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-pyridylacetonitrile (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 methyl-

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