Ultrasound-assisted synthesis of 2-amino-1,3,4-oxadiazoles through NBS-mediated oxidative cyclization of semicarbazones

N-Halo Reagents: Modern Synthetic Approaches for Heterocyclic Synthesis

“…In 2017, Santos, Machado, and co-workers successfully carried out a ultrasound-assisted oxidative cyclization of semicarbazones 265 promoted by NBS in the presence of sodium acetate to produce 2-amino-1,3,4-oxadiazoles 266 (Scheme 63). 123 Moderate to excellent yields (57-92%) were obtained in short reaction times (15 minutes). A comparison with conventional stirring conditions showed similar yields to those obtained with ultrasonic conditions.…”

“…However, a considerable reduction in the reaction time (from 90 to 15 minutes) was achieved using the sonochemical protocol. 123 On the other hand, 1,2,4-oxadiazoles 268 were readily accessed from N-acylamidines 267 via oxidative N-O bond formation promoted by NBS. 124…”

Ultrasound-assisted transition-metal-free catalysis: a sustainable route towards the synthesis of bioactive heterocycles

“…A very straightforward method to access a variety of 1,3,4-oxadiazole derivatives under ultrasound irradiation was reported by Santos, Machado, and their group ( Scheme 17 ). 87 Using NBS-NaOAc as the oxidizing system, the corresponding 1,3,4-oxadiazole products 70 derived from the cyclization of various semicarbazones 69 in the presence of acetic acid at 25–110 °C were obtained in 53–92% yield within only 15 minutes. This protocol starts with the initial NBS promoted reaction of semicarbazones 69 to form an intermediate Int-20 which, after 1,3-dipolar elimination on treatment with NaOAc, delivers nitrilimines Int-21.…”

“…Subsequent reaction of 66 with carbon disulde using KOH as the base under reux conditions afforded N-unsubstituted oxadiazoles 67, which on treatment with various amines and formaldehyde in the presence of activated Scheme 14 Ultrasound-assisted triethylamine-catalyzed assembly of functionalized thiazoles as reported by Farghaly et al A very straightforward method to access a variety of 1,3,4oxadiazole derivatives under ultrasound irradiation was reported by Santos, Machado, and their group (Scheme 17). 87 Using NBS-NaOAc as the oxidizing system, the corresponding 1,3,4-oxadiazole products 70 derived from the cyclization of various semicarbazones 69 in the presence of acetic acid at 25-110 C were obtained in 53-92% yield within only 15 minutes. This protocol starts with the initial NBS promoted reaction of semicarbazones 69 to form an intermediate Int-20 which, aer 1,3-dipolar elimination on treatment with NaOAc, delivers nitrilimines Int-21.…”

Ultrasound-Assisted Synthesis of 4-Alkoxy-2-methylquinolines: An Efficient Method toward Antitubercular Drug Candidates

“…This effect improves mass transfer and increases turbulent flow in the liquid, facilitating the chemical transformations. In our studies, the use of ultrasound has led to the production of compounds in reduced reaction times with high yields and purity [ 16 , 17 ]. Such characteristics have increased the number of compounds evaluated in pharmacological models, contributing to accelerating the hit to lead optimization process.…”