Mimicking enzymatic transaminations: Attempts to understand and develop a catalytic asymmetric approach to chiral α-amino acids

Article abstract of DOI:10.1039/b404381b

Attempts are made to build a bridge between asymmetric catalysis and enzymatic reactions by mechanistic investigations and the development of a catalytic and enantioselective approach to amination of α-keto esters by primary amines catalyzed by chiral Lewis acids as a model for transamination enzymes. Different Lewis acids can catalyze the half-transamination of α-keto esters using primary amine nitrogen sources such as pyridoxamine and 4-picolylamine. The mechanistic studies of the Lewis-acid catalyzed half-transamination using deuterium-labelled compounds show the incorporation of deuterium atoms in several positions of the α-amino acid derivative, indicating that the enol of the α-keto ester plays an important role along the reaction path. The catalytic enantioselective reactions are dependent on the pK_a-value of the solvent since enantioselectivities were only obtained in solvents with high pK_a-values relative to methanol. However, stronger acidic conditions generally gave better yields, but poor enantioselectivities. A series of chiral Lewis acids were screened as catalysts for the enantioselective half-transamination reactions and moderate yields and enantioselectivities of up to 46% ee were obtained.

Full text of DOI:10.1039/b404381b

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Mimicking enzymatic transaminations: attempts to understand and
develop a catalytic asymmetric approach to chiral ꢀ-amino acids
Stephan Bachmann, Kristian Rahbek Knudsen and Karl Anker Jørgensen*
Danish National Research Foundation: Center for Catalysis, Department of Chemistry,
Aarhus University, DK-8000 Aarhus C, Denmark. E-mail: kaj@chem.au.dk;
Fax: ϩ45 8619 6199; Tel: ϩ45 8942 3910
Received 25th March 2004, Accepted 19th May 2004
First published as an Advance Article on the web 21st June 2004
Attempts are made to build a bridge between asymmetric catalysis and enzymatic reactions by mechanistic
investigations and the development of a catalytic and enantioselective approach to amination of α-keto esters by
primary amines catalyzed by chiral Lewis acids as a model for transamination enzymes. Different Lewis acids can
catalyze the half-transamination of α-keto esters using primary amine nitrogen sources such as pyridoxamine and
4
-picolylamine. The mechanistic studies of the Lewis-acid catalyzed half-transamination using deuterium-labelled
compounds show the incorporation of deuterium atoms in several positions of the α-amino acid derivative,
indicating that the enol of the α-keto ester plays an important role along the reaction path. The catalytic
enantioselective reactions are dependent on the pK -value of the solvent since enantioselectivities were only obtained
a
in solvents with high pK -values relative to methanol. However, stronger acidic conditions generally gave better
a
yields, but poor enantioselectivities. A series of chiral Lewis acids were screened as catalysts for the enantioselective
half-transamination reactions and moderate yields and enantioselectivities of up to 46% ee were obtained.
Introduction
Mimicking biological systems in the laboratory by “simple
organic molecules and catalytic systems” is difficult to achieve
in general. First of all, the detailed reaction mechanisms
in biological systems are often only partially elucidated and
are complicated compared to the detailed insight chemists
normally have into reaction mechanisms. Second, enzym-
atically driven reactions usually have a ∆G-value close to 0 kcal
Ϫ1
mol , and therefore the reactions can proceed in both
directions. Such reactions are thus difficult to catalyze in the
desired direction, as the catalyst might also promote the reverse
reaction. Third, reactions in biological systems take place in
water, and water acting as solvent in combination with chiral
metal catalysts is generally problematic. Either the catalyst and/
or substrate are not soluble in water, or the catalyst is deactiv-
ated by water. Therefore, if one wants to perform catalytic
enantioselective reactions in water, either specifically modified
water-soluble catalysts, or different approaches in organic
Scheme
1
Mechanism of the natural enzyme controlled half-
transamination (Enz = enzyme).
1
solvents are required.
The biological transformation of α-keto acids 1 to α-amino
2a
acids 5 (also known as half-transamination ) is catalyzed by
2
vitamin B -dependent aminotransferase. This transformation
6
proceeds via ketimine 3 which is stereoselectively protonated to
give the optically active α-amino acid fragment as aldimine
Fig. 1 Influence of PLP-dependent enzymes on the lability of the
different bonds on the α-carbon atom of an amino acid substrate.
4
. The transamination of 1 to α-amino acids 5 takes place
via several protonation/isomerisation steps as outlined in
Scheme 1.
is accomplished by binding the amino acid substrate with the
The transamination enzyme ensures formation of the
optimal geometry for the protonation/isomerization steps
and incorporates the base for deprotonation. However, optimal
geometry is one of the most important factors for these
reactions and for pyridoxal phosphate (PLP) dependent
enzymes in general. The PLP-dependent enzymes labilize one
of the three bonds of the α-carbon atom of an amino acid
substrate (Fig. 1). The bond a is labilized by aminotransferase,
C –H bond perpendicular to the PLP-ring, i.e. the choice of
α
one of the three possible catalytic outcomes is determined by
3
stereoelectronic effects.
The non-enzymatic transamination of α-keto acids with
pyridoxamine and its derivatives in the presence of different
4
metal ions has been intensively studied. Different approaches
5
6
including chiral ligands, chiral pyridoxamine derivatives,
7
8
9
polymeric, dendrimeric pyridoxamines as well as others have
been achieved, but to the best of our knowledge no catalytic
enantioselective reaction has been developed until now.
