Synthesis and evaluation of new ethyl 2,3-dihydro-3-oxoisothiazolo[5,4-b]pyridine-2-alkanoate derivatives as potential herbicides

Article abstract of DOI:10.1002/(SICI)1096-9063(199702)49:2<148::AID-PS515>3.0.CO;2-U

Several ethyl 2,3-dihydro-3-oxoisothiazolo[5,4-b]pyridine-2-alkanoate derivatives were synthesized as herbicides. Only 5-methyl derivatives inhibited both hypocotyl and root growth in the lettuce (Lactuca sativa L.) seedling test at 100 mg litre^-1. Only ethyl propionate and valerate derivatives showed significant inhibition at 0.1 mg litre^-1, whereas ethyl acetate or butyrate derivatives were inactive. Contrary to unoxidized derivatives, the inhibitory effect of 1-oxide and 1,1-dioxide derivatives was strongly dependent on concentration; ethyl 2,3-dihydro-5-methyl-3-oxoisothiazolo[5,4-b]pyridine-2-propionate 1,1-dioxide inhibited 100% of germination at 100 mg litre^-1 and 45% of lettuce seedling growth at 0.1 mg litre^-1. Quantitative structure-inhibition of growth relationship analysis carried out by adaptive least-squares (ALS) method gave a good correlation with small and hydrophobic 5-substituents as well as with odd carbon-chain ethyl alkanoates in position 2. Active compounds did not show auxin-like activity from 0.1 to 100 mg litre^-1.

Full text of DOI:10.1002/(SICI)1096-9063(199702)49:2<148::AID-PS515>3.0.CO;2-U

Pestic. Sci. 1997, 49, 148È156
Synthesis and Evaluation of New Ethyl 2,3-
Dihydro-3-oxoisothiazolo[5,4-b]pyridine-
2-alkanoate Derivatives as Potential Herbicides
Maria J. Gil, Jose M. Zabalza, Javier Navarro, Maria A. Manu, Alberto Gonzalez,
V•ctor Mart•nez Merino*
Departamento de Qu•mica Aplicada, Universidad Publica de Navarra, Campus Arrosad•a,
31006 Pamplona, Spain
& Arantxa Canal, Mercedes Royuela, Pedro M. Aparicio-Tejo
Departamento de Ciencias del Medio Natural, Universidad Publica de Navarra, Campus Arrosad•a,
31006 Pamplona, Spain
(Received 8 March 1996; revised version received 8 July 1996; accepted 10 October 1996)
Abstract: Several ethyl 2,3-dihydro-3-oxoisothiazolo[5,4-b]pyridine-2-alkanoate
derivatives were synthesized as herbicides. Only 5-methyl derivatives inhibited
both hypocotyl and root growth in the lettuce (L actuca sativa L.) seedling test at
100 mg litre~1. Only ethyl propionate and valerate derivatives showed signiÐ-
cant inhibition at 0É1 mg litre~1, whereas ethyl acetate or butyrate derivatives
were inactive. Contrary to unoxidized derivatives, the inhibitory e†ect of 1-oxide
and 1,1-dioxide derivatives was strongly dependent on concentration; ethyl
2,3-dihydro-5-methyl-3-oxoisothiazolo[5,4-b]pyridine-2-propionate 1,1-dioxide
inhibited 100% of germination at 100 mg litre~1 and 45% of lettuce seedling
growth at 0É1 mg litre~1. Quantitative structureÈinhibition of growth relation-
ship analysis carried out by adaptive least-squares (ALS) method gave a good
correlation with small and hydrophobic 5-substituents as well as with odd
carbon-chain ethyl alkanoates in position 2. Active compounds did not show
auxin-like activity from 0É1 to 100 mg litre~1.
Key words: 3-Oxoisothiazolo[5,4-b]pyridines, synthesis, herbicides, plant-
growth-inhibitors, SAR analysis
1
INTRODUCTION
related 2-oxothiazolo[4,5-b]pyridine-3(2H)-acetic acid
derivatives presented auxin-like herbicidal activity (Fig.
2).6 On the other hand, a few of the proposed com-
pounds in Fig. 1 also could be rigid bioisosteric analogs
of abscisic acid (ABA) mimics7 displayed in Fig. 2.
In order to explore plant-growth-regulating activity
of the new ethyl 2,3-dihydro-3-oxoisothiazolo[5,4-b]
pyridine-2-alkanoate derivatives, three regions around
The isothiazolo[5,4-b]pyridin-3(2H)-one system has
aroused great interest in pharmacy,1 but it has been
studied very little in agronomy. Only a few derivatives
have been reported as insecticides and fungicides.2
However, our preliminary molecular modelling studies
from Sybyl software3 showed that some 3-
oxoisothiazolo[5,4-b]pyridine-2-alkanoic acid deriv-
atives (Fig. 1) match well with the auxin binding site
recently proposed.4 In this sense, 3-oxo-1,2-
benzisothiazole-2-alkanoic acid analogues were claimed
as new root auxin-like compounds (anti-auxins),5 and
Fig. 1. R \ H, NO , CH , C H , NH . n \ 0, 1, 2, x \ 0, 1,
1
2
3
6 5
2,3.
2
* To whom correspondence should be addressed.
