Transcript Document

Chapter 6
Stereochemistry
Is the study of the static and dynamic aspects of the three-dimensional shapes of molecules.
6.1 Stereogenicity and stereoisomerism
6.1.1 Basic concepts and terminology
Constitutional isomers: molecules with same molecular formular but different connectivity
between the atoms. e.g.) 1-bromo and 2-bromobutane
Stereoisomers: molecules that have the same connectivity but differ in the arrangement
of atoms in space. e.g) cis- and trans-2-butene
1. enantiomers: nonsuperimposable mirror images of each other
2. diastereomers: stereoisomers that are not enantiomers
- conformational isomers: are interconvertible by rotations about single bonds
- configurational isomers: stereochemical isomers including enantiomers and diastereomers.
configuration: the relative position or order of arrangement
of atoms in space which characterizes a particular stereoisomer.
- chiral: any object that is nonsuperimposable with its mirror images
- achiral: if an object is not chiral, it is achiral.
A molecule is achiral if it is superimposable on its mirror image.
A molecule which has a plane of symmetry, a center of symmetry
or rotation-reflection symmetry is achiral.
An axis of symmetry (C2 axis) -> achiral과 관계 없음
A molecule is achiral if it is superimposable on its mirror image.
A molecule which has a plane of symmetry, a center of symmetry
or rotation-reflection symmetry is achiral. (나중에 다시 설명)
chiral
C2
OH
Br
achiral
O
Br
OH
a plane of symmetry (s,
S1)
Br
achiral
Br
a center of symmetry (i,
S2)
meso: compounds that contain stereogenic centers but are nevertheless achiral.
Classic terminology
Optically active: refers to the ability of a collection of molecules to rotate plane polarized light
- must have an excess of one enantiomer.
Racemic mixture (or racemate): a 50:50 mixture of enantiomers and is not optically active.
However, enantiomers that do not have dramatically different refractive indices would not
result in measurable rotations.
-> in this case, they are optically inactive even though they are chiral.
따라서 optically active란 말은 사용하지 않는 것이 좋음.
Chiral center or chiral (asymmetric) carbon: an atom or specifically carbon, respectively,
that has four different ligands attached.
Chiral carbons exist in molecules that are neither asymmetric nor chiral. Many molecules can
exist in enantiomeric forms without having a chiral center.
이 말도 사용하지 않는 것이 좋음.
CO2H
H
OH
H
OH
CO2H
chiral center
achiral compound
More modern terminology
Stereocenter (stereogenic center): use this term instead of chiral center,
it is stereogenic center if the interchange of two ligands attached to it can produce a new
stereoisomer.
A non-stereogenic center is one in which exchange of any pair of ligands does not produce a
stereoisomer.
-> the term ‘stereogenic center’ is broader than the term ‘chiral center’.
A CWXYZ center does not guarantee a chiral molecule.
However, a CWXYZ group is always a stereogenic center.
CO2H
H
OH
H
OH
CO2H
stereogenic center: 두개의
치환기를 바꾸면 stereoisomers
가 생긴다
meso form
Typically, a molecule with n stereogenic, tetracoordinate carbons will have 2n stereoisomers
- 2n-1 diastereomers that exist as a pair of enantiomers.
Epimers: are diastereomers that differ in configuration at only one of the several
stereogenic centers. Carbohydrates: a- and b-anomers도 epimers의 한 형태임.
6.1.2 Stereochemical descriptors
R, S system (Cahn-Ingold-Prelog system)
2
1 R1
R2
4 R4
R3 3
1 R1
R2 2
3 R4
R3 4
S
R
rectus (right)
sinister (left)
higher atomic number: higher priority
isotopes (the one with higher mass is assigned the higher priority)
Tricoordinate -> stereogenic center
phantom atom: the lowest priority
S
CH3
CH2CH3
H3C
H3C
high energy
barrier
CH2CH3
S CH3
S
R
phantom atom: the lowest priority
P
CH3
CH2CH=CH2
R
high energy
barrier
CH2CH=CH2
P CH3
S
E, Z system
lower
higher
Opposite: E (entgegen)
(cf) same: Z (zusammen)
If an H atom is on each of the double bond,
conventionally, cis and trans can be used.
