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The rapid progress in the chemistry of carboranes and other polyhedral boron compounds makes the development of methods for the synthesis of their various functional derivatives a high priority. The formation of cyclic oxonium derivatives followed by the ring opening is one of the most efficient methods for the preparation of these compounds. This review addresses the data on the use of the nucleophilic ring opening of cyclic oxonium derivatives based on cobalt and iron bis(dicarbollides), which have been extensively studied in recent years. Reactions involving O-, N-, S-, P- and C-nucleophiles are considered as a route to the targeted synthesis of a wide range of organic boron derivatives, in particular various boron-containing biologically active compounds. Possible applications of the reaction products are discussed. The bibliography includes 197 references.

A series of novel cobalt bis(dicarbollide)—curcumin conjugates were synthesized. Two conjugates were obtained through the nucleophilic ring-opening reaction of the 1,4-dioxane and tetrahydropyran derivatives of cobalt bis(dicarbollide) with the OH group of curcumin, and using two equiv. of the oxonium derivatives, two other conjugates containing two cobalt bis(dicarbollide) units per molecule were obtained. In contrast to curcumin, the conjugates obtained were found to be non-cytotoxic against both tumor and normal cell lines. The analysis of the intracellular accumulation of the conjugates by flow cytometry showed that all cobalt bis(dicarbollide)—curcumin conjugates entered HCT116 colorectal carcinoma cells in a time-dependent manner. New non-cytotoxic conjugates contain a large amount of boron atoms in the biomolecule and can potentially be used for further biological research into boron neutron capture therapy (BNCT).

Boron neutron capture therapy (BNCT) remains an important treatment arm for cancer patients with locally invasive malignant tumors. This therapy needs a significant amount of boron to deposit in cancer tissues selectively, sparing other healthy organs. Most of the liposomes contain water-soluble polyhedral boron salts stay in the core of the liposomes and have low encapsulation efficiency. Thus, modifying the polyhedral boron core to make it hydrophobic and incorporating those into the lipid layer could be one of the ways to increase drug loading and encapsulation efficiency. Additionally, a systematic study about the linker-dependent effect on drug encapsulation and drug-release is lacking, particularly for the liposomal formulation of hydrophobic-drugs. To achieve these goals, liposomal formulations of a series of lipid functionalized cobalt bis(dicarbollide) compounds have been prepared, with the linkers of different hydrophobicity. Hydrophobicity of the linkers have been evaluated through logP calculation and its effect on drug encapsulation and release have been investigated. The liposomes have shown high drug loading, excellent encapsulation efficiency, stability, and non-toxic behavior. Release experiment showed minimal release of drug from liposomes in phosphate buffer, ensuring some amount of drug, associated with liposomes, can be available to tumor tissues for Boron Neutron Capture Therapy.

A series of novel conjugates of cobalt bis(dicarbollide) and

New conjugates of cholesterol and cobalt bis(dicarbollide) were synthesized using ‘click’ cycloaddition of 3b-(2-azidoethyl) cholest-5-ene and acetylene-equipped cobalt bis(dicarbollides). The synthesized boronated cholesterols can be of interest for liposomal drug delivery for boron neutron capture therapy of cancer.

Reactions of [8-HCCCH 2 O-8′-I-3,3′-Co(1,2-C 2 B 9 H 10 ) 2 ] - ([ 2 ] Cs ) with 5-iodo-2’-deoxyuridine under Sonogashira reaction conditions resulted in a cobalt bis(1,2-dicarbollide)(-I) 5-ethynyl-2'-deoxyuridine conjugate ([ 3 ] Cs ) linked via short spacer. Its intramolecular cyclization led to furo [2,3-d]pyrimidin-2(3H)-one isomer ([ 4 ] Cs ). Compounds [3]Cs and [4]Cs demonstrated moderate cytotoxicity, however they did not have antiviral activity against the viruses tested. A new terminal charge-compensated cobalt bis(1,2-dicarbollide)(-I) derivative with terminal alkyne group ( 5 ) was synthesized from 8,8’-μ-I-3,3′-Co(1,2-C 2 B 9 H 10 ) 2 and dimethylpropargyl amine. It was found that contrary to [2]Cs compound 5 did not react with 5-iodo-2’-deoxyuridine under Sonogashira reaction conditions. Total electrostatic potential (TEP) calculations gave explanation of this phenomenon.

8-Tetrahydrofuronium and 8-tetrahydropyronium derivatives of iron bis(dicarbollide)(-I) were synthesized. Their reactions of ring cleavage by MeOH, N3(-), amines and 1,2-bis(diphenylphosphino)ethane were investigated. 8-Tetrahydrofuronium iron bis(dicarbollide)(-I) was found to be more active in these reactions in comparison with 8-tetrahydropyronium and 8-dioxonium species, respectively. The first conjugates of iron bis(1,2-dicarbollide)(-I) with 2'-deoxyadenosine modified via the C-8 position of the purine base were synthesized. Reactions of these oxonium compounds with 1,2-bis(diphenylphosphino)ethane (dppe) led to zwitter-ionic monophosphonium salts. One of these compounds has given rise to a novel ferracarborane/cobaltacarborane hybrid complex.