Catalytic specificity exhibited by p-sulfonatocalix[n]arenes in the methanolysis of N-acetyl-L-amino acids

Article abstract of DOI:10.1021/jo026580v

Specific acid catalysis of p-sulfonatocalix[n]arenes (n = 4, Calix-S4; n = 6, Calix-S6; n = 8, Calix-S8) was observed in the alcoholysis of N-acetyl-L-amino acids in methanol. The methanolysis rates of basic amino acid substrates (His, Lys, and Arg) were

Full text of DOI:10.1021/jo026580v

Ca ta lytic Sp ecificity Exh ibited by p-Su lfon a toca lix[n ]a r en es in th e
Meth a n olysis of N-Acetyl-L-a m in o Acid s
Koichi Goto, Yoshihiro Yano, Eiji Okada, Chin-Wen Liu, Kiyoto Yamamoto, and
Ryuichi Ueoka*
Division of Applied Chemistry, Graduate School of Sojo University (former name Kumamoto Institute of
Technology), 4-22-1 Ikeda, Kumamoto 860-0082, J apan
ueoka@life.sojo-u.ac.jp
Received October 17, 2002
Specific acid catalysis of p-sulfonatocalix[n]arenes (n ) 4, Calix-S4; n ) 6, Calix-S6; n ) 8, Calix-
S8) was observed in the alcoholysis of N-acetyl-L-amino acids in methanol. The methanolysis rates
of basic amino acid substrates (His, Lys, and Arg) were markedly enhanced in the presence of
Calix-Sn, as compared with rates observed with p-hydroxybenzenesulfonic acid (pHBS), which is
a noncyclic analogue of Calix-Sn. This catalytic effect of Calix-Sn was not observed for the
1
methanolysis of Phe, Tyr, and Trp substrates. On the other hand, H NMR experiments following
the effect of Calix-Sn on N-acetyl-L-amino acid substrates in CD3OD showed that the spectrum of
a mixture of the His substrate with Calix-Sn was significantly different from the combined spectra
of the respective compounds. These changes in spectra support the formation of an inclusion complex
of Calix-Sn with basic amino acids. Furthermore, it was obvious that methanolysis of the His
substrate catalyzed by Calix-S4 and Calix-S6 obeyed Michaelis-Menten kinetics. These results
indicate that the catalytic activity of Calix-Sn originates from its forming a complex with specific
substrates (basic amino acids), similar to enzymatic reactions.
In tr od u ction
composition of coaggregates (reaction fields),1,2,9-15 and
by regulating the temperature,1,2,10,11,16-20 pH,2,5 and ionic
strength11,17,21-24 of the reaction media. In particular,
the authors attained almost complete L-enantio-
selective catalysis.11,17,21-23 This can be attributed to
optimization in the enzyme model conformation in the
coaggregate systems by controlling the reaction micro-
One of the principal subjects in biomimetic chemistry
has been the creation of artificial enzymes having high
catalytic activity and specificity comparable to those of
native enzymes. In particular, stereochemical control has
been recognized as a very important subject. It has
attracted considerable attention in connection with un-
derstanding the origins of catalytic specificity in pro-
teolytic enzymes and in creating corresponding artificial
enzymes. In the course of our study of esterase models,
remarkably stereospecific catalysis was observed in the
hydrolysis of amino acid and/or dipeptide esters carried
out by functional molecular assemblies composed of
surfactants and catalytic species. We emphasized that
stereochemical control could be established by changing
amino acid residues that were covalently introduced into
substrates and catalysts (reactants),1-9 by changing the
(8) Ueoka, R.; Matsumoto, Y.; Ninomiya, Y.; Nakagawa, Y.; Ionoue,
K.; Ohkubo, K. Chem. Lett. 1981, 785.
(9) Ueoka, R.; Matsumoto, Y.; Ihara, Y. Chem. Lett. 1984, 1807.
(10) Goto, K.; Ueoka, R.; Matsumoto, Y. J . Org. Chem. 1995, 60,
3342.
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Ueoka, R. Tetrahedron Lett. 1999, 40, 2129.
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Lett. 1986, 27, 1183.
