Helical structure of tercyclopropanedimethanol in solution

Abstract

Oligocyclopropanes with repetitive stereochemistry occur in two unusual natural products with interesting bioactivity. X-ray crystal structures are available for these compounds but with partially contradicting results. Because the H-1 and C-13 NMR spectra of oligocycloproparies are far from trivial to be assigned even at highest magnetic fields, we have prepared a specifically deuterated sample and have applied high field NMR spectroscopy and DFr calculations to determine its conformation. The helix with equal handedness shown in the stereopicture was found for tercyclopropanedimethanol. A dihedral angle of around +40 degrees is the best representation of the experimental data and characterizes, therefore, the dominating helical conformation of tercyclopropanedimethanol with a single repetitive (+)-gauche interunit dihedral angle. This is in full agreement with the crystal structure of the all sun,trans-quinquecyclopropanedimethanol with an R configuration at the termini that also adopted an all (+)-gauche conformation. However, the crystal structure of the title compound and the solution structure are different.