148
Pestic. Sci. 0031-613X/97/$09.00 ( 1997 SCI. Printed in Great Britain

New isothiazolo[5,4-b] pyridin-3(2H)-one derivative as herbicides
149
Fig. 2. (I) ABA mimics. (II) Auxin-like herbicides.
the nucleus were selected, as shown in Fig. 1. Substit-
melting points are given in Table 1. The structures of
the synthesized compounds were supported by their
infrared and proton magnetic resonance spectra, as well
as by elemental CHNS analyses. Relevant spectroscopic
data are shown in Table 2.
uents in position 5 correspond to R and inÑuence the
1
chemical stability of the system and, consequently, its
metabolism. The substituents selected for R (-H, -NO ,
1
2
-CH , -C H and -NH ) are suitable for preliminary
3
6 5
2
biological screenings, since they have a broad range of
electronic8 and hydrophobic9 properties. The inÑuence
of chain length of the ethyl alkanoates in position 2 on
plant-growth-regulating activity was analysed by
varying x (x \ 0, 1, 2, 3). Unsubstituted derivatives in
position 2 (compounds 2, 9 and 15) as well as ethyl a-
propionate derivatives (compounds 4 and 11) were addi-
tionally studied. On the other hand the region that
surrounds position 1 was analysed through sulfur oxi-
dation states (n \ 0, 1, 2).
2.1.1 Synthesis of isothiazolo[5,4-b]pyridin-3(2H)-ones
(2–17)
According to a previously described procedure,1 1,2-
dihydro-2-thioxo-3-pyridinecarboxylic acids (5É0 mmol)
were treated with SOCl in boiling xylene to give corre-
2
sponding acid chlorides. To a stirred mixture of above
crude acid chlorides in dry chloroform (30 ml), a cata-
lytic amount of iodine was added and then dry chlorine
gas was passed through the solution for 45 min at
[19¡C. The solvent was removed under vacuum and
the respective crude sulphenyl chloride (1) was dissolved
in dioxane (10 ml). To the resulting mixture, a freshly
prepared solution of ethyl aminoalkanoate hydro-
chloride (20 mmol) and sodium hydroxide (0É80 g,
20 mmol) in water (20 ml) was added dropwise with
stirring at 0¡C. Stirring was continued at room tem-
perature for a further 3 h. Subsequently, water (70 ml)
was added and then hydrochloric acid (1 M) to bring the
pH to 6. The resulting solid material was collected and
recrystallized from solvents indicated in Table 2. An
additional amount of isothiazolo[5,4-b]pyridine was
obtained by extraction of the aqueous Ðltrate with
dichloromethane (3 ] 50 ml).
The analysis of the quantitative structureÈactivity
relationship (QSAR) of these derivatives, classiÐed by
inhibition of hypocotyl and root elongation of lettuce
seedlings into three biological classes (active, moder-
ately active or inactive), was carried out by the adaptive
least-squares (ALS) method.10 The ALS method does
not assume any particular distribution of the data and
can be considered a non-parametric pattern classiÐer.
2
EXPERIMENTAL METHODS
2.1 Chemistry
New ethyl 2,3-dihydro-3-oxoisothiazolo[5,4-b]pyridine-
2-alkanoates were prepared following the procedures
described in Fig. 3. Yields, recrystallization solvents and
Compounds 2, 9 and 15 were obtained passing
ammonia through a solution of the respective crude sul-
Fig. 3. Synthesis of isothiazolo[5,4-b]pyridines and their S-oxides. Reagents and conditions: (a) R -NH , dioxane, NaOH, r.t; or
2
2
NH , dioxane; (b) NaHSO (aq), C H OH, r.t; (c) KHSO (oxone), CH OH, H O, r.t; (d) m-CPBA (2 eq.), CH Cl , r.t; (e) NaOH
3
3
2
5
5
3
2
2 2
(aq), TBAB, r.t.

150
Maria J. Gil et al.
TABLE 1
Synthesized Isothiazolo[5,4-b]pyridin-3(2H)-one Derivatives
Structure
Solvent
Compound
R
R
n
Y ield (%)
recrystna
m.p.(¡C)
1
2
2
3
CH
CH
CH
CH
CH
CH
H
NO
NO
NO
NO
NO
NO
H
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
2
2
2
2
2
È
80
61
40
35
35
30
77
60
80
85
72
67
46
65
60
60
62
90
90
91
90
85
85
84
85
85
81
90
A
B
A
A
A
D
D
A
B
B
B
B
B
A
E
C
F
C
D
D
D
B
C
D
D
D
B
A
[240
146È1481
85È86
143È144
70È71
38È40
103È10511
[240
196È1981
140È141
171È173
130È132
119È121
194È195
167È1691
87È88
163È165
86È88
75È77
69È71
53È55
3
3
3
3
3
3
CH CO C H
2 2
CH(CH )CO C H
2 2
2
5
4
3
5
5
5
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 3 2 2
(CH ) CO C H
2 4 2 2
(CH ) CO C H
2 2 2 2
5
5
5
5
6
7
8
9
H
2
2
2
2
2
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
CH CO C H
2 2
CH(CH )CO C H
2 2
2
5
3
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 3 2 2
(CH ) CO C H
2 4 2 2
5
5
5
C H
H
6
5
5
5
C H
6
CH -CO C H
2 2
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 2 2 2
CH CO C H
2 2
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 3 2 2
(CH ) CO C H
2 4 2 2
(CH ) CO C H
2 2 2 2
CH -CO C H
2 2
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 3 2 2
(CH ) CO C H
2 4 2 2
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 2 2 2
2
5
C H
6
5
5
NH
2
CH
CH
CH
CH
H
CH
CH
CH
CH
H
3
3
3
3
2
5
5
5
5
5
52È5411
78È80
73È75
88È90
65È67
87È8911
65È67
3
3
3
3
2
5
5
5
5
5
5
CH
3
a A: ethanol, B: isopropanol, C: ethyl acetate, D: ethyl acetate ] cyclohexane (2 ] 1 by
volume), E: cyclohexane, F: toluene.
