StereochemistryChapter 6

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

A molecule is achiral if it is superimposable on its mirror image. A molecule which has a plane of symmetry, a center of symmetryor rotation-reflection symmetry is achiral.An axis of symmetry (C2 axis) -> achiral

Constitutional isomers: molecules with same molecular formular but different connectivitybetween 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-butene1. enantiomers: nonsuperimposable mirror images of each other2. 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.

chiral

C2

achiral

a plane of symmetry

achiral

a center of symmetry

O

OH

OH

Br

Br

Br

Br

A molecule is achiral if it is superimposable on its mirror image. A molecule which has a plane of symmetry, a center of symmetryor rotation-reflection symmetry is achiral. ()

(, S1)

(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 notresult in measurable rotations. -> in this case, they are optically inactive even though they are chiral. optically active.

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..

H OHCO2H

H OHCO2H

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.

H OHCO2H

H OHCO2H

stereogenic center: stereoisomers

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: - and -anomers epimers.

6.1.2 Stereochemical descriptors

R, S system (Cahn-Ingold-Prelog system)

R2

R3

R1R4

1 2

34

R2

R3

R1R4

1 2

43

R S

rectus (right) sinister (left)

higher atomic number: higher priorityisotopes (the one with higher mass is assigned the higher priority)

Tricoordinate -> stereogenic center

S

CH2CH3CH3

H3CS

CH2CH3CH3

H3C

high energybarrier

R S

phantom atom: the lowest priority

P

CH2CH=CH2CH3

PCH2CH=CH2

CH3high energybarrier

R S

phantom atom: the lowest priority

E, Z system

higherlower

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

Fischer projection

Horizontal lines: bonds coming out of the plane of the paperVertical lines: bonds projecting behind the plane of the paper

The most oxidized group: topCH2OH (carbohydrates) or R (amino acids): bottom

mainly used for amino acids and carbohydrates

D: dextro, rightL: levo, left

L D D D L

Natural amino acids: L-amino acids

Important pointNo 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)

HH3CCH3

H

NO2

CH3NO2H3C

H

ClCH3H

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 reagentsNMR spectroscopy of derivatives that are diastereomericChromatography (HPLC and GC) with chiral stationary phases

R1 R2

OHH

R2 R1

OHH

Ph COCl

OMeF3C

(R)-MTPA-Clmethoxy trifluoromethyl phenylacetyl chloride

HO

O

F3C

OMeR1

R2

eclipsedS

S HO

O

F3C

OMeR2

R1

eclipsed

R

R

oror

S: R1 -> upfieldR: R2 -> upfield due to anisotropic effect of phenyl ring

NMR spectroscopy of derivatives that are diastereomeric

Methods: (R/S) racemate + (R)-MTPA-Cl 50 : 50 (R-R-MTPA : S-R-MTPA)OH, NH2, SH

ppm

sample + (R)-MTPA-Cl

R, S peak

Derivatives

R S

R S

90 10

ee 80%

(Moshers reagent)

D LD L D LD LL DL D

D LD LL DL D

D LD L

DD DDDD

DDDDDD

> 98%ee

> 98%ee

OMe-HNH

NH -H OMe

O CO2t-BuNH

F3CO

OMePh

OTBS

S D,L

O CO2Bn

Me Me

NH

F3CO

OMePh

S D,L

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 ()Specific optical activity [][]D25 -> sodium D line (589 nm emission line of sodium arc lamp)

Optical purity (%) =[] pure enantiomer

[] mixture of enantiomerx 100

6.2 Symmetry and stereochemistry6.2.1 Basic symmetry operations

Proper rotation (Cn) -> a rotation around an axis by (360/n)o that has the net effect of leavingthe 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 ()S2; equivalent to a center of inversion (i)

90o

60o

180o

6.2.2 Chirality and symmetryA 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 asymmetricmolecules are chiral, not all chiral molecules are asymmetric.

6.3.1 Homotopic, enantiotopic, and diastereotopic

6.3 Topicity relationshipTopicity: derived from the same roots as topography and topology, relating to the spatial position of an object.

Homotopic: is defined as interconvertable by a Cn axis of the molecule.

HO OHHH

homotopic hydrogens

HH

homotopic hydrogens

achiral moleculesC2

chiral influence cannot distinguish these methyl groups

CO2-

NH3+

HR HS

phenylalanine

diastereotopic

Heterotopic: the same groups or atoms in inequivalent constitutional or stereochemicalenvironment.- Enantiotopic: interconverted by an Sn axis of the molecule (n = 1 in this case).

enantiotopic groups, when exposed to a chiral influence (chiral shift reagent), become distinguishable, as if they were diastereotopic.

