8/2/2019 Sintesis Obat - 03 Carbohydrate
1/13
8/2/2019 Sintesis Obat - 03 Carbohydrate
2/13
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.
8/2/2019 Sintesis Obat - 03 Carbohydrate
3/13
8/2/2019 Sintesis Obat - 03 Carbohydrate
4/13
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)
8/2/2019 Sintesis Obat - 03 Carbohydrate
5/13
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
8/2/2019 Sintesis Obat - 03 Carbohydrate
6/13
8/2/2019 Sintesis Obat - 03 Carbohydrate
7/13
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.
8/2/2019 Sintesis Obat - 03 Carbohydrate
8/13
8/2/2019 Sintesis Obat - 03 Carbohydrate
9/13
8/2/2019 Sintesis Obat - 03 Carbohydrate
10/13
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
8/2/2019 Sintesis Obat - 03 Carbohydrate
11/13
8/2/2019 Sintesis Obat - 03 Carbohydrate
12/13
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
8/2/2019 Sintesis Obat - 03 Carbohydrate
13/13