Practical Synthesis of Optically Pure Bifunctionalized Heterohelicenes

Optically active 2-(hydroxymethyl)- and 2-(ethylthiocarbonyl)[1]benzothieno[5′,4′ : 2,3][1]benzothieno [4′,5 : 4,5]thieno[3,2-f]quinolines containing π-excessive thiophene and π-deficient pyridine units were prepared by the use of exo-3-amino-2-hydroxybornane as a chiral auxiliary. This procedure consists of separation of the helical diastereomers prepared by photocyclization of 1,2-diarylethylenes and removal of the chiral auxiliary by a thiolate ion. Large scale preparation of the helicenes can be accomplished by a modified procedure of the photocyclization reaction. Optical purities of both enantiomers of 2-(hydroxymethyl)- and 2-(ethylthiocarbonyl)[1]benzothieno[5′,4′ : 2,3][1]benzothieno[4′,5′ : 4,5]thieno[3,2-f]quinolines were >99.5%. Their absolute configurations were determined by comparison of CD spectra.