A novel synthesis of halogenated oxa[9]helicenes and dibromo spiro-lactone derivative by the reaction of helical quinones with halogenating regents

Helical Amide Derivatives: Synthesis by Insertion of Aliphatic Primary Amines into Benzofused 2,2'-Diphenoquinones and Diastereomeric Resolution

We developed a method for synthesizing novel helical amide derivatives. Nucleophilic primary amines were inserted into benzofused 2,2’-biphenoquinone to form helical amides containing seven-membered rings. When optically pure primary amines were used in this reaction, a mixture of two amide diastereoisomers was obtained and separated into diastereomerically pure products via high-performance liquid chromatography (HPLC). By contrast, the reaction of enantiomerically pure benzofused 2,2’-biphenoquinone with achiral aliphatic primary amines afforded enantiomerically pure helical amides, with stereospecificity and retention of configuration.

Synthesis of Enantiomerically Pure Oxa[9]helicene Derivatives by a Nucleophilic Cyclodehydration Reaction of Helical 1,1′-Bibenzo[c]phenanthrenylidene-2,2′-dione

Enantiomerically pure 9-substituted 11-oxa[9]helicene derivatives have been synthesized through furan-ring formation by a nucleophilic cyclodehydration reaction of enantiomerically pure helical polycondensed 2,2′-diphenoquinone derivatives (1,1′-bibenzo[c]phenanthrenylidene-2,2′-diones). (P)-2,2′-diphenoquinone derivatives afforded (P)-oxa[9]helicenes, whereas (M)-2,2′-diphenoquinone derivatives afforded the corresponding (M)-oxa[9]helicenes. Therefore, the ring-closing reaction afforded the corresponding enantiomerically pure products without decreasing the enantiomeric excess, and it proceeded stereospecifically with retention of the configuration. The thermal stability of an oxa[9]helicene was studied by determining the decrease in its enantiomeric excess at various temperatures, and its racemization barrier was found to be 165.6 kJ/mol.

Furan-containing double tetraoxa[7]helicene and its radical cation

An unprecedented furan-based double oxa[7]helicene 1 was achieved, featuring a stable twisted conformation with π-overlap at both helical ends. The excellent conformational stability allowed for optical resolution of 1, which provided a pair of enantiomers exhibiting pronounced mirror-imaged circular dichroism and circularly polarized luminescence activity. The radical cation of 1 was obtained by chemical oxidation as evidenced by UV-Vis-NIR absorption, electron paramagnetic resonance spectroscopy and in situ spectroelectrochemistry. The present work is the starting point for the investigation of open-shell oxahelicenes.