Microbial Conversion of Milbemycins: Oxidation of Milbemycin A4 and Related Compounds at the C-25 Ethyl Group by Circinella umbellata and Absidia cylindrospora.
Microbial Conversion of Milbemycins: Oxidation of Milbemycin A4 and Related Compounds at the C-25 Ethyl Group by Circinella umbellata and Absidia cylindrospora.
Microbial oxidation of milbemycin A4 at the C-25 ethyl group was performed. Milbemycin A4 was converted to 31- and 32-hydroxy derivatives by Cirdnella umbellata SANK 44272 along with 24- and 30-hydroxy derivatives. Related compounds, 5-ketomilbemycin A4 5-oxime and 13β-fluoromilbemycin A4 were similarly converted to the hydroxylated compounds by this microorganism. Absidia cylindrospora SANK 31472 converted milbemycin A4 to the corresponding 32-oic acid, 24-hydroxy derivative and a few oxygenated compounds including at the C-25 ethyl group.
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.