Synthesis and biological effects of new 3-alkylamino-4H-1,2,4- benzothiadiazine 1,1-dioxides on insulin-secreting cells

Article abstract of DOI:10.1211/096089296765556980

3-Alkylamino-4H-1,2,4-benzothiadiazine 1,1-dioxides with nitro, amino or acetylamino groups in the 7-position have been synthesized in an attempt to discover new tissue-selective K(ATP)-channel openers. The compounds were tested as putative pancreatic β-cells K(ATP)-channel openers by measuring their inhibitory activity on the insulin releasing process. The influence of the substituent in the 7-position on the acidic character (pK(a)) and on biological activity is discussed. The nitrobenzene derivatives were biologically active, but less so than the un-derivatized parent pyridothiadiazine dioxides.

Full text of DOI:10.1211/096089296765556980

Pharm. Pharmacol. Commun. 2000, 6: 89±95
Received December 9, 1999
Accepted December 22, 1999
# 2000 Pharm. Pharmacol. Commun.
Synthesis and Biological Effects of New 3-Alkylamino-4H-
1,2,4-Benzothiadiazine 1,1-Dioxides on Insulin-secreting Cells
Â
F. SOMERS, P. DE TULLIO, S. BOVERIE, J. M. DOGNE, X. DE LEVAL, M. -H. ANTOINE*,
P. LEBRUN* AND B. PIROTTE
Â
Á
à Á
Department of Medicinal Chemistry, Universite de Liege, 1 Avenue de l'Hopital, B-4000 Liege and
*Department of Pharmacology, Universite Libre de Bruxelles, 808 route de Lennik,
Â
B-1070 Brussels, Belgium
Abstract
3-Alkylamino-4H-1,2,4-benzothiadiazine 1,1-dioxides with nitro, amino or acetylamino
groups in the 7-position have been synthesized in an attempt to discover new tissue-
selective KATP-channel openers.
The compounds were tested as putative pancreatic b-cells KATP-channel openers by
measuring their inhibitory activity on the insulin releasing process. The in¯uence of the
substituent in the 7-position on the acidic character (pKa) and on biological activity is
discussed.
The nitrobenzene derivatives were biologically active, but less so than the un-derivatized
parentpyridoht iadiazine dioxides.
Potassium channels, proteinic trans-membrane
structures involved in the movement of potassium
ions, are crucially important in controlling cell
membrane potential (Cook 1988). Among these,
ATP-sensitive potassium channels (or KATP chan-
nels) have been the focus of intensive research in
the last decade. They have been identi®ed in a large
variety of cells, for example cardiac cells (Noma
1983), pancreatic b-cells (Cook & Hales 1984),
skeletal and smooth muscle cells (Standen et al
1989; Allard & Lazdunski 1993) and central neu-
rons (Bernardi etal 1988). Depending on hteir
location, KATP channels are involved in important
physiological processes. In pancreatic b-cells, they
have been shown to mediate glucose-induced
insulin secretion (Henquin 1990; Lebrun 1993) and
in smooth muscle cells they control muscle tone
and contractility (Cook & Quast 1990; O'Donnell
& Owen 1994; Quayle etal 1997).
Pinacidil and cromakalim have greater activity on
smooth muscle cells (relaxation) than on pancreatic
b-cells (inhibition of insulin release). Diazoxide,
however, is equipotent on both tissues.
We previously synthesized 3-alkylamino-4H-
pyrido[4,3-e]-1,2,4-thiadiazine 1,1-dioxides, with
different aminoalkyl side-chains in the 3-position
(Pirotte et al 1993, 1994; de Tullio et al 1996;
Lebrun etal 1996), which can be regarded as
hybrid compounds between diazoxide and pinacidil
(Figure 1). Pharmacological investigation has
recently shown that compounds such as BPDZ 42
(3-(20-butylamino)-4H-pyrido[4,3-e]-1,2,4-thiadia-
zine 1,1-dioxide) and BPDZ 44 (3-(30-methyl-20-
butylamino)-4H-pyrido[4,3-e]-1,2,4-thiadiazine 1,1-
dioxide) have the pharmacological pro®les of
K
ATP-channel openers (Pirotte et al 1994; Khelili
et al 1999). In contrast with diazoxide, which is
devoid of tissue selectivity, these two pyrido-
thiadiazine dioxides seemed to be more active on
endocrine pancreatic tissue (inhibition of insulin
release) than on vascular tissue.
