Why is asymmetric synthesis important?

Why is asymmetric synthesis important?
Nature yields an enormous variety of chiral compounds among its natural products e.g.
HO
AcO
O
5 chiral centres
O
H
H
N
H
HO
H
N
AcO
2
1
Morphine 1 and the derivative diacetylmorphine (heroine) 2 are vital for the control of severe pain
and is isolated from the opium poppy Papaver somniferum.
O
O
O
Ph
NH
O
OH
O
Ph
O
OH
O
HO
O
Ph
O
Taxol
Taxol has been used to treat cancers
such as breast and ovarian tumours for
over 14 years. Taxol is a member of a
family of drugs that affects microtubule
11 chiral centres formation, which is required before cell
mitosis (division) can occur. In the
O
present production process, a related
compound is isolated from the leaves
O
and needles of the European Ewe and
is then chemically modified.
NH
H
S
O
Penicillin G
N
O
Penicillin G is still a widely used
antibiotic, despite bacterial resistance. It
is manufactured by large scale
fermentation of the mould, Penicillium
chrysogenum.
CO2H
All these examples have been synthesised in vitro and yet the most economic route is still to
manufacture by isolation from living organisms. Obviously there is a big scope for
improvement before mankind can compete with nature’s synthetic processes.
Nature’s high molecular weight systems
The helical structure of nature’s polymeric systems (DNA/RNA) is controlled by the chirality of
the sugar-phosphate backbone made up of deoxyribose or ribose.
The 3-D structure of proteins, enzymes and receptors is partly determined by the chirality of the
amino-acids in the peptide chain.
Importance of asymmetric synthesis from the academic standpoint
Organic chemistry is the study of carbon-based compounds, the chemistry of life. If organic
chemists wish to synthesise the molecules that nature has produced then they must be able to
prepare the same enantiomer as occurs naturally. Historically, the synthesis of a racemic (50:50
mixture of both enantiomers) version was accepted as a successful outcome but that is no
longer the case. Synthetic chemists not only want to copy nature but to synthesise totally novel
chiral structures.
Importance in Biological Systems
If the only difference between enantiomers was the direction of rotation of polarised light
then asymmetric synthesis would be only of academic interest. However in living
systems, chiral drug molecules interact with receptors and enzymes which are
themselves chiral. The two drug receptor complexes are diastereomeric and so it is not
surprising that two enantiomers can have very different effects. For example O
NH
HN
OH
OH
B
A
(-)-L-DOPA is used in the
treatment of Parkinson’s disease.
The active drug is dopamine, but
it cannot cross the blood brain
barrier. L-DOPA can and is then
decarboxylated to dopamine. (+)D-DOPA is not decarboxylated
by the enzyme and there would
be a dangerous build up of the
(+) form in the body.
(-)-Propranolol A was introduced
in the 1960s -blocker used in
the treatment of heart disease.
The (+) enantiomer B is a
contraceptive.
O
HO
NH2
HO
dopamine
HO
decarboxylase
CO2H
L-DOPA
D-DOPA
CO2
HO
dopamine
NH2
What is asymmetric synthesis? In 1971 Morrison and Mosher gave a general
definition: Asymmetric synthesis is a reaction where an achiral unit is converted by a
reactant into a chiral unit, such that the stereoisomeric products are formed in unequal
amounts.
Optical Rotation
Enantiomers can denoted by the experimentally determined optical rotation (+) or (-) x°
which is defined as:
[ ]!t =
obs/l.c
[ ]!t = specific rotation,
obs
= observed rotation, l = cell path length (dm)
c = concentration (g/ml), t = temperature (°C), ! = wavelength of incident light (nm)
The experimental value varies considerably with temperature, concentration, and solvent
and provides little reliable evidence of enantiomeric purity. Most importantly it does not
give evidence of the absolute configuration.
CO2H
Me
Example of specific rotation variability
Et
CO2H
In chloroform solution:
c =0.063 g/ml [!]D = 0
c >0.063 g/ml [!]D = +ve
c <0.063 g/ml [!]D = -ve
Stereochemistry Nomenclature
Absolute configuration of a stereogenic centre is best described using the CahnIngold-Prelog rules (see previous courses for the detailed rules) used to determine
(R) or (S). Some examples to help revise the rules:
F
1
3
H
1
Br
2
2
H
(D)
Cl
3
S
1
O
1
O
3
S
Me
H
2
S
3
OH
2
R
Me
P
2
R
OMe
Topicity (face selectivity) of Enantioselective Reactions (Re and Si)
In an enantioselective reaction at a trigonal centre that generates a new asymmetric carbon,
it is possible to define the face being attacked using the same Cahn-Ingold-Prelog rules.
Such faces are given the term ‘enantiotopic’. Since the trigonal centre is not itself chiral, but
is a potential chiral centre, the term prochiral is sometimes used instead of enantiotopic.
In this case, viewing from the side of the newly formed bond, if the groups in decreasing
priority are clockwise it is called the Re face
anticlockwise – Si face.
e.g hydride attack on a ketone Re
1
O
H
F3C
OH
(R)
Si attack
2
F 3C
3
CH 3
Re attack
F 3C
CH 3
H-
H
HO
(S)
CH 3
Si
Note that there is no direct connection between Re/Si and R/S. In this example, if
hydride is replaced with EtMgBr, Re attack gives (R) and Si attack, (S).
CH3CH2
F3C
HO
OH
(S)
CH3
Si attack
EtMgBr instead of H-
F3C
CH2CH3
(R)
CH3
Re attack
Enantiotopic atoms or groups
The term prochiral is also used to describe a tetrahedral group which has two enantiotopic
atoms or groups i.e. CX2WY. To differentiate the two X groups, replace one with a dummy
group of higher priority.