(13) Ueoka, R.; Moss, R. A.; Swarup, S.; Matsumoto, Y.; Strauss,
G.; Murakami, Y. J . Am. Chem. Soc. 1985, 107, 2185.
(14) Ueoka, R.; Dozono, H.; Matsumoto, Y.; Cho, M.; Kitahara, K.;
Kato, Y. Chem. Pharm. Bull. 1990, 38, 219.
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Chem. Pharm. Bull. 1988, 36, 4660.
* To whom correspondence should be addressed. Phone: +81-96-
326-3111. Fax: +81-96-323-1331.
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Lett. 1984, 583.
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Murakami, Y. Chem. Lett. 1986, 127.
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Y.; Tetrahedron Lett. 1991, 32, 6597.
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Harada, K.; Kikuchi, J .; Murakami, Y. J . Am. Chem. Soc. 1988, 110,
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K.; Sugii, A. Tetrahedron Lett. 1990, 31, 5335.
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219.
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Extr. Conf., 1990 1992, 1649.
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10.1021/jo026580v CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/03/2003
J . Org. Chem. 2003, 68, 865-870
865

Goto et al.
environment.1,5,11,25 Furthermore, since cyclodextrins have
been utilized as useful enzyme mimics,26-32 we employed
cyclodextrins as host molecules in the enantioselective33
and diastereoselective34 hydrolysis of amino acid and
dipeptide esters. Significant stereoselective catalysis was
obtained in the hydrolysis of specific substrates mediated
by γ-cyclodextrins.
Calixarenes35-38 are macrocyclic oligomers of para-
substituted phenolic residues bridged by methylene
groups. They are also useful host molecules for efficient
enzyme models. Considerable effort has been devoted
over the past decade to developing methods to synthesize
functionalized derivatives, and to describing the confor-
mational characteristics and inclusion properties of calix-
arenes as host molecules toward various guest molecules
in solution.39 Nevertheless, little has been discovered
regarding the enzyme-like catalytic system of calix-
arenes.40-42 To extend our study of enzyme models, we
have examined the catalytic effects of p-sulfonatocalix-
[n]arenes (Calix-Sn), which were originally synthesized
as “water-soluble” calixarenes by Shinkai et al.,40,42-45 for
the hydrolysis of (Z)-amino acid esters in buffered solu-
tion. However, catalytic activity of Calix-Sn was not
observed, and the hydrolysis rates of the amino acid
esters decreased in the presence of Calix-Sn (see the
Supporting Information). On the other hand, the enzyme
mimetic catalysis by Calix-Sn in the alcoholysis of amino
acids in methanol was characterized.46 Recently, it
became obvious that the Calix-Sn catalysis could be
analyzed using Michaelis-Menten kinetics.
F IGURE 1. Time courses of the methanolysis of N-Ac-L-His-
OH in the presence of pHBS or Calix-Sn at 25 °C. [N-Ac-L-
His-OH] ) 1.0 × 10-4 M, [pHBS] ) 3.0 × 10-2 M, [Calix-S4]
) 7.5 × 10-3 M, [Calix-S6] ) 5.0 × 10-3 M, and [Calix-S8] )
3.75 × 10-3 M.
acids (Phe, Tyr, Trp, His, Lys, and Arg). The reactions
1
were monitored by kinetic and H NMR measurements.
The calixarenes and amino acid substrates employed in
this study are listed in the following section.
In this study, we report successful specific acid cataly-
sis by Calix-Sn (n ) 4, Calix-S4; n ) 6, Calix-S6; n ) 8,
Calix-S8) in the methanolysis of various N-acetyl-L-amino
(25) Ueoka, R.; Matsumoto, Y.; Nagamine, K.; Yasui, H.; Harada,
K.; Sugii, A. Chem. Pharm. Bull. 1987, 35, 3070.
(26) Komiyama, M.; Bender, M. L. J . Am. Chem. Soc. 1978, 100,
4576.
(27) Trainor, G. L.; Breslow, R. J . Am. Chem. Soc. 1981, 103, 154.
(28) Ueno, A.; Moriwaki, F.; Osa, T.; Ikeda, T.; Toda, F.; Hattori, K.
