Metal-free domino Cloke-Wilson rearrangement-hydration-dimerization of cyclopropane carbaldehydes: A facile access to oxybis(2-aryltetrahydrofuran) derivatives

Copper-Catalyzed Asymmetric Cloke-Wilson Rearrangement

Herein we describe the first transition-metal-catalyzed asymmetric Cloke-Wilson rearrangement through unprecedented propargylic alkenoxylation reaction with enol as the O-nucleophile. A set of new chiral PPBOX ligands was prepared to guarantee the high enantioselectivity of the transformation. A series of polysubstituted dihydrofuran skeletons bearing an alkyne unit was prepared in good yield and high enantioselectivity under very mild reaction conditions, and various downstream transformations were facilely conducted to access different chiral skeletons.

Stereoselective Route to 2,3,4,5-Tetraaryltetrahydrofurans via Efficient Cascade Synthesis of 2,3-Dihydrofurans with Four Different (Hetero)Aryl Substituents

Tf2NH-mediated regio- and diastereoselective skeletal metamorphosis of 1-alkynyl-2,3-diaryl-2-methoxycarbonylcyclopropyl ketones readily gave 2,3,4,5-tetraaryl-2,3-dihydrofurans. Controllable reduction and removal of the ester group then precisely afforded four types of the eight possible tetrahydrofuran (THF) diastereomers with four different (hetero)aryl substituents. Further, deuterium-labeling results revealed an unprecedented hydrogen transfer from the formyl C-H bond in tBuOK-promoted decarbonylation under tBuOH-free conditions.

p-Toluenesulfonic acid-promoted organic transformations for the generation of molecular complexity

Since the beginning of this century, p-toluenesulfonic acid (p-TSA) catalysed organic transformations have been an active area of research for developing efficient synthetic methodologies. Often, catalysis using p-TSA is associated with many advantages, such as operational simplicity, high selectivity, excellent yields, and ease of product isolation, which make organic synthesis convenient and versatile. Notably, p-TSA is a non-toxic, commercially available, inexpensive solid organic compound that is soluble in water, alcohols, and other polar organic solvents. p-TSA is a strong acid compared to many protic or mineral acids and its high acidity helps activate different organic functional groups. p-TSA-promoted conversions are fast, have a high atom and pot economy, and feature a multiple bond-forming index. Therefore, the utilization of p-TSA enables the synthesis of many important structural scaffolds without any hazardous metals, making it desirable in numerous applications of sustainable and green chemistry. Recently, this emerging area of research has become one of the pillars of synthetic organic chemistry to synthesise biologically relevant, complex carbocycles and heterocycles. This study provides a comprehensive summary of methods, applications, and mechanistic insights into p-TSA-catalysed organic transformations, covering the literature reports that have appeared since 2012.

Cloke-Wilson rearrangement: a unique gateway to access five-membered heterocycles

Cyclopropanes are of great synthetic value in heterocyclic chemistry due to their highly reactive nature. They are widely employed to synthesize various biologically active organic compounds. Generally, vinyl, carbonyl, imine, and alkylidene cyclopropanes are utilized as efficient synthetic precursors in organic synthesis. The Cloke-Wilson rearrangement of these activated cyclopropanes is carried out to achieve the synthesis of diverse heterocyclic scaffolds. Various oxygen, nitrogen, and sulfur-containing heterocyclic compounds have been synthesized employing this rearrangement. With time, Cloke-Wilson rearrangement has evolved into a high yielding enantioselective and diastereoselective approach utilizing integrated novel methods. Our review focuses on the recent approaches for Cloke-Wilson rearrangement to synthesize several five-membered heterocycles and its applicability towards the natural product syntheses reported during 2000-2020.

