Amine-protecting group is understood as meaning a group such as, for example, a C1 -C4 -alkyl, such as methyl or tert-butyl,; benzhydryl; trityl; benzoyl; a C1 -C4 -alkylcarbonyl, such as tert-butoxycarbonyl, benzyloxycarbonyl; benzyl or substituted benzyl such as p-nitrobenzyl, p-chlorobenzyl or p-methoxybenzyl.