Ultraviolet spectroscopy of anticonvulsant enaminones.

Article abstract of DOI:10.1016/S0968-0896(01)00314-5

The ultraviolet (UV) spectra of selected enaminones were determined in acidic, alkaline and neutral media and compared to their anticonvulsant activities. The wavelength of maximum absorption and molar absorptivity were compared with the anticonvulsant ac

Full text of DOI:10.1016/S0968-0896(01)00314-5

Bioorganic & Medicinal Chemistry 10 (2002) 593–597
Ultraviolet Spectroscopy of Anticonvulsant Enaminones
I. O. Edafiogho,a,* O. A. Phillips,a M. Abdel-Hamid,a A. A. M. Ali,b W. C. Matowe,a
A. El-Hashima and S. B. Kombiana
aFaculty of Pharmacy, Kuwait University, PO Box 24923, Safat 13110, Kuwait
bDepartment of Chemistry, Faculty of Science, Kuwait University, PO 5969, Safat 13060, Kuwait
Received 10 April 2001; accepted 28 August 2001
Abstract—The ultraviolet (UV) spectra of selected enaminones were determined in acidic, alkaline and neutral media and compared
to their anticonvulsant activities. The wavelength of maximum absorption and molar absorptivity were compared with the anti-
convulsant activity of the selected secondary and tertiary enaminones, and general inferences were made. The UV spectra of the
enaminones had hypsochromic shifts in acidic media in comparison with neutral media. Generally, a small hypsochromic shift
occurred in alkaline media when compared to the neutral solutions of the enaminones. The tertiary enaminones absorbed UV light
at longer wavelength than the secondary enaminones in acidic, neutral and alkaline media. In particular, the tertiary enaminones
displayed absorption at the higher end and secondary enaminones towards the lower end of the UV wavelength range 292–315 nm
in aqueous media. Tertiary enaminones (30–33) which were devoid of the NH proton were found to be uniformly inactive in a
mouse model of electroshock seizures, while some secondary enaminones (1, 5–8, 12, 16, 18, 20, 23–25, 28 and 29) had anti-
convulsant activity. Thus the NH group of secondary enaminones is very important for anticonvulsant activity, and this agrees with
an already established trend in proton NMR spectroscopy. In addition, the para-substitution on the phenyl group in some enami-
nones result in higher molar absorptivity (e) values that enhance anticonvulsant activity. These results indicate that the anti-
convulsant activity of enaminones is not due to electronic effect alone, but is probably due to a combination of factors including
electronic and steric effects, lipophilicity, and hydrogen bonding. # 2002 Elsevier Science Ltd. All rights reserved.
Introduction
Results andDiscussion
Several series of enaminones have been synthesized and
screened for pharmacological activities.1À8 Enaminones
are chemical compounds consisting of an amino group
linked through a carbon–carbon double bond to a
keto group.9 Enaminones are very stable and the
cyclic enaminones are more stable than acyclic
enaminones.10 Due to the conjugation existing in the
enaminone system, enaminones absorb ultraviolet (UV)
radiation.11
Generally, the enaminones can be prepared from reac-
tions between beta hydroxyketo compounds and pri-
mary or secondary amines (Scheme 1); and the
enaminones described in this paper were synthesized
according to the methods of Edafiogho, Scott and co-
workers.2,11
Nuclear magnetic resonance (NMR) studies had con-
firmed the general conformation of the enaminones to
be trans-S-trans conformation, and that the enaminone
system was present in all the compounds.5,15 The
enaminone compounds produced intense peaks under
UV spectroscopy, and where a compound exhibited
more than one peak, the most intense peak was repor-
ted. These peaks had e values greater than 15,000.
Generally, the UV data of enaminones 1–33 in water
indicated their absorption in neutral medium. However,
in acidic medium (1 M HCl), the enaminones were pro-
tonated; while in alkaline medium (1 M NaOH) they
were deprotonated. One common feature among the
enaminones evaluated in this study was that most of
them had similar data in neutral and alkaline solutions
The objective of this study was to determine the wave-
length of maximum absorption (lmax) and absorptivity
(e) values for selected enaminones and attempt to
establish a correlation between UV data and anti-
convulsant activity. Anticonvulsant activity of a com-
pound is the ability of the compound to protect against
seizures. Established procedures exist for screening
compounds for anticonvulsant activity.12À14
*Corresponding author. Tel.: +965-531-2300x6047; fax: +965-534-
2807; e-mail: ivanoe@hsc.kuniv.edu.kw
0968-0896/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0968-0896(01)00314-5

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I. O. Edafiogho et al. / Bioorg. Med. Chem. 10 (2002) 593–597
Scheme 1. General scheme for the synthesis of cyclic enaminones.
indicating that most of them were basic in nature.
