Singlet oxygen from endoperoxides

This is a particularly interesting reaction, since in the first stage the anthracene molecule is excited to the triplet state and produces singlet oxygen from ground-state molecular oxygen, and then finally forms an endoperoxide. 

The lactone-peroxides 17 are derived from the corresponding ot-pyrones. Singlet oxygenation at low temperature affords the unstable a-pyrone endoperoxides which, on warming up, readily decarboxylate into 1,2-diacylethylenes. However, subambient diimide reduction leads to the desired lactone peroxides 17 (Eq. 12)29). 

Singlet oxygen adds to cyclic 1,3-dienes to afford 1,4-endoperoxides 609). This type of reaction has first been applied successfully to the synthesis of ascaridole from a-terpinene 610) (6.4) and of the vesicatory compound cantharidine 6U) (6.5) some 25 years ago. 

Singlet oxygen reacts through the [4- -2]-cycloaddition mode with the sterically hindered and non-planar 1,3-diene system in l,l -dimenthene (493) to afford the cis-endoperoxide 494, albeit in moderate yield (Scheme 135) ". Cycloaddition of O2 to more planar bis(dialine) (495) proceeds smoothly leading predominantly to the trans-endoperoxide trans-496 (70%), while cis-496, which would be expected to derive from a concerted process, constitutes the minor (15%) product (Scheme 136) . 

The mechanism of singlet oxygen detoxification involves quenching by Vitamin Be (215a). After silylating this compound, in order to make it soluble in CD2CI2 to work at low temperature, the quenching process proceeds as shown in equation 75 (Section V.C.4). The main product is endoperoxide 217, probably in equilibrium with hydroperoxide 216. The structure of 217 can be established from the H and NMR spectra, e.g. 5h = 6.06 ppm (IH, d, 7 = 4.4 Hz, HCOO), 4.19 (IH, broad, NH), 5c = 96.8 ppm (MeCOO), 86.6 ppm (HCOO), and DEPT, phase-sensitive COSY, HMQC and HMBC spectra . 

The attempt to prepare a compound analogous to endoperoxide 30 from the reaction of 1,4-dimethoxyphthalazines with singlet oxygen resulted in the formation of the corresponding phthalate by elimination of N2, albeit no chemiluminescence was observed . Even though no experimental evidence indicates its existence, the endoperoxide intermediate 30 is one of the most well-accepted HEI postulated for the chemiexcitation step in luminol chemiluminescence. 

In the case of chiral base catalysis of the E2 elimination, enantioenriched 7-hydroxyenones from the corresponding endoperoxides were obtained <2006JA12658> in fact, a one-pot asymmetric 1,4-dioxygenation of 1,3-cyclohepta-diene by sequential reaction with singlet oxygen and 5 mol% chiral catalyst provided the 7-hydroxyenones 80 in 90% yield and 92% ee (Scheme 18). 

If singlet oxygen is formed by an energy transfer reaction from excited diphenylanthracene to oxygen, the quantum yield of which is the quantum yields of endoperoxide formation are given by... 

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