An efficient and practical chemoenzymatic preparation of optically active secondary amines

Article abstract of DOI:10.1021/ol051392n

(Chemical Equation Presented) An efficient and practical chemoenzymatic method was developed for the preparation of a variety of chiral secondary amines. Here, oxalamic esters were identified as unique derivatives amenable to the enzyme-catalyzed kinetic resolution of racemic secondary amines. Both enantiomers of the amines were produced in high optical purity and yields after the cleavage of the oxalamic groups.

Full text of DOI:10.1021/ol051392n

ORGANIC
LETTERS
2005
Vol. 7, No. 20
4329-4331
An Efficient and Practical
Chemoenzymatic Preparation of
Optically Active Secondary Amines
Shanghui Hu,* David Tat, Carlos A. Martinez, Daniel R. Yazbeck, and Junhua Tao
Chemical Research & DeVelopment, Pfizer Global Research and DeVelopment,
La Jolla Laboratories, 10578 Science Center DriVe, San Diego, California 92121
shanghui.hu@pfizer.com
Received June 14, 2005
ABSTRACT
An efficient and practical chemoenzymatic method was developed for the preparation of a variety of chiral secondary amines. Here, oxalamic
esters were identified as unique derivatives amenable to the enzyme-catalyzed kinetic resolution of racemic secondary amines. Both enantiomers
of the amines were produced in high optical purity and yields after the cleavage of the oxalamic groups.
Optically pure secondary amines are important building
blocks in the synthesis of biologically active compounds.
For example, many current drugs contain optically pure
secondary amine moieties. The development of general,
efficient, and practical methods for the production of
enantiomerically pure secondary amines is still a challenging
task.
Although a few methods have been reported for the
synthesis of optically pure secondary amines using chiral
auxiliaries,1 asymmetric hydrogenation,2a,b or hydrosilylation2c
of imines, classical diastereomeric resolution is still the
preferred choice due to its practicality and economic ef-
ficiency.3 On the other hand, enzyme-catalyzed hydrolysis
of secondary amides is difficult probably due to the steric
hindrance in the enzyme active site. Despite many enzymatic
methods available for the preparation of chiral primary
amines,4a,b only a few examples have been reported for the
enzymatic preparation of chiral secondary amines, and most
of them rely on the kinetic acylation of racemic amines with
narrow substrate spectrum and low reactivity.4c,d
We disclose here an efficient and highly enantioselective
enzymatic preparation of chiral aryl-alkyl secondary amines
in addition to a variety of endo- and exocyclic alkyl-alkyl
secondary amines. The oxalamic esters were identified as
the unique derivatives amenable to enzyme-catalyzed
stereoselective hydrolysis of racemic secondary amines.5a
Enzymatic resolution of oxalamic esters of a tertiary alco-
(3) (a) Sheldon, R. A. J. Chem. Technol. Biotechnol. 1996, 67, 1. (b).
Vries, T.; Wynberg, H.; van Echten, E.; Koek, J.; ten Hoeve, W.; Kellogg,
R. M.; Broxtermann, Q. B.; Minnaard, A.; Kaptein, B.; van der Sluis, S.;
Hulshof, L.; Kooistra, J. Angew. Chem., Int. Ed. 1998, 37, 2349.
(4) (a) For a review, see: van Rantwijk, F.; Sheldon, R. A. Tetrahedron
2004, 60, 501. (b). Carr, R.; Alexeeva, M.; Enright, Eve, T. S. C.; Dawson,
M. J.; Turner, N. J. Angew. Chem., Int. Ed. 2003, 42, 4807. (c). Orsat, B.;
Alper, P. B.; Moree, W.; Mak, C.-P.; Wong, C.-H. J. Am. Chem. Soc. 1996,
118, 712. (d). Morgan, W. B.; Zaks, A.; Dodds, D. R.; Liu, J.; Jain, R.;
Megati, S.; Njoroge, F. G.; Girijavallabhan, V. M. J. Org. Chem. 2000, 65,
5451 and references therein.
(1) (a) Guerrier, L.; Royer, J.; Grierson, D.; Husson, H.-P. J. Am. Chem.
Soc. 1983, 105, 7754. (b). Munchhof, M. J.; Meyers, A. I. J. Am. Chem.
Soc. 1995, 117, 5399.
(2) (a) For a review, see: Ohkuma, T.; Noyori, R. ComprehensiVe
Asymmetric Catalysis, Supplement; Springer: New York, 2004; p 43. (b).
Willoughby, C. A,; Buchwald, S. L. J. Am. Chem. Soc. 1992, 114, 7562.
(c). Verdaguer, X.; Lange, U. E. W.; Reding, M. T.; Buchwald, S. L. J.
Am. Chem. Soc. 1996, 118, 6784.
10.1021/ol051392n CCC: $30.25
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hol or a primary amine was described in the literature
previously.5b,c
by automated HPLC station to identify the ideal enzyme(s)