Deprotonation of these enaminones in alkaline solu-
tions had small hypsochromic or no significant effect on
their lmax values when compared to neutral solutions.
The enaminones that normally behave as weak bases
exhibited hypsochromic shift (from longer to shorter
wavelength) on moving from neutral to acidic media
(see Tables 1 and 2).
enaminones 1, 28 and 29 (with phenyl amino side
chain), which absorbed above 282 nm in acidic medium,
showed anticonvulsant activity while the secondary
enaminones 2 and 3 which absorbed below 282 nm in
acidic medium were inactive, irrespective of the number
of methylene groups between the unsubstituted phenyl
group and the NH group. In addition, enaminone 5
(Scheme 2) was anticonvulsant and displayed hypso-
chromic shift from neutral (311 nm) to acidic medium
(301 nm). The hypsochromic shift in alkaline medium
was not significant. The UV data for enaminone 6
In comparing UV data with anticonvulsant activity
(Tables 1 and 2), it was found that the secondary
Table 1. Anticonvulsant screening project (ASP): phase 1 test results
Compound
Mp (ꢀC)
R1
R2
R3
R4
R5
ASP
classificationa
1
2
3
416
5
6
7
8
9
118–119
163–164
90–92
–CO2CH3
–CO2CH3
–CO2CH3
–H
–CO2CH3
–H
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–CH3
–Ph
–H
–H
–H
–CH3
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–(CH2)2Ph
–(CH2)3Ph
–(CH2)4Ph
1
3
3
3
1
1
1
1
3
2
3
1
3
3
3
1
3
1
3
1
3
3
1
1
1
–
–
1
2
3
3
3
3
208–210
178–180
198–199.5
161–164
188–190
221–222
178–179
168–169
137–138
175–177
178–181
166.5–168
165–166
159–163
173–176
177–178
151–155
163–166
167–169
146–149
169–170
206–208
4-Chlorophenyl
4-Chlorophenyl
4-Chlorophenyl
4-Fluorophenyl
4-Bromophenyl
4-t–Butylphenyl
4-Nitrophenyl
Phenyl
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–H
–H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
2616
2717
28
29
30
31
32
33
–CH3
–CH3
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–CH3
–CH3
–H
–CH3
–H
–CH3
–H
–H
3-Chlorophenyl
3-Chloro-4-methoxyphenyl
3-Methoxyphenyl
3-Nitrophenyl
3-Methylphenyl
3-Bromophenyl
3-Trifluoromethoxyphenyl
4-Methoxyphenyl
3-Fluorophenyl
3-Iodophenyl
3-Trifluoromethylphenyl
3-Ethylphenyl
4-Trifluoromethylphenyl
4-Chloro-2-pyridinyl
–CH3
–H
–[CH2]4–
–CH2Ph
–CH2Ph
–[CH2]4–
154–155
139–140
138–139
112–113
168–169
194–195
–CO2CH3
–CO2CH3
–CO2CH3
–CO2CH3
–CO2C2H5
–CO2CH3
–H
–H
–[CH2]4–
–[CH2]4–
–(CH2)2–O–(CH2)2—
aAnticonvulsant activity at 100 mg/kg or less=1; at 300 mg/kg=2; at >300 mg/kg=3 (inactive).

