Glycosides oxygen requirement

An Sn 1 -like reaction, on the other hand, is much more favorable, because the glycosyl cation intermediate is stabilized by charge distribution between C-1 and the ring-oxygen atom. The unfavorable formation of an ion pair on bond cleavage can be avoided by protonation of the glycosidic oxygen atom and thus the requirement for acid catalysis (see Scheme 1). 

Benzylidene acetals, very fragile in an acidic medium, are hydrolysed by aqueous acetic acid or cleaved by hydrogenolysis. Selectivity can be observed in the hydrolysis of isopropylidene acetals. With compound 5.10, only the ring spanning positions 5 and 6 is hydrolysed by aqueous acetic acid. Hydrolysis of the acetal function which involves the anomeric oxygen requires heating in mineral acid (0.1 M) for a few hours, comparable conditions to those of hydrolysis of a glycoside. 

The proposed mechanism of a- and /3-glycosyl fluoride hydrolysis (55) is based on the assumption that catalysis requires a general acid and anionic base on opposite faces of the substrate-reactive center. The assumption is fully consistent with the operative mechanism of lysozyme, modified to catalyze a reaction with anomeric inversion. To continue the example of /3-xyIosidase, hydrolysis of the usual anomeric substrate (/3-D-xylosyl fluoride) involves water attack at the reactive carbon from the si face or below C-1. The general acid, situated above C-1 on the re face, protonates the fluoride (or glycoside oxygen) as it departs from the /3-position (57, 92) [Eq. (1)]. 

A recent phytogeny of the genus (Soltis et al., 2001) revealed that the South American taxa are most closely related to species from eastern Asia. In terms of flavonoid profiles, this requires us to reappraise the significance of the simple compounds. Thus, as suggested above, the simple flavonoid glycoside profile of C. valdivicum appears to represent a loss of biosynthetic capacities, in particular, extra oxygenation, and not a simple profile to which structural features were added in later stages of evolution. 

Interestingly, when a PMB ether is present at C2, (i-glycosides seem to predominate (see Scheme 3.3 and Ref. 11). Yan and Kahne attribute this to extreme steric requirements of the glycosyl acceptors in the examples reported.11 Boeckman suggests that the (i-sclcctivity is due to the high reactivity of the reactive intermediate, and perhaps participation by the PMB ether oxygen.9... 

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