Total synthesis and isolation of citrinalin and cyclopiamine congeners.

Many natural products that contain basic nitrogen atoms--for example alkaloids like morphine and quinine-have the potential to treat a broad range of human diseases. However, the presence of a nitrogen atom in a target molecule can complicate its chemical synthesis because of the basicity of nitrogen atoms and their susceptibility to oxidation. Obtaining such compounds by chemical synthesis can be further complicated by the presence of multiple nitrogen atoms, but it can be done by the selective introduction and removal of functional groups that mitigate basicity. Here we use such a strategy to complete the chemical syntheses of citrinalin B and cyclopiamine B. The chemical connections that have been realized as a result of these syntheses, in addition to the isolation of both 17-hydroxycitrinalin B and citrinalin C (which contains a bicyclo[2.2.2]diazaoctane structural unit) through carbon-13 feeding studies, support the existence of a common bicyclo[2.2.2]diazaoctane-containing biogenetic precursor to these compounds, as has been proposed previously.

Nonclassical ammonium ions as intermediates in cinchona alkaloid rearrangements?

The classical/nonclassical nature of cationic ammonium intermediates proposed to be involved in cinchona alkaloid solvolysis and related reactions is investigated. While these intermediates are found to possess highly distorted geometries in which the central nitrogen atom and three of the attached groups are essentially coplanar, we do not find evidence of a nonclassical bonding array. Instead, we find evidence that the intermediate resembles a classical, albeit strained, aziridinium structure, which is still able to account for experimental observations.

Toward Structural Correctness: Aquatolide and the Importance of 1D Proton NMR FID Archiving.

The revision of the structure of the sesquiterpene aquatolide from a bicyclo[2.2.0]hexane to a bicyclo[2.1.1]hexane structure using compelling NMR data, X-ray crystallography, and the recent confirmation via full synthesis exemplify that the achievement of "structural correctness" depends on the completeness of the experimental evidence. Archived FIDs and newly acquired aquatolide spectra demonstrate that archiving and rigorous interpretation of 1D (1)H NMR data may enhance the reproducibility of (bio)chemical research and curb the growing trend of structural misassignments. Despite being the most accessible NMR experiment, 1D (1)H spectra encode a wealth of information about bonds and molecular geometry that may be fully mined by (1)H iterative full spin analysis (HiFSA). Fully characterized 1D (1)H spectra are unideterminant for a given structure. The corresponding FIDs may be readily submitted with publications and collected in databases. Proton NMR spectra are indispensable for structural characterization even in conjunction with 2D data. Quantum interaction and linkage tables (QuILTs) are introduced for a more intuitive visualization of 1D J-coupling relationships, NOESY correlations, and heteronuclear experiments. Overall, this study represents a significant contribution to best practices in NMR-based structural analysis and dereplication.

Experimental and Theoretical Investigations of the Bromination of Phenols with ß and γ Aliphatic Substituents, including Rings.

Bromination reactions of substituted and ring fused phenols were studied by both experiment (t-BuNH-Br) and computation (density functional theory). The outcomes support each other, indicating a clear and predictable regioselective preference among 3,4-bis-alkylated and 3,4-ring-fused phenols.

Pentalenene formation mechanisms redux.

Quantum chemical calculations are used to assess various means of lowering the barrier for the dyotropic rearrangement previously proposed to occur during the carbocation rearrangement process promoted by pentalenene synthase. Several means of lowering this barrier, including a stepwise pathway for dyotropic rearrangement, are uncovered.

Identification and optimization of short helical peptides with novel reactive functionality as catalysts for acyl transfer by reactive tagging.

Herein we describe the screening and subsequent optimization of peptide catalysts for ester activation. A combinatorial methodology using dye-tagged substrate analogs is described for determining which components of a His-containing helical library display acyl transfer activity. We found that helical peptides display high activity, and amino acids that reinforce this propensity are advantaged. Through this approach two new structural motifs have been discovered that are capable of activating esters in organic solvents. Unlike most acyl transfer catalysts functioning in organic solvents, these catalysts are histidine- rather than N-alkyl histidine-based. Longer peptides with localization of reactive groups on the C-terminal end of the peptide were found to further enhance catalytic activity up to ∼2800-fold over background.

Effect of isotopically sensitive branching on product distribution for pentalenene synthase: support for a mechanism predicted by quantum chemistry.

