A conformational NMR analysis of methymycin aglycones: complete and unambiguous assignments of stereochemically diverse glycosylated methymycin analogs by 1D and 2D NMR techniques and molecular modeling

Structurally diverse metabolites from the rare actinobacterium Saccharothrix xinjiangensis

The bioassay-guided fractionation from cultures of the actinobacterium Saccharothrix xinjiangensis Act24Zk, collected from the Caspian Sea beach in Iran led to the isolation of three new compounds, caerulomycin M (1), saccharopyrone (2), and saccharonoic acid (3), together with the known compound, caerulomycin A (4). Their structures were elucidated from HR-ESIMS and 1D and 2D NMR data. Compound 2 displayed moderate cytotoxic activity against the human cervix carcinoma HeLa cells KB3.1 with an IC₅₀ value of 5.4 µM.

Synthesis of new cyano-substituted analogues of Tröger's bases from bromo-derivatives. A stereochemical dependence of long-range (n JHH , n = 4, 5, and 6) proton-proton and proton-carbon (n JCH , n = 1, 2, 3, 4, and 5) coupling constants of these compounds

A free-catalyst microwave-assisted cyanation of brominated Tröger's base derivatives (2a-f) is reported. The procedure is simple, efficient, and clean affording the nitrile compounds (3a-e, I) in very good yields. Complete assignment of ¹ H and ¹³ C chemical shifts of 2a-f, I and 3a-d, I was achieved using gradient selected 1D nuclear magnetic resonance (NMR) techniques (1D zTOCSY, PSYCHE, DPFGSE NOE, and DEPT), homonuclear 2D NMR techniques (gCOSY and zTOCSY), and heteronuclear 2D NMR techniques (gHSQCAD/or pure-shift gHSQCAD, gHMBCAD, bsHSQCNOESY, and gHSQCAD-TOCSY) with adiabatic pulses. Determination of the long-range proton-proton coupling constants ⁿ J_HH (n = 4, 5, 6) was accomplished by simultaneous irradiation of two protons at appropriate power levels. In turn, determined coupling constants were tested by an iterative simulation program by calculating the ¹ H NMR spectrum and comparing it to the experimental spectrum. The excitation-sculptured indirect-detection experiment (EXSIDE) and ¹ H-¹⁵ N CIGARAD-HMBC (constant time inverse-detection gradient accordion rescaled heteronuclear multiple bond correlation) were applied for determination of long-range carbon-proton coupling constants ⁿ J_CH (n = 2, 3, and 4) and for assignment of ¹⁵ N chemical shift at natural abundance, respectively. DFT/B3LYP optimization studies were performed in order to determine the geometry of 2c using 6-31G(d,p), 6-311G(d,p), and 6-311 + G(d,p) basis sets. For calculation of ¹ H and ¹³ C chemical shifts, ⁿ J_HH (n = 2, 3, 4, 5, and 6), and ⁿ J_CH (n = 1, 2, 3, and 4) coupling constants, the GIAO method was employed at the B3LYP/6-31G(d,p), B3LYP/6-31+G(d,p), B3LYP/6-311+G(d,p), B3LYP/6-311++G(2d,2p), B3LYP/cc-pVTZ), and B3LYP/aug-cc-pVTZ) levels of theory. For the first time, a stereochemical dependence magnitude of the long-range ⁿ J_HH (n = 4, 5, and 6) and ⁿ J_CH (n = 1, 2, 3, 4, and 5) have been found in bromo-substituted analogues of Tröger's bases.