Immunomodulators and immunomodulator conjugates

A compound of formula I:

• wherein:
• R1 is Rk—O—C(═O)—;
• R2 is H, NRgRh, (C1-C6)alkanoyl, (C1-C6)alkoxycarbonyl, (C1-C6)alkanoyloxy, RmRnNC(═O)—, or (C1-C6)alkyl optionally substituted with one or more hydroxy, —SH, halo, oxiranyl, (C3-C8)cycloalkyl, aryl, (C1-C6)alkoxy, (C1-C6)alkylthio, or NRgRh;
• R3 is (C1-C6)alkyl, optionally substituted with one or more hydroxy, (C1-C6)alkoxy, aryl, aryl(C1-C6)alkoxy or oxiranyl;
• Ra is H or (C1-C6)alkyl;
• Rb is H or X-Y;
• Rg and Rh are each independently H or or (C1-C6)alkyl;
• or taken together with the nitrogen to which they are attached form a aziridino, azetidino, morpholino, piperazino, pyrrolidino or piperidino ring, which ring may optionally be substituted with one or more (C1-C6)alkyl;
• Rk is H, (C1-C6)alkyl, (C2-C6)alkenyl, (C2-C6)alkynyl, (C3-C8)cycloalkyl, trifluoromethyl, aryl, or aryl(C1-C6)alkyl, wherein each (C1-C6)alkyl can optionally be substituted with one or more halo, (C1-C6)alkanoyloxy, (C1-C6)alkoxy, (C3-C8)cycloalkyl;
• Rm and Rn are each independently H or or (C1-C6)alkyl;
• or taken together with the nitrogen to which they are attached form a aziridino, azetidino, morpholino, piperazino, pyrrolidino or piperidino ring, which ring may optionally be substituted with one or more (C1-C6)alkyl;
• X is a linking group;
• and
• Y is an antigen or maleimide;
• wherein the tricyclic ring structure in formula I can optionally be further substituted on one or more carbons with one or more groups independently selected from halo, hydroxy, nitro, (C1-C6)alkyl, (C1-C6)alkenyl, (C1-C6)alkynyl, (C1-C6)alkoxy, (C1-C6)alkanoyl, (C1-C6)alkanoyloxy, (C1-C6)alkoxycarbonyl, trifluoromethyl, trifluoromethoxy, cyano, and NRpRq;
• and
• Rp and Rq are each independently H or or (C1-C6)alkyl;
• or taken together with the nitrogen to which they are attached form a aziridino, azetidino, morpholino, piperazino, pyrrolidino or piperidino ring, which ring may optionally be substituted with one or more (C1-C6)alkyl;
• or a salt thereof.