A compound, or a stereoisomer or salt thereof, wherein the compound has a structure represented by formula III: wherein one of W 1 or W 2 is OR 5 R 5 is (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 8)carbocyclyl, (C 4 -C 8)carbocyclylalkyl, aryl(C 1 -C 8)alkyl, heterocyclyl(C 1 -C 8)alkyl, (C 6 -C 20)aryl, (C 2 -C 20)heterocyclyl or heteroaryl; and the other of W 1 or W 2 is (i) each R c and R d is independently H, (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 8)carbocyclyl, (C 4 -C 8)carbocyclylalkyl, aryl(C 1 -C 8)alkyl, heterocyclyl(C 1 -C 8)alkyl, (C 6 -C 20)aryl, (C 2 -C 20)heterocyclyl or heteroaryl; or (ii) each R c is H and each R d is independently (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 8)carbocyclyl, (C 4 -C 8)carbocyclylalkyl, aryl(C 1 -C 8)alkyl, heterocyclyl(C 1 -C 8)alkyl, (C 6 -C 20)aryl, (C 2 -C 20)heterocyclyl or heteroaryl; or (iii) each R c is H and each R d is independently (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 8)carbocyclyl, (C 4 -C 8)carbocyclylalkyl, aryl(C 1 -C 8)alkyl, heterocyclyl(C 1 -C 8)alkyl, (C 6 -C 20)aryl, (C 2 -C 20)heterocyclyl or heteroaryl wherein the chirality of the carbon to which said R c and R d is attached is S; or (iv) each R c is H and each R d is independently (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 8)carbocyclyl, (C 4 -C 8)carbocyclylalkyl, aryl(C 1 -C 8)alkyl, heterocyclyl(C 1 -C 8)alkyl, (C 6 -C 20)aryl, (C 2 -C 20)heterocyclyl or heteroaryl wherein the chirality of the carbon to which said R c and R d is attached is R; or (v) each R c is H and each R d is independently (C 1 -C 8)alkyl; or (vi) each R c is H and each R d is independently (C 1 -C 8)alkyl wherein the chirality of the carbon to which said R c and R d is attached is S; or (vii) each R c is H and each R d is independently (C 1 -C 8)alkyl wherein the chirality of the carbon to which said R c and R d is attached is R; and each R 6 is independently (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 8)carbocyclyl or (C 4 -C 8)carbocyclylalkyl. (i) each R c and R d is independently H, (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 8)carbocyclyl, (C 4 -C 8)carbocyclylalkyl, aryl(C 1 -C 8)alkyl, heterocyclyl(C 1 -C 8)alkyl, (C 6 -C 20)aryl, (C 2 -C 20)heterocyclyl or heteroaryl; or (ii) each R c is H and each R d is independently (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 8)carbocyclyl, (C 4 -C 8)carbocyclylalkyl, aryl(C 1 -C 8)alkyl, heterocyclyl(C 1 -C 8)alkyl, (C 6 -C 20)aryl, (C 2 -C 20)heterocyclyl or heteroaryl; or (iii) each R c is H and each R d is independently (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 8)carbocyclyl, (C 4 -C 8)carbocyclylalkyl, aryl(C 1 -C 8)alkyl, heterocyclyl(C 1 -C 8)alkyl, (C 6 -C 20)aryl, (C 2 -C 20)heterocyclyl or heteroaryl wherein the chirality of the carbon to which said R c and R d is attached is S; or (iv) each R c is H and each R d is independently (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 8)carbocyclyl, (C 4 -C 8)carbocyclylalkyl, aryl(C 1 -C 8)alkyl, heterocyclyl(C 1 -C 8)alkyl, (C 6 -C 20)aryl, (C 2 -C 20)heterocyclyl or heteroaryl wherein the chirality of the carbon to which said R c and R d is attached is R; or (v) each R c is H and each R d is independently (C 1 -C 8)alkyl; or (vi) each R c is H and each R d is independently (C 1 -C 8)alkyl wherein the chirality of the carbon to which said R c and R d is attached is S; or (vii) each R c is H and each R d is independently (C 1 -C 8)alkyl wherein the chirality of the carbon to which said R c and R d is attached is R; and