A Pd-Catalyzed Annulation Strategy to Linearly Fused Functionalized N-Heterocycles.

Linearly fused polycyclic piperidines represent common substructures in natural products and biologically active small molecules. We have devised a Pd-catalyzed annulation strategy to these compounds that converts readily available 2-tetralones and indanones into these scaffolds with the potential for control of both enantio- and diastereoselectivity. Importantly, these compounds can be chemoselectively functionalized, providing an efficient and robust methodology to these important nitrogen-containing molecules.

Pd-Catalyzed [4 + 1] Annulation Strategy to Functionalized 4-Methyleneproline Derivatives.

A Pd-catalyzed formal [4 + 1] cycloaddition reaction of sulfur ylides and in situ-generated Pd-stabilized zwitterions offers a convenient route to a series of functionalized proline derivatives. The utility of this method is demonstrated by a gram-scale synthesis and chemoselective functionalization of a proline-based derivative.

Asymmetric Synthesis of Functionalizable Type II ß-Turn-Inducing α-Amino Acid Building Blocks.

Peptidomimetics are emerging as a promising class of potent and selective therapeutics. Among the current approaches to these compounds, the utilization of constrained lactams is a key element in enforcing the active peptide conformation, and the development of efficient and stereocontrolled methods for generating such lactam building blocks is an important objective. Current methods typically rely on the elaboration of existing α-amino acids, and in so doing, the side chain is sacrificed during the ring-forming process. We report a new asymmetric approach to lactam-constrained α-amino acid building blocks bearing a range of polar and hydrophobic side chains. The chemistry is amenable to rapidly generating di- and tripeptides, and the potential for these lactams to stabilize type II ß-turns is demonstrated in the synthesis of the melanocyte-inhibiting factor peptidomimetic.

A Dynamic Thermodynamic Resolution Strategy for the Stereocontrolled Synthesis of Streptonigrin.

We report that axially chiral biaryl boronic esters can be generated with control of atroposelectivity by a Binol-mediated dynamic thermodynamic resolution process. These intermediates can be progressed to enantioenriched products through stereoretentive functionalization of the carbon-boron bond. Finally, we have exploited this method in the first highly stereoselective total synthesis of P-streptonigrin.

Development of an Enantioselective Allylic Alkylation of Acyclic α-Fluoro-ß-ketoesters for Asymmetric Synthesis of 3-Fluoropiperidines.

The first useful enantioselective Pd-catalyzed asymmetric allylic alkylation of α-fluoro-ß-ketoesters has been achieved using the Trost family of chiral ligands yielding products in up to 92 % ee. This work provides new insights regarding the typically modest selectivities associated with acyclic α-fluoroenolates and shows experimental evidence that the typically poor levels of enantiocontrol associated with these systems are not necessarily due to the presence of E/Z enolate mixtures. Finally, this methodology allows the easy preparation of useful 3-fluoropiperidine intermediates, and it is demonstrated that these systems are applicable to a range of functionalization reactions leading to new building blocks for the discovery of bioactive products.

A Mild and Regioselective Route to Fluoroalkyl Aromatic Compounds via Directed Cycloaddition Reactions.

The synthesis of perfluoroalkyl-substituted (hetero)arenes by benzannulation strategies is complementary to ring functionalization approaches as it obviates the need for pre-existing functionality and innate regiocontrol. We report a mild and regiospecific boron-directed benzannulation method as a vehicle for accessing a range of perfluoroalkyl-substituted (hetero)aromatic building blocks that can be readily elaborated through established C-B bond functionalization processes.

A Pd-Catalyzed [4 + 2] Annulation Approach to Fluorinated N-Heterocycles.

3-Fluoro- and trifluoromethylthio-piperidines represent important building blocks for discovery chemistry. We report a simple and efficient method to access analogs of these compounds that are armed with rich functionality allowing them to be chemoselectively derivatized with high diastereocontrol.

Discovery of a First-In-Class Small Molecule Antagonist against the Adrenomedullin-2 Receptor: Structure-Activity Relationships and Optimization.

Class B G-protein-coupled receptors (GPCRs) remain an underexploited target for drug development. The calcitonin receptor (CTR) family is particularly challenging, as its receptors are heteromers comprising two distinct components: the calcitonin receptor-like receptor (CLR) or calcitonin receptor (CTR) together with one of three accessory proteins known as receptor activity-modifying proteins (RAMPs). CLR/RAMP1 forms a CGRP receptor, CLR/RAMP2 forms an adrenomedullin-1 (AM1) receptor, and CLR/RAMP3 forms an adrenomedullin-2 (AM2) receptor. The CTR/RAMP complexes form three distinct amylin receptors. While the selective blockade of AM2 receptors would be therapeutically valuable, inhibition of AM1 receptors would cause clinically unacceptable increased blood pressure. We report here a systematic study of structure-activity relationships that has led to the development of first-in-class AM2 receptor antagonists. These compounds exhibit therapeutically valuable properties with 1000-fold selectivity over the AM1 receptor. These results highlight the therapeutic potential of AM2 antagonists.

