Antiparasitic avermectin and milbemycin derivatives and process for their preparation

The invention provides novel compounds having the formula: wherein the broken line at the 22-23 position represents an optional double bond and wherein R' is H or OH and the double bond is absent, or, the double bond is present and R' is absent; R2 is an alpha-branched C3-C8 alkyl, alkenyl, alkynyl, alkoxyalkyl or alkylthioalkyl group; a C3-C8 cycloalkyl, C5-C8 cycloalkenyl or C5-C8 cycloalkylalkyl group, any 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 optionally be substituted by one or more C1-C4 alkyl groups or halo atoms; R3 is hydrogen or methyl; R4 is H or 4'-(alpha-L-oleandrosyl)-alpha-L-oleandrosyloxy, with the proviso that when R2 is alkyl it is not isopropyl or sec-butyl, and when R4 is H, R1 is OH, and the double bond is absent, R2 is not 2-buten-2-yl, 2-pentene-2-yl or 4-methyl-2-pentene-2-yl. The compounds are broad spectrum antiparasitic agents having utility as anthelmintics, ectoparasiticides, insecticides and acaricides. The invention also provides a process for producing the novel avermectin and milbemycin derivatives by adding a carboxylic acid or derivative thereof to a fermentation of an avermectin or milbemycin producing organism.