Isoprostane bicyclic endoperoxides

The dialkyl denomination also includes cyclic peroxides (endoperoxides). The most significant route for peroxide formation is probably that of autoxidation of organic materials, leading to their gradual degradation. Although hydroperoxides are the main products of this process, also peroxyesters are formed, as is the case, for example, of isoprostane bicyclic endoperoxides (25) mentioned in Section II.A.2.C. 

Yin, H., Havrilla, C. M., Morrow, J. D and Porter, N. A. 2002. Formation of isoprostane bicyclic endoperoxides from the autoxidation of cholesteryl arachidonate,... 

As in the case of linear peroxidation products, the initiation step of the formation of isoprostanes is the abstraction of a hydrogen atom from unsaturated acids by a radical of initiator. Initiation is followed by the addition of oxygen to allylic radicals and the cyclization of peroxyl radicals into bicyclic endoperoxide radicals, which form hydroperoxides reacting with hydrogen donors. 

Fig. 2.2 Rearrangement of bicyclic endoperoxides to form y-ketoaldehyde. Proposed mechanism for formation of y-ketoaldehydes from bicyclic endoperoxide such as PGFI2 and H2-isoprostanes by base-catalyzed concerted rearrangement (Salomon et al., 1984). Attack of base is shown at the top of the bridgehead, which results in formation of E2-isomers. Attack at the symmetrical position below the bridgehead forms the D2-isomers...

Exo cyclization of the 1,2-dioxolanyl radical leads to bicyclic endoperoxides and isoprostanes... 

Fragmentation of a bicyclic endoperoxide into Dj, Ej, and Fj ring isoprostanes... 

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