To discover new tissue-selective KATP-channel
openers, we have synthesized and characterized the
biological effects of new 3-alkylamino-4H-1,2,4-
benzothiadiazine 1,1-dioxides bearing a nitro group
in the 7-position and different short and branched
aminoalkyl side chains in the 3-position (Figure 1).
Such compounds are expected to be good analo-
gues of the pyridothiadiazine dioxides, because a
Hypoglycaemic sulphonylureas such as tolbut-
amide or glibenclamide are known to be KATP
channel blockers (Gylfe etal 1984; Schmid-Anot-
marchi etal 1987; Boyd 1988). In conrtas,t com-
-
pounds such as diazoxide, cromakalim, pinacidil and
nicorandil are regarded as KATP-channel openers
(Lebrun et al 1988, 1989; Hamilton & Weston 1989;
Antoine et al 1992; Longman & Hamilton 1992).
Á
Correspondence: Fabian Somers, University of Liege,
Department of Medicinal Chemistry, 1 Avenue de l'Hopital,
Ã
Á
tour 4 ( ‡ 5), B-4000 Liege, Belgium.

90
F. SOMERS ET AL
Figure 1. Diazoxide, pinacidil, 3-alkylamino-4H-pyrido[4,3-e]-1,2,4-thiadiazine 1,1-dioxides and 7-nitro-, 7-amino-, 7-
acetamido-4H-1,2,4-benzothiadiazine 1,1-dioxides related to BPDZ 42 or BPDZ 44.
2-Chloro-5-nitrobenzenesulphonamide
benzene ring with a nitro group is usually con-
sidered to be a classical isostere of the pyridinic
ring. Reduction of the nitro group to the amino
group, and subsequent acetylation, have also been
performed to give the 7-amino- and 7-acetamido-
substituted derivatives.
Glacial acetic acid (160 mL) was saturated with
gaseous sulphur dioxide and then mixed with an
aqueous solution of cupric chloride (7 g=20 mL).
A solution of 2-chloro-5-nitroaniline (10 g) in
glacial acetic acid (160 mL) and 12 M HCl (40 mL)
was cooled to À5ꢀC and a solution of sodium
nitrite (4 g) in water (20 mL) was then added
dropwise, with stirring, to the cold solution.
The diazonium saltformed was added, whit
stirring, to the cold solution of sulphur dioxide
in acetic acid prepared previously. After 10 min
the mixture was poured on ice (200 g). The
resulting precipitate was collected by ®ltration,
washed with water and suspended in 10% aqueous
ammonia (200 mL). After stirring at room tem-
perature for 30 min the mixture was concentrated to
half the volume by evaporation under reduced
pressure. The resulting suspension was adjusted to
pH 1 with 12 M HCl. The insoluble material was
collected by ®ltration, washed with water and
crystallized from methanol±water (yield 8Á8 g; mp
174±176ꢀC).
Materials and Methods
Chemistry
Melting points were determined on a BuÈchi-Tottoli
capillary apparatus and are uncorrected. IR spectra
were recorded as KBr pellets on a Perkin-Elmer
1000 FT spectrophotometer. 1H NMR spectra were
recorded on a Bruker AW-80 (80 MHz) using d6-
DMSO as solvent. Chemical shifts (d ppm) are
relative to hexamethyldisiloxane, which was used
as internal reference. NH signals appeared as broad
singlets exchangeable with D2O. In the data below,
s ˆ singlet, d ˆ doublet, t ˆ triplet, m ˆ multiplet,
b ˆ broad.

