Guolin Cheng

Highly efficient synthesis of phenols by copper-catalyzed oxidative hydroxylation of arylboronic acids at room temperature in water.

A general and efficient procedure for the preparation of phenols was developed by copper-catalyzed oxidative hydroxylation of arylboronic acids at room temperature in water.

A Metal-Free Multicomponent Cascade Reaction for the Regiospecific Synthesis of 1,5-Disubstituted 1,2,3-Triazoles†

The thermal Huisgen 1,3-dipolar cycloaddition of organic azides and alkynes to build 1,2,3-triazoles usually requires elevated temperatures and provides a mixture of regioisomers. The copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) and ruthenium(II)-catalyzed azide–alkyne cycloaddition (RuAAC) have been developed as powerful strategies for the regiospecific assembly of 1,4-disubstituted 1,2,3-triazoles and 1,5-disubstituted 1,2,3-triazoles, respectively. These reliable processes have quickly found many applications in organic synthesis, chemical biology, and materials science. Nevertheless, the contamination of heavy metals limits their potential application in the pharmaceutical industry. Several metal-free methods, including the azide–enamine cycloaddition, the condensation of azides with phosphonium ylides, and the addition of acetylide species to azides, have also been developed for the regiospecific synthesis of 1,2,3-triazoles. Considerable drawbacks, however, exist with these processes, such as poor functional-group tolerance or requiring functionalized substrates. Westermann recently reported an improved Sakai reaction to synthesize 1,4-disubstituted triazoles from primary amines and a,a-dichlorotosylhydrazones (Scheme 1a). This procedure is limited to the synthesis of 1,4-disubstituted regioisomers. Therefore, a general and metal-free procedure to synthesize 1,5-disubstituted triazoles from readily available substrates under mild reaction conditions is still of considerable interest. Herein we report the synthesis of the 1,5disubstituted triazoles 5 from a three-component reaction of aliphatic amines (1), propynones (2), and TsN3 by a Michael addition/deacylative diazo transfer/cyclization sequence (Scheme 1b). Theoretically, the enaminone compounds 4 may exist as three tautomers: the iminoenol 4a, iminoketone 4b, and ketoenamine 4c. The a-diazo iminoketone 7 and 1-tosyl triazole 8 could be obtained from the Regitz diazo transfer of 4b and 1,3-dipolar cycloaddition of 4c with TsN3, respectively. We assumed that, in the presence of suitable bases, 4a might react with TsN3 to give the a-diazoimine intermediate 6, which could further cyclize to afford the triazole 5 (Scheme 2).

Preparation of Enantiopure Substituted Piperidines Containing 2-Alkene or 2-Alkyne Chains: Application to Total Syntheses of Natural Quinolizidine-Alkaloids

A general method for the preparation of enantiopure 2-alkene- or 2-alkyne-containing chain substituted piperidines was established by using nonracemic Betti base as a chiral auxiliary. The key step is that the auxiliary residue was removed by a novel base-catalyzed N-debenzylation via a formation of o-quinone methide mechanism in stead of the traditional hydrogenolysis, by which the alkene or alkyne groups survived. By this method, ten 2-alkene- or 2-alkyne-containing chain substituted piperidines were prepared on the gram scale within a few hours. To demonstrate the efficiency of the method and the versatility of the product, total syntheses of natural alkaloids (+)-pelletierine, (-)-lasubine II, and (+)-cermizine C were achieved by using (S)-2-allyl-N-Boc-piperidine as a versatile building block.

Base-Promoted N-Pyridylation of Heteroarenes Using N-Propargyl Enaminones as Equivalents of Pyridine Scaffolds

N-Pyridylation of various N-heteroarenes, including N-heteroarene-containing peptides, was achieved using N-propargyl enaminones (isolated or generated in situ from propargyl amine and propynones) as masked polysubstituted pyridine cores. This metal-free procedure proceeds under mild reaction conditions and generates 1 equiv of H2O as the sole byproduct.

Cleavage of the C–C triple bond of ketoalkynes: synthesis of 4(3H)-quinazolinones

A novel protocol for the synthesis of 4(3H)-quinazolinones via selective cleavage of the triple bond of ketoalkynes under oxidant-, metal-, and ligand-free conditions has been developed. Various 4(3H)-quinazolinones were obtained through fragmentation of the C–C triple bond and formation of two C–N bonds.

