The present invention discloses latent thioester functionalities attached to the C-terminus of a first polypeptide, or a first fragment thereof having a Cys residue at its N-terminus, and a process using this functionality for the preparation of polypeptide thioesters, in particular of ubiquitin thioesters, this process comprising preparing a polypeptide or a fragment thereof, being attached to a latent thioester functionality, which can then be ligated with a second polypeptide fragment, followed by selective activation of the latent thioester functionality group, to provide the requested polypeptide thioester. There are also provided the polypeptides obtained by this method, specific unnatural amino acids useful to be incorporated within the polypeptide thioesters, and kits for preparing them.