Bull. Chem. Soc. J pn. 1986, 59, 3109.
(29) Kitaura, Y.; Bender, M. L. Bioorg. Chem. 1975, 4, 237.
(30) Hamasaki, K.; Ueno, A. Chem. Lett. 1995, 859.
(31) Ihara, Y.; Nakanishi, E.; Nango, M.; Koga J . Bull. Chem. Soc.
J pn. 1986, 59, 1901.
(32) Taniguchi, Y.; Makimoto, S.; Suzuki, K. J . Phys. Chem. 1981,
85, 3469.
(33) Goto, K.; Nakashima, K.; Tanoue, O.; Nukushina, S.; Toudo,
I.; Imamura, C.; Matsumoto, Y.;Ueoka, R. Chem. Pharm. Bull. 2002,
50, 1283.
(34) Ueoka, R.; Matsumoto, Y.; Harada, K.; Akahoshi, H.; Ihara, Y.;
Kato, Y. J . Am. Chem. Soc. 1992, 114, 8339.
(35) Gutsche, C. D.; Dhawan, B.; No, K. H.; Muthukrishnan, R. J .
Am. Chem. Soc. 1981, 103, 3782.
(36) Gutsche, C. D.; Bauer, L. J . J . Am. Chem. Soc. 1985, 107, 6052.
(37) Gutsche, C. D.; Bauer, L. J . J . Am. Chem. Soc. 1985, 107, 6059.
(38) Bauer, L. J .; Gutsche, C. D. J . Am. Chem. Soc. 1985, 107, 6063.
(39) For example: Gutsche, C. D. Calixarenes Revisited; Stoddart,
J . F., Eds.; Monographs in Supramolecular Chemistry; The Royal
Society of Chemistry: Cambridge, 1998.
(40) Shinkai, S.; Mori, S.; Koreishi, H.; Tsubaki, T.; Manabe, O. J .
Am. Chem. Soc. 1986, 108, 2409.
(41) Pirrincioglu, N.; Zaman, F.; Williams, A. J . Chem. Soc., Perkin
Trans. 2 1996, 2561.
(42) Shinkai, S.; Koreishi, H.; Mori, S.; Sone, T.; Manabe, O. Chem.
Lett. 1985, 62, 1033.
(43) Shinkai, S.; Araki, K.; Manabe, O. J . Am. Chem. Soc. 1988,
110, 7214.
(44) Shinkai, S.; Araki, K.; Matsuda, T.; Manabe, O. Bull. Chem.
Soc. J pn. 1989, 62, 3856.
(45) Shinkai, S.; Araki, K.; Matsuda, T.; Nishiyama, N.; Ikeda, H.;
Takasu, I.; Iwamoto, M. J . Am. Chem. Soc. 1990, 112, 9053.
(46) Goto, K.; Murakami, Y.; Ueoka, R. J . Mol. Catal., B 2001, 11,
985.
