Use of aryl chlorides as electrophiles in Pd-catalyzed alkene difunctionalization reactions

Hydroxylamines: From Synthetic Intermediates to Synthetic Targets

ConspectusSynergy between the teaching and research activities of a University should be a source of new ideas, each informing the other. A classroom discussion gave rise to our concept of using hydroxylamines as a form of "tethered nitrogen" for alkaloid synthesis. The "tether" temporarily connects a nucleophilic nitrogen atom to the substrate, rendering an intermolecular reaction intramolecular, thus providing stereo- and regiochemical control for C-N bond formation. In the context of the synthesis of 1,3-amino alcohols, this necessitated the synthesis of isoxazolidines. This concept led to the exploration of methods for the synthesis of these heterocycles beyond the well-established 1,3-dipolar cycloadditions. The first two methods developed based upon this concept were palladium catalyzed cyclocarbonylation and silver catalyzed allene cyclization. These new methods were applied to two syntheses of sedamine and the synthesis of three Nuphar alkaloids. Intramolecular aza-Michael addition, combined with the use of cross-metathesis to generate the substrates, gave access to both isoxazolidines and tetrahydro-1,2-oxazines. This new method made possible a synthesis of monomorine with high stereochemical control. The extension to more complex alkaloids required the incorporation of additional chemistry. Combining allene cyclization with iminium ion chemistry allowed the extension to the more complex piperidine alkaloids porantheridine and sedinine. In these cases, successful stereocontrol relied on the ability to predict the conformation of the intermediates and the trajectory of the nucleophilic attack, which may be sterically or stereoelectronically controlled. Inspection of our collection of methods revealed that we had good methods for cis-3,5-disubstituted isoxazolidines and trans-3,6-disubstituted tetrahydro-1,2-oxazines. Inspired by earlier work involving iminium ions, methods were developed to provide the complementary trans-isoxazolidines and cis-oxazines, using an allylation reaction. This, combined with our interest in applications of hydroformylation, led to the synthesis of 5-hydroxysedamine. Venturing away from piperidines, this method was successfully applied to the synthesis of the β-amino acid antibiotic negamycin. Use of N,O-heterocycles as intermediates naturally sparked an interest in N,O-heterocycles as synthetic targets, as there are many natural products that contain the hydroxylamine moiety, often incorporated into an isoxazolidine or 1,2-oxazine ring. As the presence of the hydroxylamine moiety cannot be fully demonstrated using the usual spectroscopic methods, total synthesis can be employed to bridge this logical gap. A synthesis of raistrickindole A, utilizing an intramolecular Mitsunobu reaction of a hydroxamic acid to form a 1,2-oxazine, confirmed the structure of this diketopiperazine natural product. A synthesis of preisomide, using an aza-Michael addition to form the required 1,2-oxazine, also confirmed the structure of this natural product.

Catalyst-free carbosilylation of alkenes using silyl boronates and organic fluorides via selective C-F bond activation

A regioselective carbosilylation of alkenes has emerged as a powerful strategy to access molecules with functionalized silylated alkanes, by incorporating silyl and carbon groups across an alkene double bond. However, to the best of our knowledge, organic fluorides have never been used in this protocol. Here we disclose the catalyst-free carbosilylation of alkenes using silyl boronates and organic fluorides mediated by tBuOK. The main feature of this transformation is the selective activation of the C-F bond of an organic fluoride by the silyl boronate without undergoing potential side-reactions involving C-O, C-Cl, heteroaryl-CH, and even CF3 groups. Various silylated alkanes with tertiary or quaternary carbon centers that have aromatic, hetero-aromatic, and/or aliphatic groups at the β-position are synthesized in a single step from substituted or non-substituted aryl alkenes. An intramolecular variant of this carbosilylation is also achieved via the reaction of a fluoroarene with a ω-alkenyl side chain and a silyl boronate.

Palladium/Xu-Phos-catalyzed asymmetric carboamination towards isoxazolidines and pyrrolidines

An efficient palladium-catalyzed enantioselective carboamination reaction of N-Boc-O-homoallyl-hydroxylamines and N-Boc-pent-4-enylamines with aryl or alkenyl bromides was developed, delivering various substituted isoxazolidines and pyrrolidines in good yields with up to 97% ee. The reaction features mild conditions, general substrate scope and scalability. The obtained products can be transformed into chiral 1,3-aminoalcohol derivatives without erosion of chirality. The newly identified Xu-Phos ligand bearing an ortho-OiPr group is responsible for the good yield and high enantioselectivity.

