Synthesis of hydrazones (1a–4a and 1b–4b), quinazolines (3c·MeOH and 3d·MeOH), and hydrazone-Schiff bases (4c and 4d) is achieved by combining suitable aldehydes (2,3- or 2,4-dihydroxybenzaldehyde) with four hydrazides (isonicotinic, nicotinic, and 2- or 4-aminobenzoic acid hydrazide). A suite of approaches for their preparation is described: solution-based synthesis, mechanosynthesis, and solid-state melt reactions. The mechanochemical approach is generally a better choice for the quinazolines, while the solid-state melt reaction is more efficient for derivatives of (iso)nicotinic based hydrazones. Crystalline amine-functionalised hydrazones 4a and 4b undergo post-synthetic modifications in reactions with 3- or 4-pyridinecarbaldehyde vapours to form hydrazone-Schiff bases 4a-3py, 4b-3py, 4a-4py, and 4b-4py. Mechanochemical and vapour-mediated reactions are followed by ex situ powder X-ray diffraction and IR-ATR methods, respectively. The chemometric analysis of these data using principal component analysis provided an insight into the reaction profiles and reaction times. Azines (5a and 5b), achieved from aldehydes and hydrazine, reversibly change colour in response to temperature changes. The structures of all products are ascertained by a combined use of spectroscopic and X-ray diffraction methods. The cytotoxic and antimicrobial activities of all compounds against selected human cancer cell lines and bacterial strains are evaluated.

Synthesis of hydrazones was performed via both conventional and solvent-free routes using the corresponding hydrazide (isonicotinic hydrazide, nicotinic hydrazide, 2-aminobenzhydrazide or 4-aminobenzhydrazide) and appropriate aldehyde (salicylaldehyde, 3-methoxysalicylaldehyde or 4-methoxysalicylaldehyde). A systematic study dedicated to solvatomorphism or polymorphism screening resulted in the formation of twelve novel crystalline forms, and eight of these were characterized via the single crystal X-ray diffraction method. In all studied structures, the molecules were assembled into endless supramolecular chains, discrete rings, chains of rings or nets. The mechanochemical synthesis employing liquid-assisted grinding was also applied and the nicotinic- and isonicotinic-based hydrazones were found to form readily from their corresponding precursors. The chemometric study using principal component analysis for mechanochemical synthesis monitoring was implemented for the first time to provide an insight into the reaction profiles. A thoughtful combination of ex situ powder X-ray diffraction and chemometric analysis was essential to identify a stepwise mechanism for the hydrazone formation via an intermediate phase. In five investigated reactions the first principal component accounted for at least 75% of the total variance, whereas in the case of two reactions this component accounted for 69.72 and 46.23% of the total variance. The hydrazones were also evaluated for cytotoxic activity in vitro. All compounds exhibited weak to moderate cytotoxicity against THP-1 and no cytotoxicity against HepG2 cells. Substantial antibacterial activity was obtained against Moraxella catarrhalis while no growth inhibition of Staphylococcus aureus, Enterococcus faecalis and Escherichia coli was observed.

Bacterial infections that do not respond to current treatments are increasing, thus there is a need for the development of new antibiotics. Series of 20 N-substituted quaternary salts of cinchonidine (CD) and their quasi-enantiomer cinchonine (CN) were prepared and their antimicrobial activity was assessed against a diverse panel of Gram-positive and Gram-negative bacteria. All tested compounds showed good antimicrobial potential (minimum inhibitory concentration (MIC) values 1.56 to 125.00 μg/mL), proved to be nontoxic to different human cell lines, and did not influence the production of reactive oxygen species (ROS). Seven compounds showed very strong bioactivity against some of the tested Gram-negative bacteria (MIC for E. coli and K. pneumoniae 6.25 μg/mL; MIC for P. aeruginosa 1.56 μg/mL). To establish a connection between antimicrobial data and potential energy surfaces (PES) of the compounds, activity/PES models using principal components of the disc diffusion assay and MIC and data towards PES data were built. An extensive machine learning procedure for the generation and cross-validation of multivariate linear regression models with a linear combination of original variables as well as their higher-order polynomial terms was performed. The best possible models with predicted R²(CD derivatives) = 0.9979 and R²(CN derivatives) = 0.9873 were established and presented. This activity/PES model can be used for accurate prediction of activities for new compounds based solely on their potential energy surfaces, which will enable wider screening and guided search for new potential leads. Based on the obtained results, N-quaternary derivatives of Cinchona alkaloids proved to be an excellent scaffold for further optimization of novel antibiotic species.

A new procedure for full conformational analyses comprising the statistical analysis of molecular dynamics trajectories was developed and applied. This method included a coordinate space for sampling using molecular dynamics simulations, reduction of dimensionality using tensor decomposition tools, determination of probability distributions in a reduced space, and finally the search for all of the strict extrema points of probability distributions. These extracted extrema points formed an initial guess for geometry optimization and clustering of conformers. A complete conformational space of 1-oxaspiro[2,5]octane and its cis- and trans-4-, 5- and 6-methyl substituted derivatives was also determined. In each case, eight conformers were found with two chair-like conformers predominant at room temperature. It was found that chair-like conformers with an epoxide ring oxygen atom in the pseudo-axial position had less strain, as well as all of their conformers with the methyl substituent in an equatorial position on a cyclohexane moiety.

The treatment of central nervous system (CNS) diseases related to the decrease of neurotransmitter acetylcholine in neurons is based on compounds that prevent or disrupt the action of acetylcholinesterase and butyrylcholinesterase. A series of thirteen quinuclidine carbamates were designed using quinuclidine as the structural base and a carbamate group to ensure the covalent binding to the cholinesterase, which were synthesized and tested as potential human acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibitors. The synthesized compounds differed in the substituents on the amino and carbamoyl parts of the molecule. All of the prepared carbamates displayed a time-dependent inhibition with overall inhibition rate constants in the 10³ M⁻¹ min⁻¹ range. None of the compounds showed pronounced selectivity for any of the cholinesterases. The in silico determined ability of compounds to cross the blood–brain barrier (BBB) revealed that six compounds should be able to pass the BBB by passive transport. In addition, the compounds did not show toxicity toward cells that represented the main models of individual organs. By machine learning, the most optimal regression models for the prediction of bioactivity were established and validated. Models for AChE and BChE described 89 and 90% of the total variations among the data, respectively. These models facilitated the prediction and design of new and more potent inhibitors. Altogether, our study confirmed that quinuclidinium carbamates are promising candidates for further development as CNS-active drugs, particularly for Alzheimer’s disease treatment.