A convenient extension of the Wessely-Moser rearrangement for the synthesis of substituted alkylaminoflavones as neuroprotective agents in vitro

Article abstract of DOI:10.1016/S0960-894X(00)00110-4

A series of 8-alkylamino-5,7-dihydroxyflavones was prepared from chrysine via a seven step sequence. The synthesis of their 6-alkylamino isomers could be subsequently accomplished through a convenient extension of the Wessely-Moser rearrangement. These compounds were found to be efficient neuroprotective agents in vitro. (C) 2000 Elsevier Science Ltd. All rights reserved.

Full text of DOI:10.1016/S0960-894X(00)00110-4

Bioorganic & Medicinal Chemistry Letters 10 (2000) 835±838
A Convenient Extension of the Wessely±Moser Rearrangement for
the Synthesis of Substituted Alkylamino¯avones as
Neuroprotective Agents In Vitro
Ronan Larget, a Brian Lockhart, b Pierre Renard c and Martine Largeron a,*
a
Â
Â
Â
Laboratoire de Chimie Analytique et Electrochimie, UMR 8638 CNRS-Universite Rene Descartes, Faculte des Sciences
Pharmaceutiques et Biologiques, 4, Avenue de l'Observatoire, F-75270 Paris Cedex 06, France
bInstitut de Recherche Servier, 125 chemin de Ronde, 78290 Croissy-sur-Seine, France
c
Â
ADIR, 1 rue Carle Hebert, 92415 Courbevoie Cedex, France
Received 5 January 2000; accepted 14 February 2000
AbstractÐA series of 8-alkylamino-5,7-dihydroxy¯avones was prepared from chrysine via a seven step sequence. The synthesis of
their 6-alkylamino isomers could be subsequently accomplished through a convenient extension of the Wessely±Moser rearrange-
ment. These compounds were found to be ecient neuroprotective agents in vitro. # 2000 Elsevier Science Ltd. All rights reserved.
There is now substantial evidence accumulating to sug-
gest that oxidative stress may play a pivotal role in the
progression of many neurodegenerative pathologies of
the central nervous system, including Parkinson's dis-
ease and Alzheimer's disease.1 Consequently, supple-
mentation with exogenous antioxidants can represent an
important therapeutic potential to minimize central
nervous system damage. So, there is considerable inter-
est in the discovery and development of ecient syn-
thetic antioxidants.
to enhance antioxidant activity and in parallel to
reduce the manifestation of intrinsic toxicity. In this
aim, the synthesis of 6-alkylamino-5,7-dihydroxy-
¯avones (10a±e) was undertaken (Scheme 1). Flavo-
noids, in particular polyhydroxy¯avones, are well
known to exhibit antioxidant activity.9 11 This raised
the question of whether or not 6-alkylamino¯avones
(10a±e) in general would be more promising derivatives
than their benzophenone analogues II.
In this paper, the synthesis of 6-alkylamino-5,7-di-
hydroxy¯avones 10a±e is reported from their original 8-
alkylamino isomers through a convenient extension of
the Wessely±Moser rearrangement. We also disclose the
neuroprotective properties of these compounds as part
of our continuing research e€orts to ®nd safe and e€ec-
tive neuroprotective agents.
In the course of our search for new neuroprotective
agents,2,3 we recently reported the synthesis of novel 8-
alkylamino-3-substituted-1,4-benzoxazine derivatives I
as well as 3-alkylamino-2,4-dihydroxybenzophenones
(II) (Scheme 1).4,5 From their capacity to inhibit oxidative
stress-mediated neuronal degeneration in vitro, these
compounds were found to be potent neuroprotective
agents with activity close to that of standard a-toco-
pherol.6,7 From these studies, substituted-1,4-benz-
oxazines (I) were identi®ed as the most promising
compounds for therapeutic potential as a result of their
low intrinsic cytotoxicity in vitro.7,8 Although 3-alkyl-
aminophenols (II) were found to be at least 10-fold more
toxic than the series of 1,4-benzoxazines, they remained
potent neuroprotective agents.6 On this basis, we envi-
saged potential useful structural modi®cations in order
Chemistry
At the outset, our synthetic strategy for the preparation
of compounds 10a±e involved the intermediary synthe-
sis of 5,7-dihydroxy-6-nitro¯avone, which appeared
deceptively simple. Indeed, currently available methods
to prepare 5,7-dihydroxy-6-nitro¯avone from 2,4,6-tri-
hydroxy-3-nitroacetophenone including Baker±Venka-
taraman rearrangement, synthesis from chalcones or via
an intramolecular Wittig strategy, as well as newer
methods using organolithium reagents or phase-transfer
*Corresponding author. Fax: +33-1-43-29-05-92; e-mail: largeron@
pharmacie.univ-paris5.fr
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00110-4

