Preparation of C60-based active esters and coupling of C60 moiety to amines or alcohols

Article abstract of DOI:10.1016/j.bmcl.2007.11.065

We report the syntheses of C₆₀-based active esters and the coupling of their C₆₀ moiety to various amines or alcohols. Methano[60]fullerene carboxylic acid was activated by esterification with N-hydroxysuccinimide (NHS) or pentafluor

Full text of DOI:10.1016/j.bmcl.2007.11.065

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 657–660
Preparation of C60-based active esters and coupling
of C60 moiety to amines or alcohols
Hiroki Tsumoto,a,b Katsumasa Takahashi,a Takayoshi Suzuki,a Hidehiko Nakagawa,a
Kohfuku Kohdab and Naoki Miyataa,*
aGraduate School of Pharmaceutical Sciences, Nagoya City University, 3-1 Tanabedori, Mizuho-ku, Nagoya, Aichi 467-8603, Japan
bFaculty of Pharmaceutical Sciences, Musashino University, 1-1-20 Shinmachi, Nishitokyo-shi, Tokyo 202-8585, Japan
Received 22 July 2007; revised 29 October 2007; accepted 17 November 2007
Available online 22 November 2007
Abstract—We report the syntheses of C60-based active esters and the coupling of their C60 moiety to various amines or alcohols.
Methano[60]fullerene carboxylic acid was activated by esterification with N-hydroxysuccinimide (NHS) or pentafluorophenol
(PFP) and the active esters were isolated. Reactions of the active esters with amines or alcohols proceeded easily to give a variety
of compounds having the C60 moiety.
Ó 2007 Elsevier Ltd. All rights reserved.
[60]Fullerene derivatives have a wide variety of biological
activities such as neuroprotective, enzyme inhibition,
photoinduced DNA cleavage, and antigenicity.1 To ap-
ply functionalized fullerenes in material science and
medicinal chemistry, many fullerene derivatives have
been synthesized by using reactions that involve addition
to C60.2 Methano[60]fullerenes are one of the most well-
studied fullerene derivatives, and several methods are
available for their synthesis: (i) the Bingel reaction, (ii)
the addition–elimination reactions of sulfonium ylides
or phosphonium ylides, and (iii) the addition-thermal
decomposition of diazo compounds.3 However, these
methods have some limitations in terms of molecular de-
sign, namely, method (i) requires strong basic condition,
and the preparation of ylides and diazo compounds is dif-
ficult in methods (ii) and (iii), respectively. These methods
also do not give a pure mono-adduct in high yield due to
the formation of isomers and multi-adducts. On the other
hand, a series of methano[60]fullerene carboxylic acids
are useful to obtain various esters and amides by conden-
sation reactions with alcohols and amines, respectively,
using a condensing reagent. Further, the in situ forma-
tion of acid chloride from methano[60]fullerene carbox-
ylic acid and the subsequent reaction with alcohol and
amine are also useful.1d,3e,4 However, the carboxylic acid
derivatives are insoluble in most organic solvents. Two
approaches have been reported to improve solubility.
One is the synthesis and isolation of the acid chloride
from methano[60]fullerene carboxylic acid, which in turn
is converted into amide or ester by reacting with amine or
alcohol, respectively.5 The other is the use of succinimide
ester that can react with molecules containing primary
and secondary amino groups.6 The acid chloride and
the succinimide ester are both isolable and soluble com-
pounds. Although these methods are convenient, it is dif-
ficult to induce the selective reaction of the acid chloride
with alcohol or amine due to its high reactivity, forming
both ester and amide. Further, the condensation reaction
of succinimide ester with alcohol has not been reported.
In this paper, we report a simple and efficient method
for the preparation of C60 derivatives coupled with
amines and alcohols. Methano[60]fullerene carboxylic
acid was chosen and its carboxylic acid group was acti-
vated to facilitate the formation of the corresponding
amide or ester. Condensation reactions using isolated
succinimide ester containing C60 moiety 1a have been re-
ported.6 According to that paper, succinimide ester 1a
was synthesized from fulleromalonic acid C61(COOH)2
and N-hydroxysuccinimide (NHS) by the DCCI method
with decarboxylation in only 30% yield.6a We synthe-
sized 1a via the condensation reaction of monoacid 1
with NHS using EDCI instead of DCCI, and achieved
an improved yield (56%) (Scheme 1). Monoacid 1 was
Keywords: Fullerenes; Amino acids; Peptides; N-hydroxysuccinimide
esters; Pentafluorophenol esters.
