Divergent Functionalizations of Azetidines and Unsaturated Azetidines

Visible light-mediated aza Paternò-Büchi reaction of acyclic oximes and alkenes to azetidines

The aza Paternò-Büchi reaction is a [2+2]-cycloaddition reaction between imines and alkenes that produces azetidines, four-membered nitrogen-containing heterocycles. Currently, successful examples rely primarily on either intramolecular variants or cyclic imine equivalents. To unlock the full synthetic potential of aza Paternò-Büchi reactions, it is essential to extend the reaction to acyclic imine equivalents. Here, we report that matching of the frontier molecular orbital energies of alkenes with those of acyclic oximes enables visible light-mediated aza Paternò-Büchi reactions through triplet energy transfer catalysis. The utility of this reaction is further showcased in the synthesis of epi-penaresidin B. Density functional theory computations reveal that a competition between the desired [2+2]-cycloaddition and alkene dimerization determines the success of the reaction. Frontier orbital energy matching between the reactive components lowers transition-state energy (ΔGǂ) values and ultimately promotes reactivity.

Stereoselective polar radical crossover for the functionalization of strained-ring systems

Radical-polar crossover of organoborates is a poweful tool that enables the creation of two C-C bonds simultaneously. Small ring systems have become essential motifs in drug discovery and medicinal chemistry. However, step-economic methods for their selective functionalization remains scarce. Here we present a one-pot strategy that merges a simple preparation of strained organoboron species with the recently popularized polar radical crossover of borate derivatives to stereoselectively access tri-substituted azetidines, cyclobutanes and five-membered carbo- and heterocycles.

Fluorinated 2-Azetines: Synthesis, Applications, Biological Tests, and Development of a Catalytic Process

An efficient and straightforward phosphine-promoted tandem aza-Michael addition/intramolecular Wittig reaction was developed for the synthesis of polyfunctionalized 2-azetines. After demonstrating that this transformation could be made catalytic in phosphine through in situ reduction of phosphine oxide with phenylsilane, different post-transformation steps have been demonstrated, including an original [2 + 2] photodimerization. Preliminary biological tests highlighted that these fluorinated 1,2-dihydroazete-2,3-dicarboxylates exhibited significant cytotoxicity against the human tumor cell line.

New strategies for the synthesis of 1- and 2-azetines and their applications as value-added building blocks

Four-membered nitrogen-containing heterocycles are highly desirable functional groups with synthetic and biological applications. Unsaturated four-membered N-heterocycles, 1- and 2-azetines, are historically underexplored, but have recently been gaining interest due to the development of new synthetic methods to access these compounds, and to their potential as reactive intermediates. This review covers both the synthesis and applications of azetines, with a focus on synthetic methods to access azetines developed since 2018, and a comprehensive review of the reactivity and applications of azetines as starting materials or intermediates to access both other heterocycles and complex products.

Synthesis of Azetidine Nitrones and Exomethylene Oxazolines through a Copper(I)-Catalyzed 2,3-Rearrangement and 4π-Electrocyclization Cascade Strategy

A variety of azetidine nitrones are prepared in moderate to good yields through copper(I) combined with 2-aminopyridine to catalyze skeletal rearrangement of O-propargylic oximes. Mechanistic studies reveal that the reaction undergoes a copper(I)-catalyzed tandem [2,3]-rearrangement, 4π-electrocyclization, ring opening, and recyclization over four steps in one pot. Substituents at the terminus of alkyne and oxime moieties have a significant impact on the formation of azetidine nitrones and exomethylene oxazolines, respectively. Furthermore, the obtained azetidine nitrone could easily participate in [3 + 2] cycloaddition with alkynoates, and a [2.2]-paracyclophane-derived azetidine nitrone is synthesized in 45% yield over five steps from bromo[2.2]-paracyclophane.

Gold-Catalyzed Nucleophilic Ring-Opening Reaction of 2-Alkynylazetidines with Alcohols

The reaction of 2-alkynylazetidines and alcohols with a gold catalyst is described. A variety of substituted δ-amino-α,β-unsaturated ketones were synthesized via gold-promoted nucleophilic attack of alcohols followed by ring-opening of azetidine ring.

Ring-Expansion Reactions of Epoxy Amides and Enamides: Functionalized Azetidines, Dihydrofurans, Diazocanes, or Dioxa-3-azabicyclonon-4-enes?

