...persilylated form, wherein: 22 P3 is hydrogen or an amino-protecting group, with an 2-substituted glycerol of the formula 23 wherein: 24 Y1 and Y2 independently are halo, C2-7-acyloxy, C1-6-alkyloxy, or aryl(lower)alkyloxy groups, and Z is a leaving group selected from C2-7-acyloxy, methoxy, isopropyloxy, benzyloxy, halo, mesyloxy or tosyloxy; optionally in the presence of a Lewis acid catalyst, to provide...