Carbohydrate α-oxoketene N,O-ketals in the synthesis of dihydroquinolines

Abstract

The carbohydrate nitro enone 1 was found to react with amines to give α-ketene ketals; the formation of carbohydrate azo derivatives could be suppressed by addition of an electron-rich aniline quenching agent. Upon reaction with benzaldehyde in the presence of trifluoroacetic acid, the α-oxoketene ketals afforded dihydroquinolines in a cyclization reaction forming two new carbon–carbon bonds. For the dihydroquinoline 18 the newly formed stereocenter was ascertained by X-ray crystallography. The two-step reaction could be carried out as one multicomponent reaction. Electron-poor anilines led to Hantzsch-type products incorporating two enaminone molecules; the crystal structure of representative 30 is reported.