Ring Expansions of Activated Aziridines and Azetidines

Catalytic Aminium Radical-Cation Salt (Magic Blue)-Initiated SN2-Type Nucleophilic Ring-Opening Transformations of Aziridines

A simple and atom economic protocol for the construction of C-X/C-C bonds via catalytic aminium radical-cation salt (Magic Blue)-initiated SN2-type nucleophilic ring-opening transformations of racemic and nonracemic aziridines with different hetero and carbon nucleophiles to afford various amino ethers, thioethers, and amines in up to 99% yield, and with perfect enantiospecificity for some substrates but reduced ee with others (for nonracemic aziridines), is developed. This aminium radical-cation salt-initiated, SN2-type nucleophilic ring-opening strategy, along with various cyclization protocols, is employed to synthesize various biologically significant compounds.

Palladium-Catalyzed, Enantioselective Desymmetrization of N-Acylaziridines with Indoles

Ring opening reactions of meso-aziridines generate chiral amine derivatives where the control of stereochemistry is possible through enantioselective catalysis. We report the use of a diphosphine-palladium(II) catalyst for the highly enantioselective desymmetrization of N-acylaziridines with indoles. The β-tryptamine products are isolated in moderate to high yield across a range of indole and aziridine substitution patterns. The synthetic utility of β-tryptamine products is demonstrated by conversion to the brominated pyrroloindoline derivative.

Catalyst-Free Visible-Light-Mediated Iodoamination of Olefins and Synthetic Applications

Herein we report a catalyst- and metal-free visible-light-mediated protocol enabling the iodoamination of miscellaneous olefins. This protocol is characterized by high yields under environmentally benign reaction conditions utilizing commercially available substrates and a green and biodegradable solvent. Furthermore, the protocol allows for late-stage functionalization of bioactive molecules and can be scaled to gram quantities of product, which offers manifold possibilities for further transformations, including morpholine, piperidine, pyrrolidine, and aziridine synthesis.

Stereoselective Synthesis of Hexahydroimidazo[1,2-a]quinolines via SN2-Type Ring-Opening Hydroarylation-Hydroamination Cascade Cyclization of Activated Aziridines with N-Propargylanilines

A novel synthetic approach for the construction of 1,2,3,3a,4,5-hexahydroimidazo[1,2-a]quinolines in good yields (up to 75%) with excellent stereoselectivity (dr up to 94:6, ee up to >99%) under one-pot domino ring-opening cyclization (DROC) conditions has been developed. The DROC protocol proceeds through a Lewis acid catalyzed S_N2-type ring-opening of activated aziridines with N-propargylanilines followed by intramolecular cyclization comprising concomitant hydroarylation and hydroamination steps in a domino fashion.

Intermolecular [3+3] ring expansion of aziridines to dehydropiperi-dines through the intermediacy of aziridinium ylides

The importance of N-heterocycles in drugs has stimulated diverse methods for their efficient syntheses. Methods that introduce significant stereochemical complexity are attractive for identifying new bioactive amine chemical space. Here, we report a [3 + 3] ring expansion of bicyclic aziridines and rhodium-bound vinyl carbenes to form complex dehydropiperidines in a highly stereocontrolled rearrangement. Mechanistic studies and DFT computations indicate that the reaction proceeds through formation of a vinyl aziridinium ylide; this reactive intermediate undergoes a pseudo-[1,4]-sigmatropic rearrangement to directly furnish heterocyclic products with net retention at the new C-C bond. In combination with asymmetric silver-catalyzed aziridination, enantioenriched scaffolds with up to three contiguous stereocenters are rapidly delivered. The mild reaction conditions, functional group tolerance, and high stereospecificity of this method are well-suited for appending piperidine motifs to natural product and complex molecules. Ultimately, our work establishes the value of underutilized aziridinium ylides as key intermediates for converting small, strained rings to larger N-heterocycles.

Synthesis of Nonracemic 1,4-Benzoxazines via Ring Opening/Cyclization of Activated Aziridines with 2-Halophenols: Formal Synthesis of Levofloxacin

Novel 3,4-dihydro-1,4-benzoxazine derivatives have been synthesized by an efficient and simple method in excellent enantio- and diastereospecificity (ee > 99%, de > 99%). The reaction proceeds via Lewis acid-catalyzed S_N2-type ring opening of activated aziridines with 2-halophenols followed by Cu(I)-catalyzed intramolecular C-N cyclization in a stepwise fashion under one-pot conditions to furnish the 3,4-dihydro-1,4-benzoxazine derivatives in excellent yields (up to 95%). The strategy offers a short and efficient synthesis to ( S)-3-methyl-1,4-benzoxazine ( S)-3v, a late stage intermediate in the synthesis of levofloxacin.