Resu lts a n d Discu ssion
Meth a n olysis of N-Acetyl-L-a m in o Acid s in th e
P r esen ce of Ca lix-Sn . To investigate the catalytic
effects of Calix-Sn on the alcoholysis of amino acids,
Calix-Sn or pHBS, which is a noncyclic analogue of Calix-
Sn, was added to methanol solutions of various N-acetyl-
L-amino acids (Phe, Tyr, Trp, His, Lys, and Arg). Ex-
amples of the time courses for methanolysis of N-Ac-L-
His-OH in the presence of Calix-Sn or pHBS are shown
in Figure 1. All of the reactions obeyed pseudo-first-order
kinetics. However, the methanolysis rates of N-acetyl-L-
amino acids were extremely slow in the absence of Calix-
Sn or pHBS, so the apparent second-order rate constant
(k2), in the concentration unit of the sulfonate group of
866 J . Org. Chem., Vol. 68, No. 3, 2003

Catalytic Specificity of p-Sulfonatocalix[n]arenes
TABLE 1. Ap p a r en t Secon d -Or d er Ra te Con sta n ts (k2)
for th e Meth a n olysis of N-Acetyl-L-a m in o Acid s in th e
P r esen ce of p HBS a n d Ca lix-Sn a
k2
catalyst
pHBS
substrate
(M-1 h-1
)
k2Calix-Sn/k2
pHBS
N-Ac-L-Phe-OH
N-Ac-L-Tyr-OH
N-Ac-L-Trp-OH
N-Ac-L-His-OH
N-Ac-L-Lys-OH
N-Ac-L-Arg-OH
N-Ac-L-Phe-OH
N-Ac-L-Tyr-OH
N-Ac-L-Trp-OH
N-Ac-L-His-OH
N-Ac-L-Lys-OH
N-Ac-L-Arg-OH
N-Ac-L-His-OH
N-Ac-L-Lys-OH
N-Ac-L-Arg-OH
N-Ac-L-His-OH
N-Ac-L-Lys-OH
N-Ac-L-Arg-OH
15
17
20
1.4
1.7
1.2
14
17
17
33
20
25
120
20
19
57
Calix-S6
0.93
1.0
0.85
24
12
16
86
12
16
41
Calix-S4
Calix-S8
95
20
56
17
a
Conditions: 25 °C, [N-Acetyl-L-amino Acid] ) 1.0 × 10-4 M,
[pHBS] ) 3.0 × 10-2 M, [Calix-S6] ) 5.0 × 10-3 M, [Calix-S4] )
7.5 × 10-3 M, [Calix-S8] ) 3.75 × 10-3 M.
Calix-Sn or pHBS, was evaluated according to eq 1. The
F IGURE 2. 1H NMR spectra of N-Ac-L-His-OH alone (A) and
in a mixture with Calix-S4 (B-D) and of Calix-S4 alone (E)
in CD3OD at room temperature. [N-Ac-L-His-OH]:[Calix-S4]
) 1:0.025 (B), 1:0.05 (C), and 1:0.25 (D).
k2 ) k1/[sulfonate unit]
(1)
rate constants in the presence of pHBS or Calix-Sn are
summarized in Table 1.
Each rate constant for the methanolysis of N-Ac-L-Phe-
OH, N-Ac-L-Tyr-OH, and N-Ac-L-Trp-OH in the presence
of Calix-S6 was almost identical with that in the presence
of pHBS (k2Calix-S6/k2pHBS values are nearly 1). Therefore,
there was no significant difference between the Calix-
S6 and pHBS catalysts in the methanolysis of these
amino acids. In contrast, the methanolysis rates of N-Ac-
L-His-OH, N-Ac-L-Lys-OH, and N-Ac-L-Arg-OH were
enhanced in the presence of Calix-S6 relative to rates
measured in the presence of pHBS (k2Calix-S6/k2pHBS ) 12-
24). Interestingly, catalytic rates for the basic N-acetyl-
L-amino acids in the presence of pHBS were significantly
smaller than those seen for pHBS plus the other three
substrates. The rate constants for methanolysis of the
basic amino acids His, Lys, and Arg in the presence of
Calix-S4 and Calix-S8 are shown in Table 1. All of these
rate constants were also greater than those measured
in the presence of pHBS. In particular, the methanolysis
of N-Ac-L-His-OH was most dramatically accelerated by
Calix-S4
pHBS
Calix-S4 (k2
) 120 M-1 h-1, k2Calix-S4/k2
) 86).
These results suggest that the catalytic activity of Calix-
Sn depends on the structure of the substrates and that
Calix-Sn acts as an efficient catalyst in the methanolysis
of basic amino acid substrates.
F IGURE 3. 1H NMR spectra of N-Ac-L-His-OH alone (A) and
in a mixture with pHBS (B-D) and of pHBS alone (E) in CD3-
OD at room temperature. [N-Ac-L-His-OH]:[pHBS] ) 1:0.1 (B),
1:1 (C), and 1:4 (D).