"
If dummy group gives (S) configuration then the atom X is pro–S
"
If dummy group gives (R) configuration then the atom X is pro–R
This is best understood by looking at an example X = H:
H2
H2
OHC
H1
HO
OHC
H2
H1
OHC
HO
OH
HO
H2
H1
OHC
H1
H1 is pro-S
H2 is pro-R
Note that there is no correlation between the designation pro-R, pro-S and the absolute
stereochemistry of the product. For example – replace the pro-R methoxy in the acetal
with a hydrogen gives an (S) isomer, replace the same methoxy with a chorine atom and
the configuration is (R).
pro-R
H
Ph
OMe
(S)
Et
MeO
Ph
OMe
Et
Cl
Ph
OMe
(R) Et
Determination of Enantiomeric Purity
•Measuring optical rotation is too unreliable and there is no guarantee that previously
published optical rotations of the same compound will be from an enantiomerically pure
source.
•The only reliable way to quantify ee is to separate the enantiomers or diasteromeric
derivatives by chromatography or to distinguish them by spectroscopic means.
•Enantiomeric purity is defined by the value of the enantiomeric excess.
•Percentage enantiomeric excess (% ee)
= [R] - [S] x 100
= %R - %S
[R] + [S]
Experimental techniques for separating enantiomers – In all these cases,
having both enantiomers available is advisable in order to test the technique.
1. Gas chromatography using a chiral stationary phase. The enantiomers to be analysed
undergo rapid and reversible diastereomeric interactions with chiral groups on the stationary
phase. These short lived diastereomeric complexes will have different stability #
separation possible
• Compounds must be sufficiently volatile and thermally stable.
•A chiral stationary phase may only work well for certain compounds types and chiral
columns are expensive.
•Quick and simple to carry out
•Measurements can be very accurate ($0.05%)
2. Chiral HPLC – The principle is the same as for g.c.
•A wider variety of compounds can be tested
•A chiral stationary phase may only work well for limited types of compounds and chiral
columns are expensive, typically £1000 for a column.
•Measurements can be very accurate ($0.05%)
•Method development can be lengthy.
Example of a typical chiral stationary phase ( similar to achiral reverse phase type):
NO2
O
Si
(CH2)3 NCO (S) NHCONH
O
NO2
Chiral group: (S)-valine linked by urea to 3,5-dinitrophenyl group
NMR Spectroscopy
Lanthanide Chiral Shift Reagents - Paramagnetic lanthanide complexes such as
the chiral camphor europium complex [Eu(hfc)3] bind reversibly via the
lanthanide metal to donor sites (e.g. NH2, OH, C=O) of the chiral molecule. This
process is much faster than the NMR timescale and what is observed is an
averaged downfield shift i.e. to higher values of ".
•Two enantiomers form different diastereomeric complexes and so NMR signals
may be non-equivalent.
•Requires electron donor groups (lone pairs e.g. OH, NH2, CO, COO)
•Paramagnetic complexes cause signal broadening so only add sufficient chiral
shift reagent to achieve signal separation but minimum signal broadening.
•Proton NMR spectroscopy is normally used
•Simple to do but accuracy is limited to ±2%, which is the limit of NMR
integration.
CF2CF2CF3
O
O
3
Eu
Eu(hfc)3 + substrate
[Eu(hfc)3...substrate]
Chiral Derivatising Agents (CDAs)
•Convert an enantiomeric mixture to a pair of diastereomers by attaching an
enantiomerically pure (homochiral) derivative.
•Diastereomeric mixture shows a larger signal separation than for chiral shift
reagents. There is no reversibility as the groups are directly bonded.
•Diastereomers can be separated on standard achiral gc and hplc columns.
•There is often a much larger peak separation on chiral gc/hplc columns.
Aspects which must be considered when using CDAs
•CDA must be enantiomerically pure.
•Reaction for both enantiomers must be 100% or results will be erroneous; why
care must be taken? - enantiomers can react at different rates with the CDA.
•No loss of stereochemical integrity can occur during the process, so all steps
must be rigorously established.
•Limited to substrate containing: RCO2H, RNH2, ROH
Example of a chiral derivatising agent
One of the most useful chiral derivatising agents for use with enantiomeric alcohols and
amines is -methoxy- -trifluoromethylphenylacetic acid (MTPA) commonly referred to as
Mosher’s acid.
F3C
H
OMe
DCC, DMAPcat.,
CH2Cl2, -10°C
F3C
(S) CO2H
HO
OMe
H
(R.S)
(S) CO2
(R.S)
Mosher's acid
in excess
N
C
N
DCC - dicycohexylcarbodiimide
a pair of diastereomeric esters
Me2N
N
DMAP dimethylaminopyridine
Difference in NMR
signals between
diastereomers:
1H
NMR spectra #" =
0.08 (alcohol methyl
signal)
19F
NMR spectra #" =
0.17 (MTPA CF3 signal)
•No -hydrogen on MTPA so configurationally stable – reaction proceeds
with retention of configuration.
•1H NMR chemical shift differences typically 0.15 ppm.
•19F NMR will give one signal for each enantiomer – simple to interprete.
•Chromatography - Diastereomers can be separated on hplc and, if they are
volatile and thermally stable, on gc. Larger peak separations are normally
achieved using chiral stationary phases.
In cases where the DCC/DMAP reaction is not high yielding, the more
reactive acid chloride can be used.