I. O. Edafiogho et al. / Bioorg. Med. Chem. 10 (2002) 593–597
595
Table 2. Ultraviolet data of enaminones
Compound
Molecular formula
MW
1 M HCl
H2O
1 M NaOH
lmax
e
lmax
e
lmax
e
1
2
3
416
5
6
7
8
9
C17H21NO3
C18H23NO3
C19H25NO3
C14H16NOCl
C15H16NO3Cl
C13H14NOCl
C15H16NO3F
C15H16NO3Br
C19H25NO3
C16H18N2O5
C16H19NO3
C15H16NO3Cl
C16H19NO4Cl
C16H19NO4
C15H16N2O5
C16H19NO3
C15H16NO3Br
C16H16NO4F3
C16H19NO4
C15H16NO3F
C15H16NO3I
C16H16NO3F3
C17H21NO3
C16H16NO3F3
C14H15N2O3
287.39
301.42
315.45
249.80
293.81
235.77
277.32
338.22
315.45
318.36
273.36
293.72
324.81
289.36
304.33
273.36
338.22
343.33
289.36
277.32
385.22
327.33
287.39
327.33
271.41
284
280
278
301
301
297
297
302
302
350
301
301
302
297
302
298
301
302
300
301
301
301
299
306
335
280
288
286
288
284
288
286
296
22,000
21,800
23,400
21,900
18,400
19,300
23,800
21,600
22,000
15,100
22,100
19,700
17,200
18,000
19,700
20,400
17,500
16,100
17,400
18,900
19,800
19,400
21,500
13,300
17,400
23,500
24,600
26,700
25,700
24,900
23,200
24,200
23,600
295
292
292
312
311
310
302
310
308
365
310
309
303
309
309
307
309
309
307
309
309
308
307
315
328
291
304
294
299
303
308
305
308
32,600
30,300
31,200
23,800
28,200
27,400
30,900
29,100
29,300
20,500
26,500
27,600
25,600
25,200
27,600
27,900
25,700
25,800
26,900
26,600
27,300
26,200
28,100
23,000
29,300
29,000
35,600
33,900
31,800
35,800
33,200
35,200
33,300
294
292
292
311
307
305
303
308
306
299
305
306
307
304
303
304
306
305
305
304
308
307
304
308
309
292
304
295
298
304
309
305
308
33,500
29,000
30,000
23,300
24,100
20,500
28,700
27,200
27,100
20,500
24,200
25,600
23,700
23,300
24,900
25,700
23,700
23,600
29,300
24,200
25,800
24,500
26,400
18,700
20,700
29,600
35,600
33,400
31,800
36,000
33,900
35,500
33,000
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
2616
2717
2811
2911
3011
3111
3211
3311
C16H19NO3
C17H21NO3
C13H19NO3
C14H21NO3
C19H23NO3
C18H21NO4
273.35
287.38
237.32
251.35
313.42
315.39
(Scheme 2) compared well with the UV data for com-
pound 5.
observed that para-substitution resulted in higher molar
absorptivity (e) values that correlated with anti-
convulsant activity. For instance, the para-bromo
enaminone 8 was anticonvulsant whereas the meta-
bromo enaminone 17 had lower e value and was inac-
tive. In moving from neutral to alkaline media, the
para-substituted enaminone 7, and unsubstituted 10
showed bathochromic shifts, while some para-sub-
stituted compounds (4–6, 8–10, 19, 24) and meta-sub-
stituted enaminones (12–18, 20–23) displayed
hypsochromic shifts. However, this property could not
be used to predict anticonvulsant activity of the enam-
inones. The bulky size of the para substituent (as in 9
and 19) tended to prevent anticonvulsant activity.
When the UV data for enaminones 7, 8 and 9 were
examined, there was a hypsochromic shift on protona-
tion in acidic medium but the shift was larger for 8
(8 nm) than for enaminone 7 (5 nm) or for enaminone 9
(6 nm). The lmax for 7 and 8 were different in all media
even though they were both halogenated enaminones.
Substitution with different halogens resulted in different
UV characteristics due to different electronic effects.
Although the UV data for 8 and 9 were close, enami-
none 8 was anticonvulsant while enaminone 9 was
inactive. This probably indicates that both steric and
electronic factors affect the anticonvulsant property of
enaminones.
Enaminones 10 (lmax 350 nm) and 25 (lmax 335 nm)
were unique in displaying lmax values higher than
315 nm in acidic solutions. They possessed anti-
convulsant activity.
Comparing the effects of para- and meta-substitutions
on the phenyl groups in some of the enaminones, it was
Scheme 2. Structures of potent anticonvulsant enaminones 5, 6, and 25.

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I. O. Edafiogho et al. / Bioorg. Med. Chem. 10 (2002) 593–597
Enaminone 25 (Scheme 2) did not behave like a typical
enaminone (weak base). Instead, it exhibited the prop-
erties of a weak acid in aqueous solutions. The lmax for
enaminone 25 showed bathochromic shift from 328 to
335 nm in acidic medium, and a hypsochromic shift
from 328 to 309 nm in alkaline medium.
308 nm in alkaline solution in comparison to the
inactive compound 4, which absorbs at higher
wavelength (311 nm) in alkaline solution.
iv. Effect of Steric Hindrance: The positioning of the
substituents on the aromatic ring plays an
important role in the activity of the analogues as
the p-bromo (8) is anticonvulsant, but the meta
analogue (17) is inactive. The e values for 8 in
acid, neutral and alkaline media are consistently
higher than those of 17. This isomeric change in
position is also observed with the active p-nitro
10, compared to the inactive m-nitro 15; and
active p-trifluoromethyl (24) compared to the
inactive m-trifluoromethyl (22). Furthermore, the
chloro- (para, 5; meta, 12) and fluoro-substituted
compounds (para 7, meta 20) are anticonvulsant.