NEW 4H-1,2,4-BENZOTHIADIAZINE 1,1-DIOXIDES
91
1
1056 cmÀ1. H NMR (DMSO-d6, HMDS; d ppm):
0Á80 (t, 3H, CH2-CH3), 1Á10 (d, 3H, CH-CH3), 1Á50
(m, 2H, CH2CH3), 3Á70 (m, 1H, NH-CH), 7Á30 (m,
2H, 5-H ‡ NH-CH), 8Á30 (m, 2H, 6-H ‡ 8-H),
10Á90 (bs, 1H, NH).
2-Amino-5-nitrobenzenesulphonamide
A suspension of 2-chloro-5-nitrobenzenesulphon-
amide (8 g) in concentrated aqueous ammonia
(80 mL) was saturated with ammonia just before its
introduction to a sealed vessel. It was then placed
in an autoclave and heated at 120ꢀC for 5 h. The
reaction mixture was then concentrated to half its
volume by evaporation under reduced pressure and
the resulting precipitate was collected by ®ltration,
washed with water and dried (yield 6Á1 g; mp 202±
204ꢀC).
3-(3-Methyl-2-butyl)amino-7-nitro-4H-1,2,4-benzo-
thiadiazine 1,1-dioxide (7)
mp 308±311ꢀC. IR (KBr): 3294, 3199, 3101, 2965,
1636, 1600, 1566, 1538, 1497, 1485, 1349, 1252,
1166, 1153, 1107 cmÀ1. 1H NMR (DMSO-d6,
HMDS; d ppm): 0Á80 (d, 6H, CH-(CH3)2), 1Á10 (d,
3H, CH-CH3), 1Á70 (m, 1H, CH-(CH3)2), 3Á65 (m,
1H, NH-CH), 7Á20 (m, 2H, 5-H ‡ NH-CH), 8Á30
(m, 2H, 6-H ‡ 8-H), 10Á80 (bs, 1H, NH).
3-(1H-Imidazol-1-yl)-7-nitro-4H-1,2,4-benzo-
thiadiazine 1,1-dioxide imidazolium salt
Thiocarbonyldiimidazole (14 g) was added to a hot
solution of 2-amino-5-nitrobenzenesulphonamide
(5 g) in dioxane (150 mL) and the mixture was
heated under re¯ux for 5 h. After cooling, the pre-
cipitate of the title compound was collected by
®ltration, washed with dioxane and dried (yield
5Á7 g; mp 246±248ꢀC).
3-(Cyclopropylmethyl)amino-7-nitro-4H-1,2,4-
benzothiadiazine 1,1-dioxide (8)
mp 276±278ꢀC. IR (KBr): 3290, 3193, 3104, 2994,
1635, 1603, 1564, 1539, 1499, 1486, 1347, 1276,
1251, 1153, 1111 cmÀ1. 1H NMR (DMSO-d6,
HMDS; d ppm): 0Á50±1Á30 (bm, 5H, 2 6
CH2 ‡ CH), 3Á10 (t, 2H, NH-CH2-CH), 7Á35 (m,
2H, 5-H ‡ NH-CH), 8Á30 (m, 2H, 6-H ‡ 8-H),
11Á15 (bs, 1H, NH).
General procedure for preparation of
3-alkylamino-7-nitro-4H-1,2,4-benzothiadiazine
1,1-dioxides
A mixture of 3-(1H-imidazol-1-yl)-7-nitro-4H-
1,2,4-benzothiadiazine 1,1-dioxide (0Á5 g) and the
appropriate alkylamine (5 mL) was heated in a
sealed vessel at150 ꢀC for 5 to 6 h until completion
of the reaction as monitored by thin-layer chroma-
tography (TLC). Excess amine was removed by
distillation under reduced pressure and the oily
residue was dispersed in water. NaOH (2Á5 M) was
added to the suspension until dissolution was
complete. The resulting solution was treated with
charcoal, ®ltered, and the ®ltrate was adjusted to
pH 5±6 with 6 M HCl. The precipitate was col-
lected by ®ltration, washed with water and recrys-
tallized from methanol±water (yield 60±70%).
