The synthesis of azabicyclo[4.2.1]nonenes by the addition of a cyclopropenone to 4-vinyl substituted 1-azetines—isomers of the homotropane nucleus

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.

Heterocycles from cyclopropenones

Great attention has been paid to cyclopropenones as they are present in many natural sources.

Synthesis of New Functionally Substituted 9-Azabicyclo[4.2.1]nona-2,4,7-trienes by Cobalt(I)-Catalyzed [6π + 2π]-Cycloaddition of N-Carbocholesteroxyazepine to Alkynes

Catalytic [6π + 2π]-cycloaddition of N-carbocholesteroxyazepine with functionally substituted terminal alkynes and 1,4-butynediol was performed for the first time under the action of the Co(acac)2(dppe)/Zn/ZnI2 three-component catalytic system. The reaction gave previously undescribed but promising 9-azabicyclo[4.2.1]nona-2,4,7-trienes (in 79-95% yields), covalently bound to a natural metabolite, cholesterol. The structure of the synthesized azabicycles was confirmed by analysis of one- and two-dimensional (1H, 13C, DEPT 13C, COSY, NOESY, HSQC, HMBC) NMR spectra.

Organocatalytic C–C bond activation of cyclopropenones for ring-opening formal [3 + 2] cycloaddition with isatins

Cyclopropenones were first applied as potential 3C synthons in ring-opening formal cycloaddition with isatins via an organocatalytic C–C bond activation strategy.

Natural product analogues: towards a blueprint for analogue-focused synthesis

A review of approaches to natural product analogues leads to the suggestion of new methods for the generation of biologically active natural product-like scaffolds.

Synthesis, conformation and antiproliferative activity of isothiazoloisoxazole 1,1-dioxides

Isothiazoles (I) reacted with 1,3-dipoles or NaN₃ to give cycloadducts (II) or thiazete (III). Thiazete (III) rearranged to give 1,2,3-oxathiazoline (IV).

Intramolecular azide to alkene cycloadditions for the construction of pyrrolobenzodiazepines and azetidino-benzodiazepines

The coupling of proline- and azetidinone-substituted alkenes to 2-azidobenzoic and 2-azidobenzenesulfonic acid gives precursors that undergo intramolecular azide to alkene 1,3-dipolar cycloadditions to give imine-, triazoline- or aziridine-containing pyrrolo[1,4]benzodiazepines (PBDs), pyrrolo[1,2,5]benzothiadiazepines (PBTDs), and azetidino[1,4]benzodiazepines. The imines and aziridines are formed after loss of nitrogen from a triazoline cycloadduct. The PBDs are a potent class of antitumour antibiotics.