Most of these investigations were conducted using primary allylic silanols. With a secondary allylic silanol, selenohydrin product (Scheme 4, Compound 4s) was formed as a single regioisomer and diastereomer in 40% yield; 48% starting material was recovered in the reaction. The relative configuration of the three stereocenters in this molecule was stablished by single crystal X-ray diffraction analysis (CCDC: 2109848). What accounts for the high diastereoselectivity in this case? We propose a model shown in Scheme 5, which suggests that the avoidance of unfavorable steric clashing between the seleniranium adduct and the bulky di-tert-butyl-silanol group may be a primary factor.