Bottom-up synthesis of optically active oligonaphthalenes: Three different pathways for controlling axial chirality

The oxidative homocoupling of optically active binaphthalenes 1a-d with a stoichiometric amount of CuCl₂ and amines afforded quaternaphthalenes 2a-d in up to 93% de. The high diastereoselectivities were achieved through three different pathways (epimerization of the axis together with diastereoselective crystallization, thermodynamic, and kinetic control pathways). The type of side chains on the naphthalene influenced which pathway dominates. Three pathways were applicable to octinaphthalenes (8a-d) and hexadecanaphthalene 10a with 46-99% de. The absolute configuration of the newly formed axial bond was determined by (1) X-ray crystallographic analysis, (2) transformation to known compounds 15 and 16, (3) CD spectra of oligonaphthalenes with two pyrene rings as exciton parts, and (4) the shift values in ¹³C NMR spectra of ¹³C-enriched derivatives 29-31 toward chiral shift reagent Eu(+tfc)₃.