Diastereoselective reaction of (MP)-pentahelicene-7,8-dione with trans-cyclohexane-1,2-diamine. Thermal and photochemical transformations of its product

Abstract The reaction of the title compounds and the transformations of the product, 3 were investigated with an emphasis on the stereochemistry. The primary interaction of the title compounds is feebly stereoselective. The diastereoisomers of product 3 exhibit free energies differing by ca. 16 kJ/mol; diastereoisomerization by helix inversion takes place during the reaction. The most stable diastereoisomers of the intermediate 8 and the product 3 show opposite helicities, which allows isolation of the product 3 in diastereoisomeric ratios from 19:81 to >99:1 depending on solvent and temperature. The free energies of activation for helix inversions of 3 were determined by time-dependent 1 H NMR. The predicted configuration of the more stable diastereoisomer of 3 was confirmed by chemical correlation to be ( M , R , R ). The four stereoisomers of 3 were separated by analytical enantioselective HPLC and characterized by on-line circular dichroism. Irradiation of 3 afforded the 2-substituted benzimidazole derivative 9 .