Dynamic kinetic resolution of amines involving biocatalysis and in situ free radical mediated racemization

Article abstract of DOI:10.1021/ol063062o

(Chemical Equation Presented) The association of lipase-catalyzed enzymatic resolution with in situ racemization mediated with the thiyl radical enables the dynamic kinetic resolution of non-benzylic amines. It leads to (R)-amides with high enantioselectivities. It can be applied either to the conversion of racemic mixtures or to the inversion of (S)-enantiomers.

Full text of DOI:10.1021/ol063062o

ORGANIC
LETTERS
2007
Vol. 9, No. 5
837-839
Dynamic Kinetic Resolution of Amines
Involving Biocatalysis and in Situ Free
Radical Mediated Racemization
Ste´phane Gastaldi,*,† Ste´phanie Escoubet,† Nicolas Vanthuyne,‡ Ge´rard Gil,*,‡ and
Miche`le P. Bertrand*,†
Laboratoire de Chimie Mole´culaire Organique, UMR 6517, Chimie, Biologie, et
Radicaux Libres, Boite 562, UniVersite´ Paul Ce´zanne, Aix-Marseille III, Faculte´ des
Sciences St Je´roˆme, AVenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20,
France, and Laboratoire de Ste´re´ochimie Dynamique et Chiralite´, UMR 6180,
Chirotechnologies: Catalyse et Biocatalyse, UniVersite´ Paul Ce´zanne, Aix-Marseille
III, Faculte´ des Sciences St Je´roˆme, AVenue Escadrille Normandie-Niemen,
13397 Marseille Cedex 20, France
michele.bertrand@uniV-cezanne.fr
Received December 19, 2006
ABSTRACT
The association of lipase-catalyzed enzymatic resolution with in situ racemization mediated with the thiyl radical enables the dynamic kinetic
resolution of non-benzylic amines. It leads to (R)-amides with high enantioselectivities. It can be applied either to the conversion of racemic
mixtures or to the inversion of (S)-enantiomers.
At a time when the state of the art in asymmetric synthesis
has reached an outstanding level,1 looking for racemization
processes may seem pointless. However, the resolution of
racemates remains one of the main industrial routes to
optically pure compounds,2 and as a consequence, the
racemization of unwanted isomers still has great economical
value. The association of chemical or enzymatic resolution
with an in situ racemization process (dynamic kinetic
resolution (DKR))3 is even more valuable because it allows
recycling steps with a 50% maximum yield to be avoided
and allows a 100% theoretical yield to be reached in one
single step.4
Racemization of nonactivated amines often necessitates
rather harsh conditions that do not tolerate the presence of
additional functional groups.2 Having elegantly solved the
problem of racemization of both benzylic and non-benzylic
unfunctionalized amines at 110 °C in the presence of a
ruthenium catalyst5 and taking advantage of the compatibility
between the enzymatic resolution and the transition-metal-
catalyzed racemization, Ba¨ckvall and co-workers have modi-
fied the catalyst in such a way that DKR could be achieved
at 90 °C.6,7 As far as we know, there are only three other
examples of DKR applied to amines. All involve transition-
(2) Ebbers, E. J.; Ariaans, G. J. A.; Houbiers, J. P. M.; Bruggink, A.;
Zwanenburg, B. Tetrahedron 1997, 53, 9417-9476.
(3) For a precise definition of the concept, see: Faber, K. Chem.-Eur.
J. 2001, 7, 5004-5010.
† Laboratoire de Chimie Mole´culaire Organique, UMR 6517, Chimie,
Biologie, et Radicaux Libres.
‡ Laboratoire de Ste´re´ochimie Dynamique et Chiralite´, UMR 6180,
Chirotechnologies: Catalyse et Biocatalyse.
(4) For general reviews, see: (a) Pellissier, H. Tetrahedron 2003, 59,
8291-8327. (b) Ward, R. S. Tetrahedron: Asymmetry 1995, 6, 1475-
1490.
(1) (a) Knowles, W. S. (Nobel Lecture) Angew. Chem., Int. Ed. 2002,
41, 1998-2007. (b) Noyori, R. (Nobel Lecture) Angew. Chem., Int. Ed.
2002, 41, 2008-2022. (c) Sharpless, B. K. (Nobel Lecture) Angew. Chem.,
Int. Ed. 2002, 41, 2024-2032.
