A site-selective modification of a vitamin D analogue (Deltanoid) through a two-step 2,3-sigmatropic rearrangement of organoselenium resin to prepare the key intermediate of calcipotriol has been developed. The polystyrene-supported selenium resins used here not only facilitate separation of product but also assist the crucial 2,3-sigmatropic rearrangement to introduce an important functional group (1α-hydroxyl) with high stereo- and regioselectivity.