Synthesis of a water soluble boron neutron capture therapy agent: 1-amino-3-[2-(7-(3-[2-(2-hydroxymethyl-ethoxy)-1-(2-hydroxy-1-hydroxymethyl- ethoxymethyl)ethoxy]propyl)-1,7-di-carba-closo-dodecaboran-1-yl)ethyl] cyclobutanecarboxylic acid

A water soluble boronated amino acid containing a cascade polyol, 1-amino-3-[2-(7-(3-[2-(2-hydroxymethyl-ethoxy)-1-(2-hydroxy-1-hydroxymethyl- ethoxymethyl)ethoxy]propyl)-1,7-di-carba-closo-dodecaboran-1-yl)ethyl] cyclobutanecarboxylic acid, has been developed. The key step in the synthesis is the alkylation of 3-[2-(1,7-dicarba-closo-dodecarboran-1-yl)ethyl]cyclobutanone hemithioketal with toluene-4-sulfonic acid 3-[2-(2-benzyloxy-1-benzyloxymethyl-ethoxy)-1-(2-benzyloxy-1-benzyloxymethyl- ethoxymethyl)ethoxy]propyl ester which gave the required precursor ketone which was then converted to the title amino acid via a Bücherer-Strecker synthesis followed by hydrogenolysis to remove the benzyl protecting groups.