Laura Iannazzo

Palladium-Catalyzed Hiyama Cross-Coupling of Aryltrifluorosilanes with Aryl and Heteroaryl Chlorides

An efficient, palladium-catalyzed Hiyama cross-coupling reaction of aryltrifluorosilanes with aryl chlorides has been developed. A wide variety of functionalized biaryl derivatives were isolated in good to excellent yields. The scope of this reaction has also been extended to heteroaryl chlorides, affording the corresponding heterobiaryl compounds in high yields.

Cobalt‐Mediated Linear 2:1 Co‐oligomerization of Alkynes with Enol Ethers to Give 1‐Alkoxy‐1,3,5‐Trienes: A Missing Mode of Reactivity

A variety of 1,6-heptadiynes and certain borylalkynes co-oligomerize with enol ethers in the presence of [CpCo(C(2)H(4))(2)] (Cp=cyclopentadienyl) to furnish the hitherto elusive acyclic 2:1 products, 1,3,5-trien-1-ol ethers, in preference to or in competition with the alternative pathway that leads to the standard [2+2+2] cycloadducts, 5-alkoxy-1,3-cyclohexadienes. Minor variations, such as lengthening the diyne tether, cause reversion to the standard mechanism. The trienes, including synthetically potent borylated derivatives, are generated with excellent levels of chemo-, regio-, and diastereoselectivity, and are obtained directly by decomplexation of the crude mixtures during chromatography. The cyclohexadienes are isolated as the corresponding dehydroalkoxylated arenes. In one example, even ethene functions as a linear cotrimerization partner. The alkoxytrienes are thermally labile with respect to 6pi-electrocyclization-elimination to give the same arenes that are the products of cycloaddition. The latter, regardless of the mechanism of their formation, can be viewed as the result of a formal [2+2+2] cyclization of the starting alkynes with acetylene. One-pot conditions for the exclusive formation of arenes are developed. DFT computations indicate that cyclohexadiene and triene formation share a common intermediate, a cobaltacycloheptadiene, from which reductive elimination and beta-hydride elimination compete.

Routes of Synthesis of Carbapenems for Optimizing Both the Inactivation of L,D-Transpeptidase LdtMt1 of Mycobacterium tuberculosis and the Stability toward Hydrolysis by β-Lactamase BlaC.

Combinations of β-lactams of the carbapenem class, such as meropenem, with clavulanate, a β-lactamase inhibitor, are being evaluated for the treatment of drug-resistant tuberculosis. However, carbapenems approved for human use have never been optimized for inactivation of the unusual β-lactam targets of Mycobacterium tuberculosis or for escaping to hydrolysis by broad-spectrum β-lactamase BlaC. Here, we report three routes of synthesis for modification of the two side chains carried by the β-lactam and the five-membered rings of the carbapenem core. In particular, we show that the azide-alkyne Huisgen cycloaddition reaction catalyzed by copper(I) is fully compatible with the highly unstable β-lactam ring of carbapenems and that the triazole ring generated by this reaction is well tolerated for inactivation of the L,D-transpeptidase LdtMt1 target. Several of our new carbapenems are superior to meropenem both with respect to the efficiency of in vitro inactivation of LdtMt1 and reduced hydrolysis by BlaC.

Synthesis of 3′-Fluoro-tRNA Analogues for Exploring Non-ribosomal Peptide Synthesis in Bacteria

Aminoacyl‐tRNAs (aa‐tRNAs) participate in a vast repertoire of metabolic pathways, including the synthesis of the peptidoglycan network in the cell walls of bacterial pathogens. Synthesis of aminoacyl‐tRNA analogues is critical for further understanding the mechanisms of these reactions. Here we report the semi‐synthesis of 3′‐fluoro analogues of Ala‐tRNAAla. The presence of fluorine in the 3′‐position blocks Ala at the 2′‐position by preventing spontaneous migration of the residue between positions 2′ and 3′. NMR analyses showed that substitution of the 3′‐hydroxy group by fluorine in the ribo configuration favours the S‐type conformation of the furanose ring of terminal adenosine A76. In contrast, the N‐type conformation is favoured by the presence of fluorine in the xylo configuration. Thus, introduction of fluorine in the ribo and xylo configurations affects the conformation of the furanose ring in reciprocal ways. These compounds should provide insight into substrate recognition by Fem transferases and the Ala‐tRNA synthetases.

Synthesis of Avibactam Derivatives and Activity on β-Lactamases and Peptidoglycan Biosynthesis Enzymes of Mycobacteria

There is a renewed interest for β-lactams for treating infections due to Mycobacterium tuberculosis and M. abscessus because their β-lactamases are inhibited by classical (clavulanate) or new generation (avibactam) inhibitors, respectively. Here, access to an azido derivative of the diazabicyclooctane (DBO) scaffold of avibactam for functionalization by the Huisgen-Sharpless cycloaddition reaction is reported. The amoxicillin-DBO combinations were active, indicating that the triazole ring is compatible with drug penetration (minimal inhibitory concentration of 16 μg mL-1 for both species). Mechanistically, β-lactamase inhibition was not sufficient to account for the potentiation of amoxicillin by DBOs. Thus, the latter compounds were investigated as inhibitors of l,d-transpeptidases (Ldts), which are the main peptidoglycan polymerases in mycobacteria. The DBOs acted as slow-binding inhibitors of Ldts by S-carbamoylation indicating that optimization of DBOs for Ldt inhibition is an attractive strategy to obtain drugs selectively active on mycobacteria.

Critical Impact of Peptidoglycan Precursor Amidation on the Activity of l,d-Transpeptidases from Enterococcus faecium and Mycobacterium tuberculosis

The bacterial cell wall peptidoglycan contains unusual l- and d-amino acids assembled as branched peptides. Insight into the biosynthesis of the polymer has been hampered by limited access to substrates and to suitable polymerization assays. Here we report the full synthesis of the peptide stem of peptidoglycan precursors from two pathogenic bacteria, Enterococcus faecium and Mycobacterium tuberculosis, and the development of a sensitive post-derivatization assay for their cross-linking by l,d-transpeptidases. Access to series of stem peptides showed that amidation of free carboxyl groups is essential for optimal enzyme activity, in particular the amidation of diaminopimelate (DAP) residues for the cross-linking activity of the l,d-transpeptidase LdtMt2 from M. tuberculosis. Accordingly, construction of a conditional mutant established the essential role of AsnB indicating that this DAP amidotransferase is an attractive target for the development of anti-mycobacterial drugs.