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Unique Applications: Novel Oxidation Reactions
By: Ian Gatfield, Jens-Michael Hilmer and Heinz-Jürgen Bertram, Symrise
Posted: October 18, 2005, from the November 2005 issue of P&F magazine.
It has long been common knowledge that the lipase from Candida antarctica can convert long chain fatty acids into the corresponding peracids in the presence of hydrogen peroxide. These peracids can bring about the oxidation of olefins, forming epoxides, which are useful synthetic intermediates. However, if ethyl acetate is used as solvent and long chain fatty acids are not present, the lipase not only hydrolyzes the ester bond but also produces peracetic acid from the liberated acetic acid. The so-formed peracetic acid can further oxidize unsaturated organic molecules, whereby the corresponding epoxides are invariably the intermediates in the resulting oxidation step. These epoxides can undergo further rearrangement reactions, thereby resulting in the formation of molecules with interesting flavor properties.
Under standard conditions, cinnamic aldehyde can be readily converted into benzaldehyde, the character impact compound of bitter almonds. Oxidation of the aldehyde group also occurs quite readily under the reaction conditions, forming benzoic acid (16 percent), cinnamic acid (19 percent) and a small amount of phenylacetaldehyde (6 percent).
The use of the less reactive cinnamic alcohol as substrate infl uences the overall reaction such that very little C-C bond cleavage takes place. Thus, the main products are derivatives of cinnamic alcohol. These derivatives arise from the enzymatic acetylation of alcohols that are either present from the start, i.e. cinnamic alcohol itself, or produced by ring opening of the epoxy intermediate. None of these products can be detected in the absence of the lipase, proving that this conversion reaction is enzyme catalyzed.
Surprisingly, this enzyme system is also able to convert isoeugenol into vanillin — not only in good yields (50 percent), but also under very mild conditions. The major byproducts formed are the corresponding diethyl acetal of vanillin (10 percent) and the methyl ketone (25 percent).
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