Studies on the chemical synthesis of antitumor saponins for the evaluation of their biological activities

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Toward the total synthesis of antitumor saponins scillascillosides, the stereoselective construction of CDE ring system of these molecules have been explored. Ireland-Claisen rearrangement of the dihydrofurancarboxylate, obtained by intramolecular O-H insertion followed by olefination, proceeded stereoselectively to give the desired stereoisomer in good yield. The product was successfully converted to the CDE ring fragment through an intramolecular 1,3-dipolar cycloaddition and introduction of an angular methyl group. With regard to the key Ireland-Claisen rearrangement, we found that the substitution pattern of the adjacent carbon atom played a pivotal role in directing the stereochemical course of the reaction, and this finding led to the development of methods for the stereodivergent syntheses of two chiral building blocks.