CA1340678C

CA1340678C - Antiparasitic avermectin derivatives

Info

Publication number: CA1340678C
Application number: CA000582721A
Authority: CA
Inventors: Shih-Jen Edward Lee; Stephen Paul Gibson; Christopher James Dutton
Original assignee: Pfizer Ltd Great Britain
Current assignee: Pfizer Ltd Great Britain
Priority date: 1988-11-10
Filing date: 1988-11-10
Publication date: 1999-07-27
Anticipated expiration: 2016-07-27

Abstract

The invention provides broad spectrum antiparasitic compounds of the formula:

(see formula I) .

The broken line at the 22-23 position represents an optional double bond wherein either R1 is H or OH and the double bond is absent, or, the double bond is present and R1 is absent. R3 is hydrogen or methyl; and R4 is H or a 4'-(alpha-L-oleandrosyl)-alpha-L-oleandrosyloxy group of the formula:

(see formula II) with the proviso that when R4 and R1 are both H and the double bond is absent, R2 is not H or CH3. Preferred values of R2 include methyl, isopropyl, sec-butyl, thiomethyl, thioethyl and 1-trifluoromethylethyl.

formula SR5 wherein R5 is C1-C8 alkyl, C2-C8 alkenyl, C2-C8 alkynyl, C3-C8 cycloalkyl, C5-C8 cycloalkenyl, phenyl or substituted phenyl wherein the substituent is C1-C4 alkyl, C1-C4 alkoxy or halo, or a 3 to 6 membered oxygen or sulphur containing heterocyclic ring which may be saturated, or fully or partially unsaturated and which may optionally be substituted by one or more C1-C4 alkyl groups or halo atoms;
R3 is hydrogen or methyl;
end R4 is H or a 4'-(alpha-L-oleandrosyl)-alpha-L-oleandrosyloxy group of the formula:
(see formula III) with the proviso that when R4 and R1 are both H and the double bond is absent, R2 is not H or CH3, are broad spectrum antiparasitic agents having utility as anthelmintics, ectoparasiticides, insecticides and acaricides.

Description

ANTIPARASITIC AGENTS
DESCRIPTION
This invention relates to antiparasitic agents and in particular to compounds related to the avermectins and milbemycins but having a novel substituent group at the 25-position and to a process for their preparation.
The avermectins are a group of broad spectrum anti-parasitic agents referred to previously as the C-076 compounds. They are produced by fermenting a strain of the microorganism Streptomyces avermitilis ATCC 31267, 31271 or 31272 under aerobic conditions in an aqueous nutrient medium containing inorganic salts and assimilable sources of carbon and nitrogen. The morphological and cultural properties of the strains ATCC 31267, 31271 and 31272 are described in detail in British Patent Specification No. 1573955 of Merck, published August 28, 1980, which also describes the isolation and the chemical structure of the eight individual components which make up the C-076 complex. The milbemycins are structurally related macrolide antibiotics lacking the sugar residues at the 13-position. They are produced by fermentation, for example as described in British Patent Specification No. 1390336 of Sankyo, published February 26, 1975 and European Patent Application Publication No. 0170006 of American Cyanamid, published June 27, 1979.
In our European Patent Application Publication No.
0214731 we disclose that by adding certain specified carboxylic acids, or derivatives thereof, to the fermentation of an avermectin producing organism it is possible to obtain ~.340~7~

novel compounds, related to the avermectins but having an unnatural substituent group at the 25-position in place of the isopropyl or sec-butyl group which is normally present.
The novel compounds produced are characterised in that the substituent group at the 25-position is alpha-branched i.e. the carbon atom attached to the C-25 ring position is a secondary carbon atom linked to two further carbon atoms.
In our European Patent Application A 0284176 published September 28, 1988 we describe and claim new mutant strains of the microorganism Streptomyces avermitilis lacking branched-chain 2-oxo acid dehydrogenase activity. Said strains have been deposited on January 16, 1987 in the American Type Culture Collection, Rockville, Maryland under the designations Streptomyces avermitilis ATCC 53567 and ATCC
53568.
We have now discovered that, by using these new mutant strains of Streptomyces avermitilis it is possible to obtain a further range of novel avermectin derivatives, not previously obtainable, wherein the C-25 substituent is linked by an unbranched (primary) carbon atom. The novel compounds are highly active antiparasitic agents having particular utility as anthelmintics, ectoparasiticides, insecticides and acaricides. The compounds can be subjected to conventional chemical transformation reactions to obtain further novel semi-synthetic derivatives. Thus, according to the present invention there are provided compounds having the formula (I):
x 13~~1~7~

