Alkyl-azonium and -pyrylium compounds

Compounds containing methylene groups activated by both a cationic ring and another electron-withdrawing group easily form stable anhydro-bases, e.g. (636) — (637), (638) — (639). Stabilization is also achieved by utilization of the aromatic character of the cyclopentadiene anion or the pyrrole anion compounds of type (640 Z = NR, O, S) and (643) readily lose protons to give the mesomeric anhydro-bases (as 641 - 642) which are called pseudoazulenes. 

Proton loss from a- and y-alkyl groups on a cationic (pyridinium, pyrylium or thiinium) ring is comparatively easy. The resulting unstable and highly reactive neutral anhydro-bases or pyridone methides cf. 645) can be isolated by using 10M sodium hydroxide, but are generally used directly. 

These anhydro-bases are heterocyclic equivalents of enamines and enol ethers and react readily with electrophilic reagents to give products which can often lose a proton to give a new resonance-stabilized anhydro-base. Thus, anhydro-l,2-dimethylpyridinium hydroxide (645) reacts with phenyl isocyanate to give an adduct (646) which is converted to the stabilized product (647 - 648). A similar sequence with carbon disulfide yields the dithio acid (644). 

Some weak electrophilic reagents, which are usually inert toward neutral pyridines and azines, also react 

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