Conjugated dienes with singlet oxygen

The thermal rearrangement of unsaturated bicyclic 1,4-peroxides, readily available from the reaction of conjugated dienes with singlet oxygen, is a convenient route fw the preparation of bisepoxides. Epoxidation of cholesteryl acetate (176) with air in the presence of a catalytic amount of dioxo(tetra-mesitylporphyrinato)ruthenium(VI) fimiishes in 8S% yield a 99 1 mixture the epoxides (177a) aitd (177b)." >... 

The thermal rearrangement of unsaturated bicyclic 1,4-peroxides, readily available from the reacdon of conjugated dienes with singlet oxygen, is a convenient route for the preparadon of bisepoxides. ... 

Reaction of trans, trara-conjugated dienes with singlet oxygen and subsequent dehydration of cyclic peroxide to cyclic ether with iron (II) sulfate. This has been used to convert the 8,10- and 9,11-dienes to the corresponding furanoid... 

Reduction of epidioxides. Schenck and Dunlap used thiourea for reduction of epidioxides, obtained by 1,4-addition of singlet oxygen to conjugated dienes at low temperatures, to the corresponding diols. Kaneko et al. report that the diols can be obtained in one step by irradiation of the diene with singlet oxygen in the presence of thiourea. Under these conditions irradiation can be conducted conveniently at room temperature. This procedure was used to prepare cw-2-cyclopentene-l,4-diol and cw-2-cyclohexene-l,4-diol. 

Conjugated dienes (and compounds that behave like conjugated dienes in the Diels-Alder reaction) react with singlet oxygen to form cyclic peroxides as if molecular oxygen acted as a dienophile. The yields of the peroxides, prepared by photochemical oxidation [13, 55] or by chemical oxidations with hydrogen peroxide and sodium hypochlorite, alkaline hydrogen peroxide and bromine, alkaline salts of peroxy acids [14, 26], or the ozonide of triphenyl phosphite [29], are comparable. 

Just as anthracene and its homologues behave like conjugated dienes in the Diels-Alder reaction, they react with singlet oxygen to form 9,10-endoperoxides (equation 135) [16, 24]. 

The oxazoles also display a number of characteristics that are typical of the furans and are explained by the structural similarity of these heterocyclic systems. The ease with which they undergo Diels-Alder reactions with dienophiles and autooxidation with singlet oxygen (see Sections IV, D and E) clearly demonstrates that oxazoles are not fully aromatic. This fact and ultraviolet data (Section III, E) suggest that oxazoles should be considered partly as conjugated dienes. 

Oxidation of polyunsaturated fatty acids (PUFA) in lipoproteins may be mediated by reactive species such as radicals, transition metals, other electrophiles, and by enzymes. Once initiated, oxidation of lipids may proceed by a chain reaction, illustrated in Fig. 4 (R5). In step I, an oxidant captures an electron from a PUFA to produce a lipid radical. In step 2, after rearrangement, the conjugated diene radical reacts rapidly with singlet oxygen to produce a lipid peroxide radical, which is the kinetically preferred reaction (step 3) (B5). The chain can be terminated if the lipid radical reacts with an antioxidant to produce a stable peroxide (step 4). Otherwise, the peroxyl radical can react with another polyunsaturated fatty acid as shown in step 5 to perpetuate a chain reaction. The chain reaction requires production of lipid peroxides, giving it the name peroxidation. Fatty acids oxidized in the core are largely triglycerides and cholesterol esters, while toward the outer layer fatty acids in phospholipids are oxidized. 

A (4 + 2) cycloaddition can take place between singlet oxygen and conjugated dienes that are free (or constrained) to adopt an s>c/s conformation (2.93). When such a reaction occurs with an aryl-conjugated alkene (2.94), the first-formed cycloadduct is itself a conjugated triene, and the overall reaction produces a bis-(4 + 2) adduct. 

Disubstituted peroxides are cleaved reductively by the same reagents as are used for hydroperoxides. The usual products are alcohols derived from the two substituents, but considerable selectivity is possible with some substrates. This is illustrated by the reduction of cyclic endoperoxides of the general formula (50) these compounds are available from the cycloaddition of singlet oxygen to cyclic conjugated dienes. The reduction of such compounds provides a good method of synthesis of cis-1,4-diols, which can be formed with retention of the double bond or with reduction of it (Scheme 29). It is also possible to prepare unsaturated epoxides by reduction with triphenylphosphine or other phosphorus(III) reagents. 

---