R~
R4 CH3 _ ~ ~,, C H3 ~Ow CH 2 R2 O
OH~
O

wherein the broken line at the 22-23 position represents an optional double bond and wherein either R1 is H or OH and the double bond is absent, or, the double bend is present and Rl is absent;
R2 is H, Cl-C8 alkyl, C2-C8 alkenyl, C2-C8 alkynyl, alkoxyalkyl or alkylthioalkyl containing from 1 to 6 carbon atoms in each alkyl or alkoxy group, wherein any of said alkyl, alkoxy alkenyl or alkynyl groups may be substituted by one or more halo atoms; or a C3-C8 cycloalkyl or C5-C8 cycloalkenyl group, either of which may optionally be substituted by methylene or one or more Cl-C4 alkyl groups or halo atoms; or a 3 to 6 membered oxygen or sulphur containing heterocyclic ring which may be saturated, or fully or partially unsaturated and which may optionally be substituted by one or more C1-C4 alkyl groups or halo atoms; or a group of the formula SR5 wherein RS is Cl-C8 alkyl, C2-C$ alkenyl, C2-C8 alkynyl, C3-C8 cycloalkyl, C5-C8 cycloalkenyl, phenyl or ~.34~Q~~

substituted phenyl wherein the substituent is C1-C4 alkyl, C1-C4 alkoxy or halo, R3 is hydrogen or methyl;
and R4 is H or a 4'-(alpha-L-oleandrosyl)-alpha-L-oleandrosyl-oxy group of the formula:
H3C~ H3 with the proviso that when R4 and R1 are both H and the double bond is absent, R2 is not H or CH3.
In the above definition, alkyl groups containing 3 or more carbon atoms may be straight or branched chain. Halo means fluoro, chloro, bromo or iodo.
The C-076 complex comprises eight distinct but closely related compounds described as C-076 Ala, Alb, A2a, A2b, Bla, Blb, B2a and B2b. The "a" series of compounds refers to the natural avermectin wherein the 25-substituent is (S)-sec-butyl and the "b" series to those wherein the 25-substituent is isopropyl. The ~.34~6'~8 designations "A" and "B" refer to avermectins wherein the 5-substituent is methoxy or hydroxy, respectively, and the numeral "1" refers to avermectins wherein a double bond is present at the 22-23 position, and numeral "2" to avermectins having a hydrogen at the 22-position and hydroxy at the 23 position.
In this application, the "a" and "b" identifiers have been dropped. Identifiers A1, A2, B1 and B2 have been retained to refer to non-natural avermectins having the structural features corresponding to those of the natural avermectins as noted above.
Preferred compounds of the formula (I) are those wherein R4 is 4'-(alpha-L-oleandrosyl)-alpha-L-oleandrosyloxy.
Also preferred are compounds of the formula (I) wherein R2 is SR5 and R5 is methyl or ethyl.
In another group of preferred compounds R2 is methyl, isopropyl or sec-butyl.
In a further group of preferred compounds R2 is branched C3-C8 alkyl group substituted by one or more halo atoms, parti-cularly 1-(trifluoromethyl)ethyl.
In accordance with the invention the compounds of formula (I) wherein R1 is OH and the double bond is absent or wherein the double bond is present and R1 is absent and R4 is 4'-(alpha-L-oleandrosyl)-alpha-L-oleandrosyloxy are prepared by fermenting a Streptomyces atrerrtiitilis mutant organism ATCC 53567 or 53568, as described in European Patent Application A 0284176, in the presence of the appropriate carboxylic acid of the formula R2CH2C02H, wherein R2 is as previously defined, or a salt, ester, or amide thereof or oxidative precursor therefor. The acid is added to the fermentation either at the time of inoculation or at intervals during the fermentation. Production of the compounds of formula (I) may be monitored by removing samples from the fermentation, extracting with an organic solvent and following the appearance of the compound of formula (I) by chromatography, for example using high pressure liquid chromatography. Incubation is continued until the yield of the compound of formula (I) has been maximised, generally for a period of from 12 to 16 days.
A preferred level of each addition of the carboxylic acid or derivative thereof is between 0.05 and 4.0 grams per litre. The best yields of the compounds of formula (I) are obtained by gradually add~.ng the acid to the fermentation, for example by daily additions of the acid or derivative thereof over a period of several days. The acid may be added as a salt, such as the sodium or ammonium salt, or as an ester, such as the methyl or ethyl ester or as an amide, but is preferably added as the free acid.
Alternative substrates which may be used iH the fermentation are derivatives which are oxidative precursors for the carboxylic acids; thus, for example suitable substrates would be alcohols of the formula R2(CH2)nOH or amine derivatives of the formula R2(CH2)nNH2, wherein n is 2, 4 or 6, substituted lower alkanoic acids of the formula 'R2(CH2)nC02H wherein n is 3 or 5 or aldehydes of the formula R2(CH2)nCHO wherein n is 1, 3 or 5 and R2 is as previously defined. The media used for the fermentation may be a conventional complex media containing assimilable sources of carbon, nitrogen and other trace elements.

