Synthesis of Charge-Compensated nido-Carboranyl Derivatives of Sulfur-Containing Amino Acids and Biotin

A new group of charge-compensated nido-carboranyl derivatives of sulfur-containing amino acids and biotin has been synthesized in which the boron atom in position 9 or 10 of carborane is attached to a positively charged sulfur atom. The possibilities of obtaining symmetrical B(10)-substituted and asymmetric B(9)-substituted nido-carboranes were studied. Using the example of (S)-methionine and D-biotin derivatives, water-soluble S-substituted charge-compensated nido-carboranes with free functional groups were prepared. The results obtained open up prospects for the development of potential boron delivery agents for BNCT as well as new bioactive compounds containing a negatively charged nido-carboranyl fragment bearing a positive charge on the sulfur atom associated with the boron cluster.