BE-43472B

An-T-biotic: The first total synthesis of the T-shaped bisanthraquinone natural product BE-43472B was accomplished and its absolute configuration assigned. Key transformations in the pivotal cascade sequence include a Diels-Alder reaction, a hemiketal formation, and a nucleophilic aromatic ipso substitution.

The bisanthraquinone antibiotic BE-43472B [(+)-1] was isolated by Rowley and co-workers from a streptomycete strain found in a blue-green algae associated with the ascidian Ecteinascidia turbinata and has shown promising antibacterial activity against clinically derived isolates of methicillin-susceptible, methicillin-resistant, and tetracyclin-resistant Staphylococcus aureus (MSSA, MRSA, and TRSA, respectively) and vancomycin-resistant Enterococcus faecalis (VRE). Described herein is the first total synthesis of both enantiomers of this bisanthraquinone antibiotic, the determination of its absolute configuration, and the biological evaluation of these and related compounds. The developed synthesis relies on a highly efficient cascade sequence involving an intermolecular Diels-Alder reaction between diene (R)-61 and dienophile 55, followed by an intramolecular nucleophilic aromatic ipso substitution. Late-stage transformations included a remarkable photochemical alpha,beta-epoxyketone rearrangement [80 --> (+)-1]. Interestingly, the unnatural enantiomer [(-)-1] of antibiotic BE-43472B exhibited antibacterial properties comparable to those of the natural enantiomer [(+)-1].

The regioselectivities of several Diels-Alder reactions utilized en route to bisanthraquinone antibiotic BE-43472B are examined using density functional theory calculations. These reactions involve highly substituted dienes and juglone dienophiles, and there is an opposite regiochemical outcome for Diels-Alder reactions with beta-aryl substituted juglones when compared to reactions of unsubstituted juglone. In this article, the effect of an aromatic conjugating group bonded to juglone is explored.