Stereogenicity/Astereogenicity as Global/Local Permutation-Group Symmetry and Relevant Concepts for Restructuring Stereochemistry

Molecules of ligancy 4 that have been derived from an allene, an ethylene, a tetrahedral, and a square-planar skeleton have been investigated to show that their symmetries are dually and distinctly controlled by point groups and permutation groups. Insomuch as the point-group symmetry was exhibited to control the chirality/achirality of a molecule, sphericity in a molecule, and enantiomeric relationship between molecules [S. Fujita, J. Am. Chem. Soc. 112 (1990) 3390], the permutation-group symmetry has been now clarified to control the stereogenicity of a molecule, tropicity in a molecule, and diastereomeric relationship between molecules. To characterize permutation groups, proper and improper permutations have been defined by comparing proper and improper rotations. Thereby, such permutation groups are classified into stereogenic and astereogenic ones. After a coset representation (CR) of a permutation group has been ascribed to an orbit (equivalence class), the tropicity of the orbit has been defined in term of the global stereogenicity and the local stereogenicity of the CR. As a result, the conventional stereogenicity has now been replaced by the concept local stereogenicity of the present investigation. The terms homotropic, enantiotropic, and hemitropic are coined and used to characterize prostereogenicity. Thus, a molecule is defined as being prostereogenic if it has at least one enantiotropic orbit. Since this definition has been found to be parallel with the definition of prochirality, relevant concepts have been discussed with respect to the parallelism between stereogenicity and chirality in order to restructure the theoretical foundation of stereochemistry and stereoisomerism. The derivation of the skeletons has been characterized by desymmetrization due to the subduction of CRs. The Cahn–Ingold–Prelog (CIP) system has been discussed from the permutational point of view to show that it specifies diastereomeric relationships only. The apparent specification of enantiomeric relationships by the CIP system has been shown to stem from the fact that diastereomeric relationships and enantiomeric ones overlap occasionally in case of tetrahedral molecules.     

Enantiomeric and Diastereomeric Relationships of Ethylene Derivatives. Restructuring Stereochemistry by a Group-Theoretical and Combinatorial Approach

To restructure stereochemistry into a systematic format, enantiomeric and diastereomeric relationships have been investigated by using ethylene derivatives as examples in the light of a new group-theoretical and combinatorial approach. On one hand, enantiomeric relationship for ethylene derivatives has been characterized by means of a point group of order 8 (D _2h ), where chirality fittingness based on the sphericity concept has been applied to the enumeration of stereoisomers. On the other hand, diastereomeric relationship for ethylene derivatives has been examined by a permutation group of order 8 (S ₉ ^[4]), which is a subgroup of the symmetric group of order 4 (S ^[4]) and isomorphic to a point group D _2d . The subgroups of S ₉ ^[4] have been classified into stereogenic and astereogenic ones. A stereogenic subgroup corresponds to a pair of diastereomers, while an astereogenic subgroup is assigned to a self-diastereomer. The relationship between diastereomers and constitutional isomers have been also discussed.     

拓扑立体化学

本文综述了拓扑立体化学发展的主要概念和实验事实,并分析了若干化合物的拓扑结构特征。     

Comment on “Bad Language”: Resolving some Ambiguities about Chirality

In this comment on the Essay entitled “Bad Language” by Dunitz I add new elements which enlighten the discussion. I hope that they contribute to suppress ambiguities and preconceived ideas about chirality.     

Inducing Propeller Chirality in Triaryl Boranes with Chiral Amines

Stabilization of chiral propeller conformations in triaryl compounds is challenging due to generally low racemization barriers. Nonetheless, it was recently found that chiral conformational preferences can be induced to triaryl boranes by incorporating point-chiral alkylether chains to the aryl blades and subsequently locking the structure with ammonia. A four-point interaction, meaning that the cooperative effects of Lewis-adduct formation and three hydrogen bonds, was proposed as stabilizing mechanism. Herein, it was shown that three such strong interactions suffice to introduce a preferential propeller handedness. Although DFT calculations predict no noteworthy preferences for either P- or M-chiral propellers for some of the investigated triarylborane–amine adducts that were prepared with chiral primary amines, vibrational circular dichroism (VCD) spectroscopic characterizations revealed that there is indeed a measurable excess of one propeller handedness. Furthermore, the steric demand of the amine was found to play a key role in the induction process and especially in preventing blade rotations.     

Chirality and Stereogenicity of Square‐Planar Complexes

Square‐planar complexes with achiral and chiral ligands have been enumerated exhaustively under the point‐group D _4h and under the symmetry group S ^[4] of degree 4, where they have been classified in terms of their symmetries and permutabilities. Thereby, their stereochemical properties and relationships have been discussed in detail. In particular, equivalency under point‐group symmetry (e.g., enantiomeric relationships for chiral complexes and prochirality for achiral complexes) and that under permutation‐group symmetry (e.g., proper and improper permutations, stereogenic and astereogenic groups, and enantiostereogenic and diastereogenic groups) have been characterized to give a systematic format for stereochemistry and stereoisomerism.     

