Palleros
Carbohydrates
1. Carbohydrates
1
Daniel Making Rings with Sugars 2. Haworth Projections 3. Going from the Fischer to the Haworth Projection 4. Going from the Haworth Projection to the Chair Conformation 5. Definition of and Anomers 6. Chair Conformations 7. Drawing Furanose Rings 8. Rotating Sugars 9. The Anomeric Effect 10. The Many Forms of Sugars 11. L sugars 12. Epimerization and Isomerization of Sugars 13. Other Important Carbohydrates 14. Biological Glycosides
14.1 Alcoholic glycosides 14.2 Phenolic glycosides 14.3 Couma rin glycosides 14.4 Cyanogenic glycosides 14.5 Cardiac glycosides 14.6 Flavonoid glycosides 14.7 Sa ponins
15. N-glycosides 16. Shorthand Notation of Sugars Additional Problems Answers to Additional Problems
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Daniel Palleros
Carbohydrates
Carbohydrates are essential biomolecules that provide the fuel that makes life possible. They are also the building blocks of structural elements such as the cell wall of bacteria and the exoskeleton of insects and crustaceans. Carbohydrates are also used by Nature to increase the solubility of other biomolecules that get attached to them. This increase in solubility facilitates their transport and excretion in the organism. You will encounter carbohydrates repeatedly in biochemistry and medicinal chemistry. The goal of this chapter is to provide you with a deeper understanding of these important compounds. When we are finished, you should be able to take any monosaccharide and draw it with ease in the Fischer and Haworth projections, chair conformations, furanose and pyranose forms and view it from different perspectives. You will also get familiar with some biological aspects of their chemistry, including isomerization reactions, biological glycosides and N-glycosides. To get started, you should first review the following concepts discussed in Chemistry 108B: aldoses, ketoses, pentoses, hexoses, Fischer projection, chair conformations, sugar tree (conformation of aldohexoses), D and L sugars, anomers, pyranoses, furanoses, hemiacetals and glycosidic bonds (sections 25.1-25.6 in McMurrys Organic Chemistry, 7th ed.) You should then proceed to the supplemental material discussed here (sections 1-16). The following sections in McMurrys Organic Chemistry (7th ed.) must also be read: 25.7 The Eight Essential Monosaccharides 25.8 Disaccharides 25.9 Polysaccharides and Their Synthesis 25.10 Some Other Important Carbohydrates
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1. Making Rings with Sugars
When one molecule of an alcohol reacts with one molecule of an aldehyde or a ketone, a hemiacetal is formed. The reaction is reversible and usually (but not always) requires the addition of an acid catalyst, HA. Under normal conditions the equilibrium is shifted toward reactants.
O R-OH an alcohol + R' C R" HA R' OH C R" + H2O OR a hemiacetal
an aldehyde or a ketone
When the carbonyl group of a sugar reacts with one of the sugars hydroxyl groups, a cyclic hemiacetal is obtained. Only five- and six-membered rings are stable enough to be formed; they are called furanoses and pyranoses, respectively. In fact these cyclic hemiacetals are so stable that the reaction is shifted toward products; furthermore, no added catalyst is needed. For aldoses, if the OH on carbon 4 reacts with the carbonyl, a furanose is obtained; if the OH on carbon 5 reacts instead, a pyranose is obtained. This is illustrated below with D-aldohexoses and D-aldopentoses. For clarity, some of the C-H bonds have been omitted. To make it general, the stereochemistry of some of the carbon atoms was left undetermined and represented by squiggly bonds.
Aldoses
closed Fischer projections anomeric carbon
H
1 2 3 4 5
O H OH OH OH OH
1
OH H
1
OH
2 3 4 5
OH OH O OH
2 3 4 5
OH OH OH O
6 CH2OH a D-aldohexose
6 CH2OH furanose of a D-aldohexose
6 CH2OH pyranose of a D-aldohexose
closed Fischer projections anomeric carbon
H
1 2 3 4
O
1
H
OH OH OH
1
OH
H OH
2 3 4
OH OH O
2 3 4
OH OH OH
5 CH2OH
5 CH2OH
5 CH2O
a D-aldopentose
furanose of a D-aldopentose pyranose of a D-aldopentose
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Daniel Palleros
For 2-ketohexoses, such as fructose, if the OH on carbon 5 reacts, a furanose is obtained; if the OH on carbon 6 reacts instead, a pyranose is obtained.
