Generation and cascade reactions of N-[1,2-bis(methoxycarbonyl)vinyl]pyridinium species

The configuration of N-[1,2-bis(methoxycarbonyl)vinyl]-pyridinium intermediate generated from stereoisomers of dimethyl dibromosuccinate or dimethyl bromobutenedioate does not depend on the precursors’ configuration, as distinct from the yields and the reaction rates. The use of various nucleophiles in these cascade reactions gives either cycloheptadieneoctacarboxylic or cycloheptatrienehepta-carboxylic esters along with pyridinium 5-dicyano-methylidene-2,3,4-tris(methoxycarbonyl)cyclopenta-1,3-dien-1-olate. In the latter case, the acyclic ‘Diels’ golden adduct’ was detected, and its structure was refined.