Chiral chromanearenes synthesised by cycloaddition reactions of o-quinomethine resorcinarenes†
We present the synthesis of chromanearenes – cavity-extended inherently chiral heterocyclic derivatives of resorcinarenes – by the Diels–Alder reaction between the o-quinomethine derivative of resorcinarene and various styrenes. Among many possible stereoisomers of chromanearenes, only C4-symmetric stereoisomers (as two diastereoisomers) are formed selectively in 44–88% yield and up to 90% de. The structures were analysed by NMR, X-ray crystallography and circular dichroism. Chromanearene derived from 4-methylstyrene forms mixed crystals with all four stereoisomers present in the crystal lattice in a non-statistical ratio (60 : 40). Diastereoisomeric molecules occupy the same position in the asymmetric unit and their different configurations are manifested as a partial disorder. Stereoisomers were separated by HPLC coupled with an ECD detector and their ECD spectra were recorded. The spectra were compared with theoretical ECD spectra (TD-DFT method). The results indicate that the signs and shapes of the ECD spectra are mainly determined by the inherent chirality of the resorcinarene scaffolds, while the configurations of the stereogenic centers at the upper rim have a minor effect.