Enantioselective lipase-catalyzed reactions of methyl pipecolinate: Transesterification and N-acylation

Article abstract of DOI:10.1016/S0040-4039(02)00288-5

The present research introduces the highly (S)-selective (E?100) acylation at the secondary ring nitrogen of methyl pipecolinate as a novel resolution method with Candida antarctica lipase A. Catalysis by lipase B leads to reactions at the methyl ester function of the substrate in an almost non-enantioselective manner.

Full text of DOI:10.1016/S0040-4039(02)00288-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 2471–2474
Enantioselective lipase-catalyzed reactions of methyl pipecolinate:
transesterification and N-acylation
Arto Liljeblad, Jutta Lindborg, Anu Kanerva, Johanna Katajisto and Liisa T. Kanerva*
Laboratory of Synthetic Drug Chemistry and Department of Chemistry, University of Turku, Lemminka¨isenkatu 2,
FIN-20520 Turku, Finland
Received 7 January 2002; revised 30 January 2002; accepted 6 February 2002
Abstract—The present research introduces the highly (S)-selective (E>>100) acylation at the secondary ring nitrogen of methyl
pipecolinate as a novel resolution method with Candida antarctica lipase A. Catalysis by lipase B leads to reactions at the methyl
ester function of the substrate in an almost non-enantioselective manner. © 2002 Elsevier Science Ltd. All rights reserved.
(S)-Pipecolic acid (piperidine-2-carboxylic acid) as a
lysine metabolite occurs widely in nature as a free
nonproteinogenic amino acid and as a constituent of
natural products,1 and hence both enantiomers are of
commercial value as building blocks for many biologi-
cally and pharmaceutically active compounds.2 (S)-
Lysine serves as a typical chiral pool reagent for the
preparation of (S)-pipecolic acid.3–5 Chemical and
chemoenzymatic asymmetric routes to pipecolic acid
enantiomers have also been reported.6–10 Resolution
methods include fractional crystallization11,12 and enzy-
matic pathways.2,13,14 Economical and environmental
benefits of enzymatic kinetic resolution over traditional
chemical methods arise from the fact that only catalytic
amounts of a reusable enzyme are needed for asymmet-
ric induction under benign reaction conditions.
CAL-A also catalyzed the acylation of the secondary
alcohol group of 1-phenyl-1,2-ethanediol in the pres-
ence of the primary alcohol function in a highly enan-
tioselective manner.20 This indicates exceptional
behavior in that the lipase preferred to act at sterically
hindered positions. On the other hand, the CAL-B-cat-
alyzed reactions of the b-amino esters with achiral
esters proceeded with relatively low chemoselectivity,
enantioselective transesterification at the ester function
rather than N-acylation being favored.17,18 Moreover,
the importance of the sterically nonhindered position as
the reaction site for CAL-B became evident.19,20 The
previous results also convinced us that highly enan-
tioselective ester alcoholyses would be possible using
CAL-B.17–20
In this paper we report a novel and highly effective
lipase-catalyzed method for the resolution of N-hetero-
cyclic amino esters using methyl pipecolinate 1 as a
model compound. For that purpose, the chemo- and
enantioselective alcoholysis and transesterification reac-
tions of 1 in the presence of CAL-B and N-acylations
in the presence of CAL-A were studied (Schemes 1 and
2).21
In our ongoing work on the synthetic applications of
lipases, we have examined the very different behaviors
of lipases A (CAL-A) and B (CAL-B) from Candida
antarctica toward polyfunctional compounds in non-
aqueous media.15–20 Thus, excellent chemoselectivity
and from good to excellent enantioselectivity of CAL-A
were observed for the N-acylation of numerous b-sub-
stituted b-amino esters as primary amine substrates.15,16
Scheme 1.
Keywords: Candida antarctica lipase A; Candida antarctica lipase B; pipecolic acid; methyl pipecolinate; resolution.
* Corresponding author. Tel.: 358 2 3336773; fax: 358 2 3337955.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00288-5

