Process for the preparation of ester N - [(2'R) -2'-deoxy-2'-fluoro-2'-methyl-p-phenyl-5'-uridilil] -L-alanine 1-methylethyl

A process for preparing a compound represented by formula 4 or a diastereomer based phosphor thereof: ** ** Formula wherein P * represents a chiral phosphorus atom, and wherein said diastereomer is based fosforo represents the RP-4 or SP-4 formulas: Formula ** ** comprising: a) reacting a alanate isopropyl, a, X'-di-phenyl phosphate, B, 2'-deoxy-2'-fluoro 2'-C-methyluridine, 3, and a base to obtain a first mixture comprising 4 or diastereomer based phosphor thereof; ** ** Formula wherein X is a conjugate base of an acid, n is 0 or 1, and X 'is a halogen b) reacting the first mixture with a protecting compound to obtain a second mixture comprising 4 or diastereomer based phosphor thereof, wherein the protective compound is selected from a t-butyldimethylsilyl protective compound and safening compound dimethoxytriphenylmethyl; and c) optionally subjecting the second mixture to crystallization, chromatography or extraction in order to obtain 4, or diastereomer based phosphor thereof.