836
R. Larget et al. / Bioorg. Med. Chem. Lett. 10 (2000) 835±838
isomer 2 was isolated as the main product (70% yield).
Nevertheless, in view of the relative ease of preparation
of 8-alkylamino-5,7-dihydroxy¯avones from 8-nitro-
chrysine 2, the synthesis of isomers 8a±e was pursued
for two reasons. Firstly, encouraged by the results of
Wessely and Moser previously demonstrating the con-
version of 5,7,8-trimethoxy¯avone into the 5,6,7-trihy-
droxy compound on demethylation,17 we thought that
8-alkylamino¯avones (8a±e) possibly could serve as the
starting materials for the preparation of their 6-isomers
10a±e. Secondly, compounds 8a±e could prove interest-
ing in exploring the relationship between the position of
the alkylamino chain and the neuroprotective activity.
Accordingly, 8-alkylamino¯avones (8a±e) were synthe-
sized from 8-nitrochrysine 2 via a six step sequence,
which involved protection and deprotection steps as
reported in Scheme 2. Methylation of 8-nitrochrysine
(2) with Me2SO4 and K2CO3, followed by reduction of
the nitro group with stannous chloride in ethanol,18
a€orded 4 in roughly 60% overall yield. The latter was
alkylated after protection with the 2,4-dinitrobenz-
enesulfonyl group.19 N-monosubstituted 2,4-dinitro-
benzenesulfonamide (5), readily prepared in 88% yield
from 8-amino-5,7-dimethoxy¯avone (4) and 2,4-di-
nitrobenzenesulfonyl chloride, could be eciently N-
alkylated with appropriate primary alkyl halides by the
conventional methods, in roughly 85% yield. For the
introduction of the isopropyl chain, compound 5 was
Scheme 1. RˆPri (a); nPr (b); Bui (c); Pei (d); CH2ˆCHCH2 (e).
catalysis, met with total failure.12 16 At this juncture, an
alternative route which involved the functionalization of
chrysine 1 was attempted for the preparation of 5,7-
dihydroxy-6-nitro¯avone (Scheme 2). Unfortunately,
the nitration of chrysine 1 with fuming HNO3, in acetic
acid, gave the desired 5,7-dihydroxy-6-nitro¯avone as a
minor product (2% yield), while the unwanted 8-nitro
Scheme 2. (a) HNO3 70%, AcOH, 65 ꢀC, 1 h, 70%; (b) Me2SO4, K2CO3, DMF, room temperature, 3 h, 81%; (c) SnCl2.2H2O, EtOH, re¯ux, 5 h,
71%; (d) 2,4-(NO2)2PhSO2Cl, pyridine, CH2Cl2, room temperature, 8 h, 88%; (e) RI, K2CO3, DMF, room temperature, 1±16 h, 83±87% or PriOH,
DEAD, PPh3, THF, re¯ux, 3 h, 77%; (f) HSCH2COOH, Et3N, CH2Cl2, room temperature, 5±16 h, 71±94%; (g) AlCl3, toluene, re¯ux, 68±92% or
BBr3, CH2Cl2, room temperature, 4±5 h, 50±70%.

