148 Heterocycles with Ring-Junction Nitrogen (Bridgehead Nitrogen)
Quinolizines are usually prepared via cyclisations of 2-substituted pyridines.
Heteropyrrolizines (pyrrolizines containing additional heteroatoms)
In these systems, electrophilic substitution in either of the rings is the most common reactivity, with fairly general
susceptibility to lithiation and with occasional examples of nucleophilic displacement.
Syntheses are usually cyclisations starting with a monocyclic compound and bear close analogy to methods used in the
other systems discussed above.
Cyclazines
The cyclazines are fused tricyclic systems containing a central nitrogen atom and a peripheral -system. Unlike the
bicyclic systems, the central nitrogen does not appear to have much infl uence and, on a simplistic view, the aromaticity
seems to follow the 4n2 rule for the peripheral -system. Hence, the (peripheral) ten-electron (3.2.2)cyclazine and its