Direct synthesis of polysubstituted aluminoisoxazoles and pyrazoles by a metalative cyclization

Alumino-heteroles are obtained from simple precursors in a fully chemo- and regioselective manner by a metalative cyclization. The carbon-aluminum bond is still able to react further with several electrophiles, without the need of transmetalation. This synthetic route provides a novel entry to heterocyclic organoaluminum reagents as well as a straightforward access to 3,4,5-trisubstituted isoxazoles and 1,3,4,5-tetrasubstituted pyrazoles.     

Novel two step synthesis of pyrazoles and isoxazoles from aryl methyl ketones

Various acetyl aromatics when reacted with dimethylformamide dimethyl acetal gave 1-aryl-3-dimethylamino-2-propen-1-ones. These intermediates are masked β-ketoaldehydes and react with hydrazine hydrate or hydroxylamine at room temperature to give in good yields pyrazoles or isoxazoles.     

Regioselective synthesis of 1,3,4,5-tetrasubstituted pyrazoles from Baylis-Hillman adducts

Tetrasubstituted pyrazole derivatives 3a-j were synthesized regioselectively in good yields from the reaction of Baylis-Hillman adducts 1a-f and hydrazine hydrochlorides 2a-d in 1,2-dichloroethane.     

Synthesis of bromodifluoromethyl substituted pyrazoles and isoxazoles

Bromodifluoromethyl substituted β-diketone 3a-3d, prepared from corresponding ketones and ethyl bromodifluoroactate in the presence of sodium methoxide, reacted with aryl hydrazine derivatives affording bromodifluoromethyl substituted pyrazoles in high regioselectivity. The reaction of 3a-3d with hydroxylamine hydrochloride gave dihydroisoxazoles, which afforded bromodifluoromethyl substituted isoxazoles through dehydration by PPA or concentrated sulfuric acid.     

Synthesis of 3-aminomethyl-substituted pyrazoles and isoxazoles

A series of new 3-(aminomethyl)pyrazoles and 3-(aminomethyl)isoxazoles was synthesized along a route involving the formation as key intermediates of esters of 5-substituted 1H-pyrazole-3-carboxylic and 1H-isoxazole-3-carboxylic acids. All compounds obtained were characterized by physicochemical constants, IR, ¹H, ¹³C NMR, and mass spectra.     

Regioselective synthesis and slow-release Suzuki-Miyaura cross-coupling of MIDA boronate-functionalized isoxazoles and triazoles

The efficient preparation of heterocycles with a range of substitutions ortho to heteroatoms remains as a challenge in organic synthesis, particularly relevant to the construction of druglike molecules due to the ubiquitous presence of such moieties in that chemical space. Modular installation of heterocyclic building blocks using Suzuki-Miyaura cross-coupling is a conceptually useful strategy to address this challenge, though this has historically been met with technical difficulty due to issues of inaccessibility and instability of the requisite heterocyclic boronates. Herein we report a mild and highly regioselective cycloaddition approach which affords convenient access to stable MIDA boronate-functionalized isoxazoles and triazoles and their subsequent efficient Suzuki-Miyaura cross-coupling. This methodology is then further applied to a set of druglike compounds in an efficient one-pot telescoped sequence in line with green chemistry principles.     

Synthesis of C-glycoside derivatives from pyrazoles and isoxazoles

Bromination of 2.3:4.5-di-O-isopropylidene-aldehydo-D -arabinose p-nitrophenyl-hydrazone affords the corresponding hydrazidic bromide. Similarly, chlorination of 3-O-benzyl-1,2-O-isopropylidene α-D -xylo-pentodialdo-1,4-furanose oxime leads to the corresponding hydroxamoyl chloride. These two compounds constitute useful new synthetic intermediates in carbohydrate chemistry. For example, they may be ethynylated, then cyclised respectively to pyrazolyl- and isoxazolyl-C-glycosides.     

Mesogenic heterocyclic pyrazoles,isoxazoles and 1,3,4-oxadiazoles

Three new series of liquid crystals 1-3 derived from heterocyclic pyrazoles, isoxazoles and 1,3,4-oxadiazoles exhibiting smectic A or C phases were reported. Compound 1a exhibited monotropic smectic A phase, while the other two compounds 2-3a were not mesogenic. In contrast, all other compounds 1-3b bearing longer alkoxy chains exhibited smectic X and C phases, which were confirmed by powder XRD diffractometer. The clearing temperatures decreased in the order of compounds 1b > 2b > 3b, in contrast the temperature range of smectic C phases increased in the order of compounds 1b < 2b < 3b. The smectic phases in 1-3b were formed by a monomeric structure, confirmed and consistent with powder X-ray diffraction data. The formation of mesomorphic behavior might be probably attributed to their exocyclic angles and/or H-bonds. All derivatives showed good stabilities at temperature below T = 346 °C on TGA analysis.     

Versatile and efficient solid-phase syntheses of pyrazoles and isoxazoles

[reaction: see text] Condensation of aromatic or aliphatic esters with resin-supported acetyl carboxylic acids 2, followed by cyclization with hydrazines or hydroxylamine, activation of the linker, and cleavage using amines provides highly substituted, isomeric pyrazoles or isoxazoles 5. This general method gives products in excellent yields and purities in which the ratio of the two isomers can be easily controlled. A variation of this scheme generates 1,4,5- and 1,3, 4-trisubstituted pyrazoles and related isoxazoles. Post-cleavage reduction with borane converts pyrazole amides to amines such as 11.     

