C4‐Regioselective Dearomatization of Quinolinium Salts via Morita–Baylis–Hillman Reaction

Traceless switch of regioselectivity: without employing any directing groups, the change of selectivity in the Morita–Baylis–Hillman reaction with N-alkylquinolinium electrophiles is attained. Simply by modifying the counterion in the azaarenium salt and the reaction conditions, a C4-addition of acrylate can be accomplished, complementing the earlier C2-selective protocol.

An intermolecular Morita–Baylis–Hillman reaction employing N-benzylquinolinium salts as electrophiles is developed. The reaction proceeds through a C4-selective dearomatization of the quinolinium substrate, contrary to the C2-regioselectivity that is observed in a previous report on a similar process. The switch in the regioselectivity is accomplished through the change of the counterion in the quinolinium salt to PF₆ ⁻ and the alteration of the reaction conditions, such as the application of a different catalyst (DABCO). The reaction allows for the preparation of functional group-rich products featuring the important 1,4-dihydroquinoline scaffold, complementing the earlier method which provides the corresponding 1,2-dihydroquinoline counterparts. The developed reaction is compatible with a range of substrates that also encompasses acridinium salts.