These data suggest that the meta-substituted
enaminones are generally restricted by steric con-
straints, but the overall effects of p and s make
some of them (12 and 20) to be active. Hence,
lipophilicity (p) is considered a predominant fac-
tor in determining the anticonvulsant effect of
enaminones.
v. Effect of meta-Substitution on Aromatic Ring:
meta-Substitution with CH3 group as in 16 results
in lower lmax values in all three solutions than
meta-substitution with CF3 group (22). Com-
pound 16 is anticonvulsant while 22 is inactive.
On the other hand, the protonation of enaminone
18 which has a meta-substitution with a tri-
fluoromethoxy group showed a hypsochromic
shift of 7 nm, compared to the meta-methoxy 14
which shows a hypsochromic shift of 12 nm on
protonation. Compound 18 which exhibits a
smaller shift is anticonvulsant, while compound
14 which exhibits a larger shift is inactive.
As shown in Table 2, the tertiary enaminones 27, 30–33
absorbed at the higher end and secondary enaminones
(1–26) generally towards the lower end of the UV
wavelength range 292–315 nm. In water, and alkaline
solution, tertiary enaminones had intense molar
absorptivity values (e 33,000–36,000) whereas the sec-
ondary enaminones had lower values (e 18,700–33,500).
The tertiary enaminones 30–33 which were devoid of the
NH proton were uniformly inactive while some second-
ary enaminones (1, 5–8, 12, 16, 18, 20, 23–25, 28 and 29)
were anticonvulsant in mice. These data validate the
trend that was previously established in the proton
NMR spectroscopy of enaminones.5,11 Thus, the NH of
secondary enaminones was found to be very important
for anticonvulsant activity, and hydrogen bonding may
contribute significantly to this activity. Although a
straight-line correlation could not be established
between the UV data and anticonvulsant activity of the
enaminones, the following general rules are suggested.
General Rules
i. Relationship Between Secondary and Tertiary
Enaminones, and Anticonvulsant Activity: Cer-
tain secondary enaminones have lower lmax and e
values than the tertiary enaminones. Delocaliza-
tion of electrons is easier in the planar secondary
enaminones and consequently secondary enami-
nones may show anticonvulsant activity while the
tertiary enaminones are inactive.
ii. Effect of Alkene Side Chain Between the Amino
and Cyclohexenone Groups: Ethylene bridge,
rather than propylene or butylene bridge between
the amino and cylohexenone groups facilitates
anticonvulsant activity of the secondary enam-
inones. Hence, compound 1 is active while com-
pounds 2 and 3 are inactive. The lmax values for 1
are longer in acid (284 nm), neutral (295 nm), and
alkaline (294 nm) media than for compounds 2
and 3.
vi. Effect of Mono Methyl Versus Geminal Methyl
Substitutions on Cyclohexenone Ring: Comparing
enaminones 4 and 6, substitution of only one CH3
group on the cyclohexenone ring as in enaminone
6 results in lower lmax values in all three media
(acidic, neutral, and alkaline), and anticonvulsant
activity. Geminal methyl groups in enaminone 4
result in decreased lmax values in all solutions,
and loss of anticonvulsant activity. Similarly,
compound 28 possessing one CH3 group exhibits
a lower lmax in acidic solution than compound 29
which has geminal CH3 groups. Compound 28 is
active while 29 is inactive.
iii. Effects of Lipophilicity (p) and Electron-with-
drawing (s) Ability of Substituents on Aromatic
Ring: Employing the Craig Plot4 to determine the
quadrant which has more anticonvulsant ana-
logues when comparing substitution on the aro-
matic ring with lipophilicity (p) and electron-
withdrawing (s) characteristics, it is found that
electron-withdrawing halogens (F, Cl, Br) in the
+p,+s quadrant for para-substitution result in
anticonvulsant activity. Accordingly, compounds
5, 7, and 8 are anticonvulsant, and they exhibit
hypsochromic shifts of at least 5 nm on protona-
tion (in moving from neutral to acidic solution).
