EP0589947A1

EP0589947A1 - Sulphonamide herbicides

Info

Publication number: EP0589947A1
Application number: EP92911736A
Authority: EP
Inventors: David Phillip Astles; Alasdair Mcarthur; Trevor William Newton
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1991-06-11
Filing date: 1992-06-10
Publication date: 1994-04-06
Also published as: WO1992022538A1; JPH06507891A; GB9112566D0

Abstract

Composés répondant à la formule (I), dans laquelle R1 et R2 représentent, indépendamment l'un de l'autre, halogène, alkyle, alcoxy, haloalkyle, haloalcoxy, alkylamino ou dialkylamino; R3 représente alkyle ou aryle ou amino éventuellement substitué; X représente oxygène ou soufre; Y représente CH, azote ou soufre; et n est 2 ou 3; leur préparation et leur utilisation à titre de désherbants.Compounds corresponding to the formula (I), in which R1 and R2 represent, independently of one another, halogen, alkyl, alkoxy, haloalkyl, haloalkoxy, alkylamino or dialkylamino; R3 represents alkyl or aryl or optionally substituted amino; X represents oxygen or sulfur; Y represents CH, nitrogen or sulphur; and n is 2 or 3; their preparation and their use as weedkillers.

Description

SULPHONAMIDE HERBICIDES

The present invention relates to sulphonamide herbicides, their preparation and their use.

An extensive range of pyrimidinyl-oxy and thio-benzoic acid derivatives are disclosed as herbicides and plant regulants in US-4 871 387 and certain such compounds are stated, in EP-B-O 223 406, to have an increased action against certain perennial grass weeds compared with a known pyrimidine-based herbicidal compound.

It has now been found that sulphonamide

derivatives of the known pyrimidine-benzoic acid compounds of EP-B-O 223 406 show a significantly greater degree of wheat selectivity both pre- and post- emergence than a representative pyrimidine-benzoic acid derivative specifically disclosed in EP-B-O 223 406.

The present invention provides a compound of the general formula

in which R¹ and R² independently represent a halogen atom or an alkyl, alkoxy, haloalkyl, haloalkoxy, alkylamino or dialkylamino group;

R³ represents an alkyl group or an optionally

substituted aryl or amino group; X represents an oxygen or sulphur atom; Y represents a group CH or a nitrogen or sulphur atom; and n is 2 or 3; or a salt thereof.

An alkyl group as a substituent or as part of a substituent may be a straight chain or branched chain group, and suitably has, for example, up to 12 carbon atoms. Preferably such an alkyl group has up to 6 carbon atoms, especially up to 4 carbon atoms. As a substituent of another group, an alkyl group suitably has 1 to 3 carbon atoms.

Halogen is fluorine, chlorine, bromine or iodine. Haloalkyl and haloalkoxy is preferably mono-, di- or tri-fluoro-alkyl or -alkoxy, especially

trifluoromethyl or trifluoromethoxy.

An aryl group is preferably a phenyl group.

Where substituents are present in the R aryl group, the substituent groups may be any of those customarily employed in the modification and/or development of pesticidal compounds and are especially substituents that maintain or enhance the herbicidal activity associated with the compounds of the present invention, or influence persistence of action, soil or plant penetration, or any other desirable property for herbicidal compounds. There may be one or more of the same or different substituents present.

Suitable examples of optional substituents for aryl or amino group R³ include halogen atoms,

especially fluorine, chlorine or bromine atoms; nitro groups; cyano groups; free or modified hydroxy groups; free or modified carboxy groups, including alkylamide or dialkylamide groups; alkyl groups unsubstituted or substituted by one or more halogen atoms, especially fluorine atoms; and aryl groups, suitably phenyl groups, unsubstituted or substituted by one or more halogen atoms, especially chlorine atoms, or nitro, cyano or haloalkyl groups. Modified hydroxy and carboxy groups are to be understood to be groups derived from hydroxy or carboxy groups by, for example, etherification, esterificiation, acylation, amidation or salt formation, as appropriate for the group in question.

