Fatty acids cyclic peroxides

Plakinastrella Atypical fatty acids, cyclic peroxides, peroxylactones... 

Plakortis Atypical fatty acids, cyclic peroxides, peroxylactones, oxylipins, terpenic glycosides, hopanoids, iodotryptophans, pyrroloacridine alkaloids... 

Cyclization requires the presence of a c/i-double bond homoallylic to a hydroperoxide (230, 269), as shown in Reaction 45. In addition, cyclization of peroxyl radicals at internal positions is considerably faster than secondary oxidations of hydroperoxides at either external position. About 25% of peroxyl radicals in lino-lenic acid and 33% of peroxyl radicals in arachidonic acid are internal (Table 4). Thus, linolenic and arachidonic acids are particularly prone to formation of cyclic peroxides. These factors together make intramolecular cyclization 4—6 fold faster than p-scission in higher polyunsaturated fatty acids (247). 

Scheme 14. Structures of cyclic peroxides from sponges. Chondrillin [1], plakorin [2], 7, 8,9, and 10 are formally derived from C22 acids, 3,4, and 5 from C18, and xestin A [11] and xestin B [12] from C24 straight chain fatty acids.

The large number of precursors of volatile decomposition products affecting the flavor of oils has been discussed in Chapter 4. Only qualitative information is available on the relative oxidative stability of hydroperoxides, aldehydes and secondary oxidation products. As observed with the unsaturated fatty ester precursors, the stability of hydroperoxides and unsaturated aldehydes decreases with higher unsaturation. Different hydroperoxides of unsaturated lipids, acting as precursors of volatile flavor compounds, decompose at different temperatures. Hydroperoxides of linolenate and long-chain n-3 PUFA decompose more readily and at lower temperatures than hydroperoxides of linoleate and oleate. Similarly, the alkadienals are less stable than alkenals, which in turn are less stable than alkanals. The short-chain fatty acids produced by oxidation of unsaturated aldehydes will further decrease the oxidative stability of polyunsaturated oils. For secondary products, dimers are less stable than dihydroperoxides, which are less stable than cyclic peroxides. 

The many variables involved in the autoxidation of fatty acid moieties make it difficult to generalize the results of a particular laboratory experiment to real-world situations. Reports of many reactions do not explain the source of the energy that drives them. Nevertheless, results tend to be accepted in those instances in which the products of the reaction have a lower total energy state than the starting reagents. In the case of the autoxidation of methyl sorbate, for example (Fig. 4.1), the reaction products are consistent with those that would be obtained with a thermodynamically unlikely dioxetane intermediate (1,2). Hydroperoxides and cyclic peroxides (not shown) are also products of this reaction (2). These products are hypothesized to arise through 1,2 addition, 1,3 addition (the ene reaction) and 1,4 cycloaddition respectively. 

Arachidonic acid (22) is a polyunsaturated fatty acid that plays a special role as a synthetic intermediate in plants and animals (Mann et al., 1994). As shown in Figure 6.6, allylic oxidation at the 11th carbon of the chain leads to the hydroperoxide (23). Further oxidation (at the 15th carbon) with two concomitant cyclization reactions gives the cyclic peroxide (24). This is a key intermediate for the biosynthesis of prostaglandins such as 6-ketoprostaglandin Fjj, (25) and... 

Hydroperoxides of polyenoic fatty acids with three or more double bonds in the molecule, which have a system of conjugated double bonds in position p to hydroperoxide group, are very unstable compounds. They tend to form more stable cyclic six-membered peroxides derived from 1,2-dioxane by 1,4-cyclisation, five-membered peroxides derived from 1,2-dioxolane by 1,3-cyclisation and endoperoxides. 

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