Compounds of formula (I) are A2B wherein R1 and R2 are independently selected from hydrogen, or optionally substituted CrC6 alkyl, C1-C6alkoxy-(C1-C6)-alkyl, C3-C8 cycloalkyl, aryl, heteroaryl, aryl-(C1-C6)-alkyl, or heteroaryl-(C1-C6)-alkyl; or R−1 and R2 taken together with the nitrogen atom to which they are attached form an optionally substituted 5- or 6-membered ring; R3 and R4 are independently selected from hydrogen, or optionally substituted C1-C6 alkyl, C1-C6 alkoxy-(C1-C6)-alkyl, C3-C8 cycloalkyl, aryl, heteroaryl, aryl-(C1-C6)-alkyl, or heteroaryl-C1-C6)-alkyl; or R3 and R4 taken together with the nitrogen atom to which they are attached form an optionally substituted 5- or 6-membered ring; R5 and R6 are independently selected from hydrogen, C1-C6 alkyl, aryl, aryl-(C1-C6)-alkyl, —NHR7—N(—R8)—R9, —NH—(C═O)—R10, —(C═O)—NH—R11, —(C═O)—O—R12, or halo; and R7, R8, R9, R10, R−11, and R12 are independently selected from C1-C6 alkyl, aryl, aryl-(C1-C6)-alkyl and heteroaryl.