Epoxides, from autoxidations

The stereoselective nature of the reaction supports the suggestion that epoxidation in this case does not occur by acylperoxy radicals but rather by peracids generated from autoxidation of aldehydes. 

Some doubt was cast oh these observations, however, when it was shown555 that vitamin A alcohol undergoes autoxidation in the dark" u give the epoxide of vitamin A alcohol. The ability of manganese iioxide to produce epoxides from olefins is therefore still open to... 

Thus, product 4 may have a dual origin, arising from autoxidation as well as from en2ymatic monooxygenation [16] However, autoxidation always yields a mixture of 3 and 4 [16]. Also 20-hydroxycholesterol can be formed both by autoxidation and by enzymatic means. 20(S)- and 22(R)-hydroxycholesterol are intermediates in the synthesis of steroid hormones [16]. Cholesterol 5a,6a-epoxide 6 is mostly associated with autoxidation, but has also been shown to be enzymatically formed in different systems. It is also formed by the action of agents such as hydroxyl radical or hydrogen peroxide on cholesterol [9]. 

The structure of the frequently occurring bicyclic sesquiterpene 6-caryophyllene was for many years a matter of doubt. After numerous investigations W. Treibs (1952) has been able to isolate the crystalline caryophyllene epoxide from the autoxidation products of clove oil and F. Sorm et al. (1950) suggested caryophyllene to have a 4- and 9-membered ring on bases of infrared (IR) investigations. This suggestion was later confirmed by the Fnglish chemist D. H. R. Barton (Barton and Lindsay, 1951), who was awarded the Nobel Prize in Chemistry in 1969. 

The primary aromatic substances in beer are derived from raw materials (barley or hops) that confer the beer s typical odour and taste. Bitter acids of hops have a bitter taste (see Section 8.3.5.1.3), but hop cones also contain 0.3-1% m/m of terpenoids (60-80% of hop essential oil), which have a considerable influence on the smell of beer. The main components of aromatic hop oils are sesquiterpenic hydrocarbons in which a-humulene, P-caryophyllene and famesene dominate. The major monoter-penic hydrocarbon is myrcene. For example, the essential oil content of fine aromatic varieties, such as Saaz, is 0.8% m/m, of which 23% is myrcene, 20.5% a-humulene, 14% famesene 6% and P-caryophyUene. Significant components of the hop aroma in beer are mainly isomeric terpenoid monoepoxides resulting from autoxidation and diepoxides of a-humulene and fS-caryophyUene, but also other terpenoids. Important components of hops odour are also various alcohols (such as geraniol and hnalool), esters (ethyl 2-methylpropanoate, methyl 2-methylbutanoate, propyl 2-methylbutanoate and esters of terpenic alcohols, such as geranyl isobutanoate), hydrocarbons, aldehydes and ketones formed by oxidation of fatty acids, such as (3E,5Z)-undeca-l,3,5-triene, (Z)-hex-3-enal, nonanal, (Z)-octa-l,5-dien-3-one, their epoxides, such as ( )-4,5-epoxydec-2-enal and sulfur compounds. Other important components of hops are so-called polyphenols (condensed tannins) that influence the beer s taste and have antioxidant effects. Less important compounds are waxes and other hpids. Hop products, such as powder, pellets and extracts (by extraction with carbon... 

There is ample evidence in the literature for conversion of reactive hydrocarbons to carbonyl compounds by autoxidation. In coals, the final products of autoxidation under the conditions used in the present study could be a mixture of carbonyl and carboxylic acid surface groups. Under mild oxidation conditions, a different set of functional groups such as ethers as proposed by Liotta et al. or epoxides as suggested in Scheme V could be formed. There are numerous examples of alkoxy radicals rearranging to epoxides . Choi and Stock have shown that ethers can be produced from benzhydrol structures, which are invoked as intermediates in Scheme IV. At higher temperatures, the epoxides and ethers are unstable and may rearrange to carbonyl compounds. 

