Oxidation reactions synthetic utility

The reaction, which is known as the Baeyer-Villiger oxidation, has synthetic utility, particularly for the oxidation of ketones to esters because ketones... 

On the whole, the cycloaddition of alkynes to nitrile N-oxides is one of the most important routes to isoxazoles, but in spite of its potentially wide application, its synthetic utility is less than that of the corresponding reaction with alkenes for the following reasons. (1)... 

Another type of (CNO-fC + C) reaction has been reported (77JCS(P1)1196). The reaction of dimethyloxosulphonium 4-nitrobenzylide (382) with benzonitrile iV-oxide gave 4,5-dihydro-4,5-bis(4-nitrophenyl)-3-phenylisoxazole (383) in 31% yield. So far, no synthetic utility for this reaction process has been reported. 

While the oxidation of ketones by peracids (Baeyer-Villiger reaction) has been used in steroids mainly for ring cleavage, it has occasionally been applied to 20-ketopregnanes for conversion to 17-acetoxy- or hydroxyandros-tanes. The synthetic utility of this method is limited since reactive double bonds and other ketones are incompatible with the reagent. 

A few studies are reported which describe the direct oxidation of sulphoxides to sulphonic acids, sulphonyl halides, thiosulphonates and sulphate. These reactions will be considered in this section but it should be noted that they are rarely of synthetic utility. 

The focus is on the primary formation of bonds, not on subsequent reactions of the products to form other bonds. These latter reactions are covered at the places where the formation of those bonds is described. Reactions in which atoms merely change their oxidation states are not included, nor are reactions in which the same pairs of elements come together again in the product (for example, in metatheses or redistributions). Physical and spectroscopic properties or structural details of the products are not covered by the reaction volumes which are concerned with synthetic utility based on yield, economy of ingredients, purity of product, specificity, etc. The preparation of short-lived transient species is not described. 

While the above examples demonstrate that product control to a significant extent is possible in oxythallation by careful choice of substrate or reaction conditions, the synthetic utility of oxythallation has been illustrated most convincingly by the results obtained with highly ionic thallium(III) salts, especially the nitrate (hereafter abbreviated TTN). Unlike the sulfate, perchlorate, or fluoroborate salts (165), TTN can easily be obtained as the stable, crystalline trihydrate which is soluble in alcohols, carboxylic acids, ethers such as dimethoxyethane (glyme), and dilute mineral acids. Oxidations by TTN can therefore be carried out under a wide variety of experimental conditions. 

The reactions of TTN with a variety of unsaturated systems have been studied systematically during the last two years, and the results obtained clearly establish the synthetic utility of the reagent as a specific oxidant. Attempts were made in 1966 by Uemura et al. 162) to oxidize a,)8-unsatur-ated carbonyl compounds with thallium(III) acetate, but were unsuccessful. In 1970, however, Ollis and his co-workers 121-123) reported that prolonged treatment of highly activated chalcones (Scheme 20) with thal-... 

The reaction that is perhaps of the greatest synthetic utility—because it proceeds at relatively low temperatures—is the Cope reaction of tertiary amine oxides, e.g. (82) ... 

Carbanions can take part in most of the main reaction types, e.g. addition, elimination, displacement, rearrangement, etc. They are also involved in reactions, such as oxidation, that do not fit entirely satisfactorily into this classification, and as specific—ad hoc—intermediates in a number of other processes as well. A selection of the reactions in which they participate will now be considered many are of particular synthetic utility, because they result in the formation of carbon-carbon bonds. 

The oxidation of aromatic aldoximes with ceric ammonium nitrate produces nitrile oxides which undergo subsequent cycloaddition to nitriles to produce 1,2,4-oxadiazoles (Equation 47) <1997PJC1093>. The anodic oxidation of aromatic aldoximes in the presence of acetonitrile has been reported to give low yields of either 3-aryl-5-methyl-1,2,4-oxadiazoles (2-25%) or 3,5-bis-aryl-l,2,4-oxadiazoles (6-28%), although the synthetic utility of this route is limited by competitive deoximation to the carbonyl being the major reaction pathway <1997MI3509>. 

When furan or substituted furans were subjected to the classic oxidative coupling conditions [Pd(OAc)2 in refluxing HOAc], 2,2 -bifuran was the major product, whereas 2,3 -bifuran was a minor product [12,13]. Similar results were observed for the arylation of furans using Pd(OAc)2 [14]. The oxidative couplings of furan or benzo[i]furan with olefins also suffered from inefficiency [15]. These reactions consume at least one equivalent of palladium acetate, and therefore have limited synthetic utility. 

This protocol is also effective for the cyclization of an allenylaldehyde, the synthetic utility of which has been demonstrated in the synthesis of (+)-testudinariol A (Scheme 16.89) [97]. Cyclization of an allenylaldehyde provides a ris-cyclopentanol bearing a 2-propenyl group at the C2 position. The reaction mechanism may be accounted for by coordination of Ni(0) with both the aldehyde and the proximal alle-nyl double bond in an eclipsed fashion with a pseudo-equatorial orientation of the side chain, oxidative cyclization to a metallacycle, followed by Me2Zn transmetalla-tion and reductive elimination. 

In addition to elimination reactions, the methoxylated amide products from electrolysis reactions have been treated with a variety of nucleophiles [47]. In recent studies, these efforts have been utilized to expand the chiral pool of starting materials available to synthetic chemists. For example, consider the reactions illustrated in Scheme 23 [52, 53]. In these efforts, Steckhan and coworkers have used the oxidation reaction to make a stable -a-methoxy amide that... 

Several reactions of imines of synthetic utility are reported. Nitric oxide reacts with A-benzylidene-4-methoxyaniline (18) in ether to give 4-methoxybenzenediazonium nitrate (19) and benzaldehyde. Two mechanisms are proposed, both involving nitrosodiazene (20), and the preferred route is suggested to involve direct electrophilic reaction of NO to the imine double bond, favoured by the polarity of the latter. 

The synthetic utility of the reaction is demonstrated by the oxidative hydrolysis of the products, giving silyloxy amides. 

In conjunction with the chiral anion TRIP (156) (10 mol%), diamine 157 (10 mol%) can be used in the catalytic asymmetric epoxidation of a,p-unsaturated ketones (>90% ee) [196], while the secondary amine 158 (10 mol%) can be used for the epoxidation of both di- and trisubstituted a,P-unsaturated aldehydes (92-98% ee) (Fig. 15) [211], The facile nature of these reactions, using commercially available peroxides as the stoichiometric oxidant, together with the synthetic utility of the epoxide products suggests application in target oriented synthesis. 

In this review, we focus mainly on the preparative utility of organic peroxides, and only few mechanistic investigations are discussed. This review covers synthetic methodologies for the preparation of alkyl hydroperoxides and dialkyl peroxides (Section II) and the synthetic use of these peroxides in organic chemistry (Section III). In Section II, general methods for the synthesis of organic hydroperoxides and dialkyl peroxides are discussed, as well as the preparation of enantiomerically pure chiral hydroperoxides. The latter have attracted considerable interest for asymmetric oxidation reactions during the last years. 

Because of the great synthetic utility, asymmetric versions of the epoxidation of allylic alcohols have been developed and will be discussed in the following. Two methods of asymmetric conduction of the reaction are known. The first one is the employment of chiral catalysts and the second possibility is the use of chiral oxidants, which will be presented separately. 

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