(5) Pa`mies, O.; Ell, A. H.; Samec, J. S. M.; Hermanns, N.; Ba¨ckvall,
J.-E. Tetrahedron Lett. 2002, 43, 4699-4702.
10.1021/ol063062o CCC: $37.00
© 2007 American Chemical Society
Published on Web 02/06/2007

metal catalysis8-10 and refer exclusively to the racemization
of benzylic amines by catalytic Pd/C. The conceptually
different chemoenzymatic deracemization using an enanti-
oselective amine oxidase in combination with ammonia
borane has also been developed.11 In all these procedures,
the racemization proceeds through an amine-imine equi-
librium.
We have recently demonstrated that alkylsulfanyl radicals
can mediate the racemization of amines via reversible
hydrogen abstraction at the chiral center in a position R
relative to nitrogen. The S-H bond dissociation enthalpy
(BDE) must match the R-C-H BDE for the reversible
hydrogen transfer to have some practical efficacy.12 In the
radical racemization process, the prochiral intermediate is
an R-amino radical. Because the R-C-H BDE is stronger
in the corresponding amide (≈17 kJ mol-1), hydrogen
abstraction at the chiral carbon is precluded after acyl-
ation.12
A large number of enzymes make use of radical mecha-
nisms.14 Furthermore, numerous studies show that proteins
are degraded under radical conditions.15
The selection of lipase B from Candida antartica (CAL-
B) as thermostable lipase to catalyze the aminolysis of an
acyl donor was facilitated by previous work.6-10 The com-
mercially available polymer-supported lipase Novozym 435
is able to catalyze the amidation of primary amines under
mild conditions.16 Enzymatic resolution by lipases is usually
conducted in low polarity aprotic solvents (hexane or
dialkylethers). This is an advantage with regard to its
compatibility with the radical reaction12 which could easily
be performed in heptane. The selection of the acyl donor
was made after the resolution of amine 1a had been tested
in the presence of several acyl donors at 80 °C. Although
enol esters are known to be highly efficient, irreversible acyl
donors,17 they were a priori rejected due to their capacity,
as electron-rich olefins, to trap alkylsulfanyl radicals via
radical addition followed by hydrogen transfer.18 Moreover,
they release carbonyl compounds that can react with the
starting amine. Ethyl laurate and lauric acid turned out to
be good candidates, giving satisfactory results for the
resolution of amines 1 at 80 °C in 8 h.19,20
Having at our disposal an efficient and selective racem-
ization process working under mild conditions at 80 °C, we
investigated the possibility of devising a DKR protocol by
associating enzymatic resolution and radical racemization
(Scheme 1). As mentioned in our previous articles, in the
The next step consisted of investigating the influence of
the thiol on the enzyme activity. Figure 1 shows the plots of
the enantiomeric excess (ee) of the remaining amine (S)-1a
vs time during the enzymatic resolution carried out at 80 °C
in the presence of lauric acid and a series of thiols (1 equiv
with regard to the amine). The graphs show that the
enzymatic resolution is slightly more efficient in the presence
of a secondary than a primary thiol (C6H11SH/n-OctSH).
However, we had previously observed that the thiyl
radical mediated racemization of non-benzylic primary
Scheme 1. Radical- and Enzyme-Catalyzed DKR
(13) Thiols might modify the enzyme through the destruction of disulfide
bridges. See: Stryer, L. Biochimie, 4th ed.; Flammarion: Paris, 1997;
Chapter 2, pp 37-39.
(14) (a) Stubbe, J.; van der Donk, W. A. Chem. ReV. 1998, 98, 705-
762. (b) Frey, P. A.; Hegeman, A. D.; Reed, G. H. Chem. ReV. 2006, 106,
3302-3316.
(15) (a) Easton, C. In Radical in Organic Synthesis; Renaud, P., Sibi,
M., Eds.; Wiley: New York, 2001; Vol.2, Chapter 6. For the involvement
of thiyl radicals, see: (b) Rauk, A.; Yu, D.; Armstrong, D. A. J. Am. Chem.
Soc. 1998, 120, 8848-8855.
(16) For general reviews, see: (a) Alfonso, I.; Gotor, V. Chem. Soc.
ReV. 2004, 33, 201-209. (b) Carrea, G.; Riva, S. Angew. Chem., Int. Ed.
2000, 39, 2226-2254. (c) Turner, N. J. Curr. Opin. Chem. Biol. 2004, 8,
114-119. (d) Turner, N. J. Curr. Opin. Biotechnol. 2003, 14, 401-406.
