Pyryliums into pyridines

Some nucleophiles open the pyrylium ring to give intermediates which are capable of spontaneous or add-induced cyclization to a new ring. The conversion of pyrylium salts into pyridines is the most thoroughly studied example and was first described in 1911 (B-64MI22300). Ammonia, ammonium acetate or carbonate react with pyrylium salts such... 

For monocyclic cations, the unique pathway of this reaction occurs regardless of conditions and is often used to identifiy pyrylium salts by converting them into pyridines. However, for 2-benzopyrylium salts, this reaction, under standard conditions (aqueous or alcoholic ammonia), is accompanied by side transformations whose extent depends on the nature of substituents in the cation. 

The products can be converted into pyridines and pyrylium salts, as formulated in equations (II) and (III). 

The susceptibility of pyrylium salts to attack at C-2 by nucleophiles and the subsequent ring opening and ring closure is of value in the synthesis of a range of heterocyclic compounds. During the course of the transformation of the primary amino group into another functionality, pyrylium salts are converted into pyridinium salts and thence into pyridine derivatives (A.R. Katritzky, Tetrahedron, 1980, 36, 679). 

Other reactants containing a primary amino gronp will also convert pyryliums into A -substituted nitrogen heterocycles iV-amino azoles are amongst several types of hydrazine derivatives to have been utilised these give iV-(heteroaryl)-pyridinium salts. Reaction of pyryliums with hydroxylamine comparably leads (predominantly) to the formation of pyridine iV-oxides. " ... 

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."... 

Pyrylium Salts.—Acetylation of isoparaffins, such as (68), under Friedel-Crafts conditions at low temperatures produces pyrylium salts (69). The ratio of reactants and the presence or absence of chloroform as a solvent affect the yield of the mixture of isomers which is often obtained, through the intermediacy of a carbenium ion. The pyrylium salts were converted into pyridines (70) by aqueous ammonia. However, 3-methylpentane gave only 3-ethyl-2,4,6-trimethyl-pyridine. 

Direct conversion of pyridines into pyrylium salts is not possible (the reverse reaction is, however, very easy). However, Klages and Trager succeeded in performing the first synthesis of the parent compound 1 by the following reaction sequence, which was recently repeated with the pentadeutero derivative. 

Method B involves the preparation of precursor of 2-alkyl-l-benzo-pyrylium salts, as shown in Scheme ll.50 2-Alkylbenzopyrylium salts have been prepared by condensation of salicylaldehyde with appropriate ketone in acetic acid or by alkylation or reduction of coumarin or chromone derivatives. Reaction of 2-alkylbenzopyrylium salts with salicylaldehyde gives directly a spirodibenzopyran or 2-vinynologue benzopyrylium salt 17 which then can be converted into the spirodibenzopyran by piperidine or pyridine. 

The pyrylium cation is isoelectronic with pyridine it has the same number of electrons and, therefore, we also have aromaticity. Oxygen is normally divalent and carries two lone pairs. If we insert oxygen into the benzene ring structure, then it follows that, by having one electron in a p orbital contributing to the aromatic sextet, there is a lone pair in an sp orbital,... 

The acid-catalyzed ot7, o-acylation of /3-oxoalkyl derivatives of 7t-excess aromatic heterocycles known as the Dorofeenko-Dulenko-Krivun reaction affords condensed pyrylium derivatives which are convenient precursors of the corresponding pyridines <2001CHE1310>. By this approach (Scheme 14), carbazole 100 is converted into carba-zolo[3,2-f]pyrylium 101, a new ISrt-electron heteroaromatic system, and 101 is readily transformed into isomer 102 of 6-methylellipticine via an Sn(ANRORC) reaction using ammonia. The yields for these transformations were not reported. 

Russian workers have reported that 3-acetonylthiophenes yield pyrylium salts when allowed to react with acid anhydrides in the presence of 70% perchloric acid. These salts are transformed into thieno[2,3-c]pyridines by treatment with ethanolic ammonia (Scheme 66 (71KGS320) compare the preparation of furo[2,3-c]pyridines). A number of variously substituted thieno-[2,3-c]- and -[3,2-c]-pyridines have been made in this way. 

Pyrylium salts are useful precursors to other heterocyclic compounds such as furanes and pyridines. The perchlorates usually used are explosive, and tetrafluoro-borates are formed only in low yield. Sulfoacetates are nonexplosive if necessary for further synthetic uses, they can be converted into perchlorates.1... 

The substituted thieno[2,3-6]thienophene (262) in a mixture of an aliphatic acid anhydride and HC104 gave thieno[2, 3 5,4]thieno[2,3-c]pyrylium perchlorates (263), which with an excess of ammonia are converted into the corresponding thieno[2, 3 5,4]thieno[2,3-c]pyridines (264) (Scheme 17) <83KGS37>. 

When 2-methyl-2-pentene is treated with acetyl chloride in the presence of a Lewis add, a mixture of substituted pyrylium salts is formed. The pyrylium salts are transformed into the corresponding pyridines by threatment with aqueous ammonia, see Scheme 8. 

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