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.

The IDA reaction of a series of boronates derived from Schiff bases with sulfolene provided a new route to 4-substituted dihydroquinolines. The reaction is stereoselective providing the product with a cis stereochemistry between the BC6H5 group and the vinylic group at position 4. The presence of the N → B dative bond polarizes the imine fragment and activates the arylimine towards the cycloaddition reaction.Figure optionsDownload as PowerPoint slide