The structure of DTMR is shown below as Formula IV. A fourth particularly compound suitable for use with the present invention, referred to herein as DROX, R1 and Y2 taken together are propylene, R2 and Y1 taken together are propylene, R3 and Y3 taken together are propylene, R4 and Y4 taken together are propylene, R5 and R6 are hydrogen, X1 is carboxylate, and one of X2 and X3 is a linking group,