Stereochemistry of optically active compounds. Problems and prospects

The review is concerned with the fundamental ideas and concepts of chiral stereochemistry, i.e., of the stereochemistry dealing with optically active compounds, from the asymmetric synthesis to the basics of mathematics, including characterization of the principal results and the current state of this branch of science.     

Control of Axial Chirality by Planar Chirality Based on Optically Active [2.2]Paracyclophane

Optically active X-shaped molecules based on the planar chiral [2.2]paracyclophane building block were prepared, in which di(methoxy)terphenyl units were stacked on the central benzene rings. At 25 °C, anisolyl rings freely rotate in solution, while in the crystal form, they are fixed by intramolecular CH–π interactions, thereby leading to the expression of the axial chirality, i.e., propeller chirality was exhibited by the planar chiral [2.2]paracyclophane moiety. The X-shaped molecule exhibited good circularly polarized luminescence (CPL) profiles with moderate Φ_PL and a large g_lum value in the order of 10⁻³ at 25 °C, in solution. In contrast, at −120 °C, dual CPL emission with opposite signs was observed. According to the theoretical studies, the rotary motion of the anisolyl units is suppressed in the excited states, and so emission from two isomers could be observed. These results demonstrate that the axial chirality was controlled by the planar chirality, leading ultimately to propeller chirality.     

HPLC Enantioseparation of Rigid Chiral Probes with Central,Axial, Helical,and Planar Stereogenicity on an Amylose (3,5-Dimethylphenylcarbamate) Chiral Stationary Phase

The chiral resolving ability of the commercially available amylose (3,5-dimethylphenylcarbamate)-based chiral stationary phase (CSP) toward four chiral probes representative of four kinds of stereogenicity (central, axial, helical, and planar) was investigated. Besides chirality, the evident structural feature of selectands is an extremely limited conformational freedom. The chiral rigid analytes were analyzed by using pure short alcohols as mobile phases at different column temperatures. The enantioselectivity was found to be suitable for all compounds investigated. This evidence confirms that the use of the amylose-based CSP in HPLC is an effective strategy for obtaining the resolution of chiral compounds containing any kind of stereogenic element. In addition, the experimental retention and enantioselectivity behavior, as well as the established enantiomer elution order of the investigated chiral analytes, may be used as key information to track essential details on the enantiorecognition mechanism of the amylose-based chiral stationary phase.     

Catalytic Enantioselective Simultaneous Control of Axial Chirality and Central Chirality in Allenes

Chiral molecules, which may contain one or more different type(s) of stereocentres, such as central, axial, planar, and helical chiralities, etc., are indispensable in chemistry, pharmaceutical industry, and life science. Despite many advances for the preparation of chiral molecules usually with a single type of chirality have been realized, simultaneous construction of different types of chiralities is still a significant challenge. Here, we wish to report a protocol for preparation of chiral allenes with both central and axial chiralities via a catalytic asymmetric allenylation of different biologically or synthetically useful fluorinated or non‐fluorinated nucleophiles with readily available racemic allenes by using a single chiral ligand. An echoing between the central chirality and axial chirality for the enantioselectivity was observed. This strategy provides a general and practical approach to functionalized optically active allenes bearing both central and axial chiralities with an excellent enantioselectivity under mild conditions.     

Louis Pasteur,Chemical Linguist: Founding the Language of Stereochemistry

Pasteur’s major discovery in chemistry was the recognition of molecular chirality, in 1848. He understood that his new science needed its own language, and introduced new terminology and nomenclature, thereby launching modern stereochemical language. He was eminently prepared for this task as a refined user of language, skills recognized by his election to the Académie française, the supreme institution for the protection and promotion of the French language. The terms chiral and chirality did not exist at the time and he adopted the French word dissymmétrie (dissymmetry) for the phenomenon of handedness. Although in his time almost nothing was known about molecular constitution and configuration, his insights allowed him to create useful language some of which is still used today in stereochemistry, e. g., racemic for the 1 : 1 mixture of the two enantiomers, and the use of the prefixes levo‐ and dextro‐ in the names of optically active substances. On the other hand, the limitations in the knowledge of organic chemistry at the time prevented him from creating some needed terms, e. g., for the phenomenon of diastereoisomerism. He also failed to adopt the enantio terminology introduced in the 1850s by German mineralogist Carl Friedrich Naumann. Analysis of Pasteur’s linguistic innovations is of interest from the point of view of the history of chemistry and is also useful in throwing light on the fundamental nature of the concepts of stereochemistry. Such understanding has acquired a new relevance due to the considerable misuse and misunderstanding of this language seen in the literature today.     

