A novel synthesis of 5-functionalized oxazolidin-2-ones from enantiomerically pure 2-substituted N-[(R)-(+)-alpha-methylbenzyl]aziridines

Accessing Rare α-Heterocyclic Aziridines via Brønsted Acid-catalyzed Michael Addition/Annulation: Scope, Limitations, and Mechanism

We report an approach to the diastereoselective synthesis of 1,2-disubstituted heterocyclic aziridines. A Brønsted acid-catalyzed conjugate addition of anilines to trisubstituted heterocyclic chloroalkenes provides an intermediate 1,2-chloroamine. Diastereocontrol was found to vary significantly with solvent selection, with computational modelling confirming selective, spontaneous fragmentation in the presence of trace acids, proceeding through a pseudo-cyclic, protonated intermediate and transition state. These chloroamines can then be converted to the aziridine by treatment with LiHMDS with high stereochemical fidelity. This solvent-induced stereochemical enrichment thereby enables an efficient route to rare cis-aziridines with high dr. The scope, limitations, and mechanistic origins of selectivity are also presented.

Expedient metal-free preparation of aryl aziridines via thermal cycloaddition reactions

A straightforward synthesis of aryl aziridines is reported from readily available azides and alkenes and using technical solvents in the presence of air. This methodology does not require any additives and the obtained compounds can be employed in ring-opening and ring-expansion reactions.

Alkylative Aziridine Ring-Opening Reactions

In this study, the highly strained three-membered aziridine ring was successfully activated as the aziridinium ion by alkylation of the ring nitrogen with a methyl, ethyl or allyl group, which was followed by ring opening with external nucleophiles such as acetate and azide. Such alkylative aziridine ring opening provides an easy route for the synthesis of various N-alkylated amine-containing molecules with concomitant introduction of an external nucleophile at either its α- or β-position.

Ring-opening functionalizations of unstrained cyclic amines enabled by difluorocarbene transfer

Chemical synthesis based on the skeletal variation has been prolifically utilized as an attractive approach for modification of molecular properties. Given the ubiquity of unstrained cyclic amines, the ability to directly alter such motifs would grant an efficient platform to access unique chemical space. Here, we report a highly efficient and practical strategy that enables the selective ring-opening functionalization of unstrained cyclic amines. The use of difluorocarbene leads to a wide variety of multifaceted acyclic architectures, which can be further diversified to a range of distinctive homologative cyclic scaffolds. The virtue of this deconstructive strategy is demonstrated by successful modification of several natural products and pharmaceutical analogues.

Acid-Mediated Ring Expansion of 2,2-Disubstituted Azetidine Carbamates to 6,6-Disubstituted 1,3-Oxazinan-2-ones

The ring expansion of 2-ester-2-arylazetidine carbamates can be achieved using Brønsted acids to form 6,6-disubstituted 1,3-oxazinan-2-ones. The reaction is rapid at room temperature with Boc or Cbz derivatives and proceeds with excellent yield (up to 96%) and broad substrate scope. Derivatives of drug compounds and natural products are incorporated. The combination of this ring expansion in a three-step N-H insertion/cyclization/expansion sequence is applied to directly access medicinally relevant scaffolds from acyclic precursors.

Catalytic, Enantioselective Synthesis of Cyclic Carbamates from Dialkyl Amines by CO2-Capture: Discovery, Development, and Mechanism

Cyclic carbamates are a common feature of small-molecule therapeutics, offering a constrained hydrogen bond acceptor that is both polar and sterically small. Methods for their preparation most often focus first on amino alcohol synthesis and then reaction with phosgene or its equivalent. This report describes an enantioselective synthesis of cyclic carbamates in which carbon dioxide engages an unsaturated basic amine, facilitated by a bifunctional organocatalyst designed to stabilize a carbamic acid intermediate while activating it toward subsequent enantioselective carbon-oxygen bond formation. Six-membered cyclic carbamates are prepared in good yield with high levels of enantioselection, as constrained 1,3-amino alcohols featuring a chiral tertiary alcohol carbon. Spectroscopic analysis (NMR, DOSY) of various substrate-reagent combinations provides insight into the dominant species under the reaction conditions. Two peculiar requirements were identified to achieve highest consistency: a "Goldilocks" amount of water and the use of a noncrystalline form of the ligand. These atypical features of the final protocol notwithstanding, a diverse range of products could be prepared. Their functionalizations illustrate the versatility of the carbamates as precursors to enantioenriched small molecules.

Atom-Economical and Metal-Free Synthesis of Multisubstituted Furans from Intramolecular Aziridine Ring Opening

Multisubstituted furans were prepared from dialkyl 2-(aziridin-2-ylmethylene)malonate and/or 1,3-dione through aziridine ring opening by the internal carbonyl oxygen with the assistance of BF3·OEt2, followed by aromatization. This synthetic method is free from any metal and is atom-economical with all of the atoms in the starting material retained in the final product.

Selective Ring-Opening of N-Alkyl Pyrrolidines with Chloroformates to 4-Chlorobutyl Carbamates

Our study shows that among aza-heterocycles of various ring sizes, including aziridines, azetidines, pyrrolidines, and piperidines, only N-alkyl pyrrolidines undergo competitive reaction pathways with chloroformates to yield N-dealkylated pyrrolidines and 4-chlorobutyl carbamates. The pathway taken depends on the substituent on the nitrogen, i.e., ring-opening with methyl and ethyl substituents and dealkylation with a benzyl substituent. Computational calculations support the substituent-dependent product formation by showing the energy difference between the transition states of both reaction pathways. Selective ring-opening reactions of N-methyl and N-ethyl pyrrolidine derivatives with chloroformates were utilized to prepare various 4-chlorobutyl carbamate derivatives as valuable 1,4-bifunctional compounds.

