Compounds are of the formula (Ia), (Ib), (Ic), or are stereoisomers thereof, wherein: R1 is hydrogen, (C1-C20)alkyl; (C3-C20)alkenyl; (C3 C20)alkynyl; (C1-C6)alkyl-O—; (C3-C20)cycloalkyl; (C1 C20)haloalkyl; (C6-C20)aryl optionally substituted; (C6-C20)heteroaryl optionally substituted; R2 and R2′ are hydrogen; (C1-C20)alkyl; (C1-C6)alkyl-O—; (C1-C6)haloalkyl; halogen;cyano; and nitro; Z1 to Z4 are diradicals of formula (III) wherein Al and A2 are O—or —NR3-, wherein R3 is selected from the group consisting of hydrogen and (C1-C20)alkyl; and G is (C1-C6)alkyl; —P(═S)R5-; —P(═O)R4; P(═O)(OR4)-; —P(═O)(NR6R7)-; —S(=0)2-; S(═O)—; or —C(═O)—; and Y1 to Y4 are (C1-C8)alkyl; (C3-C7)cycloalkyl; (C6-C20)aryl optionally substituted; or (C6-C20)heteroaryl optionally substituted; and FG1 and FG2 are H, OH, or NHR8.