Stereocontrolled synthesis of linear 22R-homoallylic sterols via a triflic acid-catalyzed 2-oxonia Cope rearrangement.

[reaction: see text] Poor stereoselectivity caused by the involvement of side reaction pathways was observed in the In(OTf)(3)-catalyzed allyl-transfer reaction (R = electron-withdrawing group). Subsequently, it was found that the employment of triflic acid (TFA) as catalyst could successfully suppress the side reaction pathways and, hence, achieve high stereoselectivity.