fenyl chloride (1) in dioxane for 30 min. The solvent was
removed under vacuum and then water (50 ml) and
hydrochloric acid (35%) were added to bring the pH to
2. The solid was Ðltered and recrystallized from ethanol.
product obtained on cooling was collected by Ðltration
and recrystallized from ethanol. The solid, a mixture of
N-(2-(ethoxycarbonyl)ethyl)-5-amino-2-thioxo-1,2-dihy-
dro-3-pyridinecarboxamide and ethyl 5-amino-2,3-dihy-
dro-3-oxoisothiazolo[5,4-b]pyridine-2-propionate, was
suspended in carbon tetrachloride (10 ml) and bromine
(0É79 g, 5 mmol) was then added. The mixture was
stirred at room temperature for 5 h and the solid
material was collected by Ðltration, treated with a solu-
tion of sodium hydrogen carbonate (10 ml, 2 M), and
extracted with chloroform (3 ] 40 ml). The organic
layer was separated, the solvent removed under vacuum
and the solid was recrystallized from toluene to give 18
(0É56 g; 62%).
2.1.2 Ethyl 5-amino-2,3-dihydro-3-oxoisothiazolo[5,4-
b]pyridine-2-propionate (18)
A suspension of 12 (1É0 g, 3É4 mmol), iron (1É16 g,
20 mmol) and ammonium chloride (0É53 g, 10 mmol) in
ethanol ] water (1 ] 1 by volume; 100 ml) was reÑuxed
for 3 h. The resulting warm suspension was Ðltered and
the solid material was extracted with boiling ethanol.
Combined Ðltrates and extracts were concentrated to
about 50 ml by evaporation in vacuum. The crystalline

New isothiazolo[5,4-b] pyridin-3(2H)-one derivative as herbicides
151
TABLE 2
Spectroscopic Data of the Isothiazolo[5,4-b]pyridin-3(2H)-one Derivatives
IR l (cm~1)a
-COO-
[1H]NMR d (ppm)b
Compound
-SO
-CON-
4-H
5-H
6-H
nv
2
3
È
È
È
È
È
È
È
È
È
È
È
È
È
È
È
È
È
È
1650
1660
1680
1650
1660
1660
1665
1660
1670
1675
1670
1670
1670
1670
1660
1660
1645
1710
1710
1710
1706
1711
1745
1735
1735
1735
1732
1645
NDc
8É17
8É06
8É12*
8É05
8É08
8É25
8É90*
8É90*
8É90*
8É90
9É02
9É02
8É50*
8É52
8É41
7É49
8É09
8É01
8É05
8É03
8É25
8É17
8É12
8É12
8É12
8É36
7É56
È
È
È
È
È
È
7É33
È
È
È
È
È
È
È
È
È
È
È
È
È
È
7É66
È
È
È
È
7É76
È
NDc
8É75*
8É58
8É71*
8É58
8É57
8É73
9É50*
9É65*
9É80*
9É50
9É50
9É58
9É14*
9É05
8É96
8É26
8É76
8É69
8É70
8É71
8É90
8É81
8É76
8É77
8É77
8É98
8É64
1730
1770
1710
1740
1720
1725
È
1735
1750
1730
1730
1730
È
1730
1730
1730
1750
1730
1737
1730
1730
1763
1735
1735
1735
1744
1730
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
1128
1107
1111
1115
1107
1336, 1157
1336, 1148
1330, 1148
1336, 1146
1325, 1170
È
a Using KBr tablets.
b Spectra were recorded in deuterochloroform except for (*) marked shifts
which were recorded in hexadeuterodimethyl sulfoxide.
c Not determined.
2.1.3 Synthesis of isothiazolo[5,4-b]pyridin-3(2H)-one
1-oxides (19È23)
8É2 mmol) in dichloromethane (10 ml) was added drop-
wise with stirring at 25¡C. After the isothiazolo[5,4-b]
pyridine was completely consumed (reaction monitored
by TLC), solid tetrabutylammonium bromide (TBAB,
0É10 g) and sodium hydroxide (2É1 ml, 4 M) were slowly
added to the stirred mixture at 25¡C. When the sul-
foxide was completely consumed, the mixture was
separated after addition of water (20 ml) and the
organic layer was washed with water (2 ] 20 ml) and
dried (sodium sulfate). The solvent was removed under
vacuum, the residue was chromatographed on a silica
gel column using ethylacetate ] cyclohexane (2 ] 1 by
volume) as eluent and recrystallized from solvents indi-
cated in Table 2.
