Ozonolysis pathways with

Scheme 6.11. A proposed pathway (after Criegee) for the ozonolysis (reaction with ozone followed by reductive hydrolysis) of alkenes. so that two equivalents of aldehyde, or two equivalents of ketone, or one equivalent of each, depending upon the substitution pattern of the alkene, results. (See Criegee, R. Angew. Chem. Int. Edit, 1975,87,745.)...

Versatile [3 + 2]-cydoaddition pathways to five-membered carbocydes involve the trimethylenemethane (= 2-methylene-propanediyl) synthon (B.M. Trost, 1986). Palladium(0)-induced 1,3-elimination at suitable reagents generates a reactive n -2-methylene-l,3-propa-nediyl complex which reacts highly diastereoselectively with electron-deficient olefins. The resulting methylenecyclopentanes are easily modified, e. g., by ozonolysis, hydroboration etc., and thus a large variety of interesting cyclopcntane derivatives is accessible. 

Conventional synthetic schemes to produce 1,6-disubstituted products, e.g. reaction of a - with d -synthons, are largely unsuccessful. An exception is the following reaction, which provides a useful alternative when Michael type additions fail, e. g., at angular or other tertiary carbon atoms. In such cases the addition of allylsilanes catalyzed by titanium tetrachloride, the Sakurai reaction, is most appropriate (A. Hosomi, 1977). Isomerization of the double bond with bis(benzonitrile-N)dichloropalladium gives the y-double bond in excellent yield. Subsequent ozonolysis provides a pathway to 1,4-dicarbonyl compounds. Thus 1,6-, 1,5- and 1,4-difunctional compounds are accessible by this reaction. 

Carbon monoxide has been used to scavenge OH fonned from the ozonolysis of alkenes. The CO2 tints generated was detected by FTIR spectroscopy and the "OH yields for individual reactions were calculated.239 The significance of the OH-induced intramolecular transformation of glutathione thiyl radicals to a-aminoalkyl radicals has been discussed with respect to its biological implications.240 The kinetics and mechanism of the process indicated that it could be a significant pathway for the selfremoval of glutathione thiyl radicals in vivo. 

The authors [23] applied the method of density fimctional (exchange-correlation functional B3LYP) and calculated the profdes of the potential energy for the interaction of ozone with ethylene. It was shown that, according to the scheme of ozonolysis of ethylene, the reaction pathway through the direct epoxidation of the double C=C bond requires high energy of activation and is improbable, both for thermochemical and photochemical reactions of ozonolysis. 

The same authors published a detailed report on the calculations of the oxygen insertion into unactivated C-H bonds by dioxiranes using DFT theory and on comparison of the transition structures for stepwise routes via radical pairs with the concerted pathway <2003JOC811>. The articles dealing with the mechanism of OH formation from ozonolysis of isoprene and a- and /3-pinene provide DFT and ab initio calculation results also for the dioxirane formation <2001CPL(358)171, 2002JA2692, 2005JCP114308>. 

Although a-dicarbonyl compounds are not known to be products of the ozonolysis of olefins, biacetyl has been isolated in photochemically initiated reactions 14, 15) which result in the net oxidation of olefins in the gas phase. For example, when a mixture of ci5-2-butene, nitric oxide, and air is irradiated, small amounts of biacetyl are isolated. One of the pathways suggested to explain the production of biacetyl involves the reaction of ozone with ci5-2-butene (14) ... 

Our first synthesis of brassinolide started from stigmasterol which was converted to (20S)-6p-methoxy-3a,5-cyclo-5a-pregnane-20-carboxaldehyde (2). This pathway involved formation of the mesylate with methanesulfonyl chloride in pyridine and tetrahydrofuran, treatment of the mesylate with potassium acetate in methanol, and ozonolysis of die i-sterol with reductive work-up. 

Costa et al. developed an efficient pathway utilizing a chiral auxiliary to enter the monobutyrolactone skeleton, Scheme (9) [56]. The carboxylic acid 48 was transformed into the corresponding acyl chloride and then esterified using a homochiral enantiomeric alcohol 49. The resulting ester 50 was deprotonated with LDA and alkylated with piperonyl iodide providing the alkylated ester 51 in 62% yield and high diastereomeric excess (94% de). Ester 51 was reduced to the homoallylic alcohol 52 and finally transformed into the desired lactone 29 either by ozonolysis of the... 

Another pathway in the formation of HO2 and probably direct H2O2 is via the ozonolysis of alkenes, which has been investigated for many years. However, several open questions remain about the initial and product relationship. It is well known that the excited Criegee radical can produce OH and HO2 radicals (Neeb et al. 1998). The stabilized Criegee radical can react with water vapor to form a-hydroxy-alkyl peroxides (RR C(OH)OOH) Gab et al. 1985, Becker et al. 1993). Recently, it... 

This summary will indicate that, even for the simplest of the molecules, isoprene, there remain uncertainties about the degradation pathways. One problem concerns poor carbon balance in almost all of the studies of attack by OH, NO3 and O3 Another concerns the effect of humidity on product distributions. Yet a further question hangs over the significance of the ozonolysis reactions as a source of OH radicals. Almost nothing is known about the mechanisms and specific pathways of reactions of the terpenes, and there are substantial experimental obstacles to investigation of these systems. Much further work is clearly warranted, in order to determine whether ozone is only lost in its reaction with biogenic VOCs or whether the reactions might constitute a source of atmospheric ozone when NOx is present. 

The rate coefficients of limonene ozonolysis, evaluated at the BH and HLYP/cc-pvdz level, and the theoretical Arrhenius expression were in reasonable agreement with the experimental values and correctly described the initial steps of ozonolysis. The ozonide formation was highly exothermic, and the decomposition channels had similar free energies of activation, AG, indicating that there was no primary pathway for ozonide decomposition. ... 

If we assume that the initiated oxidation and the ozonolysis differ only in the pathway of R02-radicals generation, then the meehanism of destmc-tion and the kinetic relationships should be similar. The experimentally found values of the ratio WAA j accounting for the occurrence of radical steps in the ozonolysis are 0.05 at 0°C, 0.2-25°C and 0.69 - 60°C (Fig. 38, 1-3). The activation energy of degradation calculated on the basis of Fig. 38 amoimts to 7.6 kcal/mol and correlates well with the activation energy of RO-radical izomerisation [120]. The ratio W./W could be estimated using the values of kj and k and the quasistationary conditions in relation to P, ... 

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