FORMATION OF AMINOSUCCINYL PEPTIDES DURING ACIDOLYTIC DEPROTECTION FOLLOWED BY THEIR TRANSFORMATION TO PIPERAZINE‐2, 5‐DIONE DERIVATIVES IN NEUTRAL MEDIA

Abstract

Prolonged acidic treatment of Boc‐Leu‐Asp(OBu t )‐Phe‐NH 2 with 4N HCl in acetic acid resulted in H‐[Leu‐Asc‐Phe‐NH 2 ] HCl(Asc, aminosuccinyl), which transformed partially to cyclo[Leu‐Asp(Phe‐NH 2 )] during its purification by column chromatography on silica gel with a mixture of ethyl acetate/pyridine/acetic acid/water = 60:20:6:11, i.e. in neutral medium. Examination of the imide formation was extended to different reaction conditions (no imide derivative was detected in trifluoroacetic acid), to several protected derivatives of L‐aspartyl‐L‐phenylalaninamide and to tripeptides containing an aspartyl residue in the middle position. It was clearly demonstrated that in strongly acidic media the imide derivatives are directly formed from the aspartyl peptides containing a free β‐carboxyl group. The influence of the C‐terminal residue was greater than the N‐terminal on both the rate of formation of the imide and its further transformation to piperazine‐2,5‐dione derivative. In aqueous ethanol the X‐Asc‐Y‐NH 2 (X, Pro, Leu; Y, Gly, Ala, Val, Phg, Phe) containing N‐terminal proline are more readily transformed to piperazine‐2,5‐dione derivatives, but compared to simple proline dipeptides the rate of this transformation is relatively slow because of the crowdedness of the tricyclic transitional state.