New non-symmetrical 2,5-disubstituted-1,3,4-oxadiazoles bearing a benzo[b]thiophene moiety

Abstract

A series of new 2,5-disubstituted-1,3,4-oxadiazoles derived from benzo [b]thiophene was obtained by dehydrative cyclization of the corresponding N,N'-diacylhydrazines, in presence of an excess of phosphorous oxychloride. The structures of the new synthesized compounds were confirmed by spectral analysis.

Unexpected Formation of N-(1-(2-Aryl-hydrazono)isoindolin-2-yl)benzamides and Their Conversion into 1,2-(Bis-1,3,4-oxadiazol-2-yl)benzenes

“…Structural investigation of the isolated product by NMR ( 1 H, 13 C, 15 N, and 2D correlations), high-resolution-mass spectrometry (HR-MS), and X-ray diffraction in the particular case of compound 10a led to identification of the cyclic compound 10Ca (Scheme 4). This compound is present in solution as mixture of Z/E isomers in a 1:0.6 molar ratio, as inferred from the 1 H NMR integration of the corresponding signals of the CH 2 protons in Z and E diastereoisomers, respectively.…”

“…The NMR spectra have been fully assigned based on spectra recorded on a spectrometer equipped with a 5 mm multinuclear inverse detection probe, operating at 400.1, 100.6, and 40.6 MHz for 1 H, 13 C, and 15 N nuclei, respectively. The 1D and 2D experiments (H−H correlation spectroscopy (COSY), ROESY, H−C heteronuclear multiple bond correlation (HMBC), H−C heteronuclear single quantum correlation (HSQC), H−N heteronuclear multiplequantum correlation (HMQC), and H−N HMBC) were recorded using standard pulse sequences in the version with z-gradients, as delivered by the spectrometer manufacturer operating software.…”

“…The 15 N chemical shifts were indirectly detected as projections from the 2D H−N HMBC spectra and are referred to external liquid ammonia (0.0 ppm) using nitromethane (380.2 ppm) as an external standard. For the major isomers (Z configuration) all 15 N signals have been detected, whereas for minor isomers, due to the smaller concentration in the mixture, often two out of the four 15 N signals have not been detected. For reaction control, various NMR spectrometers operating at 400, 500, 360, 300, and 250 MHz have been used.…”

“…Generally, closure of the 1,3,4-oxadiazole ring has been described , either through intramolecular dehydration ,− of diacylhydrazines or oxidative cyclization − of N -acyl- N ′-arylidene-hydrazines. Numerous dehydrating and oxidative reagents were reported to be very effective for this purpose including phosphorus oxychloride ,, and thionyl chloride .…”

“…Generally, closure of the 1,3,4-oxadiazole ring has been described , either through intramolecular dehydration ,− of diacylhydrazines or oxidative cyclization − of N -acyl- N ′-arylidene-hydrazines. Numerous dehydrating and oxidative reagents were reported to be very effective for this purpose including phosphorus oxychloride ,, and thionyl chloride . Recently, triflic anhydride, zirconium(IV) chloride, or [Et 2 NSF 2 ]BF 4 have been mostly used for dehydration, while cerium ammonium nitrate (CAN), lead(IV) tetracetate, − or hypervalent iodine reagents − were reported for the oxidative cyclization.…”