3
bond b by decarboxylases and bond c by aldolases. It has been
stated that an important principle for bond breaking is that the
bond being broken must be perpendicular to the π-orbitals of
the PLP-pyridine ring. In the aminotransferase (bond a) this
We recently communicated the first attempts towards the
catalytic and enantioselective formation of α-amino acid
3
2
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derivatives via half-transamination of α-keto esters in organic
solvents with zinc() and copper()-bisoxazoline complexes.
Table 1 Results of the Lewis-acid catalyzed half-transamination of
a
10
ethyl pyruvate 1a with pyridoxamine 2a
This approach includes (chiral) Lewis acid catalysts to fix the
reactants in the optimal geometry and to enhance the reactivity
of the pyridoxamine. In the in vivo system, an enzyme controls
the reaction course in a highly selective manner by binding the
substrate and PLP in the optimal geometry. This is the main
challenge in mimicking such systems.
b
Entry
Catalyst
Yield
c
1
2
3
Zn(OTf )2
AlMe3
28
35
37
22
22
38
28
55
30
43
c d
c
InCl ؒ3H O
3 2
4
5
6
7
La(OTf )3
ZrCl
In this paper we will disclose the development and mech-
anistic investigations of our attempts to mimic enzymatic
4
B(OH)3
BEt3
BEt3
B(OPh)3
BCl3
transamination reactions by
a catalytic enantioselective
e
8
approach using chiral Lewis acids as catalysts for the reaction
of α-keto esters with primary amines as the nitrogen source.
The challenge we are facing is outlined in Scheme 2: First we
have to control the formation of ketimine 3, second to control,
in a stereoselective manner, the formation of aldimine 4, and
finally the hydrolysis of 4 to release the optically active α-amino
acid. Furthermore, we have to try to drive the half-transamin-
ation in the desired direction: ketimine to aldimine to α-amino
acid. This [1,3]-proton-transfer in native or artificial systems
9
0
1
a
b
For reaction conditions see Experimental Section. Isolated yield
after protection of the free amine with (Boc) O. From reference 10.
Reaction performed in MeNO2. 10 mol% of the catalyst was used.
c
2
d
e
Table 2 Summary of the Lewis-acid mediated half-transamination of
different pyruvate derivatives 1a–1g with pyridoxamine 2a
a
9
has been extensively studied. We will show that the
1
2
b
Entry
Catalyst
R
R
Yield [%]
half-transamination can be catalyzed by Lewis acid complexes
(
ML* in Scheme 2) and in order to develop a catalytic
1
2
3
Zn(OTf )2
Zn(OTf )2
BEt3
Zn(OTf )2
Zn(OTf )2
Zn(OTf )2
Zn(OTf )2
Zn(OTf )2
Me
CH i-Pr
CH i-Pr
(CH ) C(H)᎐CH
CH -indole
CF3
CH(Br)-CH3
Me
Et 1a
Et 1b
Et 1b
Et 1c
Me 1d
Et 1e
Et 1f
5a 28
5b 6
5b 33
5c 40
5d 19
5e 0
enantioselective approach, labelling studies, reaction condition
and ligand screenings were performed. Finally, the catalytic
enantioselective developments will be outlined and some
mechanistic details discussed.
2
c
2
d
4
2
2
2
5
6
7
8
2
5f 0
5g 15
CH Ph 1g
2
a
b
For reaction conditions see Experimental Section. Isolated yield
c
after protection of the free amine with (Boc) O. 10 mol% of the cata-
lyst was used. No ligand-accelerating effect was observed as 7a gives
4
2
d
0% yield as well.
Scheme 2 The stereoselective induction in the asymmetric half-
transamination is determined by the protonation of ketimine 3 giving
aldimine 4, which is hydrolyzed to the α-amino acid derivative 5.
and 28% yield of 5a was found (entry 7). To our surprise the
yield of 5a improved to 55% when 10 mol% of BEt was used
3
(
entry 8). We believe that the presence of a higher amount
of the boron compound deactivates either the substrate, the
pyridoxamine or activates the reverse reaction. Furthermore,
Results and discussion
B(OPh) was found to be an active catalyst and shows a com-
3
Catalytic half-transaminations of ꢀ-keto esters
parable activity to BEt and Zn(OTf ) under the same reaction
3
2
Different Lewis acids can catalyze the half-transamination of
conditions. BCl , the formally strongest Lewis acid among the
3
1
ethyl pyruvate 1a with pyridoxamine 2a (Scheme 3, R = Me,
boron compounds tested, gave 43% yield of 5a (entry 10).
The Lewis-acid transamination shows a ligand accelerating
effect as e.g. Cu() and Ag() in combination with 1,2-diphenyl-
phosphinoethane are efficient catalysts and up to 51% yield of
2
R = Et). The screening results are presented in Table 1.
10
5
a was obtained. For catalyst 7a, a chiral Lewis acid (Fig. 2,
vide infra), we also observed a ligand accelerating effect and the
yield of 5a was slightly improved compared to Zn(OTf ) as the
2
catalyst.
