Biologically active mesomeric betaines and alkaloids, derived from 3-hydroxypyridine, pyridin-N-oxide, nicotinic acid and picolinic acid:: Three types of conjugation and their consequences

Betainic pyridinium alkaloids and analogs are members of the four major classes of heterocyclic mesomeric betaines, i.e. conjugated (CMB), cross-conjugated (CCMB), pseudo-cross-conjugated heterocyclic mesomeric betaines (PCCMB), and conjugated N-ylides and thus offer the possibility of detailed comparisons of the types of conjugation. The chemistry of these systems is surveyed.