Cleavage reactions ozonolysis

Alicyclic hydroxamic acids undergo several specific oxidative cleavage reactions which may be of diagnostic or preparative value. In the pyrrolidine series compounds of type 66 have been oxidized with sodium hypobromite or with periodates to give y-nitroso acids (113). Ozonolysis gives the corresponding y-nitro acids. The related cyclic aldonitrone.s are also oxidized by periodate to nitroso acids, presumably via the hydroxamic acids.This periodate fission was used in the complex degradation of J -nitrones derived from aconitine. 

SAMP/RAMP-Hydrazones are cleaved oxidatively by ozonolysis in dichloromethane at — 78 °C3. This method proceeds within 15 to 30 minutes/10 mmol in quantitative yield without racemization. The endpoint of the cleavage reaction is indicated by the green color of the yellow nitrosamine with blue ozone. Besides the desired carbonyl compound, one quivalent of (S)- or (/J)-2-methoxymethyl-l-nitrosopyrrolidine is formed. 

For the cleavage reaction two methods have been described ozonolysis at —78 °C, which can be used to recycle the chiral information (137), or acid hydrolysis in a two-phase system. No racemization of the product ketone was observed under these conditions. 

Intramolecular Dieckmann cyclization of polystyrene-bound pimelates has been used to prepare (l-keto esters (Entry 4, Table 3.41). Oxidative cleavage reactions leading to the formation of aldehydes include the ozonolysis of resin-bound alkenes, the periodate-mediated cleavage of 1,2-diols, and the oxidation of Wang resin derived ethers (Entries 5-7, Table 3.41). 

Although the distribution among these products varies, depending on the reaction conditions, particularly on the solvent used, it is evident from the data in Table II that the latter type of products in which the double bond has been immunized towards further ozone attack comprises in each case a significant part (between 22 and 30% ) of the total product mixture. This is perhaps the most remarkable result of the present investigation. In contrast to the ozonolysis of hydrocarbon olefins, the ozonolysis of this dibromosubstituted double bond cannot be viewed primarily as a double bond cleavage reaction. 

The oxidative cleavage reaction of alkenes with O3, followed by Zn in acid, produces aldehyde and ketone functional groups at sites where double bonds used to be. On ozonolysis, these two dienes yield only aldehydes because all double bonds are monosubstituted. 

Heathcock has reported an anomalous case of ozonolysis of a silyl enol ether. Usually these substrates undergo facile oxidative cleavage in the same manner as alkoies. However, in this instance the a-silyloxy ketone (61) was obtained in quantitative yield. The inteimediacy of a silyloxy epoxide was suggested. A more recent leport has indicated that a similar process is competitive with the simple cleavage reaction, (63a) versus (63b), in the ozonolysis of the steroidal enol ether (62). 

The cleavage reaction, commonly referred to as ozonolysis , is carried out by bubbling ozonized oxygen through a solution of the a ene in various solvents, including methanol, dichloromethane, carbon tetrachloride and ethyl acetate. Other solvents (ethanol, tetrahydrofiiran, acetic acid, or a combination of ethyl acetate and hexane) have also been reported for use in individual reactions.The reaction is usually performed at low temperatures (ateut 0 °C), and, since ozonides are potentially explosive compounds, the intermediates are not isolated. 

Ozonolysis (Section 12.10) An oxidative cleavage reaction in which a multiple bond reacts with ozone (O3) as the oxidant. 

The functional group transform for ozonolysis of alkenes and all oxidative cleavage reactions of alkenes can be summarized by... 

Ozonolysis of alkenes (Section 6.20) This cleavage reaction is more often seen in structural analysis than in synthesis. The substitution pattern around a double bond is revealed by identifying the carbonyl-containing compounds that make up the product. Hydrolysis of the ozonide intermediate in the presence of zinc (reductive workup) permits aldehyde products to be isolated without further oxidation. 

If the focus is on the C=C unit of 2,3-dimethyl-2-butene, the dipolar addition reaction cleaves the first bond (the n-bond), and the rearrangement of the 1,2,3-trioxolane to the 1,2,4-trioxolane cleaves the second bond (the c-bond). The conversion of an alkene to an ozonide is known as ozonolysis, and it is an example of an oxidative cleavage reaction. The ozonide is usually not isolated, but a second chemical step is performed in the same flask. When treated with hydrogen peroxide, the products of this reaction are 2-propanone (acetone) and a second molecule of acetone. This statement is phrased this way because an unsymmetrical alkene will give two different ketones. In effect, the C=C unit is cleaved and each carbon is oxidized to a C=0 unit. The mechanism described for this reaction is consistent with known chemistry of ketones and aldehydes and other carbonyl-bearing functional groups. A full discussion of carbonyl chemistry will be presented in Chapters 17 and 19. 

The oxidation reactions of either permanganate or osmium tetroxide with an alkene lead to a vicinal diol. The functional group exchange for this process is that shown. In other words, a diol is obtained by dihydro lation of an alkene. Oxidative cleavage reactions such as ozonolysis eventually lead to an aldehyde, ketone, or carboxylic acid, depending on the substituents attached to the unit... 

If the size of the carbon skeleton decreases, then a C—C bond-breaking reaction, called bond cleavage, is required. Once again, we have only seen one such reaction. Ozonolysis of an alkene (or aUtyne) achieves bond cleavage at the location of the Tt bond ... 

Because the desired compound has fewer carbons than the starting material, we know that a cleavage reaction must occur. The desired aldehyde can be prepared by ozonolysis (an oxidative cleavage) of 3-hexene. 3-Hexene can be prepared from 3-hexyne, which can be prepared from 1-butyne and ethyl bromide. 

Alkenes with at least one vinjdic hydrogen undergo oxidative cleavage when treated with ozone, yielding aldehydes (Section 7.9). If the ozonolysis reaction is carried out on a cyclic alkene, a dicarbonyl compound results. 

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