1H NMR Sp ectr oscop ic Stu d ies. 1H NMR measure-
ment is one of the most useful means for studying
1
molecular interactions in solution. Thus, H NMR mea-
observed to shift to lower magnetic fields in the presence
of pHBS or Calix-S4. These types of spectral changes
were not observed for the mixtures of N-Ac-L-Phe-OH,
N-Ac-L-Tyr-OH, or N-Ac-L-Trp-OH with Calix-Sn. The
shifts to lower magnetic fields were apparently caused
by protonation of the imidazole nitrogen with sulfonate
protons of pHBS or Calix-Sn. Figure 4 shows the chemical
shifts for the C2 and C4 protons as a function of the
surements were undertaken to explore the catalytic
properties of Calix-Sn in the methanolysis of basic amino
acids. Figures 2 and 3 show the 1H NMR spectra of N-Ac-
L-His-OH in the presence of pHBS and Calix-S4, respec-
tively, in CD3CD at room temperature. With respect to
the spectra of N-Ac-L-His-OH, the chemical shifts of the
C2 and C4 protons of the imidazole (Im) ring were
J . Org. Chem, Vol. 68, No. 3, 2003 867

Goto et al.
F IGURE 4. Chemical shifts of C2-H and C4-H of the
imidazole ring of N-Ac-L-His-OH as a function of the [sulfonate
unit]/[N-Ac-L-His-OH] ratio in CD3OD at room temperature.
∆δ ) δ(N-Ac-L-His-OH + Calix-S4 (or pHBS)) - δ(N-Ac-L-His-
OH).
F IGURE 5. Concentration dependence of Calix-Sn and pHBS
in the unit of the sulfonate group on the pseudo-first-order
rate constants (kt) for the methanolysis of N-Ac-L-His-OH.
SCHEME 2
SCHEME 1
tempted to analyze the catalysis of Calix-Sn on the basis
of Michaelis-Menten kinetics. The effect of Calix-S4,
Calix-S6, and pHBS concentrations on the pseudo-first-
order rate constant kt for the methanolysis of N-Ac-L-
His-OH was examined, and the results are shown in
Figure 5. With respect to pHBS, the kt value increased
linearly along with the concentration of pHBS. The
[sulfonate unit]/[N-Ac-L-His-OH] ratios. The ∆δ values
increased along with the increasing ratios in both sys-
tems, but the values in the presence of Calix-S4 were
smaller than in the presence of pHBS. On the other hand,
for the spectra of Calix-S4, the ArCH2Ar methylene
proton signal of Calix-S4 was broadened in the presence
of an excess amount of N-Ac-L-His-OH (Figure 2B,C).
These observations may be attributable to the inclusion
of N-Ac-L-His-OH in the Calix-S4 cavity; that is, the fixed
conformation led to broadening of the methylene
peak.38,40,44 It is plausible that the imidazole moiety of
N-Ac-L-His-OH not only was protonated but also expe-
rienced a ring current effect due to its proximity to Calix-
S4, so that the chemical shifts to lower magnetic fields
were less pronounced relative to those for pHBS.
+
apparent catalytic constant kH for the methanolysis by
pHBS, obtained from the slope, was 1.8 M-1 h-1. On the
other hand, kt values for Calix-S4 and Calix-S6 increased
rapidly, almost reaching saturation above [sulfonate unit]
) 0.03 M (the corresponding concentration of Calix-Sn,
[Calix-S4] ) 7.5 × 10-3 M, [Calix-S6] ) 5.0 × 10-3 M).
These kinetic behaviors are typical for enzymatic cataly-
sis, so the data were treated by kinetic analysis on the
basis of Michaelis-Menten principles32 (Scheme 2). Un-
der the experimental conditions of [catalyst] . [sub-
strate], the observed first-order rate constant kt,Calix-Sn
for the methanolysis in the presence of Calix-Sn is given
by
It is well-known that the acid-catalyzed methanolysis
of carboxylic acids proceeds via formation of several
cationic intermediates. In the methanolysis of N-Ac-L-
Phe-OH, N-Ac-L-Tyr-OH, and N-Ac-L-Trp-OH with Calix-
Sn and pHBS as catalysts, these sulfonic acids would act
as “simple” acid catalysts. On the other hand, in the
methanolysis of N-Ac-L-His-OH and other basic amino
acids, the imidazole or amino groups in the side chains
seem to be protonated, and the resulting positive charge
may prevent the formation of cationic intermediates, as
shown in Scheme 1, so that the rate enhancement is even
smaller than that seen for other substrates in the
presence of pHBS. In contrast, it is noteworthy that the
methanolysis rate of basic amino acids was extremely
enhanced in the presence of Calix-Sn. Calix-Sn seems to
promote complex formation with basic amino acids and
to thereby induce stabilization of cationic intermediates
with the anionic sulfonate groups.
