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Xylomolones A–D from the Thai Mangrove Xylocarpus moluccensis: Assignment of Absolute Stereostructures and Unveiling a Convergent Strategy for Limonoid Biosynthesis

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posted on 05.02.2019, 00:00 by Wan-Shan Li, Attila Mándi, Jun-Jun Liu, Li Shen, Tibor Kurtán, Jun Wu
Two new 9,10-seco limonoids with a central 3,4-dihydro-2H-pyran motif, named xylomolones A and B (1 and 2, respectively), possessing identical absolute configurations but reversed alignments of ring A, compared to the tricyclic core of rings B–D, were obtained from seeds of the Thai mangrove Xylocarpus moluccensis, together with a highly modified 1,2-seco limonoid, named xylomolone C (3), containing a novel 3-oxabicyclo[3.2.1]­octane-2,7-dione motif, and a new C11-terpenic acid methyl ester (4). The relative and absolute configurations of 1–4 were evidenced by extensive nuclear magnetic resonance (NMR) investigations combined with density functional theory and time-dependent density functional theory calculations of electronic circular dichroism, specific optical rotation, and 13C NMR data. The co-isolation of 1–4 allowed us to propose a novel convergent strategy for limonoid biosynthesis for the first time. This study demonstrates that mangroves of the genus Xylocarpus harbor new limonoid biosynthetic routes.

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