**Preface to "Chiral Auxiliaries and Chirogenesis"**

Chirality, the ability of any object to exist as a pair of non-superimposable mirror images or a unidirectional action (such as motion), is one of the most fundamental properties of nature. This phenomenon is widely seen and plays a key role in various biological structures, such as saccharides, proteins, enzymes, membranes, and DNA/RNA [1–3]. Furthermore, chirality is important for different branches of modern science, technology, and medicine. Essentially, all aspects of chirality dynamics including asymmetry generation, transfer, amplification, modulation, memorizing, and others relate to chiral auxiliaries and chirogenic processes, which are cutting-edge scientific activities [4–10]. Investigation of these effects is a rapidly growing area of research and directly connects with numerous natural processes, artificial systems, and modern industries. In addition, this research field has important practical implications in novel materials, enantioselective catalysis, chiral sensors, optical resolution, asymmetric synthesis, nanotechnology, medicine, pharmacology, biomimetic studies, and others. Many types of chemical molecules and supramolecular systems including various porphyrinoids have been well studied, as summarized in several comprehensive reviews published so far [6,11–16].

The aim of this Special Issue consisting of four review articles and one research paper was to highlight and overview all aspects of chiral auxiliary and chirogenesis in different natural/physical sciences and in modern technologies. In particular, some newly emerging classes of molecules used for these purposes have been overviewed.

Thus, in their review, Aav and Mishra [17] described chirogenic processes in cucurbituril (CB)-type hosts, which are oligomeric compounds, based on cyclic urea monomers, which are connected through methylene bridges. Indeed, while CB molecules are essentially symmetric, the following three major pathways to induce chirality in or by CB-type hosts were highlighted: first, through the incorporation of stereogenic elements into host molecules; second, through complexation with achiral guests, which leads to axial supramolecular chirality and helical structures; and third, through the formation of complexes with chiral guests in multimolecule complexes and induction of supramolecular chirality. This review is of particular importance due to the emergence of new supramolecular systems, whose selectivity can be directed by external stimuli and that can be applied for chirality sensing and enantioselective applications. Indeed, the first example of supramolecular chirogenesis in porphyrins generated by chiral CB was recently published [18].

Chiral heterocycle-based receptors as another type of chemical compound used for enantioselective recognition were comprehensively analyzed by Khose et al. in a review article [19]. Different structural motifs, intermolecular interactions, and analytical techniques employed for detecting the difference between corresponding enantiomers were found to be major components of the chiral recognition process. It is essentially important since there exists a huge global market for chiral separation technology for qualitative and quantitative estimations of chiral analytes.

In [20], Hasan and Borovkov reviewed helicene-based chiral auxiliaries as prospective chirogenic systems. Helicenes are unique helical chromophores possessing advanced and well-controlled spectral and chemical properties owing to their diverse functionalization and defined structures. Essentially, these distinct structural features of helicenes, the different synthetic and supramolecular approaches responsible for their efficient chirality control, and their employment in the chirogenic systems have been thoroughly analyzed. Moreover, the limitations, scope, and future prospects of helicene chromophores in chiral chemistry have been highlighted.

Buckybowls are polynuclear aromatic hydrocarbons that have a curved aromatic surface and are considered fragments of buckminsterfullerenes. The curved aromatic surface leads to the loss of planar symmetry of the normal aromatic plane and may cause unique inherent chirality, so-called bowl chirality, which it is possible to thermally racemize through a bowl-to-bowl inversion process. The studies concerning the special field of bowl chirality, focusing on recent practical aspects of attaining diastereo/enantioenriched chiral buckybowls through asymmetric synthesis, chiral optical resolution, selective chiral metal complexation, and chiral assembly formation have been summarized by Kanagaraj et al. in a review article [21]. While the study of chiral buckybowls is still immature yet highly promising, further investigation will provide more insight into this particular area and lead to applications in various fields such as materials, electronics, and photonics.

In the last research paper of this Special Issue [22], Hirose et al. described the syntheses and properties of optically pure [2]rotaxanes. Rotaxanes consisting of achiral axle and achiral ring components can possess supramolecular chirality due to their unique geometrical architectures. However, it is difficult to synthesize such rotaxanes as optically pure forms. To synthesize such chiral rotaxanes, a prerotaxane method based on aminolysis of a metacyclophane-type prerotaxane that had planar chirality, which is composed of an achiral stopper unit and a crown ether type ring component, was successfully applied. It was found that the rotaxane with mechanical chirality had a complexation ability against enantiopure phenylglycinol, with high enough enantioselectivity to be applied as a chiral selector of the chiral stationary phase for chiral chromatography.

Based on citations and access statistics of the published articles, this Special Issue has had a high scientific impact and attracted the considerable attention of the research community. Therefore, it was decided to launch the second Special Issue "Chiral Auxiliaries and Chirogenesis II" on the same subject in 2020, which will be completed in 2021 and then summarized in another Reprint Book.

#### **References**


**Victor Borovkov** *Editor*