kt,Calix-Sn ) (kt,RSO H + Kbk2[Calix-Sn])/(1 +
3
[Calix-Sn]) (2)
kt,RSO H ) kH+[RSO3H]
(3)
3
Kb ) k1/k-1 ) [Calix-Sn‚S]/([Calix-Sn][S]) (4)
Here, kt,RSO H is the pseudo-first-order rate constant for
3
acid-catalyzed methanolysis measured in the presence of
pHBS. The binding constant Kb of the Calix-Sn‚substrate
complex and the rate constant k2 for the complexed
substrate were determined by the least-squares method
from a modified Lineweaver-Burk plot between 1/(kt,Calix-Sn
- kt,RSO H) and 1/[Calix-Sn], as shown in eq 5. The plots
3
1/(kt,Calix-Sn - kt,RSO H) ) 1/k2 + 1/(Kbk2[Calix-Sn])
3
Kin et ic An a lysis Ba sed on Mich a elis-Men t en
(5)
1
P r in cip les. The H NMR study suggested that specific
catalysis by Calix-Sn proceeds via complex formation
with basic amino acid substrates. Therefore, we at-
of 1/(kt,Calix-Sn - kt,RSO H) against 1/[Calix-Sn] show straight
3
lines (Figure 6), indicating that the catalysis of Calix-Sn
868 J . Org. Chem., Vol. 68, No. 3, 2003

Catalytic Specificity of p-Sulfonatocalix[n]arenes
F IGURE 7. A plausible mechanism for the complexation of
Calix-S4 with N-Ac-L-His-OH.
marked rate enhancement for the methanolysis of His
substrates was obtained in the presence of Calix-S4. (b)
1H NMR results following the reaction of N-acetyl-L-
amino acids and Calix-Sn in CD3OD revealed that the
spectrum of the mixture of the His substrate plus Calix-
Sn was significantly different from the combined spectra
of the respective compounds. These changes in the
spectra suggest the formation of an inclusion complex of
Calix-Sn with basic amino acids. (c) Methanolysis of the
His substrate catalyzed by Calix-S4 and Calix-S6 obeyed
Michaelis-Menten kinetics. The results show that the
high catalytic activity of Calix-S4 in the methanolysis of
His substrates derives from both binding and reaction
processes.
F IGURE 6. Lineweaver-Burk plots for the methanolysis of
N-Ac-L-His-OH catalyzed by Calix-S4 and Calix-S6.
TABLE 2. Kin etic P a r a m eter s for th e Meth a n olysis of
N-Ac-L-His-OH Ca ta lyzed by Ca lix-Sn a
Kb
k2
k2Kb
catalyst
(M-1
)
(h-1
)
(M-1 h-1
)
Calix-S4
Calix-S6
1.8 × 103
3.5
1.3
6.3 × 103
5.1 × 102
6.6 × 102
Conditions: 25 °C, [N-Ac-L-His-OH] ) 1.0 × 10-4 M, [Calix-
a
S4] ) 1.0 × 10-3 ∼ 1.5 × 10-2 M, [Calix-S6] ) 6.7 × 10-4 ∼ 1.0 ×
10-2 M.
This is the first example of a marked substrate-specific
catalysis by calixarenes in the alcoholysis of amino acids.