The Lewis acids can catalyze the half-transamination of
1
different alkyl-substituted (R = CH (1a), CH -i-Pr (1b), (CH ) -
3
2
2 2
C(H)᎐CH (1c) or CH -indole (1d)) α-keto esters (Scheme 3).
᎐
2
2
The yield of the corresponding Boc-protected α-amino ester is
1
2
dependent on the substituents R and R and 6–40% yield of 5
Scheme 3 Half-transamination of different α-keto esters 1 with
pyridoxamine 2a.
were obtained with 20 mol% of Zn(OTf ) or 10 mol% of BEt3
2
as the catalyst (Table 2, entries 1–5). The isopropyl derivative 1b
gives low yield of the corresponding Boc-protected α-amino
ester 5b, whereas the alkene-derived 1c gives the best yield
for the Zn()-catalyzed half-transamination in MeOH. As
Lewis acids, such as Zn(OTf )2, AlMe3, InCl ؒ3H O,
3
2
La(OTf )3 or ZrCl4 can catalyze the half-transamination of
ethyl pyruvate 1a with pyridoxamine 2a (Table 1, entries 1–5).
Interestingly, it is possible to avoid the use of a metal Lewis acid
catalyst since boron compounds such as boranes, borates or
even boronic acid catalyze the half-transamination of 1a in
acceptable yields. We started testing boronic acid as the catalyst
mentioned previously, the use of BEt had a positive effect on
3
the reaction yield and for 1b as the substrate, an improvement
to 33% yield for 5b was obtained (entry 3). The yields of the
corresponding Boc-protected α-amino esters are dependent on
the steric bulk of the substituent on the α-keto function and
only n-alkylated substrates gave acceptable yields, while for
α-keto esters having a branched substituent a drop in reactivity
was observed. Substrates having a CF (1e) or CH(Br)-CH (1f )
(
20 mol%) and 38% yield of the Boc-protected alanine
derivative 5a was obtained (entry 6). Since it was not obvious if
the catalytic activity of boronic acid is due to Lewis or Brønsted
acidity, different boron compounds such as BEt , B(OPh) and
3
3
3
3
BCl were tested. BEt showed the same activity as Zn(OTf )
substituent in α-position to the keto function were found to be
3
3
2
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Fig. 2 Chiral catalysts for the half-transamination of α-keto esters.
unreactive (vide infra, entries 6, 7). Larger ester functions in the
4-nitro- and 6-nitro salicylaldehyde. They showed that only the
substrate are also tolerated and for benzyl pyruvate 1g 15%
yield of 5g was isolated (entry 8).
2-formyl- and the 3-hydroxy group attached to the ring are
necessary to obtain a reactive reverse half-transamination
system.
4d,e
We observed good catalytic activity in the Zn()-catalyzed
transamination of methyl-3-indole pyruvate 1d with 4-picolyl-
amine 2b in MeOH, where 66% isolated yield of the Boc-
protected methyl tryptophan ester 5d was obtained. This is
in contrast to the transamination of 1d with pyridoxamine
Mechanistic studies
In an attempt to obtain information about the reaction
mechanism and intermediates, we have studied (i) the influence
of different primary nitrogen sources and (ii) reactions in
deuteurated solvents.
2
a which gives only 19% yield of 5d for the Zn()-catalyzed
reaction. This indicates that the half-transamination system
using 2a as the nitrogen source is less active compared to 2b by
a factor of ca. 3. It should also be noted that the other two
nitrogen sources, 2c and 2d, were unreactive and no product
was isolated, and therefore no further simplification of the
system was possible.
Different primary amine sources
4i
4e
Matsushima and Matsumoto, and Snell et al. have studied
the influence of different primary amines as nitrogen sources in
the Al()-mediated half-transamination of ethyl pyruvate and
pyruvic acid in MeOH, respectively. They found that among
pyridoxamine 2a, 4-picolylamine 2b, 3-hydroxy-4-picolylamine,
3
-hydroxy pyridine and benzylamine, only the former was an
Deuterium experiments
4i
active nitrogen source. Our observations under catalytic con-
ditions are in accordance with these results as the testing of 2a,
2
nitrogen sources in the Zn()-catalyzed transamination of 1a in
MeOH gave only the Boc-protected alanine derivative 5a (28%
yield) when using 2a, while the other nitrogen sources were
unreactive. This shows that for the transamination of 1a by 2a
The half-transamination reaction of methyl-3-indole pyruvate
1
e with pyridoxamine 2a catalyzed by Zn(OTf ) in CD OD
2 3
b, 4-nitro benzylamine 2c and benzylamine 2d (Scheme 4) as
gives three differently labelled products: mono-, double and
triple deuterated Boc-protected methyl tryptophan ester 5d
were found in low yield (6%), according to MS and the spin
1
system in the H NMR spectrum. It is noteworthy that the
reaction of 1d with 4-picolylamine 2b catalyzed by Zn(OTf ) in
2
(
i) the pyridine N-atom plays an important role probably acting
CD OD gives at least two differently labelled products in 16%
3
as a proton acceptor and (ii) that the 3-hydroxy group is
necessary for coordination to the metal center. However, to our
surprise we have found that the half-transamination of 1a by 2b
isolated yield. In this case the mono-deuterated product was not
detected, but double and triple deuterated derivatives were
isolated. Interestingly, not only the expected position at the
newly formed stereocenter showed incorporation of
deuterium atom, but also the CH -group in the α-position to
took place in MeOH using BEt as the catalyst resulting in 9%
3
a
yield of 5a. This is, according to our knowledge, the first time
that a nitrogen source having no substituent in the 3-position of
the pyridine ring has been applied for the half-transamination
reaction. The two other nitrogen sources, 2c and 2d, are not
2
the keto function, indicating that the enol is probably the
reactive species (see Scheme 5).
useful as donors for the BEt -catalyzed reactions. These
3
findings could indicate that the boron-catalyzed transamination
reaction follows a different mechanism compared to the Zn()-
catalyzed reaction.
