Diastereoselective tandem addition-cyclization reactions of unsaturated tertiary amines initiated by photochemical electron transfer (PET)

Polycyclic molecules and tetrahydroquinoleines were obtained in a tandem re action involving the diastereoselective addition of or-aminoalkyl radicals to (5R)-5-menthyloxy-2[5H]-furanone 1. The facial diastereoselectivity on 1 is greater than or equal to 90%. The alpha -aminoalkyl radicals were produ ced from tertiary amines by photachemical-induced electron transfer. When N ,N-dialkylanilines 19 were used as starting tertiary amines, a rearomatizat ion step was involved and important side reactions of 1 were observed. A me chanistic study involving isotopic labeling of the starting amine indicated that the byproducts resulted from reduction of 1 during the rearomatizatio n process. An efficient optimization of the reaction was obtained by simply adding acetone or cyclopentanone as mild oxidants to the reaction mixture. The side products resulting from reduction of the furanone 1 were complete ly suppressed under these conditions, and the yields of the tettahydroquino lines 21a-i, 22a-f, and 26g-i were doubled.