In another embodiment, R1 is an optionally substituted group selected from the group consisting of C1-C18 alkyl; C2-C18 alkenyl; polyethyleneglycol; aryl; cycloalkyl; heterocyclyl and heteroaryl, wherein said heterocyclyl and heteroaryl include one or more heteroatoms and wherein said heteroatoms are selected from N, O and S;