Enantiopure N-Benzyloxycarbonyl-β2-amino Acid Allyl Esters from Racemic β-Lactams by Dynamic Kinetic Resolution using Candida antarctica Lipase B

Article abstract of DOI:10.1002/adsc.201500820

The dynamic kinetic resolution of α-substituted racemic β-lactams by alcoholytic ring-opening, catalyzed by immobilized lipase B from Candida antarctica is described. With this process, a variety of racemic α-substituted N-Cbz-azetidinones (Cbz=benzyloxycarbonyl) was transformed to the corresponding N-Cbz-protected β²-amino acid allyl esters with high enantioselectivity (up to 99%) and high yields (up to quantitative) at room temperature.

Full text of DOI:10.1002/adsc.201500820

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DOI: 10.1002/adsc.201500820
Enantiopure N-Benzyloxycarbonyl-b2-amino Acid Allyl Esters
from Racemic b-Lactams by Dynamic Kinetic Resolution using
Candida antarctica Lipase B
Eleonora Gianolio,+a Resmi Mohan,+a and Albrecht Berkessela,*
a
University of Cologne, Department of Chemistry, Organic Chemistry, Greinstraße 4, 50939 Cologne, Germany
Fax: (+49)-221-470-5102; e-mail: berkessel@uni-koeln.de
+
These authors contributed equally to the work.
Received: September 11, 2015; Revised: October 9, 2015; Published online: December 17, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500820.
Abstract: The dynamic kinetic resolution of a-sub-
stituted racemic b-lactams by alcoholytic ring-open-
ing, catalyzed by immobilized lipase B from Candi-
da antarctica is described. With this process, a varie-
ty of racemic a-substituted N-Cbz-azetidinones
(Cbz=benzyloxycarbonyl) was transformed to the
corresponding N-Cbz-protected b2-amino acid allyl
esters with high enantioselectivity (up to 99%) and
high yields (up to quantitative) at room tempera-
ture.
tion process, affording (in the ideal case) enantiomeri-
cally pure product in quantitative yield.
As part of our interest in the development of cata-
lytic asymmetric methods for the preparation of a-
amino acids[9] and b-amino acids,[10] we recently fo-
cused on the use of suitably substituted b-lactams (2-
azetidinones) as substrates for the preparation of
enantiopure b2-amino acid derivatives by dynamic ki-
netic resolution (DKR). 2-Azetidinones are b-amino
acid derivatives readily available in racemic form, for
which, to the best of our knowledge, no enzymatic
DKR has been reported as yet. Herein we present
a method for the enantioselective synthesis of N-Cbz-
protected b2-amino acid allyl esters by DKR of aryl-
substituted b-lactams in the presence of Candida ant-
arctica lipase B (CALB). In this approach, a catalytic
amount of base and allyl alcohol as nucleophile are
Keywords: b-amino acids; Candida antarctica lipase
B (CALB); dynamic kinetic resolution; b-lactams
b-Amino acids, although less abundant than their a- employed (Scheme 1), affording the corresponding N-
analogues, are known to be valuable building blocks Cbz b2-amino acid allyl esters in high enantiopurity
for the preparation of pharmaceuticals,[1] peptides[2] (up to 99% ee) and high yields (up to quantitative).
and natural products.[3] Established methods[4] for the
Inspired by the work of Sharma et al.,[11] we found
preparation of b2-amino acids in enanatiopure form an efficient access to compounds rac-1b–f by dehydra-
are, for example, catalytic asymmetric hydrogena- tive cyclization of the corresponding N-Cbz-3-amino-
tion,[5] conjugate additions of carbon and nitrogen nu-
cleophiles to a,b-unsaturated systems, the Mannich
reaction[6] and the kinetic resolution (KR) of suitable
racemic precursors.[7] Compared to purely chemical
methods, only few biocatalytic options for the produc-
tion of enantiopure b2-amino acids are described in
the literature. The main routes are based on the KR
of b-amino acid derivatives by lipases, lactamases,
acylases, nitrilases, and nitrile hydrolases.[8] The major
disadvantage of kinetic resolution in general, howev-
er, is the maximum yield of 50% of the desired enan-
tiopure material. Dynamic kinetic resolution (DKR)
overcomes this limitation by combining an enantiodis-
criminative transformation with an in situ racemiza-
Scheme 1. Alcoholytic dynamic kinetic resolution (DKR) of
a-substituted racemic b-lactams.
30
