Synthesis of Novel 25-Substituted Milbemycin A4 Derivatives and Their Acaricidal Activity against Tetranychus urticae

Novel 25-substituted milbemycin A4 derivatives were synthesized from 25a-hydroxymilbemycin A4 and 25b-hydroxymilbemycin A4, which had been obtained by the microbial oxidation of milbemycin A4. The acaricidal activity of each synthesized derivative was tested against Tetranychus urticae, and all of the synthesized derivatives showed higher activity than parent milbemycin A4. Some of the derivatives had higher acaricidal activity than milbemycin D, which had higher acaricidal activity than milbemycin A4. Among them, 25b-methylmilbemycin A4 was the most active derivative, with 100% mortality of the mite at a concentration of 1 ppm, and 63% mortality at 0.1 ppm.