Pyrylium ANRORC reactions

There is little or no well-defined chemistry of 2-amino-pyrones, -pyrylium salts, etc., because of the ease with which these systems undergo ANRORC reactions to give substituted pyridones (equation 88). 4-Aminopyrylium salts are reversibly deprotonated to the imines (equation 89). Amino groups in the 3- and 5-positions of pyrones can be diazotized normally. 

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. 

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. 

Almost all the known reactions of the pyrylium nucleus involve addition of a nucleophile, usually at an a-position, occasionally at C-4, as the first step. A feature of pyrylium chemistry is the ring opeinng of adducts produced by such additions, followed by cyclisation in a different manner, to give a new heterocyclic or homocyclic product (ANRORC processes). 

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. 

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