Azomethines, oxidation reduction

Attaching a Ceo cluster to an [Ru(bpy)3] + core has been achieved by 1,3-dipolar cycloaddition of azomethine ylides to the fullerene. The electrochemistry of the complex is complicated a one-electron reversible oxidation of the Ru center, five one-electron reversible reductions associated with the Ceo cage, and five more reversible reductions centered on the bpy ligands. The photophysical properties of the complex have been discussed. ... 

This chapter deals mainly with the 1,3-dipolar cycloaddition reactions of three 1,3-dipoles azomethine ylides, nitrile oxides, and nitrones. These three have been relatively well investigated, and examples of external reagent-mediated stereocontrolled cycloadditions of other 1,3-dipoles are quite limited. Both nitrile oxides and nitrones are 1,3-dipoles whose cycloaddition reactions with alkene dipolarophiles produce 2-isoxazolines and isoxazolidines, their dihydro derivatives. These two heterocycles have long been used as intermediates in a variety of synthetic applications because their rich functionality. When subjected to reductive cleavage of the N—O bonds of these heterocycles, for example, important building blocks such as p-hydroxy ketones (aldols), a,p-unsaturated ketones, y-amino alcohols, and so on are produced (7-12). Stereocontrolled and/or enantiocontrolled cycloadditions of nitrones are the most widely developed (6,13). Examples of enantioselective Lewis acid catalyzed 1,3-dipolar cycloadditions are summarized by J0rgensen in Chapter 12 of this book, and will not be discussed further here. 

For intramolecular 1,3-dipolar cycloadditions, the application of nitrones and nitrile oxides is by far most common. However, in increasing frequency, cases intramolecular reactions of azomethine ylides (76,77,242-246) and azides (247-259) are being reported. The previously described intermolecular approach developed by Harwood and co-workers (76,77) has been extended to also include intramolecular reactions. The reaction of the chiral template 147 with the alkenyl aldehyde 148 led to the formation of the azomethine ylide 149, which underwent an intramolecular 1,3-dipolar cycloaddition to furnish 150 (Scheme 12.49). The reaction was found to proceed with high diastereoselectivity, as only one diaster-eomer of 150 was formed. By a reduction of 150, the proline derivative 151 was obtained. 

These experiments clearly demonstrated that two electrons are involved in each of the two polarographic steps, which correspond to the reduction of the amine oxide and the 4,5-azomethine functional group. 

The oxidation of phenylhydrazine and 1,2-disubstituted hydrazines to hydrazones and diazenes by CI2C proceeds via formation of unstable azomethine imines.95 The conversion of alcohols into alkyl halides is achieved by reaction with CCI4 (or CBr4) in DMF under electrochemical reduction.96 The reaction of dihalocarbene X2C with DMF to form a Vilsmaier reagent (93) is proposed as the key process. The reaction of simple isocyanates (RNCO) with dimethoxycarbene normally gives hydantoin-type products. In the reaction with vinyhsocyanates such as (94), however, hydroindoles (95) are formed in good yields.97... 

The chemistry involved in the formation of these azomethine dyes has been extensively investigated by Vittum and others.244,1538,1539a The present interpretation of the reactions involved is that the primary amino group of the developer is oxidized to give a quinonediimine cation which then reacts with the anion of the coupler to form a leuco dye. Subsequent oxidation converts the leuco dye to a colored form (LIII). This may be done by silver ion or by the oxidized form of the developer. The over-all reaction requires the reduction of four silver ions. 

Minor pathways arise from hydrolytic or oxidative cleavage of the azomethine moiety and reduction to dihydro-CR (French et al., 1983b). No samples from human exposures have been reported. 

Oxidation with Sarett s reagent gave two products (1) the azomethine CCXCVIII which formed the ethiodide CCXCIX, convertible to chasmanine by reduction with sodium borohydride, and (2) the neutral V-acetyl-A-desethyl-14-dehydrochasmanine (CCC). The stereospecific course of the reduction of the five-membered ketone is indicative of the ketone carbonyl being at C-14 rather than C-12 and is consistent with the stereochemistry of these skeletons which have free access to the C-12 position, whereas the C-14 position is hindered by ring D, leaving only one side open for attack. 

Definitive evidence on this point has been provided by Dvomik and Edwards (17), who have shown that hydration of the C2o-azomethine alcohol XXXIII derived from atisine (18, 19) gave the diol XXXIV, which on reduction, acetylation, selective hydrolysis, and oxidation gave a 7,8-secomethylketo acid (XXXV). Treatment of the latter with tri-fluoroacetyl peroxide followed by hydrolysis and dichromate oxidation gave the bis norketo acid XXXVI (1712, 1602 cm i). Dibromination of... 

The atisine and Garrya (35,36) alkaloids have been interrelated by converting both atisine (XXX) and veatchine, by a parallel sequence of degradations, to the same A-acetyl ester (XCV) (35, 36). The respective azomethine acetates (LXXXVII, LXXXVIII) derived from atisine (12,16-bond) and veatchine (13,16-bond) were converted to the A-acetyl derivatives (LXXXIX, XC) by reduction, acetylation, and saponification. Oxidation of LXXXIX and CX with permanganate/periodate under controlled conditions gave the respective carboxylic acids (XCIa,... 

Ozonolysis of solasodine (1) and of the derived enone (8), with oxidative work-up, gave the keto-acids (18) and (19), respectively. These products were characterized as their methyl esters and as the acetates of the products of lithium aluminium hydride reduction of the methyl esters. Ozonolyses of (20) and (21) to give analogous products were also described. The aglycone (22) of the steroidal glycoside osladine has been synthesized from solasodine via the azomethine (23) and ketone (24). ... 

Further examples of the enhancement of those facets of pyridine chemistry associated with the azomethine electron withdrawal include a general stability towards oxidative degradation but, on the other hand, a tendency to undergo rather easy reduction of the ring. 

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