R. Larget et al. / Bioorg. Med. Chem. Lett. 10 (2000) 835±838
837
Scheme 3. (h) conc. HCl, re¯ux, 30±64 h, 60±74%.
alkylated under the Mitsunobu conditions,20 leading to
compound 6a in 77% yield. Facile deprotection of N,N-
disubstituted-2,4-dinitrobenzenesulfonamides (6a±e) was
achieved by treatment with mercaptoacetic acid and
triethylamine in dichloromethane, giving the required
compounds 7a±e (71±94% yields).19 Finally, complete
demethylation of compounds 7a±d was successfully
performed using 5 equiv of AlCl3, in toluene heated at
re¯ux, to a€ord compounds 8a±d, in yields ranging
from 68 to 92%. In the speci®c case of compound 7e
bearing an allyl chain, demethylation with 5 equiv of
BBr3 in dichloromethane, at room temperature, led to
compound 8e in 70% yield.
¯avonoids having the 5,7,8-arrangement of methoxy
groups su€ered isomeric change on demethylation,
yielding a product with the 5,6,7-arrangement. This
conversion, known as the Wessely±Moser rearrange-
ment,17 was essentially used for the elucidation of
structures of ¯avonoids and was further extended to
chromones, xanthones and derivatives.21,22 Surprisingly,
little attention has been devoted to this rearrangement
as a surrogate of direct synthesis, especially when the
latter was cumbersome and inecient.22,23 Further-
more, use of the Wessely±Moser rearrangement has
been restricted, in general, to compounds bearing
hydroxy or methoxy groups.
Having established the seven step sequence for the
synthesis of 8-alkylamino-5,7-dihydroxy¯avones (8a±e),
we next turned our attention to the hypothetical con-
version of these compounds to their 6-alkylamino iso-
mers 10a±e. It has been established for some time that
Initial attempts to perform the Wessely±Moser rearran-
gement on 8-nitro¯avone (3) were to no avail: in all cases,
we were unable to detect a trace of the expected 6-nitro
isomer. Consequently, the conversion of 8-alkylamino
derivatives 7a±e by the action of boiling hydrochloric
Table 1. In vitro neuroprotective activity of compounds 8a±e and 10a±e
Toxicitya
Protection versus 2 mM L-HCAb
Compound
Rc
MTC (mM)
TC50 (mM)
PC50 (mM)
MTT
LDH
MTT
LDH
MTT
LDH
IIa
IIb
IId
8b
8c
8d
Pri
nPr
25
25
10
5
10
10
5
10
5
5
100
100
50
25
10
10
10
100
100
100
118
78
20
14
12
12
18
18
10
16
>250
>250
82
28
12
16
11
152
176
116
5.5
5.1
4.0
7.0
4.6
5.3
>5
5.1
5.2
5.2
5.9
5.1
5.5
8.0
4.2
5.5
Pei
nPr
Bui
Pei
8e
CH2ˆCHCH2
>10
6.6
5.5
10a
10b
10d
Pri
nPr
Pei
5.2
aIn vitro neurotoxicity monitored either by reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT reduction assay) or by
quanti®cation of cellular lysis after measurement of the lactate dehydrogenase (LDH) activity released from damaged cells into the culture super-
natant.
bIn vitro neuroprotective activity estimated through their protective e€ects against L-homocysteic acid (L-HCA) cytotoxicity.25
cAbbreviations: MTC, maximum tolerated concentration; TC50, concentration producing 50% toxicity; PC50, concentration producing 50% pro-
tection; Bui, isobutyl; Pei, isopentyl; nPr, n-propyl; Pri, isopropyl.