(e) Van Rantwijk, F.; Sheldon, R. A. Tetrahedron 2004, 60, 501-519. (f)
Gotor-Fernandez, V.; Busto, E.; Gotor, V. AdV. Synth. Catal. 2006, 348,
797-812. (g) Koeller, K. M.; Wong, C.-H. Nature 2001, 409, 232-
240.
case of primary benzylic amines, the efficacy of the
racemization process is limited by the competitive oxidation
of the intermediate R-amino radical.12a Therefore, we turned
our attention to non-benzylic amines all the more because
only two examples of DKR of the latter had been reported
in the literature.6
The compatibility of lipase-mediated kinetic resolution
with a radical process involving a thiol was not obvious.13
(6) Paetzold, J.; Ba¨ckvall, J. E. J. Am. Chem. Soc. 2005, 127, 17620-
17621.
(7) For reviews on related topics, see: (a) Pa`mies, O.; Ba¨ckvall, J.-E.
Chem. ReV. 2003, 103, 3247-3261. (b) Huerta, F. F.; Minidis, A. B. E.;
Ba¨ckvall, J.-E. Chem. Soc. ReV. 2001, 303, 321-331. (c) Pa`mies, O.;
Ba¨ckvall, J.-E. Curr. Opin. Biotechnol. 2003, 14, 407-413. See also: (d)
Kim, M.-J.; Ahn, Y.; Park, J. Curr. Opin. Chem. Biol. 2002, 13, 578-
587.
(8) Reetz, M. T.; Schimossek, K. Chimia 1996, 50, 1211-1212.
(9) Parvulescu, A.; De Vos, D.; Jacobs, P. Chem. Commun. 2005, 5307-
5309.
(17) For a review, see: Hanefeld, U. Org. Biomol. Chem. 2003, 2405-
2415.
(18) (a) Bertrand, M. P.; Chatgilialoglu, C.; Ferreri, C. In The Chemistry
of Sulphur Radicals; Alfassi, Z. B., Ed.; Wiley: New York, 1999; Chapter
11, pp 311-354. (b) Bertrand, M. P.; Ferreri, C. In Radical in Organic
Synthesis; Renaud, P., Sibi, M., Eds.; Wiley: New York, 2001; Chapter
5.5, pp 485-503.
(19) The KR of amine 1a, carried out with Novozym 435 in the presence
of 1 equiv of ethyl laurate, led to (R)-2a (50% yield, ee g 99%) together
with the remaining amine (S)-1a (46% yield, 93% ee). Amides (R)-2a (ee
g 99%) and (S)-1a (99% ee) were isolated in 50% and 41% yields,
respectively, in the presence of 1 equiv of lauric acid. Ethyl acetate was
found to be far less selective at 80 °C than at 30 °C.
(10) Choi, Y. K.; Kim, M. J.; Ahn, Y.; Kim, M.-J. Org. Lett. 2001, 3,
4099-4101.
(11) Dunsmore, C. J.; Carr, R.; Fleming, T.; Turner, N. J. J. Am. Chem.
Soc. 2006, 128, 2224-2225 and previous refs cited therein.
(12) (a) Escoubet, S.; Gastaldi, S.; Vanthuyne, N.; Gil, G.; Siri, D.;
Bertrand, M. P. Eur. J. Org. Chem. 2006, 3242-3250. (b) Escoubet, S.;
Gastaldi, S.; Vanthuyne, N.; Gil, G.; Siri, D.; Bertrand, M. P. J. Org. Chem.
2006, 71, 7288-7292.
(20) The enantioselectivity for the acylation of sec-butyl amine by
Novozym 435 was shown to increase when using esters of long-chain fatty
acids. See: Goswami, A.; Guo, Z.; Parker, W. L.; Patel, R. N. Tetrahedron:
Asymmetry 2005, 16, 1715-1719.