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Enantiopure N-Benzyloxycarbonyl-b2-amino Acid Allyl Esters
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Table 1. Influence of base and solvent on the enzymatic res-
olution the b-lactam rac-1b.
Scheme 2. CALB-catalyzed kinetic resolution of the b-lac-
tams rac-1a/b.
Entry Base (equiv.)
Solvent Time Conv.[a] ee[a]
[h]
[%]
[%]
2-arylpropanoic acids with phosphoryl chloride (see
the Supporting Information). For the synthesis of rac-
1a and rac-1g, the synthesis of the N-unprotected b-
lactam followed by N-Cbz protection, as described by
Murayama et al., proved superior.[12]
In an initial screening, the racemic a-substituted b-
lactams synthesized were subjected to lipase-catalyzed
alcoholytic ring opening in order to identify potential
biocatalysts as well as suitable substrate structures for
the subsequent development of the dynamic kinetic
procedure (Scheme 2). For the a-methyl-substituted
and N-Cbz-protected lactam rac-1a, from a set of 15
lipases, only five serine hydrolases proved to be active
1
2
3
4
5
6
7
8
9
pyridine (0.5)
(i-Pr)2NEt (0.5) toluene 21
toluene 21
50
50
99
99
29
41
89
98
70
95
79
DBU (0.5)
DBN (0.5)
DBN (0.2)
DBN (0.1)
DBN (0.15)
DBN (0.15)
DBN (0.15)
toluene
toluene
4
4
quant.
82
quant.
quant.
50
toluene 17
toluene 20
THF
MTBE
MeCN
24
24
24
quant.
33
[a]
Conversion of rac-1b and ee of (R)-2b determined by
HPLC.
in the alcoholysis (see the Supporting Information). on the DKR protocol was surveyed. We were delight-
Among those, lipase B from Candida antarctica ed to see that excellent enantiomeric excesses were
showed very high activity – 85% conversion after obtained with other alcohols (such as ethanol, isobu-
30 min – but rather low selectivity for substrate rac-1a tyl alcohol and isopropyl alcohol, see the Supporting
(E=1.8). In contrast, the same enzyme provided ex- Information) as well.
cellent enantioselectivity acting on the a-phenyl-sub-
For some batches of CALB, we observed that the
stituted derivative rac-1b, giving rise to high enantio- CALB/DBN system effected clean KR, with no dy-
purity both for the product ester (R)-2b as well as for namic behavior, however. We suspected that the in-
the remaining lactam (S)-1b. As it turned out, neither hibition of racemization might be due to adventitious
N-PMP (p-methoxyphenyl) nor N-unprotected b-lac- acidic impurities in some batches of the commercial
tams underwent this enzymatic alcoholysis efficiently. acrylate resin-bound lipase. To our delight, pre-treat-
Aiming for a dynamic kinetic resolution protocol, ment of the resin-bound enzyme with NEt3 restored
various organic bases were evaluated as racemization the catalytic activity, and the alcoholytic lactam open-
catalysts in the previously optimized enzymatic ring ing proceeded with the usual excellent yield and
opening of rac-1b. The use of 0.5 equivalents of pyri- enantiopurity.
dine or Hünigꢁs base (Table 1, entries 1 and 2), did
For the determination of the absolute configuration
not effect DKR, leading in both cases to simple KR. of the product N-Cbz-b2-amino acid allyl ester 2b, the
In the presence of the stronger base 1,8-diazabicy- latter was subjected to global deprotection
clo[5.4.0]undec-7-ene (DBU), DKR was achieved, but (Scheme 3). Note that this two-step protocol resulted
the ester 2b was obtained with 29% ee only (entry 3). only in a minor decrease of enantiopurity. Compari-
1.5-Diazabicyclo[4.3.0]non-5-ene (DBN) was finally son of the specific rotation of the free amino acid 3b
chosen as racemization catalyst for the substrate rac-
1b. When enantioenriched 1b was exposed to
0.6 equiv. of this base, racemization was complete
within ca. 10 min, both in the presence and absence of
CALB. In the DKR experiments, the product enantio-
meric excess could be further increased to 98%, by
employing lower concentrations of the base, albeit at
the expense of somewhat longer reaction times (en-
tries 4–6). Variation of the solvent did not lead to fur-
ther improvement, but identified both toluene and
MTBE as the reaction media of choice (entries 6–9).
Scheme 3. Determination of the absolute configuration of
Moreover, the effect of various alcohol nucleophiles the N-Cbz-b2-amino acid allyl ester 2b.
Adv. Synth. Catal. 2016, 358, 30 – 33
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31

