sintesis obat - 03 carbohydrate

8/2/2019 Sintesis Obat - 03 Carbohydrate

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Carbohydrate

An organic compound with the empirical formula Cm(H2O)n

(where m could be different from n); that is, consists only of

carbon, hydrogen, and oxygen, with a hydrogen:oxygen

atom ratio of 2:1 (as in water).

However, there are exceptions to this. One common

example would be deoxyribose, a component of DNA,

which has the empirical formula C5H10O4.

Carbohydrates can be viewed as hydrates of carbon, hencetheir name. Structurally however, it is more accurate to view

them as polyhydroxy aldehydes and ketones.


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Classifications of Carbohydrates

Natural saccharides are generally built of simple carbohydrates called

monosaccharides with general formula (CH2O)n where n is three or

more. A typical monosaccharide has the structure H-(CHOH)x(C=O)-

(CHOH)y-H, that is, an aldehyde or ketone with many hydroxyl groups

added, usually one on each carbon atom that is not part of the

aldehyde or ketone functional group.

Examples of monosaccharides are glucose, fructose, and

glyceraldehydes. However, some biological substances commonly

called "monosaccharides" do not conform to this formula (e.g., uronic

acids and deoxy-sugars such as fucose), and there are manychemicals that do conform to this formula but are not considered to be

monosaccharides (e.g., formaldehyde CH2O and inositol (CH2O)6)


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Stereochemistry

The assignment of D or L is made according to the orientation of the

asymmetric carbon furthest from the carbonyl group: in a standard

Fischer projection if the hydroxyl group is on the right the molecule is a

D sugar, otherwise it is an L sugar. The "D-" and "L-" prefixes should

not be confused with "d-" or "l-", which indicate the direction that the

sugar rotates plane polarized light. This usage of "d-" and "l-" is nolonger followed in carbohydrate chemistry

The resulting possible pair of stereoisomers is called anomers. In the

anomer, the -OH substituent on the anomeric carbon rests on the

opposite side (trans) of the ring from the CH2OH side branch. The

alternative form, in which the CH2OH substituent and the anomeric

hydroxyl are on the same side (cis) of the plane of the ring, is called

the anomer. You can remember that the anomer is cis by the

mnemonic, "It's always better to e up". Because the ring and straight-

chain forms readily interconvert, both anomers exist in equilibrium


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Synthesis

Carbohydrate synthesis is a sub-field of organic chemistry

concerned specifically with the generation of natural and

unnatural carbohydrate structures. This can include the synthesis

of monosaccharide residues or structures containing more than

one monosaccharide, known as oligosaccharides. Scheme 1: Oligosaccharide synthesis normally consists of four

parts: preparation of the glycosyl donors, preparation of the

glycosyl acceptors with a single unprotected hydroxyl group, the

coupling of them, and the deprotection process.


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Using different bases can selectively lead to different anomeric

configurations. (Scheme 2) As to the thioglycosides, the greatest

strength is that they can offer a temporary protection to the

anomeric centre because they can survive after most of the

activation processes.Additionally, a variety of activation methodscan be employed, such as NIS/ AgOTf, NIS/ TfOH, IDCP (Iodine

dicollidine perchlorate), iodine, and Ph2SO/ Tf2O. Furthermore,

in the preparation of 1, 2-trans glycosidic linkage, using

thioglycosides and imidates can promote the rearrangement of

the orthoester byproducts, since the reaction mixtures are acidicenough


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The structures of acceptors play a critical role in the rate and

stereoselectivity of glycosylations. Generally, the unprotected

hydroxyl groups are less reactive when they are between bulky

protecting groups. That is the reason why the hydroxyl group at

OH-4 in pyranosides is unreactive. Hyperconjugation is involvedwhen OH-4 is anti-periplanar to the ring oxygen, which can also

reduce its reactivity. (Scheme 3) Furthermore, acyl protecting

groups can reduce the reactivity of the acceptors compared with

alkyl protecting groups because of their electron-withdrawing

ability. Hydroxyl group at OH-4 of N-acetylglucosaminederivatives is particularly unreactive


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sintesis obat - 03 carbohydrate

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