Carbamates benzyl, amine protecting groups

Benzyl groups are the second fiddles of the amine protection repertoire, and they are especially useful when a substrate is to be subjected to powerful orga-no metal lie reagents or metal hydrides which might attack a carbamate. Benzyl-amines are not generally cleaved by Lewis acids under preparatively useful conditions. The following discussion embraces benzylamines, diphenylmethylamines and their methoxy-substituted derivatives. 

A,r-Benzyl groups can be removed by reaction of amines with chloroformates. This can be a useful method for protecting-group manipulation if the resulting carbamate is also easily cleaved. A particularly effective reagent is a-chloroethyl chloroformate, which can be removed by subsequent solvolysis.78... 

N-Salicylideneamines, to protect amines, 370 Selective cleavage, 411-412 of benzyl carbamates, 339-340 of benzyl ethers, 49-56 of catechol protective groups, 171 of N- vs. 5-dimethylphosphinothioyl groups, 307... 

The first step in peptide synthesis is blocking (protection) of the amine functional group of an amino acid (a compound that contains both amine and carboxylic acid functional groups). Such a reaction is shown in Section 24.7C in the reaction between Ala (alanine) and benzyl chloroformate. The functional group formed in the structure labeled Z-Ala is called a carbamate (or urethane). 

Amino groups can be blocked by converting them into carbamates through simple addition-elimination reactions (p. 901).Two popular methods involve the transformation of the free amine into a carbamate by reaction with either di- -butyl dicarbonate or benzyl chloroformate. Biochemists are even more addicted to acronyms than are organic chemists, and these protecting groups are called tBoc and Cbz, respectively (Fig. 23.45). 

---