Pyranose acetal moiety

The theoretical calculations with which these experimental data can be compared are based on the use of methoxymethanol CH3OCH2OH ( 12 ) and dimethoxymethanol CH3OCH2OCH3 ( 13 ) as model compounds for the hemi-acetal and acetal moieties in the pyranose and methyl pyranoside molecules, respectively. 

Alkyl (or acyl) derivatives of the 6-amino-6-deoxy carbohydrates are examples of derivatives in which the hydrophilic and hydrophobic moieties are linked at other positions than C-1. Thus 6-amino-6-deoxy-D-galactose derivatives 34 were prepared from l,2 3,4-di-0-isopropylidene-6-0-tosyl-a-D-galacto-pyranose by the following reactions (1) substitution of the leaving group at C-6 by a phthaloyl function, (2) hydrazinolysis to afford a 6-amino-6-deoxy intermediate, (3) reaction of acyl or sulfonyl chlorides at the amino function, (4) deprotection of the acetal rings to afford the expected glycolipid 34 [56]. 

The anomeric bromination can result from photosubstitution of other positions in the ring moiety. Very recenly, bromination of fully protected I-2-O-benzylidenated pyranoses with bromotrichloromethane and U.V. light or NBS yields 2-O-benzoyl glycosylbromides which may be converted in situ to glycosides and disaccharides in good yields (Scheme 2). These reaction conditions are compatible with protective groups like esters (acetates, benzoates, tosylates) and silyl ethers. 

Benzylidene acetals are frequently used for the protection of the 4- and 6-hydroxyl groups of pyranoses, Reclamation of the diol can be accomplished by many methods that will be discussed below, but Scheme 3.53 shows an unusual cleavage of benzylidene acetals protecting the glucose moieties in 53.1 using 1% iodine in methanol (w/v),86 The reaction works equally well with isopropylidene, ethylidene and dithioacetals,87... 

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