Chapter 9 Avermectins and Related Compounds

Abstract

Publisher Summary Thin layer chromatography (TLC) of the two lipophilic concentrates provided a breakthrough in the isolation of the avermectins. A comparative analysis of the active and inactive samples on silica–gel plates containing phosphor (Analtech) showed that both samples contained many ultraviolet (UV) absorbing compounds. However, a plate developed with chloroform–methanol (19:1) exhibited two UV absorbing spots in the active concentrate which were absent in the inactive one. Aliquots of the active concentrate were streaked on TLC plates, developed with solvent, marked under UV light and assayed by two methods. First, the adsorbent from the different regions of the plate was scraped off, eluted with methanol, and the extract was dried on mouse feed. A second series of assays was prepared by simply scraping off the adsorbent from the different regions and blending it with mouse feed. Investigation of the Sephadex LH-20 avermectin A components fraction from a large scale fermentation led to the purification of six additional novel avermectin components. Purification of these components was accomplished by reverse-phase chromatography on C-18 using various proportions of methanol–water as the developing solvent, combined with normal phase chromatography on silica gel using various proportions of ethyl acetate–methylene chloride as the developing solvent.