10760
J. Am. Chem. Soc. 2001, 123, 10760-10761
ylglycine ethyl ester 3 in 5% aqueous MeOH afforded carboxa-
mide 4 in 76% yield (Scheme 1). Use of a catalytic amount of 3
(0.2 equiv) also gave 4 in 71% yield. Without use of 3, the
condensation did not take place to a discernible extent. These
findings clearly indicate that CDMT cannot react directly with
the carboxylate anion 1 by itself in the absence of 3, and therefore,
that intervention of 3 as a catalyst is essential for the condensation.
We believe that ammoniotriazine 5 should be generated from
CDMT by the reaction with amine 3 as an actual condensing
agent, as observed in the reaction of CDMT and N-methylmor-
pholine that produces reactive DMT-MM (eq 1).6a,c The reaction
Cyclodextrin-Based Artificial Acyltransferase:
Substrate-Specific Catalytic Amidation of Carboxylic
Acids in Aqueous Solvent
Munetaka Kunishima,* Kazuma Yoshimura, Hiroshi Morigaki,
Reiko Kawamata, Keiji Terao,† and Shohei Tani
Faculty of Pharmaceutical Sciences and
High Technology Research Center
Kobe Gakuin UniVersity
Nishi-ku, Kobe 651-2180, Japan
ReceiVed July 9, 2001
We report herein for the first time a new, cyclodextrin (CD)-
based artificial enzyme that efficiently mimics an acyltransferase.
The reaction involves substrate-specific condensation of aromatic
carboxylic acids that possess a strong affinity for the CD cavity,
with amines to give carboxamides. The artificial enzyme catalyzes
in situ activation of the carboxylic acid in an aqueous solvent
leading to the formation of an acyloxytriazine (activated ester),
which undergoes aminolysis to give an amide.
In the biomimetic chemistry using CD or its derivatives, many
studies on a hydrolase model, especially chymotrypsin, have been
reported.1,2 By contrast, artificial enzyme models catalyzing the
reverse reaction, dehydrating condensation between carboxylic
acids and amines, have not appeared. Aqueous media generally
increase the hydrophobic effect, and therefore, promote the
formation of an inclusion complex between CD and hydrophobic
substances.3 In fact, most reactions employing CD were carried
out in aqueous media;4 generally, CD-based enzyme-like reactions
should tolerate and enjoy the use of aqueous solvent. This
limitation is probably responsible for the lack of enzymatic models
of dehydrating condensation between carboxylic acids and amines,
which are generally carried out under dry conditions,5 via in situ
activation of an acid moiety in aqueous media.
Recently, we have introduced 4-(4,6-dimethoxy-1,3,5-triazin-
2-yl)-4-methylmorpholinium chloride (DMT-MM) as a new
dehydrating condensing agent,6 which enables us to carry out the
direct one-pot condensation of carboxylic acids and amines in
water or alcohols.7 In the course of the study, we now found a
novel catalytic system for formation of carboxamides in aqueous
solvent. Condensation of sodium 4-tert-butylbenzoate 1 with
benzylamine hydrochloride 2 using 2-chloro-4,6-dimethoxy-1,3,5-
triazine (CDMT) and a stoichiometric amount of N,N-dimeth-
of 5 and 1 would afford reactive acyloxytriazine 6,6-8 which in
turn undergoes aminolysis with amine 2 to give the carboxamide
4.9 Since 3 is regenerated by attack of carboxylate 1 on the triazino
group of 5, it enjoy the reuse in the next catalytic cycle for further
activation of carboxylates.
On the basis of this finding, we have designed a substrate-
specific, catalytic system mimicking an N-acyltransferase in which
a â-CD 7 possessing N,N-dimethylglycyl group acts as an
apoenzyme. As shown in Scheme 2, 7 is activated by coupling
with CDMT (coenzyme) through the dimethylamino group to give
a reactive holoenzyme 8 corresponding to 5. An aromatic
† Permanent address: Wacker Chemicals East Asia Ltd., 2-14-1 Nishi-
Waseda, Shinjuku-ku, Tokyo 169-0051, Japan.
carboxylate ion fitting the CD cavity preferentially interacts with
8 to form an inclusion complex (ES-complex). The included
carboxylate ion in the resulting ES-complex is brought into close
proximity with the triazino group and attacks it preferentially,
giving an EP-complex. Final aminolysis of the resulting acyloxy-
triazine takes place to precipitate the produced amides with
concomitant liberation of the apoenzyme 7, which can be recycled.
