The initial reaction of primary amines with pyrylium cations to give the ring-opened intermediate is fast for strongly basic amines and is base-catalysed for weak amines. Ring-closure of the intermediate to give the pyridinium derivative is subject to steric and electronic hindrance and is acid-catalysed. Solvent effects and the structure of the ring-opened intermediates are discussed.
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Katritzky et al. (1981) studied this question.