Synthesis of the cyclic heptapeptides axinastatin 2 and axinastatin 3

Practical total syntheses of axinastatins 2 2b and 3 2c were completed by employing Fmoc protection for the N-terminal, and tert-butyl ester blocking for the C-terminal, units of the amino acid and peptide intermediates. Generally, diethyl phosphorocyanidate proved effective for formation of the peptide bond, and in the one exception, asparagine, the o-nitrophenyl active ester proved to be useful. For the final cyclization reaction BOP-Cl was found especially effective.