Within this embodiment, both R groups or R groups are hydrogen while the remaining groups are other than hydrogen. [0019] In one embodiment, each instance of R 1 and R 2 independently is hydrogen; or optionally substituted Ci-C 20 alkyl, Ci-C 20 haloalkyl, aryl, -Ci-Ci 0 alkyl-aryl; and R 3 and R 4 are both hydrogen; with the proviso that at least one of a R 1 or a R 2 group is not hydrogen.