Scheme 5 Proposed mechanism for deuterium incorporation to the
product via enolisation of the substrate.
Scheme 4 Different nitrogen sources for the zinc() or boron()
catalyzed half-transaminations of α-keto esters.
Based on the isolated yields we can compare the reaction
Snell et al. have also investigated the use of different nitro
substituted salicylaldehydes for the reverse half-transamination
reactions, i.e. the reaction following the opposite course to the
one being currently under investigation. In contrast to our
findings, they observed conversion in the Al()-mediated
half-transaminations of glutamic acid or serine, applying
rates for the two different nitrogen sources, pyridoxamine
2a and 4-picolylamine 2b in MeOH and CD OD. The half-
3
transamination of methyl-3-indole pyruvate 1d with 2a in
MeOH is approximately three times faster than in CD OD (k /
4d,e
3
H
kD ≅ 3.2), whereas for the reaction with 2b an isotope effect
of ∼4 results (k /k ≅ 4.1). It is notable that 2a is less reactive in
H
D
2
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a
Table 3 Influence of different solvents on the enantioselective half-transamination of methyl-3-indole pyruvate 1d with 2b
b
c
Entry
Catalyst
Reaction time/h
Solvent
Yield [%]
Ee [%]
d
1
2
3
4
5
6
7
7a
7a
7a
7a
7a
7b
7b
20
40
40
40
40
40
40
MeOH
i-PrOH
MeNO2
PrNO2
Toluene
MeNO2
THF
50
18
10
15
33
15
25
rac
rac
19 (d )
5 (d )
rac
20 (d )
rac
d
d
a
b
c
For reaction conditions see Experimental Section. Isolated yield after protection of the free amine with (Boc) O. The enantiomeric excess was
2
d
determined with a Chiralpak AS column. From reference 10.
the half-transamination of 1d than 2b, because no successful
application of the latter compound has been known until now.
This could be an indication that different reaction mechanisms
for the two nitrogen sources are operating. Notably, the
are given in Table 3. The results in Table 3 indicate that the pKa-
value of the solvent affects enantioselectivity and yield of the
reaction. In nitroalkanes the yields are generally lower com-
pared to alcohols, but MeNO is the best solvent regarding the
2
reaction of ethyl pyruvate 1a with 2a catalyzed by Zn(OTf ) in
enantiomeric excess (entries 1–4). On the other hand we were
very surprised to find that the half-transamination reactions
can also proceeds in aprotic solvents such as toluene, CHCl3
and THF, and the relatively high yields obtained of 5d in tolu-
ene and THF are astonishing (entries 5,7). The reaction can
also be performed in t-BuOH, 1,2-dimethoxy ethane and N,N-
dimethyl formamide affording low yields (<10%) and racemic
2
CD OD afforded three differently deuterated products, as well
3
as a mono deuterated species as the mainly isolated compound.
This reaction is only a factor 0.7 (k /k ≅ 1.4) slower in CD OD
H
D
3
compared to the reaction in MeOH. These deuterium experi-
ments indicate that although 1d, both in the presence or absence
of Zn(OTf ) , is present in the keto form only in CD OD, it is
2
3
the enol that is active during the reaction. The low amount of
enol present in solution could be an explanation for the low
yields found for this system. In e.g. CDCl or CHCl , 1d exists
mainly in the enol, but the solvent is not protic enough to afford
acceptable yields of the α-amino acid derivative (see Table 2).
Support for the enol of the α-keto ester being involved in the
half-transamination reaction is obtained from attempts to
aminate methyl-3-indole glyoxylate and ethyl phenylglyoxylate,
where enolisation is impossible. Both compounds were found to
be unreactive under the standard reaction conditions.
products. However, in CHCl only 8% ee of 5d was obtained
3
and the product was formed with the opposite absolute con-
11
figuration compared to the reactions performed in MeNO2.
3
3
The screening shows that the solvent has an important
influence on yield and enantioselectivity and it should be noted
that enantioselectivity is observed only in combination with
low yields of the Boc-protected α-amino acid derivative. If
the reaction is performed in a solvent that gives acceptable yield
the product is formed as a racemate. One important fact to
mention is that independent of the solubility of the substrate in
the particular solvent, a slurry appears after the addition
of the primary amine (2b) which is constantly present through-
out the reaction. The low solubility might be responsible for
the poor yields. Only in DMF was a homogenous solution
observed.
