Photochemistry of 4'-benzophenone-substituted nucleoside derivatives as models for ribonucleotide reductases: competing generation of 3'-radicals and photoenols

Photo-assembly of plasmonic nanoparticles: methods and applications

In this review article, various methods for the light-induced manipulation of plasmonic nanoobjects are described, and some sample applications of this process are presented. The methods of the photo-induced nanomanipulation analyzed include methods based on: the light-induced isomerization of some compounds attached to the surface of the manipulated object causing formation of electrostatic, host-guest or covalent bonds or other structural changes, the photo-response of a thermo-responsive material attached to the surface of the manipulated nanoparticles, and the photo-catalytic process enhanced by the coupled plasmons in manipulated nanoobjects. Sample applications of the process of the photo-aggregation of plasmonic nanosystems are also presented, including applications in surface-enhanced vibrational spectroscopies, catalysis, chemical analysis, biomedicine, and more. A detailed comparative analysis of the methods that have been applied so far for the light-induced manipulation of nanostructures may be useful for researchers planning to enter this fascinating field.

Conformational control of benzophenone-sensitized charge transfer in dinucleotides

Charge transfer in DNA cannot be understood without addressing the complex conformational flexibility, which occurs on a wide range of timescales. In order to reduce this complexity four dinucleotide models 1X consisting of benzophenone linked by a phosphodiester to one of the natural nucleosides X = A, G, T, C were studied in water and methanol. The theoretical work focuses on the dynamics and electronic structure of 1G. Predominant conformations in the two solvents were obtained by molecular dynamics simulations. 1G in MeOH adopts mainly an open geometry with a distance of 12–16 Å between the two aromatic parts. In H2O the two parts of 1G form primarily a stacked conformation yielding a distance of 5–6 Å. The low-lying excited states were investigated by electronic structure theory in a QM/MM environment for representative snapshots of the trajectories. Photo-induced intramolecular charge transfer in the S1 state occurs exclusively in the stacked conformation. Ultrafast transient absorption spectroscopy with 1X reveals fast charge transfer from S1 in both solvents with varying yields. Significant charge transfer from the T1 state is only found for the nucleobases with the lowest oxidation potential: in H2O, charge transfer occurs with 3.2 × 10(9) s(-1) for 1A and 6.0 × 10(9) s(-1) for 1G. The reorganization energy remains nearly unchanged going from MeOH to the more polar H2O. The electronic coupling is rather low even for the stacked conformation with H(AB) = 3 meV and explains the moderate charge transfer rates. The solvent controls the conformational distribution and therefore gates the charge transfer due to differences in distance and stacking.

Cyclophane size drives the photochemical behaviour of benzophenone

A new series of all-heterahomocalixarene-type structures was prepared. The new hetera[2(n)]metacyclophanes obtained contain a benzophenone moiety as photoactive component. Although both forms possess identical building blocks and differ only in ring size, they are markedly different in photochemical reactivity. The cyclophane size is the driving force for the photochemical behaviour of the benzophenone.

Highly stereoselective synthesis of aristeromycin through dihydroxylation of 4-aryl-1-azido-2-cyclopentenes

Dihydroxylation of 4-aryl-1-azido-2-cyclopentenes 6, in which an aryl group is used as a synthetic equivalent of CH(2)OH, was studied to improve the low to moderate stereoselectivity previously reported for cyclopentenes 3 possessing CH(2)X and nitrogen atom-containing groups. 2-Furyl, Ph, and p-MeOC(6)H(4) groups were chosen as the aryl groups. Compounds 6a-c possessing such aryl groups were prepared by CuCN-catalyzed reaction between 2-cyclopentene-1,4-diol monoacetate 9 and the corresponding Grignard reagents followed by substitution of the hydroxyl group with (PhO)(2)P(=O)N(3). The desired diols 7a-c were obtained with higher selectivities of >7:1 when dihydroxylation of 6a-c was carried out at 0 degrees C with OsO(4) (catalyst) and NMO in a mixed solvent of MeCN, THF, t-BuOH, and H(2)O. Among them, the furyl compound recorded the highest selectivity of 14:1. The furyl and azido groups on diol 7a were converted into hydroymethyl and adeninyl groups, respectively, to produce acetonide 2, which upon hydrolysis affords aristeromycin 1.

Atropo-enantioselective total synthesis of knipholone and related antiplasmodial phenylanthraquinones

The "lactone concept" has been efficiently employed for the first atropo-enantioselective synthesis of knipholone and related natural phenylanthraquinones. Besides the regio- and stereoselective construction of the biaryl axis, another important step was the "synthetically late" introduction of the C-acetyl group, either by a Friedel-Crafts type acetylation or by an ortho-selective Fries rearrangement first tested on simplified model systems and subsequently applied to the highly atroposelective preparation of the natural products and of simplified analogs thereof for biotesting. The synthetic availability of these natural biaryls, their precursors, and their unnatural analogs permitted a broader investigation of the antiplasmodial activities of these interesting biaryls.