Reactive intermediates in peptide synthesis: Molecular and crystal structures of HOAt and HOOBt, and some ester and amide derivatives of HOBt, HOAt and HOOBt

X-ray diffraction structure determinations of HOAt and HOOBt, two well-known additives for peptide bond formation, are reported in conjunction with those of seven derivatives of HOBt, HOAt, and HOOBt, five of which originate from N-alpha-protected alpha-amino acids. In the crystal state the two additives are found in the l-hydroxy tautomeric form. In contrast to six derivatives which crystallize as esters, one, Fmoc-(N)OBt, is observed in the isomeric amide-type (urethane) form.