Catalyst-Free Ring Opening of Spiroaziridine Oxindoles by Heteronucleophiles: An Approach to the Synthesis of Enantiopure 3-Substituted Oxindoles

A simple catalyst-free method was developed for the ring opening of spiroaziridine oxindoles by three different nucleophiles, namely, amines, thiols, and methanol, to produce enantiopure (up to 99%) vicinal diaminooxindoles, β-aminosulfides, and β-amino-3-methoxyoxindole, respectively, in good to excellent yields. In contrast to the spiroepoxides, spiroaziridines are opened regio- and stereospecifically through the pseudobenzylic spirocenter under catalyst-free conditions. Moreover, unlike simple 2-substituted aziridines, these spiroaziridines are opened up with retention in configuration at the C3-spirocenter.

Stereoselective synthesis of 3-spiropiperidino indoleninesviaSN2-type ring opening of activated aziridines with 1H-indoles/Pd-catalyzed spirocyclization with propargyl carbonates

A novel synthetic route to 3-spiropiperidino indoleninesviaLewis acid catalyzed S_N2-type ring opening of activated aziridines with 1H-indoles followed by Pd-catalyzed spirocyclization with propargyl carbonates in high yields (up to 88%) with excellent diastereo- and enantiospecificity (dr >99 : 1; ee up to >99) is reported.

A Stereoselective [3+1] Ring Expansion for the Synthesis of Highly Substituted Methylene Azetidines

The reaction of rhodium-bound carbenes with strained bicyclic methylene aziridines results in a formal [3+1] ring expansion to yield highly substituted methylene azetidines with excellent regio- and stereoselectivity. The reaction appears to proceed through an ylide-type mechanism, where the unique strain and structure of the methylene aziridine promotes a ring-opening/ring-closing cascade that efficiently transfers chirality from substrate to product. The resultant products can be elaborated into new azetidine scaffolds containing vicinal tertiary-quaternary and even quaternary-quaternary stereocenters.

Conformational-dependent reaction mechanism: case of acid hydrolysis ofN-benzoyl-9-azabicyclo[6.1.0]non-4-ene

Quantum chemical studies of the acid hydrolysis of cis-cis-N-benzoyl-9-azabicyclo[6.1.0]non-4-ene in gas phase and sulfuric acid solution have been carried out. Conformational analysis of reactant and intermediates were done at DFT level of calculations, using different basis sets. Transition states were identified and IRC calculations were done at the same level. In the first step of the reaction, the protonation occurs preferentially on the nitrogen atom. Three products are obtained. The type of mechanism, stepwise or concerted, depends on the stable conformers and the cleaved N–C bond. The main conclusion is that the products of the reaction depend on the nature of the stable conformer of the reactant.

Domino ring-opening cyclization (DROC) of activated aziridines and epoxides with nitrones via dual-catalysis “on water”

A simple and green approach for the synthesis of non-racemic 1,2,4-oxadiazinanes and 1,4,2-dioxazinanes with excellent yields (up to 99%) and stereoselectivity (de/ee > 99%) in water has been described.

Synthetic route to chiral indolines via Cu(OAc)2-catalyzed ring-opening/C(sp2)–H activation of activated aziridines

A novel synthetic route to functionalized indolinesviaLewis acid catalyzed ring-opening of activated aziridines followed by Cu(OAc)₂-mediated intramolecular C–H amination in one-pot with excellent enantio- and diastereospecificity (ee 99%; de >99%).

Stereospecific Synthesis of 2-Iminothiazolidines via Domino Ring-Opening Cyclization of Activated Aziridines with Aryl- and Alkyl Isothiocyanates

Lewis acid catalyzed domino ring-opening cyclization of activated aziridines with aryl and alkyl isothiocyanates has been accomplished leading to the formation of a wide variety of highly substituted and functionalized 2-iminothiazolidines with excellent diastereo- and enantiospecificity (de, ee up to >99%). The reaction proceeds via a Lewis acid catalyzed SN2-type ring-opening of the activated aziridine followed by a concomitant 5-exo-dig cyclization in a domino fashion to furnish the 2-iminothiazolidine derivative in excellent yields (up to 99%).