Carbon acyclic monosaccharides

Although cyclic and acyclic carbonates of monosaccharides have been known for some time, little interest had been taken in such derivatives of polysaccharides, probably because the conditions could not be controlled to give any degree of specificity. The formation of cyclic carbonate rings involves treatment of the polysaccharide with ethyl chloro-formate heterogeneous reactions occur, and acyclic carbonate (ethy-oxycarbonyl) groups are also formed. The reaction conditions suitable for maximum cyclic carbonate and minimum acyclic substitution have been investigated in detail for cellulose. Other chloroformates have... 

The penultimate carbon of an acyclic monosaccharide becomes the anomeric carbon in the cyclic hemiacetal form of the molecule. (17.3)... 

The basis for the name is the structure of the parent monosaccharide in the acyclic form. Charts I and IV (2-Carb-10) give trivial names for parent aldoses and ketoses with up to six carbon atoms. 2-Carb-8.2 and 2-Carb-10.3 describe systematic naming procedures. 

The reversible reactions are initiated by an equilibrium between neutral and ionized forms of the monosaccharides (see Fig. 6). The oxyanion at the anomeric carbon weakens the ring C-O bond and allows mutarotation and isomerization via an acyclic enediol intermediate. This reaction is responsible for the sometimes reported occurrence of D-mannose in alkaline mixtures of sucrose and invert sugar, the three reducing sugars are in equilibrium via the enediol intermediate. The mechanism of isomerization, known as the Lobry de Bruyn-... 

Monosaccharides have many structural variations that correspond to local minima that must be considered. Acyclic carbohydrates can rotate at each carbon, and each of the three staggered conformers is likely to correspond to a local minimum. The shapes of sugar rings also often vary. Furanose rings usually have two major local minima and a path of interconversion. Experimental evidence shows a clear preference for only one chair form for some pyranose rings, but others could exist in several conformers. For exanqple, the and conformers must all be considered as possible structures for L-iduronate, as discussed by Ragazzi et al. in this book. 

The approach of Kunieda ami Takizawa is unique, in that elements of the carbon skeleton of the monosaccharide molecule form an acyclic frame up to the very final stage of the synthesis, and yet a high degree of selectivity is achieved, because of the all-tru ns geometry of the starting telomers. On the other hand, this situation limits the range of sugars synthesizable by this method. Only half of the aldohexoses,... 

Less than one percent of each of the monosaccharides with five or more carbons exists in the open-chain (acyclic) form. Rather, they are predominantly found in a ring form, in which the aldehyde (or ketone) group has reacted with an alcohol group on the same sugar. 

D-(+)-galactose (15) is an example of the consecutive numbering of the carbon ring atoms in a monosaccharide (disaccharide see p. 253). Carbohydrates can exist in a cyclic and an acyclic structure. For this reason there is a special position in the structure of a monosaccharide, the carbon atom C-l and so called anomeric center. You can see that there is an equilibrium between er-anomer a-15 and / -anomer /3-15 of D-(+)-glucopyranose over the acyclic aldehyde structure 16. Both are cyclic hemi-acetals. The /Tanomer is the preferred conformation, but there are a few effects, like sterical or stereoelectronical effects (anomeric effect, inverse anomeric effect), which have influence on the a /i rate. 

The monosaccharide D-glucose, whose chemistry is representative of all aldoses containing four or more carbon atoms, exists predominantly in the two pyranosc forms 4 and 5. These are six-membered hemiacetals formed by the reversible cyclization of the acyclic polyhydroxy aldehyde 3 (Eq. 23.1). In the cyclic forms 4 and 5, the ring carbon that is derived from the carbonyl group is referred to as the anomeric carbon atom. The specific rotation, [a] (Sec. 7.5), of a-D-(+)-glucose (4) is +112 whereas that of the -anomer 5 is +19°. When crystals of either pure 4 or pure 5 are dissolved in water, the [a]p changes to an equilibrium value of +52.7°. This process is termed mutarotation. At equilibrium in water, the a- and p-forms are present in the ratio of 36 64 only about 0.03% of D-glucose is in the acyclic form 3. 

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