ortho-Ethynyl group assisted regioselective and diastereoselective [2+2] cross-photocycloaddition of alkenes

A highly regioselective and diastereoselective [2+2] cross-photocycloaddition between the electron-poor and electron-rich/electron-neutral alkenes under visible-light irradiation was developed. In the absence of an external photocatalyst, the substrates 1 and 4 reacted with rigid cyclic alkoxyethenes to generate (cis,cis)-anti-head-to-head heterocoupled [2+2] products in high yields with regiospecificity and good diastereoselectivity. Meanwhile, the reactions of 1 and 4 with (Z)- and (E)-1,2-diphenylethenes yielded the desired products in good yields with a high ratio of d.r. It is important to note that no geometric isomerization of olefins was observed during the reaction. Mechanistic studies and DFT calculations suggested that 1 and 4 having ortho-ethynyl and cyano groups as a self-photocatalyst play a very important role in the reaction and a five-membered ring diradical intermediate (Int1) was formed via an intramolecular radical addition. The obtained regioselectivity and diastereoselectivity were confirmed by X-ray structural analysis of the representative products.

Photopharmacology of Azo-Combretastatin-A4: Tubulin Polymerization Inhibitors utilizing Green Chemistry Key Step

: Tubulin polymerization inhibitors (TPIs) are promising ligands utilized in chemotherapy for modern cancer treatment. However, the current TPIs exhibit many serious side effects that may pose limitations in chemotherapy. Combretastatin A-4 (CA-4) is a natural TPI that binds at the colchicine binding site located on microtubules. The only cis isomer of CA-4 is bio-active; however, due to its short half-life, it isomerizes quickly to its bio-inactive trans geometric isomer. Preventing shortcomings of CA-4, azobenzene based CA-4, called azo-CA-4 (azo-CA-4), identified as a novel TPI. The geometric isomerization of azo-CA-4 can be controlled upon exposure of ultraviolet (UV) light to remotely control its bioactivity. Cis-azo-CA-4 is 200-500 times more active (IC50 = 0.2-10 µM) than trans-azo-CA-4 (IC50 = 50-110 µM) against various cancer cell lines. Photo-pharmacology uses light to control drug activity, introduce a unique mechanism to develop novel photo-responsive TPIs. Further, the green chemistry approach using ethanol and water as a green solvent in the synthesis of azo-CA-4 delivers advanced methodology in novel TPI development .

Cobalt catalyzed isomerization of alkenes

Catalytic isomerization of alkenes is a highly atom-economical approach to upgrade from lower- to higher-value alkenes. Consequently, tremendous attention has been devoted to the development of this transformation, approaches which exploit cobalt-catalysis are particularly attractive. This short review focuses on the cobalt-catalyzed alkene isomerization, including positional isomerization, geometric isomerization and cycloisomerization. Three main types of reaction mechanism have been discussed to help the reader to better understand and make meaningful comparison between the different transformations. 1 Introduction 2 Positional isomerization 3 Geometric isomerization 4 Cycloisomerization 5 Conclusion and Outlook

Selective E to Z isomerization of 1,3-Dienes Enabled by A Dinuclear Mechanism

The E/Z stereocontrol in a C=C bond is a fundamental issue in olefin synthesis. Although the thermodynamically more stable E geometry is readily addressable by thermal Z to E geometric isomerization through equilibrium, it has remained difficult to undergo thermal geometric isomerization to the reverse E to Z direction in a selective manner, because it requires kinetic trapping of Z-isomer with injection of chemical energy. Here we report that a dinuclear PdI−PdI complex mediates selective isomerization of E-1,3-diene to its Z-isomer without photoirradiation, where kinetic trapping is achieved through rational sequences of dinuclear elementary steps. The chemical energy required for the E to Z isomerization can be injected from an organic conjugate reaction through sharing of common Pd species.

Illuminating Anti-Hydrozirconation: Controlled Geometric Isomerization of an Organometallic Species

A general strategy to enable the formal anti-hydrozirconation of arylacetylenes is reported that merges cis-hydrometallation using the Schwartz Reagent (Cp2ZrHCl) with a subsequent light-mediated geometric isomerization at λ = 400...

Boron-enabled geometric isomerization of alkenes via selective energy-transfer catalysis

Isomerization-based strategies to enable the stereodivergent construction of complex polyenes from geometrically defined alkene linchpins remain conspicuously underdeveloped. Mitigating the thermodynamic constraints inherent to isomerization is further frustrated by the considerations of atom efficiency in idealized low–molecular weight precursors. In this work, we report a general ambiphilic C3 scaffold that can be isomerized and bidirectionally extended. Predicated on highly efficient triplet energy transfer, the selective isomerization of β-borylacrylates is contingent on the participation of the boron p orbital in the substrate chromophore. Rotation of the C(sp2)–B bond by 90° in the product renders re-excitation inefficient and endows directionality. This subtle stereoelectronic gating mechanism enables the stereocontrolled syntheses of well-defined retinoic acid derivatives.

Boron-Enabled Geometric Isomerization of Small Alkene Fragments via Selective Energy Transfer Catalysis

The mammalian visual cycle epitomizes the importance of complex polyenes in biology. However, isomerization-based strategies to enable the sterodivergent construction of these important biomolecules from geometrically defined alkene linchpins remain conspicuously underdeveloped. Mitigating the thermodynamic constraints inherent to isomerization is further frustrated by the considerations of atom efficiency in idealized low molecular precursors. Herein, we report a general ambiphilic C3 scaffold that can be isomerized and bi-directionally extended. Predicated on highly efficient triplet energy transfer, the selective isomerization of β-15 borylacrylates is contingent on the participation of the boron p-orbital in the substrate chromophore. Rotation of the C(sp2)-B bond by 90° in the product renders re-excitation inefficient and endows directionality. This subtle stereoelectronic gating mechanism enables the stereocontrolled syntheses of well-defined retinoic acid derivatives.<br>

Boron-Enabled Geometric Isomerization of Small Alkene Fragments via Selective Energy Transfer Catalysis

The mammalian visual cycle epitomizes the importance of complex polyenes in biology. However, isomerization-based strategies to enable the sterodivergent construction of these important biomolecules from geometrically defined alkene linchpins remain conspicuously underdeveloped. Mitigating the thermodynamic constraints inherent to isomerization is further frustrated by the considerations of atom efficiency in idealized low molecular precursors. Herein, we report a general ambiphilic C3 scaffold that can be isomerized and bi-directionally extended. Predicated on highly efficient triplet energy transfer, the selective isomerization of β-15 borylacrylates is contingent on the participation of the boron p-orbital in the substrate chromophore. Rotation of the C(sp2)-B bond by 90° in the product renders re-excitation inefficient and endows directionality. This subtle stereoelectronic gating mechanism enables the stereocontrolled syntheses of well-defined retinoic acid derivatives.<br>