Atropisomerism of 2,2′-binaphthalenes

The synthesis of diastereo-enriched substituted (4S)-4-isopropyl-2-(2,2'-binaphthalen-1-yl)-4,5-dihydrooxazoles from substituted 2-naphthalenylmagnesium bromides and (4S)-4-isopropyl-2-(2-methoxynaphthalen-1-yl)-4,5-dihydrooxazole (4) and (4S)-4-isopropyl-2-(2,3-dimethoxynaphthalen-1-yl)-4,5-dihydrooxazole (5) is described. The product oxazolines were converted into a number of derivatives and the free energy barriers to internal rotation of several of these derivatives were determined. The determination of the X-ray crystal structures and the c.d. spectra of (S,1S)-N-[2-hydroxy-1-(isopropyl)ethyl]-3-methoxy-1',N-dimethyl-2,2'-binaphthalene-1-carboxamide (22) and (R,4S)-4-isopropyl-3-methyl-2-(1',2',4'-trimethyl-2,2'-binaphthalen-1-yl)-4, 5-dihydrooxazolium iodide (38) allowed the assignment of the absolute configurations of all the synthetic 2,2'-binaphthalenes by comparison of their c.d. spectra with those of compounds (22) and (38).