Imino Diels-Alder reaction of boronates: A new route to 3,4-dihydroquinolines

The synthesis of a series of 3,4-dihydroquinolines (3b-e, 3h and 3i) by Imino Diels-Alder reactions involving sulfolene and boronates 2a-j derived from Schiff bases is described. The reactions are regioselective leading to 4-substituted dihydroquinolines with a cis relative stereochemistry between the phenyl group on the boron atom and the vinyl substituent at position 4, as established by X-ray diffraction analyses of 3b, 3e, 3h and 3i.Boronates 2 containing substituents meta to the imino fragment lead to 4-ethenyl-dihydroquinolines while para substituted derivatives are more reactive and lead to 4-cyclohexenylquinolines 4 formed by a second Diels-Alder reaction with excess of butadiene. The results show that boronates derived from Schiff bases are electron deficient species that react with sulfolene providing a new route to 3,4-dihydroquinolines. Moreover, polarization of the imine bond activates these system toward cycloaddition.