Pyrylium salts, leaving groups

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. 

Nucleophiles react particularly easily with quaternized azines and with pyrylium and thiopyrylium slats (cf. equation 23) typical examples, including the well-known reaction of pyridinium salts with hydroxide in the presence of potassium ferricyanide to give 2-pyridones, are summarized in equations (34)-(36) (note the rather unusual orientation of addition in the last case reaction normally occurs essentially exclusively a to the heteroatom if the position is free or occupied by a leaving group). 

Alkylthio groups at C-4 are amongst the best leaving groups and are useful precursors for several 4-substituted pyrylium salts, for example 2,6-dimethyl-4-piperidinopyrylium iodide (125) (55JOC448). 

The 2- and 4-positions of pyrylium salts are readily attacked by nucleophiles (see Chapter 2.23) and in the absence of a good leaving group these reactions give rise to 2H- and 4H-pyrans, respectively. The 2-adduct is thermodynamically favoured by virtue of the... 

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