Hypervalent Iodine‐Catalyzed Synthesis of 1,2,4‐Oxadiazoles from Aldoximes and Nitriles

TFA-Mediated Synthesis of 1,2,4-Oxadiazoles from Carbamates and Nitriles

An unprecedented protocol for the synthesis of 1,2,4-oxadiazoles from carbamates has been developed by employing nitriles as both substrates and solvents. This one-pot procedure achieves the formation of C═N bonds via TFA-mediated [3+2] annulation. A series of 1,2,4-oxadiazoles are synthesized in moderate to good yields.

Organohypervalent heterocycles

This review summarizes the structural and synthetic aspects of heterocyclic molecules incorporating an atom of a hypervalent main-group element. The term "hypervalent" has been suggested for derivatives of main-group elements with more than eight valence electrons, and the concept of hypervalency is commonly used despite some criticism from theoretical chemists. The significantly higher thermal stability of hypervalent heterocycles compared to their acyclic analogs adds special features to their chemistry, particularly for bromine and iodine. Heterocyclic compounds of elements with double bonds are not categorized as hypervalent molecules owing to the zwitterionic nature of these bonds, resulting in the conventional 8-electron species. This review is focused on hypervalent heterocyclic derivatives of nonmetal main-group elements, such as boron, silicon, nitrogen, carbon, phosphorus, sulfur, selenium, bromine, chlorine, iodine(III) and iodine(V).

Self Cycloaddition of o-Naphthoquinone Nitrosomethide to (±) Spiro{naphthalene(naphthopyranofurazan)}-one Oxide: An Insight into its Formation

2-Hydroxy-1-naphthaldehyde oxime was oxidized by AgO (or Ag2O), in presence of N-methyl morpholine N-oxide (NMMO), to the title spiro adduct-dimer (±)-Spiro{naphthalene-1(2H),4'-(naphtho[2',1':2,3]pyrano[4,5-c]furazan)}-2-one-11'-oxide by a Diels-Alder(D-A) type self-cycloaddition, through the agency of an o-naphthoquinone nitrosomethide (o-NQM). Moreover, 2-hydroxy-8-methoxy-1-naphthaldehyde oxime was prepared and subjected to the same oxidation conditions. Its sterically guided result, 9-methoxynaphtho[1,2-d]isoxazole, was isolated, instead of the expected spiro adduct. The peri intramolecular H bonding in the oxime is considered to have a key contribution to the outcome. Geometry and energy features of the oxidant- and stereo-guided selectivity of both oxidation outcomes have been explored by DFT, perturbation theory and coupled cluster calculations. The reaction free energy of the D-A intermolecular cycloaddition is calculated at -82.0 kcal/mol, indicating its predominance over the intramolecular cyclization of ca. -37.6 kcal/mol. The cycloaddition is facilitated by NMMO through dipolar interactions and hydrogen bonding with both metal complexes and o-NQM. The 8(peri)-OMe substitution of the reactant oxime sterically impedes formation of the spiro adduct, instead it undergoes a more facile cyclodehydration to the isoxazole structure by ca. 4.9 kcal/mol.

Progress in organocatalysis with hypervalent iodine catalysts

Hypervalent iodine compounds as environmentally friendly and relatively inexpensive reagents have properties similar to transition metals. They are employed as alternatives to transition metal catalysts in organic synthesis as mild, nontoxic, selective and recyclable catalytic reagents. Formation of C-N, C-O, C-S, C-F and C-C bonds can be seamlessly accomplished by hypervalent iodine catalysed oxidative functionalisations. The aim of this review is to highlight recent developments in the utilisation of iodine(III) and iodine(V) catalysts in the synthesis of a wide range of organic compounds including chiral catalysts for stereoselective synthesis. Polymer-, magnetic nanoparticle- and metal organic framework-supported hypervalent iodine catalysts are also described.

Efficient Catalytic Synthesis of Condensed Isoxazole Derivatives via Intramolecular Oxidative Cycloaddition of Aldoximes

The intramolecular oxidative cycloaddition reaction of alkyne- or alkene-tethered aldoximes was catalyzed efficiently by hypervalent iodine(III) species to afford the corresponding polycyclic isoxazole derivatives in up to a 94% yield. The structure of the prepared products was confirmed by various methods, including X-ray crystallography. Mechanistic study demonstrated the crucial role of hydroxy(aryl)iodonium tosylate as a precatalyst, which is generated from 2-iodobenzoic acid and m-chloroperoxybenzoic acid in the presence of a catalytic amount of p-toluenesulfonic acid.