Functionalized azetidines, 2,3-dihydrofurans, or the unorthodox dioxa-3-azabicyclonone-4-ene motifs are the products from transition metal-free reaction between N-oxiranylmethyl benzenesulfonamide and β-chloro-cinnamaldehyde, depending on whether one uses either NaI/K₂CO₃ or LiBr/K₂CO₃. These ring expansion reactions involve enamide (X-ray evidence) derived from N-oxiranylmethyl benzenesulfonamide and β-chloro-cinnamaldehyde as an intermediate. The N-oxiranylmethyl benzenesulfonamide itself upon heating gives readily separable and crystalline isomeric diazocanes that can be characterized by X-ray crystallography.

Strain-release arylations for the bis-functionalization of azetidines

The addition of nucleophilic organometallic species onto in situ generated azabicyclobutanes enables the selective formation of 3-arylated azetidine intermediates through strain-release. Single pot strategies were further developed for the N-arylation of resulting azetidines, employing either S_NAr reactions or Buchwald-Hartwig couplings.

Synthesis of dihydropyrroles from in situ-generated zwitterions via Rh2(adc)4/TBAI dual catalysis

A convenient rhodium/iodide dual catalysis strategy for the regioselective synthesis of dihydropyrroles was established.

Development of a Continuous Flow Synthesis of 2-Substituted Azetines and 3-Substituted Azetidines by Using a Common Synthetic Precursor

The generation and functionalization, under continuous flow conditions, of two different lithiated four-membered aza-heterocycles is reported. N-Boc-3-iodoazetidine acts as a common synthetic platform for the genesis of C3-lithiated azetidine and C2-lithiated azetine depending on the lithiation agent. Flow technology enables easy handling of such lithiated intermediates at much higher temperatures compared to batch processing. Flow technology combined with cyclopentylmethyl ether as an environmentally responsible solvent allows us to address sustainability concerns.

Recent strategies used in the synthesis of saturated four-membered heterocycles

The importance and prevalance of O-, N-, and S-atom containing saturated four-membered ring motifs in biologically active molecules and potential therapeutics continues to drive efforts in their efficient synthetic preparation. In this review, general and recent strategies for the synthesis of these heterocycles are presented. Due to the limited potential bond disconnections, retrosynthetic strategies are broadly limited to cyclizations and cycloadditions. Nonetheless, diverse approaches for accessing cyclization precursors have been developed, ranging from nucleophilic substitution to C-H functionalization. Innovative methods for substrate activation have been developed for cycloadditions under photochemical and thermal conditions. Advances in accessing oxetanes, azetidines, and thietanes remain active areas of research with continued breakthroughs anticipated to enable future applications.

N-tert-Butanesulfinyl imines in the asymmetric synthesis of nitrogen-containing heterocycles

The synthesis of nitrogen-containing heterocycles, including natural alkaloids and other compounds presenting different types of biological activities have proved to be successful employing chiral sulfinyl imines derived from tert-butanesulfinamide. These imines are versatile chiral auxiliaries and have been extensively used as eletrophiles in a wide range of reactions. The electron-withdrawing sulfinyl group facilitates the nucleophilic addition of organometallic compounds to the iminic carbon with high diastereoisomeric excess and the free amines obtained after an easy removal of the tert-butanesulfinyl group can be transformed into enantioenriched nitrogen-containing heterocycles. The goal of this review is to the highlight enantioselective syntheses of heterocycles involving the use of chiral N-tert-butanesulfinyl imines as reaction intermediates, including the synthesis of several natural products. The synthesis of nitrogen-containing heterocycles in which the nitrogen atom is not provided by the chiral imine will not be considered in this review. The sections are organized according to the size of the heterocycles. The present work will comprehensively cover the most pertinent contributions to this research area from 2012 to 2020. We regret in advance that some contributions are excluded in order to maintain a concise format.

Recent advances in the synthesis and reactivity of azetidines: strain-driven character of the four-membered heterocycle

Azetidines represent one of the most important four-membered heterocycles used in organic synthesis and medicinal chemistry. The reactivity of azetidines is driven by a considerable ring strain, while at the same the ring is significantly more stable than that of related aziridines, which translates into both facile handling and unique reactivity that can be triggered under appropriate reaction conditions. Recently, remarkable advances in the chemistry and reactivity of azetidines have been reported. In this review, we provide an overview of the synthesis, reactivity and application of azetidines that have been published in the last years with a focus on the most recent advances, trends and future directions. The review is organized by the methods of synthesis of azetidines and the reaction type used for functionalization of azetidines. Finally, recent examples of using azetidines as motifs in drug discovery, polymerization and chiral templates are discussed.

