Treatment of phenols (1) with 2 mol equiv. of paraformaldehyde in aprotic and poorly electron-donating solvents in the presence of selected metal halides coupled with suitable bases produces salicylaldehydes (3) in high yields. The route is highly selective for ortho-formylation and also specific towards monoformylation. The crucial role of added bases is emphasized.
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Casiraghi et al. (1980) studied this question.