ABSTRACT The synthesis of t-butyl esters of amino acids and peptides and their use in custom peptide synthesis is described. The most convenient method was the acid-catalyzed reaction of isobutylene with benzyloxycarbonylamino acids or peptides followed by catalytic hydrogenation to produce the basic esters.
Another general method entailed the use of silver salts and t-butyl iodide. t-Butyl esters are particularly useful in custom peptide synthesis in that the ester group may be removed by acid catalysis and thus side reactions encountered in alkaline hydrolysis may be avoided. Other advantages arise from the stability of amino acid or peptide esters as free bases, particularly in allowing their storage and use as such. Comparative stabilities to ethyl esters are reported in several examples. The synthesis of several i-butyl peptides is reported as well as selective removal of this ester group or of amine-protecting groups when both are present.
Further examples of the use of 1-piperidyl esters for the synthesis of protected dipeptides have given good yields of pure product except when two valine residues were involved (in which case the reaction of the analogous p-nitrophenyl ester was also very slow), or when one residue was α-aminoisobutyric acid. The reactions of benzyloxy-carbonylamino-acid 1-piperidyl esters with the methyl esters of L-leucyl-L-leucylglycine, L-phenylalanyl-L-leucylglycine, and L-valyl-L-eucylglycine proceeded more slowly than with α-amino-esters but satisfactory yields were obtained except in one case, again involving valine.