Phosphonamidate butyrophilin ligands

A compound of formula I:

• or a salt thereof, wherein:
• R1 is aryl, 5-6 membered heteroaryl, aryl(C1-C6)alkyl, or (5-6 membered heteroaryl)(C1-C6)alkyl, wherein any aryl, 5-6 membered heteroaryl, aryl(C1-C6)alkyl, or (5-6 membered heteroaryl)(C1-C6)alkyl is optionally substituted with one or more groups Rx independently selected from the group consisting of hydroxyl, halo, nitro, cyano, carboxy, NRaRb, (C1-C6)alkyl, (C1-C6)alkoxy, (C1-C6)alkanoyl, (C1-C6)alkoxycarbonyl, (C1-C6)alkylthio, and (C2-C6)alkanoyloxy, wherein any (C1-C6)alkyl and (C1-C6)alkoxy of Rx is optionally substituted with one or more groups independently selected from halo;
• R2 is (C1-C6)alkyl that is optionally substituted with one or more groups independently selected from the group consisting of hydroxy, halo, nitro, cyano, carboxy, NRcRd, (C1-C6)alkoxy, (C1-C6)alkanoyl, (C1-C6)alkoxycarbonyl, (C1-C6)alkylthio, and (C2-C6)alkanoyloxy;
• R3 is oxo (═O) or hydroxy;
• R4 is the side-chain of a natural amino acid;
• each Ra and Rb is independently selected from the group consisting of H, (C1-C6)alkyl, (C3-C6)cycloalkyl, and (C3-C6)cycloalkyl(C1-C6)alkyl;
• or Ra and Rb together with the nitrogen to which they are attached form a aziridino, azetidino, morpholino, piperazino, pyrrolidino or piperidino, which aziridino, azetidino, morpholino, piperazino, pyrrolidino or piperidino is optionally substituted with one or more groups independently selected from the group consisting of halo and (C1-C6)alkyl;
• and
• each Rc and Rd is independently selected from the group consisting of H, (C1-C6)alkyl, (C3-C6)cycloalkyl, and (C3-C6)cycloalkyl(C1-C6)alkyl;
• or Rc and Rd together with the nitrogen to which they are attached form a aziridino, azetidino, morpholino, piperazino, pyrrolidino or piperidino, which aziridino, azetidino, morpholino, piperazino, pyrrolidino or piperidino is optionally substituted with one or more groups independently selected from the group consisting of halo and (C1-C6)alkyl.