Diacyl decarboxylation products

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). 

Diacyl peroxides are sources of alkyl radicals because the carboxyl radicals that are intitially formed lose CO2 very rapidly. In the case of aroyl peroxides, products may be derived from the carboxyl radical or the radical formed by decarboxylation. The decomposition of peroxides can also be accomplished by photochemical excitation. 

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. 

In one of the earliest and most thorough studies of this reaction, Feldhues and Shafer analyzed a wide range of diacyl peroxides, covering more than two dozen compounds. Representative examples are illustrated in Schemes 2.26 to 2.29 [58, 59]. Most reactions were carried at low temperatures with neat solid or oily compounds until the starting material had been completely consumed (up to 5 days). The product yields in the schemes represent the reaction selectivity, and are an indication of alternative reaction pathways, such as radical disproportionation and others. It was shown that the photolytic decarboxylation reaction tolerates a wide... 

The decarboxylation of diacyl peroxides was also applied in the synthesis of cydopentadecanone, a compound with a musky odor of interest in the perfume industry (Scheme 2.31) [62].The photolysis of tetraacyl diperoxides 119 led to the extrusion of four molecules of C02 to give 121 in 73% yield. When decarboxylation occurred thermally, the product was obtained in only 41% yield. While the diacyl peroxide containing a keto group (106) was shown to give side products, the ketal-protected diperoxide 120 decarboxylated efficiently to give 122 in 65% yield. 

Included in Scheme 2.32 are a series of bis(amino acids) prepared by Spantulescu et al. [63] by the decarboxylation of diacyl peroxides derived from combinations of protected aspartic and glutamic acids. In all cases, the reactions gave yields ranging from 52% to 66%. To prevent the formation of side products, the irradiation was performed on neat samples at low temperatures (—78 °C or — 96 °C). 

The decarboxylation of diacyl peroxides was also used in the synthesis of a nonapeptide 148 and 5-methyl L-arginine 151 (Scheme 2.35) [65,66]. The peptide 148 is an analogue of an oxytocin receptor antagonist. The photolysis of precursor 146 gave 34% of product 147, which was reprotected with Fmoc and then incorporated into the oligopeptide. 5-Methyl arginine 151 was synthesized as a probe to study the nitric oxide synthase (NOS) active site and mechanism. The photolysis of 149 gave 150... 

The cobalt and PTC carbonylation of dienes and trienes give acylated products [147]. Cyclohexa-1,3-diene is transformed to 1-acetylcyclohexa-1,3-diene and a mixture of isomeric diacylated cyclohexadienes. 1-Vinylcyclohexene and other substituted dienes are acylated at the dienic moiety to give the tranj -isomers of the corresponding dienones [153]. Carbonylation of a-vinylcinnamic acid in the presence of nickel cyanide gives a-ketolactone, resulting from a decarboxylation-double carbonylation reaction [154]. 

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