Propargylic compounds decarboxylation

Not least for the syntheses of natural products, alkoxycarbonylations with formation of allenic esters, often starting from mesylates or carbonates of type 89, are of great importance [35, 137]. In the case of carbonates, the formation of the products 96 occurs by decarboxylation of 94 to give the intermediates 95 (Scheme 7.14). The mesylates 97 are preferred to the analogous carbonates for the alkoxycarbonylation of optically active propargylic compounds in order to decrease the loss of optical purity in the products 98 [15]. In addition to the simple propargylic compounds of type 89, cyclic carbonates or epoxides such as 99 can also be used [138]. The obtained products 100 contain an additional hydroxy function. 

In addition to the 1,3-dienes, in situ formed allenes were applied as substrates and cyclized with 2-iodophenols as well. In 2007, Liang and co-workers reported a novel palladium-catalyzed intermolecular tandem reaction for the synthesis of tetracyclic compounds from propargylic compounds and 2-iodophenols. The corresponding tetracyclic compounds were isolated in moderate to good yields (Scheme 2.47). In respect of the reaction pathway, the possible reaction intermediate was isolated and applied. In detail, this transformation consists of two catalytic cycles. The steps are (a) initial decarboxylation of propargylic compound by palladium(O) to... 

For a-aUylation and a-propargylation of carbonyl compounds, however, the Tsuji-Trost reaction and related reactions discussed extensively in Part V (Sect. V.2.1) provide a wide range of very satisfactory procedures. Although the Tsuji-Trost reaction has mostly been carried out by using extrastabilized enolates, such as acetoacetates and malonates, subsequent decarboxylation provides more usual a-substituted ketones (Scheme 3). 

Weak base such as cesium carbonate has been utilized in this reaction to generate diazo compounds in situ from tosylhydrazones through the Bamford-Stevens reaction. The reaction is initiated by palladium-promoted decarboxylation of propargylic carbonate to form propargylpalladium complex A, which then tautomerizes to afford allenylpalladium intermediate B. Subsequently, the common carbene formation-migratory insertion-p-hydride elimination occurs to afford various vinylallenes (Fig. 30). 

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