Anomeric esters

Kusumoto and coworkers have found that the treatment of hemiacetal 1 with trifluoro- or trichloroacetic anhydride 94 (1 equiv) and trimethylsilyl perchlorate (0.2 equiv) selectively provides the corresponding anomeric ester intermediate 91 [152], Hemiacetal acylation occurs even in the presence of the alcohol acceptor. With Lewis acid assistance, the glycosyl ester intermediate is displaced to provide disaccharide products in good yields. This transformation allowed the synthesis of disaccharides 98 (81%) and 99 (91%). In some cases, acetic anhydride has been used as the electrophilic activator of hemiacetal donors and the reaction with thiol acceptors yields S-linked glycosides [153,154],... 

Lipase-catalyzed deacylations of sugars can be carried out either by hydrolysis in water or by alcoholysis in organic solvents in the presence of a nucleophilic alcohol, such as n-propanol or n-butanol. In peracylated sugars the anomeric ester is the most reactive of the molecule - probably due to the intrinsic reactivity of the... 

Selective deacylations of primary ester groups in the presence of secondary are also possible using enzymes (i.e., various lipases) [40]. Other lipases show selectivity for the anomeric position (Scheme 3.22). Also, as mentioned above, anomeric esters are more labile than other esters and can be removed selectively by mild base treatment. Furthermore, anomeric silyl ethers can be removed selectively on treatment with mild acid. 

In 1981, Acton et al. [143] performed the reaction shown in Scheme 7.41, to give a 3 1 ratio of anomeric esters. Shortly thereafter, Nicotra et al. [144] ran a similar reaction and studied the chemical shifts of the resulting a and (3 anomers. This study yielded an easy method for the assignment of the anomeric stereochemistry, with the anomeric carbon of the a anomer possessing a chemical shift of approximately 33 ppm and that of the (3 anomer possessing a chemical shift of approximately 37 ppm. 

Glycosylation with Donors Having Other Anomeric Ester Groups. 544... 

In the second approach to 1,6-anhydro sugars, anomeric esters, particularly glycosyl acetates, are activated by Lewis acids and displaced by labile ethers at 0-6. Rao has reported an efficient synthesis of tri-0-acetyl-l,6-anhydro-/3-D-glucopyranose by treatment of the trityl ether (4) with titanium chloride (O Scheme 3) [20]. 

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