with the desired reactivity and enantioselectivity.
Interestingly, in almost all cases only proteases were
identified with high enantioselectivity (Table 1). Although
The general strategy is shown in Scheme 1, using
2-ethylpiperidine (5a) as an example.6 First, the racemic
Table 1. Preparation of Optically Pure Secondary Amines via
Enantioselective Hydrolysis of Oxalamic Esters
Scheme 1. Chemoenzymatic Preparation of Enantiomerically
Pure (R)- and (S)-2-ethylpiperidines
compound 5a was acylated with ethyl chlorooxoacetate to
give 5b. The formation of oxalamic esters in general proceeds
with high yields and chemical purities, and no chromato-
graphic purification is required (see Supporting Information).
Subsequent enzymatic hydrolysis showed excellent enantio-
selectivity (E >200) for both the acid (R)-5c and ester (S)-
5b, producing optically pure amines (R)-5a and (S)-5a after
deprotection. The best enzyme Aspergillus protease highly
selectively hydrolyzes the ester bond rather than the amide
function in the oxalamic derivative, which thus produced only
the corresponding oxalamic acid, and no free amine was
detected from the reaction mixture. The pH of the reaction
was kept at 7.0-7.5, and background chemical hydrolysis
was suppressed under such conditions. The cleavage of the
oxalamic ester from the leftover starting materials or the
oxalamic acid from products by refluxing with 4 N HCl
proceeds with high yields (90-99% yields, see Supporting
Information).
The exploration of the scope of this methodology was
assisted by automated enzyme screening in 96-well plate
format developed in our lab.7 The reactions are then analyzed
(5) (a) Enzymatic hydrolysis of amides, carbonates, or other linkages
has little success for the preparation of chiral secondary amines. (b).
Brackenridge, I.; McCague, R.; Roberts, S. M.; Turner, N. J. J. Chem. Soc.,
Perkin Trans. 1 1993, 1093. (c). Chapman, D. T.; Crout, D. H. G.;
Mahmoudian, M.; Scopes, D. I. C.; Smith, P. W. Chem. Commun. 1996,
2415.
(6) Typical Procedure. To a potassium phosphate buffer (43 mL, pH
7.0, 0.1 M) was added Aspergillus species protease (2.0 g). Then, 5b (1.06
g, 5.0 mmol) in acetonitrile (5.0 mL) was added to the solution, and the
reaction mixture was stirred vigorously at 23 °C and pH 7.0 controlled by
a continuous addition of 1 N NaOH. After about 50% conversion as shown
in HPLC (within 24 h), the reaction was quenched and extracted with
CH2Cl2. After removal of the solvent, the remaining ester 5b was recovered
(0.52 g, 49%). The aqueous layer was adjusted to pH 4.0 and extracted
with CH2Cl2 to afford the acid 5c (0.41 g). Then, 5b and 5c were refluxed
separately with 4 N HCl for 5 h. The reaction mixture was washed with
CH2Cl2, and the aqueous layer was then brought to pH 10 and extracted
with CH2Cl2. After evaporation of the solvent, the amines (R)-5a and (S)-
5a were produced in good yields and optical purity (R-5a: 43%, 99% ee;
S-5a: 45%, 96% ee).
a Conversions to the corresponding oxalamic acids were calculated from
HPLC. Isolated yields for the leftover oxalamic esters are given in
parentheses. b In most cases, chiral chromatographic methods can be
developed more straightforwardly for the oxalamic esters than the corre-
sponding oxalamic acids. The accurate ee was thus only determined for
the remaining esters by chiral HPLC and GC methods. For details, please
see Supporting Information. c The E value was calculated from the ee of
the oxalamic ester and the conversion.10 d Lower yields were due to material
loss in workup. e The E value was calculated from the ee of oxalamic acid
(96% ee). f The ee was determined by chemical derivatization of free
secondary amine upon deprotection. g The absolute configurations of the
oxalamic acid were determined by comparing with a chiral standard or a
reported optical rotation in the literature.2 h Not determined.
the esters hydrolyzed are three bonds away from the chiral
center, high enantiomeric excesses (ee’s) were generally
obtained for both the oxalamic esters and acids when the
reactions were stopped at a conversion of about 50%.
(7) Yazbeck, D. R.; Tao, J.; Martinez, C. A.; Kline, B. J.; Hu, S. AdV.
Synth. Catal. 2003, 345, 524.
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Org. Lett., Vol. 7, No. 20, 2005