~..
~~~oo~s After fermentation for a period of several days at a temperature preferably in the range of from 24 to 33°C, the fermentation broth is centrifuged or filtered and the mycelial cake is extracted with acetone or methanol. The solvent extract is concentrated and the desired product is then extracted into a water-immiscible organic solvent, such as methylene chloride, ethyl acetate, chloroform, butanol or methyl isobutyl ketone. The solvent extract is concentrated and the crude product containing the compounds of formula (I) is further purified as necessary by chromatography, for example using preparative reversz phase, high pressure liquid chromatography.
The product is generally obtained as a mixture of the compounds of formula (I) wherein R4 is 4'-(alpha-L-oleandrosyl)-alpha-L-oleandrosyloxy, R1 is OH and the double bond absent or Rl is absent and the double bond is present and wherein R3 is H or CH3; however the proportions can vary depending on the particular carboxylic acid employed and the conditions used in the fermentation.
We have found that a range of carboxylic acids as defined by R2CH2C02H may be added to the fermentation to yield avermectins having a novel substituent group at the 25-position. Examples of particular acids which may be employed include the following:
methylthioacetic acid ethylthioacetic acid 3-methylbutyric acid 3-trifluoromethyl butyric acid 3-methylpentanoic acid n-butyric acid 8 1340fi78 cyclopentane acetic acid thiophene-3-acetic acid and propionic acid.
In one particular and preferred aspect of the invention, the fermentation is performed in the presence of methylthioacetic acid to yield predominantly the compound of formula (I) wherein RI is OH, the double bond is absent, R2 is SCH3, R3 is CH3 and R4 is 4'-(alpha-L-oleandrosyl)-alpha-4-oleandrosyloxy, referred to herein as 25-methylthiomethyl avermectin A2.
In another preferred aspect of the invention, the fermentation is performed in the presence of propionic acid to yield predominantly the compound of formula (I) wherein RI is OH, the double bond is absent, R2 is CH3, R3 is CH3 and R4 is 4'(-(alpha-L-oleandrosyl)-alpha-4-oleandrosyloxy, referred to herein as 25-ethyl avermectin A2.
In a further preferred aspect of the invention the fermentation is performed in the presence of 3-methylbutyric acid to yield predominantly the compound of forcqula (I) wherein R1 is absent, the double bond is present, R2 is isopropyl, R3 is H and R4 is 4'-(alpha-L-oleandrosyl)-alpha-4-oleandrosyloxy, referred to herein as 25-isobutyl avermectin B1.
In a further preferred aspect of the invention, the fermentation is performed in the presence of 3-trifluoromethyl butyric acid to yield predominantly the compounds of formula (I) wherein RI is OH, the double bond is absent, R2 is I-(trifluoro-methyl)ethyl, R4 is 4'-(alpha-L-oleandrosyl)alpha-4-oleandrosyloxy and R3 is CH3 or H, referred to herein as 25-(2-trifluoromethyl-propyl)avermectin A2 and B2 respectively.

13~06~~

Compounds of the formula (I) wherein the double bond is present and Rl is absent may alternatively be prepared from the corresponding compound of formula (I) wherein R1 is OH and the double bond is absent by a dehydration reaction. The reaction is performed by first selectively protecting the hydroxyl groups at the 5 and 4" positions, e.g. as the t-butyldimethylsilyloxy acetyl derivative, then reacting with a substituted thiocarbonyl halide, such as (4-methylphenoxy)thiocarbonyl chloride, followed by heating in a high boiling point solvent, e.g. trichlorobenzene, to effect the dehydration. The product is finally deprptected to give the unsaturated compound. These steps together with appropriate reagents and reaction conditions are described in United States patent 4328335.
The compounds of formula I wherein R3 is H may also be prepared from the corresponding compounds wherein R3 is CH3 by demethylation. This reaction is achieved by treating the 5-methoxy compound, or a suitably protected derivative thereof, with mercuric acetate and hydrolysing the resulting 3-acetoxy enol ether with dilute acid to give the 5-keto compound. This is then reduced using, for example, sodium borohydride to yield the S-hydroxy derivative. Appropriate reagents and reaction conditions for these steps are described in United States patent 4423209. ' The compounds of formula I wherein R1 is H and the double bond is absent can be prepared from the corresponding compound wherein the double bond is present and Rl is absent, by selective catalytic hydrogenation using an appropriate catalyst. For 13~' ~~7~
example the reduction may be achieved using tris(triphenyl-phosphine)rhodium (I) chloride as described in European Patent Application Publication No. 0001689.
The compounds of formula (L) wherein R4 is H are prepared from the corresponding compounds wherein R4 is 4'-(alpha-L-oleandrosyl)-alpha-L-oleandrosyloxy by removing the 4'-(alpha-L-oleandrosyl)-alpha-L-oleandrose group by mild hydrolysis with an acid in an aqueous organic solvent to yield the aglycone having a hydroxy group at the 13-position; this is then halogenated, for 10 example by reaction with a benzene sulphonyl halide, to yield the 13-deoxy-13-halo derivative which is finally selectively reduced, for example using tributyltin hydride. In order to avoid unwanted side reactions it is desirable to protect any other hydroxy groups which may be present, for example using a tert-butyldimethylsilyl group. This is then readily removed after the halogenation or reduction step by treatment with methanol containing a trace of acid. All these steps together with appropriate reagents and reaction conditions for their performance are described in European Patent Application Publication No.
0002615 of Merck, published June 27, 1979.
The compounds of the invention are highly active anti-parasitic agents having particular utility as anthelmintics, ectoparasiticides, insecticides and acaricides.
Thus the compounds are effective in treating a variety of conditions caused by endoparasites including, in particular, helminthiasis which is most frequently caused by a group of parasitic worms described as nematodes and which can cause severe economic losses in swine, sheep, horses and cattle as well as ,,:~.'~ i 1.34007 affecting domestic animals and poultry. The compounds are also effective against other nematodes which affect various species of animals including, for example, Dirofilaria in dogs and various parasites which can infect humans including gastro-intestinal parasites such as Ancylostoma, Necator, Ascaris, Strongyloides, Trichinella, Capillaria, Trichuris, Enterobius and parasites which are found in the blood or other tissues and organs such as ' filiarial worms and the extra intestinal stages of Strongyloides and Trichinella.
The compounds are also of value in treating ect~oparasite infections including in particular arthropod ectoparasites of animals and birds such as ticks, mites, lice, fleas, blowfly, biting insects and migrating dipterous larvae which can affect cattle and horses.
The compounds are also insecticides active against household pests such as the cockroach, clothes moth, carpet beetle and the housefly as well as being useful against insect pests of stored grain and of agricultural plants such as spider mites, aphids, caterpillars and against migratory orthopterans such as locusts.
The compounds of formula (I) are administered as a formulation appropriate to the specific use envisaged and to the particular species of host animal being treated and the parasite or insect involved. For use as an anthelmintic the compounds may be administered orally in the form of a capsule, bolus, tablet or preferably a liquid drench, or alternatively, they may be administered by injection or as an implant. Such formulations are prepared in a conventional manner in accordance with standard veterinary practice. Thus capsules, boluses or tablets may be ~~40~7~

prepared by mixing the active ingredient with a suitable finely divided diluent or carrier, additionally containing a disintegrating agent and/or binder such as starch, lactose, talc, magnesium stearate etc. A drench formulation may be prepared by dispersing the active ingredient in an aqueous solution together with dispersing or wetting agents etc. and injectable formulations mad be prepared in the form of a sterile solution which may contain other substances, for example, enough salts or glucose to make the solution isotonic with blood. These formulations will vary with regard to the weight of active compound depending on the species of host animal to be treated, the severity and type of infection and the body weight of the host. Generally for oral administration a dose of from about 0.001 to 10 mg per Kg of animal body weight given as a single dose or in divided doses for a period of from 1 to 5 days will be satisfactory but of course there can be instances where higher or lower dosage ranges are indicated and such are within the scope of this invention.
As an alternative the compounds may be administered with the animal feedstuff and for this purpose a concentrated feed additive or premix may be prepared for mixing with the normal animal feed.
For use as an insecticide and for treating agricultural pests the compounds are applied as sprays, dusts, emulsions and the like in accordance with standard agricultural practice.
For human use the compounds are administered as a pharmaceutically acceptable formulation in accordance with normal medical practice.

134~J~'~8 The invention is illustrated by the following Examples in which Examples 1 to 8 are Examples of the preparation of compounds of the formula (I), Example 9 is an example of a drench formulation and Examples 1~ and lI illustrate the antiparasitic and insecticidal activity of the compounds.