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Eleonora Gianolio et al.
chemicals (purchased from Sigma Aldrich, Acros or Carbo-
lution Chemicals) were used without further purification.
CALB was applied in immobilized form as received from
Sigma Aldrich. HPLC analysis was performed on Merck Hi-
tachi HPLC (Chiralpak OJ, AD, AD-H) and PDA (photo-
diode array) detector. See the Supporting Information for
the synthesis of the a-substituted racemic b-lactams rac-1a–
g.
Dynamic Kinetic Resolution
In a typical experiment, the racemic a-substituted b-lactam
rac-1b (281 mg, 1.00 mmol) was dissolved in dry toluene
(10 mL), DBN (7.4 mL, 60 mmol), allyl alcohol (102 mL,
1.50 mmol) and immobilized Candida antarctica lipase B
(CALB, 1.00 g) was added, together with ground molecular
sieves 4Š (300 mg). The reaction mixture was flushed with
argon and stirred at room temperature. After 24 h, the mix-
ture was filtered and the solvent was removed from the fil-
trate under vacuum. The crude product was purified by
column chromatography on silica gel (cyclohexane:ethyl
acetate 7:3), affording the N-Cbz b2-amino acid allyl ester
2b as a colorless solid; yield: 339 mg (quant.); 99% ee; mp
Scheme 4. Scope and limitations for the DKR of racemic a-
substituted b-lactams; yields/ees of isolated products are
given.
1
348C. H NMR (300 MHz, CDCl3): d=7.27–7.18 (m, 10H),
5.81–5.68 (m, 1H), 5.13–5.01 (m, 5H), 4.52–4.49 (m, 2H),
3.88–3.83 (m, 1H), 3.62–3.49 (m, 2H); 13C NMR (75 MHz,
CDCl3): d=172.4 (t), 156.2 (s), 136.4 (s), 136.0 (d), 131.6
(d), 128.9 (d), 128.5 (d), 127.9 (d), 118.3 (t), 66.7 (t), 65.5 (t),
51.5 (d), 43.7 (s); elemental analysis: calcd.: C 70.78, H 6.24,
N 4.13; found: C 70.71, H 6.13, N 4.11.
with literature data confirmed the (R)-selectivity of
the lipase.[13] We assume that the stereoselectivity of
CALB is analogous for the other substrates tested,
i.e., the b2-amino acid ester products 2a–g (see below)
are assumed to be of the (R)-configuration uniformly.
The scope of the chemo-enzymatic dynamic kinetic
resolution was studied using the b-lactams rac-1c–g.
In all cases, the N-Cbz b-amino acid allyl esters 2c–g
were obtained with very high optical purity and in
high yields (Scheme 4). However, this study also re-
vealed that an aromatic substituent is required for ef-
ficient dynamic kinetic resolution to occur. In the
case of the a-benzyl-substituted b-lactam rac-1g, only
kinetic resolution was observed under our optimized
conditions. Most likely, this effect is due to the lower
acidity of the lactamꢁs a-proton in the presence of an
aliphatic/benzylic substituent, as opposed to aromatic
ones.
Supporting Information
The Supporting Information contains all experimental de-
tails for the synthesis and characterization of the racemic b-
lactams (rac-1a–g) and N-Cbz b-amino acid allyl esters (2a–
g). Furthermore, the procedures for the KR and DKR,
methods used for HPLC analysis and retention times of sub-
strates and products are given.
Acknowledgements
This work was supported by the EC, Marie Curie Research
Training Network REVCAT, MRTN-CT-2006-035866 (RM),
the UoC Excellence Initiative (EG Forum and Interim Jobs),
and by the Fonds der Chemischen Industrie.
In conclusion, we report the dynamic kinetic resolu-
tion of racemic b-lactams by alcoholytic ring-opening,
catalyzed by the commercially available Candida ant-
arctica lipase B. The resulting enantiopure N-Cbz pro-
tected b2-amino acid allyl esters are valuable building
blocks for peptide synthesis, or can easily be depro-
tected to the parent b2-amino acid.
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