D, L system
mainly used for amino acids and carbohydrates
Fischer projection
Horizontal lines: bonds coming out of the plane of the paper
Vertical lines: bonds projecting behind the plane of the paper
The most oxidized group: top
CH2OH (carbohydrates) or R (amino acids): bottom
D: dextro, right
L: levo, left
L
D
D
D
L
Natural amino acids: L-amino acids
Important point
No direct relationship between the R/S and D/L and the sign of optical rotation of the molecule.
Helical descriptors – M, P system
Many chiral molecules lack a conventional center that can be described by R/s or E/Z.
-> typically helical, propeller, screw-shaped structures
-> a right-handed helix (clockwise): P (plus), a left handed helix (anti-clockwise): M (minus)
H
H
CH3
Cl
H3C
NO2
NO2
CH3
CH3
H3C
H
H
6.1.3 Distinguishing enantiomers
Chiral column chromatography
Enantiomeric excess = (Xa – Xb) x 100, Xa: mole fraction of a, Xb: mole fraction of b
High field NMR spectroscopy with chiral shift reagents
NMR spectroscopy of derivatives that are diastereomeric
Chromatography (HPLC and GC) with chiral stationary phases
NMR spectroscopy of derivatives that are diastereomeric
H S OH
R1
R2
F3C OMe
Ph
or
H R OH
R2
R1
eclipsed
eclipsed
H
S R2
COCl
(Mosher’s reagent)
Methods:
(R/S) racemate + (R)-MTPA-Cl
OH, NH2, SH 등
R2 O
R1 O
O
(R)-MTPA-Cl
methoxy trifluoromethyl
phenylacetyl chloride
H
R R1
OMe
O
or
OMe
F3C
F3C
S: R1 -> upfield
R: R2 -> upfield due to anisotropic effect of phenyl ring
50 : 50 (R-R-MTPA : S-R-MTPA)
R
S
ppm
R, S peak 결정
sample + (R)-MTPA-Cl
Derivatives
R
90
S
10
ee 80%
a-H
NH
D L
D
D L
L D
OMe
D
D
O
F3C S
Ph
OMe
N
H
O D,L CO2t-Bu
OTBS
> 98%ee
a-H
NH
D L
O
F3C S
Ph
OMe
N
H
O D,L CO2Bn
Me
> 98%ee
Me
D
D
L
L D
D
OMe
D
Optical activity and chirality
Optical activity: the ability of a sample to rotate a plane of polarized light.
A rotation to the right: + or dextrorotatory (d)
A rotation to the left: - or levorotatory (l)
Optical activity establishes that a sample is chiral,
but a lack of optical activity does not prove a lack of chirality.
Optical activity (a)
Specific optical activity [a]
[a]D25 -> sodium D line (589 nm emission line of sodium arc lamp)
Optical purity (%) =
[a] mixture of enantiomer
[a] pure enantiomer
x 100
6.2 Symmetry and stereochemistry
6.2.1 Basic symmetry operations
Proper rotation (Cn) -> a rotation around an axis by (360/n)o that has the net effect of leaving
the position of the object unchanged. C2; 180 rotation, C3; 120 rotation
Improper symmetry (Sn) -> rotation and reflection; involves a rotation of (360/n)o,
combined with a reflection across a mirror plane that is perpendicular to the rotation axis.
S1; just a mirror reflection (s)
S2; equivalent to a center of inversion (i)
90o
60o
180o
6.2.2 Chirality and symmetry
A necessary and sufficient criterion for chirality is an absence of Sn axes;
the existence of any Sn axis renders an object achiral.
C2
Asymmetric is defined as the complete absence of symmetry. However, many chiral molecules
have one or more proper rotation axes-just no improper axes are present. These compounds can
be referred to as dissymmetric, essential a synonym for chiral. Thus, while all asymmetric
molecules are chiral, not all chiral molecules are asymmetric.
6.3 Topicity relationship
Topicity: derived from the same roots as topography and topology, relating to the spatial
position of an object.
