Oxidosqualene cyclase involved in the biosynthesis of alpha-onocerin, a triterpenoid characterized by a symmetrical structure due to cyclizations at both termini of dioxidosqualene that inhibits acetylcholinesterase. Catalyzes the first half of the cyclization, exclusively from dioxidosqualene. (3S,22S)-2,3:22,23-diepoxy-2,3,22,23-tetrahydrosqualene = pre-alpha-onocerin Secondary metabolite biosynthesis; terpenoid biosynthesis. Belongs to the terpene cyclase/mutase family.