Evaluation of antioxidant properties of monoaromatic derivatives of pulvinic acids.

The natural mushroom pigment Norbadione A and three other pulvinic acids were shown by our group to display very efficient antioxidant properties by comparison with a collection of potent molecules including catechols, flavonoids, stilbenes, or coumarins. Despite numerous publications on robust and straightforward synthetic access to pulvinic acids by us and others, no report has been made to unravel the structure-activity relationships that govern the striking antioxidant activity. Herein is presented the synthesis of 18 diverse pulvinic acid derivatives and the study of their radical scavenging capacities by four different assays. The influence of each of the two phenyl rings, of their substituents and of the lateral chain on the antioxidant properties, was explored to reveal a simplified structure of excellent activity. These results, along with the absence of cytotoxicity, make the synthesized compounds interesting to evaluate for several biological activities and especially for anti-inflammatory effects and skin protection against UV induced oxidative stress.

Uncatalyzed reaction of silyl ketene acetals with oxalyl chloride: a straightforward preparation of symmetrical pulvinic acids.

[reaction: see text] Several natural pulvinic acids were synthesized. Silyl ketene acetals derived from methyl arylacetates (4 equiv) reacted with oxalyl chloride at -78 degrees C, without the need of adding a catalyst. After treatment of the crude diketones with DBU and acidification with hydrochloric acid, symmetrical pulvinic acids methyl esters were obtained. Saponification of the methyl esters afforded the corresponding pulvinic acids in 60-70% overall yields from oxalyl chloride.