Photooxidation Photooxygenation

Singlet oxygen affords a variety of regio and diastereoselective reactions with chiral allylic alcohols amines - (e.g. substrate 161, Scheme 58) and chiral cyclohexadienes - that are useful for synthetic transformations. For example, the photooxygenation of a chiral allylic alcohol was used recently as the key step in the total syntheses of plakorin 162 and ewawfio-chondrilin (Scheme 58). If the photooxidation... 

Phenyl hydroperoxide, C-O distance, 103 (y-Phenylhydroperoxides, nucleophiUc substitution cyclization, 234-5 1 -Phenyl-3-methyl-3-butene, intrazeohte photooxygenation, 874-5 Phenyl substituted alkenes, photooxidation site selectivity, 839-42... 

A similar photooxidation pathway was found for Mg(ll)TPP. It reacted readily with molecular oxygen to give the corresponding 15,16-dihydrobiliverdin, similar to the one shown for Mg(II)OEP in Scheme 1. Further studies have proposed that the photooxygenation of metallo-me o-tetrasubstituted porphyrins proceeds via a one-molecule mechanism involving only one oxygen molecule. Most likely, the first intermediates formed upon photooxygenation are short-lived peroxides. Such compounds are very unstable and a possible dioxetane structure is shown in formula 31. 

Interesting photooxidation products of betaines 363 and 364 have also been reported. Irradiation of methanol solutions (> 0.1%) of compound 364 in the presence of oxygen gives a product (CjqH, 7NO7), mp 100.5-101.5°C, which has been formulated as adduct 373 (42%). In more dilute solutions the product is berberal (374 R = H) (56%). Products 375, 376, and 374 (R = OMe) have been obtained by photooxygenation of the 8-methoxy-betaine 363. ... 

Benzo[6]furan and 2-methylbenzo[6]furan fail to undergo photooxygenation but 2,3-dimethyIbenzo[6]furan yields 2-acetoxyacetophenone (364). At -78 °C the peroxide (363) is produced which at higher temperature rearranges to the acetophenone (364 Scheme 98). 2-Vinylbenzo[6 Jfurans, e.g. trans- 2- styrylbenzo[6 Jfuran (365 Scheme 99), yield photooxides which may be isolated. Compound (366) on treatment with a catalytic amount of triethylamine yields the lactone (367) (77BCJ3026). 

Benzo[c]furans undergo very rapid photooxidation. 1,3-Diphenylbenzo[c ]furan yields the oxide (368) on sensitized photooxygenation in ether at -50 °C. Reduction of the oxide with potassium iodide in acetic acid affords 1,2-dibenzoylbenzene and reaction with methanol supplies the hydroperoxide (369). 

Photooxygenation of vinylsilane in the presence of acetic anhydride and pyridine furnishes a convenient procedure for the synthesis of a-trimethylsilylenones 185 in moderate to good yields (equation 156)278. Dye-sensitized photooxidations of allylsilane with singlet oxygen produces a mixture of (E)- and (Z)-3-trimethyl-silylallyl hydroperoxides 186 (equation 157)279. 

Reviews have appeared of the solid-state dimerisation of thiones, the photooxidation of sulphur compounds, thiocarbonyl photochemistry, the DCA sensitized photooxygenation of olefins, and of the role of intersystero crossing steps in singlet oxygen chemistry. ... 

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