Synthesis of optically active dodecaborate-containing L-amino acids for BNCT

A convenient and simple synthetic method of dodecaboratethio-L-amino acid, a new class of tumor-seeking boron carrier for BNCT, was accomplished from S-cyanoethylthioundecahydro-closo-dodecaborate (S-cyanoethyl-10BSH, [10B12H11]2−SCH2CH2CN) and bromo-L-α-amino acids by nearly one step S-alkylation. An improved synthesis of S-cyanoethyl-10BSH, a key starting compound for S-alkylation, was also performed by Michael addition of 10BSH with acryronitrile in high yield. Four kinds of new dodecaboratethio-L-amino acids were obtained in optically pure form without the need for any optical resolution.