Compound as a ubr box domain ligand

A compound having the structure of formula 1:

• or a salt thereof,
• wherein the X1 is an optionally substituted with one or more R2 or unsubstituted phenyl, cycloalkyl or heterocyclyl;
• each R2 is independently selected from alkyl, alkoxy, amino, aminoalkyl, —NO2, ═O, —NHC2H4OH, —C(═NH)NH2, —C(═O)NH2, —C(═O)NHCH3, —C(═O)OH, phenyl or heterocycloalkyl;
• X4 is an optionally substituted with one or more R3 or unsubstituted phenyl, cycloalkyl or heterocyclyl;
• each R3 is independently selected from alkyl, alkoxy, amino, halo, hydroxyl, alkylamino, dialkylamino, —NO2, —CONR′R″, —CO2R′, —NHCOR′, phenyl or heterocycloalkyl;
• each R′ and R″ is independently —H or alkyl;
• X2 is SO2 or CRaRb;
• Ra and Rb are each independently H or CHs;
• X3 is NH or CH2;
• B1 is CH2 or NH;
• A1 is CH2 or NH.