Regiodivergent photocyclization of dearomatized acylphloroglucinols: asymmetric syntheses of (—)-nemorosone and (—)-6-epi-garcimultiflorone A

Regiodivergent photocyclization of dearomatized acylphloroglucinol substrates has been developed to produce type A polycyclic polyprenylated acylphloroglucinol (PPAP) derivatives using an excited-state intramolecular proton transfer (ESIPT) process. Using this strategy, we achieved the enantioselective total syntheses of the type A PPAPs (—)-nemorosone and (—)-6-epi-garcimultiflorone A. Diverse photocyclization substrates have been investigated leading to divergent photocyclization processes as a function of tether length. Photophysical studies were performed, and photocyclization mechanisms were proposed based on investigation of various substrates as well as deuterium-labeling experiments.R35 GM118173 - NIGMS NIH HHSAccepted manuscriptSupporting documentatio

Stereoselective E/Z photoisomerization of oxazolidinone functionalized enecarbamates: direct and triplet sensitized irradiation

Oxazolidinone-functionalized enecarbamates undergo diastereoselective E/Z photoisomerization upon direct and triplet sensitized irradiations with chiral/achiral sensitizers, showing that the enhanced product diastereoselectivity depends on the solvent and temperature

Towards Upcycling Biomass-Derived Crosslinked Polymers with Light

Photodegradable, recyclable, and renewable, crosslinked polymers from bioresources show promise towards developing a sustainable strategy to address the issue of plastics degradability and recyclability. Photo processes are not widely exploited for upcycling polymers in spite of the potential to have spatial and temporal control of the degradation in addition to being a green process. In this report we highlight a methodology in which biomass-derived crosslinked polymers can be programmed to degrade at ≈300 nm with ≈60 % recovery of the monomer. The recovered monomer was recycled back to the crosslinked polymer

Modulating Photochemical Properties to Enhance the Stability of Electronically Dimmable Eye Protection Devices

The study evaluates compatibility of stabilizers with dye doped liquid crystal (LC) scaffolds that are used in electronically dimmable materials. The photodegradation of the materials was investigated and suitable stabilizers were evaluated to slow the degradation process. Various types of benzotriazole-based stabilizers were evaluated for stabilizing the liquid crystals. Based on spin trapping experiments, radicals generated upon UV exposure is likely responsible for the degradation of the system. The radical generation is competitively inhibited by the addition of stabilizers. Abbreviations: LC, liquid crystal; STB, stabilizers

ChemInform Abstract: Supramolecular Photochemistry as a Potential Synthetic Tool: Photocycloaddition

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Keeping the name clean: [2+2] photocycloaddition

Crossed [2 + 2] photocycloaddition is a specific case of intramolecular photocycloaddition reaction. Recently, the term "crossed [2 +2] photocycloaddition" is interchangeably used to represent intermolecular [2 +2] photocycloaddition reactions of two dissimilar double bonds/alkenes. To avoid confusion and to help researchers use the correct terminologies, this perspective clarifies the terminology used for different [2 +2] photocycloaddition processes based on prior literature with the hope of establishing a standard for addressing the diverse set of photocycloaddition reactions that will be helpful to the chemical community. [GRAPHICS]

Deciphering mechanism of excited state reactivity by spectroscopic methods

The Feature highlights Electron Paramagnetic Resonance (EPR) spectroscopy as an indispensable tool to understand the excited state reactivity of organic molecules

Triple-Dearomative Photocycloaddition – A Strategy to Construct Novel Caged Molecular Frameworks

An unprecedented, caged 2H-benzo-dioxo-pentacycloundecane (BDPC) framework was serendipitously obtained in a single transformation via a triple-dearomative photocycloaddition of chromone esters with furans. This caged structure was generated during the course of an effort to access a tricyclic, oxygen-bridged intermediate enroute to the dihydroxanthone natural product nidulalin A. Reaction scope and limitations were thoroughly investigated, revealing the ability to access a multitude of synthetically challenging caged scaffolds in a two-step sequence. Photophysical studies provided key mechanistic insights on the process for formation of the novel caged scaffold

Singlet Oxygen Mediated Oxidation of Olefins within Zeolites: Selectivity and Complexities

Thiazine dyes such as thionin, methylene blue and methylene green have been cation exchanged within monovalent cation exchanged Y zeolites. Depending on the water content, the dye molecules exist as either monomers ('dry') or dimers ('wet'). The monomeric dye, upon excitation with visible light, generates singlet oxygen, which has been utilized to oxidize alkenes to hydroperoxides. In the case of trisubstituted alkenes, hydroperoxidation within zeolites occurs with a certain amount of regioselectivity. The oxidation within zeolites is accompanied by photodecomposition of the dye and the product hydroperoxides and acid catalyzed rearrangement of the alkenes. In order to understand the observed selectivity, ab initio and DFT calculations on model systems have been performed. The calculations confirm fairly strong cation-alkene binding as well as additional geometric and orbital distortions. Computed activation energies for hydrogen abstraction suggest a significant rare retardation due to metal coordination. At both the MP2 and B3LYP levels, formation of the tertiary hydroperoxide by hydrogen abstraction from the methyl group (4-position) of 2-methyl 2-butene is calculated to be favored by a small margin. Between the gem-dimethyl units, abstraction from the syn methyl group is favored slightly compared to the anti counterpart. These predictions are not compatible with the observed regioselectivities. Further experimental and theoretical studies are underway to understand the observed regioselective oxidation within zeolites