An Expedient Asymmetric Synthesis of Platencin

Discovery and syntheses of "superbug challengers"-platensimycin and platencin

Bacteria have developed resistance to almost all existing antibiotics known today and this has been a major issue over the last few decades. The search for a new class of antibiotics with a new mode of action to fight these multiply-drug-resistant strains, or "superbugs", allowed a team of scientists at Merck to discover two novel antibiotics, platensimycin and platencin using advanced screening strategies, as inhibitors of bacterial fatty acid biosynthesis, which is essential for the survival of bacteria. Though both these antibiotics are structurally related, they work by slightly different mechanisms and target different enzymes conserved in the bacterial fatty acid biosynthesis. This Focus Review summarizes the synthetic and biological aspects of these natural products and their analogues and congeners.

Synthesis and antibacterial properties of (-)-nor-platencin

An asymmetric Diels-Alder reaction between acrolein and 1-benzyloxymethyl-1,3-cyclohexadiene affords a bicyclic aldehyde that was elaborated in 11 steps to nor-platencin. nor-Platencin is 4-16 times less active than platencin against several resistant strains of Staphylococcus aureus, macrolide-resistant Enterococcus faecalis, and vancomycin-resistant Enterococcus faecium.

A synthesis of the carbon skeleton of maoecrystal V

An enantioselective synthesis of the maoecrystal V (1) carbon skeleton is described. The key transformations include arylation of a 1,3-dicarbonyl compound with a triarylbismuth(V) dichloride species, oxidative dearomatization of a phenol, and a subsequent intramolecular Diels-Alder reaction.

Total syntheses of (+/-)-platencin and (-)-platencin

The secondary metabolites platensimycin and platencin, isolated from the bacterial strain Streptomyces platensis, represent a novel class of natural products exhibiting unique and potent antibacterial activity. Platencin, though structurally similar to platensimycin, has been found to operate through a slightly different mechanism of action involving the dual inhibition of lipid elongation enzymes FabF and FabH. Both natural products exhibit strong, broad-spectrum, gram-positive antibacterial activity to key antibiotic resistant strains, including methicillin-resistant Staphylococcus aureus, vancomycin-intermediate S. aureus, and vancomycin-resistant Enterococcus faecium. Described herein are our synthetic efforts toward platencin, culminating in both racemic and asymmetric preparation of the natural product. The syntheses demonstrate the power of the cobalt-catalyzed asymmetric Diels-Alder reaction and the one-pot reductive rearrangement of [3.2.1] bicyclic ketones to [2.2.2] bicyclic olefins.

Efficient synthesis and subsequent transformations of phenylsulfanylbicyclo[2.2.2]octenones and phenylselenylbicyclo[2.2.2]octenones

Inverse-electron-demand Diels-Alder reactions of masked o-benzoquinones 2 with phenyl vinyl sulfide and phenyl vinyl selenide furnished highly functionalized bicyclo[2.2.2]octenone derivatives 3 and 4, respectively, in excellent regio- and stereoselectivities and yields up to 90%. The bicyclo[2.2.2]octenone derivatives 3 with the sulfur functionality were subjected to an oxidation-elimination process to furnish bicyclo[2.2.2]octadienone systems 7 in good yields. During the reduction process, the Diels-Alder adducts 3e and 4e led to 8, whereas the carbon-centered radicals generated from the other adducts 3a-d and 4a-d provided various rearranged products 9-13 depending on the substitution pattern and reagents utilized (Raney-Ni or n-Bu(3)SnH). Surprisingly these radicals showed preference for the carbonyl functionality to the olefinic double bond, leading to interesting rearrangement reactions of mechanistic importance and possible synthetic utility. Interestingly the alcohols obtained from the reduction of Diels-Alder adducts 3a-d underwent desulfurization smoothly to give desulfurized products in high yields; thus a detoured method of "reduction-desulfurization-oxidation" provides an entry to desulfurized bicyclo[2.2.2]octenones without rearrangement.

A symmetry-based concise formal synthesis of platencin, a novel lead against "superbugs"

Quick access: A concise and efficient formal synthesis of platencin has been accomplished in nine steps from a commercially available starting material. The synthesis utilized only one protecting group. The base-catalyzed Michael cyclization of precursor 1 afforded the key diketone 2, which was converted into the desired core structure 4 via the radical intermediate 3.