Synthesis of 3,5-Disubstituted Isoxazoles through a 1,3-Dipolar Cycloaddition Reaction between Alkynes and Nitrile Oxides Generated from O -Silylated Hydroxamic Acids

Naphtho[1,8-de][1,2]Oxazin-4-ol: Precursor to 1,2,8-Trisubstituted Naphthalenes and 1-Unsubstituted Naphtho[1,2-d]isoxazole 2-Oxide: A Novel Isomerization of the N-Oxide to Nitrile Oxide en Route to Isoxazol(in)es

Naphtho[1,8-de][1,2]oxazin-4-ol and its acyl or benzyl derivatives ring open to various 2,8-dihydroxy-1-naphthonitriles, which, through (de)protection protocols and reduction, afford the target (E)-2-hydroxy-8-methoxy-1-naphthaldehyde. This was converted to its corresponding oxime, which was oxidatively o-cyclized with phenyliodine(III) diacetate (PIDA) to 9-methoxynaphtho[1,2-d]isoxazole 2-oxide. The latter, in deuterated DMSO at room temperature, was rearranged to its isomer 2-hydroxy-8-methoxy(naphthalen-1-yl)nitrile oxide. The isomerization was detected by time-course plot 1H NMR spectroscopy and further identified from its 13C NMR and HRMS spectra. The nitrile oxide was stable in (non)deuterated DMSO for at least 18 h. A 3,4-bis(2-hydroxy-8-methoxynaphthalen-1-yl)-1,2,5-oxadiazole 2-oxide, as a dimerization product or an isocyanate as a rearrangement isomer, was ruled out, the former by its HRMS spectrum and the latter by its 1,3-dipolar cycloaddition reactions to substituted isoxazoles.

Unveiling the regioselective synthesis of antiviral 5-isoxazol-5-yl-2´-deoxyuridines from the perspective of a molecular electron density theory

The regioselective synthesis of a potent antiviral sugar nucleoside isoxazole analogue in the [3+2] cycloaddition (32CA) reaction of acetonitrile- -N-oxide (ANO) and acetyl-protected 5-ethynyl-2?-deoxyuridine (EDU) has been studied at the MPWB1K/6-311G(d,p) level within perspective of the molecular electron density theory (MEDT). From an electron localization function (ELF) analysis, ANO is classified as a zwitterionic species devoid of any pseudoradical or carbenoid centre. The ortho regioisomer is energetically preferred over the meta one by the activation enthalpy of 21.7?24.3 kJ mol-1, suggesting complete regioselectivity in agreement with the experiment. The activation enthalpy increases from 53.9 kJ mol-1 in the gas phase to 71.5 kJ mol-1 in water, suggesting more facile reaction in low polar solvents. The minimal global electron density transfer (GEDT) at the TSs suggests non-polar character and the formation of new covalent bonds has not been started at the located TSs, showing non-covalent intermolecular interactions from an atoms-in- -molecules (AIM) study and in the independent gradient model (IGM) isosurfaces. The AIM analysis shows more accumulation of electron density at the C?C interacting region relative to the C?O one, and earlier C?C bond formation is predicted from a bonding evolution theory (BET) study.

Facile synthesis of O-acylhydroxamates via reaction of oxime chlorides with carboxylic acids

A simple and efficient method for the synthesis of O-acylhydroxamate derivatives from oxime chlorides and carboxylic acids was developed. The reaction affords clean and facile access to diverse O-acylhydroxamates in high yields (up to 85%). The chemical structure of a typical product was confirmed using single-crystal X-ray structure analysis.

A simple and practical method for synthesizing a broad range of functionalized O-acylhydroxamates in high yields from oxime chlorides and carboxylic acids has been achieved.

Stereoselective synthesis of spirocyclic pyrrolidines/pyrrolizidines/pyrrolothiazolidines using l-proline functionalized manganese ferrite nanorods as a novel heterogeneous catalyst

An efficient, green, one-pot, and three-component protocol has been reported for the stereoselective synthesis of a new class of spiro thiazolidines. A series of spiro-heterocycle derivatives were produced stereoselectively in high yields by the reaction of 5-arylidene thiazolidine-2,4-diones, isatin, and secondary amino acids in the presence of MnCoCuFe₂O₄@l-proline (MCCFe₂O₄@l-proline) magnetic nanorods as a novel nanocatalyst. The synthesized catalyst was fully characterized for thermal stability, magnetic properties, and other physicochemical properties via numerous techniques. It was applied as an efficient and reusable catalyst for the synthesis of endo-isomers of spirocyclic pyrrolidine/pyrrolizidine/pyrrolothiazolidine derivatives in high yield. The regioselectivity and stereochemistry of these heterocyclic spiro-compounds were established by ¹H, ¹³C, HMBC, HSQC, and COSY NMR spectroscopy techniques. The main attractive characteristics of the presented protocol are high yield, high level of diastereoselectivity, and easy recovery of catalyst without significant loss of its catalytic activity.

An efficient, green, one-pot, and three-component protocol has been reported for the stereoselective synthesis of a new class of spiro thiazolidines.

1,3-Dipolar Cycloaddition Reactions of Nitrile Oxides under "Non-Conventional" Conditions: Green Solvents, Irradiation, and Continuous Flow

The 1,3-dipolar cycloaddition reactions (DCs) of nitrile oxides (NOs) to alkenes and alkynes are useful methods for the synthesis of 2-isoxazolines and isoxazoles respectively, which are important classes of heterocyclic compounds in organic and medicinal chemistry. Most of these reactions are carried out in organic solvents and under thermal activation. Nevertheless the use of supercritical carbon dioxide (scCO₂ ) and ionic liquids (Ils) as alternative solvents and the application of microwave (MW) and ultrasound (US) as alternative activation procedures have evident advantages from the "Green Chemistry" point of view. The critical discussion on the applications of these "unconventional" activation methods and reaction conditions in the 1,3-DCs of NOs is the objective of the present Review.

Recent Advances in the Generation of Nitrilium Betaine 1,3-Dipoles

Nitrilium betaine 1,3-dipoles are ubiquitous reagents in organic chemistry, with applications ranging from natural product synthesis to materials science. Given the high reactivity of these zwitterionic motifs, they are invariably generated in situ from a suitable precursor, prior to use. This short review summarises the recent progress in the development of modern approaches towards the formation of these 1,3-dipoles, and their applications within a diverse range of fields.

1 Introduction

2 Nitrile Ylides

2.1 2H-Azirine Rearrangement

2.2 Addition of Nitriles to Carbenes

3 Nitrile Imines

3.1 2,5-Tetrazole Thermolysis

3.2 2,5-Tetrazole Photolysis

3.3 Diaryl Sydnone Photolysis

4 Nitrile Oxides

4.1 Hypervalent Iodine

4.2 The Nitroso Radical

4.3 Green Chemistry Approaches

4.4 Other Approaches

5 Conclusions

Recent advances in metal catalyzed or mediated cyclization/functionalization of alkynes to construct isoxazoles

This review summarized the recent developments in metal catalyzed or mediated cyclization/functionalization of alkynes to construct isoxazoles.