2-Ketoses
closed Fischer projections anomeric carbon
1 CH2OH 2 3 4 5
O OH OH OH
1 CH2OH 2 3 4 5
OH OH OH O
1 CH2OH 2 3 4 5
OH OH OH OH
6 CH2OH
a D-2-ketohexose
6 CH2OH
6 CH2O
furanose of a D-2-ketohexose pyranose of a D-2-ketohexose
Note that rings drawn in the Fischer projection (called closed Fischer projection) look awkward as one of the C-O bonds has been unrealistically stretched to close the ring. For this reason, chemists prefer to represent rings in the chair conformation or the Haworth projection (discussed in the next section) rather than in the closed Fischer projection. Some structural features of furanoses and pyranoses are worth remarking: the ring begins at the anomeric carbon, which can be carbon 1 (aldoses) or carbon 2 (2-ketoses); the anomeric carbon is easily identifiable by virtue of being the only carbon simultaneously attached to an OH group and the oxygen of the ring. Note that in aldoses, the anomeric carbon has also a hydrogen atom attached but in ketoses there is a CH2OH group (carbon 1) instead. These groups have been circled in the above structures. Other groups that are also dangling from the ring, but at the end of the chain rather than at the beginning, have been boxed in the above structures (terminal CH2OH and terminal CHOH-CH2OH). Also note that some pyranoses lack the terminal CH2OH group, such as those of aldopentoses and 2-ketohexoses. In principle, a carbohydrate can exist in both the pyranose and the furanose forms; it has been found experimentally that a given sugar exists in a particular and unique proportion of both forms in normal aqueous solutions. This will be discussed in section 10.
2. Haworth Projections
The Haworth projection is a convenient but simplified way of representing six-membered rings. In this projection the ring is represented as being planar and the substituents are either up or down. There are no axial or equatorial positions. D-glucose in the chair conformation and the Haworth projection is shown below.
Carbohydrates
5
Glucose
Daniel Palleros
Haworth Projection
OH H
6
CH2OH
4
HO HO
6 5
H
O
5
H
O H OH H
H
1H 3
H H
4
OH
1
OH
2
OH OH
H
3
2
OH
Note that the Haworth projection retains the same information as the chair about the substituents being above or below the plane but it does not say if the substituents are axial or equatorial. Thus, the OH group on carbon 1 is down (and axial) in the chair and down in the Haworth projection. The H atom on the same carbon is up (and equatorial) in the chair and up in the Haworth projection, and so on. Like with the chair conformation, drawing the H atoms attached to carbon is optional and oftentimes those hydrogens are omitted. The thick lines that indicate bonds closer to the viewer are also often omitted, see below.
Glucose
CH2OH O OH OH OH OH OH OH OH OH CH2OH O
Haworth projections
3. Going from the Fischer to the Haworth Projection
In this section you will learn how to transform an aldohexose from the Fischer to the Haworth projection. The steps outlined here can easily be generalized to transform any pyranose, not just an aldohexose, from the Fischer to the Haworth projection. Step 1. Draw the molecule in the Fischer projection. D-glucose in the Fischer projection is shown below. Recall that in this projection, horizontal bonds are coming towards the viewer and vertical bonds are going away. Therefore, if we look directly at any of the carbons, the rest of the molecule will be curling inside the page, away from us.
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D-Glucose Fischer projection 1 CHO 1 CHO
OH H OH H HO
Daniel Palleros
H HO H H
2 3 4
2 3
1 CHO
OH H
H 2
HO
OH H
4
H H OH
3 6
H OH
5
OH
5
OH
HOH2C H
4 5
OH
6 CH2OH
6 CH OH 2
Step 2. Take the Fischer projection and turn it on its side by rotating the whole molecule 90o, so that all the C atoms lie in the same plane, perpendicular to the page.
1 CHO H 2
HO OH H
turn on its side by rotating 90o 4
5
OH
6
CH2OH
3 6
H OH
OH
CHO 1 2
OH
HOH2C H
4 5
OH
OH
3 down
Figure 1 The OH groups on the right in the Fischer projection go down when the molecule is turned on its side. The OH groups on the left in the Fischer projection go up. The mnemonic If it is left up to me, I would be down right angry may help you remember this. Note that the OH on the farthest stereogenic carbon (the OH that defines the D series), C5, is pointing down when the molecule is turned on its side (this will be important later). For clarity, the C-H bonds have been omitted. Step 3. By rotating the appropriate carbon-carbon bond, place the OH that will form the hemiacetal (the OH on C5 in the case of aldohexoses) in the same plane as the carbon atoms and pointing toward the carbonyl group. In the case of D-glucose we do this by a 90o counterclockwise rotation of the C4-C5 bond. This puts the OH on C5 in the same plane as C1-C5 and also moves C6 up, above the plane of the molecule.
