Identification of Brain-Penetrant ATP-Competitive mTOR Inhibitors for CNS Syndromes.

The allosteric inhibitor of the mechanistic target of rapamycin (mTOR) everolimus reduces seizures in tuberous sclerosis complex (TSC) patients through partial inhibition of mTOR functions. Due to its limited brain permeability, we sought to develop a catalytic mTOR inhibitor optimized for central nervous system (CNS) indications. We recently reported an mTOR inhibitor (1) that is able to block mTOR functions in the mouse brain and extend the survival of mice with neuronal-specific ablation of the Tsc1 gene. However, 1 showed the risk of genotoxicity in vitro. Through structure-activity relationship (SAR) optimization, we identified compounds 9 and 11 without genotoxicity risk. In neuronal cell-based models of mTOR hyperactivity, both corrected aberrant mTOR activity and significantly improved the survival rate of mice in the Tsc1 gene knockout model. Unfortunately, 9 and 11 showed limited oral exposures in higher species and dose-limiting toxicities in cynomolgus macaque, respectively. However, they remain optimal tools to explore mTOR hyperactivity in CNS disease models.

Discovery of a Brain-Penetrant ATP-Competitive Inhibitor of the Mechanistic Target of Rapamycin (mTOR) for CNS Disorders.

Recent clinical evaluation of everolimus for seizure reduction in patients with tuberous sclerosis complex (TSC), a disease with overactivated mechanistic target of rapamycin (mTOR) signaling, has demonstrated the therapeutic value of mTOR inhibitors for central nervous system (CNS) indications. Given that everolimus is an incomplete inhibitor of the mTOR function, we sought to develop a new mTOR inhibitor that has improved properties and is suitable for CNS disorders. Starting from an in-house purine-based compound, optimization of the physicochemical properties of a thiazolopyrimidine series led to the discovery of the small molecule 7, a potent and selective brain-penetrant ATP-competitive mTOR inhibitor. In neuronal cell-based models of mTOR hyperactivity, 7 corrected the mTOR pathway activity and the resulting neuronal overgrowth phenotype. The new mTOR inhibitor 7 showed good brain exposure and significantly improved the survival rate of mice with neuronal-specific ablation of the Tsc1 gene. These results demonstrate the potential utility of this tool compound to test therapeutic hypotheses that depend on mTOR hyperactivity in the CNS.

NHC Ligands Tailored for Simultaneous Regio- and Enantiocontrol in Nickel-Catalyzed Reductive Couplings.

An exceptionally hindered class of enantiopure NHC ligands has been developed. While racemic forms had previously been utilized, a scalable and practical route to the enantiopure form of this ligand class is described utilizing a Buchwald-Hartwig N,N-diarylation in a highly sterically demanding environment. Using this newly accessible ligand class, nickel-catalyzed enantioselective reductive coupling reactions of aldehydes and alkynes have been developed. These studies illustrate that the newly available NHC ligands are well suited for simultaneous control of regio- and enantioselectivity, even in cases with internal alkynes possessing only very subtle steric differences between two aliphatic substituents. The steric demand of the new ligand class enables a complementary regiochemical outcome compared with previously described enantioselective processes. Using this method, a number of allylic alcohol derivatives were efficiently obtained with high regioselectivity (up to >95:5) and high enantioselectivity (up to 94% ee). The reaction conditions can also be extended to the reaction of aldehydes and allenes, providing silyl-protected allylic alcohol derivatives possessing a terminal methylene substituent. Computational studies have explained the origin of the exceptional steric demand of this ligand class, the basis for enantioselectivity, and the cooperative relationship of the aldehyde, alkyne, and ligand in influencing enantioselectivity.

Comparative Study of the Limitations and Challenges in Atom-Transfer C-H Oxidations.

A comparative study is disclosed that seeks to highlight the current limitations and challenges that exist in the field of atom-transfer C-H oxidations. State-of-the-art methods are benchmarked in order to showcase clear differences and similarities. A novel Mn-mediated method for C-H oxidation is disclosed that serves as a rapid and simple method for aliphatic C-H hydroxylation. Finally, two methods that allow for C-H oxidation in the presence of pyridine-containing substrates are studied, something that is rare in the field but of great interest to the chemical community.

Mechanistic Basis for Regioselection and Regiodivergence in Nickel-Catalyzed Reductive Couplings.

