(a) Overlays of 1H NMR spectra of 1 (C8,C9-trans,
red and green) and 8 (C8,C9-cis, blue and violet) in
DMSO-d6 and CDCl3, respectively.
The resonance of N28-H in 1 and 8 is marked
with red and blue ovals in the overlays. (b) Parts of the ROESY spectra
of 1 and 8 recorded in DMSO-d6 showing crosspeaks from N28-H. ROESY crosspeaks across
the eight-membered hydrogen-bonded ring are marked with red ovals
in the spectrum of 1. (c) 1H–15N HMBC of 1 in DMSO-d6.
The crosspeak resulting from the coupling of N28-H and N15 is marked
with a red circle.