Provided are compounds of the formula: wherein RN1 is a substituent of formula G1—NX1X2, wherein G1 is an optionally further substituted alkylene, which optionally forms, together with RN2, a cyclic group, and each of X1 and X2 is independently H or an N-substituent, or X1 and X2 together form a heterocyclic ring, or X1 together with G1 forms a cyclic group and X2 is H or an N-substituent; and each of Z1, Z2, Z3 and Z4 is H or a substituent, or two of Z1, Z2, Z3 and Z4 together form an optionally substituted ring, and further wherein at least one of Z1, Z2, Z3 and Z4 is other than H, and salts thereof, pharmaceutical compositions and methods of using the compounds. The compounds have antiviral activity.