~~~oo7g 25-Ethyl avermectin A2 A frozen inoculum (2 ml) of a culture of Streptomyces avermi~ilis mutant organism ATCC 53568 was inoculated into 50 mls of a medium containing starch (1 g), Pharmamedia (Trademark) (0.75 g), ardamine pH (0.25 g), and calcium carbonate (0.1 g) in a 300 ml flask and incubated at 28°C for 2 days. This inoculum (50 ml) was transferred to a second inoculum flask (1 litre) containing starch (20 g), Pharmamedia (15 g), ardamine pH (5 g) and calcium carbonate (2 g) and incubated at 28°C fox a further 2 Gays. This inoculum was used to inoculate 60 litres of a medium containing starch (6 kg), magnesium sulphate (60 g), Pharmamedia (300 g), dipotassium hydrogen phosphate (60 g), ferrous sulphate (0.6 g), calcium carbonate (420 g), glutamic acid (36 g), zinc sulphate (0.06 g) and manganous sulphate (0.06 g) contained in a 60 litre fermenter. The fermentation was incubated at 29°C, with agitation at 350 r.p.m. and aeration at 60 litres per minute.
Sodium propionate (140 g) was added after 9,6 hours and again after 192 hours (54 g). After 288 hours the mycelium was removed by filtration and extracted with acetone (2 x 50 litres) followed by ethyl acetate (50 litres). The acetone extract was concentrated to approximately 10 litres and extracted with the above ethyl acetate extract in three portions. The resulting ethyl acetate layers were combined and evaporated to give a brown oil (112 g).