Epoxides can also be formed from the oxidation of alkenes by molecular oxygen via in situ generation of hydroperoxides by autoxidation.251,252 An interesting example is the direct stereoselective oxidation of cyclohexene by 02 to syn-l,2-epoxycyclohexan-3-ol catalyzed by CpV(CO)4 with a 65% yield and 99% stereoselectivity (equation 78).253... 

This reaction affords much higher yields of epoxide than those obtained from the autoxidation of the olefin alone since acylperoxy radicals are more selective than alkylperoxy radicals in favoring addition relative to abstraction. 

We have demonstrated recently that epoxidation and hydroxyl-ation can be achieved with simple iron-porphine catalysts with iodosylbenzene as the oxidant (24). Cyclohexene can be oxidized with iodosylbenzene in the presence of catalytic amounts of Fe(III)TPP-Cl to give cyclohexene oxide and cyclohexenol in 55% and 15% yields, respectively. Likewise, cyclohexane is converted to cyclohexanol under these conditions. Significantly, the alcohols were not oxidized rapidly to ketones under these conditions, a selectivity shared with the enzymic hydroxylations. The distribution of products observed here, particularly the preponderance of epoxide and the lack of ketones, is distinctly different from that observed in an autoxidation reaction or in typical reactions of reagents such as chromates or permanganates (15). 

Except for a few results concerning type A and B taxanes in which either esterification or saponification was used to confirm structures by comparison with known compounds, studies of opening of the epoxide rings (36), or determination of the structure of autoxidation products (55), most of the published chemistry has been effected on baccatin Ill-type compounds (13b). Thus, two synthetic approaches for structural modifications have been described in the literature using, in one case, taxol itself as starting material or, instead, 10-deacetylbaccatin III (13a), extracted from leaves (easily collected without causing damage to the tree). 

Hydrogen abstraction also increases at elevated temperature as thermal energy decreases bond dissociation energy. Typical H abstraction rates for ROO at room temperature are < 1 M s, but this increases to 10 -10" L M s at 65°C (223). For example, in linolenic acid autoxidized neat at room temperature to PV 1113, products were not quantified, but estimates from intensities of HPLC peaks gave about 40% LnOOH, 12% dihydroperoxides, 12% hydroperoxy epidioxides, and 4% epoxides (228). At 40°C, H abstraction occurred more as a secondary process. Hydroperoxides per se were still the main products, but fewer were present as mono- and dihydroperoxides (36% total) and more had formed after cyclization or addition (31%). Data are not available to distinguish whether this... 

The ready accessibility of a-pinene epoxide (817) is frequently the reason for its use to make other compounds. A most interesting claim is that with 1 mol% of aluminum isopropoxide at 100-120°C, trans-pinocarveol (603) is obtained, while 5 mol% of the reagent at 140-170°C gives pinocamphone (818). Kane et al. have used the pinanyl oxides cis- and trans-S19 (R = K) to open a-pinene epoxide (817). These alkoxides are made from dimethylsulfmylpotassium and dy-pinanol (819, R = H) (the product of autoxidation of pinane cis here means from ci5-pinane, therefore with the rranj-hydroxyl group) and are among the... 

From 2, it was concluded that the ferryl complex is the catalytically active species. Observation 1 suggested that 80% of the epoxide product in the aerobic reaction is derived from a carbon-based radical, which is quenched by O2 (autoxidation), and this is known to produce epoxide in reactions with cyclooc-tene (325). Methanol (observation 3) is known to quench radicals. The fact that the diols formed are a mixture of cis and trans products (observation 1 this is very unusual in iron-catalyzed olefin oxidations) suggested that the diol results from the capture of OH radicals by the putative carbon-based radical. 

Epoxidation of methyl 7,8-dihydroretinoate (114) with monoperphthalic acid gave three main products (115)-(117). In the presence of acid (115) gave the spiro-derivative (118). The autoxidation of solid all-frans-retinyl acetate and palmitate, tra 5-axerophtene (82), and ll-cw-retinol has been studied. From amorphous samples dialkyl peroxides and carbonyl and hydroxy-compounds were formed, but in the crystalline state carbonyl compounds were the only products. ... 

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