F3C
R*OH
+
OMe
(S)
Cl
CH2Cl2, DMAP,
O Et3N
F3C
OMe
O
OR*
Methods of Asymmetric Synthesis
Chiral Reagents – Example – DIP chloride
Chiral reagents are consumed in the reaction; to be of practical use they must be:
inexpensive, give high ee, high chemical yields and be convenient to use. If they fail to
pass these hurdles, catalytic processes or chiral auxiliaries will always win out.
A useful reagent which has achieved widespread use in laboratory scale syntheses is
diisopinocampheylchloroborane (DIPchloride) which is used for the asymmetric
reduction of prochiral ketones.
asymmetric
O
reduction [H ]
H
OH
HO
H
or
R
R'
R
Developed by Herbert C. Brown et al – pioneer of
organoboron chemistry - awarded the Nobel Prize in
1990 for his work in the area.
R'
R
R'
BCl
2
•DIP Chloride is prepared from !-pinene (cheap,
produced in multi-ton quantities)
•Both enantiomers are available
•>90% ee often achieved
(-)-DIP-Chloride
(+)-!-pinene
The reaction is carried out in diethyl ether or neat (without solvent).
The mechanism without stereochemical detail is shown:
R
Me
R
R'
O
H
B
R'
Ipc
Cl
H
O
+
H
-
B
R'
R
Ipc
O
+
Cl
B
Ipc
Cl
Ipc = isopinocampheyl
H
N
Electron withdrawing chloro-substituent
increases the Lewis acidity of boron.
Step 1 – carbonyl bonds to electron deficent
boron (activates carbonyl to hydride attack
and generation of B- activates hydride
transfer
Step 2 – fast intramolecular hydride transfer
with good enantioselectivity (reverse of
hydroboration, see previous notes)
Step 3 – diethanolamine work-up allows
easy separation of byproducts.
OH
HO
O
Ipc
B
R'
N
+
O
crystalline
diethanolamine-boron complex
remove by filtration
H
R
OH
Enantioselectivity occurs at hydride transfer step –
•The proposed transition state shows the ketone-borane adduct arranged such that the
larger phenyl group avoids steric interaction with the pinene methyl group.
•The magnitude of ee chiefly depends on the steric differences between the groups
attached to the ketone – bigger difference = high ee
Cl
1
Ipc
B
H
OH
+
O
H
CH3
S
2
CH3
3
CH3
1
O
1
2
HMe
3
Re attack
H
S
OH
2
CH3
3
(S)-1-phenylethanol
Note that hydride attacks from the Re face but gives an S alcohol
Enantioselectivity data for reduction of unsymmetrical
ketones with (-)-DIP Chloride
Reaction
Conditions
Ketone
ether,
-25°C
1
CH3
O
ether,
-25°C
2
Ph
CF3
O
ether,
-25°C
3
Hexyl
CF3
98
90
H
HO
Ph
(S)
Ph
(S)
H
91
Hexyl
(S)
H
HO
CF3
CF3
HO
CF3
CF3
Butyl
O
4
CF3
Butyl
Controlling
group
CH3
O
Ph
Product
Isomer
%ee
ether,
-25°C
>99
(S)
H
HO
CF3
CF3
Conclusions
•Enantioselectivity increased by using low temperature – slower relative
reaction rates for competing enantiomer
•Note that for 1 and 2 the stereochemical outcome is opposite although both
are labelled S. This is due to the Cahn Ingold Prelog rules but the controlling
groups are different.
•The steric effect of CF3 is sometimes considered as similar to a tertiary butyl
group.
•Alkyne group is narrower than an alkyl group – greater steric difference
between the trifluoromethyl and the alkyne gives higher ee –this provides a
way to produce 2-trifluoromethylalcohols with high ee by hydrogenation of the
corresponding trifluoromethylalkyne alcohols.
A Quick reminder, why asymmetric synthesis is important
Et
OH
OH
Et
H
N
H
N
N
H H
OH
Et
H
(S,S) tuberculostatic
Ethambutol
Et
H
N
H H
OH
(R,R) causes blindness
Chiral Catalysts – Asymmetric reduction
The Corey-Itsuno oxazaborolidine catalyst has proved a very succesful system for the
catalytic asymmetric reduction of prochiral ketones - Elias J. Corey, Nobel Prize, 1990.
The starting material is derived from the amino-acid L-(S)-proline in two steps. Reacting the
prolinol with an akylboronic acid [RB(OH)2]gives the active oxazaborolidine.
O
CO2H
O
+
Cl
NH
Cl
Ph
PhMgCl,
THF
Ph
O
OH
N
NH
(S)-proline
O
OH
THF, R B
•The asymmetric reduction is carried out
using borane or catecholborane as the
reducing agent.
•ee from 80 to 98% with 2.5 mol% of
catalyst
OH
R = Me, Bu
Ph
oxazaborolidine
•Catechol borane A (more selective) and
BH3.SMe2 have also been used
O
N
•amenable to scale up
•catalyst can be recovered
Ph
B
R
O
B
O
catechol borane
H
Mechanism for asymmetric reduction of prochiral ketones
•The oxazaborolidine contains both a Lewis acid site (boron) and a Lewis base site (nitrogen lone pair). borane bonds to to N and becomes a more reactive hydride transfer reagent -carbonyl group bonds to
boron – activated to nucleophilic attack by hydride - hydride and carbonyl are positioned adjacent to
each other – fast intramolecular hydride transfer is ideal for good enantio-control -large difference in size
of groups attached to ketone give best enantioselectivity.