838
Org. Lett., Vol. 9, No. 5, 2007

Table 1. Dynamic Kinetic Resolution of Amines 1a-ea
product
yield (%)b
ee
(%)
amines
R
R1CO2R2
1
2
3
4
5
6
7
8
9
rac-1a Ph(CH2)2
rac-1a Ph(CH2)2
rac-1a Ph(CH2)2
(S)-1a Ph(CH2)2
rac 1b Me(CH2)5
(S)-1b Me(CH2)5
rac-1c t-BuOCOCH2
rac-1d Me2CdCH(CH2)2
rac-1e Et
CH3CO2Et
2a′, 31 (95)
74
99
C
C
C
C
C
C
C
C
11H23CO2Et 2a, 70 (71)
11H23CO2H 2a, 71 (69) >99
11H23CO2Et 2a, 58 (nd) 99
11H23CO2Et 2b, 81 (nd) >99
11H23CO2Et 2b, 62 (69) >99
11H23CO2Et 2c, 47 (54)
11H23CO2Et 2d, 68 (70)
11H23CO2H 2e, 57 (nd)
92
94
86
Figure 1. Plot of ee vs time, for Novozym 435 mediated enzymatic
resolution of amine 1a in the presence of thiol. Conditions: 1a (1
mmol) in heptane (10 mL), RSH (1.2 mmol), CAL-B (Novozym
435, 200 mg), lauric acid (2 mmol), temp ) 80 °C.
a Conditions: amine (1 mmol, 0.063 M), 3 (1.2 equiv), acyl donor (1.5
equiv), Novozym 435 (200 mg). bIsolated yield (NMR yield). cEtOAc/
heptane (2.6:8 in vol).
Table 1. As already noted, as an acyl donor, EtOAc was not
selective for the DKR of amine rac-1a (entry 1). In all cases,
the substrate was totally consumed in 8 h.
amines was slightly slower with cyclohexane thiol than with
methyl thioglycolate. Accordingly, the latter was our first
choice.12b
The racemic amines 1a-e were transformed into the
corresponding (R)-amides 2a-e (in isolated yields ranging
from 47 to 81%, with ee’s varying from 86 to >99%) in the
presence of either ethyl laurate or lauric acid (entries 2-9).
It must be noted that good results were obtained in the
presence of lauric acid, even though an equilibrium is likely
to be established between the primary amine and the
corresponding alkylammonium ion (entries 3 and 9). The
use of 1 equiv of lauric acid only slightly slowed the radical
racemization, which was completed in 5 h instead of in 2 h.
The DKR process is compatible with remote functionalities
such as the trisubstituted double bond in 1d and the sterically
hindered t-butyl ester in 1c. It could be tuned to achieve the
complete inversion of configuration of the (S)-enantiomer
(entries 4 and 6).
The problem was that this thiol could also play the role
of an acyl donor.21 Moreover, very low amine enantiomeric
excesses were observed during the kinetic resolution of amine
1a performed in the presence of methyl thioglycolate. The
corresponding N,N-diethyl amide enabled us to overcome
this problem, but this new thiol was also inhibiting the
enzymatic resolution as indicated by the absence of amide
and the low ee of the recovered amine (Figure 1). Its
secondary analogue 3 that was prepared by reduction of the
corresponding disulfide (Scheme 2) enabled us to solve both
Scheme 2. Synthesis of Thiol 3
In conclusion, as far as we know, the performance of a
DKR process associating lipase-catalyzed enzymatic resolu-
tion and in situ racemization involving a thiyl radical
mediated process is a “premiere”. This process leads to (R)-
amides with high enantioselectivities. It can be applied either
to the resolution of racemic mixtures or to the inversion of
(S)-amines.
Acknowledgment. This work was supported by the ANR
(Agence Nationale de la Recherche) (project DKR, nï.
Blan06-3_142460). A gift of CAL-B from Novo Nordisk
A/S Denmark is gratefully acknowledged.
problems, while preserving the aptitude of the thiol to
racemize amine 1a within 2 h.
The conditions for the success of a DKR were optimal
because we had at our disposal a highly selective resolution
process compatible with the presence of a thiol at 80 °C22
and a fast racemization process.
Supporting Information Available: Experimental pro-
cedures and characterization data for new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
The results obtained for the DKR experiments performed
with N,N-diethyl-2-sulfanylpropionamide (3) are reported in
OL063062O
(21) The resolution of amine 1 carried out in heptane with CALB, at
80 °C, in the presence of ethyl thioglycolate and ethyl acetate as the acyl
donor, afforded the derived 2-sulfanylacetamide and the corresponding
disulfide.
(22) E ) kR/kS > 40 in the presence of thiol 3; 42% isolated yield and
98.5% ee for amide 2a; 54% yield and 97.1% ee for the remaining amine
for the resolution of 1a in the presence of 3 and 1 equiv of ethyl laurate.
Org. Lett., Vol. 9, No. 5, 2007
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