Regioselective synthesis of 1,3,5-tri- and 1,3,4,5-tetrasubstituted pyrazoles from N-arylhydrazones and nitroolefins

Two general protocols are developed for the regioselective synthesis of 1,3,5-tri- and 1,3,4,5-tetrasubstituted pyrazoles by the reaction of electron-deficient N-arylhydrazones with nitroolefins. Studies on the stereochemistry of the key pyrazolidine intermediate suggest a stepwise cycloaddition mechanism.     

Regioselective synthesis of polysubstituted pyridines via hetero-Diels-Alder reaction of isotellurazoles with acetylenic dienophiles

Treatment of substituted isotellurazoles or their Te-oxides with acetylenic dienophiles efficiently afforded polysubstituted pyridine derivatives through a pathway involving hetero-Diels-Alder reaction of isotellurazoles and the subsequent tellurium extrusion from the intermediary cycloadducts.     

One-pot construction of pyrazoles and isoxazoles with palladium-catalyzed four-component coupling

[reaction: see text] Four-component coupling of a terminal alkyne, hydrazine (hydroxylamine), carbon monoxide, and an aryl iodide furnishes pyrazole or isoxazole derivatives in the presence of a palladium catalyst. The reaction proceeds at room temperature and an ambient pressure of carbon monoxide in an aqueous solvent system.     

Straightforward transformation of isoxazoles into pyrazoles: renewed and improved

Isoxazoles bearing alkyl or carbamoyl groups were transformed into the corresponding pyrazoles in high yields by the treatment with hydrazine in methanol in the presence of a hydrogenation catalyst, e.g., Raney nickel, at ambient temperature. For the synthesis of N-substituted pyrazoles, hydrogenolysis of isoxazole followed by the treatment with substituted hydrazine was required. 3(5)-Aryl- or acylamido-substituted isoxazoles are less suitable for such transformations.     

Regioselective carbene insertion on polysubstituted dihydroaromatic compounds

[reaction: see text] The rhodium-catalyzed CH-insertion reaction of diazo ester derivatives into dihydroaromatic compounds was expanded to the application of various polysubstituted and functionalized 1,4-dienes generated from a cobalt-catalyzed Diels-Alder reaction. The highly regioselective CH-insertion process produces after DDQ oxidation polysubstituted, polyfunctionalized aromatic compounds in good overall yields.     

Solid phase synthesis of a molecular library of pyrimidines, pyrazoles, and isoxazoles with biological potential

A small molecular library of 40 pyrimidine, pyrazole, and isoxazole derivatives, bearing structural features for a promising binding of therapeutically interesting enzymes, was designed and prepared. An efficient and straightforward solid phase synthesis was envisaged and carried out on a Rink amide resin. The assistance of microwave heating in any step reduced the reaction time, increased the reaction yields, and allowed an easy work-up and purification of the targeted compounds.     

A fast and efficient bromination of isoxazoles and pyrazoles by microwave irradiation

A fast and efficient method has been developed for the bromination of isoxazoles and pyrazoles using microwave irradiation. In this method, N-bromosuccinimide was used in different acid solvents according to the reactivity of the substrates to give mono-brominated isoxazoles and pyrazoles in good yields. Trifluoroacetic acid was found to be the best solvent for highly unreactive isoxazoles and pyrazoles.     

Regioselective synthesis of trifluoromethylated pyrazoles by selective protection of trifluoromethyl-β-diketones

1,1,1-Trifluoropentane-2,4-dione was selectively protected by treatment with pyrrolidine to give 6 . The reaction of 6 with arylhydrazines gave 5 exclusively. The structural assignment for 5 was confirmed by quenching the dianion of 8 with ethyl trifluoroacetate.     

Regioselective synthesis of some novel pyrazoles,isoxazoles, pyrazolo[3,4‐d]pyridazines and isoxazolo[3,4‐d]pyridazines pendant to benzimidazole

2‐Acetyl‐1‐methyl‐1H‐benzimidazole reacts with dimethylformamide‐dimethyl‐acetal (DMF‐DMA) to afford the corresponding E‐1‐(1‐methyl‐1H‐benzimidazol‐2‐yl)‐3‐N,N‐dimethylaminoprop‐2‐enone. The latter compound reacts regioselectively with some nitrilimines and nitrile oxides to afford the corresponding pyrazole and isoxazole derivatives, respectively. These reaction products react with hydrazine hydrate to give the novel pyrazolo[3,4‐d]pyridazine and isoxazolo[3,4‐d]pyridazine derivatives, respectively.     

Regioselective synthesis of polysubstituted benzenes from Baylis-Hillman adducts via [4+2] annulation protocol

A new and regioselective [4+2] benzannulation protocol toward polysubstituted benzenes was developed. A nitroalkane derivative, which was prepared from Baylis-Hillman adduct, served as the four-carbon unit and a Michael acceptor as a two-carbon unit. Vinyltriphenylphosphonium salt could also be used as a Michael acceptor efficiently.