Allylation of isatin-derived N-Boc-hydrazones followed by Pd-catalyzed carboamination reaction: an entry to 3-spiro-pyrazolidyl-oxindoles

The indium-mediated allylation of novel 3-(2-Boc-hydrazono)indolin-2-one derivatives, followed by a palladium-catalysed carboamination reaction, is described to afford unprecedented spirocyclic oxindoles in good yields. The method provides an efficient access to both cis and trans diastereoisomers of highly functionalized compounds, bearing an N-Boc, 5-substituted pyrazolidine ring at the C3-oxindole spiro junction. The versatility of the method is fully demonstrated starting from a series of substituted isatins and employing a variety of aryl halides in the key cyclization step.

The indium-mediated allylation of novel 3-(2-Boc-hydrazono)indolin-2-one derivatives, followed by a palladium-catalysed carboamination reaction, is described to afford unprecedented spirocyclic oxindoles in good yields.

Isoxazolidine: A Privileged Scaffold for Organic and Medicinal Chemistry

The isoxazolidine ring represents one of the privileged structures in medicinal chemistry, and there have been an increasing number of studies on isoxazolidine and isoxazolidine-containing compounds. Optimization of the 1,3-dipolar cycloaddition (1,3-DC), original methods including electrophilic or palladium-mediated cyclization of unsaturated hydroxylamine, has been developed to obtain isoxazolidines. Novel reactions involving the isoxazolidine ring have been highlighted to accomplish total synthesis or to obtain bioactive compounds, one of the most significant examples being probably the thermic ring contraction applied to the total synthesis of (±)-Gelsemoxonine. The unique isoxazolidine scaffold also exhibits an impressive potential as a mimic of nucleosides, carbohydrates, PNA, amino acids, and steroid analogs. This review aims to be a comprehensive and general summary of the different isoxazolidine syntheses, their use as starting building blocks for the preparation of natural compounds, and their main biological activities.

Organocatalytic [4+1]-annulation approach for the synthesis of densely functionalized pyrazolidine carboxylates

The present protocol provides optically active pyrazolidine carboxylates with derivatizable functional groups in high yields with excellent enantio- and diastereoselectivities.

Recent Developments in Pd-Catalyzed Alkene Aminoarylation Reactions for the Synthesis of Nitrogen Heterocycles

This short review describes new developments in Pd-catalyzed aminoarylation reactions between aryl halides and alkenes bearing pendant nitrogen nucleophiles. These transformations provide a novel and powerful method for accessing numerous 3-, 5-, 6-, and 7-membered nitrogen heterocycles.

Intercepting Wacker intermediates with arenes: C-H functionalization and dearomatization

An intramolecular cyclization cascade reaction has been developed utilizing a high valent palladium intermediate that generates a carbon-carbon and carbon-oxygen bond in a single transformation. This method provides rapid access to highly functionalized tricyclic scaffolds, including spirocyclic cyclohexadienones. Good yields and mild conditions are reported with high tolerance toward oxygen and water.

Cascade intramolecular N-arylation/intermolecular carboamination reactions for the construction of tricyclic heterocycles

A new method for the stereoselective synthesis of tetrahydropyrroloindoles and hexahydropyrroloquinolines of general structure 8 is described. These products are formed through cascade Pd-catalyzed coupling reactions between aryl chlorides and unsaturated amine substrates 5. A single catalyst effects an intramolecular N-arylation reaction followed by an intermolecular alkene carboamination reaction to generate two rings, three bonds, and one stereocenter with good chemoselectivity, diastereoselectivity, and chemical yield.

Intramolecular alkene carboamination reactions for the synthesis of enantiomerically enriched tropane derivatives

The synthesis of tropane derivatives via intramolecular Pd-catalyzed alkene difunctionalization reactions is described. Enantiopure N-aryl-γ-aminoalkenes bearing an aryl or alkenyl halide adjacent to the amino group were converted to benzo- or cycloalkenyl-fused tropane products in good yield and with no loss of enantiopurity.