Catalytic enantioselective half-transaminations
Based on the observations mentioned above, our general
interests in catalytic enantioselective reactions and the import-
ance of these reactions, we have tried to apply this information
for an asymmetric version of this reaction. Recently, we
communicated the first example of a catalytic enantioselective
half-transamination of methyl-3-indole pyruvate 1d with
Recently Wong and Kiruba reported a theoretical study
concerning the different tautomeric forms of vitamin-B
derivatives in different solvents. In accordance with our
experimental findings, they reported that the zwitterionic form
6
12
10
Zn()- or Cu()-bisoxazoline catalysts in protic solvents. For
these systems the Boc-protected methyl tryptophan ester 5d was
formed in about 20% yield and up to 37% ee. Due to the higher
reactivity of 4-picolylamine 2b compared to pyridoxamine 2a,
we decided to do the further screening exclusively with 2b. We
have studied different factors including Lewis acids, solvents,
ligands, counterions and additives for this reaction to improve
yield and enantioselectivity (Scheme 6).
(
2aЈ) predominates in polar solvents, whereas the neutral form
is dominant in apolar solvents (Scheme 7).
Scheme 7 Tautomerism model for pyridoxamine 2a.
This tautomerism is not apparent for 4-picolylamine 2b,
where only the pyridine N-atom can be protonated. Anyway,
the lack of formation of this protonated form, or the lower
solubility of this intermediate in less polar solvents could
explain the lower reactivity for the half-transamination in less
polar solvents. We know that especially the pyridine N-atom is
important for the reaction, since no product was found with
benzylamine or 4-nitro benzylamine as the nitrogen sources and
thus the protonation of the pyridine N-atom and the formation
of an ionic intermediate are important for the half-transamin-
ation reaction to take place. However, the relatively high yield
in toluene (33%), a classic apolar solvent where the neutral
form should be predominant, can not be explained by this
tautomerisation model. Therefore, we suggest that different
mechanisms are in operation when using the nitrogen sources
pyridoxamine or 4-picolylamine.
Scheme 6 Catalytic, enantioselective half-transamination of methyl-3-
indole pyruvate 1d with 4-picolylamine 2b.
Influence of the solvent
The first factor investigated was the influence of the solvent.
The screening of a variety of protic and aprotic solvents in the
reaction of methyl-3-indole pyruvate 1d with 4-picolylamine 2b
in the presence of the chiral bisoxazoline catalysts 7a,b (Fig. 2)
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Table
4
Results of the half-transamination of methyl-3-indole
pyruvate 1d with 4-picolylamine 2b catalyzed by Cu()–P–N catalysts
the P–N–Cu() complexes were able to induce enantioselectivity
a
in the reaction using 1d as the substrate. In the course of our
screening we find now that the enantioselectivity obtained with
the catalysts 10–12 shown in Fig. 2 is dependent on three
factors: (i) the steric bulk at the substituents on the phosphorus
atom, (ii) the steric demand of the oxazoline moiety and (iii)
the coordination properties of the counterion. The isolated
b
c
Entry
Catalyst
Reaction time/h
Yield [%]
Ee [%]
1
2
3
10a
10b
10b
10b
10c
11
12a
12b
12c
40
40
80
40
40
40
40
20
40
10
13
10
18
9
11
16
16
10
14 (d )
46 (d )
38 (d )
10 (d )
17 (d )
11 (d )
18 (l )
6 (l )
d
e
4
yield of 5d obtained in MeNO was generally low and varied in
2
5
6
7
8
9
a range from 10–20%. Catalyst 10b having the indane moiety at
the oxazoline and two tolyl functions at the phosphorus atom
turned out to be the best chiral complex giving 46% ee for 5d
(
Table 4, entry 2). The enantioselectivity drastically drops when
14 (l )
the steric bulk on the phosphorus part is reduced to phenyl
a
b
For reaction conditions see Experimental Section. Isolated yield
groups (catalyst 10a) instead of tolyl (catalyst 10b) (entry 1 vs.
c
after protection of the free amine with (Boc) O. The enantiomeric
excess was determined with a Chiralpak AS column. Reaction
performed at 0 ЊC. Reaction performed in toluene as the solvent.
2
Ϫ
d
2), or if a stronger coordinating counterion, such as ClO
4
Ϫ
e
instead of PF6 (catalyst 10c) is used (entry 5 vs. 2). This
indicates that stronger coordinating ions or solvents may
distort the optimal geometry of the intermediate/transition
state and therefore face-shielding is lowered, resulting in
reduced enantioselectivities.
Additives
Decreasing the reaction temperature to 0 ЊC lowered the
enantiomeric excess of adduct 5d to 38% ee (entry 3). However,
it is important to note that enantioselectivity, although low
Different additives such as HFIP (1,1,1,3,3,3-hexafluoroiso-
propanol), ethylene glycol, NEt and silica were tested for their
3
influence on the reaction (yield and selectivity). In contrast to
(
10% ee) can also be obtained with 10b as the catalyst in toluene
the half-transamination of ethyl pyruvate 1a with pyridox-9
(entry 4). This is the first time we have observed an asymmetric
induction in an aprotic solvent.