They absorb UV radiation between 303 and
Conclusion
On the whole, the results from the UV data of enami-
nones could not be used to establish a definite correla-
tion with anticonvulsant activity of the enaminones
since active and inactive compounds had similar lmax
and e values. However, the UV data gave very useful
information from which we could draw general infer-
ences. First, tertiary enaminones which absorbed at
higher wavelength were inactive while secondary enami-
nones which absorbed at lower wavelength were gen-
erally anticonvulsant. Secondly, para-substitution of the

I. O. Edafiogho et al. / Bioorg. Med. Chem. 10 (2002) 593–597
597
phenyl group of the enaminones with a variety of moi-
References andNotes
eties enhanced anticonvulsant activity. The general
conclusion of this study was that anticonvulsant activity
of enaminones was not due to electronic effect alone as
shown by UV data, but probably due to a combination
of factors including electronic and steric effects, and
hydrogen bonding.
1. Edafiogho, I. O.; Hinko, C. N.; Chang, C.; Moore, J. A.;
Mulzac, D.; Nicholson, J. M.; Scott, K. R. J. Med. Chem.
1992, 35, 2798.
2. Scott, K. R.; Edafiogho, I. O.; Richardson, E. L.; Farrar,
V. A.; Moore, J. A.; Teitz, E. I.; Hinko, C. N.; Chang, H.; El-
Assadi, A.; Nicholson, J. M. J. Med. Chem. 1993, 36, 1947.
3. Scott, K. R.; Edafiogho, I. O.; Roberts, R. R. Isoxazolyl
enaminones. US Patent 5,580,894, 1996.
4. Scott, K. R.; Rankin, G. O.; Stables, J. P.; Alexander,
M. S.; Edafiogho, I. O.; Farrar, V. A.; Kolen, K. R.; Moore,
J. A.; Sims, L. D.; Tonnu, A. D. J. Med. Chem. 1995, 38, 4033.
5. Edafiogho, I. O.; Moore, J. A.; Alexander, M. S.; Scott,
K . R.J. Pharm. Sci. 1994, 83, 1155.
6. Edafiogho, I. O.; Scott, K. R. In Burger’s Medicinal Chem-
istry and Drug Discovery, 5th ed.; Wolff, M. E., Ed.; J. Wiley
and Sons: New York, 1996; Chapter 39, p 175.
Experimental
UV spectroscopy
UV spectra of the enaminones were determined on a
Milton Roy Spectronic 1201 UV spectrophotometer
equipped with computerized programs to plot the spec-
trum as the sample was being run, and to print out the
lmax value for each enaminone. Absorptivity (e) was
then calculated for each enaminone using Beer–
Lambert law according to the method of Green-
hill.16 The most intense peak was selected for each
compound, and the e value rounded to the nearest one
hundred.
7. Edafiogho, I. O.; Muhammad, B. Y.; Unekwe, P. C.
Nigerian J. Basic Appl. Sci. 1989, 3, 35.
8. Haight, A. R.; Stuk, T. L.; Allen, M. S.; Bhagavatula, L.;
Fitzgerald, M.; Hannick, S. M.; Kerdesky, F. A. J.; Menzia,
J. A.; Parekh, S. I.; Robbins, T. A.; Scarpetti, D.; Tien, J.-H.
J. Org. Proc. Res. Dev 1999, 3, 94.
9. Cholli, A. L.; Lau, M. L.; Gala, K. J. Pharm. Sci. 1993, 83,
1275.
10. Naringrekar, V. H.; Stella, V. J. J. Pharm. Sci. 1990, 79, 138.
11. Edafiogho, I. O.; Moore, J. A.; Farrar, V. A.; Nicholson,
J. M.; Scott, K. R. J. Pharm. Sci. 1994, 83, 79.
12. Anticonvulsant Screening Project Antiepileptic Drug
Development Program; NIH 78-1093; National Institutes of
Health; DHEW Publ. (NIH) (US), 1978.
13. Porter, R. J.; Cereghino, J. J.; Gladding, G. D.; Hessie,
B. J.; Kupferberg, H. J.; Scoville, B.; White, B. G. Cleveland
Clin. Q 1984, 51, 293.
Anticonvulsant evaluation
Anticonvulsant activity of the enaminones was deter-
mined by the Antiepileptic Drug Development (ADD)
Protocol, Epilepsy Branch, Neurological Disorder Pro-
gram, National Institute of Neurological Disorders and
Stroke (NINDS) with testing procedures that have been
previously described.12À14
14. Krall, R. L.; Penry, J. K.; White, B. G.; Kupferberg, H. J.;
Swinyard, E. A. Epilepsia 1978, 19, 409.
15. Edafiogho, I. O.; Alexander, M. S.; Moore, J. A.; Farrar,
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17. Greenhill, J. V. J. Chem. Soc (C) 1971, 2699.
Acknowledgement
This research project was supported in part by Kuwait
University Research Grant PC 02/00.