*
Corresponding author. Tel./fax: +81 52 836 3407; e-mail: miyata-n@
phar.nagoya-cu.ac.jp
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.11.065

658
H. Tsumoto et al. / Bioorg. Med. Chem. Lett. 18 (2008) 657–660
NNHTs
prepared from tert-butyl [60]fullerene acetate according
to the procedure reported in the literature.3b,e,7 To im-
prove the coupling efficiency of the active ester with
amines and alcohols, pentafluorophenol (PFP) ester 1b
was prepared in 71% yield (Scheme 1). PFP ester is
highly reactive and is commonly used for the derivatiza-
tion of primary and secondary amino groups.8 Further-
more, to obtain various types of intermediates, another
type of methano[60]fullerene carboxylic acid 2 was cho-
sen and activated to give active esters 2a and 2b in 72%
and 91% yields, respectively (Scheme 1). Acid 2 was syn-
thesized from 4-benzoylbutyric acid in four steps
according to a procedure reported in the literature.5a,9
The phenyl ring in 2 has the advantage of further mod-
ification, such as introduction of various substituents
and replacement of hydrogen with deuterium. Acid 3,
whose phenyl ring hydrogens were converted into deute-
riums, was also synthesized from methyl 4-benzoylbuty-
rate-d5 5 in three steps with the procedure used for 2
(Scheme 2). Methyl ester 5 was purchased from Spectra
Stable Isotopes, Inc. Acid 4 containing bromo atom was
synthesized from methyl 4-(4-bromobenzoyl)butyrate 6
in three steps. Acids 3 and 4 were activated by esterifica-
tion with NHS or PFP using EDCI in carbon disulfide
(CS2), and active esters 3a, 4a, 3b, and 4b were isolated
in 53%, 67%, 43%, and 91% yields, respectively. These
active esters containing the C60 moiety were used as
the key intermediate to produce a variety of meth-
ano[60]fullerene derivatives efficiently.
O
a
COOMe
COOMe
R
R
5: R = C6D5
7: R = C6D5
6: R = C6H4Br
8: R = C6H4Br
R
R
COOMe
COOH
c
b
9: R = C6D5
10: R = C6H4Br
3: R = C6D5
4: R = C6H4Br
Scheme 2. Syntheses of methano[60]fullerene carboxylic acids 3 and 4.
Reagents, conditions, and yields: (a) TsNHNH2, MeOH, reflux, 81–
88%; (b) C60, NaOMe, pyridine, 1,2-dichlorobenzene, reflux, 39–47%;
(c) HCl/AcOH, toluene, reflux, 84–94%.
Table 1. Derivatization with amines
Entry
Amine
Active
ester
Equiva
Time
(h)
Yield
(%)
1
2
3
4
5
6
7
8
1a
2a
2a
4a
1b
2b
2b
4b
1.2
1.6
0.1
1.0
1.4
1.6
0.1
1.0
2
100
75
100b
97
2
1
9
H2N
1
92
25
1
48
54b
73
17
9
1a
2a
1b
0.1
0.1
0.1
19
42
22
81b
89b
76b
The derivatization reactions of the isolated active es-
ters toward various amines were demonstrated. First,
the active esters were allowed to react with benzyl-
amine to give the corresponding amides 11 (Table 1,
entries 1–8). Both NHS ester 1a and PFP ester 1b
smoothly reacted with benzylamine in quantitative
yield (Table 1, entries 1 and 5). In contrast, NHS ester
10
11
H2N
COOEt
a Equivalent amount of active ester to amine.
b The isolated yield is based on the amount of active ester.
2a reacted in moderate yield (Table 1, entry 2); how-
ever, the yield was improved by the reaction with 10
equiv of amine (Table 1, entry 3). Although PFP es-
ters were synthesized to achieve high reactivity to
amines, PFP esters 2b and 4b were less reactive than
NHS esters 2a and 4a (entries 2–4 and 6–8). These re-
sults are not consistent with our expectation that PFP
ester is more reactive than NHS ester. Next, the deriv-
atization with amino acid derivative (10 equiv) was
carried out to give the corresponding amides in mod-
erate yields (Table 1, entries 9–11).
