Acyloins unsymmetrical

Out first example is 2-hydroxy-2-methyl-3-octanone. 3-Octanone can be purchased, but it would be difficult to differentiate the two activated methylene groups in alkylation and oxidation reactions. Usual syntheses of acyloins are based upon addition of terminal alkynes to ketones (disconnection 1 see p. 52). For syntheses of unsymmetrical 1,2-difunctional compounds it is often advisable to look also for reactive starting materials, which do already contain the right substitution pattern. In the present case it turns out that 3-hydroxy-3-methyl-2-butanone is an inexpensive commercial product. This molecule dictates disconnection 3. Another practical synthesis starts with acetone cyanohydrin and pentylmagnesium bromide (disconnection 2). Many 1,2-difunctional compounds are accessible via oxidation of C—C multiple bonds. In this case the target molecule may be obtained by simple permanganate oxidation of 2-methyl-2-octene, which may be synthesized by Wittig reaction (disconnection 1). 

Acyloins undergo nucleophilic addition to /3-ethoxyvinylphosphonium salts (141) to yield an ylide (142) (74JOC584). Intramolecular Wittig reaction results in formation of the dihydrofuran (143), which is converted to the furan (144) by elimination of ethanol (Scheme 32). Symmetric acyloins give one product, but unsymmetrical ones may give two products under basic conditions due to tautomerization. 

Regioselective oxidation of 1,2-diols.1 The oxidation of di-secondary glycols to acyloins (5, 188) can be extended to oxidation of other glycols. Thus the stan-nylene of 1 is oxidized by bromine to 2 in high yield. The reaction is regioselective with unsymmetrical diols (3 — 4). 

The anion of the adduct of TMS-CN with benzaldehyde reacts with aldehydes and ketones to form acyloin silyl ethers, by way of a 1,4-0-silyl rearrangement (Scheme 10). This method gives a-hydroxy ketones in excellent yield (80-90%) and allows the selective synthesis of unsymmetrical benzoins. ... 

Acyloins are prepared in high yields by attack of a Giignard reagent on the cyano group of 0-tri-methylsilylated cyanohydrins. The method is particularly useful for the preparation of unsymmetrical acyloins. ... 

Symmetrical diketones RC(0)C(0)R (R = Ph, 4-MeCeH4, 4-MeOCeH4, or furyl) have been obtained in good yield by oxidation of the corresponding acyloins with alkaline K3Fe(CN)6, and a detailed investigation has been carried out on the reaction of unsymmetrical benzils with CN in DMSO. The latter was initiated in an attempt to clarify some ambiguities in the literature, the products... 

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