13~0~7~
is The above oil was dissolved in 160 ml of a mixture of methanol and water (95:5) and extracted with n-hexane (2 x 300 ml), the hexane extracts were discarded and the methanol layer was evaporated to give a brown oil (87 g). The latter was dissolved in methyleae chloride (250 ml) and stirred with silica gel (80 g) and charcoal (30 g) for 1 hour. The silica and charcoal were removed by filtration through Arbacel and the filtrate was evaporated to give a yellow oil (53 g). The latter was dissolved is methylene chloride (2.2 litres) and stirred with 1 0 alumina (I90 g) for two hours. The alumina was removed by filtration and the filtrate stirred with more alumina (64 g) for a further hour. The alumina was removed by filtration and the combined filter cakes from both filtrations were stirred with chloroform (1.3 litres) for 45 minutes and then the alumina was removed by filtration. The filtrate was evaporated to give a pale yellow oil (12.5 g) which was dissolved in diethyl ether and added to a column of silica gel (400 g). The column was eluted with diethyl. ether and 100 ml fractions were co~,lected. Fractions 21-28 were combined and the solvent evaporated to yield partially 20 purified material (150 mg). The product was dissolved in methanol (0.5 ml) and chromatographed on a C18 Zorbax ODS (Trademark, Dupont) column (21 mm x 25 cm) eluting with a mixture of methanol and water (77:23) at 'a flowrate of 9 mls. per minute. The relevant fractions were combined and the solvent evaporated to yield the compound of formula (I) wherein R1 is OH, the double bond is absent, R2 and R3 are both CH3 and R4 is 4'-(alpha-L-Trade-mark 1~~(~~7~
16 .
oleandrosyl)-alpha-L-oleandrosyloxy as a white powder, m.p 146-153°C. The structure of the product was confirmed by fast atom bombardment mass spectrometry performed on a VG Model 7070E mass spectrometer using a sample matrix of triethylene glycol with solid sodium chloride, (M + Na)+ observed at m/e 899 (theoretical 899).
Electron impact mass spectrometry was performed using a VG
Model 7070F mass spectrometer. The m/e values for the principal fragments were: 570, 295, 277, 275, 183, 165, 145, 127, 1I3, 95 and 87. .

25-Methylthiomethyl avermectin A2 A frozen inoculum (2 ml) of a culture of Streptomyces ' avermitilis mutant organism ATCC 53568 was inoculated into 50 mls of a medium containing starch (1 g), Pharmamedia (Trademark) (0.75 g), ardamine pH (0.25 g) and calcium carbonate (0.1 g) in a 300 ml flask and incubated for 2 days at 2$°C on a reciprocal shaker operating at 180 r.p.m. An inoculum from this flask (25 ml) was transferred to a 3 litre flask containing 600 mls 1~40~78 of the above medium (all ingredients fro rata) and was incubated for two days at 28°C with agitation on a reciprocal shaker operating at 180 r.p.m. The product from this flask (40 ml) was used to inoculate a 3 litre fermenter containing 2.5 litres of a medium consisting of starch (250 g), magnesium sulphate (2.5 g).
Pharmamedia (12.5 g), dipotassium hydrogen phosphate (2.5 g), ferrous sulphate (0.025 g), calcium carbonate (1.75 g), glutamic acid (1.5 g), zinc sulphate (0.0025 g), and manganous sulphate (1.5 g). This fermentation was incubated at 28°C with agitation at 1000 r.p.m. Methylthioacetic acid (1 g) was added at 96 hours and the fermentation continued for a further 11 days. Then the mycelium was removed by filtration and extracted with acetone (2 x 2 litres) followed by ethyl acetate (2 litres). The acetone extract was concentrated to approximately 400 mls. and extracted with the ethyl acetate extract in three portions. The resulting ethyl acetate layers were combined and evaporated to give a brown oil (4 g) which was dissolved in diethyl ether and applied to a column of silica gel (100 g). The column was eluted with diethyl ether and 50 ml fractions were collected. Fractions 11-18 were combined and evaporated to yield partially purified material which was further purified by chromatography on a C18 Zorbax ODS
(Trademark, Dupont) column (21 mm x 25 cm) eluting with a mixture of methanol and water (77:23) at a flowrate of 9 mls. per minute.
The relevant fractions were combined and evaporated to yield the compound of formula (I) wherein Rl is OH, the double bond is absent, R2 is SCH3, R3 is CH3 13~4~78 and R4 is 4'-(alpha-L-oleandrosyl)-alpha-L-oleandrosyloxy as a white powder m.p. 105-112°C. The structure of the product was confirmed by fast atom bombardment mass spectrometry was performed on a VG Model 7070E mass spectrometer using a sample matrix of triethylene glycol with solid sodium chloride. (M + Na)+ observed at m/e 931 (theoretical 931).
Electron impact mass spectrometry was performed using a VG
Model 7070F mass spectrometer. The m/e values for the principal fragments were: 327, 309, 243, 225, 215, 145, I27, 113, 95 and 87.

25-(2-Trifluoromethyl)propyl avermectins A2 and B2 The procedure of Example 1 was followed but using 3-trifluoromethyl butyric acid as substrate instead of sodium propionate. The relevant combined fractions from silica gel chromatography containing the crude A2 derivative were chromatographed on a C18 Zorbax ODS (Trademark, Dupont) column (21 mm x 25 cm) eluting with a mixture of methanol and water (75:25) at a flowrate of 9 mls/min. Fractions 167-179 were combined and evaporated to yield the compound of formula (I) wherein Rl is OH, the double bond is absent, R2 is 1-(trifluoromethyl)ethyl, R3 is CH and R4 is 4'-(alpha-L-oleandrosyl)-L-oleandrosyloxy as a white powder, m.p. 130-136°C. The structure of the product was confirmed by fast atom bombardment mass spectrometry, performed on a VG Model 7070E mass spectrometer using a sample matrix of triethylene glycol with solid sodium chloride. (M + Na)+ observed at m/e 981 (theoretical 981).