7-Nitro-3-propylamino-4H-1,2,4-benzothiadiazine
1,1-dioxide (9)
mp 268±270ꢀC. IR (KBr): 3307, 3192, 3166, 3086,
3018, 2971, 2882, 1642, 1600, 1561, 1533, 1498,
1486, 1399, 1339, 1280, 1245, 1167, 1111 cmÀ1.
1H NMR (DMSO-d6, HMDS; d ppm): 0Á85 (t, 3H,
CH3-CH2), 1Á50 (m, 2H, CH2-CH3), 3Á10 (m, 2H,
NH-CH2), 7Á35 (m, 2H, 5-H ‡ NH-CH), 8Á30 (m,
2H, 6-H ‡ 8-H), 11Á10 (bs, 1H, NH).
3-Cyclobutylamino-7-nitro-4H-1,2,4-benzo-
thiadiazine 1,1-dioxide (10)
mp 298±301ꢀC. IR (KBr): 3305, 3192, 3087, 2998,
1640, 1598, 1567, 1532, 1497, 1484, 1400, 1341,
3-Isopropylamino-7-nitro-4H-1,2,4-benzothiadiazine
1,1-dioxide (5)
1283, 1249, 1163, 1113 cmÀ1. H NMR (DMSO-
1
mp 311±313ꢀC. IR (KBr): 3363, 3221, 2980, 1657,
1646, 1615, 1601, 1572, 1532, 1494, 1339, 1284,
1267, 1155, 1106 cmÀ1. 1H NMR (DMSO-d6,
HMDS; d ppm): 1Á10 (d, 6H, 2 6 CH3), 3Á90 (m,
1H, NH-CH), 7Á35 (m, 2H, 5-H ‡ NH-CH), 8Á35
(m, 2H, 6-H ‡ 8-H), 10Á90 (bs, 1H, NH).
d6, HMDS; d ppm): 1Á30±2Á60 (bm, 6H, 3 6 CH2),
4Á20 (m, 1H, NH-CH), 7Á35 (d, 1H, 5-H), 7Á70 (bd,
1H, NH-CH), 8Á30 (m, 2H, 6-H ‡ 8-H), 10Á95 (bs,
1H, NH).
3-Alkylamino-7-nitro-4H-1,2,4-benzothiadiazine
1,1-dioxide (11)
mp 233±236ꢀC. IR (KBr): 3308, 3186, 3092, 2992,
1636, 1599, 1562, 1533, 1497, 1484, 1426, 1397,
1340, 1280, 1246, 1162, 1112 cmÀ1. 1H NMR
(DMSO-d6, HMDS; d ppm): 3Á90 (bt, 2H, NH-
CH2), 5Á15 (m, 2H, CH ˆ CH2), 5Á60±6Á10 (m, 1H,
3-(2-Butyl)amino-7-nitro-4H-1,2,4-benzothiadiazine
1,1-dioxide (6)
mp 287±288ꢀC. IR (KBr): 3354, 3223, 3111, 2969,
2935, 2877, 2853, 2361, 1601, 1573, 1529, 1489,
1452, 1401, 1383, 1280, 1245, 1178, 1130, 1104,

92
F. SOMERS ET AL
CH ˆ CH2), 7Á30 (d, 1H, 5-H), 7Á60 (m, 1H, NH-
CH2), 8Á30 (m, 2H, 6-H ‡ 8-H), 11Á20 (bs, 1H,
NH).
