Synthesis of New 1H‐Indazoles through Diels–Alder Transformations of 4‐Styrylpyrazoles under Microwave Irradiation Conditions

Abstract

Microwave irradiation under solvent-free conditions induces 1-acetyl-4-styrylpyrazoles to undergo Diels-Alder cycloaddition reactions with N-methyl-or N-phenylmaleimide to give tetrahydroindazoles in good yields and with high selectivities. With conventional heating, these reactions either do not occur or afford only traces of the cycloadducts. These cycloadducts were then converted into the corresponding 1H-

Efficient MW-Assisted Synthesis, Spectroscopic Characterization, X-ray and Antioxidant Properties of Indazole Derivatives

“…Six-membered heterocycles and their fused derivatives has been reported among the compounds synthetized via MW irradiation, using cyclocondensation, cycloaddition, multicomponents and other modular reactions, generating good yield and short reaction time in an easy and rapid way [9]. Tetrahydroindazole synthesis using MW irradiation has been previously reported [10][11][12].…”

(E)‐2‐(4‐Arylbut‐1‐en‐3‐yn‐1‐yl)chromones as Synthons for the Synthesis of Xanthone‐1,2,3‐triazole Dyads

“…On the basis of previous findings by our group [23,28] we performed the DA reaction under microwave irradiation (multimode apparatus) conditions. Several experiments were carried out in which reaction times, solvent, Lewis acids, amounts of dienophile and microwave potencies were varied (Table 1, Entries 6-15).…”

Microwave‐Assisted Cycloaddition Reactions

“…The isoxazolino-SWNT (single-walled carbon nanotube) derivative 89 (Scheme 17.11) was synthesized in 65% yield through the MW-assisted 1,3-dipolar cycloaddition between the nitrile oxide generated in situ from pyridine-4-carbaldehyde oxime (88) and the pentyl ester-SWNT [39]. (17.17) The novel N-isoxazoline-and N-isoxazole-substituted saccharin derivatives 96 and 97 (Scheme 17.12) were obtained through one-pot 1,3-dipolar cycloaddition reactions between arylnitrile oxides and N-allyl- (94) or N-propargylsaccharin (95). These 1,3-dipoles were generated in situ from aromatic oximes (93) (99) at room temperature, react with hydroximinoyl chlorides (101) to form the corresponding isoxazoles (102) through 1,3-dipolar cycloadditions with aromatic nitrile oxides generated in situ.…”

“…All transition states were determined by DFT calculations and were found to be in line with the proposed mechanism. 1-Substituted-4-stryrylpyrazoles were reported by Silva et al to undergo [4 + 2]-cycloaddition with N-methyl-and N-phenylmaleimide under MW irradiation and solvent-free conditions to give tetrahydroindazoles [95]. For example, when 1-acetyl-4-styrylpyrazoles (240) and N-methylmaleimide were subjected to MW activation under solvent-free conditions the cycloadducts 241 and 242 were obtained in moderate to high yields (Scheme 17.40).…”