Singlet oxygen preparation

Classical chemiluminescence from lucigenin (20) is obtained from its reaction with hydrogen peroxide in water at a pH of about 10 Qc is reported to be about 0.5% based on lucigenin, but 1.6% based on the product A/-methylacridone which is formed in low yield (46). Lucigenin dioxetane (17) has been prepared by singlet oxygen addition to an electron-rich olefin (16) at low temperature (47). Thermal decomposition of (17) gives of 1.6% (47). 

Many hydroperoxides have been prepared by autoxidation of suitable substrates with molecular oxygen (45,52,55). These reactions can be free-radical chain or nonchain processes, depending on whether triplet or singlet oxygen is involved. The free-radical process consists of three stages ... 

This group is prepared by the reaction of the anion of 9-hydroxyanthracene and the tosylate of an alcohol. Since the formation of this group requires an S 2 displacement on the alcohol to be protected, it is best suited for primaiy alcohols. It is cleaved by a novel singlet oxygen reaction followed by reduction of the endo-peroxide with hydrogen and Raney nickel. 

Oxazoles, prepared from carboxylic acids (benzoin, DCC NH4OAC, AcOH, BOSS % yield), have been used as carboxylic acid protective groups in a variety of synthetic applications. They are readily cleaved by singlet oxygen followed by hydrolysis (ROH, TsOH, benzene or K2CO3, MeOH ). 

Dihydro-2f/-pyran-2-one has been prepared by reductive cycliza-tion of 5-hydroxy-2-pentynoic acid [2-Pentynoic acid, 5-hydroxy-], which is obtained in two steps from acetylene [Ethyne] and ethylene oxide [Oxirane] 3 and by the reaction of dihydropyran [277-Pyran, 3,4-dihydro-] with singlet oxygen [Oxygen, singlet].4,5 2ff-Pyran-2-one has been prepared by pyrolysis of heavy metal salts of coumalic acid [2//-Pyran-5-carboxylic acid, 2-oxo-],8 by pyrolysis of a-pyrone-6-carboxylic acid [211 - Pyran-6-carboxyl ic acid, 2-oxo-] over copper,7 and by pyrolysis of coumalic acid over copper (66-70% yield).8... 

Since the corresponding endoperoxide precursors are all too unstable for isolation, the diimide reduction constitutes an important chemical structure confirmation of these elusive intermediates that are obtained in the singlet oxygenation of the respective 1,3-dienes. However, the aza-derivative 14 and the keto-derivative 15 could not be prepared,17> because the respective endoperoxides of the pyrroles 18) and cyclopentadienones suffered complex transformations even at —50 °C, so that the trapping by the diimide reagent was ineffective. 

Furthermore, ozonolysis in the presence of tetracyanoethylene (TCNE) afforded the novel bicyclic peroxide 15 which, as stated already, could not be prepared via the singlet oxygen-diimide route starting from cyclopentadienone. Peroxide 15 was too unstable for isolation, but the characteristic proton resonances at 8 2.0 (m, 4 H) and 4.38 (m, 2 H) ppm are consistent with the assignment. 

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). 

On the other hand, the thiaozonides 13 were unknown but could be prepared analogously via singlet oxygenation of 2,5-disubstituted thiophenes and subsequent diimide reduction at low temperature (Eq. 10)25). 

In addition to the parent compound 2, the derivatives 2a, b, the benzo-system 16, the lactone-peroxides 17, and the fused polycyclic derivatives 18 and 19 could be prepared via the singlet oxygen-diimide route. For example, the parent system 2 was obtained in ca. 40% yield by diimide reduction of the stable 1,3-cyclohexadiene endoperoxide in MeOH at 0 °C27,28). Dihydroascaridole 2a and dihydroergosterol endoperoxide... 

Alternatively, the (2 + 4)-tropilidene endoperoxide, which is the major product in the singlet oxygenation of cycloheptatriene 30 a) affords on diimide reduction the desired bicyclic peroxide 20. The double bond in the two-carbon bridge is reduced selectively, but on exhaustive treatment with excess diimide, the fully saturated substance is obtained. A number of substituted derivatives have been prepared in this way30). 

The first porphyrinylsugar derivatives prepared by Frank and co-workers, the bis(D-glucosyl)isohematoporphyrin 2a and bis(D-galactosyl)isohemato-porphyrin 3a, exhibit an unusual water solubility for a porphyrin derivative and also maintain the photophysical properties of their precursor 1, namely singlet oxygen production.11... 

S. L. Buell and J. N. Demas, Heterogeneous preparation of singlet oxygen using an ion exchange boundtris(2,2 -bipyridine)ruthenium(II) photosensitizer, /. Phys. Chem. 87, 4675 1681 (1983). 

Bicyclopropylidene (1) readily reacted with photochemically generated singlet oxygen at 30-35°C to give spiro[2.3lhexan-4-one (203) and 7-oxadi-spiro [2.0.2.1]heptane (202) [104,1411. Bicyclopropylidene epoxide 202 can also be prepared by epoxidation of 1 with m-chloroperbenzoic acid in the presence of Na2C03 [141] or with KHSOs/acetone [74]. Bicyclopropylidene (1) was found to quench the fluorescence of 9,10-dicyanoanthracene [81]. 

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