Gasphasen-Reaktionen, 611) Cycloalkanimine 2H-Azirin, 1-Azetin, 1-Pyrrolin, 1-Piperidein und 3,4,5,6-Tetrahydro-2H-azepin: Darstellung und Photoelektronen-Spektren2,3)

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.

1- and 2-Azetines via Visible Light-Mediated [2 + 2]-Cycloadditions of Alkynes and Oximes

Azetines, four-membered unsaturated nitrogen-containing heterocycles, hold great potential for drug design and development but remain underexplored due to challenges associated with their synthesis. We report an efficient, visible light-mediated approach toward 1- and 2-azetines relying on alkynes and the unique triplet state reactivity of oximes, specifically 2-isoxazolines. While 2-azetine products are accessible upon intermolecular [2 + 2]-cycloaddition via triplet energy transfer from a commercially available iridium photocatalyst, the selective formation of 1-azetines proceeds upon a second, consecutive, energy transfer process. Mechanistic studies are consistent with a stepwise reaction mechanism via N-O bond homolysis following the second energy transfer event to result in the formation of 1-azetine products. Characteristic for this method is its operational simplicity, mild conditions, and modular approach that allow for the synthesis of functionalized azetines and tetrahydrofurans (via in situ hydrolysis) from readily available precursors.

Conjugated imines and iminium salts as versatile acceptors of nucleophiles

Growing interests in nitrogen-containing molecules involving bioactive and functional materials have stimulated the recent development of synthetic methodologies where nucleophilic addition reactions to imino carbons are utilized in crucial steps. This article summarizes double nucleophilic addition reactions with alpha,beta-unsaturated aldimines, addition reactions using alkynyl imines, "umpoled" reactions of alpha-imino esters, and the use of iminium salts as reactive electrophiles.

Synthesis of (±)-Strychnofoline via a Highly Convergent Selective Annulation Reaction

Studies aimed at preparing (+/-)-strychnofoline by total synthesis are detailed. The route described makes use of a recently developed MgI(2)-mediated ring-expansion reaction of spiro[cyclopropan-1,3'-oxindole] with a cyclic disubstituted aldimine. The ring-expansion product was formed as a single diastereoisomer in 55 % yield, possessing the same stereochemical pattern found in strychnofoline. In addition, our synthetic effort has led to the development of new reaction methodology to access 3,4-disubstituted cyclic aldimines.

The discovery and development of compounds counteracting ethylene at the receptor level

The discovery and development of 1-methylcyclopropene (1-MCP) was not done as a single step but rather was found by studying various aspects of ethylene action and the ethylene receptors. It was first noted that 2, 5-norbornadiene seemed to counteract ethylene. Studies showed it was a competitive inhibitor of ethylene responses, and knowledge that ethylene antagonists like ethylene agonists bound to silver in the same order as they were active as inhibitors was obtained. Ring strain appeared to be a primary factor that led to trans-cyclooctene then to diazocyclopentadiene. This same concept allowed for the use of chemical concepts that lead to cyclopropenes. More recent work indicates additional factors can come into play in the development of ethylene antagonists at the receptor level and these are now being utilized to find additional and improved antagonists. 1-MCP is likely to remain a primary means of controlling ethylene responses for the immediate future.

[Chemistry of alpha-aminonitriles. Aziridine-2-carbonitrile: photochemical formation from 2-aminopropenenitrile]

2-Aminopropenenitrile in solvents such as MeCN, MeOH, or H2O is photoisomerized by UV light to racemic aziridine-2-carbonitrile (rac-2); the larger part of the starting material, however, fragments to HCN and MeCN. The observed photocyclization constitutes a structural connection within an ensemble of C3H4N2 compounds considered to be potentially relevant to prebiotic chemistry.