72 Pyryliums, Benzopyryliums, Pyrones and Benzopyrones
The reaction of pyrylium salts with ammonia (or primary amines) produces pyridines (or pyridinium salts). Here, after ring
opening of the initial hemi-aminal adduct, cyclocondensation of the amino aldehyde generates the nitrogen heterocycle.
2-Benzopyrylium salts are equally susceptible to nucleophilic additions, at C-1 on the heterocyclic ring. In the case of
ammonia, the adduct from a 2-benzopyrylium ring opens and eventually an isoquinoline is formed; ammonia adducts
from 1-benzopyryliums do not have an option to give the corresponding quinolines.
Organometallic reagents add rapidly to pyrylium salts, and the 2H-pyran adducts thus produced ring open: diene-als
or diene-ones are the isolated products, retaining the geometry of the original 4,5-bond, thus pyrylium perchlorate
reacts with n-BuLi to give (Z,E)-nona-2,4-dienal.
In another example, addition of triphenylphosphonium methylide as the nucleophile, then a Wittig condensation,
electrocyclic ring opening and double-bond equilibration produces all-trans 7-substituted 2,4,6-heptatrienals as
shown in the example below.
In reductions of pyryliums, for example 2,4,6-trimethylpyrylium perchlorate as shown, hydride adds easily and pre-
dominantly at an -position. This produces a 2H-pyran that is not isolated but undergoes ring opening to the fi nal
product, a (Z,E)-dienone. Note that the minor reduction product, a 4H-pyran, is stable.