Monosaccharide hemiacetal formation

Section 22.10 Cyclic Structure of Monosaccharides Hemiacetal Formation... 

The oligosaccharides commonly encountered in nature belong to disaccharides. The physiologically important disaccharides are maltose, lactose, trehalose and sucrose. Disaccharides consist of two monosaccharides joined covalently by an O-glycosidic bond. The hydroxyl group formed as a result of hemiacetal formation is highly reactive when compared to other hydroxyl groups. This... 

Monosaccharides can have both cyclic and straight chain structures. Cyclomonosaccharides are formed when straight-chain monosaccharides are dissolved in water. In this case, the number of asymmetrical carbon atoms increases to five. This process is known as hemiacetal formation. Let us examine the formation of the cyclic structure in the glucose molecule. 

A typical procedure calls for reaction of the hemiacetal donor with dicydohexyl carbodiimide and copper(I) chloride (0.1 equiv) at 80 °C, followed by an addition of the acceptor and continued heating. As an early demonstration of this protocol, oc-riboside 86 was prepared in moderate yield but with exclusive stereoselectivity [141]. Further measures were required for the glycosylation of monosaccharide acceptors, such as addition of p-toluenesulfonic add (0.1 equiv) to promote the formation of disaccharide 87 [144]. The method was more suitably applied to the synthesis of O-acyl glycopeptides, as evidenced by the formation of 88 in 60% yield [143,144]. Various peptides with non-nudeophilic side chains were found to be amenable to this stereoselective reaction. The [3-selectivity was suggested to arise from a preponderance of the a-isourea intermediate 85 in the activation step. 

When a monosaccharide forms a cyclic hemiacetal (or hemiketal), the carbonyl carbon becomes a stereocenter. Thus, cyclization leads to formation of two possible stereoisomers. These isomers are called anomers, and the former carbonyl carbon is called the anomeric carbon. The isomer with the anomeric OH (shown in blue) pointed down is the alpha anomer. The isomer with the anomeric OH (shown in blue) pointed up is the beta anomer. (Figure 12.14)... 

Mechanism 23-1 Formation of a Cyclic Hemiacetal 1108 23-7 Anomers of Monosaccharides Mutarotation 1112 23-8 Reactions of Monosaccharides Side Reactions in Base 1114... 

Monosaccharides Form Rings as a Consequence of Internal Hemiacetal or Hemiketal Formation. 

Monosaccharides normally exist as cyclic hemiacetals rather than as open-chain aldehydes or ketones. The hemiacetal linkage results from reaction of the carbenyl group witli an -OH group three or four carhou atoms away. A nvu-mrfn]b r cyclic hemiacetal is called a furanose, and a six-mcnibered cyclic hemiacetal is called a pyranose. Cycltzation leads to th formation of a new chirality center and production of two dia tereomerlc hemiacetaU, called a and p anomers. 

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