Ph
Ph
Ph
Ph
BH3.THF
N
O
H
s
R
N
H3B
B
R
RL
N+
H2B
H
s
R
Ph
-
B
R
O
L
R
R
s
R
Ph
N+
O
B
O
S =small
L = large
Ph
O
+
O
H3B
H
s
R
RL
Ph
O
BO+
RL
R
R
O
N
H
RS
B-
H
+
H2B
O+
-
H
Model to explain enantioselectivity of ozazaboroldine
for the asymmetric reduction of prochiral ketones.
R-large lies farthest away from the other groups
in an equatorial position.
RL
Example: The following intermediate was required in a process to synthesise a
platelet activating factor (PAF) inhibitor.
Ph Ph
O
OH
CO2Me
MeO
OMe
O cat.
N B
Me
BH3.THF
0°C
CO2Me
95% ee
MeO
OMe
Chiral Catalysts – Epoxidation of allylic alcohols
In the early 80s Barry Sharpless and coworkers developed an asymmetric
epoxidation of allylic alcohols using tertiary butyl hydroperoxide as the oxidant
and titanium tartrate catalysts. This method has been developed to an industrial
scale – Sharpless, Nobel Prize, 2001.
R1
R2
0.05 eq. (+)- or (-)-DET
0.05 eq.Ti(OPri)4
R1
1.5 eq. ButOOH
Mol. sieves
R3
O
HO
O
or
OH
OH
CO2Et
HO
R2
R3
R3
OH
R1
R2
Molecular sieves are
required for anydrous
conditions – water
contamination reduces
enantioselectivity.
CO2Et
OH
O
HO
CO2Et
R,R-(+)-DET
HO
CO2Et
tertiary butylhydroperoxide (TBHP)
S,S-(-)-DET
•DET = diethyltartrate from tartaric acid
•Both epoxide enantiomers can be synthesised in high enantiomeric purity.
•predictable absolute configuration
•The product, epoxy alcohols, are useful intermediates.
The stereochemical outcome can be predicted by drawing the allylic alcohol
vertically the OH at the bottom right. In this orientation:
(-)-diethyltartrate (DET) delivers epoxide oxygen from above
(+)-diethyltartrate (DET) delivers epoxide oxygen from below.
B
C
oxygen delivered from above
O
(-)-DET
B
A
C
OH
A
B
OH
(+)-DET
C
O
oxygen delivered from below
A
OH
A mechanism (not including stereochemical information) shows the formation of a strained
peroxonium three membered ring from the titianium tertiarybutylperoxide intermediate. This
is ideally set for a rapid intramolecular epoxidation step.
O
Ti
O
O
O+
Ti
O
O
O
Ti
O
O+
Ti
O
O
O
R'
EtO2C
R''
O
CO2Et
O
O
Ti
Ti
O
allylic alcohol
CO2Et
O
reactive oxygen
O
O+
Bu
t
EtO2C
A mechanism to explain the
enantioselectivity is proposed
to involve a di-titanium
complex as shown. Monotitanium complexes have also
been proposed so this is still
an area up for discussion.(Not
required to be known for
examination)
Typical example
H
(+)-DET
Ti(OPri)4
ButOOH
Ph
OH
O
Ph
Ph
O
OH
H
91% ee
OH
(+)-DET, attack
from below
Chiral Auxiliaries
In the topics looked at so far i.e. chiral reagents and chiral catalysts, the
enantiocontrol arises from a complex prior to the diastereoselective step.
Depending on the stability of the complex there will be a reversible equilibrium to
a greater or lesser extent.
A chiral auxiliary is a homochiral group that is temporarily directly attached
to an achiral substrate (R-X) . The modified substrate undergoes a
diastereoselective reaction and finally, the chiral auxiliary is cleaved and
recovered to give a product (R-Y*) bearing a new stereogenic centre (or in some
cases, several stereogenic centres).
R
X
Chiral
auxilary
R
X
Aux*
Diastereoselective
reaction
R
*
Y
Cleavage
Aux*
Requirements for a chiral auxiliary
•Cleavage of R-Y*-G* must occur under mild conditions and with no
racemisation.
•The auxiliary G* ideally should be low cost.
•Both enantiomers of the auxiliary should be available
R
Y* +
Aux*
•The diastereoselective step to give R-Y*-G* must proceed with very good
stereocontrol and should give a crystalline intermediate. Recrystallisation of RY*-G* generally increases the diastereomeric purity (de) and therefore gives RY* with a higher ee.
Requirements for a Chiral Auxiliary continued
Oppolzer’s Camphor Sultam
Reactive group for attaching achiral substrate
Rigid tricyclic
structure
NH
1
Chelation site - aids stereochemical control
SO2
•In the late 80s, Swiss chemist Wolfgang Oppolzer developed the camphor sultam
1, a derivative of camphor. The nitrogen is linked via an amide bond to the
carbonyl of a substrate.
•The sultam provides a rigid tricyclic structure with bulky groups close to the site of
asymmetric reaction.
•Sulphone oxygens can complex to metal ions giving further control of
diasteroeselectivity.
•Nitrogen lone pair can take part in stereolectronic control in the asymmetric step.
•Several asymmetric reactions can be carried out using this system.
Synthetic Route to Oppolzer’s Camphor Sultam
(CH3CO)2O,
H2SO4
O
D-(+)-camphor
[L-(-)-camphor also available]
CHCl3, SOCl2
SO3H
O
SO 2Cl
O
(1S)-(+)-10-camphorsulphonic acid
NH4OH
toluene,
Amberlyst 15ion exchange resin
(an acid catalyst)
LiAlH4, THF
NH
S
O2
(-)-2,10-camphorsultam
N
S
O2
O
SO2NH2
Expensive to purchase the sultam but it is easily synthesised on
a large scale and can be recycled with no loss of enantiomeric
purity
Asymmetric syntheses using Oppolzer’s camphor sultam:
Alkylation !- to a carbonyl group
Formation of an enolate by deprotonation !- to a carbonyl group and reaction
of the enolate with an electrophile is one of the most useful bond forming
reactions in organic chemistry.
