Photocleavage of dimers of coumarin and 6-alkylcoumarins

The cleavage of four coumarin dimers, the syn-head-to-tail (ht) dimer of parent coumarin (syn-ht-CC1), the anti- and syn-hh dimers of 6-methylcoumarin (anti-hh-CC2 and syn-hh-CC2, respectively) and the anti-hh dimer of 6-dodecylcoumarin (anti-hh-CC3), was studied by UV–vis and IR spectroscopy and HPLC upon direct 254 nm irradiation as well as sensitized excitation. The quantum yield of dimer splitting is Φ_sp = 0.1–0.3 in various solvents and the effects of structure and solvent polarity are small. In certain solvents some of the dimers produced CO₂ along with the monomers in the splitting reaction. Electron transfer from dimers to the triplet state of sensitizers, such as benzophenone or 9,10-anthraquinone, was observed in acetonitrile.     

Topochemical single-crystal-to-single-crystal photodimerization

The topochemical and topotactic solid-state photodimerization of 2-benzyl-5-benzylidenecyclopentanone is shown to be single-crystal-to-single-crystal, with the cell parameters of the monomer and dimer being very nearly identical (the maximum change being a contraction of 0.7% along c). For 2-benzl-5-p-bromobenzylidenecyclopentanone, the change in cell parameters is significantly larger (the maximum change being an elongation of 6.5% along c) such that the crystallinity of the final product varies with amongst other factors, the rate of reaction. A study of such benzylidenecyclopentanone derivatives should permit a greater understanding of those processes which determine the crystalline perfection of the reaction products.     

The photodimerization of aceanthrylene

Sensitized photodimerization of aceanthrylene (AA) with Rose Bengal in methanol produced both syn and anti head-to-head and head-to-tail stereoisomers with slight preference for syn adducts. The stereochemical configurations were assigned by deuterium labeling and by high field homonuclear shift correlation (COSY) experiments and these compare favorably with the assignments made for the photodimers of 1-substituted acenaphthylenes. Currently, AA has photodimerized under direct irradiation only in carbon disulfide.     

Supramolecular photodimerization of coumarins

Stereoselective photodimerization of coumarin and its derivatives in supra-molecular systems is reviewed. The enantioselective photodimerization of coumarin and thiocoumarin in inclusion crystals with optically active host compounds is also described.     

Solid-state photodimerization of steroid enones

Androst-4-ene-3,17-dione (1) and 17alpha-methyltestosterone (2) are dimerized in the solid-state by UV radiation. These substances were selected by a search in the CSD among the steroid enones presenting in the crystalline state an intermolecular short contact between a hydrogen alpha to a carbonyl group and the oxygen of an enone system. Dimerization occurs by transfer of the hydrogen to the oxygen and connection between the two involved carbons. Androst-4-ene-3,17-dione (1) affords dimer 3 and trimer 4, both formed by connection of the C-16 of a molecule with the C-3 of a near one. Irradiation of 17alpha-methyltestosterone (2) gives the isomeric trienones 5 and 6. These compounds are reasonably formed by dehydration of unisolated intermediate products 7 and/or 8 obtained by coupling of two molecules through a linkage between the C-2 and the C-3' carbons. The formation mechanisms of the photoproducts are satisfactory explained on the basis of the molecular arrangement of the monomers in the crystal state. Modeling of the dimeric molecules was done using molecular mechanics calculations. A single-crystal X-ray of the dimer of androst-4-ene-3,17-dione confirms the structural interpretation of spectral data. The conformer found in the solid-state agrees well with the results of molecular mechanics calculations.     

Theoretical study of the photodimerization of pyrimidine bases

The experimental results are correctly interpreted as from the delocalized model of the transition state and the theory of perturbations. The surfaces of the energy of interaction between both molecules in function of different coordinates of reaction make it possible to discuss the concerted nature of photodimerization and permit the two conjugated systems to be added between carbon atoms 5 and 6 of both molecules.

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Zusammenfassung Die experimentellen Resultate werden mit Hilfe eines delokalisierten Modells des Übergangszustandes und der Störungstheorie richtig interpretiert. Die Energiehyperfläche der Wechselwirkung zwischen den beiden Molekülen als Funktion verschiedener Reaktionskoordinaten erlaubt die Diskussion des konzertierten Ablaufs der Photodimerisation und der Addition der beiden konjugierten Systeme, die zwischen den Kohlenstoffatomen 5 und 6 der beiden Moleküle stattfindet.

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Résumé Les résultats expérimentaux sont correctement interprétés à partir du modèle delocalisé de l'état de transition et de la théorie des perturbations. Les surfaces de l'énergie d'interaction entre les deux molécules en fonction aux différentes coordonnées de réaction permettent de discuter le caractère concerté de la photodimerisation et que l'addition des deux systèmes conjugués se réalise entre les carbones 5 et 6 des deux molécules.

