Three-component chemoenzymatic synthesis of amide ligated 1,2,3-triazoles

Article abstract of DOI:10.1016/j.tetlet.2013.06.051

CAL-B (Candida antarctica lipase B) immobilized on an acrylic resin (Novozyme 435) smoothly catalyzes the aminolysis of methyl esters with propargyl amine furnishing propargyl amides. In the same reaction vessel these propargyl derivatives are consecutively transformed into amide ligated 1,2,3-triazoles in a Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) click reaction in good to excellent yields.

Full text of DOI:10.1016/j.tetlet.2013.06.051

Accepted Manuscript
Three-component chemoenzymatic synthesis of amide ligated 1,2,3-triazoles
Sidra Hassan, Roxanne Tschersich, Thomas J.J. Müller
PII:
S0040-4039(13)01018-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.06.051
TETL 43104
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Accepted Date:
8 May 2013
12 June 2013
Please cite this article as: Hassan, S., Tschersich, R., Müller, T.J.J., Three-component chemoenzymatic synthesis
of amide ligated 1,2,3-triazoles, Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/j.tetlet.2013.06.051
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Graphical Abstract
Three-component chemoenzymatic synthesis
of amide ligated 1,2,3-triazoles
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Sidra Hassan, Roxanne Tschersich, and Thomas J. J. Müller*

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Three-component chemoenzymatic synthesis of amide ligated 1,2,3-triazoles
Sidra Hassan, Roxanne Tschersich, and Thomas J. J. Müller*
Institut für Organische Chemie und Makromolekulare Chemie, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, 40225 Düsseldorf, Germany
Tel.: +49 (0)211 81 12298; Fax: +49 (0)211 81 14324; E-mail: ThomasJJ.Mueller@uni-duesseldorf.de
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
CAL-B (Candida antarctica lipase B) immobilized on an acrylic resin (Novozyme® 435)
smoothly catalyzes the aminolysis of methyl esters with propargyl amine furnishing propargyl
amides. In the same reaction vessel these propargyl derivatives are consecutively transformed
into amide ligated 1,2,3-triazoles in a Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC)
click reaction in good to excellent yields.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Amidation
Click reaction
Heterocycles
Lipase
One-pot reaction
1. Introduction
2. Results and discussion
The advent of the Cu(I)-catalyzed azide-alkyne cycloaddition
(CuAAC) click reaction1-3 regiospecifically furnishing 1,4-
disubstitued 1,2,3-triazoles has stimulated numerous applications
in many emerging fields ranging from materials to life
sciences.3,4 In particular, the synthesis of 1,2,3-triazole
peptidomimetics has been considerably developed by CuAAC
due to the structural and electronic similarities of 1,2,3-triazoles
to the peptide linkage.5-7 Hence, a control of the conformation in
the resulting tagged peptide sequence can easily be achieved.8
It was already shown that lipases, which are excellent
esterification and transesterification catalysts, could also catalyze
amidation reactions in the sense of ammonolysis or aminolysis.16-
18
However, propargyl amine containing an alkynyl moiety for
further functionalization has never been applied in lipase
catalyzed aminolyses of carboxylic esters so far. As a model
reaction, we chose the aminolysis of methyl p-methoxy
dihydrocinnamate (1a) with propargyl amine (2) furnishing the
propargyl amide 3a (Scheme 1, Table 1). A quick screening for a
suitable solvent system revealed that methyl t-butyl ether
(MTBE) as a solvent with a log P value of 1.4919 displayed
considerable conversion within 30 min whereas other more
hydrophobic or more hydrophilic solvents were less suited. A
screening of six commercially available lipases revealed that only
CAL-B (Candida antarctica lipase B) immobilized on
immobead® 150 or on an acrylic resin (Novozyme® 435) was
able to catalyze the model amidation of propargyl amine while
the other lipases failed. However, the reaction temperature had to
be set between 40 to 45 °C to observe conversion of the
substrates (Table 1, entries 2 and 4). Indeed Novozyme® 435 led
to full conversion within half of the reaction time (Table 1, entry
4) in comparison to CAL B immobilized immobead® 150 (Table
1, entry 2) and, therefore former was applied as a suitable CAL B
source for all further methodological studies.
