Conspectus
Natural
products containing bridged ring systems are widely identified
and show significant biological activity. The development of efficient
synthesis reactions and strategies to construct bridged ring systems
is a long-standing but very significant challenge in organic chemistry.
In 2014, our group developed a unique type II [5 + 2] cycloaddition
reaction that provides a facile and direct methodology for constructing
highly functionalized bridged bicyclo[4.3.1], bicyclo[4.4.1], bicyclo[5.4.1],
bicyclo[6.4.1], and other bicyclo[m.n.1] systems containing a strained
bridgehead double bond. In this Account, we summarize the methodology
development and report the results of application of our unique strategy
for the total synthesis of several natural products with bridged ring
systems (i.e., cyclocitrinol, cerorubenic acid-III, and vinigrol)
during the past 5 years in our laboratory. In the first part, we introduce
the logic behind the design and discovery of type II [5 + 2] cycloadditions.
The substrates can be easily synthesized by a modular approach, followed
by base-promoted group elimination under heat to form an oxidopyrylium
ylide, which can undergo cycloaddition under relatively mild conditions
with a variety of double bonds to generate bridged bicyclo[m.n.1]
frameworks in high yield. The diastereocontrol and unique endo selectivity of this methodology are favorable for further
application to the synthesis of complex natural products. In the second
part, we highlight our endeavors in the total synthesis of several
different types of molecules bearing bridged ring systems using our
methodology. The bridged bicyclo[4.4.1] system is the core structure
of two different types of natural products, cyclocitrinol and cerorubenic
acid-III, that can be efficiently constructed by type II [5 + 2] cycloadditions.
The development of suitable strategies and methods for site-selective
cleavage of the C–O bond of the oxa-[3.2.1] ring system in
the products of type II [5 + 2] cycloadditions is also discussed and
highlighted during the syntheses. Moreover, the bridged bicyclo[5.3.1]
system is the core structure of vinigrol, which can be constructed
through a novel ring contraction sequence of the bicyclo[5.4.1] system
formed by a type II [5 + 2] cycloaddition. By combining with a ring
contraction cascade, we believe that type II [5 + 2] cycloadditions
have the potential to be used as a unified approach to constructing
natural products containing bridged bicyclo[m.n.1] frameworks.
Electrochemical bubbling transfer of graphene is a technique with high industrial potential due to its scalability, time- and cost-effectiveness, and ecofriendliness. However, the graphene is often damaged due to the turbulence and the trapped bubbles formed by the direct H2O and H+permeation through the supporting polymer. We invent a graphene mechanical support of polyethylene terephthalate foil/plastic frame/poly(methyl methacrylate) sandwich, with an encapsulated air gap as the permeation stopping layer. The graphene damage is drastically reduced, as confirmed by the morphology and structural and electrical characterization, ultimately improving the controllability/reproducibility of the bubbling transfer of graphene and other two-dimensional materials.
Hydrogen binding of molecules on solid surfaces is an attractive interaction that can be used as the driving force for bond activation, material-directed assembly,p rotein protection, etc. However,t he lacko faq uantitative characterization method for hydrogen bonds (HBs) on surfaces seriously limits its application. We measured the standardG ibbsf ree energy change (DG 0 )o fo n-surface HBs using NMR. The HBaccepting ability of the surface was investigated by comparing DG 0 values employing the model biomass platform 5-hydroxymethylfurfural on aseries of Co-N-C-n catalysts with adjustable electron-richn itrogen-doped contents.D ecreasing DG 0 improves the HB-accepting ability of the nitrogen-doped surface and promotes the selectively initiated activation of OÀHb onds in the oxidation of 5-hydroxymethylfurfural. As ar esult, the reaction kinetics is accelerated. In addition to the excellent catalytic performance,the turnover frequency (TOF) for this oxidation is muchhigher than for reported non-noblemetal catalysts.
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