A total synthesis of 1α,25S,26-trihydroxycholecalciferol (2) has been accomplished via an efficient convergent approach. The remote chiral center at C-25 was introduced by a regiospecific and diastereoselective 1,3-dipolar cycloaddition of the C-23 nitrone 27 with methyl methacrylate. Subsequent transformation of the resulting isoxazolidine led to the key synthon 3, which on coupling to the anion 4 and deprotection produced the metabolite 2.
