As far as I know the problems with coupling to p-nitroaniline are even more
serious than you expect: no reaction is observed when standard coupling
procedures are employed.
In my opinion even HATU may fail with this amine. DMAP catalysis may be
more helpful in this case.
However, an old but efficient synthetic approach for p-nitoanilides exists.
The procedure is based on reaction of p-nitrophenylisocyanate with
COOH-group of protected peptide or amino acid followed by carbon dioxide
elimination.
I have no exact reference for the related original paper at hand but I am
sure that this old peptide chemistry (including detailed protocols) is well
documented in:
Synthese von Peptiden. (Muller, E. Ed.) In: Houben Weil's Methoden der
Organichen Chemie, Stuttgart: Thieme 1974, Bd.15, Teil 1-2.
A native German-speaking friend may be of tremendous help in getting
necessary information from this great book!
Best wishes,
Igor Rodionov
Laboratory Of Peptide Chemistry
Branch of Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry
Pushchino, Moscow Region
142292, Russian Federation
E-mail: <rodionov@fibkh.serpukhov.su> or <Igor_Rodionov@iname.com>
Fax: (0967)-79-05-27
Phones: (0967) 73 54 42; private: (0967) 73 58 26