3-ALKYL 1,6-DIHYDROPYRIDINES FROM 3-ALKYL 5,6-DIHYDRO-PYRIDINIUM SALTS - IMPLICATIONS IN THE BIOSYNTHESIS OF SOME MACROCYCLIC MARINE ALKALOIDS

Additions of salts 1 and dihydropyridines 2 in an intramolecular manne r have been invoked in the biosynthesis of some macrocyclic alkaloids such as manzamines, sarain A and halicyclamine A, recently isolated fr om sponges. We report conditions for the deprotonation of easily acces sible salts 1, giving a regioselective entry to 3-alkyl 1,6-dihydropyr idines 2. Also, we show that species 1 and 2 can be generated from a c ommon intermediate such as 9. Dihydropyridines 2 are unstable at high temperatures giving isomeric 1,2- and 1,4-dihydropyridines, but were c onveniently trapped by dienophiles to give synthetically useful isoqui nuclidines such as 18 or 20.