Glycosidic ether oxygens

In three dimensions, this structure forms a truncated cone where the primary hydroxyl groups are located on one face and the secondary hydroxyl groups on the other. The interior of the cone is hydrophobic due to the presence of the glycosidic ether oxygens at 0-4 and the hydrogens attached to C-3 and C-5, and thereby provides a cavity for the inclusion of hydrophobic compounds. The cavity varies in size with ot-CD being the smallest at about 5.3 A across and y-CD the largest at 8.3 A diameter (Table 6). 

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... 

The glycosidic carbon-oxygen bond in pseudouridine (AC-glycoside ) can hardly be broken by acids. It undergoes isomerization to the a- pyranoside because the ether bond is reversibly opened by acids, which form short-lived car-benium ions. Such a-pyranosides are not observed with the N-glycosides in nucleosides (Scheme 8.5.8). 

A 1,2-hydride shift has been invoked399 to account for the formation of p-methoxyphenylbutyraldehyde derivatives (337) during the treatment of />methoxy-benzyl-protected allylic alcohols (336) with zeolites. A similar C-glycosidation procedure involving Lewis acid-catalysed anomeric oxygen to carbon rearrangement of tetrahydropyranyl ether derivatives has been reported400 (see Scheme 82). It has been... 

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