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26.4.1.1.3.3 Variation 3: Of α-Hydroxy Ketones

DOI: 10.1055/sos-SD-026-00532

Landais, Y.; Vincent, J. M.Science of Synthesis, (200526676.

Oxidation of α-hydroxy ketone compounds is the method of choice for the preparation of 1,2-diketones, as a wide variety of reagents are available for the conversion of secondary alcohols into ketones.[‌128‌] Moreover, α-hydroxy ketones can be prepared in high yields from the corresponding α-methylene ketones by using the Davis oxidation,[‌129‌] or lithium diisopropylamide followed by oxodiperoxymolybdenum(pyridine)(hexamethylphosphoric triamide) (MoO5pyHMPA),[‌130‌] or the enolizationepoxidationhydrolysis reaction sequence.[‌131‌] The activated hydroxy groups of benzoins are readily oxidized into benzils in excellent yields by a variety of reagents such as thallium(III) nitrate/aqueous acid,[‌132‌] catalytic ytterbium(III) nitrate,[‌133‌] manganese(IV) oxide,[‌134‌] or ammonium chlorochromate/alumina.[‌135‌] However, most of these processes have drawbacks such as the use of corrosive acids and toxic metallic compounds that generate undesirable waste materials. In this context, the oxidation of benzoins with copper(II) sulfate supported on alumina under microwave irradiation and solvent-free conditions gives benzils in high yields; this represents an environmentally friendly protocol.[‌136‌] Another attractive procedure in terms of relatively low cost and toxicity is the bismuth(III) nitrate/copper(II) acetate catalytic system.[‌137‌] Vanadium(V) oxytrichloride is a very effective catalyst for the conversion of α-hydroxy ketones into 1,2-diketones.[‌138‌] Using this catalyst, excellent yields of 1,2-diketones 142 are obtained, under an oxygen atmosphere at room temperature, from ­benzoins or aliphatic α-hydroxy ketones 141 (Scheme 36).[‌138‌] Interestingly, alcohols without a carbonyl moiety in the α-position do not react under the same conditions, while α-hydroxy esters are efficiently oxidized to give α-oxo esters. It is proposed that oxygen acts as a co-oxidant and reoxidizes the low valent vanadium (V/IV) species formed during the reaction.

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