Planar-Chiral [7]Orthocyclophanes

Stereochemical study on planar-chiral cyclic molecules using polysaccharide-based column chromatography

The presence of planar chirality of a variety of medium-sized heterocycles, along with nine-membered cyclic ketone and its enol ester, was revealed by observation of isolable enantiomers by analytical high-performance liquid chromatography (HPLC) using a chiral stationary phase with a polysaccharide-based chiral selector. Several tens of milligrams of enantiomers of the planar-chiral molecules were successfully separated by preparative-scale HPLC, leading to the preparation of an enantioenriched sample. This in turn enabled detailed stereochemical studies, including measurement of half-lives of the optical activity and X-ray crystallography for elucidation of stereochemistry.

Planar Chirality: A mine for catalysis and structure discovery

Planar chirality is one of the most fascinating expressions of chirality, exploited by Nature to lock three-dimensional chiral conformations and, more recently, by chemists to create new chiral reagents, catalysts and functional organic materials. Nevertheless, the shortage of protocols able to induce and secure asymmetry during the generation of these unique chiral entities has dissuaded chemists to exploit their structural properties. This Minireview intends to illustrate the limited but remarkable catalytic methodologies reported for the production of planar chirality in strained molecules and serve as source of inspiration for the development of new unconventional protocols that are expected to come in the near future.

Analysis of Stereochemical Stability of Dynamic Chiral Molecules Using an Automated Microflow Measurement System

An automated microflow measurement system for the kinetic study of racemization of dynamic chiral molecules was developed. This system facilitated the analysis of fast racemization within several seconds at elevated temperatures owing to its rapid heating ability, high performance for controlling short residence times, and ease of connection to HPLC systems for direct measurement of the enantiomeric purity. A more precise analysis was realized by combination of microflow and common batch measurements over a broad range of temperatures.

Synthesis and Stereochemical Analysis of Planar-Chiral (E)-4-[7]Orthocyclophene

An efficient synthesis of (E)-4-[7]orthocyclophene (E)-1 via photochemical isomerization of (Z)-1 has been achieved. The key intermediate (Z)-1 was synthesized from commercially available 2-(hydroxymethyl)benzenepropanol (3) in five steps: (i) group-selective Mitsunobu reaction with CH₂═CHCH₂CH(SO₂Ph)₂, (ii) oxidation of alcohol, (iii) olefination, (iv) RCM, and (v) removal of sulfones in an overall yield of 73%. The photochemical isomerization of (Z)-1 was efficiently performed in the presence of AgNO₃-impregnated silica gel (AgNO₃/SiO₂). The resulting (E)-1 shows dynamic planar chirality at rt. Enantioenriched (E)-1 was prepared by the HPLC separation of enantiomers using a chiral stationary phase, and the absolute stereochemistry was determined by X-ray diffraction analysis of the Pt-coordinated crystalline derivative. The planar chirality of (E)-1 can be converted into the central chirality of carbon; e.g., the oxidation of (R)-(E)-1 using DMDO provided epoxide (8S,9S)-9 in a stereospecific manner. Furthermore, the Lewis acid-promoted reaction of (8S,9S)-9 afforded a unique tricyclic compound (8S,9S)-10 in an excellent yield and in a stereospecific manner.

Planar chiral desymmetrization of a two-layered cyclophane and control of dynamic helicity through the arrangement of two nonstereogenic centers

Dynamic helicity generated in a configurationally stable cyclophane is controlled through the arrangement of two nonstereogenic centers with different dimensions.