Type 2 reaction, singlet oxygen

Fig. 24 Comparison of Wagnerova Type I singlet oxygen reactions of sulfides in solution and in the interior of zeolite-Y.

In the termination step [eq. (13), p. 12] of type II photooxygenation reactions, singlet oxygen reacts with a suitable substrate to an oxygenation product, which may be stable or unstable. In the latter case, after-reactions such as rearrangements, decompositions, or reactions with the solvents can occur, which sometimes may obscure the mechanism of the termination step. 

While both hydrogenation and epoxidation reactions of (7) (and substituted forms) occur on the oxepin valence tautomer, cycloaddition reactions proceed more readily on the arene oxide form (where the diene is closer to planarity). Thus the dienophiles DM AD and maleic anhydride (MA) readily yielded [4 + 2] cycloadducts with (7) as shown in Scheme 22 (67AG(E)385). A similar type of singlet oxygen cycloaddition reaction gave an unstable endoperoxide (106) which upon heating yielded trans-benzene trioxide quantitatively (equation 14). (75JOC3743). 

Photo-oxygenation of acyclic dienes is complicated by the possibility that all three types of singlet oxygen reactions occur. It is, however, possible... 

Three typical water-sohible substrates including the polycyclic aromatic hydrocarbon 3, the cyclohexadienic derivative 5 and the olefin 7 were oxidized in order to assess the efficiency of the oxidizing system H202/Ca(0H)2. These substrates represent standard types of singlet oxygen reactions, namely [4 + 2] cycloaddhion and the ene reactioa The dis pearance of the substrates was monitored by HPLC and the products were identified 1 conq>arison with genuine sanq>les, photochemically obtained. E q)erimental conditions aixl results are summarized in Table 1. 

C-F bonds are normally difficult to reduce, but it has now been shown that such bonds a to the carbonyl group of alkyl perfluoroesters can be photoreduced in hexamethylphosphorotriamide in a reaction which involves electron transfer from excited phosphoramide (Portella and Iznaden). Several reports of the photoreduction of carbon dioxide have appeared for photoreduction to methane see Yamase and Sugeta, and Diirr et al. for reduction to formate see Lehn and Ziessel, and Matsuoka et al. Gollnick and Held have reported that mercurochrome is an efficient sensitizer for type II singlet oxygen photo-oxygenations. 

These reactive species will likely attack and decompose the drug or excipients in the formulation. This was demonstrated for primaquine (Kristensen et al., 1998), which can be photochemically stabilized by an inert atmosphere. Photo-oxidation reactions of type I (free radical) or type II (singlet oxygen) mechanisms can take place simultaneously in a competitive fashion. Oxygen concentration and the properties of the vehicle are factors influencing the distribution between the two processes. Free radical reactions are favored by polar vehicles such as water. 

Reaction between oxygen and butadiene in the Hquid phase produces polymeric peroxides that can be explosive and shock-sensitive when concentrated. Ir(I) and Rh(I) complexes have been shown to cataly2e this polymerisation at 55°C (92). These peroxides, which are formed via 1,2- and 1,4-addition, can be hydrogenated to produce the corresponding 1,2- or 1,4-butanediol [110-63-4] (93). Butadiene can also react with singlet oxygen in a Diels-Alder type reaction to produce a cycHc peroxide that can be hydrogenated to 1,4-butanediol. 

Another type of chemical change is initiated by light, which may trigger autolytic, that is, free radical (Type I) or singlet oxygen (Type II) reactions. These changes are routinely classified as oxidation. Rancidity in cosmetics, especially those containing unsaturated Hpids, is commonly prevented by use of antioxidants (qv). 

Consequently, the antioxidant activity of GA in biological systems is still an unresolved issue, and therefore it requires a more direct knowledge of the antioxidant capacity of GA that can be obtained by in vitro experiments against different types of oxidant species. The total antioxidant activity of a compound or substance is associated with several processes that include the scavenging of free radical species (eg. HO, ROO ), ability to quench reactive excited states (triplet excited states and/ or oxygen singlet molecular 1O2), and/or sequester of metal ions (Fe2+, Cu2+) to avoid the formation of HO by Fenton type reactions. In the following sections, we will discuss the in vitro antioxidant capacity of GA for some of these processes. 

The ability of carotenoids to act as antioxidants is closely related to their long-chain conjugated polyene structures (see Section 2.2 in Chapter 2). Two main types of antioxidant actions can be distinguished singlet oxygen quenching and reactions with radicals. The first mechanism occurs in vivo in plants and has been extensively studied in vitro. Reactions with radicals of different types have also been extensively studied in vitro under different conditions but their occurrence in vivo is still a matter of discussion. 

This is the first example of the isolation of a compound of the type postulated as intermediate in the ketene-singlet oxygen reaction. Instead... 

On a first, very broad, approximation singlet oxygen behaves somehow like ethylene. Three types of reactions of 102 are usually observed and have been utilized in organic synthesis 606 608) a) the Diels-Alder like cycloaddition to dienes (6.1) b) the ene reaction with olefins (6.2) and c) cycloaddition to activated double bonds (6.3). 

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. 

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