Catalytic Enantioselective Synthesis of α-Arylaminocyclobutanones

The Heyns Rearrangement: Synthetic Routes Beyond Carbohydrate Chemistry

This concept review describes the recent achievements on the Heyns rearrangement appeared in literature over the last decade and aims to provide the reader with a general overview of the fundamental synthetic advances in this research area.

Enantioselective α-Amination of Amides by One-Pot Organo-/Iodine Sequential Catalysis

An one-pot organo- and iodine sequential catalysis strategy for reactions of amides with pyrazole-based primary amines was described to synthesize chiral α-amino amides with a quaternary stereocenter. This methodology exhibited strong asymmetric induction, resulting in a typical enantiomeric excess value exceeding 99% and diastereoselectivity up to >99:1 dr. Moreover, the reaction was conducted without the use of any metals or strong bases.

Highly Enantioselective Brønsted Acid Catalyzed Heyns Rearrangement

Herein we report the first method for highly enantioselective Brønsted acid catalyzed Heyns rearrangements. These reactions, catalyzed by a chiral spiro phosphoric acid, afforded synthetically valuable chiral α-aryl-α-aminoketones which cannot be obtained by means of previously reported Heyns rearrangement methods. This method features low catalyst loadings, high yields and high enantioselectivities, making these reactions highly practical. We used the method to efficiently synthesize various chiral amines, including some biologically active molecules. We experimentally proved that these acid-catalyzed Heyns rearrangements proceeded via a proton-transfer process involving an enol intermediate and the stereocontrol was realized during the proton-transfer step.

Chiral Primary Amine Catalyzed Enantioselective Tandem Reactions Based on Heyns Rearrangement: Synthesis of α-Tertiary Amino Ketones

Herein, we disclose a new catalytic asymmetric tandem reaction based on the Heyns rearrangement for the synthesis of chiral α-amino ketones with readily available substrates. The rearrangement is different from the Heyns rearrangement in that the α-amino ketones were obtained without the shift of the carbonyl group. The key to success is using chiral primary amine as a catalyst by mimicking glucosamine-6-phosphate synthase in catalyzing the efficient Heyns rearrangement in organisms.

Preparation of Dihydronaphthofurans from α-Hydroxyl Ketones via a One-Pot Multicomponent Reaction Based on Heyns Rearrangement

Polysubstituted 1,2-dihydronaphthofurans were efficiently obtained in high yields and good diastereoselectivities with readily available substrates. The reaction proceeds smoothly via a series of tandem reactions, including Heyns rearrangement, oxidation, Friedel-Crafts reaction, and cyclization. The high stereoselectivity of the reaction is ascribed to the activation of the imine via an intramolecular hydrogen bond. Air is directly used as the oxidation medium, which makes the reaction safe and easy to perform. Moreover, the reaction features multiple components, which ensures the diversity of products.

A Brønsted acid catalyzed tandem reaction for the diastereoselective synthesis of cyclobuta-fused tetrahydroquinoline carboxylic esters

A novel Brønsted acid catalyzed tandem reaction provides highly functionalized cyclobuta-fused tetrahydroquinoline carboxylic esters from anilines and 2-alkylenecyclobutanones in good to high yield. During the reaction a dynamic diastereoselective cyclization is achieved, resulting in the formation of three contiguous stereocenters with high stereoselectivity.

Preparation and Application of α-Imino Ketones through One-Pot Tandem Reactions Based on Heyns Rearrangement

α-Imino ketone is a useful building block for the preparation of α-amino ketones and α-amino alcohols. However, its preparation has been seldomly seen. Herein, a metal-free and operationally simple strategy has been developed to generate α-imino ketones with high regioselectivity. Meanwhile, the method allowed for the preparation of various N,O-ketals with high regioselectivities and diastereoselectivities through cascade reactions in one pot.

Catalytic enantioselective synthesis of benzocyclobutenols and cyclobutanols via a sequential reduction/C-H functionalization

We report here a sequential enantioselective reduction/C-H functionalization to install contiguous stereogenic carbon centers of benzocyclobutenols and cyclobutanols. This strategy features a practical enantioselective reduction of a ketone and a diastereospecific iridium-catalyzed C-H silylation. Further transformations have been explored, including controllable regioselective ring-opening reactions. In addition, this strategy has been utilized for the synthesis of three natural products, phyllostoxin (proposed structure), grandisol and fragranol.

