Examples of suitable amine protective groups are an acyl of a carbonic acid such as ethoxy-carbonyl, benzyloxy-carbonyl, tert.butyloxy-carbonyl(=Boc), p-methoxy-benzyloxy-carbonyl, or fluorenylmethoxycarbonyl(=FMOC) or substituted or unsubstituted aryl or aralkyl for example benzyl or triphenylmethyl or o-nitrophenyl-sulfenyl.