Abstract
This chapter discusses the chromogenic acid-labile protecting groups for use in peptide synthesis. In the trityl butyl groups, the formation of the carbonium ion is assisted by conjugative electron displacement and, once formed, the carbonium ion is stabilized by conjugative mesomerism. The formation of the tertiary butyl carbonium ion is assisted by the inductive effect of the methyl groups, and the resultant ion is stabilized, to some extent, by hyperconjugative mesomerism. The relationship between the degree of heterolysis and the structure of the alkyl or aralkyl group is similar, allowing for differences in protonation. 4,4'-dimethoxy-benzhydryl; 3,3',4,4'-tetramethoxy-benzhydryl; and 2,2'-dimethyl-4,4'-dimethoxy-benzhydryl chlorides react readily with amino-acid esters in chloroform solution in the presence of triethylamine to yield the corresponding N-substituted amino-acid methyl esters. In a manner reminiscent of the trityl amino-acid series, benzhydryl-glycine methyl ester derivatives are readily saponified whereas the phenylalanine compounds show marked resistance to saponification. Saponification is slightly less hindered in the benzhydryl than in the trityl series.
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