1497, 1467, 1391, 1279, 1159, 1142, 1123,
1
1104 cmÀ1. H NMR (DMSO-d6, HMDS; d ppm):
1Á10 (d, 6H, 2 6 CH3), 2Á00 (s, 3H, COCH3), 3Á90
(m, 1H, NH-CH), 6Á85 (bd, 1H, NH-CH), 7Á05 (d,
1H, 5-H), 7Á60 (bd, 1H, 6-H), 8Á00 (bs, 1H, 8-H),
10Á00, (bs, 1H, CONH), 10Á10 (bs, 1H, NH).
7-Amino-3-isopropylamino-4H-1,2,4-benzo-
thiadiazine 1,1-dioxide (12)
Pd=C (10%, 0Á06 g) was added to a solution of 3-
isopropylamino-7-nitro-4H-1,2,4-benzothiadiazine
1,1-dioxide 5 (0Á6 g) in hotmehtanol (25 mL) and
the mixture was treated with hydrogen gas under
Results and Discussion
ꢀ
pressure (4 bar) for 1 h at40 C. Insoluble material
Chemistry
was removed by ®ltration and the ®ltrate was
concentrated to dryness by evaporation under
reduced pressure. The residue of the crude title
compound was recrystallized from methanol±water
(yield 0Á45 g).
2-Amino-5-nitrobenzenesulphonamide, a key inter-
mediate in the preparation of the ®nal products, was
obtained in a two-step reaction starting from 2-
chloro-5-nitroaniline (1) (Figure 2). The aniline (1)
was transformed by diazotization into the corre-
sponding 2-chloro-5-nitrobenzenesulphonamide (2)
and subsequent substitution of the chlorine atom in
the 2-position, ꢀwith concentrated ammonia under
pressure at150 C, led to the key intermediate (3).
The synthesis of the 3-(1H-imidazol-1-yl)-sub-
stituted intermediate (4) was achieved by reaction
of 2-amino-5-nitrobenzenesulphonamide (3) with
excess 1,1-thiocarbonyldiimidazole in dioxane.
The 3-alkylamino-7-nitro-4H-1,2,4-benzothia-
diazine 1,1-dioxides (5±11) were obtained by
reaction of 3-(1H-imidazol-1-yl)-7-nitro-4H-1,2,4-
benzothiadiazine 1,1-dioxide (4) with excess of the
appropriate alkylamine (Figure 2). The reaction
occurs with good yields in an hermetically closed
autoclave heated at 150ꢀC for 4 to 6 h.
mp 278±283ꢀC. IR (KBr): 3455, 3363, 3216,
2972, 1618, 1575, 1510, 1298, 1255, 1174, 1146,
1
1113 cmÀ1. H NMR (DMSO-d6, HMDS; d ppm):
1Á10 (d, 6H, 2 6 CH3), 3Á85 (m, 1H, NH-CH), 5Á15
(bs, 2H, NH2), 6Á50±6Á95 (bm, 4H, 5-H ‡ 6-H ‡ 8-
H ‡ NH-CH), 9Á75 (bs, 1H, NH).
7-Acetylamino-3-isopropylamino-4H-1,2,4-benzo-
thiadiazine 1,1-dioxide (13)
A mixture of 3-isopropylamino-7-nitro-4H-1,2,4-
benzothiadiazine 1,1-dioxide 5 (0Á18 g) and acetic
anhydride (0Á6 mL) in dioxane (2 mL) was stirred at
room temperature. The mixture was then mixed
with water (5 mL) and stirred for 1 h. The solvents
were removed by distillation under reduced pres-
sure and the residue of the crude title compound
was recrystallized from methanol±water (yield
0Á16 g).
The 7-amino-substituted derivative (12) was
prepared from 5 by reduction of the nitro function
under hydrogen gas, with palladium on activated
carbon as catalyst (Figure 3). Subsequent acetyla-
tion of 12 with acetic anhydride led to the corre-
mp 260±262ꢀC. IR (KBr): 3353, 3316, 3240,
3101, 3069, 2974, 1672, 1624, 1608, 1581, 1546,
Figure 2. Synthesis of 3-aminoalkyl-7-nitro-4H-1,2,4-benzothiadiazine 1,1 dioxides. i. NaNO2, HCl; CH3COOH, SO2, CuCl2,
NH4OH dil.; ii. NH4OH conc., 150ꢀC; iii. 1,1-thiocarbonyldiimidazole; dioxane; iv. RNH2.