Electrophile dependent mechanisms in the asymmetric trapping of α-lithio-N-(tert-butoxythiocarbonyl)azetidine

Sn-Li exchange and 'poor man's Hoffmann tests' establish asymmetric trapping of α-lithio-N-(tert-butoxythiocarbonyl) (Botc) azetidine to be controlled by dynamic thermodynamic resolution or dynamic kinetic resolution, depending on the electrophile. Unusually, different configurational stability is seen for the anion generated by lithiation compared to transmetallation. Configurational stability of α-lithio-N-Boc azetidine indicates instability with the N-Botc system is due to the C[double bond, length as m-dash]S group.

Recent advances in the chemistry of bicyclo- and 1-azabicyclo[1.1.0]butanes

Bicyclo[1.1.0]- and 1-azabicyclo[1.1.0]butanes are structurally unique compounds that exhibit diverse chemistry. Bicyclo[1.1.0]butane is a four-membered carbocycle with a bridging C(1)-C(3) bond and 1-azabicyclo[1.1.0]butane is an analog of bicyclo[1.1.0]butane featuring a nitrogen atom at one bridgehead. These structures are highly strained, allowing them to participate in a range of strain-releasing reactions which typically cleave the central, strained bond to deliver cyclobutanes or azetidines. However, despite these molecules being discovered in the 1950s and 1960s, and possessing a myriad of alluring chemical features, the chemistry and applications of bicyclo[1.1.0]- and 1-azabicyclo[1.1.0]butanes remain underexplored. In the past 5 years, there has been a resurgent interest in their chemistry driven by the pharmaceutical industry’s increasing desire for new methods to access cyclobutanes and azetidines. This short review intends to provide a timely summary of the most recent developments in the chemistry of bicyclo[1.1.0]- and 1-azabicyclo[1.1.0]butane to highlight the diverse chemistry they can access, their value as synthetic precursors to cyclobutanes and azetidines, and to identify areas for future research.

Organocatalytic Synthesis of Highly Functionalized Heterocycles by Enantioselective aza-Morita-Baylis-Hillman-Type Domino Reactions

Organocatalytic enantioselective domino reactions are an extremely attractive methodology, as their use enables the construction of complex chiral skeletons from readily available starting materials in two or more steps by a single operation under mild reaction conditions. Thus, these reactions can save both the quantity of chemicals and length of time typically required for the isolation and/or purification of synthetic intermediates. Additionally, no metal contamination of the products occurs, given that organocatalysts include no expensive or toxic metals. The aza-Morita-Baylis-Hillman (aza-MBH) reaction is an atom-economical carbon-carbon bond-forming reaction between α,β-unsaturated carbonyl compounds and imines mediated by Lewis base (LB) catalysts, such as nucleophilic phosphines and amines. aza-MBH products are functionalized chiral β-amino acid derivatives that are highly valuable as pharmaceutical raw materials. Although various enantioselective aza-MBH processes have been investigated, very few studies of aza-MBH-type domino reactions have been reported due to the complexity of the aza-MBH process, which involves a Michael/Mannich/H-transfer/β-elimination sequence. Accordingly, in this review article, our recent efforts in the development of enantioselective domino reactions initiated by MBH processes are described. In the domino reactions, chiral organocatalysts bearing Brønsted acid (BA) and/or LB units impart synergistic activation to substrates, leading to the easy synthesis of highly functionalized heterocycles (some of which have tetrasubstituted and/or quaternary carbon stereocenters) in high yield and enantioselectivity.

Synthesis and Homologation of an Azetidin-2-yl Boronic Ester with α-Lithioalkyl Triisopropylbenzoates

An α-boryl azetidine, obtained by α-lithiation-borylation of N-Botc azetidine, undergoes reaction with α-triisopropylbenzoyloxy organolithiums to give homologated boronic esters that can be further oxidized, homologated, arylated, and deprotected to give a range of α-substituted azetidines. Scalemic α-boryl azetidine-α-triisopropylbenzoyloxy organolithium pairings show stereospecific reagent control, providing access to either diastereomeric series of homologated boronic esters with very high er's.