Protease-catalyzed resolution of remote stereocenters from
the reacting carbonyl group was reported previously.8 As
shown in Table 1, proteases prefer to hydrolyze the oxalamic
esters with (R)-stereochemistry.
ary amines. The proteases disclosed here display a broad
acceptance to a variety of amines compatible with a variety
of substitutes and functional groups, including endo- and
exocylic alkyl-alkyl, aryl-alkyl 2-substituted and 3-substituted
secondary amines. The intrinsic enantioselectivity of these
proteases for most amines tested is high (E value ranging
from 40 to >200).
One limitation of the current method is that kinetic
resolution comes with a maximum of 50% yield. However,
as all of the proteases identified here and many racemic
amines are inexpensive, this method can be quite economic
and practical for large-scale production of optically pure
secondary amines and offers advantages over classic resolu-
tion where the yields are generally lower. Moreover, the
method provides a general and fast way to access both
enantiomers from one racemic amine, ideal for drug discov-
ery to identify leads, where both enantiomers are highly
desirable for activity and toxicity studies.
A variety of (R) and (S)-2-substituted pyrrolidines can be
prepared by this protocol (entries 1-3). Like the pyrrolidines,
2-alkyl and aryl piperidines can also be obtained in high ee’s
(entries 4-8). It seems that the enzymes are less sensitive
to the size of R-substituent. To further examine the scope of
this method, an alkyl-aryl amine 9a was studied, and it was
found that the hydrolysis proceeded with good enantio-
selectivity (E ) 43). Furthermore, this protocol can also be
used to prepare chiral amines where substituents are remote
from the amino group (entry 10, 3-substituted piperidine).
It is difficult to obtain these two classes of compounds by
existing strategies.1,2
In addition to endocyclic alkyl-alkyl and aryl-alkyl amines,
this methodology can also be successfully applied to the
preparation of exocyclic chiral secondary amines (entries 11,
12), where both enantiomers of the amines were produced
in high optical purity and yields.
Acknowledgment. We thank Sean Kelly and Billie Kline
for technical assistance, Jianjun Gao and Xiaobing Xiong
for chiral method developments, Min Teng and Yong Wang
for providing substrates, Van Martin and Kim Albizati for
general suggestions, and Pfizer Global R&D for awarding a
student fellowship to D.T.
In summary, we have developed an efficient chemo-
enzymatic route for the preparation of optically pure second-
(8) Lee, T.; Jones, J. B. J. Am. Chem. Soc. 1996, 118, 502.
(9) The ratio of cis and trans for the oxalamic ester (5b) is not changed
1
during the enzymatic hydrolysis by H NMR, presumably because of the
rapid equilibrium between the two conformations under the reaction
conditions.
(10) The absolute configurations of the amines were determined by
comparing with commercially available standards.
(11) Chen, C. S.; Fujimoto, Y.; Girdaukas, G.; Sih, C. J. J. Am. Chem.
Soc. 1982, 104, 7294.
Supporting Information Available: Experimental pro-
cedures and spectral characterization. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
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