R
COOH
n
1: R = H, n = 0
2: R = C6H5, n = 3
3: R = C6D5, n = 3
4: R = C6H4Br, n = 3
To investigate the reactivity of the active esters to
alcohol, the derivatization reactions with benzyl alco-
hol (10 equiv) were carried out using NHS ester 1a
or PFP ester 1b. Under the same conditions as those
used for benzylamine, corresponding ester 12 was
not obtained. However, on adding DMAP (1 equiv)
to the reaction mixture, the esterification was facili-
tated to give corresponding ester 12 in 72% and
100% yields, respectively (Fig. 1). Even when a mix-
ture of benzylamine (10 equiv) and benzyl alcohol
(10 equiv) was allowed to react with NHS ester 1a
or PFP ester 1b, only corresponding amide 11 was
obtained in 100% and 85% yields, respectively. On
the other hand, in the presence of DMAP, a mixture
of amide 11 and ester 12 was obtained at the ratio of
a
b
F
F
O
R
O
R
O
F
F
F
N
n
n
O
O
O
1a: R = H, n = 0
1b: R = H, n = 0
2a: R = C6H5, n = 3
3a: R = C6D5, n = 3
4a: R = C6H4Br, n = 3
2b: R = C6H5, n = 3
3b: R = C6D5, n = 3
4b: R = C6H4Br, n = 3
Scheme 1. Syntheses of a series of active esters containing C60 moiety
from acids 1–4. Reagents, conditions, and yields: (a) NHS, EDCI, dry
THF or CS2, rt, 53–72%; (b) PFP, EDCI, dry THF or CS2, rt, 43–91%.

H. Tsumoto et al. / Bioorg. Med. Chem. Lett. 18 (2008) 657–660
659
O
O
a
%Int.
100
H
H
N
H
O
Labeled
1315.16
80
60
Native
11
12
40
20
0
Figure 1. Structures of compounds 11 and 12.
500
700
900
1100
1300
1500
Mass/Charge
12:1. These results demonstrate that the active esters
selectively react with amine in the absence of DMAP.
b
Labeled
1a
Among the different classes of fullerene derivatives,
fullerenes coupled with amino acids or peptides are
interesting molecules for biological applications.4a,6b,10
In this regard, there is a need to develop efficient
methods for the preparation of fullerenes coupled with
amino acids or peptides to widen the applications of
fullerenes. We demonstrate here the derivatization of
fullerenes with various amino acids and peptides,
which involves the following steps. A solution of the
NHS ester in DMF was added to an aqueous solution
of amino acids or peptides. The reaction mixture was
stirred with vortex mixer for 10 s, left at room temper-
ature for 1–6 h, and subjected to matrix-assisted laser
desorption/ionization time-of-flight (MALDI-TOF)
mass spectrometry (MS) analysis without further puri-
fication. The MALDI-TOF-MS were acquired in the
negative ion mode using a-cyano-4-hydroxycinnamic
acid (CHCA) as the MALDI matrix. The results of
the MALDI-TOF-MS analysis summarized in Table
2 indicate that the NHS esters reacted with the free
amino functionality of amino acids and peptides under
mild conditions. A typical MALDI-TOF-MS is shown
in Figure 2(a). Leucine-enkephalin (YGGFL) was
derivatized with NHS ester 1a. MÀ ion of the deriva-
tized peptide was observed at m/z 1315.16 (calcd mass:
1315.26). Signals which appeared at m/z 600–800 were
those derived from the matrix and the reagent. Figure
2(b) shows the reverse-phase HPLC chromatogram of
the reaction mixture at 6 h after reaction, which was
obtained with a C4 RP column. The native peptide
(4.1 min) was not detected while the derivatized pep-
Native
0
5
10
15
20
25
30
35
Time (min)
Figure 2. (a) MALDI-TOF-MS of leucine-enkephalin (YGGFL)
derivatized with 1a. MÀ ion (calculated mass 1315.26) of the
derivatized peptide was observed at m/z 1315.16. The spectrum was
acquired in the reflectron negative mode using CHCA as the MALDI
matrix. (b) RP-HPLC chromatogram of the reaction mixture of
YGGFL derivatized with active ester 1a at 6 h. Inertsil WP300 C4
column from GL Sciences was used.
tide with 1a was detected at 24.1 min. The broad peak
at 26–31 min was derived from the unreacted reagent
1a. The results of MALDI-TOF-MS analysis and
RP-HPLC analysis indicated that NHS ester 1a re-
acted with pentapeptide quantitatively.
Spectral data of new compounds were described in Ref.
