Anhydro-bases, pyrylium

From the same point of view, 4-pyrones (8a) or 2-pyrones are anhydro bases of 4-hydroxypyrylium (9) or 2-hydroxypyrylium salts. Vinylogs (10) and phenylogs (12) (violones, after Dilthey and Burger ) of these systems are known, whose conjugate acids are monocyclic pyrylium salts (11 and 13). 

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. 

The interaction between ammonia and l-R1-3-aryl-substituted ben-zo[c]pyrylium salts 30, as well as 3,4-diaryl-substituted indeno[e l,2-]-benzo[c]pyrylium, and other substituted cations having a similar fragment is more complicated, especially in hydroxyl-containing solvents. In these cases, mixtures in different ratios are obtained that contain isoquinolines 138, 3-hydroxy-3,4-dihydroisoquinolines 137, OR-adducts 109, anhydro-bases 119, diketones 29, a-naphthylamines 140, and a-naphthols 141 (88UP2). 

Anhydro-bases, of alkyl thio- and seleno-pyrylium salts, 60, 119... 

A limitation mentioned by Dilthey consists in the possibility of an alternative ring closure when o-hydroxybenzaldehyde is employed. The pyrylium salt 130 can be obtained by acid demethylation of the corresponding o-anisylpyrylium salt its blue anhydro base (violone) 132 is unstable and cannot be isolated because it undergoes spontaneous conversion into a yellow isomer 133. HilF investigated... 

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