It is significant that the catalysis of calixarenes proceeds
via complex formation with specific substrates, and that
the accompanying stereochemical rearrangements may
induce stabilization of the reaction intermediates as in
catalysis by native enzymes.
proceeds via the formation of a 1:1 complex, and the
binding constant and rate constant obtained from the
linear relationship are presented in Table 2. The binding
constant and rate constant for Calix-S4 were 1.8 × 103
M-1 and 3.5 h-1, while the values for Calix-S6 were 5.1
× 102 M-1 and 1.3 h-1, respectively. The binding affinity
(reflected by Kb) and reaction activity (reflected by k2)
for the Calix-S4 are 3.5-fold and 2.7-fold larger than those
for Calix-S6, respectively. The differences in catalytic
activity between Calix-S4 and Calix-S6 in the metha-
nolysis of N-Ac-L-His-OH depend on both binding and
reaction processes, and the analysis clarified that Calix-
S4 forms a more stable and reactive complex with N-Ac-
L-His-OH as compared with Calix-S6. The ring size of
the Calix-Sn species plays an important role in their
catalytic activity. Calixarenes adopt some preferred
conformaitons depending on their ring size; for example,
rigid calix[4]arenes (cyclic tetramers) adopt a “cone”
conformation, and the more flexible calix[6]arenes (cyclic
hexamers) adopt “winged” or “hinged” conformations.35,36,40
It is likely that the conformation and cavity size of Calix-
S4 are more amenable to complex formation with N-Ac-
L-His-OH, and as shown schematically in Figure 7, such
a stereochemical arrangement may induce stabilization
of the cationic intermediates with the anionic sulfonate
groups.
Exp er im en ta l Section
Ma ter ia ls. Calix-Sn40,42-45 was obtained commercially and
used after purification by cation-exchange column chromatog-
raphy and drying over P2O5 in a vacuum desiccator for 24 h.
The water content of Calix-Sn was determined by thermo-
gravimetry/differential thermal analysis (TG/DTA) measure-
ments. pHBS and all of N-acetyl-L-amino acids were obtained
commercially and used without further purification.
Kin etic Mea su r em en ts. Methanolysis rates for N-acetyl-
L-amino acids were monitored by measuring the yield of ester
products on an HPLC column at room temperature. Typical
conditions were as follows: gel-packed column (4 mm φ × 250
mm); eluent, CH3CN/H2O (2:8 (v/v)); flow rate, 0.50 mL/min;
detector, UV (212 nm). Under the conditions of [Calix-Sn (or
pHBS)] . [N-acetyl-L-amino acid], the reaction followed a
pseudo-first-order rate law with the methanolysis rate con-
stant (k1) calculated by eq 6, where St and S∞ denote peak areas
of ester formation at time t and at infinite time, respectively.
Con clu sion s
log(S∞ - St) ) -k1t/2.303 + log S∞
(6)
(a) With respect to the methanolysis of N-acetyl-L-
amino acids catalyzed by Calix-Sn in methanol, the
methanolysis rate of basic amino acids (His, Lys, and
Arg) was enhanced in the presence of Calix-Sn as
compared with that in the presence of its noncyclic
analogue pHBS. It was particularly notable that a
1H NMR Mea su r em en ts. 1H NMR spectra in CD3OD were
measured at room temperature (using the internal standard
TMS) with a 270 MHz NMR apparatus. The concentration of
N-Ac-L-His-OH was maintained constant (5.0 × 10-2 M) while
that of Calix-S4 or pHBS was varied.
J . Org. Chem, Vol. 68, No. 3, 2003 869

Goto et al.
Su p p or tin g In for m a tion Ava ila ble: Kinetic results of
the hydrolysis of (Z)-amino acid (Ala, Leu, Phe, Trp, and Lys)
Ack n ow led gm en t. This work was supported in part
by a Grant-in-Aid for Encouragement of Young Scien-
tists from the J apan Society for the Promotion of Science
(No. 11750680). We are grateful to Dr. Yukito Mu-
rakami, Emeritus Professor of Kyushu University, for
his helpful discussions and valuable comments. We also
thank Hirohito Okamoto, Masaki Ohshima, Yasuyuki
Nakagaki, and Mariko Amagai for their technical as-
sistance.
1
esters in buffered solutions, H NMR spectra of the Lys ester
1
and pHBS or Calix-S8 in 15% (v/v) CD3CN-D2O, and H NMR
spectra of N-acetyl-L-amino acids (Phe, Tyr, Trp, and His) and
Calix-S6 in CD3OD. This material is available free of charge
via the Internet at http://pubs.acs.org.
J O026580V
870 J . Org. Chem., Vol. 68, No. 3, 2003