A computational study to unravel the selectivity in an iron-catalysed [3 + 2] cycloaddition of aziridine and heterocumulenes

The reaction mechanism of cycloaddition between phenyl aziridine and heterocumulene catalysed by iron salts in water has been modeled computationally to trace the origin of excellent regioselectivity toward 5-substituted product formation.

Regioselectivity in the ring opening of non-activated aziridines

In this critical review, the ring opening of non-activated 2-substituted aziridines via intermediate aziridinium salts will be dealt with. Emphasis will be put on the relationship between the observed regioselectivity and inherent structural features such as the nature of the C2 aziridine substituent and the nature of the electrophile and the nucleophile. This overview should allow chemists to gain insight into the factors governing the regioselectivity in aziridinium ring openings (81 references).

Opposite regiospecific ring opening of 2-(cyanomethyl)aziridines by hydrogen bromide and benzyl bromide: experimental study and theoretical rationalization

Ring opening of 1-arylmethyl-2-(cyanomethyl)aziridines with HBr afforded 3-(arylmethyl)amino-4-bromobutyronitriles via regiospecific ring opening at the unsubstituted aziridine carbon. Previous experimental and theoretical reports show treatment of the same compounds with benzyl bromide to furnish 4-amino-3-bromobutanenitriles through ring opening at the substituted aziridine carbon. To gain insights into the regioselective preference with HBr, reaction paths have been analyzed with computational methods. The effect of solvation was taken into account by the use of explicit solvent molecules. Geometries were determined at the B3LYP/6-31++G(d,p) level of theory, and a Grimme-type correction term was included for long-range dispersion interactions; relative energies were refined with the meta-hybrid MPW1B95 functional. Activation barriers confirm preference for ring opening at the unsubstituted ring carbon for HBr. HBr versus benzyl bromide ring opening was analyzed through comparison of the electronic structure of corresponding aziridinium intermediates. Although the electrostatic picture fails to explain the opposite regiospecific nature of the reaction, frontier molecular orbital analysis of LUMOs and nucleophilic Fukui functions show a clear preference of attack for the substituted aziridine carbon in the benzyl bromide case and for the unsubstituted aziridine carbon in the HBr case, successfully rationalizing the experimentally observed regioselectivity.

Nucleophile-dependent regioselective ring opening of 2-substituted N,N-dibenzylaziridinium ions: bromide versus hydride

The ring opening of 2-substituted N,N-dibenzylaziridinium ions by bromide exclusively occurs at the substituted aziridine carbon atom in a stereospecific way, whereas the opposite regioselectivity was observed for hydride-induced ring opening at the unsubstituted position; furthermore, this unprecedented hydride-promoted reactivity was validated by means of Density Functional Theory (DFT) calculations.

Unprecedented carbon-carbon bond cleavage in nucleophilic aziridine ring opening reaction, efficient ring transformation of aziridines to imidazolidin-4-ones

N-Styryl-3-aryl-1-methylaziridine-2-carboxamides, which were readily obtained from the cross coupling reaction between 3-aryl-1-methylaziridine-2-carboxamides and 1-aryl-2-bromoethenes catalyzed by CuI/N,N-dimethylglycine in the presence of Cs(2)CO(3), underwent a base-mediated intramolecular nucleophilic aziridine ring opening reaction effectively via the carbon-carbon bond cleavage of aziridine to afford the ring expanded imidazolidin-4-one products in good yields.

The preparation of stable aziridinium ions and their ring-openings

The reaction of enantiomerically pure 2-substituted 1-phenylethyl-aziridine with methyl trifluoromethanesulfonate generated a stable methylaziridinium ion, which was reacted with various external nucleophiles, including nitrile, to yield synthetically valuable and optically pure acyclic amine derivatives in a completely regio- and stereoselective manner.

Regioselective and stereospecific synthesis of enantiopure 1,3-oxazolidin-2-ones by intramolecular ring opening of 2-(Boc-aminomethyl)aziridines. Preparation of the antibiotic linezolid

The amide moiety of several enantiopure unactivated 1-aryl- or 1-alkylaziridine-2-carboxamides were reduced and then N-Boc-protected to afford enantiopure 2-(Boc-aminomethyl)aziridines, which were further converted into enantiopure 5-(aminomethyl)-1,3-oxazolidin-2-ones by means of a stereospecific and fully regioselective BF3.Et2O-promoted intramolecular nucleophilic ring opening. One of these oxazolidinones was transformed into the antibiotic linezolid through a CuI-catalyzed N-arylation reaction at its carbamate moiety.

Reaction of azetidines with chloroformates

The reaction of an azetidine with a chloroformate can give either the dealkylated heterocycle or the ring-opened product (gamma-chloroamine), which can further cyclize to the oxazinanone. A general study of this underrated reaction was conducted and revealed that azetidines can undergo smooth nucleophilic ring-opening reactions to highly functionalized gamma-chloroamines in the presence of a variety of alkyl chloroformates under mild reaction conditions. Yields are usually good, and parameters governing this reaction were evaluated. [reaction: see text].