Development of structurally extended benzosiloxaboroles - synthesis and in vitro biological evaluation

The synthesis of potassium 6-hydroxy-7-chloro-1,1-dimethyl-3,3-difluorobenzo-1,2,3-siloxaborolate 5b from readily available 4-bromo-2-chlorophenol was developed. This compound proved useful in various derivatizations resulting in a wide range of O-functionalized benzosiloxaboroles. Reactions of 5b with selected substituted benzoyl chlorides gave rise to a series of respective derivatives with 6-benzoate side groups attached to the benzosiloxaborole core. Furthermore, treatment of 5b with substituted benzenesufonyl chlorides afforded several benzosiloxaboroles bearing functionalized benzenesulfonate moieties at the 6 position. The synthesis of related chloropyridine-2-yloxy substituted benzosiloxaboroles was accomplished by a standard approach involving silylation/boronation of appropriate heterodiaryl ethers. Investigation of biological activity of obtained compounds revealed that some benzoate and most benzenesulfonate derivatives exhibit high activity against Gram-positive cocci such as methicillin-sensitive Staphylococcus aureus ATCC 6538P as well as methicillin-resistant S. aureus ATCC 43300 with the MIC values in the range of 0.39–3.12 mg L⁻¹. Some benzenesulfonate derivatives showed also potent activity against Enterococcus faecalis ATCC 29212 and E. faecium ATCC 6057 with MIC = 6.25 mg L⁻¹. Importantly, for the most promising cocci-active benzenesulfonate derivatives the obtained MIC values were far below the cytotoxicity limit determined with respect to human normal lung fibroblasts (MRC-5). For those derivatives, the obtained IC₅₀ values were higher than 12.3 mg L⁻¹. The results of antimicrobial activity and cytotoxicity indicate that the tested compounds can be considered as potential antibacterial agents.

The synthesis of potassium 6-hydroxy-7-chloro-1,1-dimethyl-3,3-difluorobenzo-1,2,3-siloxaborolate 5b from readily available 4-bromo-2-chlorophenol was developed.

Ethynyl-substituted benzosiloxaboroles: the role of C(π)⋯B interactions in their crystal packing and use in Cu(i)-catalyzed 1,3-dipolar cycloaddition

The synthesis and characterization of two novel 6-ethynyl-7-halogen substituted benzosiloxaboroles (Hal = F, Cl) is reported. The crystal structures of these compounds show a unique type of supramolecular assembly dictated by distinctive C(π)⋯B interactions resulting in the formation of columnar networks involving alternating ethynyl groups and boron atoms. The QTAIM, NBO and NCI analyses were performed in order to obtain a deeper quantitative insight into the nature of these interactions including energy and charge density distribution. The fluoro derivative 1c was used as a starting material in Cu-catalyzed 1,3-dipolar cycloaddition reactions with substituted benzenesulfonyl azides giving rise to benzosiloxaboroles with pendant 1-(arylsulfonyl)-1,2,3-triazole-4-yl functionalities or analogous ionic species, i.e., 1,2,3-triazolium arylsulfonates. Screening of antimicrobial activity of obtained derivatives against a wide selection of Gram-positive and Gram-negative bacteria as well as fungi strains was performed and the obtained results were compared with the data obtained previously for related benzosiloxaborole derivatives.