Pyrimidin-6-yl Trifluoroborate Salts as Versatile Templates for Heterocycle Synthesis.

We report a novel and general method to access a highly under-studied privileged scaffold-pyrimidines bearing a trifluoroborate at C4, and highlight the broad utility of these intermediates in a rich array of downstream functionalization reactions. This chemistry is underpinned by the unique features of the trifluoroborate group; its robustness provides an opportunity to carry out chemoselective reactions at other positions on the pyrimidine while providing a pathway for elaboration at the C-B bond when suitably activated.

Design and Synthesis of Fused Pyridine Building Blocks for Automated Library Generation.

We demonstrate that a diboration-electrocyclization sequence provides access to a range of pyridine-fused, small-molecule boronic ester building blocks, and that these are amenable to high-throughput synthesis leading to biaryl and ether-linked compound libraries. Moreover, the implementation of an integrated physicochemical and ADME profiling workflow allows accelerated design of novel lead compounds for application in drug-discovery projects.

Discovery of a First-in-Class Potent Small Molecule Antagonist against the Adrenomedullin-2 Receptor.

The hormone adrenomedullin has both physiological and pathological roles in biology. As a potent vasodilator, adrenomedullin is critically important in the regulation of blood pressure, but it also has several roles in disease, of which its actions in cancer are becoming recognized to have clinical importance. Reduced circulating adrenomedullin causes increased blood pressure but also reduces tumor progression, so drugs blocking all effects of adrenomedullin would be unacceptable clinically. However, there are two distinct receptors for adrenomedullin, each comprising the same G protein-coupled receptor (GPCR), the calcitonin receptor-like receptor (CLR), together with a different accessory protein known as a receptor activity-modifying protein (RAMP). The CLR with RAMP2 forms an adrenomedullin-1 receptor, and the CLR with RAMP3 forms an adrenomedullin-2 receptor. Recent research suggests that a selective blockade of adrenomedullin-2 receptors would be therapeutically valuable. Here we describe the design, synthesis, and characterization of potent small-molecule adrenomedullin-2 receptor antagonists with 1000-fold selectivity over the adrenomedullin-1 receptor, although retaining activity against the CGRP receptor. These molecules have clear effects on markers of pancreatic cancer progression in vitro, drug-like pharmacokinetic properties, and inhibit xenograft tumor growth and extend life in a mouse model of pancreatic cancer. Taken together, our data support the promise of a new class of anticancer therapeutics as well as improved understanding of the pharmacology of the adrenomedullin receptors and other GPCR/RAMP heteromers.

The influence of hypoxia and energy depletion on the response of endothelial cells to the vascular disrupting agent combretastatin A-4-phosphate.

Combretastatin A-4 phosphate (CA4P) is a microtubule-disrupting tumour-selective vascular disrupting agent (VDA). CA4P activates the actin-regulating RhoA-GTPase/ ROCK pathway, which is required for full vascular disruption. While hypoxia renders tumours resistant to many conventional therapies, little is known about its influence on VDA activity. Here, we found that active RhoA and ROCK effector phospho-myosin light chain (pMLC) were downregulated in endothelial cells by severe hypoxia. CA4P failed to activate RhoA/ROCK/pMLC but its activity was restored upon reoxygenation. Hypoxia also inhibited CA4P-mediated actinomyosin contractility, VE-cadherin junction disruption and permeability rise. Glucose withdrawal downregulated pMLC, and coupled with hypoxia, reduced pMLC faster and more profoundly than hypoxia alone. Concurrent inhibition of glycolysis (2-deoxy-D-glucose, 2DG) and mitochondrial respiration (rotenone) caused profound actin filament loss, blocked RhoA/ROCK signalling and rendered microtubules  CA4P-resistant. Withdrawal of the metabolism inhibitors restored the cytoskeleton and CA4P activity. The AMP-activated kinase AMPK was investigated as a potential mediator of pMLC downregulation. Pharmacological AMPK activators that generate AMP, unlike allosteric activators, downregulated pMLC but only when combined with 2DG and/or rotenone. Altogether, our results suggest that Rho/ROCK and actinomyosin contractility are regulated by AMP/ATP levels independently of AMPK, and point to hypoxia/energy depletion as potential modifiers of CA4P response.

