Ketones formation from ozonides

Due to the retractive forces in stretched mbber, the aldehyde and zwitterion fragments are separated at the molecular-relaxation rate. Therefore, the ozonides and peroxides form at sites remote from the initial cleavage, and underlying mbber chains are exposed to ozone. These unstable ozonides and polymeric peroxides cleave to a variety of oxygenated products, such as acids, esters, ketones, and aldehydes, and also expose new mbber chains to the effects of ozone. The net result is that when mbber chains are cleaved, they retract in the direction of the stress and expose underlying unsaturation. Continuation of this process results in the formation of the characteristic ozone cracks. It should be noted that in the case of butadiene mbbers a small amount of cross-linking occurs during ozonation. This is considered to be due to the reaction between the biradical of the carbonyl oxide and the double bonds of the butadiene mbber [47]. 

An alternative procedure for decomposing ozonides from di- or trisubstituted alkenes is to treat them with methanol (CH3OH). The use of this reagent results in the formation of an aldehyde or ketone and a carboxylic acid ... 

Another source of potential antimalarial compounds has been the isolation of stable ozonides 67, produced via the ozonolytic cleavage of the bicyclic ketones 68 (Scheme 11.19).66 Variation of the groups R within 68 allows for formation of a wide range of stable ozonides 67 as exemplified by R = n-heptyl that has a melting point of 75°C to 76°C. The ozonolysis can be carried out in dichloromethane at -5°C to 0°C, and the ozonides 67 usually crystallize directly from the reaction mixture. 

These reactions proceed by initial ozone attack on the C = C bond of the olefin. An intermediate ozonide is formed, which rapidly decomposes to a carbonyl and a biradical. The biradical can be stabilized, or it can decompose. Paulson et al. (1991b) found the products methacrolein, methyl vinyl ketone, and propene, in yields of 68%, 25%, and 7%, respectively. Based on the presence of epoxides in the ozone/isoprene system, Paulson et al. concluded that 0(3P) was being formed. Calculations indicated that 0.45 0(3P) radicals were formed for every ozone/isoprene reaction. However, Atkinson et al. (1993) recently showed that the epoxides were formed directly from the reaction with ozone rather than the reaction with 0(3P). The epoxides formed were l,2-epoxy-2-methy 1-3-butene and l,2-epoxy-3-methyl-3-butene, in yields of 0.028 and 0.011, respectively. There was also definite evidence for the formation of OH radicals in the ozone system, thus causing difficulties in product analyses. Each ozone/isoprene reaction yielded 0.68 OH radicals (Paulson et al., 1991b). 

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