Decarboxylative lactones

Biphenyl-2,2 -dicarboxylic acids are oxidatively converted into benzocoumarins with loss of carbon dioxide. A more predictable reaction of a dicarboxylic acid occurs on heating with sodium acetate-acetic anhydride C-acetylation first occurs and decarboxylative lactonization follows, but in low yield. 

Decarboxylative lactonization with cyclopropane ring opening... 

Barrelene was obtained via a double Diels-Alder reaction from a-pyrone with methyl acrylate (H.E. Zimmerman, I969A). The primarily forming bicyclic lactone decarboxylates in the heat, and the resulting cyclohexadiene rapidly undergoes another Diels-Alder cyclization. Standard reactions have then been used to eliminate the methoxycarbonyl groups and to introduce C—C double bonds. Irradiation of barrelene produces semibullvalene and cyclooctatetraene (H.E. Zimmerman. 1969B). 

Unsaturated 19-norsteroids are also obtained by thermal decarboxylation of A -19-acids (obtained by zinc reduction of the 5a-halo-6/3,19-lactones). 

CotLespondingly, tlie catalytic 1,4-addition of dialkylzinc reagents to 3-nitro-iimarin 89 iSdieme 7.24), witli a fixed irans orientation of tlie aiyl and nitro oiips, proceeds witli excellent yidds 190-99 96), bigli diastereoselectivity Id.r. up to ), and enantiosdectivities of up to 9296. Hydrolysis of tlie lactone moiety in 90 IS accompanied by decarboxylation, providing an asymmetric syntliesis of /J-aiyl-troalkane 91. 

As the mechanism, a radical and a cationic pathway are conceivable (Eq. 31). The stereochemical results with rac- or mcjo-1,2-diphenyl succinic acid, both yield only trans-stilbene [321], and the formation of a tricyclic lactone 51 in the decarboxylation of norbornene dicarboxylic acid 50 (Eq. 32) [309] support a cation (path b, Eq. 31) rather than a biradical as intermediate (path a). 

Decarboxylation of p-lactones (see 17-27) may be regarded as a degenerate example of this reaction. Unsymmetrical diacyl peroxides RCO—OO—COR lose two molecules of CO2 when photolyzed in the solid state to give the product RR. Electrolysis was also used, but yields were lower. This is an alternative to the Kolbe reaction (11-37). See also 17-29 and 17-40. 

Malonates are good reagents tor synthons (23) and (25). Lactones (2S) and (27) are formed on addition of the epoxides and give the perfumery lactones on hydrolysis and decarboxylation. 

Nair and co-workers have demonstrated NHC-catalysed formation of spirocyclic diketones 173 from a,P-unsaturated aldehydes 174 and snbstitnted dibenzylidine-cyclopentanones 175. Where chalcones and dibenzylidene cyclohexanones give only cyclopentene products (as a result of P-lactone formation then decarboxylation), cyclopentanones 175 give only the spirocychc diketone prodncts 173 [73]. Of particular note is the formation of an all-carbon quaternary centre and the excellent level of diastereoselectivity observed in the reaction. An asymmetric variant of this reaction has been demonstrated by Bode using chiral imidazolium salt 176, obtaining the desymmetrised product with good diastereo- and enantioselectivity, though in modest yield (Scheme 12.38) [74],... 

A simple application of the reaction may be mentioned. Refluxing of (I) with 48 per cent, hydrobromic acid and glacial acetic acid leads to hydrolysis and decarboxylation and the production of a mixture of the y-lactone yy-di-phenylbutyrolactone (II) and the isomeric unsaturated acid yy-dlphenyl-vinylacetic acid (III) reduction by the Clemmensen method or catalytically... 

On the other hand, y, 8-unsaturated acids give lactones that involve cyclization without decarboxylation. 

Decarboxylation, Masamune reaction, and diazotransfer Diazo 25 was prepared under optimized conditions, as summarized in Scheme 2.9. Decarboxylation of the malonate could be done under either acidic or basic conditions. Reaction of 17 under acidic conditions provided the desired mono-carboxylic acid 18 but lactone 35 was simultaneously formed (Figure 2.2). Under basic conditions,... 