Design and Synthesis of New Pyrazole-Based Heterotricycles and their Derivatization by Automated Library Synthesis.

Small-molecule heterocycles bearing orthogonal functionality have the potential to deliver diverse structural motifs that aid the drug-discovery effort. This work highlights how a readily assembled N-hydroxyethyl pyrazole trifluoroborate offers rapid access to architecturally distinct 5-6-6- and 5-7-6-fused tricyclic compounds. This chemistry is not only amenable to single compound synthesis, but also to high-throughput experimentation. It gives easy access to diverse compound arrays with various physicochemical and ADME profiles by fully automated library synthesis. The combination of the high-throughput experimentation with rapid testing of the compounds in an integrated physicochemical and ADME profiling workflow allows accelerated design of novel lead compounds in drug-discovery projects.

Exploiting Synergistic Catalysis for an Ambient Temperature Photocycloaddition to Pyrazoles.

Sydnone-based cycloaddition reactions are a versatile platform for pyrazole synthesis, however they operate under harsh conditions (high temperature and long reaction times). Herein we report a strategy that addresses this limitation utilizing the synergistic combination of organocatalysis and visible-light photocatalysis. This new approach proceeds under ambient conditions and with excellent levels of regiocontrol. Mechanistic studies suggest that photoactivation of sydnones, rather than enamines, is key to the successful implementation of this process.

Exploiting Hydrazones To Improve the Efficiency of 6π-Electrocyclization Reactions of 1-Azatrienes.

The greater geometric lability of hydrazones compared to that of oxime ethers is used as a basis to overcome the reluctance of Z-oxime ether azatrienes to undergo electrocyclization toward the synthesis of borylated (heteroaromatic) pyridines and ring-fused analogues. Such hydrazones now allow access to previously inaccessible tri- and tetrasubstituted 3-borylpyridines in high yields.

Evaluation of Sydnone-Based Analogues of Combretastatin A-4 Phosphate (CA4P) as Vascular Disrupting Agents for Use in Cancer Therapy.

The combretastatins have attracted significant interest as small-molecule therapies for cancer due to their ability to function as vascular disrupting agents. We have successfully prepared a range of combretastatin analogues that are based on a novel sydnone heterocycle core, and their potential as tubulin binders has been assessed in vitro and in vivo. The most potent candidate was found to disrupt microtubules and affect cellular morphology at sub-micromolar levels. Moreover, it was found to bind reversibly to tubulin and significantly increase endothelial cell monolayer permeability, in a similar manner to combretastatin A4. Surprisingly, the compound did not exhibit efficacy in vivo, possibly due to rapid metabolism.

Synthetic and Mechanistic Investigation of an Oxime Ether Electrocyclization Approach to Heteroaromatic Boronic Acid Derivatives.

A range of functionalized heteroaromatic boronic acid derivatives are readily accessed by a diboration/6π-electrocyclization sequence. This study revealed the surprising observation that there is a direct relationship between oxime ether stereochemistry and reactivity towards electrocyclization. Specifically, E-oxime ethers are found to be significantly more reactive than their Z-counterparts (stereochemistry relative to azatriene scaffold). In contrast, the configuration at the azatriene alkene terminus has little impact on reaction rates. Computational analysis offers a rationale for this observation; a Nlone pair →C=C π* orbital interaction lowers the energy of the transition state in the electrocyclization of E-oxime ethers. Finally, unreactive Z-oxime ethers can be converted to the corresponding heterocyclic products by a photolytically promoted E→Z isomerization and electrocyclization sequence.

Development of an Amine-Catalyzed Regioselective Synthesis of Pyrroles.

A regioselective synthesis of pyrroles has been devised through the cycloaddition of 1,3-oxazolium-5-olates and enamines. Product regiochemistry is controlled by the enamine substitution pattern. Moreover, an amine-catalyzed variant of this reaction allows aldehydes to be used directly as substrates for pyrrole synthesis.

Synthesis of Bifunctional Thiophenes via Fiesselmann Condensation of Ynone Trifluoroborate Salts.

Ynone trifluoroborate salts undergo a base-promoted condensation reaction with alkylthiols to generate thiophene boronates with complete regiocontrol. The products are isolated in high yield and can be further derivatized through conventional C-B bond functionalization reactions.

Utilizing Palladium-Stabilized Zwitterions for the Construction of N-Heterocycles.

Pd catalysis provides a convenient method for the generation of zwitterionic intermediates that offer significant opportunities for the synthesis of functionalized heterocycles. The combination of electrophilic π-allyl Pd fragments and C-, N- and O-centered nucleophiles allows these intermediates to react with readily available substrates to furnish a range of high value products with control of chemo-, regio- and stereo-selectivity.