Structure, carboxyl group Diversity

Carboxylic acids are structurally diverse, with equal concentrations of straight-chain and branched compounds. The abundances of carboxylic acids decrease as the number of carbon atoms in the structure increase. Dicarboxylic acids, containing two carboxyl groups (Fig. 10.2), also occur. These acids are abundant in Murchison. The measured S13C and 8D values are higher for branched than straight-chain acids. This isotopic difference points to synthesis of the branched and linear molecules under different conditions. 

Decarboxylation may also be required in cases other than those involving derivatives of acetoacetic or malonic esters. The usefulness of this operation stems from the tremendous synthetic potential of carboxylic acids and their derivatives as substrates employed in C-C bond-forming reactions such as a-alkylation, Michael addition, the Diels-Alder reaction, etc. As the immediate result of these reactions, acid derivatives containing diverse structural backbones are formed. Hence the scope of these methods in synthetic practice depends heavily upon the opportunity to remove the carboxyl group after it has... 

From structure-function studies it is known that esterification of the a-carboxyl group of D-iJo-Glu of microcystin generates untoxic compounds, probably as a result of the lack of inhibition of the protein phosphatases (34,53) and that the methyl ester of okadaic acid is inactive as well (86). Correspondingly, this carboxylic function seems to play an essential role. In order to account for the almost identical potency of microcystins and okadaic acid, we have proposed a location of the carboxyl function of the two diverse compounds at the same position and a wrapping of the okadaic polyether chain around part of the microcystin ring and alignment of its hydrophobic tail with the Adda residue side-chain (108). 

Carboxylic acids can be converted into amides through a transition metal-free decarboxylative amidation process (Scheme 3.40) [38]. The reaction was promoted by hypervalent iodine reagents, and moderate to good yields of the amides were obtained for a group of structurally and electronically diverse carboxylic acids. 

The reaction of tert-alkyl Grignard reagents with carboxylic acid chlorides in the presence of a copper catalyst provides ieri-alkyl ketones in substantially lower yields than those reported here.4,14 The simplicity and mildness of experimental conditions and isolation procedure, the diversity of substrate structural type, and the functional group selectivity of these mixed organocuprate reagents render them very useful for conversion of carboxylic acid chlorides to the corresponding secondary and tertiary alkyl ketones.15... 

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