The lactone-peroxides 17 are derived from the corresponding ot-pyrones. Singlet oxygenation at low temperature affords the unstable a-pyrone endoperoxides which, on warming up, readily decarboxylate into 1,2-diacylethylenes. However, subambient diimide reduction leads to the desired lactone peroxides 17 (Eq. 12)29). 

Finally, it is of interest to mention that the lactone-peroxide 17 decarboxylates at ca. 140 °C to afford succinaldehyde, but with light emission (Eq. 56)20). 

One of the best methods to synthesize cyclopentenone derivatives is the Pauson-Khand procedure. However, Shindo s group have recently developed a domino process consisting of a [2+2] cycloaddition of a ketone with anynolate, followed by a Dieckmann condensation to give a 3-lactone as 4-190 which is decarboxylated under reflux in toluene in the presence of silica gel to afford cyclopentenones [64a]. Thus, the reaction of 4-188 and 4-189 led to 4-190, which on heating furnished the linear cucumin 4-191 (Scheme 4.41). This natural product has been isolated from the mycelial cultures of the agaric Macrocystidia cucumis [65, 66]. The domino procedure described was also used to synthesize dihydrojasmone and a-cuparenone. Moreover, the [2+2] cycloaddition can be combined with a Michael reaction [64b]. 

The photolysis of carboxylic acids and derivatives as lactones, esters and anhydrides can yield decarboxylated products 253>. This reaction has been utilized in the synthesis of a-lactones from cyclic diacyl peroxides 254) (2.34) and in the synthesis of [2,2]paracyclophane by bis-decarboxylation of a lactone precursor (2.35) 255). This latter product was also obtained by photoinduced desulfurization of the analogous cyclic sulfide in the presence of triethyl phosphite 256). 

Although these reactions can be duplicated in most cases with the normal dimer of dimethylketene,11 the more reactive lactone dimer is the preferred reagent. The liquid form of this dimer is convenient to handle. A distinct difference in behavior of the dimethylketene dimers is noted when they are pyrolyzed. The normal dimer is dissociated at 600° to dimethylketene,12 but the lactone dimer is decarboxylated almost quantitatively at 450° to tetramethylallene.13... 

Several total syntheses of antirhine (11) and 18,19-dihydroantirhine (14) have been developed during the last decade. Wenkert et al. (136) employed a facile route to ( )-18,19-dihydroantirhine, using lactone 196 as a key building block. Base-catalyzed condensation of methyl 4-methylnicotinate (193) with methyl oxalate, followed by hydrolysis, oxidative decarboxylation with alkaline hydrogen peroxide, and final esterification, resulted in methyl 4-(methoxycar-bonylmethyl)nicotinate (194). Condensation of 194 with acetaldehyde and subsequent reduction afforded nicotinic ester derivative 195, which was reduced with lithium aluminum hydride, and the diol product obtained was oxidized with manganese dioxide to yield the desired lactone 196. Alkylation of 196 with tryptophyl bromide (197) resulted in a pyridinium salt whose catalytic reduction... 

Oxidative bisdecarboxylation.1 A new route to lactones is based on the ability of CAN to effect oxidative decarboxylation of a-hydroxymalonic acids to carboxylic acids (11, 143-144) and of a-alkoxymalonic acids to lactones. 

Decarboxylation of p-lactones to olefins,4 6 stereoselective reactions of p-lactones with a variety of electrophiles,6 9 and the regioselective fission of p-lactones by many different nucleophiles10 12 make these highly reactive compounds versatile intermediates for organic syntheses.13 Although several methods exist for the preparation of p-lactones, most p-lactones are now synthesized by [2+2] cycloaddition... 

Esters are hydrolysed under basic conditions in the presence of quaternary ammonium salts [e.g. 1-7], Microwave activation of basic soliddiquid systems without an added solvent enhances the rate of saponification and the reaction is not affected by steric factors [3], Microwave irradiation has also been used in the hydrolysis and decarboxylation of malonic esters [8] and p-keto esters [9] (>90%). Lactones... 

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