2.6- Disubstituted pyrylium perchlorates

Disubstituted pyrylium perchlorates could be successfully reduced to 4//-pyrans 92 (R = Ph, 4-MeOC6H4 R = H) with some tertiary amines (81JHC1235) see Section IV.A.4. 

Disubstituted pyrylium salts, such as (41), are attacked at C-4 only (74JOU2015) and in the presence of perchloric acid a new pyrylium salt (42) is formed. Flavylium (43) and xanthylium (44) salts also react at the y-position, for instance, with CH-acidic reagents such as pentane-2,4-dione, malonic acid derivatives or aromatic electron-rich compounds like IV V-dimethylaniline, 1,3-dimethoxybenzene or IV-methylindole (59CB46, 74CHE1019) some examples are shown in Scheme 1. 2,4-Disubstituted pyrylium cations, e.g. (45), react at C-6 (80JOC5160). 

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). 

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). 

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