OSW saponins, featuring a 16β, 17α-dihydroxycholest-22-one aglycon and an acylated β-d- xylopyranosyl-(1→ 3)-α-l-arabinopyranosyl residue attached to the 16-hydroxyl group, have recently been discovered from a group of lily plants, which show potent antitumor activities with a novel mechanism of action. This paper describes an aldol approach to the stereoselective construction of the 16α, 17α-dihydroxycholest-22-one structure from 16α- ...