- diastereotopic: the same connectivity, but there is no symmetry operation that interconverts them in any conformation. stereogenic center the environments of diastereotopic groups are topologically nonequivalent. -> they can be distinguished by physical probes, especially NMR spectroscopy (AB quartet)

N MeMe

PhHH

N MeMe

PhHH

meso: achiral chiral

enantiotopic diastereotopic

AB quartet

H1 H22H

6.3.2 Topicity descriptors Pro-R/Pro-S and Re/Si

pro-R pro-S

pro-Rpro-S

pro-S pro-R

O

R2R1

Si faceRe face

1

2 3

H3C OHHH

ethanol

liver alcohol dehydrogenase

O

H

H3C

acetaldehyde

pro-Spro-R pro-S

-pro-R

H3C OHD or TH

H3C OHHT ro D

alcohol dehydrogenase

O

H

H3CO

D or T

H3C

RSCoA

H

H

H

H

O

pro-R

pro-R

acyl-CoA dehydrogenaseR

SCoA

OH

H

- pro-R and H

Enzymatic reactions

6.4 Reaction stereochemistry: stereoselectivity and stereospecificity6.4.1 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 theproduct, while a different stereoisomer of the reactant gives a different stereoisomer of product. Stereospecific reaction is a special, more restrictive case of a stereoselectivereaction.Stereoselective reaction: one in which a single reactant can give two or more stereoisomericproducts, 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

Syn addition

anti elimination

Br

H Ph

H

MePh

Br

Ph H

H

MePh

anti elimination

anti elimination

-Ot-Bu

-Ot-Bu

Ph

Ph

Ph Ph

RsO

RRl

RmOR

RmRs

Rl

R O

Rm Rs

Rl

preferred

Nu:

RsOH

NuRl

Rm

R

major

RsOH

RRl

Rm

Nu

minor

Nu:

Regioselective reaction

Markovnikov addition

6.5 Symmetry and time scale

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

6.8 Stereochemical issues in chemical biology6.8.1 The linkages of proteins, nucleic acids, and polysaccharides

Proteins

planar

~4 kcal/mol preference

~19 kcal/mol rotation barrier

much smallerrotation barrier

HO2C NH2

H

HO2C NH2

CH3

HO2C NH2HO2C NH2HO2C NH2

Ala, A Val, V Ile, I

HO2C NH2 HO2C NH2HO2C NH2

OH NH

HO2C NH2

SCH3

HO2C NH2

SH

HO2C NH2

OH

HO2C NH2

OH

HO2C NH2

N

HN

NH

CO2H

H

HO2C NH2

NH2

HO2C NH2

NH

H2N NH

HO2C NH2

CO2H

HO2C NH2

CO2H

HO2C NH2

CONH2

HO2C NH2

CONH2

Ser, S Thr, T Met, M Cys, C Pro, P

Phe, F Tyr, Y Trp, W His, H Lys, K

Arg, R Asp, D Asn, N Gln, Q

Gly, G Leu, L

Glu, E

achiral

20 natural amino acids (L form)

Nucleic acidNucleic acid(RNA or DNA)(RNA or DNA)

ON

N

N

O

NN

N

NO

H

N

NN

O

O

NN

N

NNH3C

HH

HH

H

HH

O

O

O

O

O

PO O-

OPO

O-O A = TA = TG G C C

Phosphodiester bonds

Nucleic acids

55

33

33

55

OHO

HO

OH

(OH)2'-deoxyribose ribose

1'

2'3'4'

5'

BaseO

HO

HO

Base

(OH)2'-deoxyribose ribose

1'2'3'

4'

5'

Nucleosides

O

HO

Base

(OH)2'-deoxyribose ribose

1'2'3'

4'

5'

Nucleotides

O-

O

PO O-

Bases

-glycosidic linkage

Phosphodiester linkages

Carbohydrates

Structural and functional studiesof whole carbohydrates

Studies of carbohydrate-protein interactions

Structural and functional studiesStructural and functional studiesof whole carbohydratesof whole carbohydrates

Studies of carbohydrateStudies of carbohydrate--protein protein interactionsinteractions

Functional GlycomicsFunctional GlycomicsFunctional Glycomics

- Inhibitors for carbohydrate biosynthesis

- Inhibitors for carbohydrate-binding proteins

- Carbohydrate-based vaccines

- Finding disease-related markers

-- Inhibitors for carbohydrate biosynthesis Inhibitors for carbohydrate biosynthesis

