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I know salicyclic acid can be produced from salicaldehyde by treatment with $\ce{KMnO4}$.

Is there any way to reverse this, i.e- produce salicaldehyde from salicyclic acid?

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    $\begingroup$ There are reducing agents available that would reduce carboxylic acids. Trouble is that aldehydes are usually more reactive than acids, thus you often get the alcohol. But that can be selectively oxidized to the aldehyde and given the cheap starting material, the two-step procedure should be fine. $\endgroup$ – TAR86 Nov 30 '17 at 8:21
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Salicylic acid can be converted to salicylaldehyde using a triangular or square wave alternating voltage. The process of this conversion is explained in this paper$\ce{^{1}}$but it is behind paywall so here's the abstract:

The effect of superimposing sinusoidal, triangular, and square‐wave alternating voltage (aV) on the potentiostatic reduction of salicylic acid to salicylaldehyde has been studied. Polarization curves with aV modulation were measured with an amalgamated copper rotating disk electrode. Enhanced dc current densities were observed with increasing magnitude of superimposed aV. However, aV frequency and type did not have significant effect on the resulting dc current densities. Batch cell electrolysis experiments were conducted with a H‐type cell. The conversion of salicylic acid and yield of salicylaldehyde were measured as functions of electrolysis time, aV magnitude, type, and frequency. The superimposed aV significantly enhanced the reaction rate, conversion of salicylic acid, and the yield of salicylaldehyde. The energy consumption of ac electrolysis was higher than that of dc electrolysis.

  1. doi: 10.1149/1.2100342 J. Electrochem. Soc. 1987 volume 134,issue 12, 3074-3077
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It's simple. Reduce with LAH then oxidise it with PCC or PDC.

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  • $\begingroup$ That would be the first step of the reaction. It won't stop there. $\endgroup$ – user55707 Nov 30 '17 at 18:44
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    $\begingroup$ A better oxidation would be to reflux the benzyl alcohol over activated MnO2 in a neutral solvent $\endgroup$ – Waylander Nov 30 '17 at 18:49
  • $\begingroup$ I suspect that reduction of salicyclic acid with LAH can afford o-cresol. Salicyl alcohol, as its oxygen bound aluminate salts, can eliminate to form 2-methylene cyclohexadienone, which suffers reduction to o-cresol. Compare mixed anhydride reduction of salicylic acid with NaBH4. Synthesis, 2479 (2005). A fair comparison is the LAH reduction of indole-3-carboxaldehyde to 3-methylindole (skatole). $\endgroup$ – user55119 Apr 11 '18 at 21:00

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