3- Methylcyclopent-2-enone

The first group of target molecules that are considered are the cyclic a,j8-unsaturated ketones, 3-methylcyclopent-2-enone (11), 3-methylcyclohex-2-enone (12), ( + )-7a(S)-7,7a,-dihydro-7a-methyl-l,5(6H)-indanedione (13), and tetraphenylcyclopentadienone (14). 

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. 

Dissolve 2.5 g of sodium hydroxide in 250 ml of water in a 500-ml two-necked flask fitted with a reflux condenser and a dropping funnel. Bring the solution to the boil, add rapidly from the dropping funnel 28.5 g (0.25 mol) of hexane-2, 5-dione (Expt 5.104) and continue to boil steadily under reflux for exactly 15 minutes (1). Cool the resulting dark-brown solution rapidly in an ice-salt bath, saturate with sodium chloride and extract with one 100 ml and two 50 ml portions of ether. Wash the ether extract with three 5 ml portions of water, dry over anhydrous sodium sulphate and remove the ether on a rotary evaporator. Distil the residual dark oil under reduced pressure and collect the colourless 3-methylcyclopent-2-enone as a fraction of b.p. 74-76 °C/ 16mmHg, n 0 1.4818 yield 9.5 g (40%). The product thus obtained is pure enough for most purposes when perfectly pure the refractive index is 1.4893. The product may darken on storage. 

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. 

Conjugate additions. Conjugate addition of phenyllithium to 3-methylcyclopent-2-enone (1) was effected in the presence of cuprous bromide. Presumably the reactive species is diphenylcopperlithium. 

Murakami s group has developed various rhodium-catalyzed C—C bond cleavages of small rings. Recently, they reported rhodium-catalyzed carbonylation reactions of spiropentanes involving two different types of C—C bond cleavage processes (Equation 11.43) [85]. The reaction allows for the synthesis of a series of 3-methylcyclopent-2-enones, one of which has been utilized as an intermediate in the concise synthesis of ( )- 3-cuparenone. Another example is the rhodium-catalyzed intramolecular olefin insertion of 3-(o-styryl)cyclobutanone to generate... 

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... 

In 2007, a series of 3-methylcyclopent-2-enones were synthesized by a rhodium-catalyzed carbonylation of spiropentanes [33]. Here, two different types of carbon-carbon bond cleavage processes were involved to get the product in good yield (Scheme 1.13)... 

We thank Sobering AG for a gift of 3-methylcyclopent-2-enone, and the Government of West-Bengal and the SERC for Studentships (AKS and DH, respectively). 

The silyl enol ethers (42), in the presence of palladium acetate, cyclize to give the a,/8-unsaturated ketones (44), the reaction probably proceeding via the oxo-ir-allylpalladium complex (43)/ The reaction proceeds readily at room temperature in the presence of stoicheiometric amounts of Pd(OAc)2, and is particularly useful for the synthesis of 3-methylcyclopent-2-enone derivatives (45). 

The mechanism of intramolecular aldol reactions is similar to that of intermolecular reactions. The only difference is that both the nucleophilic carbonyl anion donor and the electrophilic carbonyl acceptor are now in the same molecule. One complication, however, is that intramolecular aldol reactions might lead to a mixture of products, depending on which enolate ion is formed. For example, hexane-2,5-dione might yield either the five-membered-ring product 3-methylcyclopent-2-enone or the three-memhered-ring product (2-methylcyclopropenyl)ethanone (Figure 17.11). In practice, though, only the cyclopentenone is formed. 

The photochemical dimerisation of a, P-unsalurated ketones provides a further class of [ji +jt ] cycloaddition and is demonstrated for 3-methylcyclopent-2-enone and 3-methylcyclohex-2-enone, with the formation of head to head dimers and head to tail dimers. The a, (5-unsaturated enone absorbs light of wavelength 300 nm and get n,n excitation. The reaction is believed to occur from triplet state because ISC is efficient. While it,n triplet excited enone also cause dimerisation. 

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