Mechanistic proposals for the carbocation cascade reaction leading to the tricyclic sesquiterpene pentalenene are assessed in light of the results of isotopically sensitive branching experiments with the H309A mutant of pentalenene synthase. These experimental results support a mechanism for pentalenene formation involving a 7-protoilludyl cation whose intermediacy was first predicted using quantum-chemical calculations.

Total synthesis of oxidized welwitindolinones and (-)-N-methylwelwitindolinone C isonitrile.

We report the total synthesis of (-)-N-methylwelwitindolinone C isonitrile, in addition to the total syntheses of the 3-hydroxylated welwitindolinones. Our routes to these elusive natural products feature the strategic use of a deuterium kinetic isotope effect to improve the efficiency of a late-stage nitrene insertion reaction. We also provide a computational prediction for the stereochemical configuration at C3 of the hydroxylated welwitindolinones, which was confirmed by experimental studies.

The correct structure of aquatolide-experimental validation of a theoretically-predicted structural revision.

Aquatolide has been reisolated from its natural source, and its structure has been revised on the basis of quantum-chemical NMR calculations, extensive experimental NMR analysis, and crystallography.

Prediction of the structure of nobilisitine a using computed NMR chemical shifts.

1H and 13C NMR computed chemical shifts are determined for eight diastereomers of the originally proposed structure of nobilisitine A, which has recently been shown to be incorrect. On the basis of comparison of the computed chemical shifts with those reported experimentally, we predict that the true structure of nobilisitine A is likely the diastereomer shown here or its enantiomer.

Mechanisms for formation of diazocinones, pyridazines, and pyrazolines from tetrazines--oxyanion-accelerated pericyclic cascades?

A computational approach is utilized to study the diazocinone- and pyridazine-forming cascade reactions resulting from the reaction of 1,2,4,5-tetrazines with cyclic enolates. Many of the proposed reaction steps can be formulated as oxyanion-accelerated pericyclic processes. In examining these, a unique stepwise version of a formal (4 + 2) cycloaddition/(4 + 2) cycloreversion was discovered. For the key ring-opening step in these cascades, theoretical evidence for two distinct processes is reported. Of these two possibilities, an allowed six-electron electrocyclic ring-opening is predicted to be highly favored both kinetically and thermodynamically. Evidence for an unexpected oxyanion-accelerated 1,2-sigmatropic shift was also found for certain systems, leading to the theoretical prediction that seven- and eight-membered ring-fused pyrazoline systems could be formed experimentally under conditions similar to those for diazocinone and pyridazine formation.

Computational studies on biosynthetic carbocation rearrangements leading to sativene, cyclosativene, alpha-ylangene, and beta-ylangene.

In this paper, we describe theoretical studies, using gas-phase quantum chemical calculations, on carbocationic rearrangement pathways leading to the sesquiterpenes sativene, cyclosativene, alpha-ylangene, and beta-ylangene. For all four sesquiterpene natural products, viable pathways are presented, and these are compared both to mechanistic proposals found in the literature, and in certain cases to alternative stereochemical and rearrangement possibilities, thus providing a basis for comparison to experimental results. We find that these four sesquiterpenes likely arise from a common bicyclic intermediate and, furthermore, that the computed pathways are mostly in agreement with previous mechanistic proposals, although the few differences that we have uncovered are significant. Additionally, the potential energy profiles of the pathways are found to be very flat, supporting the notion that following the initial ionization of farnesyl diphosphate, minimal enzymatic intervention may be required for the generation of such sesquiterpenes.

Constrained bithiazoles: small molecule correctors of defective ΔF508-CFTR protein trafficking.

Conformationally constrained bithiazoles were previously found to have improved efficacy over nonconstrained bithiazoles for correction of defective cellular processing of the ΔF508 mutant cystic fibrosis transmembrane conductance regulator (CFTR) protein. In this study, two sets of constrained bithiazoles were designed, synthesized, and tested in vitro using ΔF508-CFTR expressing epithelial cells. The SAR data demonstrated that modulating the constraining ring size between 7- versus 8-membered in these constrained bithiazole correctors did not significantly enhance their potency (IC50), but strongly affected maximum efficacy (Vmax), with constrained bithiazoles 9e and 10c increasing Vmax by 1.5-fold compared to benchmark bithiazole corr4a. The data suggest that the 7- and 8-membered constrained ring bithiazoles are similar in their ability to accommodate the requisite geometric constraints during protein binding.

A divergent approach to the synthesis of the yohimbinoid alkaloids venenatine and alstovenine.

