天然有機化合物討論会講演要旨集
Online ISSN : 2433-1856
第59回天然有機化合物討論会実行委員会
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7 シクロペンタジエニルアニオンを有する天然赤色色素 Juglorubin の全合成 (口頭発表の部)
*加茂 翔伍吉岡 快倉持 幸司椿 一典
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p. 37-42

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Juglorubin (1) was isolated from Streptomyces sp. 815 and GW4184 in 1993 (Figure 1). This compound has an unusual 6/6/5/9/6-fused pentacyclic ring system containing a cyclopentadienyl anion and a 9-membered lactone. Juglorubin might be biogenetically synthesized from juglomycin C (2) via juglocombins A/B (3/4), because they were isolated from the same Streptomyces strains (Scheme 1). The key steps in the proposed biosynthesis involve the dimerization reaction of juglomycin C (2) to establish the 6/6/5/6/6-fused pentacyclic flamework of juglocombins A/B (3/4), and the skeletal transformation from juglocombins A/B to juglorubin through the C–C bond cleavage reaction. Herein, the first total synthesis of juglorubin (1) by a strategy inspired by the proposed biosynthesis is reported. Juglocombins A/B (3/4), the hypothetical biosynthetic precursors, were successfully synthesized using a bioinspired dimerization of juglomycin C derivative (7) and a photoinduced reduction of epoxide in resultant dimer 6 as key reactions. The desired skeletal transformation proceeded by treatment of 3/4 with MS3A and NaCl in MeCN under an oxygen atmosphere to afford juglorubin in 26% yield. Furthermore, the reaction intermediate 16 was obtained in 78% yield by the use of NaOAc as a weak base. Juglorubin dimethyl ester (1') and 1-O-acetyljuglorubin dimethyl ester (17) were also prepared from juglocombins A/B dimethyl ester (3'/4'). Broad absorption bands in the region of 200-600 nm were observed in the UV-vis spectrum of 1'. Broad emission bands spanning from the visible to the near-infrared region (550-900 nm) were detected in the fluorescent spectrum of 1'. The maximum emission wavelength (λem. max) of 1' was 630 nm. The absolute quantum yield (φ) of 1' in acetonitrile was 5.3%.

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