2.6- Disubstituted pyrylium salts, reaction

Beckmann rearrangement of ketoketoximes 288 (R,R = alkyl, aryl) with thionyl chloride unexpectedly afforded 2-aryl(or alkyl)amino-4,6-disubstituted pyrylium salts 289 (equation 124). This reaction is the first example of rearrangement/cyclization involving carbonylic oxygen as terminator. ... 

A convenient method leading to pyrans (38) consists in the nucleophilic addition of R anions to 2,6-disubstituted pyrylium salts, in which the y-position (secondary carbonium ion) is more reactive than the a-positions (tertiary carbonium ions), in opposition to the reactivity of 2,4,6-trisubstituted pyrylium salts.1C8a Krohnke and Dickore169 as well as Dimroth and Wolf170 showed that 2,6-diphenyl-pyrylium salts add the anions R of nitromethane, 1,3-diketones, malonodinitrile, ethyl cyanoacetate, and benzoylacetonitrile. Similar reactions are known in the flavylium series.169 171 Nonactivated R ... 

Pent-2-ene-l,5-diones 23 are intermediates in this reaction. Variation of this synthesis is achieved using related bis-electrophiles. For instance, 2,6-disubstituted pyrylium salts 25 are formed from two molecules of aryl methyl ketone and orthoformic ester in the presence of strong acids (HCIO4, HBF4) ... 

Alk = Et), a-methylstyrene (156, Ar = Ph), and allyl-benzene (160, Ar = Ph) are representatives of the four types of monosubstituted propenes (olefins 157 and 158, etc. written in Table I as equilibrium pairs undergo equilibration during the course of the reaction and yield the same pyrylium salt on diacylation). Disubstituted propenes which gave pyrylium salts on diacylation are 2-pentene (163, Alk = Me) and 1,2-diphenylpropene (164, Ar = ph),305 3-ethyl-2-pentene represents a trisubstituted propene... 

Thieno[3,4-c]pyridines have also been prepared in an acylation-cyclization reaction from 2,5-disubstituted 3-acetonylthiophenes on treatment with acetic anhydride and perchloric acid via the thieno[3,4-c]pyrylium salts as intermediates (c/. Scheme 66) (72MIP31700, 75CHE551). 

The reaction of 1,3-disubstituted benzothieno[2,3-c]pyrylium salts 81 with hydrazine has been studied. It has been shown that these pyrylium salts interact with hydrazine to give Ar-amino-l, 3-dialkylbenzothieno[2,3-c]pyiidines 82. In contrast, l,3-diphenylbenzothieno[2,3-c]pyrylium perchlorate gives exclusively 5//-[2,3]benzothieno[2,3-/ ]diazc-pine 83 <1998CHE983> (Scheme 16). 

The reaction of hydrazine with 1,3-disubstituted benzothieno[2,3-c]pyrylium salts (e.g., 105) gives iV-amino-1,3-dialkylbenzothieno[2,3-c]pyridines such as 106 (Equation 6) <1998CHE983>. 

The 6-amino 88 (58%) and 6-hydroxy 90 (84%) derivatives were synthesized by the cyclization of imine 87 and ketone 89, respectively (79JHC753). 1,3-Disubstituted derivatives 92 were prepared by the reaction of pyrylium salts 91 with o-aminobenzaldehyde (71% for 92 R = Me) (71TL553). 

Pyrones also add Grignard nucleophiles at the carbonyl carbon, C-4 dehydration of the inunediate tertiary alcohol product with mineral acid provides an important route to 4-mono-substituted pyrylium salts." More vigorous conditions lead to the reaction of both 2- and 4-pyrones with two mole equivalents of organometallic reagent and the formation of 2,2-disubstituted-2H- and 4,4-disubstituted-4//-pyrans, respectively." Perhaps surprisingly, hydride (lithium aluminium hydride) addition to 4,6-dimethyl-2-pyrone takes place, in contrast, at C-b." ... 

It was reported in the late 1950s, that the reaction of pyrylium salts with hydroxylamine produces pyridine A -oxides. ° This procedure has been used to prepare 2,6-disubstituted pyridine A/ -oxides in better yield than obtained from the oxidative procedure. ... 

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