NEW 4H-1,2,4-BENZOTHIADIAZINE 1,1-DIOXIDES
93
Figure 3. Synthesis of the 7-amino- and 7-acetamido-substituted derivatives. i. H2, 4 bar, palladium; ii. (CH3CO)2O.
sponding 7-acetamido-substituted compound (13)
(Figure 3).
diazoxide and BPDZ 44. Comparison of the 7-
nitro-substituted derivative (5) with its 7-amino-
and 7-acetamido-substituted counterparts (12 and
13) showed the biological activity of 12 and 13 to
be substantially lower. Thus, reduction of the nitro
group does not seem to be a suitable means of
maintaining or increasing their activity as inhibitors
of insulin release.
The most potent compound of the series, the 3-
isopropylamino-7-nitro-4H-1,2,4-benzothiadiazine
1,1-dioxide derivative (5), markedly inhibited
insulin release. At a concentration of 10 mM, how-
ever, the drug was less potent than BPDZ 44.
Itcan be concluded ht atan isopropylamino side-
chain in the 3-position of the benzothiadiazine ring
constitutes, in this series, one of the best choices of
lateral substituent.
Pharmacological evaluation
The differentcompounds reporetd in Table 1 were
tested as inhibitors of insulin release from rat
pancreatic islets incubated in the presence of an
insulinotropic glucose concentration (16Á7 mM).
The drugs were tested at a concentration of 50 mM
and the most active compounds were also examined
at10 mM. Results were compared with those
obtained by use of three reference drugsÐpinaci-
dil, diazoxide and BPDZ 44 (a pyridothiadiazine
dioxide).
It is apparent from Table 1 that with the excep-
tion of compound 5 the activity of the newly syn-
thesized 3-alkylamino-7-nitro-4H-1,2,4-benzothia-
diazine 1,1-dioxides on pancreatic b-cells was
moderate or poor. Two compounds, 6 and 10, were
more potent than pinacidil but less potent than
Determination of pKa
The hydrogen atom in the N-4 position of the
thiadiazine ring is expected to be labile and can be
Table 1. Effects of 3-alkylamino-4H-1,2,4-benzothiadiazine 1,1-dioxides, diazoxide, pinacidil and BPDZ 44 on insulin release
from pancreatic b-cells, and pKa values of selected compounds.
Compound
X
R
% RISÆ s.e.m. (n)
pKa
50 mM
10 mM
5
6
7
8
9
10
11
12
NO2
NO2
NO2
NO2
NO2
NO2
NO2
NH2
NHCOCH3
±
CH(CH3)2
CH(CH3)CH2CH3
CH(CH3)CH(CH3)2
CH2-cyclopropyl
CH2CH2CH3
Cyclobutyl
CH2CH ˆ CH2
CH(CH3)2
CH(CH3)2
±
4Á8Æ 0Á6 (14)
63Á3Æ 3Á8 (18)
90Á4Æ 5Á2 (16)
95Á7Æ 6Á0 (16)
88Á9Æ 4Á6 (16)
59Á6Æ 4Á9 (13)
82Á7Æ 5Á8 (15)
85Á8Æ 4Á3 (15)
89Á3Æ 5Á8 (16)
28Á8Æ 2Á5 (21)a
92Á7Æ 8Á0 (14)a
7Á1Æ 0Á6 (14)a
64Á0Æ 3Á8 (37)
ND
ND
7Á91
ND
ND
ND
ND
ND
ND
10Á49
9Á80
8Á62a
ND
ꢁ 8Á1
ND
ND
ND
ND
ND
13
ND
Diazoxide
Pinacidil
BPDZ 44
70Á0Æ 3Á6 (22)a
99Á3Æ 8Á9 (13)a
26Á7Æ 1Á8 (21)a
±
±
±
±
RIS is the percentage residual insulin secretion from rat pancreatic islets incubated in the presence of 16Á7 mM glucose. ND, not
a
determined. Published results.