11. General procedure for the derivatization with amines
was described in Ref. 12.
In summary, NHS esters and PFP esters of meth-
ano[60]fullerene carboxylic acids were synthesized
and their C60 moiety was coupled to molecules con-
taining amino or hydroxyl groups. The active ester re-
acted with amine selectively and only reacted with
alcohol in the presence of DMAP. MALDI-TOF-MS
analysis indicated the potential utility of these active
esters for the preparation of fullerene-modified amino
acids or peptides for application in biological studies.
This approach features ease of coupling of the C60
moiety to a variety of molecules with amino or hydro-
xyl groups via a simple reaction under mild condi-
tions. These [60]fullerene-based active esters have the
potential to be used as building blocks for the synthe-
sis of various methano[60]fullerene derivatives, extend-
ing the application of fullerene in medicinal chemistry
and material science. Based on these ideas, expecting
the biological activities, we have examined the prepa-
ration of some amino acids and related derivatives
having C60 moiety. Results will be appeared elsewhere
soon. Also the applications of our reagents 1a–4a and
1b–4b for mass spectrometry of small amine molecules
are in progress.
Table 2. Derivatization with amino acids and peptides
Entry Amino acid
and peptide
Active
ester
Monoisotopic
MW of derivative
Expected Observed
1
Gly
1a
2a
3a
4a
1a
2a
1a
2a
1a
2a
1a
2a
835.03
953.11
958.14
835.02
953.12
958.11
1031.02
949.07
1031.03
948.94
2
3
4
5
6
Gly-Gly-Gly
Z-Lys
1067.15
1040.14
1158.22
1067.15
1185.23
1211.21
1329.29
1315.26
1433.34
1067.00
1040.13
1158.15
1067.16
1185.19
1211.04
1329.04
1315.16
1433.10
Bz-Gly-Lys
Gly-Gly-Tyr-Arg
Tyr-Gly-Gly-Phe-Leu 1a
2a

660
H. Tsumoto et al. / Bioorg. Med. Chem. Lett. 18 (2008) 657–660
3a: 1H NMR (CDCl3, 500 MHz, d; ppm) 2.99 (2H, m),
Acknowledgment
2.85–2.81 (6H, m), 2.31 (2H, m). MALDI-TOF-MS
(matrix: CHCA, reflectron negative): calculated molecular
ion MÀ: 998.13, found MÀ: 998.11.
This work was partly supported by the Sasakawa Scien-
tific Research Grant from The Japan Science Society.
4a: 1H NMR (CDCl3, 500 MHz, d; ppm) 7.75 (2H, d,
J = 8.5 Hz), 7.61 (2H, d, J = 8.5 Hz), 2.90 (2H, t,
J = 8.2 Hz), 2.78–2.75 (6H, m), 2.25–2.20 (2H, m).
MALDI-TOF-MS (matrix: CHCA, reflectron negative):
calculated molecular ion MÀ: 1071.01 and [M+2]À:
1073.01, found MÀ: 1071.01 and [M+2]À: 1072.99.
1b: 1H NMR (CDCl3, 500 MHz, d; ppm) 5.09 (1H, s).
MALDI-TOF-MS (matrix: CHCA, reflectron negative):
calculated molecular ion MÀ: 943.99, found MÀ: 944.04.
2b: 1H NMR (CDCl3, 500 MHz, d; ppm) 7.95 (2H, d,
J = 7.0 Hz), 7.57 (2H, t, J = 7.3 Hz), 7.49 (1H, t,
J = 7.3 Hz), 3.00 (2H, m), 2.88 (2H, t, J = 7.3 Hz), 2.33
(2H, m). MALDI-TOF-MS (matrix: CHCA, reflectron
negative) calculated molecular ion MÀ: 1062.07, found
MÀ: 1062.01.
References and notes
1. (a) Dugan, L. L.; Turetsky, D. M.; Du, C.; Lobner, D.;
Wheeler, M.; Almli, C. R.; Shen, C. K.-F.; Luh, T.-Y.;
Choi, D. W.; Lin, T.-S. Proc. Natl. Acad. Sci. U.S.A. 1997,
94, 9434; (b) Friedman, S. H.; De Camp, D. L.; Sijbesma,
P. R.; Srdanov, G.; Wudl, F.; Kenyon, G. L. J. Am. Chem.
Soc. 1993, 115, 6506; (c) Tokuyama, H.; Yamago, S.;
Nakamura, E.; Shiraki, T.; Sugiura, Y. J. Am. Chem. Soc.
