Martin D. Burke.

Amphotericin B (AmB) is the archetype for small molecules that form ion channels in living systems and has recently been shown to replace a missing protein ion transporter and thereby restore physiology in yeast. Molecular modeling studies predict that AmB self-assembles in lipid membranes with the polyol region lining a channel interior that funnels to its narrowest region at the C3-hydroxyl group. This model predicts that modification of this functional group would alter conductance of the AmB ion channel. To test this hypothesis, the C3-hydroxyl group was synthetically deleted, and the resulting derivative, C3deoxyAmB (C3deOAmB), was characterized using multidimensional NMR experiments and single ion channel electrophysiology recordings. C3deOAmB possesses the same macrocycle conformation as AmB and retains the capacity to form transmembrane ion channels, yet the conductance of the C3deOAmB channels is 3-fold lower than that of AmB channels. Thus, the C3-hydroxyl group plays an important role in AmB ion channel conductance, and synthetic modifications at this position may provide an opportunity for further tuning of channel functions.

Deficiencies of protein ion channels underlie many currently incurable human diseases. Robust networks of pumps and channels are usually responsible for the directional movement of specific ions in organisms ranging from microbes to humans. We thus questioned whether minimally selective small molecule mimics of missing protein channels might be capable of collaborating with the corresponding protein ion pumps to restore physiology. Here we report vigorous and sustainable restoration of yeast cell growth by replacing missing protein ion transporters with imperfect small molecule mimics. We further provide evidence that this tolerance for imperfect mimicry is attributable to collaboration between the channel-forming small molecule and protein ion pumps. These results illuminate a mechanistic framework for pursuing small molecule replacements for deficient protein ion channels that underlie a range of challenging human diseases.

Amphotericin B (AmB) is a clinically vital antimycotic but is limited by its severe toxicity. Binding ergosterol, independent of channel formation, is the primary mechanism by which AmB kills yeast, and binding cholesterol may primarily account for toxicity to human cells. The leading structural model predicts that the C2′ hydroxyl group on the mycosamine appendage is critical for binding both sterols. To test this, the C2′-OH was synthetically deleted, and the sterol binding capacity of the resulting derivative, C2′deOAmB, was directly characterized via isothermal titration calorimetry. Surprisingly, C2′deOAmB binds ergosterol and, within the limits of detection of this experiment, does not bind cholesterol. Moreover, C2′deOAmB is nearly equipotent to AmB against yeast but, within the limits of detection of our assays, is nontoxic to human cells in vitro. Thus, the leading structural model for AmB/sterol binding interactions is incorrect, and C2′deOAmB is an exceptionally promising new antifungal agent.

Problem solved: An air‐stable 2‐pyridyl borane that can effectively couple to a wide range of aryl and heteroaryl halides and pseudohalides has evaded the synthesis community for decades. The discovery that Cu(DEA)₂ powerfully enables palladium‐mediated cross‐couplings with air‐stable boronates 1 has finally provided a general solution to this problem. DEA=diethanolamine, DMF=N,N′‐dimethylformamide, Tf=trifluoromethanesulfonyl.

Amphotericin B (AmB) is a prototypical small molecule natural product that can form ion channels in living eukaryotic cells and has remained refractory to microbial resistance despite extensive clinical utilization in the treatment of life-threatening fungal infections for more than half a century. It is now widely accepted that AmB kills yeast primarily via channel-mediated membrane permeabilization. Enabled by the iterative cross-coupling-based synthesis of a functional group deficient derivative of this natural product, we have discovered that channel formation is not required for potent fungicidal activity. Alternatively, AmB primarily kills yeast by simply binding ergosterol, a lipid that is vital for many aspects of yeast cell physiology. Membrane permeabilization via channel formation represents a second complementary mechanism that further increases drug potency and the rate of yeast killing. Collectively, these findings ( i ) reveal that the binding of a physiologically important microbial lipid is a powerful and clinically validated antimicrobial strategy that may be inherently refractory to resistance, ( ii ) illuminate a more straightforward path to an improved therapeutic index for this clinically vital but also highly toxic antifungal agent, and ( iii ) suggest that the capacity for AmB to form protein-like ion channels might be separable from its cytocidal effects.

Efficient access to chiral C(sp³) boronates in stereochemically pure form is critical for realizing the substantial potential of such building blocks in complex-molecule synthesis. We herein report that a pinene-derived iminodiacetic acid (PIDA) ligand enables the highly diastereoselective synthesis of a wide range of oxiranyl C(sp³) boronates from the corresponding olefins. These oxiranyl PIDA boronates, in turn, can be readily transformed into unprecedented stable α-boryl aldehydes via a novel 1,2-migration of the boronate group that proceeds with complete maintenance of stereochemical purity. B-Protected haloboronic acids containing dual sp³-hybridized C centers are readily accessible via this platform, and the herein demonstrated capacity for stereocontrolled iterative C(sp³) cross-coupling with this novel type of bifunctional reagent to access a medicinally important chiral small-molecule target in highly enantioenriched form represents a substantial advance for the building-block-based approach to synthesis.