Synthesis of diamino-furostan sapogenins and their use as scaffolds for positioning peptides in a preorganized form

The synthesis of peptide-furostane conjugates from natural steroidal sapogenins is reported. The approach comprises the introduction of α-oriented amino groups into spirostanic sapogenins followed by reductive opening of the spiroketal chain, thus producing diamino-furostanic scaffolds suitable for further functionalization. Solid and solution-phase coupling processes were utilized for the incorporation of various α-amino acids and peptides into the furostanic skeletons. The attachment position depends on the steroidal sapogenin originally used, i.e., diosgenin or hecogenin. The resulting furostanic skeletons feature a trans A/B-ring fusion and hold the peptides in axial disposition. This characteristic ensures a preorganized alignment of the peptidic motifs, an important structural feature for future applications in molecular recognition and catalysis. A macrocyclic tripeptide-furostane conjugate was also produced by a combination of peptide coupling, Staudinger ligation, and a cyclization protocol. This work constitutes the first report on the use of furostanic sapogenins as scaffolds for positioning natural amino acids and (cyclo)peptides.