epi-Avermectin B1a is a base-catalysed intermediate in the decomposition of avermectin B1a. epi-Avermectin B1a is formed by epimerisation at the 2-position which ultimately rearranges irreversibly to the isomeric alkene analogue, Δ2-avermectin B1a. epi-Avermectin B1a is only very weakly active as a nematocide showing ~100-fold loss of biological activity compared with the parent avermectin. epi-Avermectin B1a is formed in vivo following treatment with avermectin and is an environmental degradation product.