Chemoenzymatic transformations have become increasingly
important over the years and many applications have been
focusing on chiral resolution of racemic or meso substrates as
catalytic accesses to enantiomerically enriched chiral building
blocks in organic syntheses.9 With the exception of
chemoenzymatic Suzuki sequences by Gröger,10 and a very
recent chemoenzymatic CuAAC by Gotor,11 the one-pot
combination of enzymes and transition metal catalysts is
dominated by dynamic kinetic resolution as an important tool in
asymmetric synthesis.12 As an alternative a chemoenzymatic
continuous flow process has just recently become important.13
Chemoenzymatic syntheses of azides and their applicability in
other reactions have been reported14 one-pot multicomponent
enzyme-CuAAC sequences have remained unexplored to date.
Inspired by the idea to concatenate several catalytic events in a
one-pot process in the sense of sequential, consecutive, domino
or tandem catalysis,15 we set out to combine lipase-catalyzed
aminolysis of methyl esters with propargyl amine to be
consecutively transformed via CuAAC click reaction in a one-pot
fashion. Here we report the design of a novel three-component
chemoenzymatic synthesis of amide ligated 1,2,3-triazoles.
Scheme 1. CAL-B catalyzed aminolysis of methyl p-methoxy
dihydrocinnamate (1a) with propargyl amine (2).

2
Tetrahedron Letters
Table 1. Screening of conditions for the CAL-B catalyzed
While methyl esters 1 lacking an -heteroatom substituent
aminolysis of p-methoxy dihydro cinnamate (1a) with
(Table 2, entries 1, 2, and 8-10) have to be reacted for 24 h at 45
°C, methyl esters containing oxygen atoms in the -position, as
already previously observed for CAL-B catalyzed amidations,3
displayed complete conversion already after 4 h. In addition,
other heteroatoms in -position such as sulfur (Table 2, entries 5
and 6) and nitrogen (Table 2, entries 7, and 13-17) are
transformed with comparable rates.
propargyl amine (2).[a]
Entry
CAL-B
T [°C]t [h] Yield of
amide 3a [%]
1
2
3
4
CAL-B immobilized on immobead® 150 21 48
CAL-B immobilized on immobead® 150 45 48
n. i.[b]
68
Novozyme® 435[c]
21 24
n. i.[b]
67
Novozyme® 435[c]
45 24
Interestingly, the -heteroatom substitution can nicely be
exploited in the regioselective transformation of the diester 1k to
the exclusive formation of product 3k (Table 2, entry 11) in very
good yield. Amino acid methyl esters with Cbz (carboxylbenzyl)
as N-protection group can be efficiently employed (Table 2,
entries 14-17). Lipases, unlike proteases, do not possess amidase
activity and, therefore, lipases can occasionally act as catalysts
for peptide syntheses.20 Although, the synthesis of a -dipeptide
has been reported using CAL-A as an acylation catalyst for -
amino esters,21 CAL-B has not previously been employed as an
enzyme of choice for acylating amino acids to the best of our
knowledge.
[a]A molar ratio of 1.2 for 1a/2 was chosen to achieve full conversion. [b]Not
isolated. [c]CAL B immobilized on an acrylic resin.
With these optimized conditions in hand, the scope of suitable
methyl carboxylates 1 furnishing the corresponding propargyl
amides 3 was tested on a preparative scale (Scheme 2, Table 2).
Scheme 2. CAL-B catalyzed aminolysis of methyl carboxylates 1
with propargyl amine (2) (Reaction Conditions: c0(1) = 0.6
mmol/mL, c0(2) = 0.5 mmol/mL, Novozyme® 450 (50 % w/w with
respect to 1), 2.0 mL of MTBE, 45 °C, shaker @ 290 rpm).