R¹ and R² may be different, for example one of R¹ and R² may be an alkoxy group, eg. methoxy, and the other a halogen atom, e.g. chlorine, or one may be an alkylamino group, e.g. methylamino, and the other an alkyl group, e.g. methyl. Preferably, however, either, R¹ and R² are the same and each represents a C_1-4 alkyl, for example methyl, or C_1-4 alkoxy, especially methoxy, group, or R¹ represents a C_1-4 alkoxy, especially methoxy, group and R² represents a halogen, e.g. chlorine, atom.

R³ is preferably a C_1-6 alkyl, for example methyl or ethyl, group, a phenyl group optionally substituted by one or more halogen atoms, especially a chlorine atom, or by one or more nitro groups, or an amino group substituted by one or more C_1-6 alkyl or phenyl groups.

Salt formation can occur at the -NH- of the sulphonamide group of formula I. Suitable salts are agrochemically acceptable salts and include alkali metal, e.g. sodium, salts and ammonium, e.g.

tetrabutylammonium, salts.

The present invention further provides a process for the preparation of the compounds of the present invention, which comprises (a) reacting an acid of the general formula

in which R¹, R², X, Y and n are as defined above, or a corresponding ester, acid chloride or acid anhydride,

with a sulphonamide of the general formula

R³SO₂NH₂ (III) in which R³ is as hereinbefore defined, or a salt thereof, in the presence of a carboxyl-activating agent, if appropriate,

or

(b) reacting a compound of the general formula

in which R¹ and R² are as previously defined and

L¹ represents a leaving group, with a di-salt of a compound of the general formula

in which R³, Y and n are as previously defined and L² represents a group XH where X is as previously defined,

and, if required or desired, converting a resulting compound into another compound of the invention.

A leaving group is any group that will, under the reaction conditions, cleave from the starting material thus promoting reaction at a specified site.

The leaving group in a compound of general formula IV is conveniently a halogen atom, for example a bromine, chlorine or iodine atom, or an

alkanesulphonyl group, for example methanesulphonyl.

A di-salt of compound V is suitably a di(alkali metal) salt, preferably a di-sodium salt.

Compounds of formulae II and III are either known or preparable by conventional methods. The acids of formula II are known from or can be prepared by the methods disclosed in, for example, EP-B-O 223 406.

The sulphonamides of formula III are well known or easily preparable by standard techniques; benzene sulphonamide, for example, features in Beilstein

11,39.

The starting pyrimidines of general formulae IV may be prepared by conventional techniques, for example those described in Heterocyclic compounds, 16 "The Pyrimidines", edited by D.J. Brown, Interscience, 1962.

Compounds of general formula V may suitably be prepared from salicylic acid, or a suitable derivative thereof, for example salicylamide, by conventional techniques.

Process (a) of the invention is suitably carried out at ambient or elevated temperature, i.e. at a temperature above 20°C. A preferred temperature range in which to carry out the reaction is from 20ºC to 80°C; an especially suitable reaction temperature is in the range of from 20°C to 50°C. The molar ratio of reactant II to reactant III may, for example, be in the range of from 1.0 to 5.0 preferably from 1.0 to 2.5.

The reaction (a) is suitably carried out in an inert organic solvent such as a hydrocarbon solvent, e.g. benzene or toluene, a chlorinated hydrocarbon, e.g. dichloromethane or chloroform, an alcohol, e.g. methanol or ethanol, an ether, e.g. diethyl ether, tetrahydrofuran or 1,4-dioxane, a ketone, e.g. acetone or methyl ethyl ketone, an ester, e.g. ethyl acetate, an aprotic polar solvent, e.g. dimethylformamide, dimethylacetamide or dimethylsulphoxide, or a nitrile, e.g. acetonitrile.

