Procedure:  100 mL 1-neck flask, stirbar, septum, condenser, N₂ inlet

                  Prepared a suspension of 0.44 g of NaH in 20 mL of dry THF.  Stirred at rt.  Added 4.789 g diol I.  Heated to reflux.  After 2 h the reaction was cooled back to rt and a solution of 2.5 mL of PMBCl in 5.0 mL of THF was added.  The reaction was heated back to reflux.  After 48 h, TLC (20:80 EtOAc-hexanes, PMA) still showed some unreacted PMBCl.  Quenched with sat. aq. NH₄Cl and extracted with Et₂O.  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 30:70 EtOAc-hexanes as eluant.  The product was a clear, pale yellow oil. 

 

notes

 

This reaction involves the mono-protection of a diol as it's PMB ether.  Refluxing the diol in the presence of NaH forms the insoluble anion which precipitates thus preventing the formation of the dianion and allowing for a better than statistical average formation of the mono-PMB ether. 

 

PMB (para-methoxybenzyl) is also abbreviated MPM for para-methoxyphenylmethyl.

 

PMB ethers are generally more susceptible to cleavage by acid than benzyl ethers.