Efficient Approach to 4-Sulfonamidoquinolines via Copper(I)-Catalyzed Cascade Reaction of Sulfonyl Azides with Alkynyl Imines‡

A novel and efficient approach to 4-sulfonamidoquinolines via copper-catalyzed cascade reaction of sulfonyl azides with alkynyl imines has been developed in which a 1,3-dipole cycloaddition/ketenimine formation/6π-electrocyclization/[1,3]-H shift cascade reaction was involved. Various 4-sulfonamidoquinolines were afforded in up to 84% yield for 19 examples. This synthetic strategy features with atom economy, concise steps, easy operation, and mild reaction conditions.

Palladium-Catalyzed Oxidative Cross-Coupling of N-Tosylhydrazones with Indoles: Synthesis of N-Vinylindoles

A general and efficient palladium-catalyzed oxidative cross-coupling reaction of N-tosylhydrazones with indoles providing N-vinylindoles has been developed. The reaction proceeds smoothly with various indoles and N-tosylhydrazones in a stereocontrolled manner, and a wide variety of N-vinylindoles were obtained up to 99% yields for 26 examples.

Practical Pd(II)-Catalyzed C–H Alkylation with Epoxides: One-Step Syntheses of 3,4-Dihydroisocoumarins

Pd(II)-catalyzed ortho-alkylation of benzoic acids with both terminal and internal epoxides affords 3,4-dihydroisocoumarins in one step. The presence of potassium countercations is crucial for this reaction. Monoprotected amino acid ligands significantly promote this reaction, enabling the development of a practical C-H alkylation reaction using 0.5 mol % Pd catalyst. The inversion of stereochemistry in the C-H alkylation step is consistent with a redox-neutral SN2 nucleophilic ring-opening process as opposed to a Pd(II)/Pd(IV) pathway.

Total synthesis of (-)-cocaine and (-)-ferruginine.

Total synthesis of tropane alkaloids (-)-cocaine and (-)-ferruginine were accomplished in nine steps each and in 55% and 46% overall yields, respectively, starting from the known Betti base derivative (+)-(7aR,10R,12S)-10-(1H-benzotriazol-1-yl)-7a,8,9,10-tetrahydro-12-phenyl-12H-naphtho[1,2-e]pyrrolo[2,1-b][1,3]oxazine. In this novel route, RCM reaction and 1,3-dipolar cycloaddition were employed as key steps for the enantioselective construction of tropane skeleton and the regioselective introduction of 3-bromo-2-isoxazoline ring as masked cis-2,3-disubstituents. To obtain the desired precursor (2S,5R)-2-allyl-5-vinylpyrrolidine for RCM reaction, we developed a general and practical method for the preparation of enantiopure cis-2,5-disubstituted pyrrolidines bearing alkene- and/or alkyne-containing substituents. We also offered two highly efficient pathways for the conversion of the 3-bromo-2-isoxazoline ring into the desired cis-2,3-disubstituted groups in (-)-cocaine and (-)-ferruginine.

Ligand-Enabled meta-Selective C–H Arylation of Nosyl-Protected Phenethylamines, Benzylamines, and 2-Aryl Anilines

A Pd-catalyzed, meta-selective C-H arylation of nosyl-protected phenethylamines and benzylamines is disclosed using a combination of norbornene and pyridine-based ligands. Subjecting nosyl protected 2-aryl anilines to this protocol led to meta-C-H arylation at the remote aryl ring. A diverse range of aryl iodides are tolerated in this reaction, along with select heteroaryl iodides. Select aryl bromides bearing ortho-coordinating groups can also be utilized as effective coupling partners in this reaction. The use of pyridine ligands has allowed the palladium loading to be reduced to 2.5 mol %. Furthermore, a catalytic amount of 2-norbornene (20 mol %) to mediate this meta-C-H activation process is demonstrated for the first time. Utilization of a common protecting group as the directing group for meta-C-H activation of amines is an important feature of this reaction in terms of practical applications.