A Simple Synthesis of 9-Ketodihydro-α-damascone

The compounds, 9-keto-8,9-dihydro-damascones or damascenones, are used as the ingredients in the production of perfumes and aromas. Since these compounds are able to be converted into damascones and damascenones. 9-keto-8,9-dihydro-α-damascone [2,6,6-trimethyl-1-(1,3-dioxobutyl)cyclohex-2-ene] (I) may be considered as a precursor of α-damascone. Until now, (I) and its analogues were synthesized by the multistep reactions.

The authors synthesized (I) by a simple reaction of α-cyclogeranoyl chloride(II) and the carbanion(III) which was prepared from acetone with base.

(II) was prepared from α-cyclogeranoic acid by the action of thionyl chloride (bp 43-45°C/0.4 mm Hg, 81% yield).

The reaction procedure was as follows: Sodium amide(15 mmol) was suspended in ether(10 ml) and refluxed for 30 minutes under nitrogen. Into this suspension, acetone(8.6 mmol) was dropped at room temperature with stirring. After 20 minutes, (II) (2.7 mmol) in dry ether(l.5 ml) was added at once to the mixhne at 0°C and stirred vigorously for 5 minutes, and for 15 minutes at 20°C. Then the reaction mixture was poured into ice water and extracted with ether. Column chromatography (silica gel/benzene) of the extract gave (I) (1.07 mmol). The spectroscopic data agreed with those in the literature. The aqueous layer was acidified with phosphoric acid, and α-cyclogeranoic acid was recovered(l.4 mmol).

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