838
R. Larget et al. / Bioorg. Med. Chem. Lett. 10 (2000) 835±838
acid was investigated. Then, the isomeric change took
place leading to 6-alkylamino¯avones (9a±e), in good
yields ranging from 60 to 74%. Finally, synthesis of 6-
alkylamino isomers 10a±e could be achieved through
demethylation at the 7-position, under the conditions
reported above (Scheme 3).24
References and Notes
1. Halliwell, B.; Gutteridge, J. M. C. In Free Radicals in Biol-
ogy and Medicine, 3rd ed.; Oxford University Press, Oxford,
1999; pp 246±350.
2. Fleury, M.-B.; Maurette, J.-M.; Largeron, M. PCT Int.
Appl. WO 95 18114/1995 (Chem. Abstr. 1995, 123, 340 160x).
3. Largeron, M.; Dupuy, H.; Fleury, M.-B. Tetrahedron 1995,
51, 4953.
4. Largeron, M.; Fleury, M.-B. Tetrahedron Lett. 1998, 39,
8999.
5. Largeron, M.; Neudor€er, A.; Fleury, M.-B. Tetrahedron
Lett. 1998, 39, 5035.
6. Larget, R.; Lockhart, B.; Pfei€er, B.; Fleury, M.-B.; Lar-
geron, M. Bioorg. Med. Chem. Lett. 1999, 9, 2929.
7. Largeron, M.; Lockhart, B.; Pfei€er, B.; Fleury, M.-B. J.
Med. Chem. 1999, 42, 5043.
8. Fleury, M.-B.; Largeron, M.; Lestage, P.; Lockhart, B.
ADIR and Cie Fr. Patent Appl. 9806809 May 29, 1998; PCT/
FR 99/01254, May 28, 1999.
9. Bors, W.; Saran, M. Free Radical Res. Commun. 1987, 2, 289.
10. Cotelle, N.; Bernier, J.-L.; Catteau, J.-P.; Pommery, J.;
Wallet, J.-C.; Gaydou, E.-M. Free Radical Biol. Med. 1996,
20, 35.
11. Gao, D.; Tawa, R.; Masaki, H.; Okano, Y.; Sakurai, H.
Chem. Pharm. Bull. 1998, 46, 1383.
12. Baker, W. J. Chem. Soc. 1933, 1381.
13. Iinuma, M.; Tanaka, T.; Mizuno, M. Chem. Pharm. Bull.
1987, 35, 660.
14. Le Floc'h, Y.; Lefeuvre, M. Tetrahedron Lett. 1986, 27, 2751.
15. Cushman, M.; Nagarathnam, D. Tetrahedron Lett. 1990,
31, 6497.
In Vitro Biological Results
The intrinsic neurotoxicity as well as the neuroprotec-
tive activity of 8-alkylamino¯avones (8a±e) and 6-alkyl-
amino¯avones (10a±e) were assessed in vitro on murine
HT-22 hippocampal cell cultures and compared with
those of benzophenone analogues II.25 The results are
presented in Table 1.
Taking into account that optimum activity was dis-
played by alkyl substituents in the benzophenone series
II,6 only substituted alkylamino¯avones bearing iso-
propyl, n-propyl, isobutyl, isopentyl and allyl chains
were evaluated. As reported in Table 1, all tested ¯avo-
noids showed signi®cant in vitro neuroprotective activ-
ity, with PC50 (MTT) values between 4.6 and 7.0 mM
and PC50 (LDH) values between 4.2 and 10.0 mM.
Based on the PC50 values, it appeared that 6-alkylami-
no¯avones (10a±d), as well as their benzophenone ana-
logues IIa±d, demonstrated equivalent neuroprotective
activities (for instance, compare 10a and IIa). Further-
more, as the 6-alkylamino¯avones did not exhibit a
reduced toxicity compared to their benzophenone ana-
logues IIa±d, it could be concluded that alkylamino-
¯avone derivatives could not be considered as promising
compounds for therapeutic potential.
16. Saxena, S.; Makrandi, J. K.; Grover, S. K. Synthesis 1985,
697.
17. Wessely, F.; Moser, G. H. Monatsh. Chem. 1930, 56, 97.
18. Bellamy, F. D.; Ou, K. Tetrahedron Lett. 1984, 25, 839.
19. Fukuyama, T.; Cheung, M.; Jow, C. K.; Hidai, Y.; Kan,
T. Tetrahedron Lett. 1997, 38, 5831.
20. Mitsunobu, O. Synthesis 1981, 1.
21. Philbin, E. M.; Swirski, J.; Wheeler, T. S. J. Chem. Soc.
1956, 4455 and references cited therein.
22. Seshadri, T. R. Tetrahedron 1959, 6, 169 and references
cited therein.
In summary, we have synthesized a series of novel 8-
alkylamino¯avone derivatives from chrysine via a seven
step sequence. The synthesis of their 6-alkylamino iso-
mers was successfully accomplished through a con-
venient extension of the Wessely±Moser rearrangement.
To the best of our knowledge, such a rearrangement has
as yet not been reported for alkylamino¯avones.
Finally, the results of our biological evaluation revealed
that replacement of the benzophenone skeleton by the
¯avone ring did not markedly a€ect the neuroprotective
activity in vitro.
23. Shaw, S. C.; Azad, R.; Mandal, S. P.; Gandri, R. S. J.
Indian Chem. Soc. 1988, 65, 107.
24. 5,7-dihydroxy-2-phenyl-8-propylamino-4H-1-benzopyran-4-
1
one 8b. H NMR (300 MHz, DMSO d6): d 0.90 (t, 3H, Me,
J=6 Hz), 1.50 (sextet, 2H, CH2, J=6 Hz), 3.15 (t, 2H, N-
CH2, J=6 Hz), 6.35 (s, 1H, 6-H), 6.95 (s, 1H, 3-H), 7.60 (m,
3H, H meta and H para, Ph), 8.10 (m, 2H, H ortho, Ph), 12.50
(s, 1H, 5-OH); MS (DCI): m/z=312 (MH+).
5,7-dihydroxy-2-phenyl-6-propylamino-4H-1-benzopyran-4-one
1
10b. H NMR (300 MHz, DMSO d6): d 0.85 (t, 3H, Me, J=6
Acknowledgements
Hz), 1.45 (sextet, 2H, CH2, J=6 Hz), 3.20 (t, 2H, N-CH2,
J=6 Hz), 6.60 (s, 1H, 8-H), 6.95 (s, 1H, 3-H), 7.55 (m, 3H, H
meta and H para, Ph), 8.05 (m, 2H, H ortho, Ph), 12.95 (s, 1H,
5-OH); MS (DCI): m/z=312 (MH+).
25. In vitro biological assays are extensively detailed in refs 6
and 7.
We would like to thank Professor M.-B. Fleury, in
whose laboratory this work was carried out, for his help
and encouragement, and Nadege Villain for excellent
technical assistance.