Bioactive Phloroglucinyl Heterodimers: The Tautomeric and Rotameric Equlibria of Arzanol

Arzanol (1) exemplifies a large number of heterodimeric bioactive natural products based on an alkylidene-bridged β-dicarbonyl-acylphloroglucinyl structural motif. The NMR spectra of these compounds are complicated by tautomeric and rotational equilibria that, despite growing synthetic and biomedical interest in the area, have not been systematically investigated. By combining information from NMR spectroscopy and DFT calculations, we have comparatively characterized the solution structures and conformations of the anti-inflammatory agent arzanol (1), its inactive monomethyl derivative 4, and its corresponding homodimers [helipyrone (5) and demethylmallotojaponin C (6)]. The results highlight the relevance of a network of intramolecular hydrogen bonds for the tautomeric and rotameric equilibria of these compounds, with arzanol (1) existing in two almost equally populated rotameric forms of a 2-pyrone tautomer, and its phloroglucinyl homodimer 6 in four major forms. Conversely, the monomethyl derivative 4 of arzanol and its dipyrone homodimer 5 exist in essentially a single solution form. Interestingly, this conformational bias was associated with a marked decrease of bioactivity.