The third factor affecting the enantioselectivity is the size of
the oxazoline moiety. For this part of the chiral ligand, the
trend is inverted compared to the phosphorus part. An
enlargement of the steric bulk on the oxazoline by an additional
amine 2a, where HFIP had no influence on the reaction yield,
the reactivity could be improved for the half-transamination of
methyl-3-indole pyruvate 1d with 4-picolylamine 2b catalyzed
by 7a and 38% yield of the Boc-protected methyl tryptophan
ester 5d was obtained. The use of HFIP has a positive effect on
the yield of 5d, but unfortunately no enantiomeric excess was
formed for the isolated product, whereas the reaction without
CH -group in the ring next to the oxazoline portion (ligand 11)
2
has a negative effect on the enantiomeric excess and only 11%
ee was observed of the Boc-protected methyl tryptophan ester
HFIP gave 19% ee. The use of NEt in toluene did not give any
3
improvement of yield or ee (20% yield of racemate). However,
performing the half-transamination reaction catalyzed by 7b in
5
d (entry 6) compared to 46% ee when catalyst 10b is used. The
indane moiety seems to be optimal for this half-transamination,
as not only the increase of steric bulk, but also the introduction
of i-Pr or t-Bu substituents on the oxazoline ring affects the
enantioselectivity in a negative manner (catalysts 12a–c, entries
THF with 20 mol% of NEt improved the yield of 5d to 34%,
3
whereas the analogous reaction catalyzed by 7a gave no
improvement compared to the reaction in THF in the absence
of additives and the product was formed as a racemate. In the
experiments applying ethylene glycol as the co-solvent (1 : 1
7
–9).
It is notable that B() and Al() complexes which turned
mixture of MeNO : ethylene glycol) and applying catalyst 8
2
out to be effective catalysts for different half-transamination
reactions, in combination with chiral ligands induced no
enantioselectivity for this reaction.
(
Fig. 2) product 5d was formed in 19% yield and 8% ee and,
most notably, with the opposite absolute configuration
compared to the one observed in the absence of an additive.
In general, additives have a negative effect on the enantio-
selectivity and only HFIP in MeNO and NEt in THF in
2
3
Conclusion
combination with 7a or 7b as the catalyst are able to improve
We have shown that Lewis acid catalysts can promote the
transamination of α-keto esters with different nitrogen sources
in protic solvents. For the linear alkylated α-keto esters, boron
compounds in MeOH can catalyze the reaction to an
acceptable level. Enantioselectivity in the reaction is dependent
on several factors: (i) the pKa of the solvent, (ii) the
coordination properties of counterion or solvents and (iii)
the optimal geometry of the transition state determined
by the structure of the catalyst. We have found that chiral
Cu()–P–N complexes are the best catalysts for the enantio-
selective reactions. Interestingly different nitrogen sources
could be applied for this reaction type and the reaction
proceeds even in aprotic solvents such as toluene.
the yield.
Chiral catalysts
The chiral ligands/catalysts shown in Fig. 2 were tested for
the catalytic enantioselective half-transamination of methyl-3-
indole pyruvate 1d with 4-picolylamine 2b.
Unfortunately, none of the ligands 9a–9g in combination
with e.g. Cu()- or Zn()-induced enantioselectivity in the
half-transamination reaction. Bernauer et al. reported an
enantioselective half-transamination of phenyl pyruvic acid
with a Cu()-ligand 9e complex (using 5 equiv. of 9e!) giving up
to 54% ee (measured by circular dichroism) for the alanine
5
derivative in a buffered aqueous solution. It was therefore
surprising for us that we did not observe any enantioselectivity
for the Cu()- or Zn()-catalyzed half-transamination of
13
Experimental section
methyl-3-indole pyruvate 1d in MeOH or MeNO using 9e as
2
General
the chiral ligand. A crucial fact for a successful half-transamin-
ation in the present system seems to be an imine-functionality
in the ligand, and amine or amide functionalities are not
tolerated if enantioselectivity should be obtained.
1
13
The H and C NMR spectra were measured on a Varian
Mercury spectrometer at 400 MHz and 100 MHz, respectively.
The chemical shifts are reported in ppm downfield from CHCl3
(δ = 7.26) for H NMR and relative to the central CDCl
resonance (δ = 77.0) for C NMR. All solvents used were of
1
However, it is well known that Cu(I) is a good Lewis acid for
3
14
13
(
chiral) P–N ligands and we therefore turned our attention to
P–N ligands (10–12). To our delight we found that several of
p.a. quality and were dried according to standard procedures.
2
048
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 2 0 4 4 – 2 0 4 9

View Article Online
Flash chromatography (FC) was carried out using Merck silica
gel 60 (230–400 mesh). The ee was determined by HPLC using a
Chiralpak AS column with i-PrOH/hexane as eluent. All cata-
References
1
For catalytic asymmetric reactions in water see e.g.: Adv. Synth.
Catal., 2002, 344, pp. 219–451.
lytic reactions were carried out under an atmosphere of N or Ar.
2
2
For an overview on enzymatic transaminations see: (a)
Y. Murakami, J. Kikuchi, Y. Hisaeda and O. Hayashida, Chem. Rev.,
1996, 96, 721–758; (b) A. E. Braunstein, in The Enzymes, ed.
P. D. Boyer, Academic Press, New York, 1973, vol. 9; (c) see
e.g. Transaminases, eds. P. Christen and D. E. Metzler, John Wiley,
New York, 1985; (d ) Biochemistry of Vitamin B6 and PQQ, eds.