Preferably, the reaction is carried out in the presence of a tertiary amine, for example

triethylamine. Other suitable tertiary amines include pyridine and 1,8-diazabicyclo[5.4.0]undec-7-ene.

When the reactant II is in the form of a free carboxylic acid, the carboxy group needs to be

activated for the reaction to proceed. Suitable carboxyl-activating agents include 2-chloro-N-methyl Pyridinium iodide, dicyclohexylcarbodiimide and carbonyldiimidazole. Suitably the acid reactant II is activated by the carboxyl-activating agent in the presence of an inert organic solvent at ambient or elevated temperature, for example at a temperature in the range of from 20°C to the reflux temperature of the mixture, prior to the addition of reactant III and, if desired, the tertiary amine.

Process (b) is suitably carried out at a

temperature in the range of from ambient to the reflux temperature of the reaction medium, preferably in the range of from 100 to 150ºC, for example at 120ºC. The molar ratio of the reactants IV:V is suitably in the range of from 1.0 to 2.5.

In reaction (b) the di-salt may suitably be prepared from a compound V in which L² is a hydroxy group by the action of an alkali metal, such as metallic sodium or potassium, or, conveniently, a strong base, for example, an alkali metal hydride, such as sodium or potassium hydride, an alkaline earth metal hydride, such as calcium hydride, an alkali metal alkoxide, such as potassium t-butoxide, or an alkali metal hydroxide, such as sodium or potassium hydroxide. Suitably conversion of a hydroxy compound V to the di-salt occurs in situ.

Suitably, the reaction (b) is carried out in the presence of a solvent; typical solvents are, for example, the same as noted above for process (a).

A compound of general formula I obtained by either of the methods (a) or (b) may be converted to a further compound of general formula I by methods known to a man skilled in the art, provided that suitable care is taken to ensure that the sulphonamide group is not affected. Thus for example, a compound of general formula I where R¹ and/or R² represents a halogen atom, suitably chlorine, may be transformed into other derivatives by nucleophilic displacement, for example by reaction with two equivalents of an amine, such as dimethylamine, to give the corresponding compound of general formula I in which R¹ and/or R² represents a substituted amino group. Acid and salt conversion reactions may be carried out using conventional techniques as appropriate.

The prepared compounds of formula I may, if desired, be isolated and purified using conventional techniques.

Compounds of the general formula I have been found to have interesting activity as herbicides having a wide range of pre- and post-emergence

activity against undesirable species.

The present invention therefore provides a herbicidal composition which comprises a compound of the present invention in association with a carrier.

The present invention additionally encompasses the preparation of such a herbicidal composition by the process of bringing a carrier into association with a compound of the present invention.

Preferably there are at least two carriers in a composition of the present invention, at least one of which is a surface-active agent.

The present invention further provides the use of a compound according to the invention as a herbicide.

Further, in accordance with the invention there is provided a method of combating undesired plant growth at a locus by treating the locus with a

composition or compound according to the invention. The locus may, for example, be the soil or plants in a crop area; typical crops being cereals, for example wheat. The compounds of the invention are especially suited for the control of broad-leaf weeds in cereal crops. Application to the locus may be pre-emergence or post-emergence. The dosage of active ingredient used may, for example, be in the range of from 0.01 to 10kg/ha, preferably 0.1 to 1kg/ha.

A carrier in a composition according to the invention is any material with which the active ingredient is formulated to facilitate application to the locus to be treated, which may for example be a plant, seed or soil, or to facilitate storage, transport or handling. A carrier may be a solid or a liquid, including a material which is normally gaseous but which has been compressed to form a liquid, and any of the carriers normally used in formulating herbicidal compositions may be used. Preferably compositions according to the invention contain 0.5 to 95% by weight of active ingredient.

