An Effective Procedure for the Synthesis of 1α,25-Dihydroxycholecalciferol (Calcitriol) Starting with 1α,3β, 25-Trihydroxycholesta-5,7diene (Pro-Calcitriol). -An effective synthesis of the important vitamin D metabolite calcitriol (III) via double wavelength irradiation of pro-calcitriol ( I) and thermal isomerization of the obtained photoisomer (II) is given. -(REICHENBAECHER, M.; GLIESING, S.; LANGE, C.; GONSCHIOR, M.; SCHOENECKER, B.; J. Prakt. Chem./Chem.