The Vilsmeier formylation of tertiary and secondary enamides leads to 2-pyridones and 2-chloropyridines, respectively. The reaction appears to be quite general allowing substitution in the 1-, 3-, 5-, or 6-position or combinations of these. The major limitation arises with enamides which are unsymmetrically substituted on the double bond with alkyl groups, when mixtures can result. Attempts to introduce a 4-substitutent by a variation of the Vilsmeier reagent had limited success.
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Meth‐Cohn et al. (1984) studied this question.