Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Monosaccharide hemiacetal forms

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). [Pg.985]

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. [Pg.97]

Monosaccharides commonly form internal hemiacetals or hemiketals, in which the aldehyde or ketone group joins with a hydroxyl group of the same molecule, creating a cyclic structure this can be represented as a Haworth perspective formula. The carbon atom originally found in the aldehyde or ketone group (the anomeric carbon) can assume either of two configurations, a and /3, which are interconvertible by mutarotation. In the linear form, which is in equilibrium with the cyclized forms, the anomeric carbon is easily oxidized. [Pg.247]

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-... [Pg.643]

When a monosaccharide exists in the heterocyclic intramolecular hemiacetal form, the size of the ring is indicated by the suffixes -furanose, -pyranose, and -septanose for five-, six-, and seven-membered rings, respectively. [Pg.85]

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. [Pg.1045]

In the structures, introduced so far, an O4 rhomb or O5 patterns turned out to be ideal building blocks for carbohydrate-metal complexes. The most important monosaccharide, D-glucose, in any of its hemiacetal forms does not exceed the simple 02 diol pattern ( a consequence is the epimerization reaction of glucose to 8-mannofuranose in the presence of trivalent metal ions). [Pg.1128]

All these facts indicate the same thing (4 )-maltose contains a carbonyl group that exists in the reactive hemiacetal form as in the monosaccharides we have studied. It contains only one such "free carbonyl group, however, since... [Pg.1112]

It is worth to note that P-acetoxy-a-diazoesters were also readily accessible through the reaction of monosaccharides in their hemiacetal form. However, such a reaction required the use of an equimolecular amount of diethylzinc creating likely very reactive zinc diazointermediates which react smoothly with aldehydes [l 02]. Applying this procedure KO and its 3-epimer were prepared, starting from 2,3 5,6-di-0-isopropylidene-D-mannofuranose [101]. [Pg.448]

It is obvious that monosaccharides in their hemiacetal form are the best substrates if they are able for the introduction of a-keto acid unit at the anomeric center. These substrates do not require tedious multistep selective protection-deprotection procedures to liberate the carbonyl function. A series of reactions were examined, what succeeded in developing of diverse routes to the construction of 3-deoxy-ulosonic acids. [Pg.452]

In Section 4.3, we learned that hemiacetals and hemiketals can react with alcohols in acid solutions to yield acetals and ketals, respectively. Thus, cyclic monosaccharides (hemiacetals and hemiketals) readily react with alcohols in the presence of acid to form acetals and ketals. The general name for these carbohydrate products is glycosides. The reaction of a-D-glucose with methanol is shown in Reaction 7.6. [Pg.244]

Dissolved in an aqueous environment, monosaccharides exist primarily in the cyclic or hemiacetal form, which is more stable in solution than the open chain form. If an aldohexose is the precursor of cyclic hemiacetal, it forms a six-membered pyranoside (Figure 3) if a ketohexose or aldopentose is the precursor of the cyclic hemiacetal, a five-membered furanoside is formed. [Pg.138]

When carrying out reactions with monosaccharides, it is usually important to prevent these isomerizations and thereby to preserve the stereochemistry at all of the chirality centers. One way to do this is to convert the monosaccharide to the methyl glycoside first. We can then safely carry out reactions in basic media because the aldehyde group has been converted to an acetal and acetals are stable in aqueous base. Preparation of the methyl glycoside serves to protect the monosaccharide from undesired reactions that could occur with the anomeric carbon in its hemiacetal form. [Pg.990]

Furanose A five-membered cyclic hemiacetal form of a monosaccharide. [Pg.591]

As we saw in Section 12.6A, treating an aldehyde or a ketone with one molecule of alcohol yields a hemiacetal, and treating the hemiacetal with a molecule of alcohol yields an acetal. Treating a monosaccharide, all forms of which exist as cyclic hemiacetals, with an alcohol gives an acetal, as illustrated by the reaction of jS-D-glucopyranose (jS-D-glucose) with methanol ... [Pg.596]

A six-membered cyclic hemiacetal form of a monosaccharide is called a pyranose a five-membered cyclic hemiacetal form is called a furanose. [Pg.607]

The penultimate carbon of an acyclic monosaccharide becomes the anomeric carbon in the cyclic hemiacetal form of the molecule. (17.3)... [Pg.608]

We learned earlier that alcohols undergo rapid and reversible addition to the carbonyl group of aldehydes and ketones, to form hemiacetals (review Sec. 9.7). This can happen intramolecularly when the hydroxyl and carbonyl groups are properly located in the same molecule (eqs. 9.14 and 9.15), which is the situation in many monosaccharides. Monosaccharides exist mainly in cyclic, hemiacetal forms and not in the acyclic aldo- or keto-forms we have depicted so far. [Pg.466]

Mutarotation in Solution. In solution, equilibrium exists between the linear conformation and the two anomeric forms of pyranose and fiiranose cyclic hemiacetal forms (Fig. 9) the percentage of each form as well as the ratio a/p is characteristic of the monosaccharide considered. [Pg.6542]

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. [Pg.788]

Other monosaccharides also form five-membered cyclic hemiacetals. Following are the five-membered cyclic hemiacetals of fructose. [Pg.1097]

In the same way that a hemiacetal reacts with an alcohol to form an acetal (Section 17.12), the cyclic hemiacetal formed by a monosaccharide can react with an alcohol to form two acetals. [Pg.1034]


See other pages where Monosaccharide hemiacetal forms is mentioned: [Pg.989]    [Pg.123]    [Pg.136]    [Pg.291]    [Pg.28]    [Pg.1085]    [Pg.116]    [Pg.989]    [Pg.244]    [Pg.600]    [Pg.1100]    [Pg.2]    [Pg.1300]    [Pg.202]   
See also in sourсe #XX -- [ Pg.873 ]




SEARCH



Hemiacetal

Hemiacetalization

Monosaccharides hemiacetal

© 2024 chempedia.info