Ethers hydrogenolysis

The oxirane ring in 175 is a valuable function because it provides a means for the introduction of the -disposed C-39 methoxy group of rapamycin. Indeed, addition of CSA (0.2 equivalents) to a solution of epoxy benzyl ether 175 in methanol brings about a completely regioselective and stereospecific solvolysis of the oxirane ring, furnishing the desired hydroxy methyl ether 200 in 90 % yield. After protection of the newly formed C-40 hydroxyl in the form of a tert-butyldimethylsilyl (TBS) ether, hydrogenolysis of the benzyl ether provides alcohol 201 in 89 % overall yield. 

Bamford-Stevens decomposition of tosylhy-drazones, 351 p-Benzoquinone, 308 Benzyl ether hydrogenolysis, 139 Benzyl thioenol ethers, 87 Birch reduction, 11, 49, 50 Birch reduction of estrone methyl ether diethyl ketal, 51... 

Catalytic hydrogenolysis offers the mildest method for deprotecting benzyl ethers. Hydrogenolysis of 2°- and 3°-benzyl ethers may be sluggish. Protection of alcohols using (benzyloxy)methyl chloride produces the corresponding (benzyloxy)methyl ethers (RO-BOM), which are cleaved more readily than the corresponding ROBn ethers. ... 

The following table shows how substituents can affect the relative rate of benzyl ether hydrogenolysis ... 

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