Advances in the synthetic strategies of benzoxazoles using 2-aminophenol as a precursor: an up-to-date review

Abstract

Benzoxazole is a resourceful and important member of the heteroarenes that connects synthetic organic chemistry to medicinal, pharmaceutical, and industrial areas.

Unsymmetrical Bis‐Coumarin Based on o‐Hydroxy Aldehydes and 4‐Hydroxy Coumarin: Expedient Synthesis, Single Crystal X‐rays and DFT Studies

“…The solid so formed was collected through decantation followed by washing with cold alcohol and dried under vacuum. The dried sample showed following spectral data: δ 10.44 ppm (1H, D 2 O exchangeable), 12 numbers of aromatic protons and δ 5.40 ppm a methine proton in its 1 H NMR spectra; in 13 C NMR spectra it shows altogether 25 signals among which only one is aliphatic carbon (methine). These NMR data indicated that the product can be either 4 a or 5 a.…”

“…The hydroxy aldehydes bearing tert-butyl group prepared by Duff's formylation method from 4-tert butyl phenol and coumarin moiety prepared from 4-methyl/phenyl 7-hydroxy coumarin via Duff's method [31] all reacted in a similar fashion (Figure 5, 5 n-p) albeit moderate yield in some cases. The identity of all the products (5 c-p) were verified by their 1 H, 13 C NMR spectra and HRMS analyses (ESI).…”

“…General 1 H NMR and 13 C NMR spectra were recorded at ambient temperature using 400 MHz spectrometers (400 MHz for 1 H and 100 MHz for 13 C) on a Bruker Ascend 400 spectrophotometer. Chemical shifts were reported in parts per million from the tetramethylsilane internal reference, and coupling constants were reported in Hertz.…”

“…They have shown lot of biological applications [8] and some of the derivatives of bis‐coumarin have been extensively studied as analytes [9] . Recent research [10–13] also emerges the significant importance of the bis‐coumarin type compounds in various occasions. Most of the reported methods rely on the involvement of a three component condensation of 4‐hydroxy coumarin ( 1 ) units and an aldehyde ( 2 ) through Knoevenagel condensation followed by Michael addition in the presence of various catalysts under room temperature or elevated temperature [5b,6b,15] .…”

TEMPO-immobilized metal–organic frameworks for efficient oxidative coupling of 2-aminophenols and aldehydes to benzoxazoles

“…44–47 The coupling of aldehyde and aminophenol, used as a model reaction in many benzoxazole studies, represents a strategy for synthesizing benzoxazole from a simple molecular structure without the need to synthesize a specific precursor. 48 Han et al demonstrated the oxidative cyclization of aldehydes with aminophenols using methoxy-functionalized TEMPO (Scheme 1a), 49 while Wang et al employed MOFs as the primary metal catalyst in combination with molecular TEMPO (in homogeneous media) as a co-catalyst for this reaction (Scheme 1b). 50 Although both studies utilized catalytic amounts of TEMPO derivatives, the TEMPO radicals were not recovered or recycled after the main reaction.…”