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

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.

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Dinaphtho[2,1-b:1′,2′-d]furan-2-yl((1R,2S,5R)-2-isopropyl-5-methylcyclohexyl)carbonateC31H30O4[α]D28 = −40.0 (c 0.3, chloroform)Source of chirality: (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl carbonochloridateAbsolute configuration: (1R,2S,5R)

(1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl(1-nitrodinaphtho[2,1-b:1′,2′-d]furan-2-yl)carbonateC31H29NO6[α]D28 = +84 (c 0.06, chloroform)Source of chirality: (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl carbonochloridateAbsolute configuration: (1R,2S,5R)