Synthesis of new C60 derivatives containing biologically active 4-aryl-1,4-dihydropyridines

Article abstract of DOI:10.1016/S0040-4039(02)00762-1

Water-soluble pyrrolidino[3,4:1,2][60]fullerene derivatives bearing the biologically active 1,4-dihydropyridine (1,4-DHP) system have been synthesized through different chemical approaches depending upon the linkage position of the fulleropyrrolidine to the 1,4-DHP ring.

Full text of DOI:10.1016/S0040-4039(02)00762-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 4133–4136
Synthesis of new C60 derivatives containing biologically active
4-aryl-1,4-dihydropyridines
Beatriz Illescas,a M. Angeles Mart´ınez-Grau,a M. Luisa Torres,a Javier Ferna´ndez-Gadeab and
Nazario Mart´ına,*
aFacultad de Qu´ımica, Departamento de Qu´ımica Orga´nica, Universidad Complutense, E-28040 Madrid, Spain
bDepartamento de Investigacio´n Ba´sica, Janssen-Cilag S.A., C/Jarama s/n, Pol´ıgono Industrial, E-45007 Toledo, Spain
Received 14 March 2002; revised 16 April 2002; accepted 19 April 2002
Abstract—Water-soluble pyrrolidino[3,4:1,2][60]fullerene derivatives bearing the biologically active 1,4-dihydropyridine (1,4-DHP)
system have been synthesized through different chemical approaches depending upon the linkage position of the fulleropyrrolidine
to the 1,4-DHP ring. © 2002 Elsevier Science Ltd. All rights reserved.
Fullerenes and their derivatives have shown a broad
range of promising biological activities,1,2 especially in
the fields of photodynamic therapy,3 inhibition of HIV-
proteasa,4,5 neuroprotection6 and apoptosis.1,2 How-
ever, an important feature when dealing with fullerenes
is the lack of solubility in polar solvents for their
biological study. This problem can be overcome by
means of chemical modification of fullerenes in such a
way that they acquire solubility in polar media. In this
regard, Prato et al.7 have recently shown that the
covalent attachment of polyether chains to [60]fullerene
brings about the formation of water-soluble
fulleropyrrolidines.
dimethoxycarbonyl-2,6-dimethyl-4-(2-nitrophenyl)-1,4-
dihydropyridine). These fullerene derivatives have been
synthesized by 1,3-dipolar cycloaddition of the in situ
generated azomethine ylides to C60, by reaction of the
corresponding formyl substituted DHP with sarcosine
and [60]fullerene. In order to improve the solubility of
the new compounds in polar solvents, we have also
synthesized the N-(3,6,9-trioxadecyl)fulleropyrrolidines,
thus introducing a hydrophilic chain in the structures.
As there is not a general method for the preparation of
formyl substituted 1,4-dihydropyridines (DHPs), it was
necessary to employ a different synthetic approach
depending on the position of the formyl group (Scheme
1).
On the other hand, there is unabated interest in the
study of 4-aryl-1,4-dihydropyridines (DHPs) as a conse-
quence of their pharmacological activity as the most
important class of the calcium channel modulators.8 A
big effort has been devoted to the synthesis of DHPs
with different substituents9 or heteroatoms,10 thus
enabling the definition of a structure–activity relation-
ship for these types of compounds. The knowledge of
Thus, the preparation of 3,5-diethoxycarbonyl-4-(4-
formylphenyl)-2,6-dimethyl-1,4-dihydropyridine 6 was
carried out in three synthetic steps, as depicted in
Scheme 1. The Koevenagel condensation13 of ethyl
acetoacetate 1 and p-diethoxymethylbenzaldehyde 2
yielded compound 3 as a Z/E isomer mixture in a 1/1
proportion. Both isomers were separated by flash chro-
matography and the Z-isomer was assigned by com-
parison of the chemical shifts of the vinyl hydrogens
with other a-acetylcinnamates.14 Reaction of Z-3 with
ethyl 2-aminocrotonate gave the DHP 5 in good yield
(82%). Acidic hydrolysis of 5 under smooth conditions
led to the formyl substituted DHP 615 in 89% yield.
