Abstract: The invention provides compounds of the formula ##STR1## or a pharmaceutically acceptable salt thereof, wherein [D] is the residue of a drug having a reactive functional group, said functional group being attached, directly or through a bridging group, via an oxygen-phosphorus bond to the phosphorus atom of the ##STR2## moiety; R.sub.1 is C.sub.1 -C.sub.8 alkyl, C.sub.6 -C.sub.10 aryl or C.sub.7 -C.sub.12 aralkyl; R.sub.2 is hydrogen, C.sub.1 -C.sub.8 alkyl, C.sub.6 -C.sub.10 aryl, C.sub.4 -C.sub.9 heteroaryl, C.sub.3 -C.sub.7 cycloalkyl, C.sub.3 -C.sub.7 cycloheteroalkyl or C.sub.7 -C.sub.12 aralkyl; and R.sub.3 is selected from the group consisting of C.sub.1 -C.sub.8 alkyl; C.sub.2 -C.sub.8 alkenyl having one or two double bonds; (C.sub.3 -C.sub.7 cycloalkyl)--C.sub.r H.sub.2r --wherein r is zero, one, two or three, the cycloalkyl portion being unsubstituted or bearing 1 or 2 C.sub.1 -C.sub.4 alkyl substituents on the ring portion; (C.sub.6 -C.sub.10 aryloxy)C.sub.1 -C.sub.