The total synthesis of (-)-ilimaquinone, a metabolite isolated from sea sponges, is described. The key step of the synthesis is the attachment of the quinone moiety to the drimane skeleton. Alkylation of enone 11 obtained in four steps from the readily available diketone 8, with tetramethoxybenzyl bromide 15 as the alkylating agent, led to addition product 16 in excellent yield. The presence of the tetramethoxybenzyl group induced stereoselective hydrogenation of the exo olefin 18, leading to the required isomer in a 9:1 ratio. Treatment of compound 21 with ceric ammonium nitrate (CAN) afforded formation of the quinone and deprotection of only one methyl ether in one step to furnish the desired ilimaquinone 1.
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.
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