O
B:
R
X
E+
O
R
or
X
X
or E+
O
O
R
X
*
R
E
-carbon
Enolates can have different geometries and, in addition, an electrophile can
attack from or below the double bond.
This gives scope for enantioselective control of the electrophilic addition if there is
a single enolate stereoisomer and it is in a chiral environment:
Cl
NaH
+
NH
O
N
S
O2
S
O2
O
1. BuLi, -78°C
Formation of an !-substituted-carboxylic acid with high ee
NH
+
HO
(S)
H
I
2.
LiOH, H2O2
H2O, THF
H
N
S
O2
O
S
O2
O
1st step: NaH deprotonates N-H prior to N-acylation
2nd step: formation of enolate using butyllithium followed by diastereoselective
alkylation with an akyl halide- recrystallisation will normally increased ee
3rd step: cleavage of sultam – SO2N-CO is easier to hydrolyse than a typical
amide bond due to electron withdrawing sulphone. However, the compound is
sensitive to base catalysed racemisation due to the presence of a carbonyl acidic
!-proton. LiOH is more covalent than NaOH so hydroxide is less basic and more
nucleophilic. Adding H2O2 generates HOO- which is a much better nucleophilic
than OH- and so the cleavage can be performed at low temperature.
Rationalisation of diastereoselective enolate alkylation
Deprotonation at low temperature gives a (Z)-enolate, lithium chelates to the
sulphone and oxygen giving a fixed stereochemistry. Electrophilic attack comes
from the bottom (Re) face. The direction of attack may be controlled by the steric
hindrance of the dimethyl group or due to the nitrogen lone pair.
BuLi, THF
-78 deg C
N
S
O2
N
SO2
O
O
Li
I
R-I attack from bottom (Re) face.
N
SO2
H
O
Li
I
Alternative explanation of
stereoelectronic control – electrophile
attacks from opposite face to nitrogen
lone pair
N
SO2
(S)
O
Conjugate addition to N-!% -enoyl sultams
Conjugate (Michael) addition of a nucleophile to an !%& - unsaturated carbonyl
group is a reaction that can be exploited using chiral auxilairies for asymmetric
synthesis. Addition of the nucleophile at the 'position of the prochiral double
bond can generate a new stereogenic centre. The resulting enolate may be
quenched with water to give A or it may be trapped with an electrophile to
produce a second chiral centre at the !-position i.e.B
O
X
NuR
O
-
O
Nu
E = R1-Hal
*
X
Nu
*
X
R
*
R1
E+
E = H+
O
X
Nu
*
A
R
The nucleophiles are normally organomagnesium e.g. Grignard or
organocopper reagents. As with alkylation in the previous example,
chelation to the sulphone and carbonyl oxygen plays a key role in
contolling the stereochemical outcome.
B
R
Example - Grignard addition to an N-!% -enoyl camphor sultam and trapping the enolate with MeI
n-BuMgCl, Et2O,
-78°C
Me
SO2
H
N
N
SO2
O
O
Mg
Bu
Me
H
MeI
ClH
Bu
Mg
N
Bu
SO2
O
Me
Mg
Cl
L
L
I
Re face
HCl aq.
Me
NH
SO2
+
HO
Y
Me
LiOH, H2O,
H2O2, THF
N
O
Bu
SO2
O
Bu
Explanation of diastereoselectivity
•Chelation of Mg2+ to sulphone and carbonyl oxygen gives fixed stereochemistry.
•Intramolecular addition of n-butyl anion to lower face of (E)-double bond gives high
diasteroselectivity.
•Trapping of chelated enolate at lower Re-face
•Steric hindrance of dimethyl groups may be controlling factor in face selectivity of
Me-I attack or it occurs at the opposite face to the nitrogen lone pair.
•Recrystallisation of diastereomer Y increases diastereomeric purity.
Chiral Auxiliaries - Summary
Oppolzer’s camphor sultam has been exploited in many more reactions than
shown in this course. Other asymmetric reactions carried out on the enoylsultam
system include: Diels-Alder cycloaddition, catalytic hydrogenation (Pd/C),
dihydroxylation of an alkene using osmium tetroxide, 1,3-dipolar additions. Other
diastereoselective reactions of the sultam enolate include: aldol, Mannich and !bromination.
Example
1. EtAlCl2, -78°C
SO2
R'
Me +
N
2. LiOH, THF, H2O
or LiOOH, THF,H2O
O
Diels-Alder
*
*
*
*
O
4 chiral centres
created in one
reaction
OH
98% de
There are also many more equally noteworthy chiral auxiliaries that could have
been discussed given time e.g.
HN
O
O
O
O
HN
O
HN
Me
Ph
O
Ph
oxazolidinones,
Evans, similar
applications to
Oppolzer’s
sultam
H2N
Me
HO
Ph
aminoalcohols
e.g. (-)-ephedrine
HO
HO
1,2-diols
with C2 symmetry
A Quick reminder, why asymmetric synthesis is important
Penicillamine
NH2
NH2
CO2H
SH
Antidote for Pb,
Au, Hg
CO2H
SH
Can cause optic
atrophy blindness
Enzymes in asymmetric synthesis
Nature’s catalysts – enzymes - are proteins that have evolved, in most cases,
over millions of years to carry out a particular synthetic step. Rate increases
over the uncatalysed rate of 1017 have been achieved by evolution, better
then any catalysts developed by human endeavour.