To
a
stirred suspension of the corresponding
isothiazolo[5,4-b]pyridin-3(2H)-one (3, 5È8) (9É0 mmol)
in methanol ] water (1 ] 1 by volume; 30 ml) at 20¡C,
oxone} (0É83 g, 13É5 mmol of KHSO ) was added in
5
small portions according to a previously described pro-
cedure.11 When the reaction had been completed
(reaction monitored by thin layer chromatography,
TLC), the reaction mixture was poured into water
(100 ml) and extracted with dichloromethane
(3 ] 50 ml). Organic layers were dried over sodium
sulfate and evaporated. The residue was recrystallized
to give the 1-oxides (19È23) (Table 2).
2.1.4 Synthesis of isothiazolo[5,4-b]pyridin-3-(2H)-one
1,1-dioxides (24È28)
To a suspension of the corresponding isothiazolo[5,4-b]
pyridin-3-(2H)-one 3, 5È8) (4É0 mmol) in dichloro-
methane (10 ml), a solution of 98% m-CPBA (1É44 g,
2.1.5 N-(2-(ethoxycarbonyl)ethyl)-1,2-dihydro-5-methyl-
2-thioxo-3-pyridinecarboxamide (29)
A solution of 5 (1É30 g, 5 mmol) in ethanol (20 ml) was
treated with an aqueous solution of sodium hydrogen
sulÐte (10 ml, 1 M). The mixture was stirred at room

152
Maria J. Gil et al.
temperature for 30 min and then the solvent was
removed under vacuum. The solid residue was washed
with water, collected by Ðltration and recrystallized
from ethanol to give (1É18 g; 90%).
Ðlter paper (two discs of Whatman No. 1), and 4 ml of
test solution was added. Each product was tested at 0É1,
1, 10 and 100 mg litre~1. Seeds were maintained at
26¡C in darkness. After four days, root and hypocotyl
lengths were measured to the nearest millimetre. All
treatments were replicated three times. As the same
results were obtained in roots and hypocotyls, Table 3
summarizes the e†ect of the di†erent products on the
whole seedlings expressed as percentage of the control.
Average seedling length (^SE) of the control was
6É6(^0É1) cm.
2.2 Biological assays
Three di†erent bioassays were carried out to evaluate
the plant-growth-regulating activity of the synthesized
compounds. In all cases compounds were dissolved in
7É6 mM potassium phosphate bu†er, pH 7É2 ] acetone
(99 ] 1 by volume) and this solvent system was used as
the control.
2.2.2 Pea stem bioassay
Di†erent compounds were tested for auxinic activity in
the pea stem bioassay.12 Seeds of Pisum sativum L. cv.
Frilene were grown in perlite ] vermiculite (2 ] 1 by
mass) in total darkness at 26¡C for seven days. When
the fourth internode was less than 2 mm long, 6 mm
2.2.1 Assay of lettuce seed germination and root and
hypocotyl elongation
Groups of 10 seeds of L actuca sativa L. cv. Crispilla
were placed in Petri dishes (9 cm) containing pads of
TABLE 3
E†ect of Isothiazolo[5,4-b]pyridin-3(2H)-ones Derivatives on the Growth of L actuca sativa
Seedings
Growth of L. Sativa (% of control)
Structure
Concentration (mg litre~1)
Compound
R
R
n
0É1
1
10
100
1
2
2
3
CH
CH
CH
CH
CH
CH
H
NO
NO
NO
NO
NO
NO
H
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
2
2
2
2
È
Na
N
N
80
N
40
N
N
N
N
N
N
N
N
N
N
N
N
N
N
55
N
90
N
N
N
N
72
N
43
N
N
N
N
N
N
N
N
N
N
N
N
91
N
55
N
78
N
N
N
N
67
N
41
N
N
N
N
N
N
N
N
N
N
N
N
60
N
10
N
74
N
N
86
86
66
78
39
N
N
N
N
N
N
N
N
N
N
N
N
60
N
0
3
3
3
3
3
3
CH CO C H
2 2
CH(CH )CO C H
2 2
2
5
4
3
5
5
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 3 2 2
(CH ) CO C H
2 4 2 2
(CH ) CO C H
2 2 2 2
5
5
5
5
6
7
8
9
H
2
2
2
2
2
2
10
11
12
13
14
15
16
17
18
19
20
24
25
26
27
29
CH CO C H
2 2
CH(CH )CO C H
2 2
2
5
3
5
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 3 2 2
(CH ) CO C H
2 4 2 2
5
5
5
C H
H
6
5
5
5
C H
6
CH CO C H
2 2
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 2 2 2
CH CO C H
2v 2 2
(CH ) CO C H
2 2 2 2
CH CO C H
2 2
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 3 2 2
(CH ) CO C H
2 4 2 2
(CH ) CO C H
2 2 2 2
2
5
C H
6
5
5
NH
2
CH
CH
CH
CH
CH
CH
CH
3
5
3
3
3
3
3
3
5
2
5
5
5
5
5
N
67
N
a N \ not signiÐcantly di†erent from control at the 5% signiÐcance level.

New isothiazolo[5,4-b] pyridin-3(2H)-one derivative as herbicides
153
segments were cut from the apical end of the third inter-
node. The excised segments were washed in water for
30 min at 25¡C with orbital shaking and placed in
sterile plastic Petri dishes (9 cm diameter, six segments
per dish) containing 10 ml of test solution. Each
product was tested on three individual samples at 0É01,
0É1, 1, 10 and 100 mg litre~1. Petri dishes were incu-
bated in darkness at 26¡C for 24 h. Measurement of the
Ðnal segment lengths with an estimation to 0É01 mm
was facilitated by using an optical microscope Ðtted
with a video monitor. All manipulations, except the
measurement itself, were carried out in dim red light.
solution and leaves kept in this solution for 60 min,
allowing intact leaves to take in the test solution
through the petiole. Stomatal conductance measure-
ments were performed during the latter period at 0, 10,
20, 30, 45 and 60 min with a Delta-T AP4 porometer.