     

Unsymmetrical photodimerization of a 9-aminomethylanthracene in the crystalline salt

By the salt formation with particular nonaromatic dicarboxylic acids, rapid and selective photodimerization of 9-(N,N-dimethylaminomethyl)anthracene (1) was accomplished in the solid state. For instance, the salt with trans,trans-muconic acid or acetylenedicarboxylic acid was led quantitatively to the 9,10:4',1' photodimer usy-ht-2, the first example of the unsymmetrical [4+4] photodimerization of anthracene in the solid state. The reactions were rationalized by the relevant C...C distances between the reacting carbons.     

Exploring two-state reaction pathways in the photodimerization of cyclohexadiene.

The ground- (S0) and lowest triplet-state (T1) pathways associated with dimerization of cyclohexadiene to give [2+2] and [4+2] cycloadducts have been theoretically studied at the UBLYP and UB3LYP levels of theory with the 6-31G* basis set. The DFT energies were validated by CCSD(T) single-point energy calculations. These cycloaddition reactions follow stepwise mechanisms with formation of bis-allylic biradical (BB) intermediates. In the S0 ground state, the interaction between two cyclohexadiene molecules with formation of BB intermediate IN(S0) has a large activation enthalpy of 32.0 kcal mol(-1). On the other hand, C-C bond-formation in the lowest triplet state (T1) leading to BB intermediate IN(T1) has a low activation enthalpy of 5.0 kcal mol(-1), but the subsequent ring closure involves a very large activation enthalpy of 43.4 kcal mol(-1). Triplet-to-singlet intersystem crossing from IN(T1) to IN(S0) favors cyclization to give the corresponding [2+2] and [4+2] cycloadducts.     

Supramolecular photocatalysis: insights into cucurbit[8]uril catalyzed photodimerization of 6-methylcoumarin

Guest induced shape change of the cucurbit[8]uril cavity is likely rate limiting in the supramolecular photocatalytic cycle for CB8 mediated photodimerization of 6-methylcoumarin.     

以活性酰化试剂选择性制备蔗糖-6-酯

由于具有良好的表面活性和生物活性,蔗糖-6-酯被广泛应用到表面活性剂、食品添加剂等方面[1,2],它们还是合成超级甜味剂三氯蔗糖的关键中间体[3].     

Context-dependent photodimerization in isolated thymine-thymine steps in DNA

The effect of 280 nm irradiation on a family of synthetic DNA hairpins possessing an alkane linker connecting a six-base pair stem having a single T-T step located at different positions within the hairpin has been investigated. A single adduct assigned to the product of 2+2 dimerization is obtained except in the case of a T-T step located adjacent to the linker, in which case both 2+2 and 6-4 adducts are obtained. The efficiency of dimerization is similar for three hairpins having a T-T step located within the duplex interior. Lower efficiency is observed for a T-T step located at the open end of the hairpin and in T overhangs, whereas higher efficiency is observed for the T-T step adjacent to the linker and in a single T bulge. The context-dependence of dimerization efficiency is discussed.     

Structure of the product of the sensitized photodimerization of peucedanin

Chemistry of Natural Compounds - On the basis of the NMR spectrum, a most probable structure has been proposed for the product of the sensitized photodimerization of the natural furocoumarin...     

A theoretical study of the reaction pathway in the photodimerization of anthracene

The approximation proposed by Salem to calculate the interaction energy between conjugated systems has been used to obtain different potential surfaces for anthracene photodimerization in order to establish the reaction pathway.     

Structure of the product of the sensitized photodimerization of peucedanin

Summary On the basis of the NMR spectrum, a most probable structure has been proposed for the product of the sensitized photodimerization of the natural furocoumarin peucedanin. It has been shown that the latter is formed at the expense of the double bonds of the -pyrone ring of one and of the furan ring of the second molecule in the head to tail manner.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR. Translated from Khimiya Prirodynkh Soedinenii, No. 6, pp. 680–683, November–December, 1970.     

The role of orientational defects in the photodimerization of 1,5-dichloroanthracene

The photodimerization of 1,5-dichloroanthracene is discussed with particular reference to the mechanism by which the ratio of head-to-head and head-to-tail dimer differs between reaction in the solid and in solution. It is shown that in certain crystallographic space groups linear and planar imperfections will not, of themselves, produce regions of packing suitable for the formation of the “non-topochemical” dimer. A novel type of point defect which may account for the solid state reactivity is described and the extra lattice energy that is associated with this defect calculated.     

Photochemical assembly of gold nanoparticles utilizing the photodimerization of thymine

Irradiation of UV light to the solution of gold nanoparticles modified with thymine units resulted in the formation of aggregates comprising chemical cross-linking gold nanoparticles through the photodimerization of the thymine units. Transmission electron microscopy and UV-visible absorption measurement showed the aggregates consisting of the gold nanoparticles. The effect of thymine unit density on the nanoparticle surface and the concentration of the gold nanoparticles in solution to the aggregation process were studied by UV-visible absorption measurement.     

Coerced photodimerization reaction in the solid state through amine salt formation

Photodimerization of fumaric or several γ-form trans-cinnamic acids proceeded successfully in the solid state through amine salt formation with ammonia or some aromatic heterocyclic amines (especially, imidazole). It appears that this success is due to a small size or a planar structure of the amine. A layered or a channel-type clathrate crystal structure was revealed, respectively.