Suitable solid carriers include natural and synthetic clays and silicates, for example natural silicas such as diatomaceous earths; magnesium

silicates, for example talcs; magnesium aluminium silicates, for example attapulgites and vermiculites; aluminium silicates, for example kaolinites,

montmorillonites and micas; calcium carbonate; calcium sulphate; ammonium sulphate; synthetic hydrated silicon oxides and synthetic calcium or aluminium silicates; elements, for example carbon and sulphur; natural and synthetic resins, for example coumarone resins, polyvinyl chloride, and styrene polymers and copolymers; solid polychlorophenols; bitumen; waxes; and solid fertilisers, for example superphosphates.

Suitable liquid carriers include water; alcohols, for example isopropanol and glycols; ketones, for example acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; ethers; aromatic or

araliphatic hydrocarbons, for example benzene, toluene and xylene; petroleum fractions, for example kerosine and light mineral oils; chlorinated hydrocarbons, for example carbon tetrachloride, perchloroethylene and trichloroethane. Mixtures of different liquids are often suitable. Agricultural compositions are often formulated and transported in a concentrated form which is subsequently diluted by the user before application. The presence of small amounts of a carrier which is a surface-active agent facilitates this process of dilution. Thus preferably at least one carrier in a composition according to the invention is a

surface-active agent. For example the composition may contain at least two carriers, at least one of which is a surface-active agent.

A surface-active agent may be an emulsifying agent, a dispersing agent or a wetting agent; it may be nonionic or ionic. Examples of suitable

surface-active agents include the sodium or calcium salts of polyacrylic acids and lignin sulphonic acids; the condensation of fatty acids or aliphatic amines or amides containing at least 12 carbon atoms in the molecule with ethylene oxide and/or propylene oxide; fatty acid esters of glycerol, sorbitol, sucrose or pentaerythritol; condensates of these with ethylene oxide and/or propylene oxide; condensation products of fatty alcohol or alkyl phenols, for example

p-octylphenol or p-octylcresol, with ethylene oxide and/or propylene oxide; sulphates or sulphonates of these condensation products; alkali or alkaline earth metal salts, preferably sodium salts, of sulphuric or sulphonic acid esters containing at least 10 carbon atoms in the molecule, for example sodium lauryl sulphate, sodium secondary alkyl sulphates, sodium salts of sulphonated castor oil, and sodium alkylaryl sulphonates such as dodecylbenzene sulphonate; and polymers of ethylene oxide and copolymers of ethylene oxide and propylene oxide.

The herbicidal composition of the invention may also contain other active ingredients, for example compounds possessing, insecticidal or fungicidal properties, or other herbicides.

The following Examples illustrate the invention. The structures of the compounds of the invention prepared in the following Examples were confirmed by mass spectrometry and NMR.

Example 1

2-(4,6-dimethoxypyrimidin-2-yl)oxy-N-benzenesulphonyl benzamide

A solution of 2-(4,6-dimethoxypyrimidin-2-yl)oxybenzoic acid (1.0g, 3.6 mmol) in tetrahydrofuran (10ml) was added dropwise to a solution of carbonyl diimidazole (0.59g, 3.6 mmol) in tetrahydrofuran (15 ml). The mixture was stirred for 30 minutes at room temperature and then refluxed for 30 minutes. Benzene sulphonamide (0.55g, 3.6 mmol) was added to the cooled mixture followed by a solution of

diazabicyclo[5.4.0]undec-7-ene (0.54g, 3.6 mmol) in tetrahydrofuran (10 ml), which was added dropwise. After stirring the mixture overnight, the

tetrahydrofuran was evaporated in vacuo, water (20 ml) was added and the solution was acidified to pH2 using dilute hydrochloric acid. The crude product was extracted into chloroform (2× 20 ml) and the organic layer was dried (using sodium sulphate) and evaporated in vacuo. Flash chromatography of the crude product, eluting with a mixture of chloroform (95%) and

methanol (5%) gave the product,

2-(4,6-dimethoxypyrimidin-2-yl) oxy-N-benzenesulphonyl benzamide (0.65g, 44%) as colourless crystals of melting point 107°C.

