Novel Asymmetric Formylation of Aromatic Compounds: Enantioselective Synthesis of Formyl 7,8-Dipropyltetrathia[7]helicenes

Asymmetric formylation of aromatic compounds is virtually unexplored. We report the synthesis and evaluation of a library including 20 new chiral formamides in the kinetic resolution of 7,8-dipropyltetrathia[7]helicene, affording the corresponding formyl- or diformylhelicenes in up to 73% ee, making enantiopure compounds available by recrystallisation. With the N,N-disubstituted formamides used in this study, the best enantioselectivity has been achieved with R¹=iPr, R²=Me, R³=H, R⁴=1-naphthyl or its 1-pyrenyl equivalent. Why have chiral formamides been overlooked for so long as tools in asymmetric synthesis? Herein new and efficient procedures are described in which enantioselective delivery of the formyl group achieves kinetic resolution and double kinetic resolution of helicenes, a class of chiral fused aromatic structures that are notoriously difficult to prepare in enantiopure form.