5
OH
6
CH2OH OH
CH2OH
6
rotate C4-C5 bond CHO 1 2
OH
5 4
OH
OH OH
4
OH
by 90o
CHO 1 2
OH
3
3
Figure 2
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Daniel Palleros
Step 4. We close the ring by reacting the OH with the carbonyl to form the cyclic hemiacetal. The hemiacetal OH that results from the reaction, called the anomeric OH, can point up or down. Two anomers are formed. Anomers are discussed in section 5.
Haworth projections of D-g lucose
CH2OH
6
5 4
OH
OH OH
6 CH2OH form hemiacetal between 4 CHO 1 OH on C5 and C1 2
OH OH
6 CH O H 2
O HO
5
HO
4
5
HO
O
+
1 2
HO OH
1 2
HO OH
3
3
3
anomeric C with two possible configurations
Figure 3 Note that the Haworth projections of D-glucose have C1, C2, C3, C4, C5 and the oxygen atom all in the same plane. By convention, the oxygen atom usually occupies the topright corner and C1 is drawn on the right. The molecule can, of course, be rotated and represented from different perspectives (this is discussed in section 8) but the standard representation is the one shown in Figure 3 with the oxygen in the top-right corner and carbon 1 on the right. In this representation, D-aldohexoses have the terminal CH2OH group (C6) pointing up and L-aldohexoses have the terminal CH2OH group pointing down as indicated in the figure below (L sugars are discussed in section 11.)
Scaffolds for D and L-aldohexoses in the Haworth projections 6 5 4 1 3 D 2
CH2OH O
5 4 6 3 L
CH2OH
O
1 2
Figure 4 In summary, to go from the Fischer projection to the Haworth projection for any pyranose we follow step 1 though step 4. For aldohexoses, we can take a shortcut. We first decide if the aldohexose is D or L. For D-aldohexoses we draw the scaffold as shown in Figure 4, left, and for L-aldohexoses we draw the scaffold as shown on the right. Then, to place the OH groups on carbons 2-4, we follow the rule: right in the Fischer projection, down in the Haworth projection; left in the Fischer projection, up in the Haworth projection. For C1, the anomeric C, two configurations are possible giving rise to two anomers.
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Daniel Palleros
Example I, D-galactose:
Fischer projection 1 CHO Haworth projections
H HO HO H
2 3 4 5
OH H
4
6
CH2OH O OH
1 2
6
OH 5 HO
3
CH2OH O
1 2
OH 5
4
H OH
HO
3
OH
OH
OH
6 C H 2O H anomers of D-galactose D-galactose
Example II, L-mannose:
Fischer projection 1 CHO Haworth projections
H H HO HO
2 3 4 5
OH OH OH
4
5 6 C H 2O H 3
O
OH
1 2
OH 5
4 6 CH2OH 3
O
1 2
H H HO OH
OH
HO
OH
6 CH2OH anomers of L-mannose L-mannose
4.Going from the Haworth Projection to the Chair Conformation
The carbon atoms in a saturated six-membered ring are never in the same plane as the Haworth projection shows, but rather alternate above and below an imaginary plane in a conformation known as the chair. To go from the Haworth projection to the chair, move C4 up and C1 down. Move the atoms adjacent to those carbons in the opposite direction: C3 and C5 (adjacent to C4) down, and C2 and the oxygen (adjacent to C1) up.
6
CH2OH
5
H
O H OH
OH
H
6
CH2OH H
4
OH
4
H
1 2
H
5 2
O
HO HO
1 OH 3
H H OH H
3
H
OH
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Daniel Palleros
To draw the perfect chair keep these ideas in mind: All axial bonds must be perfectly vertical. The axial bonds follow the carbons: if the carbon points up, the axial bond points up; if the carbon points down, the axial bond points down. An equatorial bond points in the opposite direction to the axial bond: if the axial bond points down, the equatorial bond attached to the same carbon points up, and vice versa. All equatorial bonds are slanted and should not be drawn inside the ring. To place the substituents in the chair, follow the simple rule: if a substituent is up in the Haworth projection, it must be up in the chair (it can be axial or equatorial, depending on the carbon); if a substituent is down in the Haworth projection, it must be down in the chair (it can be axial or equatorial depending on the carbon).
