Protection for the Hydroxyl Group Ethers

Reactivity Chart 1. Protection for the Hydroxyl Group Ethers 

9-(9-Phenyl-10-oxo)anthryl Ether (Tritylone) 43. Trimethylsilyl Ether (TMS) 

---

The t-butyl group is useful also for the protection of the hydroxyl group of serine or tyrosine. The ethers are cleaved by HBr-AcOH or by HCI-CHCI3. The group is not useful for protection of sulfhydryl groups because drastic conditions are required for cleavage. 

Protection of the hydroxyl group will be needed during the Friedel-Crafts reaction and a methyl ether is easiest as acid (30) is available. The benzofuran (29) is reactive enough for a Friedel-Crafts catalyst to be unnecessary. Methyl ether protecting groups are usually removed with HBr or HI (see Table 9.1) but these strong acids would destroy the furan. Pyridinium chloride proves satisfactory in practice. 

Tetrahydropyranyl (THP) ethers are frequently used as a protecting group for the hydroxyl group. Their formation and cleavage from alcohols and phenols have been successfully catalyzed by Bi(N03)3 5H20 [92] and Bi(0Tf)3 %H20 [93]. The conversion of THP ethers to acetate and formate esters has been reported to be efficiently catalyzed by Bi(0Tf)3 xH20 [94]. 

---