Synthesis and lithiation of oxazolinylaziridines: the N-substituent effect

Recent updates and future perspectives in aziridine synthesis and reactivity

In this review, selected recent advances in the preparation and reactivity of aziridines using modern synthetic approaches are highlighted, while comparing these new strategies with more classical approaches. This critical analysis is designed to help identify current gaps in the field and is showcasing new and exciting opportunities to move the chemistry of aziridines forward in the future.

Aza-Quasi-Favorskii Reaction: Construction of Highly Substituted Aziridines through a Concerted Multibond Rearrangement Process

A new molecular rearrangement, the aza-Quasi-Favorskii rearrangement, has been developed for the construction of highly substituted aziridines. Electron-deficient O-sulfonyl oximes react readily with α,α-disubstituted acetophenone-derived enolates to furnish highly substituted aziridines via this unprecedented domino process. In-depth computational studies reveal an asynchronous yet concerted nitrenoid-type rearrangement pathway.

Stereo- and Enantioselective Addition of Organolithiums to 2-Oxazolinylazetidines as a Synthetic Route to 2-Acylazetidines

A new synthetic route to N-alkyl-2-acylazetidines was developed through a highly stereoselective addition of organolithiums to N-alkyl-2-oxazolinylazetidines followed by acidic hydrolysis of the resulting oxazolidine intermediates. This study revealed an unusual reactivity of the C=N bond of the oxazoline group when reacted with organolithiums in a non-polar solvent such as toluene. The observed reactivity has been explained considering the role of the nitrogen lone pair of the azetidine ring as well as of the oxazolinyl group in promoting a complexation of the organolithium, thus ending up with the addition to the C=N double bond. The high level of stereoselectivity in this addition is supported by DFT calculations and NMR investigations, and a model is proposed for the formation of the oxazolidine intermediates, that have been isolated and fully characterized. Upon acidic conditions, the oxazolidine moieties were readily converted into 2-acylazetidines. This synthetic approach has been applied for the preparation of highly enantioenriched 2-acylazetidines starting from chiral not racemic N-alkyl-2-oxazolinylazetidines.

Synthesis of allyl-aziridines from α-halo oxime ethers and allyl zinc bromides

A novel method for the preparation of allyl aziridines by reacting α-halo oxime ethers with allylic zinc reagents under mild conditions. Some advantages of using organozinc reagents are that they are easily prepared, non-toxic, and more selective than Grignard reagents.

Lithiated oxazolinyloxiranes and oxazolinylaziridines: key players in organic synthesis

This paper focuses on α-lithiated oxazolinyloxiranes and oxazolinylaziridines, their generation, reactions, and synthetic applications. The ability of the oxazoline ring in providing stabilization to such α-heterosubstituted carbanions either through electronic effects and coordinative action has been stressed as well as the contribution to the configurational stability or instability of such species. IR spectroscopic data, multinuclear NMR investigations, and ab initio calculations planned to get insights on chemical properties and structural features have been carried out. A number of new reactions including alkylations, addition reactions, hydroxyalkylations, cyclopropanations, lactonizations, and rearrangements have been discovered, giving access to a variety of substances including: aziridinolactones, epoxylactones, aryl alkanones, polysubstituted cyclopropanes, cyclopropanefused lactones, dihydro-oxazoloisoquinolines, diversely functionalized oxazolines, and products that can be derived from them by synthetic elaboration.

α,β-Aziridinylphosphonates by lithium amide-induced phosphonyl migration from nitrogen to carbon in terminal aziridines

N-Phosphonate terminal aziridines undergo lithium 2,2,6,6-tetramethylpiperidide-induced N- to C-[1,2]-anionic phosphonyl group migration under experimentally straightforward conditions, to provide a stereocontrolled access to synthetically valuable trans-α,β-aziridinylphosphonates. The utility of this chemistry has been demonstrated in the asymmetric synthesis of a β-aminophosphonate.

Synthesis of optically active arylaziridines by regio- and stereospecific lithiation of N-bus-phenylaziridine

Alpha,alpha-disubstituted aziridines can be produced in good yields by selective lithiation of N-tert-butylsulfonyl-2-phenylaziridine (n-BuLi/TMEDA, Et(2)O) at the benzylic position and subsequent trapping with a range of electrophiles. Repetition of the lithiation/electrophilic trapping sequence provides a stereocontrolled route to trisubstituted aziridines. Using (R)-N-tert-butylsulfonyl-2-phenylaziridine, the alpha,alpha-disubstituted aziridines can be produced as single enantiomers (er >98:2), indicating that the intermediate organolithium is configurationally stable. Efficient aziridine ring-opening reactions leading to 1,2-diamines and 1,4-diamines are also reported.

Terminal aziridines by alpha-deprotonation/electrophile trapping of N-protected aziridine

N-tert-Butylsulfonyl and N-tert-butylsulfinyl aziridine undergo alpha-lithiation/electrophile trapping providing a new entry to terminal aziridines. With N-tert-butylsulfinyl aziridine complete asymmetric induction is observed alpha to nitrogen.

Regio- and Stereoselective Lithiation of Terminal Oxazolinylaziridines: The Aziridine N-Substituent and the Oxazolinyl Group Effect

The regioselective lithiation of terminal oxazolinylaziridines has been investigated. The steric hindrance of the nitrogen substituent in 1-trityl-2-oxazolinylaziridine 3a, combined with the coordinating ability of the oxazolinyl group, causes beta-lithiation, whereas a completely regioselective alpha-lithiation is observed with the much less sterically demanding 1-benzyl-2-oxazolinylaziridine 3c and a competition between alpha- and beta-lithiation occurs with 1-cumyl-2-oxazolinylaziridine 3b in which the N-substituent has a steric hindrance in between the trityl and the benzyl groups. The application of the lithiation-trapping sequence for the preparation of enantioenriched 2,3-cis-disubstituted oxazolinylaziridines and aziridino-gamma-lactones is also reported.

Regio- and Stereoselective Lithiation and Electrophilic Substitution Reactions of N-Alkyl-2,3-diphenylaziridines: Solvent Effect

[structure: see text]. The lithiation reaction of cis- and trans-N-alkyl-2,3-diphenylaziridines has been investigated. While cis-diphenylaziridines do not undergo any lithiation upon treatment with organolithiums, the lithiation reaction of the trans counterparts is completely alpha-regioselective and the stereochemical course of the lithiation-trapping sequence is solvent dependent: inversion of configuration in coordinating solvents (THF or toluene/crown ether) and retention in hexane, ether, or toluene. The preparation of stereodefined functionalized N-alkyl-2,3-diphenylaziridines is described.

Intramolecular Cyclopropanation of Unsaturated Terminal Aziridines

Regio- and stereoselective deprotonation of bishomoallylic terminal N-Bus (Bus=tert-butylsulfonyl)-protected aziridines generate aziridinyl anions that undergo diastereoselective intramolecular cyclopropanation giving trans-2-aminobicyclo[3.1.0]hexanes in good to excellent yields.

Directed Ortho Lithiation of N-Alkylphenylaziridines

The ortho lithiation-trapping sequence of phenylaziridines is described. This methodology, which counts on the ability of the aziridino group to act as a directed metalation group (DMG), provides an easy access to functionalized arylaziridines as well as to phthalans and phthalides. The importance of the aziridine N-substituent in this DoM reaction is stressed as well. [reaction: see text]