As model reaction for the catenation of the Cu(I) catalyzed
click reaction in a one-pot fashion, the CAL-B catalyzed
aminolysis of methyl ester 1c with propargyl amine (2)
furnishing the corresponding propargyl amide 3c and its
subsequent Cu-catalyzed alkyne-azide cycloaddition with benzyl
azide (4a) as a substrate to give the triazole 5c was chosen
(Scheme 3, Table 3).
Table 2. Propargyl amides 3 synthesized by CAL-B
catalysis.[a]
Entry
Methyl ester 1
Propargyl amide 3 (yield
after isolation)
1
2
R1 = p-MeOC6H4CH2CH2 (1a)
R1 = PhCH2CH2 (1b)
R1 = PhOCH2 (1c)
3a (68%)[a]
3b (62%)[a]
3c (87%)[b]
3d (86%)[b]
3e (77%)[b]
3f (82%)[b]
3g (81%)[b]
3h (77%)[a]
3i (71%)[a]
3j (80%)[a]
3k (80%)[b]
3l (80%)[a]
3m (82%)[b]
3n (87%)[b]
3o (43%)[b]
3p (68%)[a]
3q (70%)[b]
3
4
R1 = PhCH2OCH2 (1d)
R1 = PhSCH2 (1e)
5
Scheme 3. Model reaction for the optimization of the Cu-AAC step
in the chemoenzymatic one-pot sequence furnishing propargyl amide
5a.
6
R1 = PhCH2SCH2 (1f)
R1 = PhNHCH2 (1g)
R1 = 2-thienyl (1h)
R1 = 2-furyl (1i)
7
8
Whereas the classical Medal-Sharpless system, i.e. CuSO4 · 5
H2O and sodium ascorbate, in various added solvent systems
gave only moderate yields (Table 3, entries 1-6), even in added
dichloromethane,22 the addition of L-proline as a ligand23 proofed
to accelerate the reaction and led to higher isolated yields (Table
3, entries 9-11). However, even simpler than cupric sulfate,
cupric acetate (Table 3, entry 7),24 and cuprous iodide (Table 3,
entry 8), and yet comparably efficient turned out to be Cu2O as
direct Cu(I) source in the presence of benzoic acid as a bidentate
Cu(I) stabilizing ligand (Table 3, entries 12 and 13).25
9
10
R1 = PhCC (1j)
11 R1 = p-(MeO2CCH2CH2)C6H4OCH2 (1k)
12
13
14
15
16
17
R1 = (R)-H3CCH(NHCOCF3) (1l)
R1 = (R)-H3CCH(NHCbz) (1m)
R1 = (S)-H3CCH(NHCbz) (1n)
R1 = (S)-HOCH2CCH(NHCbz) (1o)
R1 = (S)-N-Cbz-pyrrolidin-2-yl (1p)
R1 = (CH2)5NCH2 (1q)
[a]Reaction time of 24 h to complete conversion. [b]Reaction time of 4 h to
complete conversion. Cbz: carboxybenzyl
Table 3. Screening of conditions for the Cu-catalyzed click step in the chemoenzymatic aminolysis-azide addition sequence of
methyl -phenoxy acetate (1c) with propargyl amine (2) and benzyl azide (4a).[a]
Entry
1
Cu catalyst
Na ascorbate [mol%]
20
Ligand
-
Base
-
Solvent
THF/H2O (2:1)
T [°C] t [h]
Yield of triazole 5c [%]
CuSO4 · 5 H2O[b]
21
48
24
24
24
48
24
42
24
24
19
19
8
35
CuSO4 · 5 H2O[b]
CuSO4 · 5 H2O[c]
CuSO4 · 5 H2O[c]
CuSO4 · 5 H2O[b]
CuSO4 · 5 H2O[b]
Cu(OAc)2 · H2O[e]
CuI[f]
20
20
20
15
15
6
-
-
EtOH/H2O (1:1) 21
EtOH/H2O (1:1) 21
EtOH/H2O (2:1) 21
CH2Cl2/H2O (1:1) 21
CH2Cl2/H2O (1:1) 45
MeCN/H2O (1:1) 21
52
42
38
60
63
72
61
61
85
82
61
83
2
[d]
[d]
-
Na2CO3
3
-
Na2CO3
4
-
-
-
-
-
5
6
-
7