Leaves were illuminated at 350 kmol m~2 s~1 during
feeding. Compound 5 and ABA were tested at 0É1, 1, 10
and 100 mg litre~1 in triplicate. Stomatal resistance of
the control was 2É02(^0É41) s cm~1 at 60 min.
2.3 Calculations
Table 4 shows the average elongation of treated seg-
ments expressed as percentage of the control. Average
elongation (^SE) of the control pea stem was
0É62(^0É04) mm.
2.3.1 Observed activity
From the lettuce assay, the growth rates were calculated
as a percentage of the averaged length of treated plant
hypocotyls and roots of those of the control. Activities
(L) were classiÐed in three categories for the ALS
method:
2.2.3 Stomatal conductance bioassay
This bioassay was used to detect abscisic acid (ABA)-
type activities.13 French bean seeds (Phaseolus vulgaris
L. cv. Tendergreen) were sown in 1-litre plastic pots in
perlite ] vermiculite (2 ] 1 by mass). The plants were
grown for 15 days under a 16 : 8 h light : dark regime
with day/night temperature and relative humidity of
28/20¡C and 60/80%, respectively, in a controlled
environmental chamber. The light was provided by a
combination of incandescent bulbs and cool-white Ñuo-
rescent lamps (Sylvania VHO), with a light intensity of
400 kmol m~2 s~1 active radiation. The plants were
watered with Hoagland solution.14
Activity class 2 for active inhibitory compounds,
when the percentage change at a concentration of
0É1 mg litre~1 was statistically signiÐcant compared to
that of the control.
Activity class 1 for moderately active inhibitory com-
pounds, when percentage change compared to that of
the control was only statistically signiÐcant at 100 mg
litre~1.
Activity class 0 for inactive compounds, when no sta-
tistically signiÐcant di†erence compared to the control
was observed in the range 0É1È100 mg litre~1.
For this bioassay, the Ðrst trifoliate leaf was used. The
leaves were excised under water and immediately fed
with water for 2 h. Water was then changed for test
2.3.2 Substituent parameters
Electronic (p), hydrophobic (n) and steric (MR) substit-
uent constants serve as general descriptors for the R
1
TABLE 4
E†ect of Isothiazolo[5,4-b]pyridin-3(2H)-ones, on Elongation of Pea Stem Segments
Average elongation of stem (% of control)a
Structure
Concentration (mg litre~1)
Compound
R
R
n
0É01
1
100
1
2
AIA
5
131*
122
106
102
100
99
171*
107
115
107
121
88
240*
71*
93
93
109
91
CH
CH
H
(CH ) CO C H
0
0
0
0
2
2
2
3
3
2 2
2 2
5
5
5
5
5
5
5
7
(CH ) CO C H
2 4 2 2
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 4 2 2
(CH ) CO C H
2 2 2 2
8
17
25
27
28
C H
6
5
CH
3
CH
3
118
101
126*
97
92
93
H
a * \ di†erence from control statistically signiÐcant at 5% level.

154
Maria J. Gil et al.
TABLE 5
Structural Features, Inhibitory Activity (L) and ALS Recognition (L )and Prediction (L )Analysis of Ethyl 2,3-
rec pred
Dihydro-3-oxoisothiazolo[5,4-b]pyridine-2-alkanoate Derivatives
Compound
R
R
p
n
MR
I
I
L
L
L
1
2
e
p
rec
pred
2
3
CH
CH
CH
CH
CH
CH
H
NO
NO
NO
NO
NO
NO
H
[0É17
[0É17
[0É17
[0É17
[0É17
[0É17
0É00
0É78
0É78
0É78
0É78
0É78
0É78
[0É01
[0É01
[0É01
[0É66
0É56
0É56
0É56
0É56
0É56
5É7
5É7
5É7
5É7
5É7
5É7
1É0
7É4
7É4
7É4
7É4
7É4
7É4
25É4
25É4
25É4
5É4
0
0
0
1
1
2
1
2
0
0
0
0
0
0
0
0
0
0
0
0
1
1
2
1
2
1
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
3
3
3
3
3
3
CH CO C H
2 2
CH(CH )CO C H
2 2
1
1
1
1
1
1
0
1
1
1
1
1
0
1
1
1
1
1
0
1
0
0
0
1
1
0
1
0
0
1
0
0
2
5
4
3
5
5
5
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 3 2 2
(CH ) CO C H
2 4 2 2
(CH ) CO C H
2 2 2 2
5
5
5
5
6
7
0É56
0É00
8
9
H
[0É28
[0É28
[0É28
[0É28
[0É28
[0É28
1É96
1É96
1É96
[1É23
2
2
2
2
2
2
10
11
12
13
14
15
16
17
18
CH CO C H
2 2
CH(CH )CO C H
2 2
2
5
3
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 3 2 2
(CH ) CO C H
2 4 2 2
5
5
5
C H
H
6
5
5
5
C H
6
CH CO C H
2 2
(CH ) CO C H
2 2 2 2
(CH ) CO C H
2 2 2 2
2
5
C H
6
5
5
NH
2
physicochemical properties. Structural modiÐcations on
substituents in position 2 of the isothiazolo[5,4-b]
pyridin-3(2H)-one system were represented by two
presenceÈabsence independent variables I and I : I \
where L is the discriminant score for the classiÐcation,
x (k \ 1, 2, . . . p) is the kth descriptor for the structure
k
and w (k \ 1, 2, . . . p) is the weight coefficient. The
k
value of w is determined by the least squares adapta-
e
p
e
k
1, when the substituent has an ester group; I \ 1, when
tion using the starting score a (j \ 1, 2, . . . m in the “mÏ
p
j
the alkanoate has an even number of carbon atoms
group case) and the correction term C . In this study
i
measured from heterocyclic system to ester group.
