Algorithm-aided engineering of aliphatic halogenase WelO5* for the asymmetric late-stage functionalization of soraphens.

Late-stage functionalization of natural products offers an elegant route to create novel entities in a relevant biological target space. In this context, enzymes capable of halogenating sp3 carbons with high stereo- and regiocontrol under benign conditions have attracted particular attention. Enabled by a combination of smart library design and machine learning, we engineer the iron/α-ketoglutarate dependent halogenase WelO5* for the late-stage functionalization of the complex and chemically difficult to derivatize macrolides soraphen A and C, potent anti-fungal agents. While the wild type enzyme WelO5* does not accept the macrolide substrates, our engineering strategy leads to active halogenase variants and improves upon their apparent kcat and total turnover number by more than 90-fold and 300-fold, respectively. Notably, our machine-learning guided engineering approach is capable of predicting more active variants and allows us to switch the regio-selectivity of the halogenases facilitating the targeted analysis of the derivatized macrolides' structure-function activity in biological assays.

Chiral Atropisomeric Indenocorannulene Bowls: Critique of the Cahn-Ingold-Prelog Conception of Molecular Chirality.

Chiral corannulenes abound, but suffer generally from configurational lability associated with bowl-to-bowl inversion, thus obviating questions of stereogenicity and stereoelement construction. In contrast, peri-annulated corannulenes show greatly increased barriers for bowl-to-bowl inversion; specifically indenocorannulenes invert on a time scale too slow to observe by normal NMR methods and raise the possibility of creating chiral atropisomeric bowl-shaped aromatics. Two methods for preparing indenocorannulene from simple 2-haloarylcorannulenes-silyl cation C-F activation, and Pd-mediated C-Cl activation[5] -enable the synthesis of an array of such chiral atropisomeric indenocorannulenes. Resolution of the enantiomers by high-performance liquid chromatography over chiral support phases motivates the study of chiroptical properties, the assignment of absolute "Cartesian" configuration, and the assessment of configurational stability. These studies bring into question any systematic assignment of nontrivial stereoelements (i.e. not the molecule in its entirety) and refute any assertion of congruence between "Cahn-Ingold-Prelog elements" and the physical or "Cartesian" basis of chirality.

1,2,3- versus 1,2-Indeno Ring Fusions Influence Structure Property and Chirality of Corannulene Bowls.

Annulated corannulenes 3-5 form via distinct synthetic pathways: (i) Pd-catalyzed sp3 CH insertion, (ii) Pd-catalyzed aryl coupling, and (iii) silyl cation-promoted C-F activation/CH insertion. Crystal structure, redox, and photophysical studies elucidate the differing influence of 1,2,3- versus 1,2-indeno ring fusions. Mono and dianions of 3-5 are characterized. Resolution of 4 gives enantiopure forms, allowing assessment of the bowl-inversion barrier.

Pauli Repulsion Versus van der Waals: Interaction of Indenocorannulene with a Cu(111) Surface.

Modification of metal electrode surfaces with functional organic molecules is an important step toward organic electronics. The interaction of the buckybowl indenocorannulene with a Cu(111) surface and the two-dimensional self-assembly on the same surface was studied by means of scanning tunneling microscopy and dispersion-enabled density functional theory. Based on the conjecture of maximizing van der Waals interaction with the surface one would expect the indeno group to be aligned parallel to the surface. Theoretical investigations predict a nonparallel arrangement with the benzo ring of the indeno group located higher above the surface than the bowl rim connected to the indeno group. This adsorbate geometry is due to strong electronic interaction between molecule and surface, including substantial Pauli repulsion. The long-range ordered monolayer shows differences for two molecules of the unit cell in scanning tunneling microscopy contrast, suggesting either different polar alignments, and therefore a different tilt of the indeno group, or occupation of different adsorption sites.

Key analogs of a uniquely potent synthetic vinblastine that contain modifications of the C20' ethyl substituent.

