Spontaneous Stereoselective Oxidation of Crystalline Avermectin B_1a to Its C‑8a‑(S)‑Hydroperoxide

Prolonged storage of technical abamectin as well as avermectin B_1a samples yielded a previously unknown derivative, designated here as compound 1. Detailed NMR analysis and X-ray crystallography allowed us to determine the structure of this compound and revealed the presence of a hydroperoxide group (−OOH) attached stereoselectively with configuration S to the C-8a carbon. This surprising result involves the formation of the peroxide bond in solid crystalline avermectin B_1a upon exposure to air with no involvement of light or recognized catalytic factors and is consistent with a topotactic mechanism for the oxidation reaction. Compound 1 is stable in the absence of reducing agents and has potential as a starting point in structural modification of the tetrahydrofuran ring of avermectin B_1a. It could also serve as a marker in assessing the quality of stored technical abamectin.