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

Abstract

The procedures used previously for the preparation of 3-chlorosalicylaldehyde^1,2, 3-methylsalicylaldehyde^2,3 (2,3-cresotaldehyde) and 5-methylsalicylaldehyde^2,3,4 (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 preparation^5,6. 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.