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Concept
Oxazole
Summary
Oxazole is the parent compound for a vast class of heterocyclic aromatic organic compounds. These are azoles with an oxygen and a nitrogen separated by one carbon. Oxazoles are aromatic compounds but less so than the thiazoles. Oxazole is a weak base; its conjugate acid has a pKa of 0.8, compared to 7 for imidazole.
Classical oxazole synthetic methods in organic chemistry are
the Robinson–Gabriel synthesis by dehydration of 2-acylaminoketones
the Fischer oxazole synthesis from cyanohydrins and aldehydes
the Bredereck reaction with α-haloketones and formamide
the Van Leusen reaction with aldehydes and TosMIC
Other methods:
Oxazolines can also be obtained from cycloisomerization of certain propargyl amides. In one study oxazoles were prepared via a one-pot synthesis consisting of the condensation of propargyl amine and benzoyl chloride to the amide, followed by a Sonogashira coupling of the terminal alkyne end with another equivalent of benzoylchloride, and concluding with p-toluenesulfonic acid catalyzed cycloisomerization:
In one reported oxazole synthesis the reactants are a nitro-substituted benzoyl chloride and an isonitrile:
In biomolecules, oxazoles result from the cyclization and oxidation of serine or threonine nonribosomal peptides:
Oxazoles are not as abundant in biomolecules as the related thiazoles with oxygen replaced by a sulfur atom.
With a pKa of 0.8 for the conjugate acid, oxazoles are far less basic than imidazoles (pKa = 7).
Deprotonation of oxazoles at C2 is often accompanied by ring-opening to the isonitrile.
Electrophilic aromatic substitution takes place at C5 requiring activating groups.
Nucleophilic aromatic substitution takes place with leaving groups at C2.
Diels–Alder reactions with oxazole dienes can be followed by loss of oxygen to form pyridines.
The Cornforth rearrangement of 4-acyloxazoles is a thermal rearrangement reaction with the organic acyl residue and the C5 substituent changing positions.
Various oxidation reactions.
Official source
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