the algorithm was carried out using the computer
Additionally, an indicator variable (I ) which shows the
number of oxygen atoms attached in position 1, was
program Data Desk.17
n
Moriguchi and coworkers10 proposed a “riditÏ as a
standard numerical score for ordered categories. The
choice of ridit as the numerical score is based on the
assumption that only the potency order groups are reli-
able, i.e. quantitative di†erences in potency between the
di†erent groups and between the di†erent compounds
within a group are uncertain in the data to be analysed.
It is deÐned in eqn (2)
analysed. HammettÏs constant8 (p), hydrophobic
character9 (n) and molar refractivity (MR)15 were taken
from the literature (Table 5).
All combinations of independent variables described
in the above paragraph were tried in order to obtain the
best possible correlation. The data processing was done
on a Macintosh IIci machine. Subsequently the ALS
iteration was performed, and the best discriminant func-
tion was selected according to the Spearman rank
correlation coefficient, as will be detailed below.
C A BN D
j~1
a \ 2 n ] 2 ; n n [ 2
(2)
j
j
i
i/1
where a is the ridit for group “jÏ and n and n are the
j
i
j
2.3.3 AL S method
size of groups “iÏ and “jÏ respectively. From eqn (2) the
The ALS method includes an error-correcting feedback
algorithm and the details have been described else-
where.10,16 The equation (discriminant function) is for-
mulated by a feedback adaptation procedure in a linear
form as eqn (1)
mean value of a over “nÏ compounds becomes zero and
j
a \ [1 and a \ ]1 for two groups of the same size.
1
2
The procedure begins with the setting of forcing
factors S , which are taken to be S \ a . By use of S in
i
i
j
i
place of L in eqn (1), the ordinary least-squares estimate
w (k \ 1, 2, . . . p) is used as the initial weight vector.
k
L \ w ] w x ] w x ] É É É ] w x
1 1 2 2 p p
(1)
Then L for each substance is calculated from eqn (1).
0
i

New isothiazolo[5,4-b] pyridin-3(2H)-one derivative as herbicides
155
All substances are classiÐed on the basis of the values of
anoates (3È7) showed statistically signiÐcant inhibition
of L . sativa growth at 100 mg litre~1. The ester group
seems to be necessary to produce activity, so the 5-
methyl-2-unsubstituted compound (2) was inactive. The
3-oxoisothiazolo[5,4-b]pyridine nucleus is also neces-
sary for inhibitory activity; thus 3-pyridinecarboxamide
(29), obtained by reduction of the 5-methyl-2-propi-
onate derivative (5), was inactive. Chain length of the
ethyl alkanoate was also another important inhibitory
factor. Ethyl 5-methyl-2-propionate (5) and valerate (7)
derivatives inhibited lettuce growth at 0É1 mg litre~1.
However, ethyl 5-methyl-2-acetate (3) and butyrate (6)
derivatives lost activity at 10 mg litre~1. Ethyl a-2,3-
dihydro-3-oxoisothiazolo[5,4-b]pyridine-2-propionate
(4) was also inactive at 10 mg litre~1, and its behaviour
was more like that of acetate (3) than b-propionate (5).
On the other hand, ethyl 2,3-dihydro-5-methyl-3-
oxoisothiazolo[5,4-b]pyridin-2-alkanoate 1-oxides (19
and 20) and 1,1-dioxides (24È27) di†ered notably from
the unoxidized analogues (3, 5È7): only ethyl propionate
(20 and 25) and valerate (27) derivatives inhibited
lettuce growth at 100 mg litre~1, and their resulting
inhibition was more dependent on the concentration
than in the case of 5 and 7. The most active of the 1,1-
dioxides was the ethyl propionate (27), whereas, for
unoxidized derivatives, it was the ethyl valerate (7).
L and the cuto† point by b \ 0É5(a ] a ) as follows:
i
j
j
j`1
if L O b assigns the ith substance to class 0; if b \ L
i
1
1
i
O b , assigns ith the substance to class 1 and if L [
2
i
b , assigns the ith substance to class 2.
2
At iteration 2 and thereafter, the forcing factor S is
i
adapted as S \ L (when the ith substance is correctly
i
i
classiÐed) or S \ L [ C (when misclassiÐed), where
i
i
i
the sign “[Ï is chosen to correspond with S [ L . The
i
i
correction term C for the misclassiÐed compound “iÏ at
i
each iteration is given as eqn (3), where the b cuto†
point is nearer to L .16 From the newly adapted S , the
least-squares estimate of w is computed and the new L
j
i
i
k
i
is calculated from eqn (1).