The artificial apoenzyme 7 was readily prepared from mono-
6-hydroxy permethylated â-cyclodextrin10 by coupling with N,N-
dimethylglycine.11 Substrate specificity of the condensation with
amines was examined using a competitive reaction between 1
and sodium 3,5-di-tert-butylbenzoate 9a. As shown in Table 1, a
methanolic solution of ammonium salt 2 (1.0 equiv) and CDMT
(1.0 equiv) was added to an aqueous solution of sodium
(1) Breslow, R.; Dong, S. D. Chem. ReV. 1998, 98, 1997-2011 and
references therein.
(2) For recent chymotrypsin models using CD, see: (a) Breslow, R.; Nesnas,
N. Tetrahedron Lett. 1999, 40, 3335-3338. (b) Ohe, T.; Kajiwara, Y.; Kida,
T.; Zhang, W.; Nakatsuji, Y.; Ikeda, I. Chem. Lett. 1999, 921-922. (c)
Fukudome, M.; Okabe, Y.; Yuan, D.-Q.; Fujita, K. Chem. Commun. 1999,
1045-1046. (d) Yan, J.-M.; Atsumi, M.; Yuan, D.-Q.; Fujita, K. Tetrahedron
Lett. 2000, 41, 1825-1828. (e) Gadosy, T. A.; Boyd, M. J.; Tee, O. S. J.
Org. Chem. 2000, 65, 6879-6889.
(3) (a) Harrison, J. C.; Eftink, M. R. Biopolymers 1982, 21, 1153-1166.
(b) Nemethy, G.; Scheraga, H. A. J. Chem. Phys. 1962, 36, 3401-3417.
(4) (a) Connors, K. A. Chem. ReV. 1997, 97, 1325-1357. (b) Rekharsky,
M. V.; Inoue, Y. Chem. ReV. 1998, 98, 1875-1917. (c) Takahashi, K. Chem.
ReV. 1998, 98, 2013-2033.
(5) For reviews on amide formation, see: (a) Challis, B. C.; Challis, J. A.
In ComprehensiVe Organic Chemistry; Barton, D. H. R., Ollis, W. D., Eds.;
Pergamon Press: Oxford, 1979; Vol. 2, p 957. (b) Benz, G. In ComprehensiVe
Organic Synthesis; Trost, B. M., Ed.; Pergamon Press: Oxford, 1991; Vol.
6, p 381. (c) Bailey, P. D.; Collier, I. D.; Morgan, K. M. In ComprehensiVe
Organic Functional Groups Transformations; Katrizky, A. L., Meth-Cohn,
O., Rees, C. W., Eds.; Pergamon: New York, 1995; Vol. 5, p 257.
(6) (a) Kunishima, M.; Kawachi, C.; Iwasaki, F.; Terao, K.; Tani, S.
Tetrahedron Lett. 1999, 40, 5327-5330. (b) Kunishima, M.; Morita, J.;
Kawachi, C.; Iwasaki, F.; Terao, K.; Tani, S. Synlett 1999, 1255-1256. (c)
Kunishima, M.; Kawachi, C.; Morita, J.; Terao, K.; Iwasaki, F.; Tani, S.
Tetrahedron 1999, 55, 13159-13170.
(8) (a) Kaminski, Z. J. Tetrahedron Lett. 1985, 26, 2901-2904. (b)
Kaminski, Z. J. Int. J. Pept. Protein Res. 1994, 43, 312-319. (c) Kaminska,
J. E.; Kaminski, Z. J.; Gora, J. Synthesis 1999, 593-596. (d) Luca, L. D.;
Giacomelli, G.; Taddei, M. J. Org. Chem. 2001, 66, 2534-2537.
(9) We separately synthesized and isolated 5 and found that 5 was able to
act as a condensing agent giving 4 in 76% yield.
(10) Chen, Z.; Bradshaw, J. S.; Lee, M. L. Tetrahedron Lett. 1996, 37,
6831-6834.
(11) For experimental details, see Supporting Information.
(7) Kunishima, M.; Kawachi, C.; Hioki, K.; Terao, K.; Tani, S. Tetrahedron
2001, 57, 1551-1558.
10.1021/ja011660m CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/03/2001