120(P62) メソ化合物の非対称化を利用した新規不斉ラクトンおよびシクロプロパン化反応(ポスター発表の部)

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There are many natural products possessing lactone and cyclopropane as partial structure and also they are important as building blocks for the synthesis of natural procucts and derivatives. So that it is desirable to make a simple protocol for their asymmetric induction. This work present new paradigm for asymmetric lactonization and cyclopropanation with exceptionally high ee by the use of the ability to desymmetrize meso diester using an asymmetric palladium catalyzed allylic alkylation. Reaction of meso dibenzoate 1a with dibenzyl malonate in the presence of [η^3-C_3H_5PdCl]_2 and ligand 2 gave mono alkylated product 3 in 81% yield and 99% ee. Cyclization of 3 to cyclopropane 4 using triphenylphosphine as ligand, [η^3-C_3H_5PdCl]_2 and DBU as base proceeded moderately but significant racemization accompanied. Use of cesium carbonate with a catalyst formed from [η^3-C_3H_5PdCl]_2 and dppp in THF gave 4 in 90% yield and 94% ee. Along the almost same way, seven-membered and five-membered ring substrate (5a and 5b) gave cyclopropane 7a and 7b via mono alkylated product 6a and 6b in 96% ee and 89% ee, respectively. One of the explanation of the cause of racemization stemmed from reversible cyclization to a symmetrically bridged bicyclic system 16. Switching to Meldrum's acid as pronucleophile gave similar results wherein the monoalkylation product 8a was obtained in 60% yield and 98% ee. Resubjection of 8a to the same condition gave none of the cyclopropane 9 and only lactone 10a in 75% yield. Switching to dicarbonate 1b in the absence of base led directly to the lactone 10c but only in 30% yield (83% ee). After examining the various factors, the best conditions involved performing the alkylation with 1mol% [η^3-C_3H_5PdCl]_2, 2.5mol% 2, and 0.5 equiv potassium carbonate with 3 equiv methanol in THF at 0℃ to give 10a in 70% yield and >99% ee. Seven-membered and five-membered bicyclic lactone (12a and 12b) was also obtained in both 96% ee with some modified conditions. The formation of the lactone presumably arose from an unprecedented O-alkylation to produce 13 which may react with alcohol to give product 15.
天然有機化合物討論会の論文
1998-08-31

120(P62) メソ化合物の非対称化を利用した新規不斉ラクトンおよびシクロプロパン化反応(ポスター発表の部)

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