6.3.1 Homotopic, enantiotopic, and diastereotopic
Homotopic: is defined as interconvertable by a Cn axis of the molecule.
homotopic hydrogens homotopic hydrogens
H
HO
H
H
chiral influence cannot
distinguish these methyl groups
H
OH
achiral molecules
C2
Heterotopic: the same groups or atoms in inequivalent constitutional or stereochemical
environment.
- Enantiotopic: interconverted by an Sn axis of the molecule (n = 1 in this case).
enantiotopic groups, when exposed to a chiral influence (chiral shift reagent를 사용할 시),
become distinguishable, as if they were diastereotopic.
- diastereotopic: the same connectivity, but there is no symmetry operation that
interconverts them in any conformation. 이미 stereogenic center를 갖고 있음
the environments of diastereotopic groups are topologically nonequivalent. -> they can be
distinguished by physical probes, especially NMR spectroscopy (AB quartet)
diastereotopic
HR
HS
CO2-
NH3+
phenylalanine
meso: achiral
Me
N
Me
H
enantiotopic
H
Ph
2H
chiral
Me
N
H
Me
H
Ph
H 1 H2
AB quartet
diastereotopic
6.3.2 Topicity descriptors – Pro-R/Pro-S and Re/Si
1
O
Si face
Re face
pro-S
pro-R
pro-R
pro-S
pro-S
pro-R
R1
2
R2
3
Enzymatic reactions
pro-S H
H3C
pro-S
pro-R
H
liver alcohol dehydrogenase
H
OH
-pro-R
H3C O
ethanol
acetaldehyde
H
H3C
D or T
OH
T ro D
H3C
H
OH
alcohol dehydrogenase
D or T
H
H3C
pro-R
H H
SCoA
R
H
H O
pro-R
O
H3C
O
H
acyl-CoA dehydrogenase
- pro-R aand bH
SCoA
R
H
O
6.4 Reaction stereochemistry: stereoselectivity and stereospecificity
6.4.1 Simple guidelines for reaction stereochemistry
1. Homotopic groups cannot be differentiated by chiral reagents.
2. Enantiotopic groups can be differentiated by chiral reagents.
3. Diastereotopic groups are differentiated by achiral and chiral reagents.
6.4.2 Stereospecific and stereoselective reactions
Stereospecific reaction: one stereoisomer of the reactant gives one stereoisomer of the
product, while a different stereoisomer of the reactant gives a different stereoisomer of
product. Stereospecific reaction is a special, more restrictive case of a stereoselective
reaction.
Stereoselective reaction: one in which a single reactant can give two or more stereoisomeric
products, and one or more of these products is preferred over the others-even if the
preference is very small.
Regioselective reaction; when more than one site reacts, this reaction is one where an excess
of one of the possible products results.
stereospecific
stereoselective
stereoselective
stereospecific
inversion
Br
Me
Ph
H
Ph
H
Syn addition
Ph
anti elimination
Ph
-Ot-Bu
Br
Me
Ph
Ph
H
Ph
H
-Ot-Bu
anti elimination
anti elimination
Ph
Nu:
Rm
Rs
Rl
O
Rm
Rs
R
Rm
Nu:
O
R
Rl
preferred
Rl
R
Rm
O
Rs
Regioselective reaction
Markovnikov addition
Rs
Rl
OH
R
Nu
major
Rm
Rs
Rl
OH
Nu
R
minor
6.5 Symmetry and time scale
Time scale is important.