Pros:
•Enzymes exist to perform almost every possible reaction.
•Enzyme reactions are normally carried out in aqueous conditions – green
credentials.
•However, in some cases enzyme reactions also proceed perfectly well in
organic solvents.
•Enzymes are derived from poly-amino-acids so they are inherently chiral i.e.
reactions are often highly enantioselective.
Cons:
•Only one enantiomer of an enzyme will exist.
•Enzymes sometimes limited to work with a small range of substrates.
•Modification of an enzyme to enable its use with different substrates requires
gene manipulation – difficult.
•Cofactors (coenzymes) often required e.g NADPH, SAM, Coenzyme A.
•Allergic reactions sometimes occur.
•Fermenting systems – difficult to work up (emulsions) – yield by-products –
cannot accept high substrate concentrations.
Classes and function of Enzymes
Enzyme major
classes
Reaction type and enzyme subclasses
Oxidoreductases Oxidation- reduction, transferring hydrogen e.g
dehydrogenases (H- transfer), oxidases (electron transfer to
molecular oxygen), oxygenases (oxygen transfer from
molecular oxygen) and peroxidases (electron transfer to
peroxide)
Transferases
Transfer groups or atoms: amino, acetyl, phosphoryl from a
donor to a suitable acceptor
Hydrolases
Hydrolytic cleavage of bonds, R-CO2R, RCONR e.g.
proteases, amylases, acylases, lipases and esterases
Lyases
Non-hydrolytic cleavage of small molecules from C-C, C-O,
C-N by elimination to give C=C, C=O, C=N etc, e.g.
fumarase, aspartase, decarboxylase, dehydratase, aldolase
Isomerases
Catalyse isomerisation and transfer reactions within one
molecule e.g racemisation, epimerisation
Ligases
Catalyse the joining of two molecules via C-O, C-S, C-N, CC bonds with the concomitant hydrolysis of an energy rich
triphosphate (ATP)
Enzyme Cofactors and their actions
NADP+/NADPH + H+
NAD+/NADH + H+
FAD, FMN
Redox reactions and
hydrogen transfer
Coenzyme A
Transfer of acyl groups
ATP
Metabolic energy,
Phosphate-,
pyrophosphatetransfer
Pyridoxal phosphate
(PLP)
Transamination, amino
acid decarboxylation
Tatrahydrofolic acid
Transfer of C1 groups
S-Adenosyl
methionine,
Methyl-cobalamine
Methylation
The problem of cofactors
(coenzymes)
Nature uses reagents in the
same way as chemists do for
many reactions. If cofactors
are involved, they bind to the
enzyme and are intimately
involved in the catalytic cycle.
They are often complex
molecules and as they are
consumed in the reaction,
there must be a recycle system
This severely limits the range
of isolated enzymes that can
be used as part of a simple
chemical process. However
recycling systems have been
developed for most cofactors.
Example of a Hydrolase (Lipase PS from Pseudomonas cepacia, supplied
by The Amano Enzyme Company)
•Lipase enzymes hydrolyse esters.
•Work on a wide range of substrates
•Often exhibit excellent steroeselectivity
•Accept nucleophiles other than H2O (i.e. alcohols, amines)
•Do not require a cofactor
•Easy to use and inexpensive
O
O
lipase,H2O
pH 7
OEt
F
O
OEt
F
OH
F
This reaction process is not strictly asymmetric synthesis but rather an enzyme
catalysed kinetic resolution of a racemic ester.
•In order to achieve high ee the enzyme must differentiate between two groups
of very similar size, F and H
•Very unlikely that a non-enzymatic system could achieve this.
Synthesis of racemic ethyl 2-fluorohexanoate
O
OEt
O
KF, acetamide
Bu4N+F-, 140 °C
O
= acetamide
OEt
Br
Me
NH2
F
Bu4N+F- is a phase transfer catalyst
•Enzymic resolution – only requires ca 60 mg of enzyme for 10 g of the fluoroester.
•As the hydrolysis proceeds the formation of RCO2H causes the pH to fall.
•0.1 M NaOH solution is added by syringe pump (on laboratory scale) to maintain
the pH at 7. This provides an accurate means of monitoring the degree of
conversion.
O
OEt
F
(R,S)
lipase,H2O
0.05 M phosphate buffer
pH 7, 0.1 M NaOH, 5 °C
O
O
( R)
( S)
O -Na+
OEt
60% conversion
F
>99% ee
Ethyl (R)-2-fluorohexanoate (99% ee) - At 60%
conversion, the mixture is partitioned in an ether–
water mixture, the sodium salt remains in the
aqueous phase and the ethyl fluorohexanoate
(99% ee) is extracted into the ether layer.
F
69% ee
3M HCl
O
( S)
OH
F
69% ee
Ethyl (S)-2-fluorohexanoate (99% ee)
This enzyme preferentially hydrolyses the (S)-isomer, but selectivity is not 100%.
This means that a more lengthy procedure is required to give the pure (S)-ethyl
ester. It can be summarised as follows:
esterify
75% conv. (S)-fluoroacid esterify
40% conv.