手性杯芳烃及其超分子手性

杯芳烃是由苯酚单元通过亚甲基连接而成的空腔型分子,具有衍生位点多,构象丰富等特点,被称为第三代主体分子。在分子层次,依手性因素的结构特点不同,可将手性杯芳烃分为具有手性亚单元的杯芳烃、固有手性杯芳烃和桥手性杯芳烃。在超分子层次,杯芳烃自身或杯芳烃与其他分子或离子在溶液中、晶态中或二维表面可通过非共价键力形成多种拓扑结构的纳米手性聚集体。研究手性杯芳烃和基于杯芳烃的超分子手性组装体的合成、结构和性能,不仅在理解手性起源、手性结构等方面具有理论意义,而且有望获得以分子识别为基础的手性传感器、手性催化剂、手性分离材料、手性载体和手性纳米材料。本文综述近十年来有代表性的分子手性杯芳烃和以杯芳烃为组分的超分子手性聚集体的设计、合成、结构和功能。着重展示杯芳烃骨架在形成新颖分子手性和超分子手性上的优势,以及杯芳烃单元在实现特定功能如手性识别时发挥的作用。相信随着杯芳烃合成技术和杯芳烃超分子设计的发展,必将进一步发挥杯芳烃的结构优势,涌现出更多性能优异的手性杯芳烃功能分子和超分子手性杯芳烃功能材料。     

Systematic Enumeration and Symmetries of Cubane Derivatives

The feasibilities of Fujita's unit‐subduced‐cycle‐index (USCI) approach, Fujita's proligand method, and Fujita's stereoisogram approach have been demonstrated by applying them to cubane derivatives as probes. They provide us with a new set of theoretical foundations for comprehensive investigation of geometric and stereoisomeric features of stereochemistry. The new set of theoretical foundations is based on mathematical formulations so as to explore mathematical stereochemistry as a new interdisciplinary field of stereochemistry.

     

Chirality Transfer and Memory of Chirality in Gold‐Catalyzed Reactions

Over the last few years, gold‐catalyzed reactions that involved chirality transfer and memory of chirality (MOC) have emerged as a powerful tool in enantioselective synthesis. This technique has allowed for the single‐step synthesis of enantioenriched compounds from readily available starting materials. This Focus Review discusses this emerging field with an emphasis on mechanistic aspects and their applications in synthetic organic chemistry.     

Synthesis of Bis(indole) Alkaloids from Arundo donax: The Ynindole Diels–Alder Reaction,Conformational Chirality,and Absolute Stereochemistry

Bis(indole) alkaloids from Arundo donax were synthesized using the first ynindole Diels–Alder reaction. The alkaloids are chiral, having stable enantiomeric conformations with half‐lives of racemization of t_1/2=4150–25100 seconds at room temperature. Their absolute stereochemistry was determined using the exciton chirality method.     

Combinatorial enumeration of ethane derivatives as three-dimensional chemical structures,not as graphs. A systematic approach for enumerating and characterizing stereoisomers of tartaric acids and related compounds

Fujita’s proligand method is applied to the enumeration of ethane derivatives, where the counting of stereoisomers of tartaric acids is examined in detail as a probe for testing the versatility of the method. The cycle index with chirality fittingness (CI-CF) for enumerating ethane derivatives is obtained by Fujita’s proligand method and compared with the CI-CF derived alternatively by the direct calculation of permutations of substitution positions. The two CI-CFs are identical with each other so that the methodology underlying in Fujita’s method is demonstrated in a concrete fashion. The enumeration results are compared with those derived by Pólya’s corona. Fujita’s proligand method is shown to be capable of enumerating stereoisomers, whereas Pólya’s corona is concluded to enumerate graphs, but not stereoisomers. The conceptually change from graphs to three-dimensional (3D) chemical structures is discussed, where the superiority of Fujita’s proligand method is demonstrated.     

Sensing Remote Chirality: Stereochemical Determination of β‐, γ‐, and δ‐Chiral Carboxylic Acids

Determining the absolute stereochemisty of small molecules bearing remote nonfunctionalizable stereocenters is a challenging task. Presented is a solution in which appropriately substituted bis(porphyrin) tweezers are used. Complexation of a suitably derivatized β‐, γ‐, or δ‐chiral carboxylic acid to the tweezer induces a predictable helicity of the bis(porphyrin), which is detected as a bisignate Cotton Effect (ECCD). The sign of the ECCD curve is correlated with the absolute stereochemistry of the substrate based on the derived working mnemonics in a predictable manner.     

胆色素类化合物光学活性及其立体化学研究进展

本文综述了20 年来胆色素光学活性及其立体化学的研究进展, 着重讨论了分子内和分子间相互作用对色素构象的选择性及其光活性的影响。对研究前景进行了展望。