Dimerization reductions

The Mo(V) dimer reduction of iodine has been studied in the pH range 1.65 to 7.20 in phosphate buffer in the presence of excess iodide. The stoichiometry is simple, viz. 

If, however, compound 390 is treated with a stronger acid (TFA), it undergoes diastereoselective dimerization reduction of the initial reaction product gives 391 (Scheme 113). It is suggested that in vivo dimerization of myrmicarin 215B may provide the key to the biosynthesis of the more complex myrmicarins such as M430 <2006T5287>. 

Photochemical reactions, like any chemical reaction, can be classified into various groups, depending on the reactants and products, for example, elimination, isomerization, dimerization, reduction, oxidation, or chain reaction. One important practical field of photochemistry is organic photochemistry. In solution photochemical reactions, the nature of the solvent can markedly influence the reaction. The absorbtion of the solvent and of the reaction products is an important parameter for the choice of the reaction conditions. It is useful to have a solvent with a relatively low absorption in the desired wavelength. Sometimes photosensitizers are used these are substances that absorb light to further activate another substance, which decomposes. 

Aryltriazenes can also be decomposed by hydrogen fluoride in organic solution after extraction from their aqueous mother phase. In this case, hydrogen fluoride can be used in small excess but the nature of the solvent is crucial for example, tetrahydrofuran gives complex mixtures, dichloromethane promotes radical reactions (dimerizations, reductions) and acetic acid favors triazene decomposition before fluorination. Aromatic and haloaromatic compounds seem to be the best solvents.283 Such a technique, especially suited for the rapid introduction of an 18F atom, has been employed to produce [ 8F]haloperidol (3), the specific receptors of which have been localized in the brain by positron emission transaxial tomography.298... 

Cyanoquinoline is reduced polarographically in ethanol in two one-electron waves the product from the first reduction is the 4,4 -dimer reduction at the plateau of the second wave produced 3-cyano-l,4-... 

Summarizing all these results, it can be said that three different kinds of products can be obtained due to reaction conditions113. The alternatives are (a) pure C-silylation 113 under reduction (b) dimerizing reduction ( duplication reductrice 113 ) (c) pure reduction ( simple reduction 113 ). 

Photochemistry of Pyrimidone-2 Dimer Reduction Product 141 III. 1.2 Tetrameric Photoproduct of Thymine and Pyrimidone-2. 141... 

In an aqueous buffered medium, over the pH range 1-12, pyrimidone-2 exhibits a single one-electron wave. Preparative electrolysis, at a potential corresponding to the initial limiting current, led to formation of an insoluble product, isolated as a white amorphous powder, and shown by various physico-chemical criteria to correspond to a dimer consisting of two molecules of reduced pyrimidone-2. This was further confirmed by H NMR spectroscopy, which also established the structure of the product as 6,6 (or 4,4 )-bis-(3,6(4)-dihydropyrimidone-2), shown in Scheme 2, below. The structure of the dimer reduction product, and its solid state conformation, were subsequently further established by X-ray diffraction (see Sect. III.3.). 

In aqueous 0.1 M (CH3)4NBr, pyrimidone-2 was found to exhibit two reduction waves of equal height, with E1/2 values of —0.75 V and —1.55 V for waves I and II, respectively 1,2). (Fig. 1) Preparative electrolysis under these conditions at the potential of wave I resulted in formation of the same dimer reduction product as in aqueous buffered medium. By contrast, electrolysis on wave II led to formation of two products, one of which was identical with that formed on wave I. The other, readily soluble in aqueous medium, was identified as 3,6-dihydropyrimidone-2, identical with that synthesized chemically and described earlier by Skaric75). 

The dimer reduction product of wave I, in turn, exhibited a polarographic anodic wave with E1/2 = —0.27 V in neutral aqueous medium. Anodic electrolysis at the crest of this wave led to regeneration of the parent pyrimidone-22). 

The UV absorption spectra, in neutral aqueous medium, of pyrimidone-2 and its dimer reduction product, 6,6 -6u-(3,4-dihydropyrimidone-2), are exhibited in Fig. 2 Particularly interesting was the finding that irradiation of the dimer at 254 nm under these conditions led to the stepwise disappearance of its characteristic absorption spectrum, with the simultaneous appearance o the spectrum of the parent monomer. Additional evidence for the identity of the photoproduct with the parent pyrimidone-2 was furnished by chromatography and polarographic behaviour. The photochemical conversion reaction was shown to be quantitative (under these conditions pyrimidone-2 itself is quite radiation resistant), and to proceed with a quantum yield of 0.1, both in the presence and absence of oxygen 2). This value was unchanged when irradiation was conducted in 2H20 the absence of an isotope effect is clearly of relevance to the mechanism of the photodissociation reaction. 

Of additional, and particular, interest is the finding that the dimer reduction product of pyrimidone-2 is an integral component of a tetrameric photoproduct resulting from the photodimerization of a photoadduct of thymine and pyrimidone-2 77), as shown in Scheme 3. This tetrameric photoproduct has, furthermore, been isolated from an irradiated frozen aqueous solution of cytosine and thymine 8), and, more... 

Macroelectrolysis on wave I yielded as product the dimer Id (see Scheme 6)80), apparently identical with the photoproduct resulting from UV-irradiation of 4,6-dimethylpyrimidone-2 in isopropanol at 254 nm81). The dimer reduction product... 

This value is to be compared with that of 0.1 for photochemical regeneration of pyrimidone-2 from its dimer reduction product (Sect. III.l.l). 

The polarographic behaviour of 1-methylcytosine and 1, N4-dimethylcytosine at pH 3-7 was, as expected, similar to that for cytosine. Reduction led to formation, from each compound, of a dimer of l-methylpyrimidone-2, accompanied by quantitative elimination of ammonia and dimethylamine, respectively. The dimer reduction product in each case underwent photochemical conversion to 1-methylpyrimidone-2 84). 

The postulated dimer reduction product is very unstable, even in the absence of oxygen. Attempts to identify it directly, and to examine its potential susceptibility to photooxidation, have been unsuccessful. 

The foregoing suggests that the structure of the reduction product is a dimer, as shown in Scheme 22, and supported by comparison with the dimer reduction product of pyrimidone-2 (Scheme 2), since the reduced pyrimidine rings in both dimers should be similar. The foregoing interpretation is confirmed by the results of earlier studies on 1 1 photoadducts of alcohols to purine 156157> the products were identified as substituted 1,6-dihydropurines, and the site of addition of the alcohols was readily established as C(6) on the basis of the 1H NMR spectra prior to, and following, selective deuteration at C(6) or C(8) 156,157). 

In neutral aqueous medium the dimer reduction product, but not the product of reduction on wave II, underwent photodissociation to the parent 2-oxopurine, with a quantum yield at 254 nm of 0.03, as compared to 0.1 for photodissociation of pyrimidine-2. 

The consequent similarities in polarographic reduction of 2-thiopurine and 2-oxo-purine are analogous to the similarities in photochemical reduction of pyrimidone-2 and pyrimidone-2-thione, and of photodissociation of their dimer reduction products. The dimer reduction product of 2-thiopurine, obtained by electrolysis at pH 5, undergoes photodissociation at 254 nm to the parent monomer with a quantum yield comparable to that for the dimer reduction product of 4,6-dimethylpyrimidine-2-thione (Scheme 26). 

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