2‑oxazoline, 3‑oxazoline, and 4‑oxazoline (from left to right)
Three structural isomers of oxazoline are possible depending on the location of the double bond, however only 2‑oxazolines are common. 4‑Oxazolines are formed as intermediates during the production of certain azomethine ylides but are otherwise rare. 3‑Oxazolines are even less common but have been synthesised photochemically and by the ring opening of azirines. These three forms do not readily interconvert and hence are not tautomers.
A fourth isomer exists in which the O and N atoms are adjacent, this is known as isoxazoline.
The synthesis of 2-oxazoline rings is well established and in general proceeds via the cyclisation of a 2-amino alcohol (typically obtained by the reduction of an amino acid) with a suitable functional group. The overall mechanism is usually subject to Baldwin's rules.
From acid chlorides
A routine route to oxazolines entails reactions of acyl chlorides with 2-amino alcohols. Thionyl chloride is commonly used to generate the acid chloride in situ, care being taken to maintain anhydrous conditions, as oxazolines can be ring-opened by chloride if the imine becomes protonated. The reaction is typically performed at room temperature. If reagents milder than SOCl2 are required, oxalyl chloride can be used.
Aminomethyl propanol is the classical precursor to oxazolines using acid chloride method. As applied to fatty acids, the resulting dimethyloxazoline (DMOX) derivatives are amenable to analysis by gas chromatography.
From carboxylic acids
Modification of the Appel reaction allows for the synthesis of oxazoline rings. This method proceeds under relatively mild room temperature conditions, however, owing to the large amounts of triphenylphosphine oxide produced, the method is not ideal for large-scale reactions. The use of this method is becoming less common, due to carbon tetrachloride being restricted under the Montreal protocol.
The use of catalytic amounts of ZnCl2 to generate oxazolines from nitriles was first described by Witte and Seeliger, and further developed by Bolm et al. The reaction requires high temperatures to succeed and is typically performed in refluxing chlorobenzene under anhydrous conditions. A precise reaction mechanism has never been proposed, although it is likely similar to the Pinner reaction; preceding via an intermediate amidine. Limited research has been done into identifying alternative solvents or catalysts for the reaction.
Ligands containing a chiral 2-oxazoline ring are used in asymmetric catalysis due to their facile synthesis, wide range of forms and effectiveness for many types of catalytic transformation.
2-Substituted oxazolines can be prepared by many methods and possess a moderately hard N-donor. Chirality is easily incorporated by using 2-amino alcohols prepared by the reduction of amino acids; which are both optically pure and inexpensive. As the stereocentre in such oxazolines is adjacent to the coordinating N-atom, it can influence the selectivity of processes occurring at the metal centre. The ring is thermally stable and resistant to nucleophiles, bases, radicals, and weak acids as well as being fairly resistant to hydrolysis and oxidation; thus it can be expected to remain stable in a wide range of reaction conditions.
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