The yohimbinoid alkaloids continue to receive considerable attention from the synthetic community because of their interesting chemical structures and varied biological activity. Although there are several elegant syntheses of certain members of this group of alkaloids, a truly unified approach has yet to be developed. In short, general approaches to this compound class are hampered by a lack of complete control in setting the C(3) stereocentre at a late stage. Herein, we report that a functionalized hydrindanone enables a divergent strategy that builds on existing precedent to address this long-standing challenge. Utilizing an aminonitrile intermediate, the stereochemistry at C(3) of the yohimbinoid skeleton can be controlled effectively in a Pictet-Spengler reaction. We applied this approach to the first total syntheses of the C(3) epimeric natural products venenatine and alstovenine.

Structure-activity relationships of cyanoquinolines with corrector-potentiator activity in ΔF508 cystic fibrosis transmembrane conductance regulator protein.

Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. The most common CF-causing mutation, ΔF508-CFTR, produces CFTR loss-of-function by impairing its cellular targeting to the plasma membrane and its chloride channel gating. We recently identified cyanoquinolines with both corrector ("Co", normalizing ΔF508-CFTR targeting) and potentiator ("Po", normalizing ΔF508-CFTR channel gating) activities. Here, we synthesized and characterized 24 targeted cyanoquinoline analogues to elucidate the conformational requirements for corrector and potentiator activities. Compounds with potentiator-only, corrector-only, and dual potentiator-corrector activities were found. Molecular modeling studies (conformational search ⇒ force-field lowest energy assessment ⇒ geometry optimization) suggest that (1) a flexible tether and (2) a relatively short bridge between the cyanoquinoline and arylamide moieties are important cyanoquinoline-based CoPo features. Further, these CoPo's may adopt two distinct π-stacking conformations to elicit corrector and potentiator activities.

Cyclopenta[b]pyrroles from triazines: synthetic and mechanistic studies.

The pyrrolidine-mediated reactions of 3,5-disubstituted 1,2,4-triazines with cyclobutanone lead to cyclopenta[b]pyrroles, which can be derivatized into hydrazones and oximes. The cyclopenta[b]pyrrole ring system likely arises through a tandem [4 + 2] cycloaddition/cycloreversion/ring rearrangement reaction. In contrast, 3,6-disubstituted 1,2,4-triazines undergo a simple nucleophilic 1,4-addition with cyclobutanone to give 1:1 adducts.

Synthesis of substituted chromanones: an organocatalytic aldol/oxa-Michael reaction.

A diastereoselective organocatalytic aldol/oxa-Michael reaction has been developed to efficiently deliver medicinally relevant 2,3-ring-substituted chromanones. Development of this synthetic strategy revealed an unexpected kinetic anti-Saytzeff elimination; an origin for the observed selectivity is suggested on the basis of the results of quantum chemical calculations. This unusual kinetic selectivity necessitated an isomerization protocol that in turn led to the discovery of an intriguing Pd-mediated isomerization/intramolecular Friedel-Crafts-type alkylation.

Potent s-cis-locked bithiazole correctors of DeltaF508 cystic fibrosis transmembrane conductance regulator cellular processing for cystic fibrosis therapy.

N-(5-(2-(5-Chloro-2-methoxyphenylamino)thiazol-4-yl)-4-methylthiazol-2-yl)pivalamide 1 (compound 15Jf) was found previously to correct defective cellular processing of the cystic fibrosis protein DeltaF508-CFTR. Eight C4'-C5 C,C-bond-controlling bithiazole analogues of 1 were designed, synthesized, and evaluated to establish that constraining rotation about the bithiazole-tethering has a significant effect on corrector activity. For example, constraining the C4'-C5 bithiazole tether in the s-cis conformation [N-(2-(5-chloro-2-methoxyphenylamino)-7,8-dihydro-6 H-cyclohepta[1,2- d:3,4- d']bithiazole-2'-yl)pivalamide, 29] results in improved corrector activity. Heteroatom placement in the bithaizole core is also critical as evidenced by the decisive loss of corrector activity with s-cis constrained N-(2-(5-chloro-2-methoxyphenylamino)-5,6-dihydro-4 H-cyclohepta[1,2- d:3,4- d']bithiazole-2'-yl)pivalamide 33. In addition, computational models were utilized to examine the conformational preferences for select model systems. Following our analysis, the " s-cis-locked" cycloheptathiazolothiazole 29 was found to be the most potent bithiazole corrector, with an IC50 of approximately 450 nM.