Procedure:  5 mL 1-neck flask, stirbar, cap

                  Dissolved 0.054 g of I in 1.0 mL of acetone and 0.1 mL of water.  The mixture was stirred at rt and 0.031 g of NMO followed by 0.02 mL of OsO₄ (2.5 wt% in isobutanol) was added.  After 3 h, TLC (10:90 EtOAc-hexanes, CAM) showed product spot at Rf 0.50.  The reaction mixture was quenched with sat. aq. NaHSO₃, stirred for 30 min and extracted with EtOAc.  The organic layer was dried over MgSO₄, filtered and the solvent was removed by rotary evaporation.  The product was isolated by flash chromatography on silica gel using 10:90 EtOAc-hexanes as eluant. 

 

notes

 

In this particular reaction, the protected syn diol functional groups adopt an antiperiplanar orientation due to the steric repulsion between the bulky TBS protecting groups.  This orientation forces a gauche relationship between the alkene and the alkyne and, due to a preference for conformer A over conformer B, effectively shields one face of the double bond (see Gung, B.W.; Melnick, J.P.; Wolf, M.A.; Marshall, J.A.; Beaudoin, S.  J. Org. Chem.  1994, 59, 5609).   

 

The osmate ester is then stereoselectively formed on the exposed face of the double bond leading to a product which is a single diastereomer.