1340~7g Electron impact mass spectrometry was performed using a VG
Model 7070F mass spectrometer. The m/e values for the principal fragments were: 652, 377, 359, 293, 275, 265, 257, 247, 223, 179, 145, 127, 113, 111 and 87.
The relevant fractions from silica gel chromatography containing the crude B2 derivative were combined and chromatographed on a C-18 Dynamax (Trademark Rainin) column (41.4 mm x 25 cm) eluting with a mixture of methanol and water (73:27) at a flowrate of 60 mls/min. Relevant fractions were combined to yield the compound of formula (I), wherein R1 is OH,.the double bond is absent, R2 is 1-(trifluoromethyl)ethyl, R3 is H and R4 is 4'-(alpha-L-oleandrosyl)-L-oleandrosyloxy as a white powder, m.p.
158-160°C. The structure of the product was confirmed by fast atom bombardment mass spectrometry performed on a VG Model 7070E
mass spectrometer using a sample matrix of triethylene glycol with solid sodium chloride. (M + Na)+ observed at m/e 967 (theoretical 967).
Electron impact mass spectrometry was,performed using a VG
Model 7070F mass spectrometer. The m/e values for the principal fragments were: 638, 377, 359, 293, 275, 265, 261, 257, 247, 223, 145, 127, 113, 111, 95 and 87.

25-Ethylthiomethyl avermectin A2 The procedure of Example 1 was followed but using ethylthioacetic acid as substrate instead of sodium propionate.
After chromatography on a Zorbax ODS (Trademark, Dupont) column fractions 24-72 were combined to yield the compound of formula (I) 1344~7~
wherein R1 is OH, the double bond is absent, R2 is ethylthio, R3 is CH3 and R4 is 4'-(alpha-L-oleandrosyl)-L-oleandrosyloxy as a white powder, m.p. 265-270°C (dec). The structure of the product was confirmed by fast atom bombardment mass spectrometry performed on a VG Model 7070E mass spectrometer using a sample matrix of triethylene glycol with solid, sodium chloride. (M + Na)+ observed at,m/e 945 (theoretical 945).
Electron impact mass spectrometry was performed using a VG
Model 7070F mass spectrometer. The m/e values for the principal fragments were: 616, 473, 341, 323, 257, 239, 229, zll, I87, 179, 145, 113 and 87.

25-Isobutvl avermectin B1 The procedure of Example 1 was followed but using 3-methylbutyric acid as substrate instead of sodium propionate.
The relevant fractions from silica gel chromatography were combined and chromatographed on a C18 Zorbax ODS (Trademark, Dupont) column (21 mm x 25 cm) eluting with a mixture of methanol and water (81:19) at a flowrate of 9 mls/min. Fractions 93-98 were combined and evaporated to yield the compound of formula (I) wherein R1 is absent, the double bond is present, R' is isopropyl, R3 is H and R4 is 4'-(alpha-L-oleandrosyl)-L-oleandrosyloxy, as a white powder, m.p. 120-123. The structure of the product was confirmed by fast atom bombardment mass spectrometry performed on a VG Model 7070E mass spectrometer using a sample matrix of 1~4~6~~

triethylene glycol with solid sodium chloride. (M + Na)+ observed at m/e 895 (theoretical 895).
Electron impact mass spectrometry was performed using a VG
Model 7070F mass spectrometer. The m/e values for the principal fragments were: 565, 319, 305, 221, 193, 169, 145, 127, 113 and 87.

25-(2-Methylbutyl) avermectins A2 and B1 The procedure of Example I was followed but using 3-methylpentanoic acid as substrate instead of sodium propionate.
The relevant fractions from silica gel chromatography containing the crude A2 derivative were combined and chromatographed on a C-18 Zorbax ODS (Trademark Dupont) column (21 mm x 25 cm) eluting with a mixture of methanol and water (80:20) at a flowrate of 9 mls/min. Relevant fractions were combined to yield the compound of formula (I), wherein Rl is OH, the double bond is absent, R2 is sec-butyl, R3 is methyl and R4 is 4'-(alpha-L-oleandrosyl)-L-oleandrosyloxy as a white powder, m.p. 120-I25°C. The structure of the product was confirmed by fast atom bombardment mass spectrometry performed on a VG Model 7070E mass spectrometer using a sample matrix of triethylene glycol with solid sodium chloride.
(M + Na)+ observed at m/e 941 (theoretical 941).
Electron impact mass spectrometry was performed using a VG
Model 7070F mass spectrometer. The m/e values for the principal fragments were: 337, 319, 253, 235, 225, 207, 179, 145) 113 and 87.

