3-Substituted pentadienals derivatives from condensation of imines anions to malonaldehyde equivalents. A C–C–C + C–C + N type entry to 3-alkyl substituted pyridinium salts

In order to model a biogenetic hypothesis concerning the origin of 3-substituted natural pyridinium type alkaloids extracted from sponges, the reactions of imine anions with malonaldehyde equivalents were investigated. Use of malonaldehyde monoacetals or dimethylaminoacrolein resulted in formation of glutaconaldehyde or aminopentadienal derivatives in moderate yields. Improved yields were observed using β-silyl imines. The so obtained glutaconaldehyde or aminopentadienal derivatives react with primary amines to give 3-alkyl substituted pyridinium salts. Therefore the reported sequence constitutes a C–C–C + C–C + N type entry to 3-alkyl substituted pyridinium salts. 3-Alkylglutaconaldehydes were also shown to dimerize, giving substituted cinnamic dialdehydes.