Fig. 2: HSQC and HSQC-TOCSY spectra of caylobolide A at 700 MHz in pyridine-d5.
a, HSQC spectra with 128 f1 increments over 31,645.6 Hz, giving a spectral resolution of ∼247 Hz. b, Pure-shift HSQC spectra with 1,024 f1 increments linear predicted to 3,072 and zero-filled to 4,096, over 6,443.3 Hz, giving a spectral resolution of ∼1.5 Hz. c, 3JCH correlations observed by HSQC-TOCSY were used to ‘walk’ around the backbone of caylobolide A, leading to a regiochemical reassignment. The originally proposed structure suggested that the triol was at C25–C29 and has been repositioned to be at C17–C21. d, Tabulated chemical shifts (ppm) for the (S) and (R) Mosher esters of caylobolide A with the difference between ΔδS/R values giving ΔΔδSR, the sign of which directly correlates to the absolute stereochemistry of the stereogenic carbinol of interest; illustrated in the caylobolide A carbinol, with ΔΔδSR values indicated at all six stereogenic secondary 1,5-alcohols. Atom numbers are highlighted in bold. Green ΔΔδSR values depict stereochemistry as ‘up’ and red ΔΔδSR values depict stereochemistry as ‘down’ in the orientation shown. It is worth noting that the 13C ΔΔδSR values of the 1,3,5-triol did not align to the relative stereochemistry suggested by the Kishi analysis, ascribed to perturbation by the proximal stereocentres, and therefore analysis was performed using 1H chemical shift analysis with consideration of the pairwise additive effect of poly-Mosher esters (Supplementary Information, section 2.4.4)23,24.
