Acetates anomerization

Bu3SnOMe, CICH2CH2CI, 1 h, 77% yield.These conditions selectively cleave the anomeric acetate of a glucose derivative in the presence of other acetates. 

LiEt3BH, THF, —78°, 2 h, 98% yield. An anomeric acetate can be selectively cleaved in the presence of a secondary acetate. 

H2NNH2, MeOH, 92% yield. An anomeric acetate was cleaved selectively in the presence of an axial secondary acetate. ... 

The 3-deoxyhexoses were obtained crystalline after separation on a column of cellulose, a rather tedious operation recently separation of these sugars by fractional crystallization has been reported (41). From the major unsaturated ester 27, 3-deoxy-D-ribo- and -arabino-hexose were shown to have been produced respectively in the ratio 66 34, and from ester 28 in the ratio 24 76. Saturation of the enol grouping of the esters 27 and 28 gives, therefore, the 1,2-cis products preferentially. However, some hydrogenolysis of the anomeric acetate group accompanies simple saturation of the double bond and poses another separation problem. 

This reduction technique also applies to the benzaldehyde acetals of sugars with reduction of the benzaldehyde acetals taking place in preference to reduction of the anomeric acetal (Eq. 305).497... 

A number of conditions are available for the reduction of the anomeric acetals to the reduced sugars. Among these are the combinations Et3SiH/BF3-OEt2/ TFA483 and Et3SiH/TMSOTf,503,504 although some isomerization is found to... 

Elimination of sulfonic esters on anomeric acetal derivatives... 

Anomeric acetates 104 have been also used as electrophiles toward alkynylindium reagents 105 under Barbier conditions. Good yields and 1,2-cA-stereoselectivities were observed during the reaction (Scheme 36). The alkynylation was applied to the synthesis of an a-(l —>6)-C -disaccharide (110) analogue of methyl isomaltoside.108... 

Glucose is first esterified to penta-O-acetylglucose using acetic anhydride. Note that the hemiacetal hydroxyl is also esterified, and thus any equilibration with an aldehyde form is now not possible (see Section 7.2). When this penta-acetate is treated with HBr, the anomeric acetate is preferentially lost under the acidic conditions, due to the stabilization conferred by the heterocyclic oxygen. Note that this is the same type of intermediate we implicated... 

The enolate reactivity associated with this approach to C-glycoside synthesis was, however, first developed with octulosonic acid derivatives, such as 283 [114], 284 [115], and 285 [116], and a series of examples involving aldehyde and halide-based electrophiles are shown in Scheme 74. Related studies involving stereoselective protonation of this class of exocyclic enolate have also been described [13] and Scheme 75 illustrates this with an example of reductive samariation (using ethane-1,2-diol, but no HMPA) of an anomeric acetate 286 [117]. 

The procedure also replaces anomeric acetate groups. 

The most widely used approaches in the synthesis of the O-glycosylated amino acids, Koenigs-Knorr, activation of anomeric acetate, trichloroacetim-idate, and thioglycosides will be explained in detail. 

When anomeric acetate moiety is GlcNAc it is desirable to protect the A-acetyl group (100). 

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