5. Definition of and Anomers
The general definition and of anomers says that an anomer has the anomeric OH and the oxygen on the farthest stereogenic carbon (C5 in the case of hexoses) on the same side in the Fischer projection. Conversely, the anomer has the anomeric OH and the oxygen on the farthest stereogenic carbon on opposite sides in the Fischer projection. This definition applies to both D and L sugars. The two anomers of D-glucose in the closed Fischer projection are shown below. Anomers of D-glucose in the closed Fischer projection
1 2 3 4 5 1
HO H HO H H H
H H HO H H
OH OH H OH O
2 3 4 5
OH H OH O
6 C H 2O H same side: "
6 C H 2O H opposite sides: !
Since sugars are hardly ever represented in the closed Fischer projection, to actually apply this definition we must be able to transform the closed Fischer projection into Haworth projections, back and forth. To go from a closed Fischer projection to the Haworth projection we follow the steps outlined in section 3. For aldohexoses, we first draw the scaffold for the sugar (D or L) as shown in Figure 4 and then position the OH groups following the usual rule: right in the Fischer projection, down in the Haworth projection; left in the Fischer projection, up in the Haworth projection. This also applies to the anomeric OH.
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Anomers of D-glucose in the Haworth projection 6 5
H CH2OH O H OH H H H
Daniel Palleros
6 CH O H 2 5 1
OH O H OH H OH
4
OH
4
OH
1 2
H
H
3 ! anomer
2
OH
H
3
down " anomer
OH
Note that in agreement with the general definition of anomers, the anomer has the anomeric OH pointing down in the Haworth projection like the OH on the farthest stereogenic center, C5, was in the Fischer projection turned on its side (right structure in Figure 1, page 6). As precise as the general definition of and anomers is, it is sometimes hard to apply because, as already mentioned, it requires transforming the sugar, from the Fischer to the Haworth projection. Thus, other definitions that can be directly applied to rings are often used. A second definition of and anomers that is found in many textbooks, including the 6th edition of McMurrys Organic Chemistry (but not in the 7th) and which can be considered a useful shortcut of the general definition, says that the anomer has the anomeric OH and the last CH2OH group trans to each other in the Haworth projection or chair conformation, and the anomer has the two groups cis. This definition is true for D and L sugars.
D-glucose
trans: ! anomer
CH2OH H O H OH H OH H OH OH H OH H H CH2OH O H OH H
cis: " anomer
OH
OH
H
! anomer
" anomer
This definition is very useful for rings but has limitations because it relies on the presence of a terminal CH2OH group which may be absent in some sugars such as arabinopyranose (discussed below). There is a third definition of and anomers that can be used as long as the sugar is drawn in the standard representation with the oxygen atom in the top and carbon 1 on the right. According to this definition, the anomer of a D sugar has the anomeric OH pointing down and the anomer pointing up.
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Daniel Palleros
Scaffold for D hexopyranoses
CH2OH O CH2OH O OH
pointing up
OH
! anomer
pointing down
" anomer
For L sugars, it is the opposite.
Scaffold for L hexopyranoses pointing up
O CH2OH OH C H 2O H OH O
! anomer
" anomer
pointing down
An important application of the third definition is that it can be used for sugars that lack a terminal CH2OH group such as arabinopyranose. D-arabinopyranose is D-arabinose, a pentose, in which the six-membered ring is formed between carbon 1 and the oxygen on the last carbon (C5). The and anomers in the Haworth projections are shown below. Note the absence of a terminal CH2OH group. The anomer has the anomeric OH pointing down and the anomer pointing up.
5 4
OH OH O OH
1
4
OH
5 3
OH OH O OH
OH
3
2
!-D-arabinopyranose
down D series conventional representation
1
!
2
up D series conventional represenation
!
!-D-arabinopyranose
A limitation of the third definition is that the sugar must be in the standard representation. If the molecule has been rotated (rotation of sugars is explained in section 8) this definition may not apply. When applied properly all three definitions must lead to the same result. Whether we apply one or the other should be a matter of convenience. For example, in the case of arabinopyranose we cannot apply the second definition because the molecule lacks a terminal CH2OH group but we can apply the first and the third definition, which lead to the same result. We have already seen the third definition in action with arabinopyranose, lets now apply the first. The farthest stereogenic carbon is carbon 4. In the anomer both the oxygen on the farthest stereogenic carbon and the anomeric OH point in the same direction as the general definition demands. In the anomer they point in opposite directions.
