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.

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.

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.

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.

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.

A Mild and Regiospecific Synthesis of Pyrazoleboranes.

Alkynylboranes show unprecedented reactivity in their [4+2] cycloaddition of sydnones offering access to fully substituted pyrazoles within a few hours at room temperature. This method delivers synthetically valuable pyrazoleboranes with complete control of regioselectivity, and these intermediates can be further elaborated through functionalization of the C-B bond.

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.

Synthesis and Modular Reactivity of Pyrazole 5-Trifluoroborates: Intermediates for the Preparation of Fully Functionalized Pyrazoles.

The regioselective condensation of hydrazines and ynone trifluoroborates provides access to a range of pyrazole 5-trifluoroborates. The stability of the borate unit allows chemoselective halogenation of the heteroaromatic ring, thereby delivering pyrazole scaffolds that allow orthogonal functionalization at C5 and C4. The modular reactivity of these intermediates is exemplified by cross-coupling reactions, enabling regiocontrolled synthesis of fully functionalized pyrazole derivatives.

A Pd-Catalyzed Synthesis of Functionalized Piperidines.

A readily available cyclic carbamate 1 functions as a general precursor to a range of functionalized piperidine products via a new Pd-catalyzed annulation strategy. An asymmetric catalytic variant provides a rapid and efficient means to access these heterocycles with high to excellent levels of enantiocontrol. Finally, these richly functionalized compounds are amenable to further chemoselective elaboration.

A Modular Class of Fluorescent Difluoroboranes: Synthesis, Structure, Optical Properties, Theoretical Calculations and Applications for Biological Imaging.

Ten borylated bipyridines (BOBIPYs) have been synthesized and selected structural modifications have been made that allow useful structure-optical property relationships to be gathered. These systems have been further investigated using DFT calculations and spectroscopic measurements, showing blue to green fluorescence with quantum yields up to 41 %. They allow full mapping of the structure to determine where selected functionalities can be implemented, to tune the optical properties or to incorporate linking groups. The best derivative was thus functionalised with an alkyne linker, which would enable further applications through click chemistry and in this optic, the stability of the fluorophores has been evaluated.

Oxazoline-Promoted Rh-Catalyzed C-H Amidation of Benzene Derivatives with Sulfonamides and Trifluoroacetamide. A Comparative Study.

A Rh-catalyzed ortho-amidation of 2-aryloxazolines offers an efficient and direct route to a range of sulfonamides. The scope of the reaction is very broad with respect to sulfonamide substrate, but the position and electronic nature of the substituents on the aryl moiety of the oxazoline lead to a surprising modulation of reactivity. The reactivity of sulfonamides in comparison to trifluoroacetamide is compared, the latter undergoing Rh-catalyzed amidation more rapidly.

An Alkyne Diboration/6π-Electrocyclization Strategy for the Synthesis of Pyridine Boronic Acid Derivatives.

A new and efficient synthesis of pyridine-based heteroaromatic boronic acid derivatives is reported through a novel diboration/6π-electrocyclization strategy. This method delivers a range of functionalized heterocycles from readily available starting materials.

A mild and regioselective route to functionalized quinazolines.

A Rh-catalyzed ortho-amidation cyclocondensation sequence gave a range of 4-aminoquinazolines in high yield. The method features a remarkably mild C(sp(2) )H activation step and can be exploited to rapidly access compounds with established biological activity.

Cu-promoted sydnone cycloadditions of alkynes: scope and mechanism studies.

Cu salts have been found to promote the cycloaddition reaction of sydnones and terminal alkynes, providing significant reduction in reaction times. Specifically, the use of Cu(OTf)2 is found to provide 1,3-disubstituted pyrazoles, whereas simply switching the promoter system to Cu(OAc)2 allows the corresponding 1,4-isomers to be produced. The mechanism of the Cu-effect in each case has been investigated by experimental and theoretical studies, and they suggest that Cu(OTf)2 functions by Lewis acid activation of the sydnone, whereas Cu(OAc)2 promotes formation of reactive Cu(I) acetylides.

Investigation of alkyne regioselectivity in the ni-catalyzed benzannulation of cyclobutenones.

A Ni-catalyzed benzannulation reaction of cyclobutenones and alkynes provides a rapid synthesis of heavily substituted phenols. The regioselectivity of this reaction can be modulated by variation of substituents on the alkyne. Though the incorporation of Lewis basic donors provides modest selectivities, the use of aryl substituents can provide high levels of regiocontrol. Finally, alkynylboronates derived from alkyl-substituted acetylenes provide both high yields and regioselectivities. This study suggests that alkynes bearing one sp(2) - and one sp(3) -based substituent can undergo benzannulation with high levels of regiocontrol whereby the sp(3) -based group is incorporated ortho-to the phenolic OH.

Direct arylation of sydnones with aryl chlorides toward highly substituted pyrazoles.

The direct arylation of the C4 position of both N-alkyl- and N-arylsydnones with aryl/heteroaryl chlorides has been realized. The reaction is quite general and allows access to a wide range of 4-substituted sydnones. Yields of more challenging substrates can be improved through the use of aryl bromides.

A mechanistic study of the Lewis base-directed cycloaddition of 2-pyrones and alkynylboranes.

Significant rate enhancements in the Diels-Alder reaction of alkynes and 2-pyrones bearing a Lewis basic group are observed when a combination of alkynyltrifluoroborates and BF3·OEt2 is used. This process generates functionalized aromatic compounds with complete regiocontrol. The observed rate enhancement was studied by density functional theory methods and appears to originate from coordination of the diene substrate to a mixture of alkynylborane intermediates, followed by a Lewis acid-mediated product equilibration step. Evidence for this mechanism is presented, as is the enhanced promotion of the cycloaddition via the use of alternative Lewis acid promoters.