Decarboxylation 1-lactones

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). 

Recently, the Biju group demonstrated a highly enantioselective NHC-eatalyzed synthesis of functionalized cyclopentenes via o,p-unsaturated acyl azolium intermediates. This organo cascade reaetion of modified enals with malonic ester derivatives having a y-benzoyl group involves the Michael/ intramolecular aldol/p-lactonization/decarboxylation sequence to deliver functionalized cyclopentenes in good yields and excellent ee values (up to 85% yield and >99% ee) (Scheme 7.105). 

Treatment of the ascorbigens (63) with base (pH 11-12, 20°, 12 h) gave C-1 substituted L-sorbopyranoses (64) by ring-cleavage of the lactone, decarboxylation, and rearrangement. ... 

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]. 

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