[h]
[d]
-
ethylenediamine[g]
L-proline[i]
L-proline[i]
L-proline[i]
benzoic acid[j]
benzoic acid[j]
EtNiPr2
THF
21
8
CuSO4 · 5 H2O[b]
CuSO4 · 5 H2O[b]
CuSO4 · 5 H2O[b]
Cu2O[c]
20
20
20
-
Na2CO3
MeOH/H2O (1:1) 21
MeOH/H2O (1:1) 45
CH2Cl2/H2O (1:1) 45
9
[d]
[d]
Na2CO3
10
11
12
13
Na2CO3
-
H2O
21
Cu2O[c]
-
-
H2O
45
4
[a]The reaction conditions for the CAL-B catalyzed aminolysis was chosen as in Scheme 2. [b]5 mol% of the catalyst. [c]4 mol% of the catalyst. [d]20 mol% of the
base. [e]6 mol% of the catalyst. [f]10 mol% of the catalyst. [g]100 mol% of the ligand. [h]100 mol% of the base. [i]20 mol% of the ligand. [j]8 mol% of the ligand.

3
Therefore, this straightforward copper catalyst system was
employed to scout the scope of the consecutive three-component
chemoenzymatic synthesis of the amido methylsubstituted 1,2,3-
triazoles 5 on a preparative scale (Scheme 4, Table 4).
The authors cordially thank CLIB graduate college for the
financial support (scholarship of S. H.), and the Fonds der
Chemischen Industrie.
References and notes
1. For seminal publications, a) Rostovtsev, V. V.; Green, L. G.;
Fokin, V. V.; Sharpless, K. B. Angew. Chem. Int. Ed. 2002, 41,
25962599. b) Tornøe, C. W.; Christensen, C.; Meldal, M. J. Org.
Chem. 2002, 67, 30573064.
2. For recent reviews on CuAAC, see: a) Fokin, V. V. in Catalyzed
Carbon-Heteroatom Bond Formation, Wiley, Weinheim, 2011,
chpt. 7. b) Hein, J. E.; Fokin, V. V. Chem. Soc. Rev. 2010, 39,
13021315. c) Spiteri, C.; Moses, J. E. Angew. Chem. Int. Ed.
2010, 49, 3133. d) Meldal, M.; Tornøe, C. W. Chem. Rev. 2008,
108, 29523015.
Scheme 4. Consecutive three-component chemoenzymatic synthesis
of the amido methylsubstituted 1,2,3-triazoles 5.
Table 4. Amido methylsubstituted 1,2,3-triazoles 5
synthesized by CAL B catalysis.
3. For representative reviews on click chemistry, see: a) A special
issue on click chemistry, Finn, M. G.; Fokin, V. V. in Chem. Soc.
Rev. 2010, 39, issue 4. b) Hawker, C. J.; Fokin, V. V.; Finn, M.
G.; Sharpless, K. B. Aust. J. Chem. 2007, 60, 381383. c) Kolb,
H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem. Int. Ed. 2001,
40, 20042021.
4. For recent reviews on the applications of click chemistry, see: a)
Franc, G.; Kakkar, A. K. Chem. Soc. Rev. 2010, 39, 15361544.
b) Moses, J. E.; Moorhouse, A. D. Chem. Soc. Rev. 2007, 36,
12491262. c) Gil, M. V.; Arévalo, M. J.; López, Ó. Synthesis
2007, 15891620. d) Lutz, J. F. Angew. Chem. Int. Ed. 2007, 46,
10181025. e) Kolb, H. C.; Sharpless, K. B. Drug. Discov. Today
2003, 8, 11281137.
5. Angell, Y. L.; Burgess, K. Chem. Soc. Rev. 2007, 36, 1674–1689.
6. Wu, P.; Fokin, V. V. Aldrichim. Acta 2007, 40, 7–17.
7. Bock, V. D.; Heimstra, H.; van Maarseveen, J. H. Eur. J. Org.
Chem. 2006, 5168.