Regio- and Stereospecific Hydrative Cloke-Wilson Rearrangement

The Cloke-Wilson rearrangement of unsymmetrical β-diketone-derived cyclopropanes inevitably yields a mixture of two 4-acylated 2,3-dihydrofuran regiomers. By using alkynes as masked acyls, Tf2NH-promoted Cloke-Wilson rearrangement of polysubstituted 1-(1-alkynyl)cyclopropyl ketones followed by alkyne hydration is described, regioselectively affording 2,3-dihydrofurans bearing 4-acyls nonequivalent to that involved in the Cloke-Wilson rearrangement. The 2,3-dihydrofuran rings with cis 2,3-diaryls are unexpectedly more stable than their trans diastereomers under the reaction conditions, guaranteeing the regiospecificity of this hydrative Cloke-Wilson rearrangement with high fidelity.

Organocatalytic (3+3)-cycloaddition of ortho-substituted phenyl nitrones with aryl cyclopropane carbaldehydes: a facile access to enantioenriched 1,2-oxazinanes

The first asymmetric (3+3)-cycloaddition of ortho-substituted phenyl nitrones with aryl cyclopropane carbaldehydes has been demonstrated by secondary amine catalysts. While the other ortho-substituents gave 1,2-oxazinanes, ortho-hydroxy ones provided a novel class of tetrahydrochromeno-1,2-oxazine cores via rare 1,3-aryl migration, followed by cyclization. An unusual type of asymmetric approach was also recognized.

Organocatalytic Activation of Donor-Acceptor Cyclopropanes: A Tandem (3 + 3)-Cycloaddition/Aryl Migration toward the Synthesis of Enantioenriched Tetrahydropyridazines

An organocatalytic enantioselective (3 + 3)-cycloaddition reaction of racemic cyclopropane carbaldehydes and aryl hydrazones has been demonstrated for the first time. A wide range of enantioenriched tetrahydropyridazines with an exocyclic double bond were obtained with moderate to good yields and good to excellent enantiomeric excesses. Mechanistic investigations hinted toward a matched/mismatched kinetic resolution, and control experiments and DFT calculations unveiled that 1,3-aryl migration was concerted and intramolecular and proceeds via a four-membered transition state.

Organocatalytic Cloke-Wilson Rearrangement: Carbocation-Initiated Tandem Ring Opening/Cyclization of Cyclopropanes under Neutral Conditions

We report a metal-, acid-, and base-free 2-(bromomethyl)naphthalene (2-BMN)-promoted organocatalytic Cloke-Wilson rearrangement of chain doubly activated cyclopropanes for the construction of 2,3-dihydrofurans via a carbocation-initiated tandem intramolecular ring-opening/recyclization process. The strategy is especially suitable for the construction of furan units in complex molecules, providing a solution to the problem of heavy-metal residues in dihydrofuran-containing drugs synthesized by traditional metal-based protocols. Thus, it is of potential interest in synthetic and medicinal chemistry.

Donor-Acceptor Cyclopropanes: Activation Enabled by a Single, Vinylogous Acceptor

A novel class of highly activated donor-acceptor cyclopropanes bearing only a single, vinylogous acceptor is presented. These strained moieties readily undergo cycloadditions with aldehydes, ketones, thioketones, nitriles, naphth-2-ols and various other substrates to yield the corresponding carbo- and heterocycles. Diastereocontrol can be achieved through the choice of catalyst (Brønsted or Lewis acid). The formation of tetrahydrofurans was shown to be highly enantiospecific when chiral cyclopropanes are employed. A series of mechanistic and kinetic experiments was conducted to elucidate a plausible catalytic cycle and to rationalize the stereochemical outcome.

Accessing Complex Tetrahydrofurobenzo-Pyran/Furan Scaffolds via Lewis-Acid Catalyzed Bicyclization of Cyclopropane Carbaldehydes with Quinone Methides/Esters

Herein, we report a straightforward one-pot synthesis of tetrahydrofurobenzopyran and tetrahydrofurobenzofuran systems via an in situ ring-expansion of the cyclopropane carbaldehydes followed by a [2 + n] cycloaddition with the quinone derivatives. The transformation not only unveils a new reaction mode of cyclopropane carbaldehydes with quinone methides/esters, but also promotes a step-efficient diastereoselective route to the sophisticatedly fused oxygen tricycles that can be further dehydrogenated to access the valued dihydro-2H-furo[2,3-b]chromene frameworks.