stereochemical/conformational
requirements
for
activity11 makes it necessary the study of other ana-
logues of the DHP ring.12
Here we report the synthesis of new fulleropyrrolidines
(7a–b, 10, 16a–b) bearing DHPs as biologically active
substituents related to the well-known nifedipine (3,5-
The formyl-DHP 9 was obtained by reaction of benzyli-
denemalonaldehyde (8), which was in turn prepared in
* Corresponding author. Tel.: 34-91-3944227; fax: 34-91-3944103;
e-mail: nazmar@quim.ucm.es
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00762-1

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B. Illescas et al. / Tetrahedron Letters 43 (2002) 4133–4136
Scheme 1. Reagents and conditions: (i) Piperidine, toluene, D; (ii) EtOH, D; (iii) pTsOH, acetone; (iv) a: C60, N-methylglycine,
toluene, D, b: C60, N-(3,6,9-trioxadecyl)glycine, ODCB, D; (v) C60, N-methylglycine, toluene, D; (vi) 4, EtOH, D; (vii) acetone, 6N
HCl.
multistep synthetic procedure,16 with ethyl 2-
using piperidine as the catalyst. Compound 13 was
employed in the next synthetic step without further
purification yielding, on reaction with ethyl 2-
aminocrotonate 4, the formyl protected DHP 1418 in
28% yield. Acidic hydrolysis of the acetal group in 14
with 6N HCl afforded compound 15 in 79% yield.
a
aminocrotonate 4 in a low yield (10%).
Finally, the 2-formyl DHP 15 was prepared starting
from the Knoevenagel condensation of ethyl g-
diethoxyacetoacetate17 (12) with benzaldehyde (11),

B. Illescas et al. / Tetrahedron Letters 43 (2002) 4133–4136
4135
1
The H NMR spectra for the formyl substituted DHPs
behavior of the NH proton in compounds 16a–b we
were not able to observe the restricted rotation on
them.
6, 9 and 15 show the signal for the aldehyde group at
ꢀ9.2–10.5 ppm. The amino group gives rise to a broad
singlet at 5.7 ppm for 6, 6.9 ppm for 15 and a broad
doublet at 6.6 ppm for compound 9. The signal for the
proton (H-4) attached to the sp3 carbon of the 1,4-dihy-
dropyridine ring appears at l ꢀ5.0–5.1 as a singlet. For
compound 9, the hydrogen atom at C-6 of the DHP is
observed as a doublet at 6.87 ppm with a coupling
constant of 5.7 Hz.
It is worth mentioning that the non-planar geometry of
the 1,4-DHP ring could account for the different
dynamic behavior observed in compounds 7a–b bearing
the phenyl ring adjacent to the pyrrolidine moiety and
compounds 10 and 16a–b in which the 1,4-DHP ring is
directly linked to the fulleropyrrolidine.
The signals in the 13C NMR spectra of the DHPs 6, 9
and 15 are also in agreement with the proposed struc-
tures. Thus, the aldehyde group is observed around
183–192 ppm, the carbonyl group of the ester appears
at l ꢀ167 and the C-4 of the DHP ring at l ꢀ37–42.
13C NMR spectra could be recorded for the more
soluble compounds, that is, 7a–b and 16b. The number
of signals of these spectra show the lack of symmetry in
the compounds. The carbonyl carbon of the ester group
appears at l ꢀ167 and the signals for the sp3 carbons
of the pyrrolidine ring and those at the 6,6-ring junc-
tion of the C60 cage are observed at l ꢀ81–84 and l
ꢀ70–72, respectively.
The target fulleropyrrolidines were synthesized by
refluxing in toluene a mixture of the corresponding
formyl substituted DHP (1 equiv.), C60 (1 equiv.) and
sarcosine during a variable period of time (4 h for 7a,
48 h for 10 and 24 h for 16a). In order to increase the
solubility of the fullerene derivatives in polar solvents,
we also prepared the N-(3,6,9-trioxadecyl)glycine fol-
lowing the procedure previously reported by Prato et
al.7 It is worth mentioning that, with this amino acid,
the best yields were obtained using ODCB instead of
toluene as the solvent. For DHP 9, no reaction was
observed after refluxing in ODCB for 48 h when N-
(3,6,9-trioxadecyl)glycine was employed instead of
sarcosine.
In order to carry out a biological study of DHP deriva-
tives (7a–b, 10, 16a–b) their solubility in polar solvents
is an important aspect. In this sense, we performed
solubility tests finding that the N-oligoether fullero-
pyrrolidines are quite soluble in a 9/1 ratio of H2O/
DMSO, thus paving the way for further biological
evaluation.
In this regard, preliminary biological tests are in pro-
gress to determine the range of potential activity of this
class of compounds. Work is currently directed to
prepare other fullerene derivatives containing differ-
ently substituted 1,4-dihydropyridines covalently
attached to the C60 core as promising candidates as
calcium channel modulators.
