2-Hydroxy-3-methylcyclopent-2-enone

The radical reductive cyclisation of diesters to acyloins (see also Section 5.9.1, p. 628) is an important method of synthesis for ring sizes from four-membered upwards. The example selected here is 2-hydroxy-3-methylcyclopent-2-enone ( corylone ) (29) (Expt 7.10), which is an important perfumery and flavouring material.53 In the first step (i), methyl acrylate is converted into its dimer with tris(cyclohexyl)phosphine in pyridine solution.5b Step (ii) is the protection of the double bond by conversion into the dimethylamino adduct. The acyloin reaction is step (iii), and the product is trapped as its bis(trimethylsilyl)ether. Finally, in step (iv), the protecting dimethylamino and trimethylsilyl groups are removed by passage down a column of silica gel. 

The alicyclic ketones have been identified as being important flavor precursors by Flament et al. (15) and more recently by MacLeod and Ames (44), who identified a similar compound (3-methyl-cyclo-pentanone) in heated ground beef. Flament et al. (15) singled out the importance of these compounds, particularly in the presence of alhylpyrazines. Nishimura et al. (41) produced a meaty odor by heating 2-hydroxy-3-methylcyclopent-2-enone with cyclotene and HaS. Two volatile compounds described as having meaty odor were 2-methyl-cyclopentanone and 3-methylcyclopentanone. Thus, cyclotene appears to be a key precursor of "roast beef" flavor. 

Rethrolones and Related Compounds.—Shono and his co-workers have extended their studies of the synthesis of cyclopentenones via the electrochemical oxidation of substituted furans, and effected a synthesis of 2-alkylrethrolones (Scheme 17), and also of 2-hydroxy-3-methylcyclopent-2-enone (Scheme 18), which is an important flavour compound. 

Synonyms 2-Cyclopenten-1-one, 2-hydroxy-3-methyl- Cyclotene 2-Hydroxy-3-methylcyclopent-2-enone 2-Hydroxy-3-methyl-2-cyclopenten-1-one Hydroxy-3-methylcyclopent-2-en-1 -one Kentonarome Maple lactone 3-Methylcyclopentan-1,2-dione 3-Methylcyclopentane-1,2-dione 3-Methyl-2-cycl opentene-2-ol-one Methyl cyclopentenolone Methyl-2-cyclopenten-2-ol-1-one Empirical C6Hs02... 

Regio- and diastereoselectivity in 1,3-dipolar cycloadditions of nitrile oxides to 4-substituted cyclopent-2-enones was studied (238, 239). The reactions are always regioselective, while the diastereofacial selectivity depends on the nature of the substituents. Thus, 4-hydroxy-4-methylcyclopent-2-enone (75) gives preferably adducts 76a, the 76a 76b ratio warying from 65 35 to 85 15 (Scheme 1.22). 

A reported diastereoselective synthesis of precursor A of vitamin D3 involved the use of 2-methylcyclopent-2-enone as starting material. The Mukaiyama-Michael conjugate addition of ketene acetal 269 in the presence of trityl hexachloroaniimonate afforded the adduct 270. The lateral chain was introduced, according the procedure of Tsuji, by the treatment of crude 270 with allyl carbonate and palladium dibenzylideneacetone " (Scheme 63). The expected product 271 was obtained in 63% yield from 269. Reduction of 271 with LAH afforded a mixture of diols that was selectively tosylated at the primary hydroxy group. The secondary hydroxy group was protected with the methoxymethyl group and further functional modifications afforded the lactone 272. The reaction of lithium dimethyl methylphosphonate with the lactone 272 completed the synthesis of the AB-des-cholestane derivative 273. 

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