Ozonides determination

Gas electron diffraction (GED) acyl peroxides, 702 peroxynitrates, 743-4 see also Electron diffraction Gas-hqiud chromatography (GLC) hydroperoxide determination, 684-5 ozonide determination, 719 Gas-phase addition, hydroperoxides, 157 Gas-phase epoxidation, alkenes, 58 gauche arrangement, acyclic organic peroxides, 102, 104-5 GED (gas electron diffraction), 743-4 Geminal regioselectivity... 

Ozone adds readily to unsaturated organie eompounds and ean eause unwanted eross-linking in rubbers and other polymers with residual unsaturation, thereby leading to brittleness and fraeture. Addition to alkenes yields ozonides whieh ean be reduetively eleaved by Zn/H20 (or I /MeOH, ete.) to yield aldehydes or ketones. This smooth reaetion, diseovered by C. D. Harries in 1903, has long been used to determine the position of double bonds in organie moleeules, e.g. ... 

This discussion of the structures of diene polymers would be incomplete without reference to the important contributions which have accrued from applications of the ozone degradation method. An important feature of the structure which lies beyond the province of spectral measurements, namely, the orientation of successive units in the chain, is amenable to elucidation by identification of the products of ozone cleavage. The early experiments of Harries on the determination of the structures of natural rubber, gutta-percha, and synthetic diene polymers through the use of this method are classics in polymer structure determination. On hydrolysis of the ozonide of natural rubber, perferably in the presence of hydrogen peroxide, carbon atoms which were doubly bonded prior to formation of the ozonide... 

The hydroperoxide (POOH) concentration was determined iodometri-cally after decomposition of the ozonides with excess of alcoholic sodium hydroxide. 

The use of both ozonation and ozonolysis is reviewed32. Ozonation leads to ozonide and ozonolysis leads to oxidized fragments, showing the use of both oxidative (AgN03) or reductive [(CH3)2S or PI13P] methods to produce the FAME (fatty acids methyl esters) that by subsequent GC analysis enabled determination of the position of the double bonds in the original molecule (equations 2-4). 

An inner-sphere electron reduction has been proposed as a possible mechanism for the Fe(II)-induced decomposition of 1,2,4-trioxolanes (ozonides) (75) and (76). Benzoic acid was found to be the major product. The nucleophilic Ee(II) species attack the ozonide from the less hindered side of the electrophilic 0-0 a orbital to generate exclusively the Ee(III) oxy-complexed radical (inner-sphere electron transfer). After selective scission of the C-C bond, the resulting carbon-centred radical produced the observed product. The substituent effect determine the regioselective generation of one of the two possible Fe(III)-complexed oxy radicals. The bond scission shown will occur if R is bulkier than R. ... 

O NMR resonances for several 1,2,4-trioxolanes have been reported <91CC816> (Table 6). The ether and peroxide signals are very distinct proving the value of O NMR as an analytical tool for characterization of ozonides. It can be used to determine unequivocally whether a compound is an ozonide (peroxide 6 295-327 ppm) or tetroxane ((26), for example d = 256 ppm). This is often a competing product from the ozonolysis reaction of alkenes (Section 4.16.8.2). 

Several 1,2,4-trioxolanes are known which contain a double bond linked in some way to the ring, with interest focusing on furan endoperoxides. The alkene moiety of 2,5-dimethylfuran endoperoxide (41) can be selectively functionalized in the presence of the 1,2,4-trioxolane ring. Reaction with diimide gives the saturated ozonide <81TL3509> and a single addition product was obtained with p-nitrophenylazide, although the stereochemistry of the reaction was not determined. 

The ozonide (176) was prepared as part of a synthetic study towards the antimalarial natural product artemisinin (177) <92JCS(Pl)325l>. It proved stable enough to allow x-ray crystal structure determination (see Section 4.16.3.1). Other, more complex, polycyclic ozonides were prepared in order to investigate possible antimalarial properties. Compounds were evaluated in vitro for antimalarial activity using a multiresistant strain of Plasmodium falciparum. Most were found to be weakly active although a thousand times less active than artemisinin. 

The second ozonide is that from fraw -stilbene, i.e. the 3,5-diphenyl trioxolane derivative. The ozonation experiment was done on both the cis and trans olefins resulting in reaction enthalpies of —471.5 and —428.9 kJmoU, respectively. The difference between these two values is ca 43 kJ moU, which is close to the difference between the two olefins enthalpies of formation (one as liquid and the other as solid), 46 2 kJ moU. This result vindicates the original authors suggestion of using the ozonation enthalpies as a method of determining the difference between enthalpies of formation of a Z/ pair of olefins. 

Acid-catalyzed condensation of bicyclic ozonides with aldehydes and ketones, in the presence of hydrogen peroxide, leads to the formation of bicyclic peroxide analogs of acetals in low yields, as shown in equation 91 for the condensation of the ozonide of 1-phenylcyclopentene (266) with benzaldehyde. The structure of compound 267, with the preferred ring conformation as shown, was determined by XRD analysis . The same method served to demonstrate that the condensation compound 16c is unique, with structure 254 . 

The ozonides of choline and ethanolamine phosphatides subjected to reduction with PhsP yield the corresponding core aldehydes. After hydrolysis with phospholipase C to eliminate the polar group and silylation with trimethylsilyl chloride, the core aldehydes can be determined by GLC-FID using temperature programming to high temperatures . ... 

The ozonide of 2,3-diphenyl-2-bomene can be prepared by photosensitized addition of oxygen to the corresponding epoxide (313), as shown in equation 103. The structure of product 314 was determined by single-crystal XRD crystallographic analysis. It should be pointed out that both 302 and 314 have Oe envelope conformation for the 1,2,4-trioxolane... 

This is an additive constitutive property that is easy to determine experimentally and has theoretical significance, but it has been superceded by spectral methods that offer a deeper insight into the molecular structure. Nevertheless, it has historical value in the early elucidation of the structure of secondary ozonides as substituted 1,2,4-trioxolanes . ... 

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