(S)-fluoroester
(S)-fluoroester
(S)-fluoroacid
± fluoroester
100% ee
100% ee
90% ee
90% ee
O
OEt
1. lipase,H2O
0.05 M phosphate bufer
pH 7, 0.1 M NaOH, 5 °C
2. 3MHCl
40% conversion
OH
+
F
O
(S)
OEt
F
OEt
OH
90% ee
F
EtOH, cat conc. H2SO4
reflux
O
75% conversion
(S)
OEt
F
EtOH, cat conc. H2SO4
reflux
ca 100% ee
1. lipase,H2O
0.05 M phosphate bufer
pH 7, 0.1 M NaOH, 5 °C
2. 3MHCl
O
(S)
+
F
O
(S)
O
OEt
F
ca 100% ee
O
90% ee
F
Rationale of the enantioselective hydrolysis
The similar size of H and F suggest that electronic factors must be involved, in
addition to the usual steric factors, in order to give such high enantioselectivity.
During hydrolysis the ester undergoes nucleophilic attack from an alkoxide, usually
from the amino acid serine. A diastereomeric complex is formed as the attack of
RO- takes place. Ab initio calculations show that one diasteroemeric complex is
favoured by lone pair donation of the incoming alkoxide to the * orbital of the
C-F bond. The result is backside attack of the alkoxide approaching antiperiplanar
to the fluorine atom, before attack at the carbonyl occurs.
HN
N
R
H
O
O
EnzO
-
O
R
O
N
H
H
F
1st step of serine mediated
hydrolysis
R
O
Initial histidine assisited attack of the
serine OH residue on the ester
carbonyl
Esterification using lipases
Lipases in organic solvents can be used to catalyse the reverse process,
kinetic resolution by esterification with high enantioselectivity. Typically vinyl
acetate is used as the by-product tautomerises to ethanal and prevents the
reverse reaction. e.g.
O
OH
O
+
O
vinyl acetate
H
lipase CAL
(from Candida)
hexane
F3C
25% conv.
O
+
(S)
HO
F3C
>99% ee
tautomerises
H
Lyases - Hydroxynitrile lyases, often called oxynitrilases catalyse the reversible
enantioselective addition of HCN to aldeydes and ketones. Most of the enzymes
are obtained from plants that release HCN from cyanogenic-glycoside or -lipid.
Almonds are a well known source of this enzyme. No cofactor is required
H
HCN +
Conditions
H2O/EtoH
iPr O/Avicel
2
OH
(R)-Prunus amygdalus HNL
O
Ph
O
O
Ph
yield 99%, ee 11%
yield 99%, ee 98%
O
H
(R) CN
Avicel is a cellulose membrane
for enzyme immobilisation.
Oxoreductases
Enzymatic oxidation and reduction requires a cofactor in most cases.
Nevertheless, they are still very useful and can perform reactions not readily
available by conventional chemical synthesis.
For example a dioxygenase from Pseudomonas putida catalyses the oxidation
of arenes with oxygen to cis-cyclohexanediols. This is an NADH/NADPH
dependant system so the reactions are generally carried out with whole cells to
avoid the need for added cofactors – Examples:
MeO
P. putida
O
OMe
OH
O
O
Diels-Alder
O
+ O2
N
O
OH
O
O
From toluene to a highly functionalised single enantiomer
containing 4 stereogenic centres in 3 steps – other examples:
Cl
CO2H
Cl
P. putida
Ph
Ph
N
O
CO2H
OH
OH
P. putida
+ O2
+ O2
OH
OH
R
R
Catalytic antibodies (Abzymes)
Proposal - an antibody generated to bind to a stable analogue of the transition
state for the rate determining step of a chemical reaction should be a catalytic
protein for that reaction.
Enzymes vs antibodies
•Enzymes evolve to bind to high energy transition states.
•Antibodies evolve to bind to molecules in their ground state.
•Enzyme evolution – millions of years
•immunological evolution of an antibody – weeks
•Antibodies can be created for both enantiomers.
•Antibodies can be created for reactions for which no known enzyme exists.
Immune system has the ability to generate more than 1012 unique
antibodies to any antigen (foreign protein, virus, bacteria, parasite, fungi).
Antibodies are glycoproteins.
Antigen binding
sites
membrane
bound
antibodies
free antibodies
antigen
plasma
cell
B lymphocyte
Antibody production in vivo
Antibody protein
structure
1.
Macrophage and B-lymphocytes recognise foreign antigen
2.
Absorb the material into cell and cut protein into smaller peptide units - a unique
system of rapid mutation called gene translocation is able to ‘evolve’ an antibody to
any possible antigen. In B-lymphocytes this is catalysed by an enzyme called
antibody recombinase. The mutation rate is 100,000 faster than occurs in other cells
and generates membrane bound markers on the cell surface.
3.
These makers bind to helper T-cells which release lymphokynes
4.
Lymphokynes switch on B-lymphocytes to mature and produce antibody producing
plasma cells
5.
Plasma cells release large amounts of free antibodies into the serum (2000 antigen
molecules per second from one cell)
6.
Free antibodies-bind to antigen – quickly recognised and eliminated
How can we use immune system to generate catalysts?
•Design transition state analogue – a hapten
•Conjugate (link) the hapten to a larger antigen e.g. a protein such as BSA, bovine
serum albumin or keyhole limpet hemocyanin (KLH).
•Raise antibodies
•Screen for hapten-bound antibodies
•Screen for catalytic activity
•Select monoclonal antibody and amplify production of the protein catalyst to give
typically 30 mg of purified antibody.
This method required innoculation of animals. Now most of the process can be
performed in vitro. This requires cloning the immunological system of a hyperimmunised mouse into a bacteriophage which infects E.coli. The result is that
the enormous number of possible antibody proteins are encoded in the bacteria.
In some cases the initial animal immunisation can be avoided.
A phage is a single strand dna virus which infects bacteria.