8. Pedersen, D. S.; Abell, A. Eur. J. Org. Chem. 2011, 23992411.
9. For an excellent recent monography, see e. g. Enzyme Catalysis in
Organic Synthesis, Drauz, K.; Gröger, H.; May, O., eds, Wiley-
VCH, 3rd ed., 2012.
Entry
Methyl ester 1
Azide 3
1,2,3-
Triazoles 5
(yield after
isolation)
1
2
R1 = p-MeOC6H4CH2CH2 (1a)
R1 = PhCH2CH2 (1b)
R2 = Ph (3a) 5a (71%)[a]
3a
3a
5b (61%)[a]
5c (83%)[b]
5d (70%)[b]
5e (74%)[b]
5f (85%)[b]
5g (68%)[a]
5h (71%)[a]
5i (51%)[b]
5j (73%)[b]
5k (70%)[b]
3
R1 = PhOCH2 (1c)
4
R1 = PhCH2OCH2 (1d)
R1 = PhSCH2 (1e)
3a
5
3a
6
R1 = PhNHCH2 (1g)
3a
7
R1 = 2-furyl (1i)
3a
8
R1 = PhCC (1j)
3a
9
R1 = p-(MeO2CCH2CH2)C6H4OCH2 (1k)
R1 = (R)-H3CCH(NHCbz) (1m)
R1 = (S)-H3CCH(NHCbz) (1n)
3a
10
11
3a
3a
R2 = PhS
(3b)
3b
12
R1 = PhSCH2 (1e)
5l (59%)[b]
13
14
R1 = PhNHCH2 (1g)
R1 = 2-furyl (1i)
5m (78%)[b]
5n (85%)[a]
10. Burda, E.; Hummel, W.; Gröger, H. Angew. Chem. Int. Ed. 2008,
3b
47, 95519554.
[a]Reaction time of 24 h of the aminolysis. [b]Reaction time of 4 h of the
aminolysis. Cbz: carboxybenzyl
11. Cuetos, A.; Bisogno, F. R.; Lavandera, I.; Gotor, V. Chem.
Commun. 2013, 49, 2625-2627.
12. For reviews, see e. g. a) Pàmies, O.; Bäckvall, J.-E. Chem. Rev.
2003, 103, 32473261. b) Lee, J. H.; Han, K.; Kim, M.-J.; Park, J.
Eur. J. Org. Chem. 2010, 9991015. c) Hoyos, P.; Pace, V.;
Alcántara, A. R. Adv. Synth. Catal. 2012, 354, 25852611.
13. For coupled chemo(enzymatic) reactions in continuous flow, see
e. g. Yuryev, R.; Strompen, S.; Liese, A. Beilstein J. Org. Chem.
2011, 7, 14491467.
14. a) de la Sovera, V.; Bellomo, A.; Gonzalez, D. Tetrahedron Lett.
2011, 52, 430433. b) Groothuys, S.; Kuijpers, B. H. M.;
Quaedflieg, P. J. L. M.; Roelen, H. C. P. F.; Wiertz, R. W.;
Blaauw, R. H.; van Delft, F. L.; Rutjes, F. P. J. T. Synthesis 2006,
31463152.
15. a) Fogg, D. E.; de Santos, E. N. Coord. Chem. Rev. 2004, 248,
23652379. b) Ajamian, A.; Gleason, J. L. Angew. Chem. Int. Ed.
2004, 43, 37543760. c) Wasilke, J.-C.; Obrey, S. J.; Baker, R. T.;
Bazan, G. C. Chem. Rev. 2005, 105, 10011020. d) Müller, T. J. J.
Top. Organomet. Chem. 2006, 19, 149-205. e) Lebel, H.; Ladjel,
C.; Bréthous, L. J. Am. Chem. Soc. 2007, 129, 1332113326. f)
Zhong, C.; Shi, X. Eur. J. Org. Chem. 2010, 29993025. g)
Wende, R. C.; Schreiner, P. R. Green Chem. 2012, 14,
18211849.