Examples 2 to 25

Following procedures similar to that described in Example 1, further examples of compounds of the invention were prepared. Details of the compounds are given in Table I below, in which the compounds are identified by reference to the substituents of the following formula.

Table I

Melting Ex R¹ R² R³ X Y n Point

(ºC)

2 OCH₃ OCH₃ CH₃ O CH 3 139

3 OCH₃ OCH₃ 2-Cl-phenyl O CH 3 154

4 OCH₃ OCH₃ N(CH₃)₂ O CH 3 114-115

5 OCH₃ OCH₃ N(CH₃)₂ O N 3 142-146

6 OCH₃ OCH₃ NHphenyl O CH 3 174

7 CH₃ CH₃ N(CH₃)₂ O CH 3 156-158

8 OCH₃ OCH₃ C₂H₅ O CH 3 114-116

9 OCH₃ OCH₃ CH₃ O N 3 141-144

10 OCH₃ Cl CH, O CH 3 155-158

11 CH₃ CH₃ phenyl O CH 3 207-210

12 OCH₃ OCH₃ NHCH₃ O CH 3 169-173

13 CH₃ CH, NHCH₃ O CH 3 154-155

14 OCH₃ OCH₃ 2-NO₂-phenyl O CH 3 119-123

15 CH₃ CH, CH, O CH 3 *

16 OCH₃ OCH₃ NHCH₃ O N 3 123-126

17 OCH₃ OCH₃ CH₃ S CH 3 123-124

18 OCH₃ OCH₃ N(CH₃)₂ S CH 3 119-122

19 OCH₃ OCH₃ phenyl S CH 3 138-145 Table 1 (cont'd)

Melting

Ex R¹ R² R³ X Y n Point

(°C)

20 OCH₃ OCH₃ phenyl O N 3 163-167

21 OCH₃ OCH₃ NHnC₃H₇ O N 3 134-136

22 OCH₃ OCH₃ NHiC₃H₇ O N 3 126-130

23 OCH₃ OCH₃ NHC₂H₅ O N 3 99-104

24 OCH₃ OCH₃ CH₃ O S 2 115-123

25 OCH₃ OCH₃ N(CH₃)₂ O S 2 154-157

Example 26

Herbicidal Activity

To evaluate their herbicidal activity, compounds according to the invention were tested using as representative range of plants: maize, Zea mays (Mz); rice, Oryza sativa (R); barnyard grass, Echinochloa crusgalli (BG); oat, Avena sativa (O); linseed, Linum usitatissimum (L); mustard, Sinapsis alba (M); sugar beet, Beta vulgaris (SB) and soya bean, Glycine max (S).

The tests fall into two categories, pre-emergence and post-emergence. The pre-emergence tests involved spraying a liquid formulation of the compound onto the soil in which the seeds of the plant species mentioned above had recently been sown. The post-emergence tests involved two types of test, viz., soil drench and foliar spray tests. In the soil drench tests the soil in which the seedling plants of the above species were growing was drenched with a liquid formulation containing a compound of the invention, and in the foliar spray tests the seedling plants were sprayed with such a formulation. The soil used in the tests was a prepared horticultural loam.

The formulations used in the tests were prepared from solutions of the test compounds in acetone containing 0.4% by weight of an alkylphenol/ethylene oxide condensate available under the trade mark TRITON X-155. These acetone solutions were diluted with water and the resulting formulations applied at dosage levels corresponding to 5 kg or 1 kg of active

material per hectare in a volume equivalent to 600 litres per hectare in the soil spray and foliar spray test, and at a dosage of level equivalent to 10 kilograms of active material per hectare in a volume equivalent to approximately 3,000 litres per hectare in the soil drench tests.

