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Abstract
This work develops a research methodology based on modern NMR spectroscopy techniques for the complete structural characterization of Skimmianine, a furoquinoline alkaloid isolated from plants of the Rutaceae family. Experiments were conducted in a DMSO-d6 + CCl4 solvent mixture at a frequency of 600 MHz. The ¹H and ¹³C NMR chemical shifts (δ, ppm) of the compound, as well as long-range proton-carbon correlations from the HMBC (Heteronuclear Multiple Bond Correlation) spectrum, were analyzed in detail. The HMBC experiment unambiguously determined the connectivity of the furan ring (H-2, H-3) to the quinoline skeleton, the position of the ortho-substituted protons (H-5, H-6) on the benzene ring, and most importantly, the precise positions of the three methoxy groups (4-OCH₃, 7-OCH₃, 8-OCH₃). For example, the HMBC correlations demonstrated 4-OCH₃ (4.44 ppm) with C-4 (156.34 ppm), 7-OCH₃ (3.98 ppm) with C-7 (151.49 ppm), and 8-OCH₃ (3.96 ppm) with C-8 (141.51 ppm).
The presented methodology demonstrates the advantage of combining 1D NMR with 2D NMR (HSQC, HMBC) methods for elucidating the structure of complex natural compounds. This approach ensures the complete and reliable structural assignment of polymethoxylated furoquinoline alkaloids such as Skimmianine and also allows for the correction of previous misassignments found in the literature.
References
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