A compound of formula: wherein: either: Y 1 is X a and Y 2 is hydrogen, or: Y 2 is X a and Y 1 is hydrogen; wherein: X a is selected from carboxylic acid, carboxyl ester, heterocyclyl, and heteroaryl; or X a is is selected from heterocyclyl or heteroaryl, and is substituted with X 1, wherein X 1 is C 1-6 alkyl or a progroup; R 1 is selected from hydrogen, alkyl, alkenyl, alkynyl, cycloalkyl, heterocyclyl, aryl, heteroaryl, and -R 15, wherein R 15 comprises a linking group and a compound of formula; or R 1 is alkyl substituted with 1 to 5 substituents selected from halogen, alkyl, alkenyl, alkynyl, alkoxy, hydroxyl, carboxyl, carboxyl ester, amino, acyl, aminoacyl, acylamino, thioalkoxy, sulfonyl, aminosulfonyl, sulfonylamino, cycloalkyl, heterocyclyl, aryl, and heteroaryl; or R 1 is: wherein: X 1b is selected from C 1-6 alkyl and a progroup; or R 1 has the formula: wherein: R d and R d' are independently selected from H and C 1-6 alkyl; or a compound of formula: wherein: either: Y 11 is -C(=O)-O-X 1 and Y 12 is hydrogen, or: Y 12 is -C(=O)-O-X 1 and Y 11 is hydrogen; wherein: X 1 is a H, C 1-6 alkyl or a progroup; R 1 is selected from hydrogen, alkyl, alkenyl, alkynyl, cycloalkyl, heterocyclyl, aryl, heteroaryl, and -R 15, wherein R 15 comprises a linking group and a compound of formula; or R 1 is alkyl substituted with 1 to 5 substituents selected from halogen, alkyl, alkenyl, alkynyl, alkoxy, hydroxyl, carboxyl, carboxyl ester, amino, acyl, aminoacyl, acylamino, thioalkoxy, sulfonyl, aminosulfonyl, sulfonylamino, cycloalkyl, heterocyclyl, aryl, and heteroaryl; or R 1 is: wherein: X 1b is selected from C 1-6 alkyl and a progroup; and wherein: the linking group is -(CH 2) n -Z y -(CH 2) m -, wherein: n is an integer from 1 to 6; m is 0 or an integer from 1 to 6; y is 0 or 1; and Z is O, NH, -O-P -O-, S, S, SO 2, or -O-S 2 -O-; and wherein: each progroup is: a hydroxyl functional group masked as a sulfonate, ester, or carbonate; an amino functional group masked as an amide, carbamate, imine, urea, phosphenyl, phosphoryl, or sulfenyl; or a carboxyl group masked as an ester, silyl ester, thioester, amide, or hydrazide; and additionally X 1 may be a progroup of the formula: wherein: L 1 is C 1-6 alkyl or C 1-6 alkyl substituted with one or more groups selected from alkyl, alkenyl, alkynyl, cycloalkyl, heterocyclyl, aryl, and heteroaryl; X 2 is optional and selected from O, N and S; R 2 is selected from hydrogen, alkyl, alkenyl, alkynyl, acyl, aminoacyl, carboxyl, carboxyl ester, cycloalkyl, heterocyclyl, aryl, and heteroaryl; and R 3 and R 4 are each independently optional and selected from hydrogen, alkyl, alkenyl, alkynyl, oxo, cycloalkyl, heterocyclyl, aryl, and heteroaryl; or R 2 together with X 2 and either R 3 or R 4 form a heterocyclyl or heteroaryl; or a compound of the following formula: or a salt or stereoisomer thereof. either: Y 1 is X a and Y 2 is hydrogen, or: Y 2 is X a and Y 1 is hydrogen; X a is selected from carboxylic acid, carboxyl ester, heterocyclyl, and heteroaryl; or X a is is selected from heterocyclyl or heteroaryl, and is substituted with X 1, wherein X 1 is C 1-6 alkyl or a progroup; R 1 is selected from hydrogen, alkyl, alkenyl, alkynyl, cycloalkyl, heterocyclyl, aryl, heteroaryl, and -R 15, wherein R 15 comprises a linking group and a compound of formula; or R 1 is alkyl substituted with 1 to 5 substituents selected from halogen, alkyl, alkenyl, alkynyl, alkoxy, hydroxyl, carboxyl, carboxyl ester, amino, acyl, aminoacyl, acylamino, thioalkoxy, sulfonyl, aminosulfonyl, sulfonylamino, cycloalkyl, heterocyclyl, aryl, and heteroaryl; or R 1 is: wherein: X 1b is selected from C 1-6 alkyl and a progroup; or R 1 has the formula: wherein: R d and R d' are independently selected from H and C 1-6 alkyl; either: Y 11 is -C(=O)-O-X 1 and Y 12 is hydrogen, or: Y 12 is -C(=O)-O-X 1 and Y 11 is hydrogen; X 1 is a H, C 1-6 alkyl or a progroup; R 1 is selected from hydrogen, alkyl, alkenyl, alkynyl, cycloalkyl, heterocyclyl, aryl, heteroaryl, and -R 15, wherein R 15 comprises a linking group and a compound of formula; or R 1 is alkyl substituted with 1 to 5 substituents selected from halogen, alkyl, alkenyl, alkynyl, alkoxy, hydroxyl, carboxyl, carboxyl ester, amino, acyl, aminoacyl, acylamino, thioalkoxy, sulfonyl, aminosulfonyl, sulfonylamino, cycloalkyl, heterocyclyl, aryl, and heteroaryl; or R 1 is: wherein: X 1b is selected from C 1-6 alkyl and a progroup; n is an integer from 1 to 6; m is 0 or an integer from 1 to 6; y is 0 or 1; and Z is O, NH, -O-P -O-, S, S, SO 2, or -O-S 2 -O-; a hydroxyl functional group masked as a sulfonate, ester, or carbonate; an amino functional group masked as an amide, carbamate, imine, urea, phosphenyl, phosphoryl, or sulfenyl; or a carboxyl group masked as an ester, silyl ester, thioester, amide, or hydrazide; L 1 is C 1-6 alkyl or C 1-6 alkyl substituted with one or more groups selected from alkyl, alkenyl, alkynyl, cycloalkyl, heterocyclyl, aryl, and heteroaryl; X 2 is optional and selected from O, N and S; R 2 is selected from hydrogen, alkyl, alkenyl, alkynyl, acyl, aminoacyl, carboxyl, carboxyl ester, cycloalkyl, heterocyclyl, aryl, and heteroaryl; and R 3 and R 4 are each independently optional and selected from hydrogen, alkyl, alkenyl, alkynyl, oxo, cycloalkyl, heterocyclyl, aryl, and heteroaryl; or R 2 together with X 2 and either R 3 or R 4 form a heterocyclyl or heteroaryl;