16. Zaks, A.; Klibanov, A. M. Proc. Natl. Acad. Sci. 1985, 82,
31923196.
17. For a review on CAL-B catalyzed ammonolysis and aminolysis,
see e.g. Gotor-Fernández, V.; Busto, E.; Gotor, V. Adv. Synth.
Catal. 2006, 348, 797812.
18. a) Adamczyk, M.; Grote, J. Tetrahedron Lett. 1996, 37,
79137916. b) Adamczyk, M.; Grote, J. Tetrahedron Asymmetry
1997, 8, 20992100. c) Adamczyk, M.; Grote, J. Tetrahedron
Asymmetry 1997, 8, 25092512. d) Adamczyk, M.; Grote, J.
Bioorg. Med. Chem. Lett. 1999, 9, 245248.
19. Rosche, B.; Breuer, M.; Hauer, B.; Rogers, P. L. Biotech.
Bioengin. 2004, 86, 788794.
This novel chemoenzymatic multicomponent synthesis of
especially decorated 1,2,3-triazoles combines the mild reaction
conditions of the CAL-B catalyzed propargyl aminolysis of
methyl esters and the efficient Medal-Sharpless Cu-AAC triazole
synthesis with a high level of functional group tolerance.
The regio- and chemoselective click reaction easily allows
distinguishing between terminal and internal triple bonds as
demonstrated by the synthesis of compound 5h, and although the
alkynoate is even activated towards the thermal [3+2]
cycloaddition, the click reaction proceeds faster. Other interesting
showcases are the efficient formation of the compounds 5j and
5k derived from Cbz-protected D- and L-alanine, respectively.
3. Conclusion
All this accounts for a suitable application of this enzyme-
metal catalyzed methodology for the application to more
sophisticated peptides and azides as substrates for the efficient
generation of peptidomimetics in a one-pot fashion. Additional
studies directed to expand and to develop further
chemoenzymatic sequences with this reactivity principle, are
currently underway.
Acknowledgments

4
Tetrahedron Letters
20. a) Margolin, A. L.; Klibanov, A. M. J. Am. Chem. Soc. 1987, 109,
38023804. b) Margolin, A. L.; Tai, D. F.; Klibanov, A. M. J. Am.
Chem. Soc. 1987, 109, 78857887. For enzyme catalysis in
peptide synthesis: c) Gardossi, L.; Bianchi, D.; Klibanov, A. M. J.
Am. Chem. Soc. 1991, 113, 63286329. d) Chinsky, N.; Margolin,
A. L.; Klibanov, A. M. J. Am. Chem. Soc. 1989, 111, 386388. e)
Kitaguchi, H.; Klibanov, A. M. J. Am. Chem. Soc. 1989, 111,
92729273.
21. Li, X. G.; Kanerva, L. T. Org. Lett. 2006, 8, 55935596.
22. Lee, B. Y.; Park, S. R.; Jeon, H. B.; Kim, K. S. Tetrahedron Lett.
2006, 47, 5105-5109.
23. Feldman, A. K.; Colasson, B.; Fokin, V. V. Org. Lett. 2004, 6,
38973899.
24. Hein, J. E.; Krasnova, L. B.; Iwasaki, M.; Fokin, V. V. Org. Synth.
2011, 88, 238247.
25. a) Shao, C.; Wang, X.; Xu, J.; Zhao, J.; Zhang, Q.; Hu, Y. J. Org.
Chem. 2010, 75, 70027005. b) Shao, C.; Wang, X.; Zhang, Q.;
Luo, S.; Zhao, J.; Hu, Y. J. Org. Chem. 2011, 76, 68326836. c)
Shao, C.; Zhu, R.; Luo, S.; Zhang, Q.; Wang, X.; Hu, Y.
Tetrahedron Lett. 2011, 52, 37823785.
Supplementary Material
Electronic Supplementary Information (ESI) available.