Katritzky pyrylium-pyridinium

This is an Sn2 process, since inversion is found at R. Another good leaving group is NTS2 ditosylamines react quite well with acetate ion in dipolar aprotic solvents RNTs2 4- OAc — ROAc. Ordinary primary amines have been converted to acetates and benzoates by the Katritzky pyrylium-pyridinium method (p. 447). Quaternary ammonium salts can be cleaved by heating with AcO in an aprotic solvent. Oxonium ions can also be used as substrates RsO -f R COO —> R COOR R2O. 

Alkyl halides or sulfuric or sulfonic esters can be heated with sodium or potassium thiocyanate to give alkyl thiocyanates, though the attack by the analogous cyanate ion (10-66) gives exclusive N-alkylation. Primary amines can be converted to thiocyanates by the Katritzky pyrylium-pyridinium method (pp. 447, 489). "... 

Primary alkyl amines RNHi can be convertedto alkyl halides by (1) conversion to RNTs2 (p. 447) and treatment of this with I or Br in DMF, or to N(Ts)—NH2 derivatives followed by treatment with NBS under photolysis conditions, (2) diazotization with terr-butyl nitrite and a metal halide such as TiCU in DMF, or (3) the Katritzky pyrylium-pyridinium method (pp. 447,489). Alkyl groups can be cleaved from secondary and tertiary aromatic amines by concentrated HBr in a reaction similar to 10-71, for example,... 

The extensive studies of the reactions of pyrylium and pyridinium salts by Katritzky and his co-workers have led to an appreciation of the value of these salts in organic synthesis. Chromenylium salts, which are readily available from 1-tetralones, react with aqueous ammonia at room temperature to give 5,6-dihydrobenzo[h]quinolines in very high yield, providing an exceptionally good route to the nitrogen heterocycle. Considerable variation in the substituent pattern is possible and further rings may be annelated (A.R. Katritzky et al, J. chem. Soc. Perkin II, 1984, 857 and earlier papers). 

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). 

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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