Synthesis and study of the structural properties of oxa[5]helicene derivatives

Abstract

The preparation of the (−)-menthyl carbonate of 2-hydroxy-7-oxa[5]helicene and its structural characterization is reported. Single crystal X-ray diffraction analysis established the relatively flat structure of oxa[5]helicene unit thus indicating the absence of stable helical isomers. The unit cell of the crystal includes one molecule in the P-conformation and another in the M-isomer. However, its nitro derivative, 1-nitro-2-hydroxy-7-oxa[5]helicene, provides sufficient steric crowding to observe two helical isomers at low temperature, as confirmed by 1H NMR at −20°C.