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    • The molecular formula of was assigned

    2019-11-04

    The molecular formula of 2 was assigned as C12H12N2O2 by the positive HRESIMS at m/z 217.0973 [M+H]+ (calcd for 217.0972), 13C NMR and DEPT spectra, having 8 degrees of unsaturation. The 1H NMR spectrum contains an ABX spin system comprised of resonances at δH 7.47 (d, J=1.9Hz, H-2′), 6.86 (d, J=8.3Hz, H-5′) and 7.35 (dd, J=8.3, 1.9Hz, H-6′), indicating the presence of a 1,3,4-trisubstituted benzene ring. In addition, there exists a signal at δH 8.58 (s, H-6) and two methyl singlets respectively at δH 2.57 (s, H-7) and δH 2.53 (s, H-8). The 13C NMR and DEPT spectra (Table 1) demonstrate 12 signals ascribe to two methyl, four olefinic methine, and six olefinic quaternary carbons. The HMBC correlations of H-6, H-8/C-5 and ROESY correlation of H-6/H-8 indicate the substructure of C-6–C-5–C-8 (A). HMBC correlations of H-2′, H-6′/C-2 and H-7/C-2, C-3 allow establishing structure fragment of C-1′–C-2–C-3–C-7 (B). A pivotal HMBC correlation of H-6/C-2 in consideration of the chemical shifts of C-2, C-3, C-5, and C-6 suggest fragments A and B are connected via two N-atom bridges, in accordance with the requirement of degrees of unsaturation. Therefore, the structure of 2 was established and named as plancypyrazine A. The molecular formula of 3 was determined to be C20H19NO8 (12 degrees of unsaturation) deduced from its positive HRESIMS at m/z 402.1182 [M+H]+ (calcd for 402.1183), 13C NMR and DEPT spectra. The 1H NMR spectrum of 3 (Table 1) reveals the presence of two methyl [δH 2.16 (s, H-10); δH 1.88 (s, H-10′)], and six olefinic protons. The 13C NMR and DEPT spectra (Table 1) give 20 carbon signals classified into two methyl, one oxygenated methylene, eight methine (six olefinic and two oxygenated), and nine quaternary carbons. Inspection of these NMR data discloses that the structure of 3 extremely resembles that of molossusamide A [11], differing in that C-7 of 3 is oxidized into a ketone gaining support from HMBC correlations of H-6, H-8/C-7 (δC 193.0). Another difference between 3 and molossusamide A is that the side chain is positioned at C-1 in 3 instead of C-6 in molossusamide A evident from HMBC interactions of H-6, H-7′/C-4. Thus far, the planar structure of 3 was constructed. As far as the relative configuration of 3 is concerned, it is unambiguously assigned as trans-form by a coupling constant between H-7′/H-8′ (J=7.2Hz). The fact that syk inhibitor 3 is optically inactive. Separation of 3 on chiral HPLC afforded 3a and 3b, whose absolute configuration was further determined by ECD comparison with those of previously reported data [12], allowing 3a to be 7′R,8′S. Therefore, the structure of 3 was finally deduced and named as plancyamide B. Compound 4 possesses a molecular formula C15H14O6 as deduced from its negative HRESIMS and NMR data, indicating 9 degrees of unsaturation. The 1H NMR spectrum of 4 shows an ABX system characteristic of signals at δH 7.27 (d, J=1.9Hz, H-2′), 6.81 (d, J=8.3Hz, H-5′) and 7.15 (dd, J=8.3, 1.9Hz, H-6′). Two aromatic/olefinic singlets at δH 6.74 (s, H-3) and δH 6.79 (s, H-6) are indicative of a 1,2,4,5-tetrasubstituted benzene ring. The 13C NMR and DEPT spectra display 15 carbons, of which twelve are olefinic carbons belonging to two phenyl groups. Besides, the residue signals accounting for two methylene (one oxygenated) and one ketone are also observed. The 1H-1H COSY spectrum gives cross peaks of H-7/H-8 (δH 3.63), in conjunct with HMBC correlations of H-7/C-2, C-6 and H-8/C-1, and chemical shifts of C-4 (δC 143.7), C-5 (δC 148.7), revealing the west part of 4. The east part of 4 is a 1,2,4-trisubstituted benzene with 1,2-dihydroxyl groups supported by the above mentioned an ABX spin system. Two parts are connected via a ketone (C-7′) aided by HMBC correlations of H-2′, H-6′, H-3/C-7′. The structure of 4 was therefore identified and named as plancyol A. Compound 5 was found to have the molecular formula C15H18O9 (7 degrees of unsaturation) derived by analysis of its negative HREIMS ([M−H]−, m/z 341.0877, calcd 341.0878), 13C NMR and DEPT spectra. The 1H NMR spectrum of 5 shows a signal at δH 5.06 (d, J=3.7Hz) typical for the presence of an anomeric proton. The 13C NMR and DEPT spectra exhibit signals for five oxygenated methine (one ketal at δC 106.0) and one oxygenated methylene, strongly supporting the presence of an α-glucose moiety. In addition to this moiety, the 1H NMR spectrum also shows one aromatic proton at δH 7.42 (s, H-7) and one methyl at δH 2.27 (s, H-8), and the 13C NMR and DEPT spectra also give nine carbons including six olefinic carbon signals, one methyl, one oxygenated methylene and one carbonyl. To assemble the structure of the aglycone, the data from HMBC experiment were completely analyzed, which shows correlations of H-7 (δH 7.42, s)/C-1, C-3a; H-3/C-1, C-3a, C-4, C-7a, indicating the presence of a five-membered lactone ring as shown. In addition, HMBC correlations of H-8/C-5 (δC 155.5), C-6, C-7 indicate the substituted pattern in the benzene ring. Finally, HMBC correlation of H-1′ (δH 5.06)/C-4 evidently implies the position of glucose moiety at the benzene ring.