consisting of: ???OH; -halogen; ???N(R5)2; ???CO???N(R5)2; ???CO???C1-10 alkyl; ???CO???O???C1-0 alkyl; ???N3; -aryl; -heteroaryl; -heterocyclyl; ???CO-aryl; and ???CO-heteroaryl; each R3 is ???O or ???S; each R4 is independently alkyl or alkenyl, which may be interrupted by one or more ???O??? groups; each R5 is independently H or C1-10 alkyl; R6 is a bond, or is alkyl, or alkenyl, which may be i