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Oxazole
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Oxazole
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.
The classic synthetic route the Robinson–Gabriel synthesis by dehydration of 2-acylaminoketones:
The Fischer oxazole synthesis from cyanohydrins and aldehydes is also widely used:
Other methods are known including the reaction of α-haloketones and formamide and the Van Leusen reaction with aldehydes and TosMIC.
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 (oxazolium salts), oxazoles are far less basic than imidazoles (pKa = 7). Deprotonation of oxazoles occurs at C2, and the lithio salt exists in equilibrium with the ring-opened enolate-isonitrile, which can be trapped by silylation. Formylation with dimethylformamide gives 2-formyloxazole.
Electrophilic aromatic substitution takes place at C5, but requiring electron donating groups.
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Oxazole
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.
The classic synthetic route the Robinson–Gabriel synthesis by dehydration of 2-acylaminoketones:
The Fischer oxazole synthesis from cyanohydrins and aldehydes is also widely used:
Other methods are known including the reaction of α-haloketones and formamide and the Van Leusen reaction with aldehydes and TosMIC.
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 (oxazolium salts), oxazoles are far less basic than imidazoles (pKa = 7). Deprotonation of oxazoles occurs at C2, and the lithio salt exists in equilibrium with the ring-opened enolate-isonitrile, which can be trapped by silylation. Formylation with dimethylformamide gives 2-formyloxazole.
Electrophilic aromatic substitution takes place at C5, but requiring electron donating groups.
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