Solvent effect on helicity induction of zinc bilinone bearing a chiral auxiliary at the helix terminal

Solvent effects on helicity induction in zinc bilinone (ZnBL) derivatives bearing chiral auxiliaries at their 19-positions were investigated by using circular dichroic spectroscopy and (1)H NMR experiments. In ZnBLs 1 and 2, which possess (R)-2-methyl-1-phenylpropyloxy and (R)-1-phenylethyloxy groups at their 19-positions, respectively, the efficiency of helicity induction was significantly affected by employed solvents (78-95% de in 1 and 33-89% de in 2). The free energy changes of the P-M interconversion of 1 and 2 were linearly in proportion to reduction in polarizability of solvents: lower polarizability of solvents led to better efficiency of helicity induction in 1 and 2. With the support of the (1)H NMR study in addition to the molecular modeling previously reported, it was indicated that the solvophobic van der Waals interaction between the alkyl group in the chiral auxiliary and the A-ring of the bilinone skeleton in the preferred conformer plays a crucial role in determining the efficiency of helicity induction in 1 and 2.