three Hs -> equivalent due to fast rotation
of C-C bond
three Hs -> equivalent but at low temperature (-90 oC),
inequivalent due to slow rotation (very clowded system)
achiral <- fast inversion
chiral <- slow inversion
6.8 Stereochemical issues in chemical biology
6.8.1 The linkages of proteins, nucleic acids, and polysaccharides
Proteins
planar
~19 kcal/mol
rotation barrier
~4 kcal/mol
preference
Much smaller
cis-trans preference
20 natural amino acids (L form)
achiral
HO2C
NH2
HO2C
H
HO2C
HO2C
NH2
OH
OH
NH2
NH2
HO2C
HO2C
NH2
HO2C
NH2
Tyr, Y
HO2C
NH2
NH2
CO2H
N
H
SH
HO2C
HO2C
H
Pro, P
Cys, C
N
H
OH
Phe, F
NH2
Ile, I
Leu, L
SCH3
Met, M
Thr, T
Ser, S
HO2C
HO2C
Val, V
Ala, A
NH2
HO2C
NH2
CH3
Gly, G
HO2C
HO2C
NH2
NH2
NH2
H
N
N
Trp, W
NH2
Lys, K
His, H
NH2
HO2C
NH
NH2
HO2C
NH2
HO2C
NH2
NH2
CONH2
CO2H
CONH2
CO2H
H2N
Arg, R
HO2C
NH
Asp, D
Glu, E
Asn, N
Gln, Q
3’
Nucleic acids
H
N H
5’
N
N
H3C
O P O-
3’
O
O
H
H N
A=T
G≣C
O
N
5’
N
N
N
N
O
O P O
H N
H
N H
O
-
N
O
O
O
N
H N
N
O
Phosphodiester bonds
O
O
O
OO P O-
HO
HO
5'
1'
O
4'
3'
5'
OH
2'
HO (OH)
2'-deoxyribose
ribose
Base
Base
O
4'
3'
1'
2'
HO (OH)
O
5'
Base
O
4'
3'
1'
2'
HO (OH)
2'-deoxyribose
ribose
2'-deoxyribose
ribose
Nucleosides
Nucleotides
Nucleic acid
(RNA or DNA)
Bases
b-glycosidic linkage
Phosphodiester linkages
Carbohydrates
Functional Glycomics
Functional Glycomics
Carbohydrate-protein Interactions
Toxin
• Structural and functional studies
of whole carbohydrates
• Studies of carbohydrate-protein
interactions
Bacteria
Antibody
Hormone
• Understanding biological processes
Virus
• Development of therapeutic agents
Protein
(Tumor) Cell
Biological processes
- Inhibitors for carbohydrate biosynthesis
- Inhibitors for carbohydrate-binding proteins
- Carbohydrate-based vaccines
- Finding disease-related markers
• Fertilization, development, differentiation, growth, aging
Diseases
• Tumor metastasis
• Inflammation
• Bacterial and viral infection
Glycoconjugates
Carbohydrates exist in the forms of glycoconjugates such as
glycolipids and glycoproteins
Glycoproteins: glycans attached to proteins
Glycolipids: glycans attached to lipids
Cell surface carbohydrares
anomeric center
- Homopolysaccharides
- heteropolysaccharides
- Complex carbohydrates in which many simple sugars are linked.
- Cellulose and starch are the two most widely occurring polysaccharides in plants.
Polysaccharides
Cellulose (-Glcb1,4Glc-)n
4
- Consists of thousands of D-glucopyranosyl-1,4-b-glucopyranosides.
- form a large aggregate structures held together by hydrogen bonds.
- is the main component of wood and plant fiber.
- is not digested in human body but is digested in herbivore (초식동물).
Starch (녹말 綠末 또는 전분 澱粉)
- is digested into glucose.
- can be separated into two fractions
1) amylose, insoluble in cold water, 20% by weight of starch, 1,4-a-glycoside polymer
2) amylopectin, soluble in cold water, 80% by weight of starch
contains 1,6-a-glycoside branches approximately every 25 glucose units in addition to
1,4-a-links.
amylose (-Glca1,4Glc-)n
Amylopectin
In human, glycosidases highly selectively hydrolyze 1,4-a-linkage
in starch but not 1,4-b linkage in cellulose.