1340~7~

The relevant fractions from silica gel chromatography containing the crude B1 derivative were combined and chromatographed on a C-18 Ultrasphere (Trademark Beckman) column (10 mm x 25 cm) eluting with a mixture of methanol and water (85:15) at a flowrate of 4 mls/ min. Relevant fractions were combined to yield the compound of formula (I), wherein Rl is H, tha double bond is present, R2 is sec-butyl, R3 is H and R4 is 4'-(alpha-L-oleandrosyl)-L-oleandrosyloxy as a white powder, m.p.
158-164°C. The structure of the product was confirmed by fast atom bombardment mass spectrometry performed on a VG. Model 7070E
~hass spectrometer using a sample matrix of triethylene glycol with solid sodium chloride. (M + Na)+ observed at m/e 909 (theoretical 909).
Electron impact mass spectrometry was performed using a VG
Model 7070F mass spectrometer. The m/e values for the principal fragments were: 319, 235, 207, 183, 145, 113, 95 and 87.
EYAMPLE 7 , 25-n-Propyl avermectin A2 The procedure of Example 1 was followed but using n-butyric acid as substrate instead of sodium propionate. The relevant fractions from silica gel chromatography were combined and chromatographed on a C-18 Dynamax (Trademark Rainin) column (41.4 mm x c25 cm) eluting with a gradient of methanol and water from (75:25) to (100:0) over 170 minutes at a flowrate of 40 mls/min.
One minute fractions were collected and fractions 36 and 37 were combined to yield the compound of formula (I), wherein Rl is OH, the double bond is absent, R2 is ethyl, R3 is methyl and R4 is ~~~oo~ts 4'-(alpha-L-oleandrosyl)-L-oleandrosyloxy as a white powder, m.p.
150-155°C. The structure of the product was confirmed by fast atom bombardment mass spectrometry performed on a VG Model 7070E
mass spectrometer using a sample matrix of triethylene glycol with solid sodium chloride. (M+Na)+ observed at m/e 913 (theoretical 913).
Electron impact mass spectrometry was performed using a VG
Model 7070 F mass spectrometer. The m/e values for the principal fragments were: 584, 309, 291, 225, 207, 197, 179, 145, 113 and 87.

25-Cyclonentylmethyl avermectins B1 and B2 The procedure of Example 1 was followed but using cyclopentane acetic acid instead of sodium propionate. The relevant fractions from silica gel chromatography containing the crude B1 derivative were combined and chromatographed on a C-18 Dynamax (Trademark Rainin) column (41.4 mm,x 25 cm) eluting with a mixture of methanol and water (84:16) at a flowrate of 60 mls/min.
Relevant fractions were combined to yield a compound of formula (I), wherein R1 is absent, the double bond is present, R2 is cyclopentyl, R3 is H and R4 is 4'-(alpha-L-oleandrosyl)-L-oleandrosyloxy, as a 'white powder, m.p. 140-146°C. The structure of the product was confirmed by fast atom bombardment mass spectrometry performed on a VG Model 7070E mass spectrometer using a sample matrix of triethylene glycol with solid sodium chloride.
(M + Na) observed at m/e 921 (theoretical 921).

..-Electron impact mass spectrometry was performed using a VG
Model 7070F mass spectrometer. The m/e values for the principal fragments were: 592, 331, 295, 257, 247, 218, 195, 145, 127, 113, 111, 95 and 87.
The relevant fractions from silica gel chromatogrpahy containing the crude B2 derivative were combined and chromatographed on a C-18 Ultrasphere (Trademark Beckman) column (10 mm x 25 cm) eluting with a mixture of methanol and water (80:20) at a flowrate of 4 mls/min. Relevant fractions were combined to yield the compound of formula (I), wherein R1 is OH, the double bond is absent, R2 is cyclopentyl, R3 is H and R4 is 4'-(alpha-L-oleandrosyl)-L-oleandrosyloxy as a white powder, m.p.
155-165°C. The structure of the product was confirmed by fast atom bombardment mass spectrometry performed on a VG Model 7070E
mass spectrometer using a sample matrix of triethylene glycol with solid sodium chloride. (M + Na)+ observed at m/e 939 (theoretical 939).
Electron impact mass spectrometry was,performed using a VG
Model 7070F mass spectrometer. The m/e values for the principal fragments were: 349, 335, 331, 289, 265, 261, 257, 247, 237, 219, 195, 179, 145, 127, 113, 111, 95 and 87.

Drench Formulation The product of any one of the preceding Examples was dissolved in polyethylene glycol (average molecular weight 300) to give a solution containing 400 micrograms/ml for use as a drench formulation.

Anthelmintic Activity Anthelmintic activity was evaluated against Caenorhabditis elegans using the in vitro Screening test described by K. G.
Simpkin and G. L. Coles in Parasitology, 1979, 79, 19. The products of Examples 1 to 8 all killed 100% of the worms at a well concentration of 0.1 micrograms per ml.

Insecticidal Activity , ' Activity against the larval stage of the blowfly Lucilia cuprina (Q strain) is demonstrated using a standard procedure in which first instar larvae are kept in contact with filter paper treated with test compound. The test compound is first applied to the paper as an acetone solution. The treated filter papers are then placed into tubes containing I ml of newborn calf serum and the first instars are added. The products of Examples 1 to 8 killed 100% of the larvae when applied to the filter paper at a level of 1 milligram per square metre.

