Oxidative cleavage, degradation

Alicyclic hydroxamic acids undergo several specific oxidative cleavage reactions which may be of diagnostic or preparative value. In the pyrrolidine series compounds of type 66 have been oxidized with sodium hypobromite or with periodates to give y-nitroso acids (113). Ozonolysis gives the corresponding y-nitro acids. The related cyclic aldonitrone.s are also oxidized by periodate to nitroso acids, presumably via the hydroxamic acids.This periodate fission was used in the complex degradation of J -nitrones derived from aconitine. 

Stndies of the antoxidation of carotenoids in liposomal suspensions have also been performed since liposomes can mimic the environment of carotenoids in vivo. Kim et al. stndied the antoxidation of lycopene," P-carotene," and phytofluene" " in liposomal snspensions and identified oxidative cleavage compounds. Stabilities to oxidation at room temperature of various carotenoids incorporated in pig liver microsomes have also been studied." The model took into account membrane dynamics. After 3 hr of reactions, P-carotene and lycopene had completely degraded, whereas xanthophylls tested were shown to be more stable. 

In recent years a number of new isoquinoline alkaloids of untypical structures, i.e., structures with open heterocyclic rings, have been isolated from natural sources. These bases were named secoisoquinoline alkaloids because they were believed to be produced in vivo from classic isoquinoline alkaloids as a result of various degradation processes causing oxidative cleavage of some bonds. 

This arrangement of subgroups is due to the hypothetical biosynthetic sequence. It assumes that precursors for these alkaloids are the Af-methylphth-alideisoquinolinium salts, whose presence in plants is well documented. Enol lactones may be the initial degradation products formed in a Hofmann-type jft-elimination process. They could be hydrated to the keto acids and in the next step oxidated in air to the diketo acids. Diketo adds may undergo further oxidative cleavage to yield simple alkaloids of the fumariflorine (87) type 85,86), which seem to be the final products of the metabolism of phthalideiso-quinoline alkaloids. 

The strong oxidants Cr(VI) and Mn04 can also be used for oxidative cleavage of double bonds, provided there are no other sensitive groups in the molecule. The permanganate oxidation proceeds first to the diols and ketols, as described earlier (p. 757), and these are then oxidized to carboxylic acids or ketones. Good yields can be obtained provided care is taken to prevent subsequent oxidative degradation of the products. Entries 5 and 6 in Scheme 12.17 are illustrative. 

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