Targeting RNAs with Tobramycin Analogues

Enantioselective Total Synthesis of Fortimicin B

Fortimicins, featuring a pseudodisaccharide scaffold, are an unusual class of aminoglycosides (AGs) with potent efficacy against several aminoglycoside-resistant bacterial strains. Notably, these molecules also exhibit lower inherent ototoxicity and nephrotoxicity than common aminoglycosides. Consequently, fortimicins are a promising type of protoypical molecules for the development of the next generation of aminoglycoside antibiotics. Here, we report the asymmetric total synthesis of fortimicin B in 12 steps (longest linear sequence, LLS) from readily available starting materials. An enantioselective Cu(II)-catalyzed inverse-electron-demand Diels-Alder (IEDDA) reaction of 2-pyrones and N-substituted 2-oxazolones was developed for the efficient synthesis of the fortamine fragment, which previously required a lengthy multistep synthesis owing to its complex stereochemistry. The 6-epi-purpurosamine B fragment was efficiently synthesized through a Cr(II)/Co(I)-mediated C─C bond coupling between aldehydes and alkyl halides. Within these two fragments, the stereoselective construction of the α-glycosidic bond of fortimicin B was realized via the gold(I)-catalyzed glycosylation. Overall, this study provides an efficient synthetic platform for future investigations into the structure-activity relationships of fortimicins.

Tobramycin Variants with Enhanced Ribosome-Targeting Activity

With the increased evolution of aminoglycoside (AG)-resistant bacterial strains, the need to develop AGs with 1) enhanced antimicrobial activity, 2) the ability to evade resistance mechanisms, and 3) the capability of targeting the ribosome with higher efficiency is more and more pressing. The chemical derivatization of the naturally occurring tobramycin (TOB) by attachment of 37 different thioether groups at the 6''-position led to the identification of generally poorer substrates of TOB-targeting AG-modifying enzymes (AMEs). Thirteen of these displayed better antibacterial activity than the parent TOB while retaining ribosome-targeting specificity. Analysis of these compounds in vitro shed light on the mechanism by which they act and revealed three with clearly enhanced ribosome-targeting activity.

Synthetic and structural studies on macrocyclic amino cyclitols--conformational chameleons

Starting from quinic acid the synthesis of 1,4-butanediol-linked macrocyclic aminocyclitols 30, 32, 34, 36 and 38 is described. Assembly was achieved by olefin cross-metathesis of appropriate cyclohexyl allyl ethers followed by ring-closing metathesis of bis-O-allyl homodimers. In all five cases studied, the only products that were formed were those resulting from direct ring-closing metathesis; the formation of larger rings was not detected. These macrocycles exhibited diverse conformational behaviour which included formation of stable separable conformers 31a and 31b as well as conformationally dynamic macrocycles 35 in which a ring flip in one cyclohexane chair conformer induces a ring flip of the other cyclohexane ring through the linking chains of the macrocycles. The activation energy for the inversion of the chair conformation in this process was determined to be about 38 kJ mol(-1), which is about 7 kJ mol(-1) lower than the activation energy for the ring flip of the unsubstituted cyclohexane ring. In all cases, the conformational studies strongly suggest that intramolecular H-bonding between 1,3-diaxially oriented amido and alcohol or ether groups exerts a decisive contribution to the overall stabilisation of the preferred cyclohexane chair conformation.

Differential selectivity of natural and synthetic aminoglycosides towards the eukaryotic and prokaryotic decoding A sites

The lack of absolute prokaryotic selectivity of natural antibiotics is widespread and is a significant clinical problem. The use of this disadvantage of aminoglycoside antibiotics for the possible treatment of human genetic diseases is extremely challenging. Here, we have used a combination of biochemical and structural analysis to compare and contrast the molecular mechanisms of action and the structure-activity relationships of a new synthetic aminoglycoside, NB33, and a structurally similar natural aminoglycoside apramycin. The data presented herein demonstrate the general molecular principles that determine the decreased selectivity of apramycin for the prokaryotic decoding site, and the increased selectivity of NB33 for the eukaryotic decoding site. These results are therefore extremely beneficial for further research on both the design of new aminoglycoside-based antibiotics with diminished deleterious effects on humans, as well as the design of new aminoglycoside-based structures that selectively target the eukaryotic ribosome.

Synthetic access to spacer-linked 3,6-diamino-2,3,6-trideoxy-α-d-glucopyranosides—potential aminoglycoside mimics for the inhibition of the HIV-1 TAR-RNA/Tat-peptide complex

The synthesis of spacer-linked neoaminoglycoside 5 is described. Key steps of the synthesis are the introduction of nitrogen functionalities at C-3 and C-6 and the olefin cross metathesis of allyl glycoside 16. Although it is known that Grubbs catalysts tolerate nitrogen functionalities, difficulties were encountered in the cross metathesis reaction. Factors that govern this dimerization are the steric and electronic demands of the catalyst and the substrate. Preliminary biological evaluation of homodimer 5, by studying the inhibition of HIV-1 TAR-RNA/Tat-peptide complex using a method based on fluorescence titration, revealed an inhibitory effect of 5.