Synthesis of 2,3-dihydroimidazo[2,1-b]thiazole derivatives via cyclization of N-allylimidazoline-2-thiones

Assembly of isoxazol-5-one with 2-unsubstituted imidazole N-oxides and aldehydes

A new synthetic protocol has been developed to construct polyfunctional isoxazol-5-one derivatives with imidazole moiety. It is based on the Knoevenagel condensation of isoxazol-5-one with aldehydes and subsequent Michael addition of imidazole N-oxides as part of a tandem three-component process. This reaction proceeds smoothly in moderate-to-good yield under catalyst-free conditions.

Synthesis and Selected Transformations of 2-Unsubstituted Imidazole N-Oxides Using a Ball-Milling Mechanochemical Approach

Synthetically relevant 2-unsubstituted imidazole N-oxides were obtained by using the ball-milling mechanochemical method. The presented approach extended the scope of the known method and enabled the preparation of hitherto little known N(1)-aryl-substituted derivatives, which are of interest as starting materials for the synthesis of more complex imidazole-based organic materials, generally in good to excellent yields. In addition, selected one-pot mechanochemical transformations including N- and O-alkylations as well as sulfur transfer reactions based on either (3+2)-cycloaddition reaction with 2,2,4,4-tetramethylcyclobutane-1,3-dithione or sulfurization of the transient imidazol-2-ylidenes, generated from corresponding imidazolium salts, were studied. The reported results can be considered as a continuation of long-term studies focused on the synthesis and applications of 2-unsubstituted imidazole N-oxides.

A novel approach to bis(1,3-azol-2-yl)acetonitriles and bis(1,3-azol-2-yl)methanes via the [3 + 2]-dipolar cycloaddition of imidazole N-oxides and 2-heteroaryl-3,3-dimethylacrylonitriles

A new synthetic approach for obtaining previously unknown bis(1,3-azol-2-yl)acetonitriles and bis(1,3-azol-2-yl)methanes has been developed. It is based on 1,3-dipolar cycloaddition between 2-unsubstituted imidazole N-oxides and 2-(1,3-azol-2-yl)-3,3-dimethylacrylonitriles, which are easily available through the condensation of (1,3-azol-2-yl)acetonitriles with acetone. The method allows for the construction of various unsymmetric derivatives based on imidazole, oxazole, thiazole, and 1,3,4-thiadiazole cyclic molecules. Its potential has been demonstrated via the synthesis of 24 diverse derivatives with yields of 29-92%. Bis(1,3-azol-2-yl)acetonitriles can be converted to the corresponding bis(1,3-azol-2-yl)methanes via simple acid hydrolysis followed by subsequent spontaneous decarboxylation at nearly quantitative yields.

The Use of Electrophilic Cyclization for the Preparation of Condensed Heterocycles

Condensed heterocycles are well-known for their excellent biological effects and they are undeniably important compounds in organic chemistry. Electrophilic cyclization reactions are widely used for the synthesis of mono-heterocyclic compounds. This review highlights the utility of electrophilic cyclization reactions as an effective generic tool for the synthesis of various condensed heterocycles containing functional groups that are able to undergo further chemical transformations, such as nucleophilic substitution, elimination, re-cyclization, cleavage, etc. This review describes the reactions of unsaturated derivatives of different heterocycles with various electrophilic agents (halogens, arylsulfanyl chlorides, mineral acids) resulting in annulation of an additional partially saturated heterocycle. The electrophilic reaction conditions, plausible mechanisms and the use of such transformations in organic synthesis are also discussed. The review mainly focuses on research published since 2002 in order to establish the current state of the art in this area.

1 Introduction

2 Electrophilic Cyclization Pathways Involving a Nitrogen Nucleo­philic Center

3 Electrophilic Cyclization Pathways Involving a Chalcogen Nucleophilic Center

3.1 Sulfur Centers

3.2 Oxygen Centers

3.3 Selenium Centers

4 Strategies and Mechanisms

5 Conclusion

Halogenoheterocyclization of terminally substituted 2-allylthio(seleno)quinolin- 3-carbaldehydes

Electrophilic heterocyclization of 2-alkenyllthioquinolin-3-carbaldehydes or 2-alkenylselenoquinolin-3-carbaldehydes 2, 3 under the action of iodine or bromine non-regioselectively leads to the formation of fused quinolinium trihalogenides 4, 5. Type of the chalcogen atom does not affect regiochemistry of halogenation.

Halogenoheterocyclization of 2-(allylthio)quinolin-3-carbaldehyde and 2-(propargylthio)quinolin-3-carbaldehyde

The reaction of 2-allyl(propargyl)thioquinolin-3-carbaldehyde with halogens (Br or I) results in formation of 1-halogenomethyl(halogenomethylidene)-4-formyl-1,2-dihydrothiazolo[3,2-

Stereoselective Lewis acid mediated (3+2) cycloadditions of N-H- and N-sulfonylaziridines with heterocumulenes

Alkyl and aryl isothiocyanates and carbodiimides are effective substrates in (3+2) cycloadditions with N-sulfonyl-2-substituted aziridines and 2-phenylaziridine for the synthesis of iminothiazolidines and iminoimidazolidines. Additionally, the stereoselective (3+2) cycloaddition of N-H- and N-sulfonylaziridines with isothiocyanates can be accomplished, allowing for the synthesis of highly enantioenriched iminothiazolidines. Evidence for an intimate ion-pair mechanism is presented herein in the context of these chemo-, regio-, and diastereoselective transformations. The demonstrated ability to remove the sulfonyl group from the heterocyclic products displays the utility of these compounds for further derivatization and application.