The control of regiochemistry is a considerable challenge in the development of a wide array of catalytic processes. Simple π-components such as alkenes, alkynes, 1,3-dienes, and allenes are among the many classes of substrates that present complexities in regioselective catalysis. Considering an internal alkyne as a representative example, when steric and electronic differences between the two substituents are minimal, differentiating among the two termini of the alkyne presents a great challenge. In cases where the differences between the alkyne substituents are substantial, overcoming those biases to access the regioisomer opposite that favored by substrate biases often presents an even greater challenge. Nickel-catalyzed reductive couplings of unsymmetrical π-components make up a group of reactions where control of regiochemistry presents a challenging but important objective. In the course of our studies of aldehyde-alkyne reductive couplings, complementary solutions to challenges in regiocontrol have been developed. Through careful selection of the ligand and reductant, as well as the more subtle reaction variables such as temperature and concentration, effective protocols have been established that allow highly selective access to either regiosiomer of the allylic alcohol products using a wide range of unsymmetrical alkynes. Computational studies and an evaluation of reaction kinetics have provided an understanding of the origin of the regioselectivity control. Throughout the various procedures described, the development of ligand-substrate interactions plays an essential role, and the overall kinetic descriptions were found to differ between protocols. Rational alteration of the rate-determining step plays a key role in the regiochemistry reversal strategy, and in one instance, the two possible regioisomeric outcomes in a single reaction were found to operate by different kinetic descriptions. With this mechanistic information in hand, the empirical factors that influence regiochemistry can be readily understood, and more importantly, the insights suggest simple and predictable experimental variables to achieving a desired reaction outcome. These studies thus present a detailed picture of the influences that control regioselectivity in a specific catalytic reaction, but they also delineate strategies for regiocontrol that may extend to numerous classes of reactions. The work provides an illustration of how insights into the kinetics and mechanism of a catalytic process can rationalize subtle empirical findings and suggest simple and rational modifications in procedure to access a desirable reaction outcome. Furthermore, these studies present an illustration of how important challenges in organic synthesis can be met by novel reactivity afforded by base metal catalysis. The use of nickel catalysis in this instance not only provides an inexpensive and sustainable method for catalysis but also enables unique reactivity patterns not accessible to other metals.

Non-Directed Allylic C-H Acetoxylation in the Presence of Lewis Basic Heterocycles.

We outline a strategy to enable non-directed Pd(II)-catalyzed C-H functionalization in the presence of Lewis basic heterocycles. In a high-throughput screen of two Pd-catalyzed C-H acetoxylation reactions, addition of a variety of N-containing heterocycles is found to cause low product conversion. A pyridine-containing test substrate is selected as representative of heterocyclic scaffolds that are hypothesized to cause catalyst arrest. We pursue two approaches in parallel that allow product conversion in this representative system: Lewis acids are found to be effective in situ blocking groups for the Lewis basic site, and a pre-formed pyridine N-oxide is shown to enable high yield of allylic C-H acetoxylation. Computational studies with density functional theory (M06) of binding affinities of selected heterocycles to Pd(OAc)2 provide an inverse correlation of the computed heterocycle-Pd(OAc)2 binding affinities with the experimental conversions to products. Additionally, 1H NMR binding studies provide experimental support for theoretical calculations.

Efficient search of chemical space: navigating from fragments to structurally diverse chemotypes.

We introduce a novel strategy to sample bioactive chemical space, which follows-up on hits from fragment campaigns without the need for a crystal structure. Our results strongly suggest that screening a few hundred or thousand fragments can substantially improve the selection of small-molecule screening subsets. By combining fragment-based screening with virtual fragment linking and HTS fingerprints, we have developed an effective strategy not only to expand from low-affinity hits to potent compounds but also to hop in chemical space to substantially novel chemotypes. In benchmark calculations, our approach accessed subsets of compounds that were substantially enriched in chemically diverse hit compounds for various activity classes. Overall, half of the hits in the screening collection were found by screening only 10% of the library. Furthermore, a prospective application led to the discovery of two structurally novel histone deacetylase 4 inhibitors.

Functionalized chromans and isochromans via a diastereoselective Pd(0)-catalyzed carboiodination.

A diastereoselective approach to isochromans and chromans via Pd(0)-catalyzed carboiodination is reported. The transformations using this methodology display excellent yields and diastereoselectivities as well as broad functional group compatibility. The selectivity observed in these cyclizations, forming isochroman or chroman targets, is postulated to originate from the minimization of A(1,2) strain and axial-axial interactions, respectively. This method has also been used to highlight the concept of reversible oxidative addition to carbon-iodine bonds in polyiodinated substrates.

A general strategy for regiocontrol in nickel-catalyzed reductive couplings of aldehydes and alkynes.

A strategy for catalyst-controlled regioselectivity in aldehyde-alkyne reductive couplings has been developed. This strategy is the first where either regiochemical outcome may be selected for a broad range of couplings, without relying on substrate biases or directing effects. The complementary use of small cyclopropenylidene carbene ligands or highly hindered N-heterocyclic carbene ligands allows the regiochemical reversal with unbiased internal alkynes, aromatic internal alkynes, conjugated enynes, or terminal alkynes.

Cooperativity of regiochemistry control strategies in reductive couplings of propargyl alcohols and aldehydes.

The nickel-catalyzed reductive coupling of propargyl alcohols and alkynes proceeds with excellent regiochemical control with an underlying electronic preference that can be supplemented by ligand size effects. The products obtained may be readily converted to substructures that are not directly available by an aldehyde-alkyne reductive coupling. A simple model for how steric and electronic factors are both important in governing regiochemistry in couplings of this type is presented, along with examples of how the effects can combine in either a constructive or destructive manner.

Oxone as an inexpensive, safe, and environmentally benign oxidant for C-H bond oxygenation.

[reaction: see text] This paper describes the application of peroxide-based oxidants in the Pd(OAc)(2)-catalyzed acetoxylation and etherification of arene and alkane C-H bonds. Oxone in acetic acid and/or methanol proved particularly effective, and these transformations were applied to a wide variety of substrates.