Novel chiral norbornadiene-quadricyclane systems: Substituent effect on chiroptical properties and stereochemical consequence of photochemical interconversion

For efficient absolute asymmetric synthesis (AAS) by circularly polarized light (CPL), a series of 3-substituted 2-methoxycarbonylnorbornadiene (N) and -quadricyclane (Q) derivatives (R = H, Me, Ph, 4-biphenylyl, 1- and 2-naphthyl) were prepared and optically resolved, and the chiroptical properties of the enantiomeric pairs were determined in hexane and acetonitrile. All of the N and Q derivatives gave moderate to large circular dichroism intensities (Δε) and specific rotations ([α]_D). In particular, aromatic Ns (R = 4-biphenylyl and 2-naphthyl) gave high [α]_D values of 216° and 206°, while non-aromatic Qs (R = H and Me) afforded high [α]_D's of 322° and 271°. Depending on the nature of the substituent introduced at the 3-position, the maximum anisotropy factor (g = Δε/ε) varied over a wide range from 0.0141 (R = H) to 0.0022 (R = 1-naphthyl) in the N series and from 0.0091 (R = Me) to 0.0012 for (R = 2-naphthyl) in the Q series. Thus, non-aromatic N/Q pair is absolutely advantageous for the CPL-induced AAS reactions. The relatively high g_max values observed for non-aromatic Ns and Qs may be rationalized by the contribution of the n, π* transition of the ester chromophore to the N and Q chromophores. Stereochemical consequence of the photochemical interconversion of the chiral N and Q was also revealed; all of the (-)-Ns examined photoisomerized to the corresponding (+)-Qs without any side reactions.