40 対称構造をもつトリテルペンエーテル類の不斉全合成(口頭発表の部)

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Previously, we have synthesized various natural products enantioselectively by using baker's yeast reduction of acyclic α-hydroxyketones as the chilarity induction method. In this paper, we describe the enantioselective synthesis of two triterpene ethers possessing symmetrical structure, e.g. glabrescol with C_s symmetry and testudinariol A with C_2 symmetry. 1. What is the structure of glabrescol? Glabrescol, isolated from Spathelia glabrescens, is a triterpene having five continuously linked tetrahydrofuran (THF) rings, to which novel meso-type structure 5 has been assigned. Starting from (+)-9 obtained in high enantiomeric purity by yeast reduction, two 15-carbon segments 7 and 8 were synthesized. Coupling of them, followed by diastereoselective oxygenation and THF ring formation resulted in the synthesis of a compound having the structure 5 and one of its diastereomers 21. However, their NMR spectra were not identical to those of natural glabrescol. Thus, the structure of glabrescol remains to be clarified. 2. Synthesis of testudinariol A Testudinariol A has been isolated from the marine mollusc Pleurobrancus testudinarius and C_2 symmetric structure 6 has been assigned. Because of its C_2 symmetry, 6 could be synthesized by the homocoupling of the segment such as 22. Starting from the compound 23 which has been prepared during our synthesis of hippospongic acid A (4) and employing hydroboration and ene reaction, the bromide 40 was prepared and treated with Ni(cod)_2. The coupling product was a mixture of (Z,Z)-isomer and (Z,E)-isomer (1:1). Although the separation of them was unsuccessful, NMR spectra revealed the presence of testudinariol A (6) in the mixture. Further study for other method to combine the segments without formation of undesired isomer is under proceeding.
天然有機化合物討論会の論文
2000-10-01

40 対称構造をもつトリテルペンエーテル類の不斉全合成(口頭発表の部)

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