Oxadiazol-based mTOR inhibitors with potent antiproliferative activities: synthetic and computational modeling

Series of N-aryl-1,3,4-oxadiazole-2-amines and 3-aryl-1,2,4-oxadiazole-5-carboxamides derivatives were synthesized as novel chemotherapeutic agents. Synthesized compounds were evaluated for their anticancer activities against several cancer cell lines. Many analogues of 1,3,4-oxadiazole scaffold showed potent antiproliferative activities against breast cancer cell lines, with higher activities toward the metastatic breast cancer cell line (MDA-MB-231). Active analogues were profiled using in-house pharmacophore database in search for molecular target. Active analogues (2j and 2k) were found to fit the pharmacophoric map of ATP-competitive inhibitors of mTOR. The mTOR inhibitory activities of the most active compounds were confirmed with IC₅₀ values in nanomolar range. The N-aryl-1,3,4-oxadiazole-2-amines linked to a basic head is a novel ATP-competitive inhibitors of mTOR with potential activities for treatment of different types of cancer.

Practical Synthesis of 2-Iodosobenzoic Acid (IBA) without Contamination by Hazardous 2-Iodoxybenzoic Acid (IBX) under Mild Conditions

We report a convenient and practical method for the preparation of nonexplosive cyclic hypervalent iodine(III) oxidants as efficient organocatalysts and reagents for various reactions using Oxone® in aqueous solution under mild conditions at room temperature. The thus obtained 2-iodosobenzoic acids (IBAs) could be used as precursors of other cyclic organoiodine(III) derivatives by the solvolytic derivatization of the hydroxy group under mild conditions of 80 °C or lower temperature. These sequential procedures are highly reliable to selectively afford cyclic hypervalent iodine compounds in excellent yields without contamination by hazardous pentavalent iodine(III) compound.

Hypervalent-iodine mediated one-pot synthesis of isoxazolines and isoxazoles bearing a difluoromethyl phosphonate moiety

A one-pot, regioselective 1,3-dipolar cycloaddition of in situ generated (diethoxyphosphoryl)difluoromethyl nitrile oxide toward selected alkenes and alkynes is reported. This protocol enables facile access to 3,5-disubstituted isoxazolines and isoxazoles bearing a CF2P(O)(OEt)2 moiety in good to excellent yields, under mild reaction conditions.

2-Iodosylbenzoic acid activated by trifluoromethanesulfonic anhydride: efficient oxidant and electrophilic reagent for preparation of iodonium salts

2-Iodosylbenzoic acid in the presence of trifluoromethanesulfonic anhydride is an efficient oxidant and electrophilic reagent useful for preparation of the corresponding alkenyl and aryliodonium salts.

Iron Nitrate-Mediated Selective Synthesis of 3-Acyl-1,2,4-oxadiazoles from Alkynes and Nitriles: The Dual Roles of Iron Nitrate

A direct strategy for the selective synthesis of 3-acyl-1,2,4-oxadiazoles from alkynes and nitriles has been developed under iron(III) nitrate-mediated conditions. The mechanism includes three sequential procedures: iron(III) nitrate-mediated nitration of alkynes leads to α-nitroketones, dehydration of α-nitroketones provides the nitrile oxides, and 1,3-dipolar cycloaddition of nitrile oxides with nitriles produces 3-acyl-1,2,4-oxadiazoles under iron-mediated conditions. Iron(III) nitrate plays dual roles in the nitration of alkynes and the activation of nitriles, while the formation of pyrimidine/isoxazole byproducts can be efficiently inhibited.

Facile synthesis of N-1,2,4-oxadiazole substituted sulfoximines from N-cyano sulfoximines

A divergent approach has been successfully developed for the synthesis of N-1,2,4-oxadiazole substituted sulfoximines starting from N-cyano sulfoximines. This method has a wide degree of substrate scope that includes aryl, heteroaryl, alkyl, fluoroalkyl and saturated heterocyclic compounds. Excellent functional group tolerability was also observed. Extension of this methodology to nucleosides, amino acids and dipeptides was found to be successful. A gram scale reaction was also established. The major part of this method is metal free and the utility of environmentally friendly solvents such as 2-methyl THF and ionic liquids is an added advantage.

One-step formation of dihydrofuranoindoline cores promoted by a hypervalent iodine reagent

Treatment of aniline derivatives in the presence of a hypervalent iodine reagent and furan produces dihydrofuranoindoline cores in one step.