2472
A. Liljeblad et al. / Tetrahedron Letters 43 (2002) 2471–2474
Scheme 2.
Reaction of 1 as an ester. Our high expectations for the
CAL-B-catalyzed alcoholysis of 1 were disappointed
when low enantioselectivities were found in terms of the
observed E (enantiomer ratio) values22 (Scheme 1;
Table 1, rows 1–4). The reaction in 2-propanol (R=iPr)
with E=15 at 8°C was the most promising (row 4).
Reactions at the ester function of the substrate pro-
ceeded smoothly in the formation of enantiomerically
enriched (R)-2 in neat alcohols as shown by high
conversions [C(II)/%]. Chemical ester alcoholysis in
1-butanol clearly competed with the enzymatic one
although its proportion was insignificant at short reac-
tion times as shown by the conversion values C(I)/% for
the chemical reaction compared to the C(II)/% values
in the presence of CAL-B after 40 minutes (rows 1 and
2). This allows the calculation of the E values for
enzymatic reactions. On the other hand, the CAL-A-
catalyzed alcoholysis of 1 gave racemic 2 (R=Bu) at
20% total conversion after 24 h, the proportion of the
simultaneous chemical reaction then being already
approximately 10% as determined separately by moni-
toring the reaction, under otherwise identical condi-
tions, but in the absence of the enzyme.
after 8 days) and the reaction was mostly chemical in
nature. The increased steric size of the N-acetylated
substrate could explain why CAL-B does not work. It is
worth emphasizing that lipases typically leave amide
bonds untouched.
Previously it was proposed that secondary amines are
not substrates for lipases.25 In accordance with this,
chemical or CAL-B-catalyzed N-acylation was not
detected in the case of the studied acyl donors shown in
Table 1 (rows 5–7). On the other hand, chemoselective
transesterification by CAL-B catalysis proceeded
smoothly at the ester function of 1 with butyl acetate
and butyl butanoate, leading to the formation of
slightly enantiomerically enriched butyl esters (R)-2
(Scheme 2, lower part, R=Bu; Table 1, rows 5 and 6).
2,2,2-Trifluoroethyl butanoate as an acyl donor gave
enzymatic transesterification product 2 (R=CH2CF3,
row 7) in an extremely slow reaction. Chemical reaction
was observed only in the case of butyl acetate as an
acyl donor (row 5). Practically no transesterification by
CAL-B was observed with sterically hindered rac-3
(R%=Me) in butyl butanoate (conversion <5% after 5
days).
As was observed for the lipase-catalyzed resolutions of
various piperidine derivatives23–25 and in our previous
work for those of amino acid esters,17–19 N-protection
was expected to make 1 a better substrate for CAL-B
catalysis. However, the CAL-B-catalyzed reaction of
the N-acetylated substrate (rac-3; Scheme 2, R%=Me)
with neat butanol was extremely slow (conversion <5%
Reactions of 1 as a secondary amine. At this point with
unsuccessful trials in directing enantioselective enzy-
matic reactions to the ester function of 1 it was time to
reconsider our experience with lipases. Thus, we came
to consider CAL-A and its above-mentioned excep-
tional behaviors as a lipase.15,16,20 In contrast to CAL-
Table 1. Reactions of methyl pipecolinate (0.1 M) in neat alcohol and ester: C(I) (%) is conversion in the absence of the
enzyme and C(II) (%) in the presence of CAL-B after 40 minutes at room temperature (25°C).
Solvent
CAL-B (mg/ml)
C(I) (%)
C(II) (%)
E(II)
1
2
3
4
5
6
7
BuOH
10
10
10
10
5
0.3
0.08
0.01
–
0.2
–
82
55
22
11
28
55
2
3
4
9
15
2
3
BuOHa
iPrOH
iPrOHa
AcOBu
PrCO2Bu
PrCO2CH2CF3
5
10
b
–
1
a Temperature 8°C.
b 0.2 M in TBME.

