alkaloids. The current reaction also proves to
be very practical and was successfully applied to the gram-scale total
synthesis of strychnine and deethylibophyllidine in short routes.
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