Reduction of azides to primary amines in substrates bearing labile ester functionality. Synthesis of a PEG-solubilized, "Y"-shaped iminodiacetic acid reagent for preparation of folate-tethered drugs.

[formula: see text] Anhydride 3 is a useful reagent for the synthesis of triply linked drug conjugates. Examples using paclitaxel are provided. Conversion of the azido moiety to a primary amine in the presence of substrates bearing labile ester functionality requires the use of tin/mercaptan reducing system which includes methanol exchange equilibration to effect nitrogen-tin bond scission.