We saw in Section 25.6 that DCC can be used to form a peptide bond. We explored the mechanism, and we saw that DCC activates the COOH moiety so that it readily undergoes nucleophilic acyl substitution. An alternative method for activating a COOH group involves converting it into an activated ester, such as a para€‘nitrophenyl ester:

The activated ester is readily attacked by a suitably protected amino acid to form a peptide bond:

(a) Explain how the p-nitrophenyl ester activates the carbonyl group toward nucleophilic acyl substitution.

(b) What is the function of the nitro group?

(c) A meta-nitrophenyl ester is less activating than a paranitrophenyl ester. Explain.

Transcribed Image Text:

н "ОН Boc NO2 Boc A para-nitrophenyl ester R. NO2 Н.N Boc LOR N. Н Boc R. R-