Solvent Effects in Hydrogen Abstraction from Cholesterol by Benzophenone Triplet Excited State

Mechanisms and energetics for hydrogen abstraction of thymine photosensitized by benzophenone from theoretical principles

The significant role of hydrogen abstraction in chemistry and biology has inspired many theoretical works to link its practical phenomena and mechanistic properties. Here, the photophysical processes and hydrogen abstraction mechanisms of benzophenone (BZP) photosensitized thymine damage were systematically investigated from theoretical principles. It was found that the BZP photosensitizer upon UV irradiation undergoes vertical excitation, internal conversion, vibrational relaxation and intersystem crossing into a triplet excited state. Then the triplet BZP damages thymine by a hydrogen abstraction process. However, the reverse reaction easily occurs due to the lower reaction energy, which causes a low yield of hydrogen abstraction products. We hope this comprehensive work can provide a deeper understanding of photosensitive DNA damage from hydrogen abstraction.

Hydrogen Abstraction from the C15 Position of the Cholesterol Skeleton

Cholesterol (Ch) is an integral part of cell membrane, where it is prone to oxidation. In humans, oxidation of Ch is commonly linked to various pathologies like Alzheimer's disease, atherosclerosis, and even cancer, which proceed via mechanisms involving enzymatic and free radical pathways. The latter begin with hydrogen abstraction (HA) from Ch by a reactive free radical. It has been established that the most efficient HA from Ch occurs at C7, although HA from C4 by peroxyl radicals has recently been observed. Conversely, HA from Ch positions other than the thermodynamically preferred C7 or C4 has never been reported. We have designed a Ch derivative where a benzophenone moiety is linked to C7 by a covalent bond. This mirrors a specific orientation of Ch within a confined environment. Product analysis and time-resolved spectroscopic studies reveal an unprecedented HA from C15, which is a thermodynamically unfavorable position. This indicates that a specific topology of reactants is crucial for the reactivity of Ch. The relative orientation of the reactants can also be relevant in biological membranes, where Ch, polyunsaturated fatty acids, and numerous oxidizing species are confined in highly restricted and anisotropic environments.

Evolution of a Strategy for the Enantioselective Total Synthesis of (+)-Psiguadial B

(+)-Psiguadial B is a diformyl phloroglucinol meroterpenoid that exhibits antiproliferative activity against the HepG2 human hepatoma cancer cell line. This full account details the evolution of a strategy that culminated in the first enantioselective total synthesis of (+)-psiguadial B. A key feature of the synthesis is the construction of the trans-cyclobutane motif by a Wolff rearrangement with in situ catalytic, asymmetric trapping of the ketene. An investigation of the substrate scope of this method to prepare enantioenriched 8-aminoquinolinamides is disclosed. Three routes toward (+)-psiguadial B were evaluated that featured the following key steps: (1) an ortho-quinone methide hetero-Diels-Alder cycloaddition to prepare the chroman framework, (2) a Prins cyclization to form the bridging bicyclo[4.3.1]decane system, and (3) a modified Norrish-Yang cyclization to generate the chroman. Ultimately, the successful strategy employed a ring-closing metathesis to form the seven-membered ring and an intramolecular O-arylation reaction to complete the polycyclic framework of the natural product.

Hydrogen bonding tunes the early stage of hydrogen-atom abstracting reaction

The spontaneous and collision-assisted hydrogen-atom abstracting reaction (HA) dynamics of triplet benzil are investigated through the combination of transient absorption spectroscopy with TD-DFT calculations. HA dynamics exhibit a remarkable dependence on the hydrogen donor properties. The effects of the triplet-state hydrogen bonding on the reaction dynamics are illustrated. In particular, it is experimentally observed that strengthened triplet-state hydrogen bonding could accelerate the HA, whereas weakened triplet-state hydrogen bonding would postpone the HA. The triplet-state hydrogen bonding has great influences on the early stage of the HA reaction, while the bond dissociation energy of the hydrogen donors determines the subsequent reaction pathways. Protic solvents could sustain longer lifetimes of the excited-state intermediate formed after HA than non-protic solvents by 10 μs. This investigation provides insights into the HA dynamics and guidance to improve the product efficiency of photochemical reactions.

Recent advances in cholesterol chemistry

This review article presents advances in cholesterol chemistry since 2000. Various transformations (chemical, enzymatic, electrochemical, etc.) of cholesterol are presented. A special emphasis is given to cholesterol oxidation reactions, but also substitution of the 3β-hydroxyl group, addition to the C5-C6 double bond, C-H functionalization, and C-C bond forming reactions are discussed.

Organic aspects. Oxygen-containing functions

In this chapter, most of the reported work deals with the photochemistry of carbonyl compounds; however, the photoreactions of other functions, such as the photo-Claisen rearrangement or the photocleavage of cyclic ethers, are also included. In the present volume, time coverage is 2010–2011, and only original research articles are quoted. In general, reviews or purely theoretical calculations are not systematically included. As usually, the material is organized according to established types of reactions (e.g., Norrish I/II, hydrogen abstraction, Paternò-Büchi, photoelimination, photo-Fries/photo-Claisen, etc.). After presenting the basic photochemical aspects, more specific findings are reported. They include synthetic applications, stereoselectivity, and biological or technological implications. Next, the attention is focused on photochemical reactions in anisotropic media, including (micro)heterogeneous or supramolecular systems, solid matrixes or fully organized crystals. Finally, mechanistic studies based on direct experimental evidence are highlighted, especially when transient absorption spectroscopy or related ultrafast detection are employed.

Understanding kinetic solvent effects on hydrogen abstraction reactions from carbon by the cumyloxyl radical

A kinetic study of the hydrogen abstraction reactions from tetrahydrofuran (THF) and cyclohexane (CHX) by the cumyloxyl radical was carried out in different solvents. With THF, a 4.5-fold decrease in rate constant (k(H)) was observed on going from isooctane to 2,2,2-trifluoroethanol. An opposite behavior was observed with CHX, where k(H) increased by a factor 4 on going from isooctane to 2,2,2-trifluoroethanol. The important role of substrate structure and of the solvent hydrogen bond donor ability is discussed.