Preparation of Salicylaldehydes via the Ortho-Lithio Derivatives of Methoxymethyl-Protected Phenols

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Abstract
The procedures used previously for the preparation of 3-chlorosalicylaldehyde1,2, 3-methylsalicylaldehyde2,3 (2,3-cresotaldehyde) and 5-methylsalicylaldehyde2,3,4 (2,5-cresotaldehyde) comprises processes by which the formyl group is introduced directly into the nucleus of o-chlorophenol, o- and p-cresol by a number of established methods - as for example the Reimer-Tiemann-, the Gatter-mann- and the Duff-reaction - as well as processes in which the formyl group is introduced indirectly through reduction of the correspondingly substituted salicylic acids or through oxidation of the chloro- or methyl-substituted 2-hydroxybenzyl alcohols. The only process by which the salicylaldehydes in question have been obtained in high yields is by oxidation of the correspondingly substituted 2-hydroxy-benzyl alcohols but these starting materials are not available commercially and various problems are encountered in their preparation5,6. The two steps of this method have been conducted in succession without intermediate work up in the case of salicylaldehyde2 which was obtained in a yield of 63%. We attempted to adapt this procedure to the preparation of 3-chlorosalicylaldehyde from o-chlorophenol but the results were discuraging. 3-fluorosalicylaldehyde has recently been prepared from o-fluorophenol by a related procedure6 but the yield obtained was low.