Synthetic Routes to Oxazolines

In this mini-review, the main synthetic routes used in the preparation of oxazolines is presented. The review is systematically carried out and the syntheses are presented in terms of precursors utilised (nitriles, aldehydes and carboxylic acids). Additionally, the reported synthesis of all chiral and achiral oxazolines involve either the use of amino alcohols as essential building blocks or some form of intramolecular cyclisation reactions. A comparison of the effectiveness of various reaction initiators such as Lewis acids, bases, oxidants and metals as well as their respective reaction conditions is also described. Lewis acid catalysts such as zinc chloride, zinc oxide and indium (III) chloride as well as triflic acid and ruthenium complexes are presented as effective catalysts in the formation of oxazolines from nitrile precursors. Oxidising agents such as N-bromosuccinimide, hypervalent iodine reagents and reducing agents such as butyllithium have been used in the formation oxazolines from aldehydes. While carboxylic acids have been used effectively as good precursors to oxazolines when using reagents such as cyanuric chloride as well as transition metal containing catalysts such as copper, ruthenium and titanium. In some cases, catalyst free reaction conditions have also been reported offering substituted oxazolines through microwave and ultrasonic irradiation as well as under standard reflux conditions.