The side chain of brassinolide (1) was stereoselectively synthesized, in which the 20(S)- and the 22(R)-configurations were introduced by the alkylation (SN2) of the 20(R)-tosyloxy steroid 7 with the protected cyanohydrin 8 followed by the stereoselective reduction of the 23-en-22-one 10 with Dibal-H.