Tetrachlorovancomycin: Total Synthesis of a Designed Glycopeptide Antibiotic of Reduced Synthetic Complexity.

A technically straightforward total synthesis of a new class of vancomycin analogues of reduced synthetic complexity was developed that provided tetrachlorovancomycin (1, LLS = 15 steps, 15% overall yield) and its precursor aglycon 29 (nearly 20% overall yield). The class retains all the intricate vancomycin structural features that contribute to its target binding affinity and selectivity, maintains the antimicrobial activity of vancomycin, and achieves the simplification by an unusual addition, not removal, of benign substituents to the core structure. The modification, accomplished by addition of two aryl chloride substituents to provide 1, permitted a streamlined total synthesis of the new glycopeptide antibiotic class by removing the challenges associated with CD and DE ring system atropisomer stereochemical control. This also enabled their simultaneous and further-activated SNAr macrocyclizations that establish the tricyclic skeleton of 1. Key elements of the approach include catalyst-controlled diastereoselective formation of the AB biaryl axis of chirality (>30:1 dr), an essentially instantaneous macrolactamization of the AB ring system free of competitive epimerization (>30:1 dr), racemization free coupling of the E ring tetrapeptide, room temperature simultaneous CD and DE ring system cyclizations, a highly refined 4-step conversion of the cyclization product to the aglycon, and a protecting-group-free one-pot enzymatic glycosylation for disaccharide introduction. In addition to the antimicrobial evaluation of tetrachlorovancomycin (1), the preparation of key peripherally modified derivatives, which introduce independent and synergistic mechanisms of action, revealed their exceptional antimicrobial potency and provide the foundation for future use of this new class of synthetic glycopeptide analogues.

Divergent Total Synthesis and Characterization of Maxamycins.

A new streamlined and scaled divergent total synthesis of pocket-modified vancomycin analogs is detailed that provides a common late-stage intermediate [Ψ[C(═S)NH]Tpg4]vancomycin (LLS = 18 steps, 12% overall yield, >5 g prepared) to access both existing and future pocket modifications. Highlights of the approach include an atroposelective synthesis of [Ψ[C(═S)NH]Tpg4]vancomycin aglycon (11), a one-pot enzymatic glycosylation for direct conversion to [Ψ[C(═S)NH]Tpg4]vancomycin (12), and new powerful methods for the late-stage conversion of the embedded thioamide to amidine/aminomethylene pocket modifications. Incorporation of two peripheral modifications provides a scalable total synthesis of the maxamycins, all prepared from aglycon 11 without use of protecting groups. Thus, both existing and presently unexplored pocket-modified analogues paired with a range of peripheral modifications are accessible from this common thioamide intermediate. In addition to providing an improved synthesis of the initial member of the maxamycins, this is illustrated herein with the first synthesis and examination of maxamycins that contain the most effective of the pocket modifications (amidine) described to date combined with two additional peripheral modifications. These new amidine-based maxamycins proved to be potent, durable, and efficacious antimicrobial agents that display equipotent activity against vancomycin-sensitive and vancomycin-resistant Gram-positive organisms and act by three independent synergistic mechanisms of action. In the first such study conducted to date, one new maxamycin (21, MX-4) exhibited efficacious in vivo activity against a feared and especially challenging multidrug-resistant (MRSA) and vancomycin-resistant (VRSA) S. aureus bacterial strain (VanA VRS-2) for which vancomycin is inactive.

Discovery of small-molecule enzyme activators by activity-based protein profiling.

Activity-based protein profiling (ABPP) has been used extensively to discover and optimize selective inhibitors of enzymes. Here, we show that ABPP can also be implemented to identify the converse-small-molecule enzyme activators. Using a kinetically controlled, fluorescence polarization-ABPP assay, we identify compounds that stimulate the activity of LYPLAL1-a poorly characterized serine hydrolase with complex genetic links to human metabolic traits. We apply ABPP-guided medicinal chemistry to advance a lead into a selective LYPLAL1 activator suitable for use in vivo. Structural simulations coupled to mutational, biochemical and biophysical analyses indicate that this compound increases LYPLAL1's catalytic activity likely by enhancing the efficiency of the catalytic triad charge-relay system. Treatment with this LYPLAL1 activator confers beneficial effects in a mouse model of diet-induced obesity. These findings reveal a new mode of pharmacological regulation for this large enzyme family and suggest that ABPP may aid discovery of activators for additional enzyme classes.

N-Acyl pyrazoles: Effective and tunable inhibitors of serine hydrolases.

A series of N-acyl pyrazoles was examined as candidate serine hydrolase inhibitors in which the active site acylating reactivity and the leaving group ability of the pyrazole could be tuned not only through the nature of the acyl group (reactivity: amide > carbamate > urea), but also through pyrazole C4 substitution with electron-withdrawing or electron-donating substituents. Their impact on enzyme inhibitory activity displayed pronounced effects with the activity improving substantially as one alters both the nature of the reacting carbonyl group (urea > carbamate > amide) and the pyrazole C4 substituent (CN > H > Me). It was further demonstrated that the acyl chain of the N-acyl pyrazole ureas can be used to tailor the potency and selectivity of the inhibitor class to a targeted serine hydrolase. Thus, elaboration of the acyl chain of pyrazole-based ureas provided remarkably potent, irreversible inhibitors of fatty acid amide hydrolase (FAAH, apparent Ki = 100-200 pM), dual inhibitors of FAAH and monoacylglycerol hydrolase (MGLL), or selective inhibitors of MGLL (IC50 = 10-20 nM) while simultaneously minimizing off-target activity (e.g., ABHD6 and KIAA1363).

Computational and NMR Spectroscopic Evidence for Stereochemistry-Dependent Conformations of 2,2,6,6-Tetramethylpiperidinyl-Masked 1,2-Diols.

2,2,6,6-Tetramethylpiperidinyl-masked 1,2-diols exhibited stereochemistry-dependent hydroxyl proton chemical shifts: ca. 7 ppm for the syn diastereomer and ca. 2 ppm for the anti diastereomer. A computational search for low energy geometries revealed that the syn isomer favors a six-membered ring hydrogen bond to nitrogen and the anti isomer favors a five-membered ring hydrogen bond to oxygen. The computed low energy conformations were found to have a large difference in hydroxyl proton shielding that was reflected in the experimental chemical shift difference. This chemical shift difference was observed in a broad range of solvents, and thus may be useful as a stereochemical probe. The stereochemistry-dependent conformation and chemical shift signature appeared to be due to a syn pentane interaction between the gem-dimethyl groups on the 2,2,6,6-tetramethylpiperidinyl moiety.

anti-Diols from α-oxyaldehydes: synthesis and stereochemical assignment of oxylipins from Dracontium loretense.

Differentially protected 1,2-diols were synthesized by enantioselective aldehyde α-oxygenation followed by organomagnesium or -lithium addition. Contrary to a previous report, the resultant diols possess an anti configuration. Good selectivity was achieved regardless of the hybridization state of the nucleophile or the presence or absence of branching. This method was applied to short syntheses of all possible stereoisomers of two oxylipins from Dracontium loretense with incomplete stereochemical assignments. Spectroscopic comparisons between the synthetic and natural oxylipins led to unambiguous assignments.