Cyclic hemiacetals monosaccharides

Some monosaccharides also exist in a five-mem be red cyclic hemiacetal form called a furanose form. D-Fructose, for instance, exists in water solution as 70% /Tpvranose, 2% a-pyranose, 0.7% open-chain, 23% /3-furanose, and 5% a-furanose. The pyranose form results from addition of the -OH at C6 to the carbonyl group, while the furanose form results from addition of the —OH at C5 to the carbonyl group (Figure 25.5). 

Monosaccharides normally exist as cyclic hemiacetals rather than as open-chain aldehydes or ketones. The hemiacetal linkage results from reaction of the carbonyl group with an —OH group three or four carbon atoms away. A... 

This is a modified form of the 1980 recommendations [4]. Priority is now given to naming cyclic forms, since in most cases branched-chain monosaccharides will form cyclic hemiacetals or hemiketals. 

If the branched monosaccharide forms a cyclic hemiacetal or hemiketal, the chain which includes the ring atoms rather than any alternative open chain must be the basis of the name. Otherwise the parent is chosen according to the principles given in 2-Carb-2.1. 

The aldehyde or ketone group of monosaccharides can undergo an intramolecular reaction with one of its own hydroxyl groups to form a cyclic, hemiacetal, or hemiketal structure, respectively (Figure 1.26). In aqueous solutions, this cyclic structure actually predominates. The open-chain aldehyde or ketone form of monosaccharides is in equilibrium with the cyclic form, but the open structure exists less than 0.5 percent of the time in aqueous environments. It is the... 

Monosaccharide structures may be depicted in open-chain forms showing their carbonyl character, or in cyclic hemiacetal or hemiketal forms. Alongside the Fischer projections of glucose, ribose, and fructose shown earlier, we included an alternative... 

The cyclic hemiacetal and hemiketal forms of monosaccharides are capable of reacting with an alcohol to form acetals and ketals (see Section 7.2). The acetal or ketal product is termed a glycoside, and the non-carbohydrate portion is referred to as an aglycone. In the nomenclature of glycosides we replace the suffix -ose in the sugar with -oside. Simple glycosides may be synthesized by treating an alcoholic solution of the monosaccharide with an acidic catalyst, but the reaction mixture usually then contains a mixture of products. This is an accepted problem with many carbohydrate reactions it is often difficult to carry out selective transformations because of their multifunctional nature. 

Dehydrations have been observed to be caused by water-accepting reagents. Thus cyclic hemiacetal 259 was dehydrated with oxalic acid to 2,2,4-trimethyl-2//-pyran (260) in a 50% yield289 (for similar examples see Section IV,G,1 and IV,G,4). The dehydration of easily accessible hemi-acetals 263 to appropriate condensed 4//-pyrans, e.g., 263a - 27 and 263b - 264, proceed smoothly in an acidic medium.59,290 Similarly, dehydration 28 - 27 has been mentioned in Section III,A. A more complex dehydration agent consisting of phthalimide, triphenylphosphine, and diethyl azodicar-boxylate was used for the transformation of stereoisomeric unsaturated monosaccharides 261 to 2//-pyran 262 (15 to 26%).291... 

The halogen in the acylglycosyl halide is reactive and may be readily displaced, for example, by an alkoxy group on reaction with an alcohol under anhydrous conditions in the presence of a silver or mercury(n) salt. In this case the products are glycosides which are the mixed cyclic acetals related to the cyclic hemiacetal forms of the monosaccharides. In the case of the D-glucose derivative shown below (and of other 1,2-cis acylglycosyl halides) the replace-... 

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)... 

Because glucose and the other monosaccharides contain both a carbonyl group and hydroxy groups, they exist predominantly in the form of cyclic hemiacetals. 

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... 

Drawing Cyclic Monosaccharides Cyclic hemiacetal structures may seem complicated at first glance, but they can be drawn and recognized by following the process illustrated in Figure 23-6. 

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. 

Cyclization forms the more stable ring size in a given molecule. The most common monosaccharides, the aldohexoses like glucose, typically form a pyranose ring, so our discussion begins with forming a cyclic hemiacetal from D-glucose. 

Because monosaccharides contain OH groups, they undergo reactions typical of alcohols—that is, they are converted to ethers and esters. Because the cyclic hemiacetal form of a monosaccharide contains an OH group, this form of a monosaccharide must be drawn as the starting material for any reaction that occurs at an OH group. 

The third descriptor for monosaccharides only comes into play when sugars internally react to form rings. Monosaccharides react internally by nucleophilic attack of the carbonyl group by a OH group, to form a cyclical hemiacetal . 

The carbonyl and alcohol groups within the same monosaccharide may react together if the carbon chain is long enough. The result is a cyclic hemiacetal. A new chiral center is formed at the carbon which was previously the carbonyl. The two optical isomers that can result are called anomers. Five- and six-membered cyclic structures predominate with the alcohol oxygen as the last member of the ring. These are referred to as furanoses and pyranoses, respectively. Cyclic structures exist in equilibrium with the open-chain form. 

Hemiacetals and hemiketals react with alcohols to form acetals and ketals, respectively. When the cyclic hemiacetal or hemiketal form of a monosaccharide reacts with an alcohol, the new linkage is called a glycosidic linkage, and the compound is called a glycoside. 

Hemiacetals and hemiketals are readily formed in carbohydrates. Monosaccharides contain several hydroxyl groups and one carbonyl group. The linear form of a monosaccharide quickly undergoes an intramolecular reaction in solution to give a cyclic hemiacetal or hemiketal. 

Two different procedures can be used to determine what size ring a monosaccharide forms. In the first procedure, treatment of the monosaccharide with excess methyl iodide and silver oxide converts all the OH groups to OCH3 groups (Section 22.12). Acid-catalyzed hydrolysis of the acetal then forms a hemiacetal, which is in equilibrium with its open-chain form. The size of the ring can be determined from the structure of the open-chain form because the sole OH group is the one that had formed the cyclic hemiacetal. 

The actual structure of monosaccharides such as o-glucose is not what has been shown up to this point. We previously learned that aldehydes and ketones react reversibly with alcohols to form hemiacetals and hemiketals, respectively. (Refer back to Sec. 14.7 to review this reaction.) This reaction takes place intramolecularly with monosacchardes since both OH and C = O are present in the same molecule. The structures of aldoses and ketoses are cyclic hemiacetals and hemiketals, respectively. 

Ans. The cyclic hemiacetal and hemiketal structures have no carbonyl function but are in equilibrium with the open-chain carbonyl structure. As the small amount of the open-chain carbonyl structure reacts, some of the a- and /3-anomers undergo ring-opening to replenish the equilibrium concentration of the open-chain structure. The newly formed open-chain structure undergoes reaction. More a- and -anomers open up. There is a continuous shifting of a- and /3-anomers to the open-chain carbonyl structure until all of the monosaccharide initially present has reacted. 

---