-- Inhibitors for carbohydrateInhibitors for carbohydrate--binding proteinsbinding proteins

-- CarbohydrateCarbohydrate--based vaccinesbased vaccines

-- Finding diseaseFinding disease--related markers related markers

Biological processes Fertilization, development, differentiation, growth, aging

Diseases Tumor metastasis Inflammation Bacterial and viral infection

Biological processesBiological processes Fertilization, development, differentiation, growth, aging Fertilization, development, differentiation, growth, aging

DiseasesDiseases Tumor metastasisTumor metastasis InflammationInflammation Bacterial and viral infectionBacterial and viral infection

Carbohydrate-protein InteractionsCarbohydrateCarbohydrate--protein Interactionsprotein Interactions

Understanding biological processes Development of therapeutic agents

Understanding biological processesUnderstanding biological processes Development of therapeutic agentsDevelopment of therapeutic agents

Toxin

Bacteria

Hormone

Virus

Antibody

(Tumor) CellProtein

Functional GlycomicsFunctional Glycomics

Carbohydrates exist in the forms of glycoconjugates such as glycolipids and glycoproteins

Carbohydrates exist in the forms of glycoconjugates such as glycolipids and glycoproteins

Cell surface carbohydrares

GlycoconjugatesGlycoconjugates

Glycoproteins: glycans attached to proteinsGlycolipids: glycans attached to lipids

Polysaccharides- Complex carbohydrates in which many simple sugars are linked.- Cellulose and starch are the two most widely occurring polysaccharides in plants.

- Consists of thousands of D-glucopyranosyl-1,4--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 ().

Cellulose (-Glc1,4Glc-)n

4

anomeric center

- Homopolysaccharides- heteropolysaccharides

Starch ( )

- is digested into glucose.- can be separated into two fractions

1) amylose, insoluble in cold water, 20% by weight of starch, 1,4--glycoside polymer2) 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--links.

In human, glycosidases highly selectively hydrolyze 1,4--linkage in starch but not 1,4- linkage in cellulose.

amylose (-Glc1,4Glc-)n Amylopectin

Monosaccharides in mammalian glycoconjugatesMonosaccharides in mammalian glycoconjugatesMonosaccharides in mammalian glycoconjugates

OHOHO

OH

OHOH

OHOHO

OH

NHAcOH

OOH

HO

OH

OHOH

OOH

HO

OH

NHAcOH

OHOHO

OH

HO

OH

OHOHO

HO2C

OHOH

OHOHO

OHOHO

CO2H

OH

OH

OH OH

OHAcHN

O OH

OHOH

OH

H3C

D-Glucose (Glc) D-N-acetyl glucosamine (GlcNAc) D-Galactose (Gal)

D-N-acetyl galactosamine (GalNAc) D-Mannose (Man) D-Glucuronic acid (GlcA)

D-Xylose (Xyl) L-Fucose (Fuc)N-Acetylneuraminic acid (NeuAc)

O

OH

HOO

OHO

OHO

OHO

O

OH

HO

OHO

O

HOO

HOHO

OH

Glycosidic Bonds

OOHO

OH

NHAcOR

OHO

O

OH

O

O

OHOH

OH

H3C

OHO

HO

OH

AcHN OOHO

OH

NHAcOR

OHO

O

OH

O

O

OHOH

OH

H3C

OHO

HO

OH

HO

OOHO

OH

NHAcOR

OHO

OH

O

O

OHOH

OH

H3C

HO

Blood group A Blood group B Blood group O

Blood type

Pathogen Infection by CarbohydratePathogen Infection by Carbohydrate--protein Interactionsprotein Interactions

DNA or RNA

pathogens

Human influenza viruses (haemagglutinin protein) preferentially adhere to NeuNAca2,6Gal residues on epithelial cells () of the lungs and upper respiratory tract.

Avian influenza viruses (AI, ) 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 ( )

TamifluTamiflu: a drug for influenza : a drug for influenza

Transition state for actionof influenza neuraminidase

O

CO2H

OH

OH

OH OH

OHAcHN

N-acetyl neuraminic acid

O-sugar O

CO2H

OH

OH

OH OH

OHAcHN

neuraminidase

essential for influenza virus

Stereochemical Terminology

Absolute configuration. A designation of the position or order of arrangement of the ligandsof 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 and ).

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 morestereogenic 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 stereochemicalenvironments.

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 propersymmetry 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 ligandsproduces 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 stereoisomericproducts, 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 Agives stereoisomeric product B. There can be degrees of stereospecificity. Stereosprcific does notmeans 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.