During the synthesis of styryl ketonic Mannich bases, a one-pot method was used, in which ketone, secondary-amine hydrochloride and paraformaldehyde were refluxed together in ethanol. In most cases, the normal Mannich product (MA) was obtained, but the side-product α-methylene-β-aminoketones (IM), instead of MA, was obtained when certain amines were used as the reactant. IR, MS and ¹H NMR spectra confirmed that the compound IM was a new type of Mannich base which had a terminal double bond at α-position of the ketone group. This modified Mannich reaction was further explored and the formation mechanism of product IM was postulated and discussed based on the evidence of deuterium labeling. Fourteen IM were prepared by the modified Mannich reaction. Based on isotopic labeling, a plausible way of cleav-age was proposed for the formation of M^＋－17 fragment signal in the mass spectra of the α-methylene-β-aminoketones. The characteristic chemical shift of the olefinic protons in ¹H NMR was also discussed.

Key words: styryl ketonic Mannich base, α-methylene-β-aminoketone, Mannich reaction, mechanism, side-product