Pyrylium salts reactions with primary amines

The preparation of highly functionalized cyclopentadiene derivatives may by achieved by using oxalenes or their pyrylium salts (90 and 92, respectively), (see Scheme 14, where X = O). In this way, azalenes 26,51 29,8Z 88 32,51 or 3651 can be obtained by heating salts (90) with primary amines in dimethyl-formamide (Eq. 6). The reaction with sodium hydrogen sulfide gives thialenes... 

A specific behavior in reactions with primary amines was described for l-aryl-3-carboxy-benzo[c]pyrylium salts 62 (89KGS454). While their monocyclic analogs undergo decarboxylation with heteroatom exchange,... 

It was shown by Katritzky (80T679) that acetic acid promotes recyclization of triaryl-pyrylium salts in reactions with primary amines. 

Addition of an ammonia source to pyrylium salts readily affords pyridine derivatives and provides a good method for the preparation of the pyridine moiety if the corresponding pyrylium salt is accessible. The carbon oxygen double bond present in the pyrylium salt is an oxonium ion however, owing to aromatic stabilization they are easily formed by a variety of methods. The reactivity of pyrylium salts toward nucleophiles makes them useful reagents for the preparation of structurally diverse heterocyclic compounds. Thus pyrylium salts afford pyridines by reaction with ammonia, pyridine-A -oxides by reaction with hydroxylamine and pyridinium salts by reaction with primary amines. 

The reactions in Methods A, B and C, which all start from pyrylium salts are analogous to the well-known conversions of 2.4.6-substituted pyrylium salts 27 with ammonia, primary amines, hydrogen sulfide or the anions of CH activated compounds to the corresponding heterocyclic or isocyclic aromatic systems The first step involves addition of the basic phosphine at C-2 (or C-6) to form 2S. Ring-opening, ring-closure and elimination of water are likely steps in the formation of the product 2. 

The effect of the structure of the alkyl substituent in primary amines on the course of reaction with these amines was observed also for monocyclic pyrylium salts [66T(57)9 87TL3143 89RRC1425]. The interaction of ben-zo[c]pyrylium-4-oxide 15 with aniline follows a different recyclization pathway, which is interpreted as shown here (64JA3814). 

The reactions of primary amines with pyrylium salts produce pyridinium salts. Pyridinium salts like 23 are useful intermediates that have... 

The addition of ammonia or primary and secondary amines to pyrylium salts is usually accompanied by a rapid ring opening without detection of the expected 2//-pyran intermediates. The exceptional behavior of the salt 160a was observed in its reaction with Af./V-dimethylaniline, affording 85% of 4H-pyran 210, whereas analogous addition products from toluene and methoxy-benzene were too unstable to be isolated.214... 

Amines also react with thiinium salts to give pyridinium salts, but reaction goes less easily than with the pyrylium analogues. 1,3-Oxazinium and 1,3-thiazinium cations react with ammonia and primary amines to give pyrimidines and pyrimidinium cations, respectively. 

It is important to realise that 1-benzopyrylium salts cannot be converted into quinolines or quinolinium salts by reaction with ammonia or primary amines (cf. pyryliums to pyridines, 11.1.2.2), whereas 2-benzopyrylinm salts are converted, efficiently, into isoqninolines or isoquinolinium salts, respectively."... 

Reaction of pyrylium salts with hydroxylamines and primary amines... 

The reaction of pyrylium salts with hydroxylamines and primary amines produces pyridine Moxides and pyridinium salts, respectively. These pyridine derivatives have interesting properties and are also useful intermediates for the preparation of a variety of new materials. For example, nucleophilic substitution on the pyridine ring is facilitated by the formation of an A -oxide or pyridinium salt. 

When the nucleophile has hydrogen atoms, e.g., with ammonia, primary amines, hydroxylamine, hydrazine derivatives, phosphine, hydrogen sulfide, nitromethane (EWG = NO2), acetonitrile (EWG = CN), an ANRORC (attack by nucleophile, ring opening, ring closure) reaction results in the conversion of substituted pyrylium salts into a large variety of six-membered aromatic carbocyclic or heterocyclic compounds. 

Katritzky converted pyrylium salts 44 (formed from the reaction of enone 42 and ethyl pyruvate 43) to the corresponding pyridiniums (45) via ANRORC reaction with a primary amines. This strategy serves as an alternative to the Zincke-Konig method for the formation of substituted pyridinium salts. 

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