1.2- Dioxetanes synthesis singlet oxygenation

Enamines which cannot undergo ene-like reactions add efficiently to singlet oxygen. The intermediate dioxetane can be cleaved under mild conditions to afford a ketone (and an amide). Such a sequence has been applied in a synthesis of testosterone (6.18) 624). 

As already hinted at above, chiral dioxetanes, obtained through the highly stereoselective [2 + 2] cycloaddition of singlet oxygen to the chiral enecarbamate, provide a convenient preparation of optically active 1,2 diols as building blocks for asymmetric synthesis (Scheme 5) . Reduction of the dioxetane 2c by L-methionine, followed by release of the oxazolidinone auxiliary by NaBH4/DBU reduction, affords the enantiomerically pure like-5 diol (for additional cases, see Table 4 in Reference 19e). 

Kopecky s synthesis of trimethyldioxetane employed the base-mediated dehydrohalogenation of 2-methyl-2-hydroperoxy-3-bromobutane. Subsequently, this type of eliminative cyclization (14) has been applied to the preparation of scores of dioxetanes. Additionally, many dioxetanes have been prepared by the addition of singlet oxygen to electron-rich olefins which do not possess allylic hydrogens (15), a method discovered first by Bartlett and Schaap... 

However, in recent years, it has become apparent that several photoinduced oxidation reactions do not involve singlet oxygen as the reactive intermediate, and, consequently, the reaction products cannot be accounted for the mechanisms shortly reported above. Moreover, since the first report on the photochemical stereospecific synthesis of the most fascinating peroxide derivatives, i.e., 1,2-dioxetanes [32], it clearly appeared, with a few notable exceptions [33,38], that only electron-rich olefins, such as enamines, enol ethers, and thio-substituted... 

Reviews on the following aspects of four-membered heterocycles have appeared 1,2-dioxetans and the reaction of singlet oxygen with alkenes/ /8-lactam antibiotics, small ring compounds of sulphur and selenium," recent developments in the synthesis of azetidines, the chemistry of benzazetes and of thiacyclobutenes (thietes) and their valence tautomers, and conformational barriers and interconversion pathways in some small ring systems. ... 

A preliminary report has been published on a stereoselective approach to medium-sized ketolactones involving an alumina-catalysed cyclization of epoxydihydropyrans followed by addition of singlet oxygen and thermal decomposition of the resulting dioxetan (Scheme 26). Previous studies (1, 134, 135) have been extended to a synthesis of keto-thiolactones. ... 

Singlet Oxygen.—Singlet oxygen reacts with dienes to give e do-peroxides and with alkenes to give allylic hydroperoxides or dioxetans. " The mechanism of the latter reaction is still under investigation. The use of these reactions in natural products synthesis has been comprehensively reviewed, "— " and attention will be concentrated on recent work. 

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