In the pre-emergence tests untreated sown soil and in the post-emergence tests untreated soil bearing seedling plants were used as controls.

The herbicidal effects of the test compounds were assessed visually twelve days after spraying the foliage and the soil, and thirteen days after

drenching the soil and were recorded on a 0-9 scale. A rating 0 indicates growth as untreated control, a rating 9 indicates death. An increase of 1 unit on the linear scale approximates to a 10% increase in the level of effect.

The results of the tests are set out in Table II below, in which the compounds are identified by reference to the preceding examples. Absence of a numeral in the following table indicates a zero rating; an asterisk indicates that no result was obtained.

Example 27

Herbicidal Selectivity

A further series of biological evaluations were carried out to investigate the selectivity of a compound of the invention, and to compare that selectivity with a representative pyrimidine

specifically described in EP-B-O 223 406. The compounds tested were the compound of Example 1 herein and for comparison the compound designated as compound 17 of said European publication; the comparison compound being designated hereafter as compound A.

Two tests were conducted as spray tests, a pre-emergence test in which soil in which seeds had been sown was sprayed and a post-emergence foliar spray test in which seedling plants were sprayed with the test compounds. In each test the effect of the test compounds on wheat and on ranges of weed species commonly affecting wheat crops, was evaluated.

The test plant species for the pre-emergence test were wheat (WH), blackgrass (AM), wild oat (WO), couchgrass (CO), meadowgrass (PA), oil-seed rape (RA), cleavers (GG) , speedwell (SW), chickweed (ST), field pansy (FP), mayweed (MW), pale persicaria (PP), and bindweed (CV).

The test plant species for the foliar spray test were wheat (WH), blackgrass (AM), wild oat (WO), couchgrass (CO), meadowgrass (PA), oil-seed rape (RA), cleavers (GG), chickweed (ST), field pansy (FP), and pale persicaria (PP).

The soil used in the test was a prepared

horticultural loam.

The compounds were tested, as technical materials and formulated in a 1:1 acetone:water mix containing up to 0.2% of the wetting agent, Triton X155, and applied as single dose sprays in a total volume of 600 litres/hectare. Application was at different dosage levels in the range of from 0.001 to 5 kg/ha designed to produce a range of responses. Three replicate pots were used for each treatment. Untreated seedling plants were used as controls.

Phytotoxicity compared with the untreated control was assessed visually using the standard 0-9 scale, 0 indicating no effect and 9 indicating death, 19 days after treatment.

The results were subjected to a standard probit analysis by computer to calculate the dosage of each compound in kg/ha required to kill 50% of the weed species and to produce 10% level of effect on the crop species of wheat. These dosages are referred to as the GID₅₀ and GID₁₀ dosage respectively.

These GID₅₀ and GID₁₀ were then used to calculate (to 2 decimal places) the selectivity factors for wheat by dividing the GID₁₀ of the crop by the GID₅₀ of each weed species. The numbers indicate

selectivity between the crop and weed and the greater the value the better the selectivity.

To further facilitate comparison between the compounds tested, the selectivity data have been standardised to Compound A for each test, by dividing the selectivity factor for each weed species by the value of the selectivity factor for Compound A for that species. With the selectivity factor thus set at 1.0 for Compound A for each weed species, the

standarised selectivity factor (calculated to 1 decimal place) for the compound of Example 1 indicates the number of times greater the selectivity of the compound of the invention is compared with that of the comparison Compound A.

