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Amino acids with unusual types of chirality and their derivatives have recently attracted attention as precursors in the synthesis of chiral catalysts and peptide analogues with unique properties. In this study, we have synthesized a new nido-carborane-based planar-chiral amino acid, in the molecule of which the amino group is directly bonded to the B(3) atom, and the carboxyl group is attached to the B(9) atom through the CH2S+(Me) fragment. 3-Amino-9-dimethylsulfonio-nido-carborane, prepared in three steps from 3-amino-closo-carborane in a high yield, was a key intermediate in the synthesis of the target planar-chiral amino acid. The carboxymethyl group at the sulfur atom was introduced by the demethylation reaction of the dimethylsulfonio derivative, followed by S-alkylation. The structure of new 3,9-disubstituted nido-carboranes was studied for the first time using NMR spectroscopy. The resonances of all boron atoms in the 11B NMR spectrum of 3-amino-9-dimethylsulfonio-nido-carborane were assigned based on the 2D NMR correlation experiments. The nido-carborane-based planar-chiral amino acid and related compounds are of interest as a basis for peptide-like compounds and chiral ligands.

The design of highly selective low-toxic, low-molecular weight agents for boron delivery to tumour cells is of decisive importance for the development of boron neutron capture therapy (BNCT), a modern efficient combined method for cancer treatment. In this work, we developed a simple method for the preparation of new closo- and nido-carborane-containing folic acid bis-amides containing 18–20 boron atoms per molecule. Folic acid derivatives containing nido-carborane residues were characterised by high water solubility, low cytotoxicity, and demonstrated a good ability to deliver boron to tumour cells in in vitro experiments (up to 7.0 µg B/106 cells in the case of U87 MG human glioblastoma cells). The results obtained demonstrate the high potential of folic acid–nido-carborane conjugates as boron delivery agents to tumour cells for application in BNCT.