Monosaccharides in mammalian glycoconjugates
OH
OH
O
HO
HO
OH
OH
O
HO
HO
OH
O
OH
HO
OH
NHAc
OH
OH
D-N-acetyl glucosamine (GlcNAc)
D-Glucose (Glc)
D-Galactose (Gal)
OH
OH
OH
HO
O
HO
HO
HO
OH
HO2C
O
O
HO
HO
OH
OH
OH
NHAc
D-N-acetyl galactosamine (GalNAc)
OH OH
CO2H
O
AcHN
D-Mannose (Man)
O
HO
HO
OH
D-Glucuronic acid (GlcA)
H3C
OH
N-Acetylneuraminic acid (NeuAc)
D-Xylose (Xyl)
HO
HO
HO
OH
HO
O
O HO
O
OH
OH
OH
OH
OH
OH
OH OH
O
L-Fucose (Fuc)
OH
O
OH
O
O HO
O
OH
O
O
OH
Glycosidic Bonds
Blood type
HO
HO
OH
HO
O HO OH
OH
O
AcHN O
O
HO
O
H3C
O
OH
OH
OH
Blood group A
O
OR
NHAc
HO
OH
HO
O HO OH
OH
O
HO O
O
OH
O
HO
O
HO
O
H3C
OH
OH
O
OR
NHAc
H3C
O
HO
O
O
OH
OH
OH
OH
OH
Blood group B
Blood group O
O
OR
NHAc
Pathogen Infection by Carbohydrate-protein Interactions
pathogens
DNA or RNA
√ Human influenza viruses (haemagglutinin protein) preferentially adhere to NeuNAca2,6Gal
residues on epithelial cells (상피세포) of the lungs and upper respiratory tract.
√ Avian influenza viruses (AI, 조류독감 바이러스) are specific for NeuNAca2,3Gal residues
on intestinal epithelial cells.
√ Some of Helicobacter pyroli expresses Leb-binding adhesin (BabA) and sialyl Lex-binding
adhesin (SabA) and thus adhere to the human gastric mucosa expressing these glycans.
√ Cholera toxin adheres to ganglioside GM1 in host cells.
Tamiflu: a drug for influenza
OH
OH
O
HO
HO
OH
OH
OH
O
HO
HO
O
OH
NHAc
OH
D-N-acetyl glucosamine (GlcNAc)
D-Glucose (Glc)
OH
OH
OH
HO
Transition state forOaction
HO
OH
of influenza neuraminidase
HO
HO
O
OH
NHAc
D-Mannose (Man)
CO2H
O
AcHN
OH
O
OH
HO
HO
OH
OH
D-Galactose
D-Glucose (Gal)
(Glc)
D-N-acetyl glucosa
OH
HOTamiflu
(독감 치료제)
2COH
O
HO
O OH
HO
HO
OH
OH
OH
HO
HO
HO
NHAc
D-N-acetyl galactosamine (GalNAc)
OH OH
HO
HO
HO
OH
OH
HO
OH
O-sugar HO
OH OH
N-Acetylneuraminic
acid (NeuAc)
N-acetyl
neuraminic
acid
O
neuraminidase
OH
OH
essential
for
influenza virus
D-Xylose (Xyl)
D-Glucuronic
acid (GlcA) (GalNAc)
D-N-acetyl galactosamine
H3C
OH OH
O
AcHN
OH
OH OH
D-Mannos
CO2H
OH
OH
O
OH
HO
HO
OH
L-Fucose (Fuc) acid (NeuAc)
N-Acetylneuraminic
D-Xylos
Stereochemical Terminology
Absolute configuration. A designation of the position or order of arrangement of the ligands
of a stereogenic unit in reference to an agreed upon stereochemical standard.
Achiral Not chiral. A necessary and sufficient criterion for achirality in a rigid molecule is the
presence of any improper symmetry element (Sn including σ and ί).
A chirotopic. The opposite of chirotopic. See “ chirotopic” below.
Anomers. Diastereomers of glycosides or related cyclic forms of sugars that are specifically
epimers at the anomeric carbon (C1 of an aldose, or C2, C3, etc., of a ketose).
Anti. Modern usage is to describe relative configuration of two stereogenic centers along a
chain. The chain is drawn in zigazg form, and if two substituent s are on opposite sides of the
plane of the paper, they are designated anti. See also “syn”, “antiperiplanar”, and “ anticlinal”.
Anticlinal. A term describing a conformation about a single bond. In A-B-C-D, A and D are
anticlinal if the torsion angle between them is between 90 and 150 or -90 and -150. See Figure
2.7.
