Furan, ozonides

Diazene, furan ozonide decomposition, 730 Diazo compounds, ozone adducts, 734 Diazonium salts, TEARS assay, 667 Dibenzoyl peroxide, determination, 698 Z-Dibenzoylstilbene, tetracyclone bleaching, 734-5... 

Polycyclic aromatic hydrocarbons such as anthracenes, tetracenes, and pentacenes, as well as cyclopentadienes, cyclohexa-l,3-dienes, cyclo-hepta-l,3-dienes, and furans, have been found to be suitable diene systems to which the singlet oxygen adds as a dienophile in a 1,4-cycloaddition reaction. Thus, endoperoxides (transannular peroxides) and, in the case of furans, ozonides of the corresponding cyclobutadienes are the primarily produced, more or less stable addition products (2, 21, 22). 

A number of the bicyclic ozonides 12 were prepared in good yield (45-65 %) by diimide reduction of furan singlet oxygenates (Eq. 9) 23>. Again, low temperature were essential because the furan endoperoxides readily transform into 1,2-diacyl-ethylenes. Of course, the bicyclic ozonides 12 can alternatively be prepared via ozonolysis of the appropriate 1,2-disubstituted cyclobutene 24). 

This, the ozonide of cyclobutadiene (and also a likely peroxide of furan), may be stored in liquid nitrogen. It can explode with considerable force on warming. 

Cyclobutenes have been applied as latent functionality of 1,4-dicarbonyl systems. Photolysis of (466) gave a 1.5 1 mixture (60%) of (467a) and (467b), which were cleaved by ozone and subsequent reduction of the ozonides yielded the epimeric (468). Cyclization and dehydration process converted (468) to the furan (469) 160). Furan (469) was converted to hibiscone C in a few steps, 60). 

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 reactions of a number of ozonides and alkenes in the presence of BF3 Et20 resulted in the formation of E- and Z-l,2-dioxolanes in 12-70% yields (Scheme 30) <83JA6279>. 2,5-Dimethyl-thiophene endoperoxide can act as a sulfur- or oxygen-atom transfer agent in analogous reactions to those shown by furan endoperoxides <86JA2775>. 

SCHEME 18. Generation of an ozonide (317) from a furan derivative in solution and paths for its disappearance, alone or in the presence of an alcohol... 

Diaryl- and tetraaryl-substituted furans such as tetraphenylfuran (384) for example, yield generally cw-diaroylethylenes such as 380, probably via intermediate ozonide formation.257 Secondary reactions seem to depend very much on the nature of the solvents. Thus, cis-dibenzoylstilbene (380) has been observed by direct photooxygenation of 384 in CS2 as well as by methylene blue-sensitized photooxygenation of 384 in methanol.257,258 However, when the latter reaction was carried out in acetone, epoxide 386 and the enolbenzoate 387 were obtained.258... 

Evidence for ozonides to be primary products in furan and alkyl-substituted furan oxygenation reactions has been obtained. Thus, furan (388) is converted to a highly explosive substance at low temperatures,... 

The < a/o-peroxides of aromatic oxygen-containing five-membered heterocycles such as furan and oxazole are actually ozonides (1,2,4-trioxolanes), and by a reverse dipolar [3+2] cycloaddition they can be a source of carbonyl oxides. 

D. Addition of Ozone to the Furan Ring Furan and its derivatives can readily by ozonized, and the ozonides converted into various products.276 277... 

Bailey and Colomb278 described an ozonolysis of 2,5-diphenylfuran in methanol-acetone, with two equivalents of ozone, which gave 14% phenylglyoxal and 81% benzoic acid. Abnormal ozonizations such as this can be explained if we consider that the initial step is the normal 2,5-addition of ozone on the furan ring. Then 26 or the primary ozonides 27 or 28 could give the resulting ketonic product ... 

The action of oxygen on furan and furan derivatives in light leads to 1,4 addition with formation of ozonides.197 However, the only case in which the ozonide has been isolated is that of 1,4-diphenylisobenzo-furan this gives the crystalline ozonide (176), which explodes at 18°.198 In most cases only the decomposition products of the ozonides (carbonyl compounds, acids, and their derivatives) are isolated. 

At a temperature of —20° C. and in chloroform solution, the y-pyrones take up 2 molecules of ozone. This means that only the C=C bonds react with ozone, which was also found when ozone reacted with other heterocyclic rings, such as pyridine, pyrrole, and furan. The ozonides were decomposed by reduction with hydriodic acid, whereby carbonyl compounds are formed, as well as acetic acid and in some cases formic acid. By reacting the carbonyl compounds with p-nitrophenylhydrazine, the corresponding nitrophenylhydrazones or p-nitrophenylosazones are formed. The separation of the mixture of hydrazones and osazones is carried out by a chromatographic method. The quantitative determination of the separated nitrophenylosazones (or hydrazones) is carried out by means of spectrographic measurements. 

Two other approaches to the furan derivative 67 have been described (a) from perfluoro-2-butyne and involving the novel cyclopropenyl ketone derivative 74154 and (b) by ozonolysis of hexakistrifluoromethylbenzvalene (75), followed by treatment of the ozonide (76) with triphenylphosphine and... 

The addition of oxygen to furans to form ozonides and hydroxyfuranones (Scheme 2.21) has been largely exploited in view of the ensuing easy elaboration of these versatile intermediates [33-35]. 

Obtained (poor yield) by hydrolysis of the ozonide formed from 4,6-dihydroxy-2,3,7-trimethylbenzo-furan (m.p. 178°(d)) with dilute ozone (7%) [3196] in ethyl acetate at -30° (13%) [3197]. 

Photo-oxygenation of furan gives the ozonide which decomposes explosively at -10°C, in the presence of triphenyl phosphine to yield butendial. 

The photooxygenation of furans was investigated initially by Schenck in the mid-forties. A crystalline and highly explosive material 32 was isolated from the parent furan by Schenck and Koch. This secondary ozonide rearranges thermally to an epoxybutanolide and a fcis-epoxide reduction with triphenylphos-phine leads to maleic dialdehyde, methanolysis to a methoxy butenolide. 

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