Palladium-catalyzed intramolecular alpha-arylation of alpha-amino acid esters

Palladium- and Copper-Catalyzed Synthesis of Medium- and Large-Sized Ring-Fused Dihydroazaphenanthrenes and 1,4-Benzodiazepine-2,5-diones. Control of Reaction Pathway by Metal-Switching

Methods for the synthesis of dihydroazaphenanthrene fused to macrocycles (2) and medium-ring heterocycles (4), as well as 1,4-benzodiazepine-2,5-diones (5), are developed. A distinctly different catalytic property of palladium and copper catalysts was uncovered that leads to the development of a divergent synthesis of two different heterocyclic scaffolds from the same starting materials, simply by metal-switching. Thus, starting from linear amide 3, palladium acetate triggers a domino intramolecular N-arylation/C-H activation/aryl-aryl bond-forming process to provide 4, while copper iodide promotes only the intramolecular N-arylation reaction leading to 5. In combination with the Ugi multicomponent reaction (Ugi-4CR) for the preparation of the linear amides, a two-step synthesis of either the 5,6-dihydro-8H-5,7a-diazacyclohepta[jk]phenanthrene-4,7-dione (4) or 1,4-benzodiazepine-2,5-diones (5), by appropriate choice of metal catalyst, is subsequently developed from very simple starting materials.

Synthesis of Fused Heteroarylprolines and Pyrrolopyrroles

Fused heteroarylprolines were prepared starting from 4-oxo-N-(PhF)proline benzyl ester (6, PhF = 9-(9-phenylfluorenyl)) following two approaches. First, allylation of oxoproline 6 followed by Wacker oxidation gave 1,4-dione 8 that was selectively converted to pyrroloproline 10b, pyrrolopyrrole 12, and pyridazinoproline 9. Second, aldol condensation of oxoproline 6 with a series of N-(Boc)-alpha-amino aldehydes 15a-e and acid-catalyzed cyclization gave pyrroloprolines 17a-e possessing a variety of pyrrole 5-position substituents. Conditions for the selective deprotection and alkylation of the pyrrole nitrogen of pyrroloprolines 17 were developed to expand the diversity of the heteoaryl systems. Finally, hydrogenolytic cleavage of the PhF and benzyl groups followed by subsequent protection with Boc, Fmoc, and Moz groups was performed to obtain analogues suitable for peptide synthesis. The enantiomeric purity of N-(Boc)pyrrolo-proline 21a was ascertained by coupling to l- and d-phenylalanine methyl ester and examination of the diastereotopic pyrrole protons, which demonstrated the dipeptides to be of >99% diastereomeric purity. These approaches have thus delivered the first series of enantiopure fused arylprolines for application as arylglycine-proline chimeras in structure-activity studies of biologically active compounds.

Palladium-catalyzed alpha-arylation of azlactones to form quaternary amino acid derivatives

[reaction: see text] The synthesis of alpha-aryl-alpha-alkyl amino acid derivatives from alpha-amino acids by the arylation of azlactone derivatives is reported. Arylation of azlactones derived from alanine, valine, phenalanine, phenyl glycine, and leucine all provided good yields of the arylated product. Mechanistic studies of this reaction revealed that a stable complex containing a ligand formed by reaction of dba with the azlactone accounts for a new inhibiting effect of dba when reactions are initiated with Pd(dba)(2).

Palladium-catalyzed alpha-arylation of carbonyl compounds and nitriles

The palladium-catalyzed alpha-arylation of ketones has become a useful and general synthetic method. In this process, an enolate is generated from a ketone and base in the presence of an aryl halide, and a palladium catalyst couples this enolate with the aryl halide. With the advent of new catalysts composed of sterically hindered, electron-rich alkylphosphine and N-heterocyclic carbene ligands, this process now encompasses a broad range of enolates and related anions, including those derived from amides, esters, aldehydes, nitriles, malonates, cyanoesters, nitroalkanes, sulfones, and lactones. In the proposed mechanism for this reaction, the carbon-carbon bond of the product is formed by reductive elimination from an arylpalladium enolate intermediate. The structures and reactions of arylpalladium complexes of enolate, cyanoalkyl, and malonate ions have been studied to determine how the binding mode and electronic and steric parameters influence the rate and mechanism of reductive elimination.

Intramolecular Pd-mediated processes of amino-tethered aryl halides and ketones: insight into the ketone alpha-arylation and carbonyl-addition dichotomy. A new class of four-membered azapalladacycles

An exploration of the scope and limitations of Pd(0)-catalyzed intramolecular coupling reactions of amino-tethered aryl halides and ketones has been conducted. Two different and competitive reaction pathways starting from omega-(2-haloanilino) alkanones, enolate arylation and addition to the carbonyl group, have been observed, while omega-(2-halobenzylamino) alkanones exclusively underwent the enolate arylation process. The dichotomy between ketone alpha-arylation and carbonyl-addition in the reactions of omega-(2-haloanilino) alkanones has been rationalized by the intermediacy of unprecedented four-membered azapalladacycles, from which X-ray data and chemical behavior are reported.

Copper- and palladium-catalyzed intramolecular aryl guanidinylation: an efficient method for the synthesis of 2-aminobenzimidazoles

[reaction: see text] The formation of 2-aminobenzimidazoles via intramolecular C[bond]N formation between an aryl halide and a guanidine moiety can be achieved using either copper or palladium catalysis. Inexpensive copper salts such as CuI are generally superior to the use of palladium catalysts. Regioselective cyclizations, where R(3) = H, can be achieved in high yield under CuI/1,10-phenanthroline-catalyzed conditions, whereas palladium catalysis results in the formation of regioisomeric products.