Oxygen cleavage

The saponification of 0 labeled ethyl propanoate was desenbed in Section 20 11 as one of the significant expenments that demonstrated acyl-oxygen cleavage in ester hydrolysis The 0 labeled ethyl propanoate used in this expenment was prepared from 0 labeled ethyl alcohol which in turn was obtained from acetaldehyde and 0 enriched water Wnte a senes of equations... 

Oxiranones (a-lactones) (Section 5.05.3.2.1), e.g. (6), in accord with their putative facile opening to zwitterions (Scheme 18), usually react readily with nucleophiles by alkyl-oxygen cleavage (Scheme 41) (71JA557), rather than the acyl-oxygen cleavage common for esters. 

Therefore, transesterification reactions frequently fail when R is tertiary, since this type of substrate most often reacts by alkyl-oxygen cleavage. In such cases, the reaction is of the Williamson type with OCOR as the leaving group (see 10-14). With enol esters, the free alcohol is the enol of a ketone, so such esters easily... 

In the special case of 3-lactones, where small-angle strain is an important factor, alkyl-oxygen cleavage is observed (Bal2 mechanism, as in the similar case of hydrolysis of P-lactones, 10-10), and the product is not an amide but a P-amino acid ... 

After a short presentation of naturally occurring oxygenated cleavage compounds, we describe different ways by which they can be formed starting from the parent carotenoid, and we give some information on their mechanisms of formation when available in the literature. 

Studies on the autoxidation of carotenoids in liposomal suspensions have also been performed since liposomes can mimic the environment of carotenoids in vivo. Kim et al. have studied the autoxidation of lycopene (Kim et al. 2001), -carotene (Kim 2004), and phytofluene (Kim et al. 2005) in liposomal suspensions and identified oxygenated cleavage compounds. The stability to oxidation at room temperature of various carotenoids has also been studied when incorporated in pig liver microsomes (Socaciu et al. 2000), and taking into account membrane dynamics. After 3 h of reaction, P-carotene and lycopene had completely degraded, whereas the xanthophylls tested were shown to be more stable. 

The ozonolysis of carotenoids was employed in order to obtain oxygenated cleavage products for biological tests, for example, for lycopene. In this case, among a series of products, one product formed by a double oxidative cleavage was purified and characterized as ( , ,/ )- 4 - methyl - 8 -oxo-2,4,6-nonatrienal, and it was shown to be active in the induction of apoptosis in HL-60 cells (Zhang et al. 2003). 

It should be noted that partial or total organic synthesis was used to produce carotenoid oxygenated cleavage products such as, for example, apo-8 -lycopenal (Surmatis et al. 1966). 

Esters, RC02R, where the alkyl group R can form a relatively stable carbocation, e.g. (177) from (178), have been shown—by lsO labelling experiments—to undergo alkyl-oxygen cleavage ... 

BF3 is also used for ether carbon-oxygen cleavage in the presence of sulfide or thiol (Scheme 62). 

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