Acetals vicinal diols

Reaction is acid catalyzed Equilibrium constant normally favorable for aide hydes unfavorable for ketones Cyclic acetals from vicinal diols form readily... 

The reaction is based, on the one hand, on the oxidative cleavage of vicinal diols by lead(IV) acetate and, on the other hand, on the reaction of dichlorofluorescein with lead(IV) acetate to yield a nonfluorescent oxidation product. The dichlorofluorescein only maintains its fluorescence in the chromatogram zones where the lead(IV) acetate has been consumed by the glycol cleavage reaction [1],... 

Although lead tetraacetate can attack many polar and nonpolar functions in the steroid molecule, its greatest reactivity is towards vicinal diols. These diols are generally cleaved so rapidly under stoichiometric conditions that other alcohol functions in the molecule need not be protected. Thus lead tetraacetate in acetic acid at room temperature splits the 17a,20-diol group in (9) to yield the 17-ketone (10), the allylic A -3jS-alcohol remaining intact during this oxidation. Since lead tetraacetate is solublein many anhydrous... 

The acetates of vicinal diols undergo reductive elimination on treatment with metal-ammonia yields of olefin are only significant if one ester is tertiary and the arrangement is tran -diaxial. ... 

In 1970, it was disclosed that it is possible to achieve the conversion of dimethylformamide cyclic acetals, prepared in one step from vicinal diols, into alkenes through thermolysis in the presence of acetic anhydride." In the context of 31, this two-step process performs admirably and furnishes the desired trans alkene 33 in an overall yield of 40 % from 29. In the event, when diol 31 is heated in the presence of V, V-dimethylforrnamide dimethyl acetal, cyclic dimethylformamide acetal 32 forms. When this substance is heated further in the presence of acetic anhydride, an elimination reaction takes place to give trans olefin 33. Although the mechanism for the elimination step was not established, it was demonstrated in the original report that acetic acid, yV, V-dimethylacetamide, and carbon dioxide are produced in addition to the alkene product."... 

Alcohols (diols, polyols, sugan) Lead(IV) acetate -dichloro fluorescein Diol cleavage of vicinal diols, e. g. sugars, sugar alcohols. The lead tetraacetate consumed is no longer available to decompose the fluorescent dichlorofluorescein. [3, 8]... 

Acyloins were converted to mixtures of stereoisomeric vicinal diols by catalytic hydrogenation over copper chromite [972]. More frequently they were reduced to ketones by zinc (yield 77%) [913, 914], by zinc amalgam (yields 50-60%) [975], by tin (yields 86-92%) [173], or by hydriodic acid by refluxing with 47% hydriodic acid in glacial acetic acid (yields 70-90%) [916], or by treatment with red phosphorus and iodine in carbon disulfide at room temperature (yields 80-90%) [917] Procedure 41, p. 215). Since acyloins are readily accessible by reductive condensation of esters (p. 152) the above reductions provide a very good route to ketones and the best route to macro-cyclic ketones [973]. 

The groups of Burczyk, Takeda, and others have made thorough studies of cyclic acetals, such as 1,3-dioxolane (five-membered ring) and 1,3-dioxane (six-membered ring) compounds, illustrated in Fig. 13. They are typically synthesized from a long-chain aldehyde by reaction with a diol or a higher polyol. Reaction with a vicinal diol gives the dioxolane [40-42] and 1,3-diols yield dioxanes [43,44]. 

Interesting progress has also been made on that part of the studies concerning the synthesis of acetals that deals with the strained ring formed from diequatorial, vicinal diols. 

Synthesis of the cyclic acetal 25 in 63% yield by reaction of the unsaturated bromo-nitrilo sugar 24 (E configuration) with ethylene glycol in the presence of sodium hydroxide has been reported.40 An extension of this work was given for sugars having a vicinal diol... 

There are several methods reported in the literature for transforming vicinal diols into ct-diketones while avoiding the risk of C-C bond cleavage.26 Examples include the standard Swem conditions (dimethyl sulfoxide and oxalyl chloride followed by triethylamine), or the use of DMSO activated by acetic anhydride, pyridine-sulfur trioxide complex, or dicyclohexylcarbodiimide (Mq/J-att oxidation). Diones are also obtained by treatment with benzalacetone as a hydride acceptor in the presence of catalytic amounts of tris(triphenylphosphine)ruthenium dichlonde [(PPh RuCFl.27 Recent developments include the use of w-iodoxyben/.oic acid28 or the oxoammonium salt of 4-acctamidoletramethylpipcridine-1-oxyl and y -toluencNulfonic acid.29... 

The phase-transfer technique is a simple and efficient tool for the benzylation of carbohydrates. With benzyltriethylammonium chloride or tetrabutylammonium bromide as a catalyst, a mixture of aqueous, 50 % sodium hydroxide and benzyl bromide or chloride in benzene or dichloromethane solution gives a good yield of the fully protected product [103, 104], such as methyl 2,3-di-0-benzyl-4,6-0-benzylidene c-D-glucopyranoside, when stirred at room temperature for several hours. The latter catalyst is slightly more efficient. Dichloromethane has been observed to produce methylene acetals from cis vicinal diols under comparable conditions [103]. 

First write out the chemical equation for the reaction that takes place. Vicinal diols (1,2-diols) react with aldehydes to give cyclic acetals. 

Vicinal diols are converted into acetals by reaction with 4-pentenal catalyzed by camphorsulfonic acid. The acetals are converted on reaction with NIS into an... 

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