1993, 115, 7918; (d) Chen, B.-X.; Wilson, S. R.; Das, M.;
Coughlin, D. J.; Erlanger, B. F. Proc. Natl. Acad. Sci.
U.S.A. 1998, 95, 10809.
2. (a) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc.
1993, 115, 9798; (b) Prato, M.; Maggini, M.; Giacometti, C.;
Scorrano, G.; Sandona, G.; Farnia, G. Tetrahedron 1996,
52, 5221; (c) Yamakoshi, Y.; Yagami, T.; Sueyoshi, S.;
Miyata, N. J. Org. Chem. 1996, 61, 7236; (d) Nakamura, E.;
Yamago, S. Acc. Chem. Res. 2002, 35, 867.
3. (a) Bingel, C. Chem. Ber. 1993, 126, 1957; (b) Wang, Y.;
Cao, J.; Schuster, D. I.; Wilson, S. R. Tetrahedron Lett.
1995, 36, 6843; (c) Bestmann, H. J.; Hadawi, D.; Roder, T.;
Moll, C. Tetrahedron Lett. 1994, 35, 9017; (d) Isaacs, L.;
Wehrsig, A.; Diederich, F. Helv. Chim. Acta. 1993, 76, 1231;
(e) Isaacs, L.; Diederich, F. Helv. Chim. Acta. 1993, 76,
2454.
4. (a) Prato, M.; Bianco, A.; Maggini, M.; Scorrano, G.;
Toniolo, C.; Wudl, F. J. Org. Chem. 1993, 58, 5578; (b)
Toniolo, C.; Bianco, A.; Maggini, M.; Scorrano, G.;
Prato, M.; Marastoni, M.; Tomatis, R.; Spisani, S.; Palu´,
G.; Blair, E. D. J. Med. Chem. 1994, 37, 4558; (c)
Giacalone, F.; Segura, J. L.; Martin, N. J. Org. Chem.
2002, 67, 3529.
5. (a) Hummelen, J. C.; Knight, B. W.; LePeg, F.; Wudl, F.;
Yao, J.; Wilkins, C. L. J. Org. Chem. 1995, 60, 532; (b) Ito,
H.; Tada, T.; Sudo, M.; Ishida, Y.; Hino, T.; Saigo, K.
Org. Lett. 2003, 5, 2643.
6. (a) Lamparth, I.; Schick, G.; Hirsch, A. Liebigs Ann./
Recueil 1997, 1, 253; (b) Jung, G.; Redemann, T.; Kroll,
K.; Meder, S.; Hirsch, A.; Boheim, G. J. Peptide Sci. 2003,
9, 784.
7. (a) Sun, Y.-P.; Lawson, G. E.; Riggs, J. E.; Ma, B.; Wang,
N.; Moton, D. K. J. Phys. Chem. A 1998, 102, 5520; (b)
Ma, B.; Bunker, C. E.; Guduru, R.; Zhang, X.-F.;
Sun, Y.-P. J. Phys. Chem. A 1997, 101, 5626.
8. Kovacs, J.; Kisfaludy, L.; Ceprini, M. J. Am. Chem. Soc.
1967, 89, 183.
3b: 1H NMR (CDCl3, 500 MHz, d; ppm) 3.00 (2H, m),
2.88 (2H, m), 2.33 (2H, m). MALDI-TOF-MS (matrix:
CHCA, reflectron negative): calculated molecular ion MÀ:
1067.10, found MÀ: 1067.07.
4b: 1H NMR (CDCl3, 500 MHz, d; ppm) 7.76 (2H, d,
J = 8.5 Hz), 7.63 (2H, d, J = 8.5 Hz), 2.91 (2H, t,
J = 8.2 Hz), 2.82 (2H, t, J = 7.3 Hz), 2.26–2.22 (2H, m).
MALDI-TOF-MS (matrix: CHCA, reflectron negative):
calculated molecular ion MÀ: 1139.98 and [M+2]À:
1141.98, found MÀ: 1139.22 and [M+2]À: 1141.92.
3: MALDI-TOF-MS (matrix: CHCA, reflectron negative):
calculated molecular ion MÀ: 901.12, found MÀ: 901.08.
