Synthesis of Nitrogen-Containing Heterocycles through Catalytic Dehydrative Cyclization Reactions

CuI-Catalyzed Radical Reaction of Benzimidates to Form Valuable 4,5-Dihydrooxazoles through Regioselective Aerobic Oxidative Cross-Coupling

A straightforward Cu(I)-catalyzed oxidative cross-coupled organic transformation has been developed under mild conditions for the construction of functionalized 4,5-dihydrooxazoles through a four-bond-forming regiocontrolled C-C/C-N/C-O coupling strategy emerging benzimidates, paraformaldehyde, and 1,3-diketo analogues using eco-friendly O2 as the sole oxidant. The fundamental features of these designed approaches involve operational simplicity, selectivity, generality, and a broad substrate scope with high yields under the same reaction conditions.

Oxytrofalcatin Puzzle: Total Synthesis and Structural Revision of Oxytrofalcatins B and C

The previously reported structures of oxytrofalcatins B and C possess a benzoyl indole core. However, following synthesis and NMR comparison of both the proposed structure and the synthesized oxazole, we have revised the structure of oxytrofalcatins B and C as oxazoles. The synthetic route developed herein can further our understanding of the biosynthetic pathways that govern the production of natural 2,5-diaryloxazoles.

A Facile Synthesis of 2-Oxazolines via Dehydrative Cyclization Promoted by Triflic Acid

2-oxazolines are common moieties in numerous natural products, pharmaceuticals, and functional copolymers. Current methods for synthesizing 2-oxazolines mainly rely on stoichiometric dehydration agents or catalytic dehydration promoted by specific catalysts. These conditions either generate stoichiometric amounts of waste or require forcing azeotropic reflux conditions. As such, a practical and robust method that promotes dehydrative cyclization while generating no byproducts would be attractive to oxazoline production. Herein, we report a triflic acid (TfOH)-promoted dehydrative cyclization of N-(2-hydroxyethyl)amides for synthesizing 2-oxazolines. This reaction tolerates various functional groups and generates water as the only byproduct. This method affords oxazoline with inversion of α-hydroxyl stereochemistry, suggesting that alcohol is activated as a leaving group under these conditions. Furthermore, the one-pot synthesis protocol of 2-oxazolines directly from carboxylic acids and amino alcohols is also provided.

One-pot double annulations to confer diastereoselective spirooxindolepyrrolothiazoles

A novel four-component reaction in one pot as an atom- and step-economic process was developed to synthesize diastereoselectively spirooxindolepyrrolothiazoles through sequential N,S-acetalation of aldehydes with cysteine and decarboxylative [3 + 2] cycloaddition with olefinic oxindoles. High synthetic efficiency, operational simplification and reaction process economy using EtOH as solvent, and only releasing CO2 and H2O as side products confer this approach favorable in green chemistry metrics analysis.

Direct activation of alcohols via perrhenate ester formation for an intramolecular dehydrative Friedel–Crafts reaction

A general and highly efficient intramolecular dehydrative Friedel–Crafts reactions via Re2O7 mediated hydroxyl group activation is described for the syntheses of tetrahydronaphthalene, tetrahydroquinoline, tetrahydroisoquinoline, chromane, and isochromane derivatives.

Solvent Effects and Mechanistic Studies for Re2O7-Catalyzed Allylative Annulation Reactions

The sequence of allylic alcohol transposition, carbonyl group trapping, oxocarbenium ion formation, and nucleophilic addition results in the formation of a ring while serving as a fragment-coupling and stereocenter-generating reaction. Successful applications of these processes require a balancing of the kinetics of numerous productive and unproductive steps. This work describes the manner in which solvent changes can be used to expand the scope and change the stereochemical outcomes of these processes. Mechanistic studies provide greater insight into the nuances of the transformations and the reactive species that are generated.

Mechanochemical Rearrangements

Molecular rearrangements are a powerful tool for constructing complex structures in an atom- and step-economic manner, translating multistep transformations into an intrinsically more sustainable process. Mechanochemical molecular rearrangements become an even more appealing eco-friendly synthetic approach, especially for preparing active pharmaceutical ingredients (APIs) and natural products. Still in their infancy, rearrangements promoted by mechanochemistry represent a promising approach for chemists to merge molecular diversity and green chemistry perspectives toward more selective and efficient syntheses with a reduced environmental footprint.

One-Pot and Two-Chamber Methodologies for Using Acetylene Surrogates in the Synthesis of Pyridazines and Their D-Labeled Derivatives

Acetylene surrogates are efficient tools in modern organic chemistry with largely unexplored potential in the construction of heterocyclic cores. Two novel synthetic paths to 3,6-disubstituted pyridazines were proposed using readily available acetylene surrogates through flexible C₂ unit installation procedures in a common reaction space mode (one-pot) and distributed reaction space mode (two-chamber): (1) an interaction of 1,2,4,5-tetrazine and its acceptor-functionalized derivatives with a CaC₂ -H₂ O mixture performed in a two-chamber reactor led to the corresponding pyridazines in quantitative yields; (2) [4+2] cycloaddition of 1,2,4,5-tetrazines to benzyl vinyl ether can be considered a universal synthetic path to a wide range of pyridazines. Replacing water with D₂ O and vinyl ether with its trideuterated analog in the developed procedures, a range of 4,5-dideuteropyridazines of 95-99% deuteration degree was synthesized for the first time. Quantum chemical modeling allowed to quantify the substituent effect in both synthetic pathways.

Perrhenate Esters as Intermediates in Molecular Complexity-Increasing Reactions

Allylic alcohols form perrhenate esters upon reaction with Re2O7 or HOReO3. These species undergo nonstereospecific and nonregiospecific alcohol-transposition reactions through cationic intermediates. Sequencing these nonselective processes with reversible trapping by electrophiles results in cyclization reactions where regio- and stereocontrol are dictated by thermodynamics. The cationic intermediates can also be utilized as electrophiles in intra- or intermolecular dehydrative reactions with nucleophiles. These processes serve as the basis for applications in catalytic syntheses of a wide range of heterocyclic and carbocyclic structures that often show considerable increases in molecular complexity. This Account describes a sequence of events that started from a need to solve a problem for the completion of a natural product synthesis and evolved into a central element in the design of numerous new transformations that proceed under mild conditions from readily accessible substrates.

1 Introduction

2 Exploratory Studies

3 Application to Spiroketal Synthesis

4 Reactions with Epoxides as Trapping Agents

5 Development of Dehydrative Cyclizations

6 Bimolecular Reactions

7 Spirocyclic Ether Formation

8 Conclusions