Enantioenriched selenonium ylides have been generated by addition of benzyl bromide to C2 symmetric (2R,5R)-2,5-dimethylselenolane in the presence of NaOH, and subsequently reacted with a variety of aldehydes to give oxiranes with excellent enantiomeric excesses (a catalytic version has been achieved); also, an aliphatic cyclic hypervalent dibromoselenurane structure has been demonstrated by X-ray analysis.