G. Marino, G. Sannia and F. Bossa, Birkhauser Verlag, Basel, 1994;
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Materials
All commercially available compounds were used without any
further purification. 2,2Ј-Isopropylidenebis[(4R)-4-phenyl-2-
oxazoline], TMSCHN2 (2.0 M in hexane), 4-picolylamine,
pyridoxamine dihydrochloride hydrate, ethyl pyruvate, indole-
Vitamin B Catalysis, A. R. Liss, New York, 1984; ( f ) A. E. Martell,
3
-pyruvic acid, BEt , B(OH) , BCl , B(OPh) , Zn(OTf ) ,
6
3
3
3
3
2
Acc. Chem. Res., 1989, 22, 115–124.
See e.g. L. Streyer, in Biochemistry, W. E. Freeman and Company,
New York, 1999, p. 633.
Selected papers: (a) D. E. Metzler and E. E. Snell, J. Am. Chem.
Soc., 1952, 74, 979–983; (b) M. Ikawa and E. E. Snell, J. Am. Chem.
Soc., 1954, 76, 637; (c) D. E. Metzler, J. Olivard and E. E. Snell,
J. Am. Chem. Soc., 1954, 76, 644–648; (d ) D. E. Metzler, M. Ikawa
and E. E. Snell, J. Am. Chem. Soc., 1954, 76, 648–652; (e) M. Ikawa
and E. E. Snell, J. Am. Chem. Soc., 1954, 76, 653–655; ( f )
J. B. Longenecker and E. E. Snell, Proc. Natl. Acad. Sci. USA, 1956,
[
Cu()(CH CN) ]PF and the ligands 9f and 9g were purchased
3 4 6
3
4
from Aldrich. The indane-derived ligand and its Zn() complex
1
5
8
9
9
were synthesised according to a literature method. Ligand
c was purchased from Strem Chemicals, whereas the ligands
a, 9b, 9d and 9e were synthesised according to literature
16
17
18
5
methods. The P–N ligands and the corresponding Cu() com-
19
plexes were synthesised according to described methods.
General procedure for the catalytic (enantioselective)
transamination of ꢀ-keto esters
4
2, 221–227; (g) Y. Matsushima and A. M. Martell, J. Am. Chem.
Soc., 1967, 89, 1331–1335; (h) O. A. Gansow and R. H. Holm,
J. Am. Chem. Soc., 1968, 90, 5629–5631; (i) S. Matsumoto and
Y. Matsushima, J. Am. Chem. Soc., 1972, 94, 7211–7213; (j) L. Liu
and R. Brelow, J. Am. Chem. Soc., 2002, 124, 4978–4979;
(k) E. Fasella, S. D. Dong and R. Breslow, Bioorg. Med. Chem.,
1999, 7, 709–714.
The Lewis acid (0.1 mmol, 20 mol% versus the nitrogen source)
and the chiral ligand (0.11 mmol, 22 mol%) were stirred for
3
0 min under vacuum. Then 2 ml of solvent was added and
the solution was stirred for 1 h before the nitrogen source
0.5 mmol) and the α-keto ester (1 mmol, 2 eq.) were added.
5
6
(a) K. Bernauer, R. Deschenaux and T. Taura, Helv. Chim. Acta,
(
1
983, 66, 2049–2057; (b) R. Deschenaux and K. Bernauer, Helv.
The resulting mixture was stirred for 20 h and hydrolysed by the
addition of H O (2 eq.) and CF COOH (1 eq.). The free amino
Chim. Acta, 1984, 67, 373–377.
2
3
(a) Y. Tachibana, M. Ando and H. Kuzuhara, Chem. Lett., 1982,
1765–1768; (b) Y. Tachibana, M. Ando and H. Kuzuhara, Chem.
Lett., 1982, 1769–1772; (c) Y. Tachibana, M. Ando and
H. Kuzuhara, Bull. Chem. Soc. Jpn., 1983, 56, 2263–2266;
group was protected by adding NEt (210 µl, 3 eq.) and (Boc) O
3
2
(
5
218 mg, 2 eq.) to the MeOH solution and stirred for 30 min at
0 ЊC. After cooling the mixture to ambient temperature the
20
(
d ) Y. Tachibana, M. Ando and H. Kuzuhara, Bull. Chem. Soc. Jpn.,
solvent was removed and the oily residue was purified by FC
1
983, 56, 3652–3656; (e) S. C. Zimmermann and R. Breslow, J. Am.
using CH Cl /Et O 9 : 1 as eluent. The purity of the products
2
2
2
1
Chem. Soc., 1984, 106, 1490–1491.
was checked by H NMR and TOF MS and the values of the
7
L. Liu, M. Rozenman and R. Breslow, J. Am. Chem. Soc., 2002, 124,
21
22
21
known compounds 5a, 5e and 5h were compared with the
1
2660–12661.
products described in the literature.
8 L. Liu and R. Breslow, J. Am. Chem. Soc., 2003, 125, 12110–12111.
(a) A. Hjelmencrantz and U. Berg, J. Org. Chem., 2002, 67, 3585–
594; (b) V. A. Soloshonok, I. V. Soloshonok, V. P. Kukhar
9
3
Ethyl N-tert-butyloxycarbonyl-2-amino-4-methyl-pentanoate 5b
and V. K. Svedas, J. Org. Chem., 1998, 63, 1878–1884; (c)
V. A. Soloshonok and O. Taizo, J. Org. Chem., 1997, 62, 3030–3031;
(d ) D. A. Jaeger, M. D. Broadhurst and D. J. Cram, J. Am. Chem.
