Reaction acyloin

A spectacular application of the acyloin ester condensation was the preparation of catenaries like 11. These were prepared by a statistical synthesis which means that an acyloin reaction of the diester 10 has been carried out in the presence of an excess of a large ring compound such as 9, with the hope that some diester molecules would be threaded through a ring, and would then undergo ring closure to give the catena compound ... 

The acyloin reaction is so useful for making medium and large rings that cyclic ketones are often made this way. Reduction of acyloin (32) with zinc in acid solution or HI removes the unwanted hydroxyl group to give cyclo-decanone (33) in good yield. 

One route uses the acyloin reaction to make (26) hich can be converted into (25) by FGI. 

D-erythro-Pentulose 5-phosphate (XLIV) has been formed by the action of transketolase on hydroxypyruvate (XLII) and D-glycerose 3-phosphate, the hydroxypyruvate being decarboxylated196 to active glycolaldehyde which then reacts with the triose phosphate by an acyloin reaction.28 The active glycolaldehyde is also formed from L-glycero-tetrulose, d-altro-heptulose 7-phosphate, D-fructose 6-phosphate, and D-i/ireo-pentulose 5-phosphate and it reacts with various aldehydes (acceptors) to give ketoses.198, 200 Thus, substitution of L-gfh/cero-tetrulose for hydroxypyruvate in the above experiment also resulted in formation of D-en/i/iro-pentulose... 

Acrylic acid, /ro i-/3-(o-NiTROPHEira.)-a-PHENYL-, 35, 89 Acrylonitrile, 30, 80 36, 6 Acrylonitrile, triphenyl-, 31, 52 Acylation of ethanolamine with phthalic anhydride, 32, 19 Acyloin reaction, 36, 79 2-Acylpyridines, phenylhydrazones of,... 

The two most important natural pentoses, 1 -arabinose and 1 -xylose, occur in nature as polymeric anhydrides, the so-called pentosans, viz. araban, the chief constituent of many vegetable gums (cherry gum, gum arabic, bran gum), and xylan, in wood. From these pentapolyoses there are produced by hydrolysis first the simple pentoses which are then converted by sufficiently strong acids into furfural. This aldehyde is thus also produced as a by-product in the saccharification of wood (cellulose) by dilute acids. Furfural, being a tertiary aldehyde, is very similar to benzaldehyde, and like the latter undergoes the acyloin reaction (furoin) and takes part in the Perkin synthesis. It also resembles benzaldehyde in its reaction with ammonia (p. 215). 

A useful method of forming carbon-carbon bonds involves reduction of esters with sodium metal in aprotic solvents such as ether or benzene and is called the acyloin reaction ... 

An asymmetric C-C coupling, one of the most important and challenging problems in synthetic organic chemistry, seems to be most appropriate for the creation of a complete set of diastereomers because of the applicability of a convergent, combinatorial strategy [38-40]. In Nature, such reactions are facilitated by lyases which catalyze the (usually reversible) addition of carbo-nucleophiles to C=0 double bonds, in a manner mechanistically most often categorized as aldol and Claisen additions or acyloin reactions [41], The most frequent reaction type is the aldol reaction, and some 30 lyases of the aldol type ( aldolases ) have been identified so far [42], of which the majority are involved in carbohydrate, amino acid, or hydroxy acid metabolism. This review will focus on the current state of development of this type of enzyme and will outline the scope and limitations for their preparative application in asymmetric synthesis. 

Acyl imidazoles take part in a silyl acyloin reaction to give the corresponding silyl enol ethers in moderate yields. A possible mechanism is outlined in Scheme 2092. The silyl enol ethers could be hydrolysed to acyl silanes by treatment with acid. 

Lithium alkoxides of bis(trimethylsilyl) carbinols react with benzophenone to produce silyl enol ethers of acyl silanes in good yields93. The alcohols were prepared in reasonable yields by hydrolysis of the bis(trimethylsilyl) carbinol silyl ethers94,95, which in turn were produced from the corresponding esters using another silyl acyloin reaction, which itself, ironically, proceeds through an acyl silane intermediate (Scheme 21)94. 

A much more general synthesis of these silyl enol ethers, however, is based on the reductive cleavage of the carbon-sulphur bond of the silyl enol ether of a thiolester using sodium metal and chlorotrimethylsilane, once again in a silyl acyloin reaction (Scheme 22)97,98. 

A modification involves the addition to the reaction mixture of chlorotrimethyl-silane which gives the bis-(trimethylsilyloxy)alkene which is hydrolysed to yield the acyloin.145 A preparative example, which also illustrates an intramolecular acyloin reaction to form an alicyclic ring system, is given in Expt 7.10. 

The radical reductive cyclisation of diesters to acyloins (see also Section 5.9.1, p. 628) is an important method of synthesis for ring sizes from four-membered upwards. The example selected here is 2-hydroxy-3-methylcyclopent-2-enone ( corylone ) (29) (Expt 7.10), which is an important perfumery and flavouring material.53 In the first step (i), methyl acrylate is converted into its dimer with tris(cyclohexyl)phosphine in pyridine solution.5b Step (ii) is the protection of the double bond by conversion into the dimethylamino adduct. The acyloin reaction is step (iii), and the product is trapped as its bis(trimethylsilyl)ether. Finally, in step (iv), the protecting dimethylamino and trimethylsilyl groups are removed by passage down a column of silica gel. 

The acyloin reaction is a similar radical dimerisation but at the ester oxidation level.11 At first it looks just like a pinacol electrons are added to the carbonyl groups to give 47, the radicals combine to give a new C-C bond and the ethoxide groups are lost 48 to give the 1,2-diketone 49. 

A closely allied reductive linking of carbonyl groups is an intramolecular version with esters, called the acyloin reaction, which again gives a 1, 2-dioxygenated skeleton... 

Esters undergo pinacol-type coupling the acyloin reaction... 

On quenching the reaction with acid, this dianion is protonated twice to give the enol of an a-hydroxy-ketone, and it is this a-hydroxy-ketone that is the final product of the acyloin reaction. The yield in this example is a quite respectable 70%. However, in many other cases, this usefulness of the acyloin reaction is hampered by the formation of by-products that arise because of the reactivity of the enediolate dianion. It is, of course, quite nucleophilic, and is likely to be formed in the presence... 

The solution to these problems is to add trimethylsilyl chloride to the reaction mixture. The silyl chloride silylates the enediolate as it is formed, and the product of the acyloin reaction becomes a bis-silyl ether. 

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