Catalytic asymmetric synthesis of N-substituted tetrahydroquinoxalines via regioselective Heyns rearrangement and stereoselective transfer hydrogenation in one pot

N-Substituted tetrahydroquinoxalines (37 examples) were step-economically obtained in good yield (<97%) and ee (<99%) with readily available substrates. The reaction proceeds through an interesting regioselective Heyns rearrangement/enantioselective transfer hydrogenation in one pot. The substrate scope and the reaction mechanism were systematically investigated.

N-Substituted tetrahydroquinoxalines were step-economically obtained in good yield and ee with readily available substrates.

Recent advances in the synthesis of α-amino ketones

Due to the importance of the amino ketone motif in synthetic and medicinal chemistry, the number of protocols developed in recent years has considerably increased. This review serves to collate and critically evaluate novel methodologies published since 2011 towards this high value synthon. The chapters are divided by the requisite functionality in the starting material, and an emphasis is placed on discussing functional group compatibility and resultant product substitution patterns. Throughout, applications to medicinal targets are highlighted and mechanistic details are presented, and we further provide a short outlook for future development and emerging potential within this area.

Diastereo- and Enantioselective CuH-Catalyzed Hydroamination of Strained Trisubstituted Alkenes

Amine-substituted cyclobutanes and cyclopropanes are important substructures in biologically active compounds. Moreover, many of the cycloalkane units bear multiple substituents and stereocenters. Therefore, synthetic methods that produce polysubstituted aminocyclobutanes and aminocyclopropanes in a highly diastereo- and enantioselective manner are of importance. Herein, we describe the diastereo- and enantioselective synthesis of various types of polysubstituted aminocyclobutanes and aminocyclopropanes through CuH-catalyzed hydroamination of 1-substituted cyclobutenes and cyclopropenes. These strained trisubstituted alkenes exhibit much higher reactivity compared to their unstrained analogues in the initial hydrocupration step of the reaction. Moreover, an interesting reversal of regioselectivity was observed in the hydroamination of 1-aryl-substituted cyclobutenes compared to the cyclopropene analogues. The origins of the enhanced reactivity of strained trisubstituted alkenes as well as the differences in the regio- and enantioselectivity between reactions with cyclobutenes and cyclopropenes were investigated computationally.

Pictet–Spengler reaction based on in situ generated α-amino iminium ions through the Heyns rearrangement

A useful tandem reaction via the Heyns rearrangement and Pictet–Spengler reaction was developed which ensured the synthesis of complex N-heteropolycycles containing tetrahydro-β-carboline with high yield (up to 96%) and dr (99 : 1).

Tandem Wittig Reaction-Ring Contraction of Cyclobutanes: A Route to Functionalized Cyclopropanecarbaldehydes

An original tandem reaction consisting of a Wittig reaction-ring contraction process between α-hydroxycyclobutanone and phosphonium ylides has been developed. Highly functionalized cyclopropanecarbaldehydes are obtained in good to high yield.

Synthesis of β-sulfinyl cyclobutane carboxylic amides via a formal α to β sulphoxide migration process

An original tandem reaction consisting of a thermal elimination-addition process was developed. Highly substituted β-sulfinyl cyclobutane carboxylic acid derivatives were obtained from isomeric α-sulfinyl derivatives in a single operation in good to high yields and with high trans diastereoselectivity.

Asymmetric synthesis of polysubstituted methylenecyclobutanes via catalytic [2+2] cycloaddition reactions of N-allenamides

A highly enantioselective [2+2] cycloaddition reaction of alkylidene malonates with the internal C[double bond, length as m-dash]C bond of N-allenamides was developed with a MgII/N,N'-dioxide complex as a catalyst. Various polysubstituted methylenecyclobutanes were afforded in good yields (up to 99%) and excellent enantioselectivities (up to 96% ee) under mild conditions. The utility of the donor-acceptor cyclobutane product was demonstrated as a masked 1,4-dipole in the formal [4+2] annulation reaction with a silyl enol ether.

Acid-catalyzed synthesis of functionalized arylthio cyclopropane carbaldehydes and ketones

A general strategy for the synthesis of arylthio cyclopropyl carbaldehydes and ketones via acid catalysed arylthiol addition/ring contraction reaction sequence has been exploited. The procedure led to a wide panel of cyclopropyl carbonyl compounds in high yields and broad substrate scope.

