A compound of formula: or a pharmaceutically acceptable salt or boronic acid anhydride thereof, wherein: A is 0, 1, or 2; P is hydrogen or an amino-group-blocking moiety; R a1 is C 1-6 aliphatic, C 1-6 fluoroaliphatic, -(CH 2) m -CH 2 -R B, -(CH 2) m -CH 2 -NHC(=NR 4)NH-Y, -(CH 2) m -CH 2 -CON(R 4) 2, -(CH 2) m -CH 2 -N(R 4)CON(R 4) 2, -(CH 2) m -CH(R 6)N(R 4) 2, -(CH 2) m -CH(R 5)-OR 5, or -(CH 2) m - CH(R 5)-SR 5; each R a2 independently is hydrogen, C 1-6 aliphatic, C 1-6 fluoroaliphatic, -(CH 2) m -CH 2 -R B, -(CH 2) m -CH 2 -NHC(=NR 4)NH-Y, -(CH 2) m -CH 2 -CON(R 4) 2, -(CH 2) m - CH 2 -N(R 4)CON(R 4) 2, -(CH 2) m -CH(R 6)N(R 4) 2, -(CH 2) m -CH(R 5)-OR 5, or -(CH 2) m -CH(R 5)-SR 5; each Y independently is hydrogen, -CN, -NO 2, or -S 2 -R 10; each R B independently is a substituted or unsubstituted mono- or bicyclic ring system; each R 4 independently is hydrogen or a substituted or unsubstituted aliphatic, aryl, heteroaryl, or heterocyclyl group; or two R 4 on the same nitrogen atom, taken together with the nitrogen atom, form a substituted or unsubstituted 4- to 8-membered heterocyclyl ring having, in addition to the nitrogen atom, 0-2 ring heteroatoms independently selected from N, O, and S; each R 5 independently is hydrogen or a substituted or unsubstituted aliphatic, aryl, heteroaryl, or heterocyclyl group; each R 6 independently is a substituted or unsubstituted aliphatic, aryl, or heteroaryl group; each R 10 independently is C 1-6 aliphatic, C 6-10 aryl, or -N(R 4) 2; m is 0, 1, or 2; Z 1 and Z 2 are each independently hydroxy, alkoxy, aryloxy, or aralkoxy; or Z 1 and Z 2 together form a moiety derived from a boronic acid complexing agent. A is 0, 1, or 2; P is hydrogen or an amino-group-blocking moiety; R a1 is C 1-6 aliphatic, C 1-6 fluoroaliphatic, -(CH 2) m -CH 2 -R B, -(CH 2) m -CH 2 -NHC(=NR 4)NH-Y, -(CH 2) m -CH 2 -CON(R 4) 2, -(CH 2) m -CH 2 -N(R 4)CON(R 4) 2, -(CH 2) m -CH(R 6)N(R 4) 2, -(CH 2) m -CH(R 5)-OR 5, or -(CH 2) m - CH(R 5)-SR 5; each R a2 independently is hydrogen, C 1-6 aliphatic, C 1-6 fluoroaliphatic, -(CH 2) m -CH 2 -R B, -(CH 2) m -CH 2 -NHC(=NR 4)NH-Y, -(CH 2) m -CH 2 -CON(R 4) 2, -(CH 2) m - CH 2 -N(R 4)CON(R 4) 2, -(CH 2) m -CH(R 6)N(R 4) 2, -(CH 2) m -CH(R 5)-OR 5, or -(CH 2) m -CH(R 5)-SR 5; each Y independently is hydrogen, -CN, -NO 2, or -S 2 -R 10; each R B independently is a substituted or unsubstituted mono- or bicyclic ring system; each R 4 independently is hydrogen or a substituted or unsubstituted aliphatic, aryl, heteroaryl, or heterocyclyl group; or two R 4 on the same nitrogen atom, taken together with the nitrogen atom, form a substituted or unsubstituted 4- to 8-membered heterocyclyl ring having, in addition to the nitrogen atom, 0-2 ring heteroatoms independently selected from N, O, and S; each R 5 independently is hydrogen or a substituted or unsubstituted aliphatic, aryl, heteroaryl, or heterocyclyl group; each R 6 independently is a substituted or unsubstituted aliphatic, aryl, or heteroaryl group; each R 10 independently is C 1-6 aliphatic, C 6-10 aryl, or -N(R 4) 2; m is 0, 1, or 2; Z 1 and Z 2 are each independently hydroxy, alkoxy, aryloxy, or aralkoxy; or Z 1 and Z 2 together form a moiety derived from a boronic acid complexing agent.