Proton nuclear magnetic resonance evidence for two configurations of the hemiacetals of aflatoxin B1 and sterigmatocystin

Apoptotic Activity of New Oxisterigmatocystin Derivatives from the Marine-Derived Fungus Aspergillus nomius NC06

“…Compound 2 afforded similar 13 C and 1 H NMR spectral data, which diverged at less than 0.1 and 0.5 ppm to those of oxisterigmatocystin C, respectively, while the coupling constants were also comparable at a maximum difference of 0.4 Hz between both derivatives, which indicated an identical spatial conformation. The β-configuration of the methoxy unit on C-4 of compound 2 was similar to that found in oxisterigmatocystins A and C [18], which was based on the coupling pattern observed for an α-hemiacetal sterigmatocystin [18,24]. The coupling constant between H-2 at 4.19 ppm and H-3 A at 2.39 ppm gave 9 Hz (with actual values at 9.3 and 9.5 Hz, respectively) for a dihedral angle of about 20 • [24].…”

Novel Aflatoxin Derivatives and Protein Conjugates

“…Only the spectrum of the major diastereomer of AFB2-GA is discussed in detail including two-dimensional NMR spectra (see Supporting Information). A comparison of the chemical shifts and couplings constants of the proton signals of the two diastereomers of AFB2a (including Karplus analysis) [28,29] and AFB2-GA strongly suggests that the main isomer has an 8S configuration due to attack of glycolic acid on the aflatoxin's protonated enol ether group from the sterically less hindered side, as observed elsewhere [29,30] (note: the α/β nomenclature in [28] and [29] is not consistent). The NMR spectra of AFG2-GA are similar (not shown, see Supporting Information).…”

Sterigmatocystins from the deep-sea-derived fungus Aspergillus versicolor

“…Similarly, the dihedral angel between H-3¢b and H-2¢ should be 801-1001, and the angle between H-3¢a and H-2¢ was close to 201 ( Figure 3b). 12 For compound 2, the relative configuration of 4¢-OMe should be a. However, the coupling constant pattern was not completely consistent with that of sterigmatocystin, which had the a configuration of the hemiacetal.…”

“…However, the coupling constant pattern was not completely consistent with that of sterigmatocystin, which had the a configuration of the hemiacetal. 12 Based on the coupling constants of J 3¢a,2¢ and J 3¢b,2¢ , the dihedral angel between H-3¢a and H-2¢, and H-3¢b and H-2¢ should be about 201 and 1201, respectively ( Figure 3c). Furthermore, the dihedral angel between H-4¢ and H-3¢a should not be larger than 1201 was clearly proven by molecular modeling, and the dihedral angel between H-3¢a and H-4¢, and H-3¢b and H-4¢ were both close to 601 (Figure 3d) based on the coupling constants of them.…”