C \ 0É1/[0É1 ] (0É45 ] o L [ b o)2]
(3)
i
i
j
The adaptation is repeated until all substances are cor-
rectly classiÐed or repeated a maximum of 15 times, and
the best discriminant function is selected.
The results of the ALS calculation were validated by
the leave-one-out prediction. The measure of the predic-
tive ability is obtained by leaving out one compound
and using the remaining compounds as the training set.
The discriminant function developed from the training
set is used to predict the potency class of the compound
left out. This procedure is continued until each com-
pound of the data set has been left out of the training
set once. The predictive results were given as the mis-
classiÐed number and the Spearman rank correlation
coefficient for the overall leave-one-out classiÐcation.
The above qualitative conclusions were reÐned by
means of a QSAR analysis. When the previous ALS dis-
criminant method was applied on tested 3-
oxoisothiazolo[5,4-b]pyridine derivatives (2È27, Table
3), the oxidation indicator variable I was not signiÐ-
n
cant statistically. Thus 1-oxides and 1,1-dioxides were
discarded for the Ðnal analysis to prevent statistical
redundancy. Compounds analysed by ALS are listed in
Table 5 along with descriptors and activity ratings
(Section 2.3). The resulting discriminant function is
expressed as eqn (4), where the Ðgure in parentheses
beside coefficients is the contribution index
( o coef o ] SD of descriptor),10 “nÏ stands for the number
3
RESULTS AND DISCUSSION
Isothiazolo[5,4-b]pyridin-3(2H)-ones (2È17) were syn-
thesized in a single step by reacting 2-chlorothio-3-pyri-
dinecarbonyl chloride derivatives1 (1) with amines as
described in Fig. 3. The 5-amino derivative 18 was
obtained by reduction of compound 12 with iron. The
oxidation of isothiazolo[5,4-b]pyridin-3(2H)-ones (3,
5È8) to their 1-oxides (19È23) was accomplished with 1É5
of compounds, “n Ï is the number misclassiÐed and the
mis
Ðgure in parentheses after the value of “n Ï is the
mis
number misclassiÐed by two categories and “R Ï is the
s
equivalents of KHSO in the form of oxone} dissolved
in 50% aqueous methanol at 20¡C, giving good yields
Spearman rank coefficient correlation.18
5
(B90%) in 1 h. Isothiazolo[5,4-b]pyridin-3(2H)-one 1,1-
dioxides (24È28) were prepared in a single pot from
isothiazolo[5,4-b]pyridin-3(2H)-ones (3, 5È8). When
compounds 3, 5È8 had been reacted with 98% m-CPBA
(1 : 2É2 equiv.) in dichloromethane, the subsequent addi-
tion of aqueous sodium hydroxide (1 : 2É1 equiv) and
TBAB (cat) to the reaction mixture at room tem-
perature gave 1,1-dioxides (24È28) in good yields
(B85%). This procedure gave similar yields and was
simpler than the two-pot oxone}/NaOCl method
recently reported.11
L \ 0É392 ] 1É516(1É367)n [ 0É142(1É100)MR
] 1É224(0É481)I [ 0É454(0É230)I
(4)
e
p
n\17 recognition: n \1 (0), R \0É907 (p\0É000)
mis
s
prediction: n \4 (0), R \0É760 (p\0É002)
mis
s
On the basis of eqn (4) for the inhibition of lettuce
growth of compounds 2È18, hydrophobic (n [ 0) and
small (MR B 1) R substituents will have favourable
1
e†ects on activity. Also eqn (4) conÐrms that an ester
As seen from Table 3, only 5-methyl derivatives of
ethyl 2,3-dihydro-3-oxoisothiazolo[5,4-b]pyridin-2-alk-
group (I \ 1) and a chain with an odd number of
e
carbons (I \ 0) are preferred for inhibition. In this
p

156
Maria J. Gil et al.
pyridine-2-acetate. Heterocycles, 38 (1994) 333È44 and ref-
erences cited therein.
2. Rainey, J. L. & Seidel, M. C., US Patent 3965 107, 1976.
3. Sybyl 6.1 molecular modelling program (1994). Tripos
Inc., 1699 S. Hanley Rd, St. Louis, Missouri 63144È2913
(USA).
4. Edgerton, M. D., Tropsha, A. & Jones, A. M., Modelling
the auxin-binding site of auxin binding protein I of maize.
Phytochemistry, 35 (1994) 1111È23.
5. Ricci, D., Maggiali, C. C., Morini, G. & Ronchini, F.,
Anti-auxin e†ects of 3-oxo-1,2-benzisothiazolin-2-yl alka-
noic acids. Phytochemistry, 29 (1990) 2787È91.
6. Hedge, S. G. & Mahoney, M. D., Synthesis and herbicidal
activity of 5-(haloalkyl)-substituted thiazolo[4,5-b]
pyridine-3(2H)-acetic acid derivatives. J. Agric. Food
Chem., 41 (1993) 2131È4.
7. Yoshikawa, H., Fujimoto, E. & Doi, K., Synthesis and
biological activity of benzaldehyde O-alkoximes as absci-
sic acid mimics. Biosci. Biotech. Biochem., 56 (1992) 256È
60.
8. Bowden, K., Electronic e†ects in drugs. In Comprehensive
Medicinal Chemistry, Vol. 4, ed. C. Hansch. Pergamon
Press, Oxford, 1990, pp. 205È40.
