Synthesis of 24a-Substituted Milbemycin A₄ Derivatives and Their Acaricidal Activity against Tetranychus uvticae

A series of 24a-substituted milbemycin A₄ derivatives were synthesized from 24a-hydroxymilbemycin A₄, which had been obtained by the microbial oxidation of milbemycin A₄. The acaricidal activity of each synthesized derivative against Tetranychus urticae was tested and some of the derivatives showed higher activity than parent milbemycin A₄. Among them, 24a-methylmilbemycin A₄ (22) was the most active derivative, with 100% mortality of the mite at a concentration of 1 ppm and 50% mortality at a concentration of 0.1 ppm.