The natural stereochemistry at the 13-position of avermectin aglycones, normally α-oriented or below the plane of the molecule, is inverted or epimerized into the β-position. The procedure starts with the avermectin aglycone compounds where the 13α-hydroxy group is substituted with a leaving group. Treatment of the substituted compound with tetra alkyl ammonium nitrate displaces the leaving group, substitutes a nitrate ester or a nitrooxy group, for the hydroxy and inverts the 13-stereochemistry to β. Reduction of the nitrooxy group returns the 13-hydroxy group and retains the 13-β configuration. The intermediate 13-nitrooxy compounds are themselves potent anti-parasitic agents.