Novel Synthesis of 4H-Quinolizine Derivatives Using Sulfonyl Ketene Dithioacetals

In the synthesis of 4H-quinolizine derivatives involving the use of a sulfonyl ketene dithioacetal, we found a novel reaction in which the remaining methylsulfanyl group was replaced with a proton after the ring-closure reaction in the quinolizine skeleton under mild conditions, without the use of any metallic reagent. The reaction of 3,3-bis(methylsulfanyl)-2-phenylsulfonylacrylonitriles (1a,b) with 2-pyridyl-acetonitrile (2a) in the presence of potassium carbonate as a base in DMSO afforded 4-imino-2-methylsulfanyl-3-phenylsulfonyl-4H-quinolizine-1-carbonitriles (3a,b). The methylsulfanyl group at the 2-position of 3a,b was readily removed under methanol reflux conditions to afford 4-imino-3-phenylsulfonyl-4H-quinolizine-1-carbonitriles (4a,b) in good yields. Alkyl 3-phenylsulfonyl-4H-quinolizine-1-carboxylates (4c-f) were directly synthesized from sulfonyl ketene dithioacetal (1a,b) with alkyl 2-pyridylacetates (2b,c) and involved desulfanylation by simple hydrolysis. In addition, the fluorescent properties of these compounds were investigated. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)