Towards the preparation of 2″-deoxy-2″-fluoro-adenophostin A. Study of the glycosylation reaction

Towards the synthesis of a 2-deoxy-2-fluoro-d-mannose building block and characterisation of an unusual 2-S-phenyl anomeric pyridinium triflate salt via 1 → 2 S-migration

Regio- and stereo-selective synthetic routes to 2-deoxy-2-fluoro-d-mannose building blocks are often experimentally challenging when using Selectfluor with the corresponding glycal. We targeted a late-stage method to introduce fluorine in a stereospecific manner using inversion via a triflate. Accordingly, synthesis of a conventionally protected 2-deoxy-2-fluoro-d-mannose β-thioglycoside donor, directly applicable to oligosaccharide synthesis, was attempted using C2-triflate inversion of the corresponding d-glucoside with TBAF. Unexpectedly, an anomeric pyridinium salt was isolated when attempting to form the C2-triflate using Tf2O in pyridine. Indicatively, this proceeds via a 1 → 2 S-migration delivering a 1,2-trans product with α-d-manno configuration and the anomeric pyridinium in a pseudo-equatorial position. The structure of this unexpected intermediate was confirmed in the solid-state using X-ray crystallography. Omission of the pyridine solvent led to dimer formation. Switching the aglycone to an O-para-methoxyphenyl enabled smooth C2 inversion to the desired 2-deoxy-2-fluoro d-mannose system, suitably equipped for further anomeric manipulation.

Orthogonal Active-Site Labels for Mixed-Linkage endo-β-Glucanases

Small molecule irreversible inhibitors are valuable tools for determining catalytically important active-site residues and revealing key details of the specificity, structure, and function of glycoside hydrolases (GHs). β-glucans that contain backbone β(1,3) linkages are widespread in nature, e.g., mixed-linkage β(1,3)/β(1,4)-glucans in the cell walls of higher plants and β(1,3)glucans in yeasts and algae. Commensurate with this ubiquity, a large diversity of mixed-linkage endoglucanases (MLGases, EC 3.2.1.73) and endo-β(1,3)-glucanases (laminarinases, EC 3.2.1.39 and EC 3.2.1.6) have evolved to specifically hydrolyze these polysaccharides, respectively, in environmental niches including the human gut. To facilitate biochemical and structural analysis of these GHs, with a focus on MLGases, we present here the facile chemo-enzymatic synthesis of a library of active-site-directed enzyme inhibitors based on mixed-linkage oligosaccharide scaffolds and N-bromoacetylglycosylamine or 2-fluoro-2-deoxyglycoside warheads. The effectiveness and irreversibility of these inhibitors were tested with exemplar MLGases and an endo-β(1,3)-glucanase. Notably, determination of inhibitor-bound crystal structures of a human-gut microbial MLGase from Glycoside Hydrolase Family 16 revealed the orthogonal labeling of the nucleophile and catalytic acid/base residues with homologous 2-fluoro-2-deoxyglycoside and N-bromoacetylglycosylamine inhibitors, respectively. We anticipate that the selectivity of these inhibitors will continue to enable the structural and mechanistic analyses of β-glucanases from diverse sources and protein families.

Synthesis of substrate analogues as potential inhibitors for Mycobacterium tuberculosis enzyme MshC

Mycothiol cysteine ligase (MshC) is a key enzyme in the mycothiol (MSH) biosynthesis and a promising target for developing new anti-mycobacterial compounds. Herein, we report on the synthesis of substrate analogues, as potential inhibitors, for the MshC enzyme. The target molecules were synthesized employing a Schmidt glycosylation strategy using an enantiomerically pure inositol acceptor and 2-deoxy trichloroacetimidate glycosyl donors with glycosylation yields greater than 70% and overall yields >5%. The inositol acceptor was obtained via chiral resolution of (±)-myo-inositol.

Investigation and conformational analysis of fluorinated nucleoside antibiotics targeting siderophore biosynthesis

Antibiotic resistance represents one of the greatest threats to public health. The adenylation inhibitor 5'-O-[N-(salicyl)sulfamoyl]adenosine (SAL-AMS) is the archetype for a new class of nucleoside antibiotics that target iron acquisition in pathogenic microorganisms and is especially effective against Mycobacterium tuberculosis, the causative agent of tuberculosis. Strategic incorporation of fluorine at the 2' and 3' positions of the nucleoside was performed by direct fluorination to enhance activity and improve drug disposition properties. The resulting SAL-AMS analogues were comprehensively assessed for biochemical potency, whole-cell antitubercular activity, and in vivo pharmacokinetic parameters. Conformational analysis suggested a strong preference of fluorinated sugar rings for either a 2'-endo, 3'-exo (South), or a 3'-endo,2'-exo (North) conformation. The structure-activity relationships revealed a strong conformational bias for the C3'-endo conformation to maintain potent biochemical and whole-cell activity, whereas improved pharmacokinetic properties were associated with the C2'-endo conformation.

Structure-guided design of fluorescent S-adenosylmethionine analogs for a high-throughput screen to target SAM-I riboswitch RNAs

Many classes of S-adenosylmethionine (SAM)-binding RNAs and proteins are of interest as potential drug targets in diverse therapeutic areas, from infectious diseases to cancer. In the former case, the SAM-I riboswitch is an attractive target because this structured RNA element is found only in bacterial mRNAs and regulates multiple genes in several human pathogens. Here, we describe the synthesis of stable and fluorescent analogs of SAM in which the fluorophore is introduced through a functionalizable linker to the ribose. A Cy5-labeled SAM analog was shown to bind several SAM-I riboswitches via in-line probing and fluorescence polarization assays, including one from Staphylococcus aureus that controls the expression of SAM synthetase in this organism. A fluorescent ligand displacement assay was developed and validated for high-throughput screening of compounds to target the SAM-I riboswitch class.