The results are set out in Tables III & IV below. Table III

PRE- -EMERGENCE

Selectivity Standardised

GID Values Factor Selectivity

Test (wheat) Factor Species 1 A 1 A 1 A

WH 0.80 0.001 - - - -

AM 0.20 0.007 4.00 0.14 28.6 1

WO 0.46 0.020 1.74 0.05 34.8 1

CO 0.08 0.001 10.00 1.00 10.0 1

PA 0.42 0.001 1.90 1.00 1.9 1

RA 0.65 0.040 1.23 0.03 41.0 1

GG 0.55 0.023 1.45 0.04 36.3 1

SW 0.20 0.002 4.00 0.50 8.0 1

ST 1.02 0.006 0.78 0.16 4.9 1

FP 0.04 0.001 20.00 1.00 20.0 1

MW 0.34 0.066 2.35 0.02 117.5 1

PP 0.50 0.013 1.60 0.08 20.0 1

CV 1.84 0.049 0.43 0.02 21.5 1

Table IV

POST -EMERGENCE FOLIAR SPRAY

Selectivity Standardised

GID Values Factor Selectivity

Test (Wheat) Factor Species 1 A 1 A 1 A

WH 0.90 0.002

AM 0.87 0.018 1.03 0.11 9.4 1

WO 2.00 0.030 0.45 0.07 6.4 1

CO 0.98 0.060 0.92 0.03 30.7 1

PA 0.35 0.008 2.57 0.25 10.3 1

RA 0.08 0.137 11.25 0.01 1125.0 1

GG 0.17 0.012 5.29 0.17 31.1 1

ST 1.12 0.143 0.80 0.01 80.0 1

FP 0.08 0.004 11.25 0.50 22.5 1

PP 0.48 0.034 1.88 0.06 31.3 1 From the results of Tables III and IV it can be clearly seen that the compound of the present

invention has a higher selectivity of action both pre- and post- emergence against each of the test weed species for the same level of effect on the wheat crop species, than has the comparison Compound A.

Claims

CLAIMS 1. A compound of the general formula

in which

R¹ and R² independently represent a halogen atom or an alkyl, alkoxy, haloalkyl, haloalkoxy, alkylamino or dialkylamino group;

R³ represents an alkyl group or an optionally substituted aryl or amino group; X represents an oxygen or sulphur atom; Y represents a group CH or a nitrogen or sulphur atom; and n is 2 or 3; or a salt thereof.

2. A compound as claimed in claim 1, in which R¹ and R² are the same and each represents a C_1-4 alkyl or C_1-4 alkoxy group, or R¹ represents a C_1-4 alkoxy group and R² represents a halogen atom.

3. A compound as claimed in claim 2, in which each of R¹ and R² represents a methyl or methoxy group, or R¹ represents a methoxy group and R² represents a chlorine atom.

4. A compound as claimed in any one of claims 1 to 3, in which R³ represents a C_1-6 alkyl group or a phenyl group optionally substituted by one or more halogen atoms or by one or more nitro groups, or represents an amino group substituted by one or more C_1-6 alkyl or phenyl groups.

5. A compound as claimed in claim 4, in which R³ represents a methyl, ethyl, phenyl, chlorophenyl, nitrophenyl, methylamino, ethylamino,

propylamino, phenylamino or dimethylamino group.

6. A process for the preparation of a compound as claimed in claim 1, which comprises

(a) reacting an acid of the general formula

in which R¹ and R² are as defined in claim 1, or a corresponding ester, acid chloride or acid anhydride, with a sulphonamide of the general formula

R³SO₂NH₂ (III) in which R³ is as defined in claim 1, or a salt thereof, in the presence of a carboxyl-activating agent, if appropriate,

or

(b) reacting a compound of the general formula in which R¹ and R² are as previously defined and

and, if required or desired, converting a resulting compound into another compound as claimed in claim 1.

7. A compound as claimed in claim 1, whenever

prepared by a process as claimed in claim 6.

8. A herbicidal composition which comprises a

compound as claimed in any one of claims 1 to 5 and 7, in association with a carrier.

9. A method of combating undesired plant growth at a locus which comprises treating the locus with a compound as claimed in any one of claims 1 to 5 and 7, or with a composition as claimed in claim 8.

10. The use of a compound as claimed in any one of claims 1 to 5 and 7 in combating undesired plant growth.