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!-D-arabinopyranose
Daniel Palleros
H HO H H
1 2 3 4
!-D-arabinopyranose
HO
OH
H OH OH O
1 2 3 4
H H OH OH O
5 4
OH OH
3
O OH
1
OH
HO H
5 4
OH OH
3
O OH
OH
1
2
H
2
5 H 2C
5 H 2C
same side: ! same side: !
opposite sides: !
opposite sides: !
All three definitions of and anomers also apply to sugars in the five-membered, furanose, rings (discussed in section 7).
6. Chair Conformations
Sugars can exist in two chair conformations in equilibrium with one another (chair flip). In one of them carbon 1 is pointing down and in the other is pointing up. In the chair with carbon 1 pointing down, D-aldohexoses have the terminal CH2OH group equatorial. This chair conformation is called C1. The other chair with carbon 1 pointing up and the CH2OH group axial is called 1C.
OH HO
46
equatorial 5 2 C1 chair pointing down
O
axial 6 5 1
O
pointing up
1 4 3 3
2
1C chair
The names C1 and 1C may seem confusing. To remember which one is each, the following phrase may help: 1 before is up, 1 after is down. This means that by placing the 1 before the C (1C) we represent the chair with carbon 1 pointing up, and by placing it after the C (C1), the chair with carbon 1 pointing down. Because a CH2OH group in the axial position reduces the stability of the chair, 1C chairs are usually (but not always) less stable than C1 chairs. Their stability ultimately depends on the position of all the substituents on the ring. As a result, D-glucopyranose exists almost exclusively in the C1 chair conformation because in this conformation all of the ring substituents occupy equatorial positions.
HO
6 5 OH
O
OH
!"D-glucopyranose 4
OH
6 5 2 3
O OH OH
1 2
OH
HO HO
4
OH
3
1
1C chair
C1 chair More stable
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Daniel Palleros
For other sugars, such as D-idopyranose, the situation is different; D-idopyranose exists mainly in the 1C chair (see below). In the C1 chair, D-idopyranose has three of its OH groups (those on carbon 2, 3 and 4) in the least stable axial positions. By being in the 1C chair, those three OH groups occupy the more stable equatorial positions and this relieves the steric repulsion caused by the axial groups (terminal CH2OH and OH on carbon 1).
!"D-idopyranose
HO
6 5
O
OH
OH OH
1 4
OH
6 5 3
OH
OH O OH
HO
4
HO
3
2
2 1
1C chair More stable
C1 chair
7. Drawing Furanose Rings
As we discussed in previous sections, sugars in a five-membered ring are called furanoses. To draw a furanose ring beginning with a Fischer projection we follow similar steps to those outlined before when drawing the Haworth projection. Lets first consider D-talose, an aldohexose. To form a furanose ring, the OH on C4 must react with the carbonyl.
1 CHO these two groups must react
HO HO HO H
2 3 4 5
H H H OH
6 CH2OH D-Talose
We turn the molecule on its side by rotating it 90o. The OH groups that are on the left in the Fischer projection go up, and those on the right go down.
1 CHO HO 2 HO 3 HO 4 OH H H H
turn on its side by rotating 90o 4
5
OH OH
CH2OH OH OH
6
CHO 1
6 HOH2C H
3
2
5
D-Talose
To form a five-membered ring, the carbonyl (C1) must react with the OH on C4 which must be in the same plane as C1, C2, C3 and C4. To that effect, the C3-C4 bond must be rotated clockwise 90o. This results in the C4-C5 bond moving down, below the plane.
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R 5
CH2OH OH OH
Daniel Palleros
6
OH OH
4 2 3
CHO 1
rotate C3-C4 90o
H HO
OH
4 5 6
CH2OH
OH
OH
CHO 1
3
2
R
The configuration at C5 must be clearly (and correctly) shown once the C3-C4 bond is rotated. The best way to assure that the substituents on C5 are in the correct positions is to assign R/S configuration to this carbon before the rotation and stick to it after the C3-C4 bond has been rotated. The configuration of C5 is R. The anomer of D-talofuranose is shown below in four different representations. The first structure is the closed Fischer projection. Note that the anomeric OH and the OH on the last stereogenic carbon, C5, are both on the same side as it should be for an anomer. In the other structures the anomeric OH is pointing down in agreement with the third definition of anomers in the D series. Note that in all four structures, C5 is R. In the last structure, C5 (and only C5) is in the Fischer projection.
