ChemInform Abstract: The Cyclization Route to the Calcitriol A-Ring: A Formal Synthesis of ( +)-1α,25-Dihydroxyvitamin D3

An efficient asymmetric synthesis of the A-ring of 1α,25-dihydroxyvitamin D 3 from α-bromoacrolein is described. The key steps of the synthesis are the Evans type syn -selective asymmetric aldol reaction of bromoacrolein with the boron enolate of 3-chloroacetyl-4(S)-isopropyl oxazolidinone and ring closure by Heck type reaction of a vinyl bromide onto an α,β-unsaturated ester in the exo-mode. This paper is warmly dedicated to Professor Sir Derek H.R. Barton, FRS, inventor of chemical reactions, scholar, teacher and mentor par excellence, on the happy occasion of his seventy-fifth birthday.