3,4,5-trimethoxy-benzaldehyde from 3,4,5-trimethoxy-benzoic acid
Turn your attention to Patent US4240984 (http://l2.espacenet.com/dips/viewer?PN=US4240984&CY=gb&LG=en&DB=EPD)
for procedure details.
Abstract:
A process for preparing aromatic aldehydes starting from the corresponding aromatic acid, comprising two steps, the first one consisting of the reaction of said acid with ethyl or isobutyl chlorocarbonate to obtain the corresponding anhydride, and the second one to hydrogenate the thus-obtained anhydride to yield to aldehyde.
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