1
H OH
HO HO O H
2 3 4 5
H H H
HO
H
H O HO OH
H
H O HO OH
H
H O HO OH
same side: !
H
4 5
1
OH
6
HOH2C
4 5
1
OH H
4 5
1
OH
3
OH
3 6 R
CH2OH
2
3
2
OH
2
OH
H
CH2OH
R !-D-talofuranose
R
6
6 CH2OH
R
Lets now make the furanose ring of fructose, a ketohexose. To form a furanose, the OH on C5 must react with the carbonyl.
1 CH2OH 2
HO H H O H OH OH
3 4 5
these two groups must react
6 CH2OH D-Fructose
We turn the molecule on its side by rotating it 90o.
1 C H 2O H
O
2
HO
H
6 turn on its side 5
OH CH2OH
3 6
H OH
1
CH2OH
by rotating 90o
4
OH
OH
H O H 2C H
4 5
OH
3
2
O
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Daniel Palleros
To form the five-membered ring, the carbonyl (C2) must react with the OH on C5, and thus we rotate the C4-C5 bond counterclockwise to put that OH in the same plane as C2, C3, C4 and C5.
5
OH
6
CH2OH
6 CH2OH 1
CH2OH
rotate C4-C5 bond by 90o 4
OH
5
OH OH
1
CH2OH
4
OH
OH
3
2
O
3
2
O
The two anomers are shown below. Note that in this case, because the sugar is a ketose, the anomeric carbon is C2 instead of C1 and has a CH2OH group attached instead of an H. The anomer has the anomeric OH pointing down, like the OH on C5 in the Fischer projection turned on its side (see left structure above). The anomer has the OH on C2 pointing up. Also note that the anomer has the anomeric OH and the terminal CH2OH (C6) trans to one another. These two groups are cis in the anomer. This is in keeping with the second definition of anomers.
6 CH2OH 5 4
OH OH O
1
CH2OH
6 CH2OH 5 4
OH OH O
cis: "
OH
2
OH
2 1
CH2OH
3 ! anomer
3 " anomer
trans: !
8. Rotating Sugars
Pyranoses are usually represented with the oxygen in the top-right corner, carbon 1 on the right, and carbon 4 on the left. This is what we call the standard representation. Consider, for example, -D-mannopyranose:
c H 4 HO HO 3 H
!-D-mannopyranose
b 6 CH2OH 5
OH
O a
H
2 H 1 OH
H
Out of necessity, it is often required to view a sugar molecule from a different perspective. This can be done by rotating it along one of its many axes. An axis is a line that passes through the center of the molecule as shown in the figure above. A common rotation is along the horizontal axis contained on the C1-C4 plane (plane of the page, axis a). A 180o rotation yields the following structure.
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H
4
HO HO
6 CH2OH 5
H H
H OH O rotate 180o around the axis on the C1-C4 plane
OH
3
HO HO
H
2
OH
H
1
H
a
3
2
H H
1
OH
4
H
5 6
CH2OH
O
As a result of this rotation, C1 and C4 stay in the same plane. The carbons that were closer to us (C2 and C3) move away from us and the ones farther away (C5, C6 and the oxygen) move closer to us. Substituents that were pointing up, now point down, and vice versa. Note that substituents that were axial stay axial and substituents that were equatorial stay equatorial. Another useful rotation is a 180o rotation along the axis that cuts through the C2-C3 and C5-O bonds (axis b).
H
6
CH2OH
b
OH O rotate 180o around the axis that goes through C2-C3 and C5-O bonds H H H
4
HO HO
5
H H
OH
1
O H
H
5
6
CH2OH OH
3
2
1
OH
2
OH
3
OH H
4
As a result of this rotation, C3, C4, C5 and C6 move to the right and C1, C2 and the oxygen move to the left. The carbons that were closer to us (C2 and C3) stay closer to us. Substituents that were up, are now down and vice versa; axial stays axial and equatorial stays equatorial. Another useful rotation is that along the vertical axis (axis c). A clockwise 120o rotation is shown below. In this rotation, up stays up, down stays down, equatorial stays equatorial and axial stays axial. Only the absolute position of the atoms changes and, as a result, C1 moves to the left and the oxygen moves from the top-right corner to the bottom-right corner, and so on.
c
H
4
HO HO
6 CH O H
2
OH OH O rotate 120o along the verical axis H H H
H HO
5
H H
2
H
3
O H
4
OH
3
2
5
H
CH2OH
1
OH
1
OH
6