Claims

1. A compound having the formula:

wherein the broken line at the 22-23 position represents an optional double bond and wherein either R1 is H or OH and the double bond is absent, or, the double bond is present and R1 is absent;
R2 is H, C1-C8 alkyl, C2-C8 alkenyl, C2-C8alkynyl, alkoxyalkyl or alkylthioalkyl containing from 1 to 6 carbon atoms in each alkyl or alkoxy group, wherein any of said alkyl, alkoxy alkenyl or alkynyl groups may be substituted by one or more halo atoms; or a C3-C8 cycloalkyl or C5-C8 cycloalkenyl group, either of which may optionally be substituted by methylene or one or more C1-C4 alkyl groups or halo atoms; or a 3 to 6 membered oxygen or sulphur containing heterocyclic ring which may be saturated, or fully or partially unsaturated and which may optionally be substituted by one or more C1-C4 alkyl groups or halo atoms; or a group of the Formula SR5 wherein R5 is C1-C8 alkyl, C2-C5 alkenyl, C2-C8 alkynyl, C3-C8 cycloalkyl, C5-C8 cycloalkenyl, phenyl or substituted phenyl wherein the substituent is C1-C4 alkyl, C1-C4 alkoxy or halo;
R3 is hydrogen or methyl;
and R4 is H or a 4'-(alpha-L-oleandrosyl)-alpha-L-oleandrosyl-oxy group of the formula:
with the proviso that when R4 and R1 are both H and the double bond is absent, R2 is not H or CH3.

2. A compound as claimed in claim 1 wherein R4 is 4'-(alpha-L-oleandrosyl)-alpha-L-oleandrosyloxy.

3. A compound as claimed in claim 2 wherein R2 is SR5 and R5 is methyl or ethyl.

4. A compound as claimed in claim 2 wherein R2 is methyl, isopropyl or sec-butyl.

5. A compound as claimed in claim 2 wherein R2 is 1-(trifluoromethyl)ethyl.

6. A compound as claimed in claim 2 wherein R1 is OH and the double bond is absent, and R2 and R3 are both methyl.

7. A compound as claimed in claim 2 wherein R1 is OH and the double bond is absent, R2 is thiomethyl and R3 is methyl.

8. A compound as claimed in claim 2 wherein R1 is OH and the double bond is absent, R2 is 1-(trifluoromethyl)ethyl and R3 is methyl.

9. A compound as claimed in claim 2 wherein R1 is OH and the double bond is absent, R2 is 1-(trifluoromethyl)ethyl and R3 is hydrogen.

10. A compound as claimed in claim 2 wherein R1 is OH and the double bond is absent, R2 is ethylthio and R3 is methyl.

11. A compound as claimed in claim 2 wherein R1 is OH and the double bond is absent, R2 is 1-methyl-propyl and R3 is methyl.

12. A compound as claimed in claim 2 wherein R1 is OH and the double bond is absent, R2 is ethyl and R3 is methyl.

13. A compound as claimed in claim 2 wherein R1 is OH and the double bond is absent, R2 is cyclopentyl and R3 is hydrogen.

14. A compound as claimed in claim 2 wherein R1 is absent and the double bond is present, R2 is 1-methyl-ethyl and R3 is hydrogen.

15. A compound as claimed in claim 2 wherein R1 is absent and the double bond is present, R2 is 1-methyl-propyl and R3 is hydrogen.

16. A compound as claimed in claim 2 wherein R1 is absent and the double bond is present, R2 is cyclopentyl and R3 is hydrogen.

17. A process for preparing a compound of the formula (I) as claimed in claim 1 which comprises fermenting a Streptomyces avermitilis mutant organism ATCC 53567 or 53568, in the presence of the appropriate carboxylic acid of the formula R2CH2CO2H, wherein R2 is as defined in claim 1, or a salt, ester, or amide thereof or oxidative precursor therefor, and isolating the compound of formula I wherein R1 is OH and the double bond is absent or wherein the double bond is present and R1 is absent and R4 is 4'-(alpha-L-oleandrosyl)-alpha-L-oleandrosyloxy and, if required, hydrogenating the product wherein the double bond is present and R1 is absent to obtain the compounds of formula (I) wherein the double bond is absent and R1 is H, or hydrolyzing followed by halogenation and reduction to obtain the compounds of formula (I) wherein R4 is H.

18. A process as claimed in claim 17 wherein R4 is 4'-(alpha-L-oleandrosyl)-alpha-L-oleandrosyloxy and the optional hydrolysis, halogenation and reduction steps are not performed.

19. A process as claimed in claim 18 wherein R2 is SR5 and R5 is methyl or ethyl.

20. A process as claimed in claim 18 wherein R2 is methyl, isopropyl or sec-butyl.

21. A process as claimed in claim 18 wherein R2 is 1-(tri-fluoromethyl)ethyl.

22. A composition for the treatment and prevention of parasitic infections in humans and animals, including ectoparasiticidal, insecticidal, acaricidal and anthelmintic compositions, which comprises a compound of the formula (I) as claimed in any one of claims 1 to 5 together with an inert diluent or carrier.

23. A composition as claimed in claim 22 in the form of a liquid drench or an oral or injectable formulation or in the form of an animal feedstuff or a premix or supplement for addition to animal feed.

24. A compound of the formula (I) as claimed in any one of claims 1 to 16 for use in the treatment or prevention of parasitic infections in humans and animals.

25. Use of a compound of the formula (I) as claimed in any one of claims 1 to 16 for combating insect or parasite infections or infestations.

26. A commercial package containing as active pharmaceutical ingredient a compound of the formula (I) as claimed in any one of claims 1 to 16, together with instructions for the use thereof in the treatment or prevention of parasitic infections in humans and animals.