a) Planar structure of 1 showing TOCSY correlations from H-2, H-6, H-8, H-10b, H-13, H-17, H-21, H-1′, H-2′ (black circles) indicated as bolded lines and key HMBC correlations represented by single-headed arrows. b) The assignment of geometry around the C-13/C-14 bond in 1 using key ROESY correlations indicated by double-headed arrows.