4:1H NMR (CDCl3, 500 MHz, d; ppm) 7.78 (2H, d,
J = 8.5 Hz), 7.64 (2H, d, J = 8.5 Hz), 2.88 (2H, t,
J = 8.2 Hz), 2.54 (2H, t, J = 7.3 Hz), 2.20–2.13 (2H, m).
MALDI-TOF-MS (matrix: CHCA, reflectron negative):
calculated molecular ion MÀ: 973.99 and [M+2]À: 975.99,
found MÀ: 973.99 and [M+2]À: 975.97. 7: 1H NMR
(CDCl3, 500 MHz, d; ppm) 9.20 (1H, s), 7.92 (2H, d,
J = 8.2 Hz), 7.30 (2H, d, J = 8.2 Hz), 3.81 (3H, s), 2.63
(2H, m), 2.41 (3H, s), 2.33 (2H, m), 1.69 (2H, m). MS (EI)
m/z 379 (M). Mp: 125–126 °C.
1
8: H NMR (CDCl3, 500 MHz, d; ppm) 9.24 (1H, s), 7.90
(2H, d, J = 8.5 Hz), 7.52 (2H, d, J = 8.5 Hz), 7.46 (2H, d,
J = 8.5 Hz), 7.30 (2H, d, J = 8.5 Hz), 3.81 (3H, s), 2.59
(2H, t, J = 8.2 Hz), 2.41 (3H, s), 2.33 (2H, t, J = 6.1 Hz),
1.69–1.64 (2H, m). MS (EI): m/z 452 (M), 454 (M + 2).
Anal. Calcd for C19H21BrN2O4S: C, 50.34; H, 4.67; N,
6.18. Found: C, 50.36; H, 4.71; N, 6.08. Mp: 151–155 °C.
1
9: H NMR (CDCl3, 500 MHz, d; ppm) 3.68 (3H, s), 2.91
(2H, m), 2.53 (2H, t, J = 7.6 Hz), 2.19 (2H, m). MALDI-
TOF-MS (matrix: CHCA, reflectron negative): calculated
molecular ion MÀ: 915.13, found MÀ: 915.14.
10: 1H NMR (CDCl3, 500 MHz, d; ppm) 7.82 (2H, d,
J = 8.5 Hz), 7.68 (2H, d, J = 8.5 Hz), 3.69 (3H, s), 2.89
(2H, t, J = 8.5 Hz), 2.54 (2H, t, J = 7.6 Hz), 2.20–2.15 (2H,
m). MALDI-TOF-MS (matrix: CHCA, reflectron nega-
tive): calculated molecular ion MÀ: 988.01 and [M+2]À:
990.01, found MÀ: 987.91 and [M+2]À: 989.87.
´
9. Gonzalez, R.; Hummelen, J. C.; Wudl, F. J. Org. Chem.
1995, 60, 2618.
10. (a) Bianco, A.; Da Ros, T.; Prato, M.; Toniolo, C.
J. Pept. Sci. 2001, 7, 208; (b) Pantarotto, D.; Bianco, A.;
Pellarini, F.; Tossi, A.; Giangaspero, A.; Zelezetsky, I.;
Briand, J.-P.; Prato, M. J. Am. Chem. Soc. 2002, 124, 12543;
(c) Bianco, A.; Pantarotto, D.; Hoebeke, J.; Briand, J.-P.;
Prato, M. Org. Biomol. Chem. 2003, 1, 4141.
11. All the new compounds were characterized on the basis of
NMR and mass spectral data. Selected spectroscopic data:
2a: 1H NMR (CDCl3, 400 MHz, d; ppm) 7.94 (2H, m),
7.58–7.49 (3H, m), 3.00 (2H, m), 2.85–2.81 (6H, m), 2.31
(2H, m). MALDI-TOF-MS (matrix: CHCA, reflectron
negative): calculated molecular ion MÀ: 993.10, found
MÀ: 993.01.
12. General procedure for the derivatization with amine. To a
solution of 2a (15 mg, 0.015 mmol) in CHCl3 (4 mL) was
added 0.18 M solution of benzylamine in CHCl3 (0.85 mL,
0.15 mmol). The reaction mixture was stirred at room
temperature for 1 h and then subjected to flash column
chromatography on silica gel using a toluene/CHCl3
mixture (1:1) as eluent to give the corresponding amide
as a brown solid (16.6 mg, quant). MALDI-TOF-MS
(matrix: CHCA, reflectron negative): calculated molecular
ion MÀ: 985.15, found MÀ: 985.10.