Cyclopentane-1,2,3-trione

The terminal double bond of 9-decenoic acid (i), prepared from commercially available 10-undecenoic acid, is oxidized with PdCl2/CuCl2 to give 9-oxodecanoic acid (2), which is converted to 3-(6-methoxycarbonylhexyl) cyclopentane-1,2,4-trione (3). This is an important intermediate in the prostaglandin synthesis5 (Scheme 2). 

The reaction of diethyl oxalate with ketones in the presence of sodium ethoxide, or other bases, has been used extensively examples are given in Scheme 66. Reaction may occur with esterketone ratios of 1 1, 2 1, or 1 2, but only the 1 1 case finds substantial use in modem synthetic practice. Frequently the a-oxalyl ketone is thermally decarbonylated to give the 3-heto ester.An early example of this was provided by Bachmann s synthesis of equilenin. The mechanism of this reaction has teen examined labeling studies showed that it was the ester carbonyl that was eliminated. The intact oxalyl group has teen used as a directing group in steroid methylation while, more recently, 2-oxalylcyclohexanone has provided a route to (R)-(-)-hexahydromandelic acid (Scheme 67). The products of acylation of suitable acyclic ketones can cyclize to form (enolic) cyclopentane-1,2,4-triones (equation 39). ... 

Saponification and decarboxylation of (521) with formic acid gave cyclopentane-1, 2,4-trione in good yield. ° Reaction of (522) with DMSO gave (523), which was converted into (524) by aqueous HCl. Cyclopentane-1,2,4-trione was converted by... 

AT-methylaniline into (525) with diazomethane O-methylation of the trione occurs. From a consideration of the u.v. spectrum of the 0-methyl derivative, cyclopentane-1, 2,4-trione was considered to have a preferred structure (526). 

Hulupone or 2-(3-methylbutanoyl)-5,5-bis(3-methyl-2-butenyl)-3-hydroxy-2-cyclo-pentene-1,4-dione (56, Figs. 23 and 83) is a yellow oil with a boiling point of 110°C at 10 mm Hg. It forms a quinoxaline with 1,2-diaminobenzene (melting point 110 C) (4). The pK/ value in methanol water 1 1 is 2.6 and the molecular formula C20H28O4. The UV absorption maxima are situated at 277 nm in acidic methanol and at 254 nm and 310 nm in alkaline methanol. These features are ascribed to an enolized cyclopentane-1,2,4-trione system (5). The H NMR spectrum shows the absorption signals for the protons of a 3-methylbutanoyl group and two 3-methyl-2-butenyl side... 

This behaviour is typical for a non-chelated hydroxyl function. The oxidation product therefore is 5,5-bis(3-methyl-2-butenyl)-3-hydroxy-3-(2-methylpropanoyl)-cyclopentane-1,2,4-trione (222, Fig. 93), which is also available by direct oxidation of cohulupone in hexane. Compound 222 occurs in old hops, as confirmed by extraction, trimethylsilylation and gas chromatographic analysis. The oxidation of the triacylmethane chromophore in mild conditions is rather unusual. On the other hand, the carbon atom is readily oxidized in strong base (see 13.2.3.) or via catalysis by... 

The polyhydrazones of cyclopentane-l,3-diones and -1,2,3-triones, like their cyclohexane counterparts, are accessible from 1,3-diones. For example, cyclopentane- 1,3-dione yields cyclopentane-1,2,3-trione 3-phenylhydrazone... 

Reactions of cyclic and acyclic 1,2,3-triones with phenylhydrazine give rise to mono- and bis-(phenylhydrazones). The central carbonyl group of a vicinal tricarbonyl system is electron-deficient and highly electrophilic,255 which is why treatment with an aryldiazonium ion affords 2-phenylhydrazones. The structure of such bis(phenylhydrazones) as cyclopentane-1,2,3-trione 1,3-bis(phenylhydrazone), dehydroascorbic acid 2,3-bis(phenylhydrazone), and cy-clobutanetetraone l,3-bis(phenylhydrazone) has been studied by UV, IR,1H-, 13C-, and 15N-NMR spectroscopy.265-268 Quantum-mechanical calculations to predict the most stable tautomeric forms of some 1,2- and l,3-bis(phenylhydrazones) revealed that the chelated bis(hydrazone) structure was usually more stable than the azoene-hydrazine structure.269-271 This does not mean that such structures do not exist, for example cyclobutanetetraone l,2,3-tris(phenylhydrazone) exists in a stable phenylazoene-hydrazine structure (see 49 Scheme 8).257-263... 

Scheme 11. Chelated structures of cyclohexane- and cyclopentane-1.2.3-trione tris (phenylhydrazones).270 271...

The first result in this field was the reduction of 2-(6-carbomethoxy-hexyl)cyclopentane-l,3,4-trione with [Rh(COD)(ACMP)2]" BF4 as catalyst to 2-(6-carbomethoxy-hexyl)-4-(R)-hydroxycyclopentane-l,3-dione, an intermediate of the Ei prostaglandine synthesis (12) ... 

The synthesis began with cyclopentan-l,3-dione 75. A Robinson annela-tion gave 77 via intermediate trione 76. Reduction of the more electrophilic of the two ketones gave 78, which was protected as tetrahydropyranyl ether 79. Deconjugative alkylation of 79 gave 80. Hydrolysis of the THP protecting group and reduction of the ketone (pseudo-axial delivery of hydride) provided 81/82. 

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