94
F. SOMERS ET AL
removed by aqueous alkali. The pKa of 1,2,4-ben-
zothiadiazine 1,1-dioxides is modulated by the
nature and position of the substituent on the ben-
zene ring. Electron-withdrawing groups should
increase acidity whereas electron-donor groups
should reduce it.
an electron-withdrawing group seems to promote
activity.
In conclusion, although the nitrobenzene analo-
gues of the active 3-alkylamino-4H-pyrido[4,3-e]-
1,2,4-thiadiazine 1,1-dioxides are acceptable bioi-
sosteres, their biological activity on b-cells was less
pronounced.
The pKa values of compounds 5, 12 and 13 were
determined by UV spectrophotometry, using the
Debye±HuÈckel (Albert& Serjeant1971) relaiton-
ship linking pKa with the experimental optical
density. The results are reported in Table 1. As
expected, the pKa values correlated directly with
the electron-withdrawing character of the sub-
stituent in the 7-position. The 7-nitro-substituted
derivative (5), representative of the nitrobenzene
isosteres, was found to be more acidic (pKa 7Á91)
than diazoxide (pKa 8Á62). Compared with 3-alky-
lamino-4H-pyrido[4,3-e]-1,2,4-thiadiazine 1,1-di-
oxides (pKa values ꢁ 8Á1) (de Tullio etal 1996), at
physiological pH (7Á4) the nitrobenzene isosteres
(5±11) are probably present in solution to a greater
extent as ionic species. Such species are expected
to be ineffective because it can be predicted that
only the non-ionic form of the drug should be able
to interact favourably with the receptor binding site
(de Tullio etal 1996). Thus, exceptfor hte 3-iso-
propylamino-substituted compound (5), the poor
biological activity of the 3-alkylamino-7-nitro-4H-
1,2,4-benzothiadiazine 1,1-dioxides (6±11) could
be explained, at least in part, by their more pro-
nounced acidicity than their pyridothiadiazinic
isosteres.
Acknowledgements
This study was supported by grants from the
National Fund for Scienti®c Research (F.N.R.S.,
Belgium) of which P. Lebrun is Director of
Research, and B. Pirotte and P. de Tullio are Senior
Research associates. The assistance of Mrs P.
Neven, D. Dewalque, M.-L. Pirard, J. Sergooris and
M. Hermann is gratefully acknowledged.
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‡
The 7-amino-substituted compound (12) had the
highestpK (10Á49). Thus, reduction of the nitro
a
group to the amino electron-donor group reduces
the acidic character of the molecule. Finally, the 7-
acetamido-substituted derivative (13) with inter-
mediate electron-withdrawing character was found
to have an intermediate pKa (9Á80). Although these
two compounds (12, 13) would existin soluiton
mainly as non-ionized species, they are poorly
active in contrast with their 7-nitro-substituted
counterpart (5). Thus 12 and 13 are notaccepatble
bioisosteres of the active pyridothiadiazines.
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Conclusions
Some of the newly synthesized 3-alkylamino-7-
nitro-4H-1,2,4-benzothiadiazine 1,1-dioxides inhi-
bitinsulin release and could ht us be considered as
putative KATP-channel openers. Their biological
activity is modulated by the nature of the amino-
alkyl side chain. Shortbranched chains resultin
the maximum effect (isopropyl > cyclobutyl ꢁ 2-
butyl). Biological activity is also affected by the
nature of the substituent in the 7-position. Indeed,
È
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Ã
Â
Â
A61±A81

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95
‡
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‡