Soc., 1979, 101, 717–732; (e) M. D. Toney and J. F. Kirsch,
Biochemistry, 1993, 32, 1471–1479.
0 K. R. Knudsen, S. Bachmann and K. A. Jørgensen, Chem.
Commun., 2003, 2602–2603.
1
H NMR (CDCl ): δ 4.89 (br d, 1H, J = 8.4 Hz, NH ), 4.28 (m,
3
1
H, CH ), 4.18 (q, 1H, J = 6.8 and 14.0 Hz, OCH ), 1.69 (m, 1H,
2
CH ), 1.59 (m, 1H, CH ), 1.48 (m, 1H, CH(CH ) ), 1.44 (s, 9H,
2
2
3 2
C(CH ) ), 1.27 (t, 3H, J = 7.6 Hz, CH ), 0.94 (d, 3H, J = 3.2 Hz,
3
3
3
1
13
CH ), 0.93 (d, 3H; J = 3.2 Hz, CH ); C NMR (CDCl ):
3
3
3
δ 173.5, 155.4, 79.7, 61.1, 52.1, 41.9, 28.3, 24.8, 22.8, 21.9, 14.1;
1
1 M. P. Sibi and M. Liu, Curr. Org. Chem., 2001, 5, 719–755.
ϩ
ϩ
HRMS (ES TOF) calcd for C H NO [M ϩ Na] 282.1681,
12 G. S. M. Kiruba and M. W. Wong, J. Org. Chem., 2003, 68,
2874–2881.
13
25
4
found 282.1679.
1
3 The use of Cu(OTf )2 as catalyst for the half-transamination of
different α-keto esters 1 with pyridoxamine 2a or 4-picolylamine 2b
did not yield any transamination product. See also reference 10.
Ethyl N-tert-butyloxycarbonyl-2-amino-5-hexenoate 5c
1
H NMR (CDCl ): δ 5.77 (m, 1H, C(H )᎐CH ), 5.02 (m, 3H,
14 For P–N ligands in combination with Cu() see for instance: (a)
Y. C. Hu, X. M. Liang, J. W. Wang, Z. Zheng and X. Q. Hu, J. Org.
Chem., 2003, 68, 4542–4545; (b) L. Bernardi, A. S. Gothelf,
R. G. Hazell and K. A. Jørgensen, J. Org. Chem., 2003, 68, 2583–
2591; (c) X. M. Fang, M. Johannsen, S. L. Yao, N. Gathergood,
R. G. Hazell and K. A. Jørgensen, J. Org. Chem., 1999, 64,
4844–4849; (d ) J. Andrien, B. R. Steele, C. G. Screttas, C. J. Cardin
and J. Fornies, Organometallics, 1998, 17, 839–845.
3
᎐
2
CH᎐CH and NH ), 4.28 (dd, 1H, J = 8.0 and 13.2 Hz, CH ),
᎐
2
4
1
1
1
.18 (q, 2H, J = 6.8 and 14.4 Hz, OCH ), 2.12 (m, 2H, CH ),
2
2
.90 (m, 1H, CH ), 1.71 (m, 1H, CH ), 1.43 (s, 9H, C(CH ) ),
2
2
3 3
13
.27 (t, 3H, J = 7.2 Hz, CH ); C NMR (CDCl ): δ 172.8, 155.3,
37.0, 115.6, 79.8, 61.3, 53.0, 32.0, 29.4, 14.1; HRMS (ES TOF)
3
3
ϩ
ϩ
calcd for C H NO [M ϩ Na] 280.1525, found 280.1529.
13
23
4
1
5 D. M. Barnes, M. Ji, M. G. Fickes, M. A. Fitzgerald, S. A. King, H. E.
Morton, F. A. Plagge, M. Preskill, S. H. Wagaw, S. J. Wittenberger
and J. Zhiang, J. Am. Chem. Soc., 2002, 124, 13097–13105.
N-(tert-Butyloxycarbonyl)-2-(3-indolyl)glycin-methylester 5d
1
1
1
6 I. Alfonso, F. Rebolledo and V. Gotor, Tetrahedron: Asymmetry,
The ee was determined by HPLC on a Chiralpak AS column
1
999, 10, 367–374.
7 J. T. Seiders, W. D. Ward and R. H. Grubbs, Org. Lett., 2001, 3,
225–3228.
with hexane/i-PrOH 80:20 as eluent. R (min), 5.7 (d-isomer),
t
7
.3 (l-isomer). The absolute configuration was assigned
by comparison with a commercially available sample of
l )-tryptophan that has been derivatised to the Boc-protected
3
8 D. Seebach, A. K. Beck, R. Imwinkelried, S. Roggo and
(
A. Wonnacott, Helv. Chim. Acta, 1987, 70, 954–974.
19 See reference 14b and literature cited therein.
tryptophan methylester.
2
0 If the reaction was performed in MeNO , then the solvent had to be
2
removed under high vacuum before protecting the free amine,
otherwise a part decomposition of the desired product occurs.
1 O. Hayashida, L. Sebo and J. Rebek Jr., J. Org. Chem., 2002, 67,
Acknowledgements
2
This work has been made possible by a grant of the Danish
National Research Foundation.
8
291–8298.
22 B. Jiang and X.-H. Gu, Heterocycles, 2000, 53, 1559–1568.
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 2 0 4 4 – 2 0 4 9
2049