Out of interest, how would one go about reducing carboxylic acids, to aldehydes?

The easiest way is to make an ester and hit them with stoichiometric DIBAH (diisobutylaluminum hydride).

 

Or you can reduce them to the alcohol (LiAlH4, B2H6*THF, or the acid chloride with NaBH4) and then use PCC or PDC.

 

Or convert the alcohol to the halide, treat with silver nitrite (for best yields. lower yields are obtained with alkali nitrites) and seperate the mixture of alkyl nitrite (generally the minor product) and nitroalkane.

 

Use the nef reaction to cleave the nitroalkane to an aldehyde.

 

You'll have to do some research, because it works decently with some alkyl carboxylates and horribly with others (and very well with benzoic acids), but the Stephen aldehyde synthesis takes a nitrile to the aldehyde by treatment with anhydrous SnCl2 in ether followed by aqueous quenching. There are a number of ways to make the nitrile from the acid.

 

http://en.wikipedia.org/wiki/Stephen_aldehyde_synthesis

Edited October 8, 200916 yr by UC

The best way would be to reduce it all the way down to the alcohol either directly with borane or via the ester. The primary alcohol can then easily be oxidised to the aldehyde with either a Swern oxidation or Dess-Martin conditions.