Process for the preparation of calcipotriol

A process for the preparation of calcipotriol at least including: (a) reacting a C-22 phenylsulfonyl derivative of cholecalciferol of Formula 2, wherein R1 and R2 are the same or different and represent hydroxyl protecting groups, with a silyl derivative of alfa-hydroxy aldehyde of Formula 3, wherein R3 represents silyl group of formula Si(R4)(R5)(R6), where R4-R6 are the same or different and represent C1-6 alkyl or phenyl groups, in the presence of a strong organic base in an aprotic solvent, followed by reductive desulfonation of the obtained diastereomeric mixture of alfa-hydroxysulfones with sodium amalgam, removal of the hydroxyl protecting groups, and purification of the product. The process leads to the formation of calcipotriol containing less than 0.3% of the (22Z)-isomer. The obtained calcipotriol is free of mercury traces.