A key series of vinblastine analogs 7-13, which contain modifications to the C20' ethyl group, was prepared with use of two distinct synthetic approaches that provide modifications of the C20' side chain containing linear and cyclized alkyl groups or added functionalized substituents. Their examination revealed the unique nature of the improved properties of the synthetic vinblastine 6, offers insights into the origins of its increased tubulin binding affinity and 10-fold improved cell growth inhibition potency, and served to probe a small hydrophobic pocket anchoring the binding of vinblastine with tubulin. Especially noteworthy were the trends observed with substitution of the terminal carbon of the ethyl group that, with the exception of 9 (R=F vs H, equipotent), led to remarkably substantial reductions in activity (>10-fold): R=F (equipotent with H)>N3, CN (10-fold)>Me (50-fold)>Et (100-fold)>OH (inactive). This is in sharp contrast to the maintained (7) or enhanced activity (6) observed with its incorporation into a cyclic C20'/C15'-fused six-membered ring.

Synthesis of a Potent Vinblastine: Rationally Designed Added Benign Complexity.

Many natural products, including vinblastine, have not been easily subjected to simplifications in their structures by synthetic means or modifications by late-stage semisynthetic derivatization in ways that enhance their biological potency. Herein, we detail a synthetic vinblastine that incorporates added benign complexity (ABC), which improves activity 10-fold, and is now accessible as a result of advances in the total synthesis of the natural product. The compound incorporates designed added molecular complexity but no new functional groups and maintains all existing structural and conformational features of the natural product. It constitutes a member of an analogue class presently inaccessible by semisynthetic derivatization of the natural product, by its late-stage functionalization, or by biosynthetic means. Rather, it was accessed by synthetic means, using an appropriately modified powerful penultimate single-step vindoline-catharanthine coupling strategy that proceeds with a higher diastereoselectivity than found for the natural product itself.

Chiral conflict among different helicenes suppresses formation of one enantiomorph in 2D crystallization.

Diastereomeric interactions in 2D crystals formed at solid surfaces serve as excellent models for understanding molecular recognition in biomineralization. Adsorption of a pentahelicene racemate on a Au(111) surface leads to 2D conglomerate crystallization, i.e., homochiral mirror domains, as observed by scanning tunneling microscopy. Upon mixing 26% of (M)-heptahelicene into the racemate monolayer, only the (M)-pentahelicene enantiomorph is observed. This effect is explained by a preferred heterochiral interaction between the different helicene species, suppressing the formation of the pure (P)-pentahelicene enantiomorph. These results shine new light onto stereoselective molecular recognition mediated by van der Waals forces.

Catechol-O-methyltransferase in complex with substituted 3'-deoxyribose bisubstrate inhibitors.

The biological activity of catechol neurotransmitters such as dopamine in the synapse is modulated by transporters and enzymes. Catechol-O-methyltransferase (COMT; EC 2.1.1.6) inactivates neurotransmitters by catalyzing the transfer of a methyl group from S-adenosylmethionine to catechols in the presence of Mg²âº. This pathway also inactivates L-DOPA, the standard therapeutic for Parkinson's disease. Depletion of catechol neurotransmitters in the prefrontal cortex has been linked to schizophrenia. The inhibition of COMT therefore promises improvements in the treatment of these diseases. The concept of bisubstrate inhibitors for COMT has been described previously. Here, ribose-modified bisubstrate inhibitors were studied. Three high-resolution crystal structures of COMT in complex with novel ribose-modified bisubstrate inhibitors confirmed the predicted binding mode but displayed subtle alterations at the ribose-binding site. The high affinity of the inhibitors can be convincingly rationalized from the structures, which document the possibility of removing and/or replacing the ribose 3'-hydroxyl group and provide a framework for further inhibitor design.

Proton-catalyzed, silane-fueled Friedel-Crafts coupling of fluoroarenes.

The venerable Friedel-Crafts reaction appends alkyl or acyl groups to aromatic rings through alkyl or acyl cation equivalents typically generated by Lewis acids. We show that phenyl cation equivalents, generated from otherwise unreactive aryl fluorides, allow extension of the Friedel-Crafts reaction to intramolecular aryl couplings. The enabling feature of this reaction is the exchange of carbon-fluorine for silicon-fluorine bond enthalpies; the reaction is activated by an intermediate silyl cation. Catalytic quantities of protons or silyl cations paired with weakly coordinating carborane counterions initiate the reactions, after which proton transfer in the final aromatization step regenerates the active silyl cation species by protodesilylation of a quaternary silane. The methodology allows the high-yield formation of a range of tailored polycyclic aromatic hydrocarbons and graphene fragments.