Antiperiplanar. A term describing a conformation about a single bond. In A-B-C-D, A and D
are antiperiplanar if the torsion angle between them is between +150° to -150° . See Figure 2.7.
Apical, axial, basal, and equatorial. Terms associated with the bonds and positions of
ligands in trigonal bipyramidal structures.
Asymmetric. Lacking all symmetry elements (pointing group C1). All asymmetric
molecules are chiral.
Asymmetric carbon atom. Traditional term used to describe a carbon with four
different ligands attached. Not recommended in modern usage.
Atactic. A term describing the relative configuration along a polymer backbone. In an
atactic polymer, the stereochemistry is random-no particular pattern or bias is seen.
Atropisomers. Stereoisomers ( can be either enantiomers or diastereomers) that can be
interconverted by rotation about single bonds and for which the barrier to rotation is
large enough that the stereoisomers can be separated and do not interconvert readily at
room temperature.
Chiral. Existing in two forms that are related as non-congruent mirror images. A
necessary and sufficient criterion for chirality in a rigid molecule is the absence of any
improper symmetry elements.
Chiral center. Older term for a tetracoordinate carbon or similar atom with four
different substituents. More modern, and preferable, terminology is “stereogenic center”
(or “stereocenter”)
Chirotopic. The term used to denote that an atom, point, group, face, or line resides in a
chiral environment.
Cis. Describing the stereochemical relationship between two ligands that are on the same
side of a double bond or a ring system. For alkenes only, Z is preferred.
Configuration. The relative position or order of the arrangement of atoms in space that
characterizes a particular stereoisomer.
Conformers or conformational isomers. Stereoisomers that are interconverted by rapid
rotation about a single bond.
Constitutionally heterotopic. The same groups or atoms with different connectivities.
D and L.
An older system for identifying enantiomers, relating all stereocenters to the sense of
chirality of D- or L-glyceraldehyde. See discussion in the text. Generally not used anymore,
except for biological structures such as amino acids and sugars.
Diastereomers. Stereoisomers that are not enantiomers.
Diastereomeric excess (de). In a reaction that produces two diastereomeric products in
amounts A and B, de = 100% (|A – B|) / (A + B).
Diastereotopic. The relationship between two regions of a molecule that have the same
connectivity but are bit related by any kind of symmetry operation.
Dissymmetric. Lacking improper symmetry operations. A synonym for “chiral”, but not the
same as “asymmetric” .
Eclipsed. A term describing a conformation about a single bond. In A-B-C-D, A and D are
eclipsed if the torsion angle between them is approximately 0°.
Enantiomers. Molecules that are related as non-congruent mirror images.
Enantiomeric excess (ee). In a reaction that produces two enantiomeric products in amounts
A and A´ , ee = 100% (|A – A´|) / (A + A´).
Enantiotopic. The relationship between two regions of a molecule that are realated only by
an improper symmetry operation, typically a mirror plane.
Endo. In a bicyclic system, a substituent that is on a bridge is endo if it points toward the
larger of the two remaining bridges. See also “exo” .
Epimerization. The interconversion of epimers.
Epimers. Diastereomers that have the opposite configuration at only one of two or more
stereogenic centers.
Erythro and threo. Descriptors used to distinguish between diastereomers of an acyclic
structure having two stereogenic centers. When placed in a Fischer projection using the
convention proper for carbohydrates, erythro has the higher priority groups on the same side of
the Fischer projection, and threo has them on opposite sides.
Exo. In a bicyclic system, a substituent that is on a bridge is exo if it points toward the smaller
of the two remaining bridges. See also “endo” .
E, Z. stereodescriptors for alkenes (see discussion in the text).
Gauche. A term describing a conformation about a single bond, In A-B-C-D, A and D are
gauche if the torsion angle between them is approximately 60°(or -60°). See section 2.3.1.
Geminal. Attached to the same atoms. The two chlorines of 1,1-dichloro-2,2-difluoroethane
are geminal. See also “vicinal”.
Helicity. The sense of chirality of a helical or screw shaped entity ; right (P) or left (M).
Heterochiral. Having an oppsite sense of chirality. For example, D-alanine and L-leucine are
heterochiral. See also “homochiral”.
