The sulfa-Michael addition reaction was applied for the two-step synthesis of o-carboranyl cysteine 1-HOOCCH(NH2)CH2S-1,2-C2B10H11 from the trimethylammonium salt of 1-mercapto-o-carborane and methyl 2-acetamidoacrylate. To avoid the decapitation of o-carborane into its nido-form, the “free of base” method under mild conditions in a system of two immiscible solvents toluene-H2O was developed. The replacement of H2O by 2H2O resulted in carboranyl-cysteine containing a deuterium label at the α-position of the amino acid 1-HOOCCD(NH2)CH2S-1,2-C2B10H11. The structure of the protected o-carboranyl cysteine was determined by single-crystal X-ray diffraction. The obtained compounds can be considered as potential agents for the Boron Neutron Capture Therapy of cancer.