Acid-catalyzed reaction of 2-hydroxycyclobutanone with benzylic alcohols

The acid-promoted syntheses of 2-(benzyloxy)cyclobutanones and bis(benzyloxy)dioxatricyclo decanes were achieved starting from 2-hydroxycyclobutanone and variously functionalized benzyl alcohols. The reaction sequences afforded the desired products in good to high yields and in a solvent-dependent chemoselective fashion.

Synthesis of 2,2-bis(pyridin-2-yl amino)cyclobutanols and their conversion into 5-(pyridin-2-ylamino)dihydrofuran-2(3H)-ones

A two-step protocol is presented for the preparation of 5-(pyridin-2-ylamino)dihydrofuran-2(3H)-ones from 2-hydroxycyclobutanone and some 2-aminopyridines via a catalyst-free synthesis of 2,2-bis(pyridin-2-ylamino)cyclobutanols followed by Dess-Martin periodinane mediated ring expansion.

Cyclobutanone Analogues of β-Lactam Antibiotics: β-Lactamase Inhibitors with Untapped Potential?

β-Lactam antibiotics have been used for many years to treat bacterial infections. However the effective treatment of an increasing range of microbial infections is threatened by bacterial resistance to β-lactams: the prolonged, widespread (and at times reckless) use of these drugs has spawned widespread resistance, which renders them ineffective against many bacterial strains. The cyclobutanone ring system is isosteric with β-lactam: in cyclobutanone analogues, the eponymous cyclic amide is replaced with an all-carbon ring, the amide N is substituted by a tertiary C-H α to a ketone. Cyclobutanone analogues of various β-lactam antibiotics have been investigated over the last 35 years, initially as prospective antibiotics in their own right and inhibitors of the β-lactamase enzymes that impart resistance to β-lactams. More recently they have been tested as inhibitors of other serine proteases and as mechanistic probes of β-lactam biosynthesis. Cyclobutanone analogues of the penam ring system are the first reversible inhibitors with moderate activity against all classes of β-lactamase; other compounds from this family inhibit Streptomyces R61 dd-carboxypeptidase/transpeptidase, human neutrophil elastase and porcine pancreatic elastase. But has their potential as enzyme inhibitors been fully exploited? Challenges in synthesising diversely functionalised cyclobutanone derivatives mean that only a limited number have been made (with limited structural diversity) and evaluated. This review surveys the different synthetic approaches that have been taken to these compounds, the investigations made to evaluate their biological activity and prospects for future developments in this area.

Deracemizing organocatalyzed Michael addition reactions of 2-(arylthio)cyclobutanones with β-nitrostyrenes

Organocatalyzed Michael addition reactions of 2-(arylthio)cyclobutanones with trans-β-nitrostyrenes have been carried out using a bifunctional thiourea-primary amine catalyst, providing diastereoisomerically and enantiomerically enriched 2-alkyl-2-(arylthio)cyclobutanones having two contiguous stereocenters of which one is a chiral quaternary center. The absolute configuration of these novel adducts was assigned by X-ray diffraction analysis and a transition-state model is proposed to explain the observed stereoselectivities.

A Nucleophilic Strategy for Enantioselective Intermolecular α-Amination: Access to Enantioenriched α-Arylamino Ketones

The enantioselective addition of anilines to azoalkenes was accomplished through the use of a chiral phosphoric acid catalyst. The resulting α-arylamino hydrazones were obtained in good yields and excellent enantioselectivities and provide access to enantioenriched α-arylamino ketones. A serendipitous kinetic resolution of racemic α-arylamino hydrazones is also described.

Disulfide-based metal-free α-sulfanylation of ketones

An eco-friendly methodology for the direct α-sulfanylation of ketones has been developed by using diaryl disulfides as sulfanyl source affording ketosulfides in good to excellent yields and high selectivity.

Synthesis of chiral cyclobutanes via rhodium/diene-catalyzed asymmetric 1,4-addition: a dramatic ligand effect on the diastereoselectivity

A highly diastereo- and enantioselective rhodium-catalyzed arylation of cyclobutene-1-carboxylate esters for the efficient synthesis of chiral cyclobutanes has been developed.

Synthesis of functionalized tryptamines by Brønsted acid catalysed cascade reactions

A new solvent-free Brønsted acid catalysed cascade reaction has been established, allowing access to highly substituted tryptamines under mild conditions.