A. Liljeblad et al. / Tetrahedron Letters 43 (2002) 2471–2474
2473
B, the CAL-A-catalyzed reaction of 1 with 2,2,2-tri-
fluoroethyl butanoate in organic solvents proceeded
exclusively at the secondary ring nitrogen, leading to
the formation of (S)-3 (Scheme 2, upper part, R%=Pr;
Table 2). Chemical transesterification/alcoholysis at the
ester function (Table 1, row 7) or N-acylation was not
observed. tert-Butyl methyl ether (TBME) was chosen
as the best solvent due to high enantioselectivity (E>
100) and short reaction time needed to reach 50%
conversion (Table 2, row 10). There was no reaction in
dichloromethane, chloroform or DMF as a solvent.
This is the first time that lipase-catalyzed highly enan-
tioselective acylation at a secondary nitrogen has been
described. Thus, e.g. for the acylation of hydroxy-
methyl piperidines with various hydrolytic enzymes,
the observed N-acylated product was proposed to be
the result from a fast O“N acyl migration in the
initially formed O-acyl derivative.25
enzymatic formation of allyl (8% from the observed
24% conversion after 24 h) and ethyl esters (traces after
24 h), respectively, in addition to the N-acylation prod-
ucts (S)-3 (R%=CH2=CHCH2O and R%=Pr,
respectively).
To this end, the gram-scale resolution of 1 in TBME
was performed at room temperature. For that purpose
1 (1.50 g, 10.5 mmol) and 2,2,2-trifluoroethyl butanoate
(21 mmol) were dissolved in TBME (105 ml) and
CAL-A (7.9 g containing 20% (w/w) of the enzyme on
Celite) was added. At 48% conversion the enzyme was
filtered off. (R)-1 (0.72 g, 4.02 mmol, ee 90%, [h]D24
+7.09 (c 1.00, MeOH)) was precipitated as the hydro-
chloride salt. (S)-3 (R%=Pr, 0.88 g, 4.12 mmol; ee 98%,
[h]2D4 −77.7 (c 1.01, MeOH)) was purified by column
chromatography on silica (elution with ace-
tone:petroleum ether 1:1).
The reactions of 1 with various achiral acyl donors in
TBME were tested and the results are shown in Table
3. 2,2,2-Trifluoroethyl butanoate, 3-butenoate and 4-
pentenoate (rows 2–4) are the most suitable for resolu-
tion purposes. Only in the case of 2,2,2-trifluoroethyl
chloroacetate (row 5) as an acyl donor, did chemical
N-acylation (10% in 96 h) compete with the enzymatic
one and led to somewhat lowered enantiopurity (ee=
92%) for (S)-3 (R%=CH2Cl). The use of diallyl carbon-
ate and ethyl butanoate as acyl donors resulted in the
In conclusion, we have reported the application of
CAL-A as a highly enantio- and chemoselective catalyst
for the acylation of the secondary amine 1 under condi-
tions where CAL-B leads chemoselectively to reaction
at the ester function. Noteworthy are the opposite
enantioselectivities of CAL-A for N-acylation and
CAL-B for transesterification and alcoholysis as shown
in Schemes 1 and 2. Absolute configurations were
determined using commercially available (S)-pipecolic
acid as a reference.
Table 2. CAL-A-catalyzed (75 mg/ml) acylation of methyl pipecolinate (0.1 M) with trifluoroethyl butanoate (0.2 M) in
different solvents at room temperature (25°C).
Solvent
Time (h)
ee(R)-1 (%)
ee(S)-3 (%)
C (%)
E
1
2
3
4
5
6
7
8
CH3CN
CH2Cl2
CHCl3
DMF
Hexane
Toluene
THF
22
24
24
24
24
72
72
72
4
62
1
2
99
\99
\99
\99
\99
\99
\99
\99
99
39
1
1
\100
–
–
–
2
2
24
16
22
84
39
95
20
13
18
46
28
49
\100
\100
\100
\100
\100
\100
Et2O
9
10
iPr2O
TBME
9
99
Table 3. Effect of an acyl donor (0.2 M) for the CAL-A-catalyzed (75 mg/ml) acylation of methyl pipecolinate (0.1 M) in
TBME at room temperature (25°C)
Acyl donor
Time (h)
ee(R)-1 (%)
ee(S)-3 (%)
C (%)
E
1
2
3
4
5
6
7
8
2,2,2-Trifluoroethyl acetate
2,2,2-Trifluoroethyl butanoate
2,2,2-Trifluoroethyl 3-butenoate
2,2,2-Trifluoroethyl 4-pentenoate
2,2,2-Trifluoroethyl chloroacetate
Diallyl carbonate
24
9
43
6
24
24
24
24
40
95
96
94
18
18
12
20
\99
99
95
96
92
\99
97
98
29
49
50
49
17
24
11
17
\100
\100
\100
\100
a
b
b
b
–
–
–
–
Ethyl butanoate
Ethyl butanoatec
a Chemical N-acylation significant.
b Enzymatic reaction at the ester function observed parallel with the N-acylation as the main reaction.
c Ethyl butanoate as the solvent.

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A. Liljeblad et al. / Tetrahedron Letters 43 (2002) 2471–2474
15. Kanerva, L. T.; Csomo´s, P.; Sundholm, O.; Berna´th, G.;
Acknowledgements
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Thanks are due to the Technology Development Center
in Finland (TEKES) for financial support.
18. Gedey, S.; Liljeblad, A.; La´za´r, L.; Fu¨lo¨p, F.; Kanerva,
L. T. Can. J. Chem. 2002, in press.
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