9. Taylor, P. J., Hydrophobic properties of drugs. In Com-
prehensive Medicinal Chemistry, Vol. 4, ed. C. Hansch.
Pergamon Press, Oxford, 1990, pp. 241È94.
10. Moriguchi, I., Komatsu, K. & Matsushita, Y., Adaptive
least-squares method applied to structureÈactivity corre-
lation of hypotensive N-alkyl-NA-cyano-N@-pyridylguani-
dines. J. Med. Chem., 23 (1980) 20È6.
equation the scores were a \ [ 0É5882, a \ 1É1765,
1
2
a \ 1É7647, and cuto† points were b \ 0É2941 and
3
1
b \ 1É4706. The recognition of eqn (4) was very good
2
(n \ 1) and leave-one-out prediction gave a good sig-
mis
niÐcance level of 0É2%. MisclassiÐed compounds were
by only one category (Table 5). Oxidized compounds,
which were not included in the analysis, also followed
the tendency suggested by eqn (4).
The most active compounds in the lettuce test, the
5-methyl 2-propionate (5) and 2-valerate (7) derivatives,
the corresponding 1,1-dioxides (25 and 27), as well as
the related 5-H derivatives (8 and 28) and the
unoxidized 5-phenyl-2-propionate (17) derivative were
investigated on cell elongation in pea stem bioassay in
order to ascertain potential auxinic activity. Table 4
shows that pea segments have a high degree of response
to auxin (AIA) giving a large increase in length at
0É01 mg litre~1 or higher concentrations. Tested com-
pounds did not show auxin-like activity from 0É01 to
100 mg litre~1. Weak or marginal elongation was
observed at ¹ 1 mg litre~1. Additionally, compound 5,
analogous to the ABA-mimic displayed in Fig. 2, was
assayed by the stomatal conductance test and compared
with the ABA e†ect. No signiÐcant di†erences were
observed between the control and compound 5 in the
0É1 to 100 mg litre~1 range, although ABA increased
stomatal resistance six-fold at 100 mg litre~1.
11. Martinez-Merino, V., Gil, M. J., Zabalza, J. M. & Gonza-
lez, A., Synthesis of 2-substituted isothiazolo[5,4-b]
pyridin-3(2H)-one 1,1-dioxides. Heterocycles, 41 (1995)
2737È44.
Unlike
3-oxo-1,2-benzisothiazole
anti-auxin
analogues5 ethyl 2,3-dihydro-3-oxoisothiazolo[5,4-b]
pyridin-2-alkanoate derivatives inhibited root growth,
and their activity depended on the number of carbon
atoms in the alkyl chain. The absence of auxin or anti-
auxin activities in ethyl 2,3-dihydro-3-oxoisothiazolo[5,
4-b]pyridin-2-alkanoate derivatives provides new struc-
tural features to reÐne existing models of receptors.4,19
The Ðndings described above indicate that derivatives 7
and 25 can be leads to a promising series of herbicidal
compounds despite this activity not being related to the
auxinic mode of action.
12. McDougall, G.
& Fry, S. C., Inhibition of auxin-
stimulated growth of pea stem segments by a speciÐc non-
asaccharide of xyloglucan. Planta, 175 (1988) 412È16.
13. MansÐeld, T. A., Hormones as regulators of water
balance. In Plant Hormones and their role in plant growth
and development, ed. P. J. Davies. Martinus Nijho† Publi-
shers, New York, 1987, pp. 411È30.
14. Arnon, D. I. & Hoagland, D. R., Crop production in arti-
Ðcial solutions and in soils with special reference to factors
inÑuencing yields and absorption of inorganic nutrients.
Soil Sci., 50 (1940) 463È85.
15. Silipo, C. & Vittoria, A., Three-dimensional structure of
drugs. In Comprehensive Medicinal Chemistry, Vol. 4, ed.
C. Hansch. Pergamon Press, Oxford, 1990, pp. 153È204.
16. Kawashima, Y., Amaruma, F., Sato, M., Okuyama, S.,
Nakashima, Y., Sota, K. & Moriguchi, I., StructureÈ
ACKNOWLEDGEMENTS
activity
studies
of
4,6-disubstituted
2-
(morpholinocarbonyl)furo[3,2-b]indol derivatives with
analgesic and antiinÑammatory activities. J. Med. Chem.,
29 (1986) 2284È90.
Financial support from National Plan of R&D of Spain
through project AGR 90-0892 is gratefully acknow-
ledged. We are also indebted to “Navarra GovernmentÏ
for an award (O.F. 516/92) to one of us (M.A. Manu).
17. Data Desk} Professional 2.0, Odesta Corporation, North-
brook IL, USA, 1988.
18. Koch, G., Carr, G., Amara, I., Stokes, M. & Uryniak, T.,
Categorical data analysis. In Statistical Methodology in
the Pharmaceutical Sciences, ed. D. Berry. Marcel Dekker
Inc., New York, 1990, pp. 410È12.
19. Katekar, G. F., Auxins: On the nature of the receptor site
and molecular requirements for auxin activity. Phyto-
chemistry, 18 (1979) 223È33.
REFERENCES
1. Martinez-Merino, V., Gil, M. J., Gonzalez, A., Zabalza, J.
M., Navarro, J. & Manu, M. A., New 5-substituted deriv-
atives of ethyl 2,3-dihydro-3-oxoisothiazolo[5,4-b]