Total synthesis of myxovirescin A1

Bacterial Lipoprotein Biosynthetic Pathway as a Potential Target for Structure-based Design of Antibacterial Agents

BACKGROUND

Antibiotic resistance is currently a serious problem for global public health. To this end, discovery of new antibacterial drugs that interact with novel targets is important. The biosynthesis of lipoproteins is vital to bacterial survival and its inhibitors have shown efficacy against a range of bacteria, thus bacterial lipoprotein biosynthetic pathway is a potential target.

METHODS

At first, the literature that covered the basic concept of bacterial lipoprotein biosynthetic pathway as well as biochemical characterization of three key enzymes was reviewed. Then, the recently resolved crystal structures of the three enzymes were retrieved from Protein Data Bank (PDB) and the essential residues in the active sites were analyzed. Lastly, all the available specific inhibitors targeting this pathway and their Structure-activity Relationship (SAR) were discussed.

RESULTS

We briefly introduce the bacterial lipoprotein biosynthetic pathway and describe the structures and functions of three key enzymes in detail. In addition, we present much knowledge on ligand recognition that may facilitate structure-based drug design. Moreover, we focus on the SAR of LspA inhibitors and discuss their potency and drug-likeness.

CONCLUSION

This review presents a clear background of lipoprotein biosynthetic pathway and provides practical clues for structure-based drug design. In particular, the most up-to-date knowledge on the SAR of lead compounds targeting this pathway would be a good reference for discovery of a novel class of antibacterial agents.

Total Synthesis of Myxovirescin A1

A convergent total synthesis of the antibiotic macrolide myxovirescin A1 (1) is described that is largely based on reagent- and catalyst-controlled transformations. This includes a highly regioselective Negishi reaction of dibromo-alkene 48 with an alkynylzinc reagent, and a palladium catalyzed alkyl-Suzuki coupling of the resulting enyne derivative 12 with the 9-BBN-adduct derived from alkene 61. The latter was obtained via an asymmetric hydrogenation of the chlorinated beta-ketoester 49 and an anti-selective oxyallylation of the functionalized aldehyde 53 as the key steps. The preparation of the bis-borylated allyl-donor 57 used in the oxyallylation step, however, required careful optimization and led to important insights into the nature of the classical hydroborating agent "di(isopinocampheyl)borane (Ipc2BH)". It was unambiguously shown by X-ray crystallography that in the solid state this compound is dimeric, but it is prone to undergo an essentially quantitative mono-deborylation when dissolved in CH2Cl2 or benzene; its composition in ethereal solvents is even more complex as evident from 11B NMR data. Product 71 derived from 12 and 61 was elaborated into the enyne-yne derivative 75, which served as the substrate for an exquisitely selective ring closing alkyne metathesis reaction (RCAM) catalyzed by the molybdenum tris-amido complex 20 activated in situ with CH2Cl2. The resulting cyclic enyne 76 was subjected to a ruthenium catalyzed trans-hydrosilylation/proto-desilylation tandem. Although [Cp*Ru(MeCN)3]PF6 had previously been recommended as catalyst of choice for trans-hydrosilylation reactions of internal alkynes, this complex failed to afford the desired product, whereas its sterically less hindered congener [CpRu(MeCN)3]PF6 permitted the reaction to be performed in appreciable yield, but at the expense of a lower stereoselectivity. AgF-mediated proto-desilylation of the isomeric silanes 79 and 80 followed by cleavage of the remaining acetal protecting groups afforded myxovirescin A1 and its hitherto unknown 14Z-isomer 81, respectively.

Efforts toward the Total Synthesis of (−)-Kendomycin

[structure: see text] The synthesis of an advanced component leading to (-)-kendomycin is described. The synthetic scheme features the application of asymmetric conjugate addition methodology for the early generation of the C13-C14 (E)-trisubstituted olefin, providing an efficient assembly of the ansa chain. Condensation reactions probe two strategies for attachment of the aromatic system.

Asymmetric Conjugate Addition for the Preparation of syn-1,3-Dimethyl Arrays: Synthesis and Structure Elucidation of Capensifuranone

The synthesis of capensifuranone (1) has been achieved by the application of developments for asymmetric conjugate addition reactions of organocopper reagents with nonracemic N-enoyl-4-phenyl-1,3-oxazolidinones for the preparation of 1,3-syn-dimethyl arrays. The assignment of relative and absolute stereochemistry of 1 has been made following the high-field NMR characterizations of synthetic diol derivatives. The previously unassigned C4 stereochemistry of 1 was determined to be of the (S)-configuration. The thermodynamic equilibration of capensifuranone and its C4 diastereomer has been examined.

Myxovirescin analogues via macrocyclic ring-closing metathesis

A short, efficient route has been developed to analogues of myxovirescin using ring-closing metathesis whereby the antibacterial activity has been retained.

Construction of 4-hydroxy-2-pyridinones. Total synthesis of (+)-sambutoxin

Total synthesis of (+)-sambutoxin has been achieved, establishing the relative and absolute stereochemistry of the naturally occurring mycotoxin. Efforts feature methodology for enantiocontrolled construction of 1,3-anti-dimethyl arrays and a novel Saegusa oxidation to provide a pyridinone methide leading to formation of the central dihydropyran ring.