Concept first successfully demonstrated -1986
Examples
Key to success - choice of reaction and design of the hapten
Hydrolytic antibodies have achieved 106 x rate of uncatalysed reactions.
Kinetic resolution of an ester by enantioselective hydrolysis
•Hapten design – transition state has a tetrahedral shape with three oxygens,
two of which have a partial negative charge.
•A phosphonate ester has a similar shape, charge distribution and is stable.
R
H
H (O
O
O
P
O
(-
O
O
R
O (R)
R
R
R
RDS of ester hydrolysis
-
BSA
R
O
O
O
hapten – homochiral tetrahedral phosphonate
ester linked to bovine serum albumin antigen
OH
O
CHF2
HO (R)
+
cat. antibody
49% conversion
kinetic resolution
O
CHF2
CHF2
O
99% ee
O
+/+
O
CHF2
unreacted
•Catalytic antibodies are not as efficient catalysts as enzymes, but are useful for
reaction for which enzymes are very rare and they can be obtained for almost any
substrate. Anti-bodies for both enantiomers can be generated.
•Pericyclic reactions – almost no enzyme counterparts exist, examples have only
recently been discovered.
•Example: A Diels-Alder reaction between a mono-substituted diene and a
dienophile could theoretically yield 8 stereoisomers.
The aim is to control: regio- and stereo- and enantio-selectivity. Two rigid bicyclic
haptens were designed, with the boat-shape of the Diels-Alder transition state.
One was the favoured endo-transition state and the other for the disfavoured
exo.
Reaction
CONMe2
H
CONMe2
HN
O
O
HN
endo-hapten
HN
O
endo gives
cis
O
O
H
exo-hapten
O
O
CO2-
CONMe2
O
O
O
O
O
exo gives
trans
Diels Alder adducts obtained using the catalytic antibodies raised against the
endo and exo haptens
CONMe2
O
NH
O
exo antibody
22CB
'endo' antibody
7D4
HN
O
98% ee
O
CO2-
CONMe2
CONMe2
CO2-
O
NH
98% ee
O
CO2-
•High regioselectivity
•High stereoselectivity: relative configuration of amides, cis (normally
favoured endo product) versus trans (normally disfavoured exo
product)
•High enantioselectivity
Aldolase catalytic antibodies – now commercially available from Aldrich,
match the activity of natural enzymes – accept a wider range of substrates.
Aldol reaction
:O-
O
H R
R1
R
OH
1
R
2
O
R
R2
H
R1
*
*
R
R, R1, R2 = H, alkyl, aryl
B:
O
R" = H can form1 new asymmetric
centre
R = alkyl or aryl, 2 new asymmetric
centres possible
The two step aldol reaction in the lab can have problems of by-products due to
cross condensation. One way round this is to prepare an enamine as a ‘masked’
enolate. Enzymic aldolases do the same trick. A lysine residue on the enzyme
first reacts with a carbonyl and the imine residue tautomerises to an enamine.
This reacts with an incoming aldehyde.
O
N
NH
N+
N:
+
H
H2O
OH
R
Laboratory enamine reaction
O
R
R
O-
Enamine
O
To copy nature’s mechanism, ‘reactive immunisation’ was used: The diketone
of the hapten acts a chemical trap for lsyine residues during the antibody
H
evolution and yielded successful aldolase antibodies.
O
N
Ab
O
O
O
O
X
HO2C
N
H
N
H
amine
'trap'
O
HN
Ab
lysine
residue
hapten
O
conjugate to
antigen protein
X
Ab = absyme
N
H
Examples of using catalytic aldolase antibody 38C2:
O
OH
O
H
+
Antibody
38C2
Si face
attack
O2 N
98% ee
O2N
O
O
+
O
Antibody
38C2
OH
O
H
OH
>98% ee OH
Hydroxyacetone gives a regioselective reaction at the less favoured hydroxy-carbon
Separation of racemates - Resolution
•Despite the continuing advances in asymmetric synthesis techniques
available, many industrial processes still employ classical resolution of a
diastereomeric salt or or ‘covalent’ diastereomer.
•Important consideration that may lead to a resolution process being preferred
•Practicality of synthesis
•speed to market
•economics and short effective patent life
Synthesis of enantiopure drugs at
Lilly in 1993
‘Chiral pool’ or
semi-synthesis
5.5 (20%)
Asymmetric
synthesis
6.0 (22%)
Resolution (ionic
or covalent)
15.5 (57%)
Data gathered from 40 drug
candidates at phase I to phase III of
development
In any resolution the compound must
have a ‘handle’ with which to form a
salt or covalent link.
The ideal resolution system also
involves an in-situ racemisation
process, in which the unwanted
diastereomer is soluble and the
desired diasteromer crystallises out.
This drives the equilibrium to give
100% of the desired diastereomer.
Merck devised an elegant in situ racemisation system:
•small amount of aldehyde forms an imine, lowers pKa of acidic proton
•free amine present deprotonates and thus racemises the chiral centre
•resolving agent (+)-(S)-camphor-10-sulphonic acid (CSA) crystallises out the
diastereomerically pure salt containing only the (S)-amine..
Me
N
N
Me
Me
pKa=20
O
H
3 mol%
ArCHO
NH2
-ArCHO
pKa=12
O
N
N
N
N
base
H
O
N
CHAr
CHAr
H
N
unwanted (R) - amine
-ArCHO
+ArCHO
Me
O
N
HO
Cl
Ar =
O
precipitates
as (S)-CSA
salt
SO3H
Cl
(+) camphor-10-sulphonic acid
NH2
(+)-CSA
(S)
N
H