The spectroscopic data for the novel fulleropyrrolidines
(7a–b, 10, 16a–b) are in agreement with the proposed
structures.19 Thus, these compounds showed a typical
weak absorption band at around 430 nm in the UV–vis
spectra, similar to that of most dihydrofullerenes. The
1H NMR spectra showed the presence of the pyrro-
lidine protons in the region between 4.2 and 5.3 ppm,
as one singlet and two doublets with a coupling con-
stant of ꢀ9.3–9.9 Hz. The proton attached to the C-4
of the DHP ring gives rise to a singlet around ꢀ5.0–5.3
ppm and the N–H appears as a broad singlet for 7a–b
and 16a–b at l ꢀ5.5–6.4 and for compound 10 it is
observed as a broad doublet with a coupling constant
of 5.1 Hz. It is important to note that compound 10 is
isolated as a diastereomeric mixture in 60:40 proportion
which could not be separated by column chromatogra-
phy. This diastereomeric mixture was not observed by
Acknowledgements
Support of this work by DGESIC of Spain (PB98-0818)
is gratefully acknowledged.
References
1. Da Ros, T.; Prato, M. Chem. Commun. 1999, 663–669.
2. Jensen, A. W.; Wilson, S. R.; Schuster, D. I. Bioorg.
Med. Chem. 1996, 4, 767–779.
3. Tokuyama, H.; Yamago, S.; Nakamura, E.; Shiraki, T.;
Sugiura, Y. J. Am. Chem. Soc. 1993, 115, 7918–7919.
4. Friedman, S. H.; Decamp, D. L.; Sijbesma, R. P.;
Srkanov, G.; Wudl, F.; Kenyon, G. L. J. Am. Chem. Soc.
1993, 115, 6506–6509.
5. Sijbesma, R.; Srdanov, G.; Wudl, F.; Castoro, J. A.;
Wilkins, C.; Friedman, S. H.; DeCamp, D. L.; Kenyon,
G. L. J. Am. Chem. Soc. 1993, 115, 6510–6512.
6. 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. USA 1997,
94, 9434.
1
TLC or H NMR analysis for 7 and 16 due, probably,
to the longer distance between both stereogenic centers.
A restricted rotation of phenyl substituents on the
pyrrolidine ring has been described for phenylfullero-
pyrrolidine derivates.20 We were able to observe this
dynamic effect in compounds 7a–b which showed broad
signals for the ortho aromatic protons (l=7.60) close to
the fullerene surface. In contrast, a sharp doublet (J=
5.1 Hz) was observed for the vinyl proton close to the
C60 surface in compound 10. This vinyl proton is
slightly shifted (l ꢀ6.77) in comparison with its precur-
sor 9 (l=6.87, J=5.7 Hz), thus showing the influence
of the close C60 surface. Finally, due to the dynamic
7. (a) Da Ros, T.; Prato, M.; Novello, F.; Maggini, M.;
Banfi, E. J. Org. Chem. 1996, 61, 9070–9072; (b) see also:

4136
B. Illescas et al. / Tetrahedron Letters 43 (2002) 4133–4136
Rio, Y.; Nicoud, J.-F.; Rehspringer, J.-L.; Nierengarten,
J.-F. Tetrahedron Lett. 2000, 41, 10207.
CODEN: USXXAM US 4500527 A 19850219 CAN 102:
184980 AN 1985: 184980 CAPLUS (Copyright 2001
ACS).
8. (a) Janis, R. A.; Silver, P. J.; Triggle, D. J. Adv. Drug
Res. 1987, 16, 309–591; (b) Bossert, F.; Vater, W. Med.
Res. Rev. 1989, 9, 291–324; (c) For a review on calcium
channel modulators, see: Mart´ın, N.; Seoane, C. Qu´ım.
Ind. 1990, 36, 115–127.
9. (a) Eisner, U.; Kuthan, J. Chem. Rev. 1972, 72, 1–42; (b)
Stout, D. M.; Meyers, A. I. Chem. Rev. 1982, 82, 223–
243; (c) Bossert, F.; Meyer, H.; Wehinger, E. Angew.
Chem., Int. Ed. Engl. 1981, 210, 762–769; (d) Kuthan, J.;
Kurfu¨rst, A. Ind. Eng. Chem. Prod. Res. Dev. 1982, 211,
191–261.
10. (a) Chorvat, R. J.; Rorig, K. J. J. Org. Chem. 1988, 53,
5779–5781; (b) Kappe, C. O.; Fabian, W. M. F. Tetra-
hedron 1997, 53, 2803–2816; (c) Kappe, C. O. Tetrahedron
1993, 49, 6937–6963.
