Syrbactin macrolactams and unnatural analogs as proteasome inhibitors for the treatment of multiple myeloma and chemoenzymatic synthesis thereof

A compound of formula:

• or salt thereof, wherein:
• R1 is optionally substituted alkyl, optionally substituted cycloalkyl, or optionally substituted aryl;
• R2 is optionally substituted alkyl or optionally substituted aryl;
• R3 is optionally substituted alkyl;
• and
• each of R10, R11, R12, and R13 is hydrogen or —OR′, or R10 and R11 are taken together to form ═O, or R12 and R13 are taken together to form ═O, wherein each instance of R′ is independently hydrogen, optionally substituted alkyl, or optionally substituted acyl;
• provided that if R2 is —CH3, R3 is —CH3, R10 is hydrogen, R11 is hydrogen or —OH, R12 is hydrogen, and R13 is hydrogen, R1 is not —CH(CH3)2 or —CH2CH3.