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"6-nonenal" is a monounsaturated, nine carbon aldehyde with two isomers based on the stereochemistry of the double bond at the 6 position. The cis isomer is often listed as the principal component in the aromas of muskmelon fruits. The trans isomer is included in aromas of milk foams, and thought to be a possible polypropylene odorant.

Biosynthesis
6-nonenal is thought to be synthesized in nature from γ-lineolenic acid through a lipoxygenase system. The lipoxygenase system cleaves carbon-carbon double bonds into hydroperoxides, which can be cleaved by hydroperoide lysase into the corresponding cis-aldehydes. The rationale behind this mechanism is due to the observed odor properties of muskmelons based on air exposure. In the ripe, unmodified muskmelon there are small concentrations of cis-6-nonenal. A large increase in the concentration of 6-nonenal is noticed when the cells are lysed and exposed to air. This is attributed to enzymatic activity of hydroperoxide lysases when exposed to a high oxygen environment not present in the cells.

Laboratory Synthesis
Synthetically, 6-nonenal can be produced from 1-bromo-5-octene. Depending on the desired product stereochemistry, either the cis or trans isomer is used. Because 1-bromo-5-octene is not very readily available, it must be first synthesized by brominating 5-octene-1-ol. To prepare 6-nonenal, first the 5-octene-1-ol is reacted with magnesium in ether to form the Grignard reagent. Then, triethyl orthoformate is added and the mixture is refluxed, yielding 1,1,6-diethoxynonene. The product is hydrolyzed with cold sulfuric acid, yielding 6-nonenal.