11. (a) Goldman, S.; Geiger, W. Angew. Chem., Int. Ed. Engl.
1984, 23, 301–302; (b) Goldman, S.; Born, L.; Kazda, S.;
Pittel, B.; Schramm, M. J. Med. Chem. 1990, 33, 1413–
1418; (c) Goldman, S.; Stoltefuss, J. Angew. Chem., Int.
Ed. Engl. 1991, 30, 1559–1578.
12. For some recent structural studies on related DHPs, see:
(a) Sua´rez, M.; Ochoa, E.; Verdecia, Y.; Pita, B.; Mora´n,
L.; Mart´ın, N.; Quinteiro, M.; Seoane, C.; Soto, J. L.;
Novoa, H.; Blaton, N.; Peters, O. M. Tetrahedron 1999,
55, 875–884; (b) Sua´rez, M.; Verdecia, Y.; Ochoa, E.;
Salfra´n, E.; Mora´n, L.; Mart´ın, N.; Mart´ınez, R.; Quin-
teiro, M.; Seoane, C.; Soto, J. L.; Novoa, H.; Blaton, N.;
Peters, O. M.; De Ranter, C. Eur. J. Org. Chem. 2000,
2079–2088.
13. Horning, E. C.; Koo, J.; Fish, M. S.; Walker, G. N. Org.
Synth. 1951, 31, 56.
14. Daniou-Bougot, R.; Carrie´, R. Bull. Soc. Chim. Fr. 1968,
2526.
16. de Lucas, A. I.; Ferna´ndez-Gadea, J.; Mart´ın, N.;
Seoane, C. Tetrahedron 2001, 57, 5591–5595 and refer-
ences cited therein.
17. Johnson, T. B.; Mikeska, L. A. J. Am. Chem. Soc. 1919,
810–817.
18. Liepins, E.; Zolotoyabko, R. M.; Cekavicius, B.; Sausins,
A.; Lusis, V.; Duburs, G. Khim. Geterotsikl. Soedin. 1989,
9, 1238.
19. Selected spectroscopic data for compound 7b:
IR FT (KBr) w 3400, 2923, 2850, 1701, 1618, 1460, 1408,
1
1207, 527 cm−1; H NMR (CDCl3, 300 MHz) l 7.60 (d,
2H, J=7.83 Hz), 7.26 (m, 2H), 5.57 (s, 1H), 5.22 (d, 1H,
J=9.6 Hz), 5.09 (s, 1H), 4.90 (s, 1H), 4.27 (d, 1H, J=9.6
Hz), 3.97 (m, 6H), 3.72 (m, 6H), 3.54 (m, 2H), 3.36 (m,
4H), 2.84 (m, 1H), 2.33 (s, 6H), 1.14 (t, 3H, J=7.1 Hz),
1.03 (t, 3H, J=7.3 Hz); 13C NMR (CDCl3, 75 MHz) l
166.95, 166.64, 166.55, 156.31, 156.04, 154.86, 154.06,
153.83, 152.94, 152.68, 147.87, 147.02, 146.63, 146.24,
146.07, 145.95, 145.88, 145.71, 145.66, 145.50, 145.26,
145.08, 145.02, 144.98, 144.90, 144.50, 144.45, 144.36,
144.28, 144.19, 144.12, 143.31, 143.23, 142.93, 142.87,
142.80, 142.74, 142.37, 142.32, 142.00, 141.83, 141.69,
141.48, 141.41, 140.00, 139.92, 139.85, 139.75, 139.57,
139.15, 138.99, 137.37, 136.89, 136.52, 136.37, 136.22,
135.73, 135.51, 135.42, 134.36, 128.52, 128.41, 126.56,
103.85, 82.14, 81.95, 71.90, 70.55, 70.26, 68.92, 67.61,
67.51, 60.95, 59.38, 59.32, 58.70, 52.13, 39.41, 29.81,
22.58, 19.27, 14.31, 14.10, 13.85; UV–vis (CHCl3), umax
255, 310, 430, 695.
20. (a) De la Cruz, P.; De la Hoz, A.; Font, L. A.; Langa, F.;
Pe´rez-Rodr´ıguez, M. C. Tetrahedron Lett. 1998, 39,
6053–6056; (b) Gu, T.; Bourgogne, C.; Nierengarten,
J.-F. Tetrahedron Lett. 2001, 42, 7249–7252.
15. Loev, B.; Shroff, J. R.; Desai, R. US 1985, 6 pp,