Practical Deoxygenation of Oxazole N-Oxides by PCl3/Collidine (original) (raw)
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2016
Broad bio-spectrum of 1,3-oxazole and benzoxazole has created an attractive platform for synthetic chemists to introduce structural modification in its nucleus by straight forward access to new approaches. Owing to its fascinating features and interesting pharmacological activities, many researchers have proved that oxazole is an active agent in treating different diseases. Based on this fact, this article outlines intramolecular and two component intermolecular cyclization to oxazoles and benzoxazoles. The theme is well documented in this review article covering the era from 2007 till present. Moreover, bioactivity and mechanistic insights are provided with different synthetic approaches, encompassing various pathways.
Journal of Taibah University for Science, 2020
We herein report a modified approach for the synthesis of some pharmacologically relevant oxazole derivatives linked with amides under mild conditions. The utilization of polymer-supported triphenylphosphine (poly-TPP) as a phosphine ligand for generating the key iminophosphorane intermediate was found to be vital for achieving the synthesis of oxazole in room temperature. This alternative approach relying on the cyclization reaction of readily available phenacyl azide and phenyl isothiocyanate rendered the oxazole derivative 3a in excellent yield. This methodology has been applied for the synthesis of a more decorated oxazole derivative 3b having ester functionality and was further converted to different amides under microwave irradiation in good to excellent yields in the later stage.
Journal of Organic Chemistry, 2006
The reaction of 1-(methylthio)acetone with different nitriles in the presence of triflic anhydride led to the one-pot formation of 2-substituted 5-methyl-4-methylthio-1,3-oxazoles in good yield. 1,2-and 1,4-Bisozaxolyl-substituted benzenes were obtained when the reaction was carried out using aromatic dinitriles. The methylthio group at the C4 position of the oxazole ring was easily removed with Raney nickel to form 2-substituted 5-methyl-1,3-oxazoles in good yields. 4-Methylsulfonyl derivatives were prepared by the oxidation of the MeS group with m-CPBA. The proposed mechanism for the formation of oxazoles involves an unstable 1-(methylthio)-2-oxopropyl triflate, which was detected from the low-temperature NMR spectra.
New perspectives in oxazole chemistry
Tetrahedron, 1999
Addition -ring opening of the nitrooxazole 2a with diverse amino nucleophiles afforded directly the polfinctionalized nitroenamines 7a-g in good to excellent yields. At the same time, highly diastereoselective cascade reactions, easily performed both on 2a-c with the ynamine 8 and 2b with the en01 ether 13, led to the novel poly~yclic systems 12a-c and 15, respectively. 0
2014
DFT calculations have shown that the N-O dipole of benzene-and naphthalene-fused 1,2-oxazole N-oxides causes a distortion of their r and p frame, concentrated on the 1,2-oxazole ring, such that it increases its susceptibility to opening. The distortion forces the benzene ring into some diene geometry, thus, reducing p delocalization over the bi-or tricyclic structure and ultimately their aromatic character. C-3 substitution has a marked influence mainly on the naphthalene-fused N-oxides. C-5 and particularly C-6 substitution, as the position of most extended interaction with the N-O dipole through the p ring density, contribute to the distortion of the 1,2-oxazole geometry and thereby to the decrease of aromaticity of the structure. Bond uniformity (I A ), average bond order (ABO) and Harmonic Oscillator Model of Aromaticity (HOMA) indices have been recruited to measure aromaticity changes. I A and ABO appear to be more credible to 1,2-benzoxazole N-oxides and 1,2-naphthoxazole N-oxides, respectively, while HOMA has been found equally reliable to both. Hardness and dipole moments follow similar trends. Energies, localization and separation of the four frontiers orbitals, i.e. HO, HO-1, and LU, LU?1, indicate a rather notable aromatic character of the N-oxides. Their reactivity profile, portrayed by descriptors such as Fukui and electro(nucleo)philicity Parr functions, shows good agreement with experimental outcomes towards electrophiles but succumbs to discrepancies towards nucleophiles due to the susceptibility of the hetero-ring to opening. The ''push-pull'' character of the N-O dipole and more importantly the extent of its double bonding direct site selectivity.
J. Org. Chem. 2015, 80, 18, 9016–9027, 2015
The higher propensity of C−N over C−S bond forming ability was demonstrated, through formal C−H functionalization during the construction of 4,5-disubstituted 1,2,4-triazole-3-thiones from arylidenearylthiosemicarbazides catalyzed by Cu(II). However, steric factors imparted by the o-disubstituted substrates tend to change the reaction path giving thiodiazole as the major or an exclusive product. Upon prolonging the reaction time, the in situ generated thiones are transformed to 4,5-disubstituted 1,2,4-triazoles via a desulfurization process. Two classes of heterocycles viz. 4,5-disubstituted 1,2,4-triazole-3-thiones and 4,5-disubstituted 1,2,4-triazoles can be synthesized from arylidenearylthiosemicarbazides by simply adjusting the reaction time. Desulfurization of 1,2,4-triazole-3thiones is assisted by thiophilic Cu to provide 1,2,4-triazoles with concomitant formation of CuS and polynuclear sulfur anions as confirmed from scanning electron microscope and energy dispersive X-ray spectroscopy measurements. A one-pot synthesis of an antimicrobial compound has been successfully achieved following this strategy.
Tetrahedron Letters, 2017
A facile and practical protocol has been developed for the synthesis of 3,5-disubstituted 1,2,4oxadiazoles and 1,4,2-oxathiazoles through oxidative cyclization of amidoximes and thiohydroximic acids, respectively at room temperature. Use of mild reaction conditions, inexpensive reagents, broad substrate scope and good to excellent yield of the products are the salient features of this protocol.