Heterotopic. The same groups or atoms in inequivalent constitutional or stereochemical
environments.
Homochiral. Having the same sense of chirality. For example, the 20 natural amino acids are
homochiral – they have the same arrangement of amino, carboxylate, and side chain groups.
Has also been used as a synonym for “enantiomerically pure”, but this is not recommended,
because homochiral already as a well-defined term before this alternative usage became
fashionable.
Homotopic. The relationship between two regions of a molecule that are related by a proper
symmetry operation.
Isotactic. A term describing the relative configuration along a polymer backbone. In an
isotactic polymer, all stereogenic centers of the polymer backbone have the same sense of
chirality.
Meso. A term describing a achiral member of a collection of diastereomers that also includes at
least one chiral member.
Opitcally active. Rotating plane polarized light. Formerly used as a synonym for “chiral”, but
this is not reconmmended.
Prochiral. A group is prochiral if it contains enantiotopic or diastereotopic ligands or faces, such
that replacement of one ligand or addition to one face produces a stereocenter. See section 6.3.2.
R, S. The designations for absolute stereochemistry (see earlier discussion in the text).
Racemic mixture or racemate. Comprised of a 50:50 mixture of enantiomers.
Relative configuration. This refers to the configuration of any stereogenic center with respect to
another stereogenic center. If one center in a molecule is known as R, then other centers can be
compared to it using the descriptors R* or S*, indicating the same or opposite stereochemistry,
respectively.
Resolution. The separation of a racemic mixture into its individual component enantiomers.
Scalemic. A synonym for “non-racemic” or “enantiomerically enriched”. It has not found
general acceptance, but is used occasionally.
S-cis and s-trans. Descriptors for the conformation about a single bond, such as the C2-C3 bond
in 1,3-buadiene, or the C-N bond of an amide. If the substituents are synperiplanar, they are
termed s-cis (“s” for “single”); if they are antiperiplanar, they are termed s-trans.
Stereocenter. See “stereogenic center”.
Stereogenic center. An atom at which interchange of any two ligands produces a new
stereoiosmer. A synonym for “stereocenter”.
Stereogenic unit. An atom or grouping of atoms at which interchange of any two ligands
produces a new stereoisomer.
Stereoisomers. Molecules that have the same connectivity, but a different arrangement of atoms in
space.
Stereoselective. A term describing the stereochemical consequences of certain types of reactions.
A stereoselective reaction is one for which reactant A can give two or more stereoisomeric products,
B and B’, and one or more product is preferred. There can be degrees of stereoselectivity. All
stereospecific reactions are stereoselective, but the converse is not true.
Stereospecific. A term describing the stereochemical consequences of certain types of reactions. A
stereospecific reaction is one for which reactant A gives product B, and stereoisomeric reactant A’
gives stereoisomeric product B’. There can be degrees of stereospecificity. Stereosprcific does not
means 100% stereoselective.
Syn. Modern usage is to describe the relative configuration of two stereogenic centers along a
chain. The chain is drawn in zigzag form, and if two substituents are on the same side of the
plane of the paper, they are syn. See also “anti”, “synperiplanar”, and “synclinal”.
Synclinal. A term describing a configuration about a single bond. In A-B-C-D, A and D are
synclinal if the torsion angle between 30° and 90° (or -30° and -90°). See Figure 2.7.
Syndiotactic. A term describing the relative configuration along a polymer backbone. In a
syndiotactic polymer, the relative configuration of backbone stereogenic centers alternate
along the chain.
Synperiplanar. A term describing a conformation about a single bond. In A-B-C-D, A and D
aresynperiplanar if the torsion angle between them is between + 30° and –30°. See Figure 2.7.
Tacticity. A generic term describing the stereochemistry along a polymer backbone. See
“atactic”, “isotactic”, and “syndiotactic”.
Trans. A term describing the stereochemical relationship between two ligands that are on
opposite sides of a double or a ring system. For alkenes only. E is preferred.
Vicinal. Attatched to adjacent atoms. In 1,1-dichloro-2,2-difluoroethane, the relationship of
either chlorine to either fluorine is vicinal. See also “geminal”.