Heterocycles with Ring-Junction Nitrogen (Bridgehead Nitrogen) 1471,2,3-Triazolo[1,5-a]pyridines and tetrazolopyridines are usually synthesised via generation of their ring-open forms.
(Caution: Ts N 3 (tosyl azide) is a widely used reagent but is potentially explosive. Safer diazo-transfer reagents, for
example 4-acetamidobenzenesulfonyl azide, are available and serve the same purpose.)
Compounds with additional nitrogens in the six-membered ring are again formed by cyclocondensations or cycload-
ditions starting with a higher azine, often with exact analogy to the pyridine-derived systems. However, sometimes a
route starting with the fi ve-membered component is preferred.
Quinoliziniums and quinolizinones
Quinoliziniums are about as reactive to nucleophilic addition as are pyridiniums, the quinazoline products undergoing
spontaneous electrocyclic ring opening to give 2-substituted pyridines. Quinolizinones are susceptible to electrophilic
attack in the ring bearing the carbonyl function, there being a clear analogy to the reactivity of pyridones.