Diols, acid catalyzed reagents

The cyclohexyloxy(dimethyl)silyl unit in 8 serves as a hydroxy surrogate and is converted into an alcohol via the Tamao oxidation after the allylboration reaction. The allylsilane products of asymmetric allylboration reactions of the dimethylphenylsilyl reagent 7 are readily converted into optically active 2-butene-l, 4-diols via epoxidation with dimethyl dioxirane followed by acid-catalyzed Peterson elimination of the intermediate epoxysilane. Although several chiral (Z)-y-alkoxyallylboron reagents were described in Section 1.3.3.3.3.1.4., relatively few applications in double asymmetric reactions with chiral aldehydes have been reported. One notable example involves the matched double asymmetric reaction of the diisopinocampheyl [(Z)-methoxy-2-propenyl]boron reagent with a chiral x/ -dialkoxyaldehyde87. 

Depending upon the conditions, the reaction of benzylmagnesium chloride with benzal-dehyde can yield benzylphenylmethanol, dibenzoylphenylmethane and 1,3-diphenyliso-chroman. The last product arises from the acid-catalyzed dehydration of the 1,5-diol (296), which is the predominant product when the Grignard reagent is added to benzaldehyde at room temperature (Scheme 81) (51 3163). There was no evidence for the formation of o-methylbenzhydrol, consistent with the lack of this type of product with citronellal and various simple aliphatic aldehydes (44JA354,51JA3237). 

Discussion In principle, acetals are cleaved by acid-catalyzed hydrolysis. In most cases aqueous acetic acid, aqueous trifluoracetic acid, dilute HC1 in THF or DOWEX 50W (H+) resin are used. Thus, treatment of 6 with DOWEX ion exchange resin in methanol rapidly furnishes the corresponding 1,2-diol without any further chromatographic purification steps. Generally, polymer supported reagents benefit from the ease of removal from the reaction mixture just by filtration of the insoluble resin. The resulting diol is acetylated by addition of acetic anhydride and pyridine. Final acetal exchange is achieved by acetic anhydride and catalytic amounts of concentrated sulfuric acid. A mixture (2 1) of anomers is obtained. 

Anti hydroxylation of an alkene is readily achieved with peroxycarboxylic acids. - Acid-catalyzed ting opening of the initial product, an oxirane (epoxide), forms the monoester of a 1,2-diol, hydrolysis of which affords the parent diol. Alternative reagents which are often used for anti hydroxylation of alkenes are hydrogen peroxide with oxides of timgsten - or selenium, - and iodine-silver benzoate (Prdvost reaction). ... 

Besides stoichiometric epoxidation and subsequent hydrolysis to diols, metal-catalyzed methods for converting olefins to glycols are also known in the literature. The classical method utilizes hydrogen peroxide in the presence of catalytic amounts of acidic metal oxides (Milas reagents) [2]. Typically, strong oxidants such as osmium [3] and ruthenium tetroxides [4], permanganate [5], and chro-... 

Some thermally forbidden [2 + 2]-cycloaddition reactions can be promoted by Lewis acids1-6. With chirally modified Lewis acids, the opportunity for application in asymmetric synthesis of chiral cyclobutanes arises (for a detailed description of these methods see Sections D.l. 6.1.3.. D.l. 61.4. and references 7, 28-30). Thus, a chiral titanium reagent, generated in situ from dichloro(diisopropoxy)titanium and a chiral diol 3, derived from tartaric acid, catalyzes the [2 + 2]-cycloaddition reaction of 2-oxazolidinone derivatives of a,/ -unsalurated acids 1 and the ketene thioacetal 2 in the presence of molecular sieves 4 A with up to 96 % yield and 98% ee. Fumaric acid substrates give higher yields and enantiomeric excesses than acrylic acid derivatives8. Michael additions are almost completely suppressed under these reaction... 

In the following reaction, the aldehyde reacts with the diol because aldehydes are more reactive than ketones. The Grignard reagent will now react only with the keto group. The protecting group can be removed by acid-catalyzed hydrolysis. 

Protection of 1,2-diols. These reagents are prepared from the diketones and HC(OMe)j in methanol containing a little sulfuric acid. By an acid-catalyzed exchange reaction, 2,3-dimethoxy-l,4-dioxane derivatives are formed in the reaction with 1,2-diols. Diequatorial diols are selectively protected. Actually, the protection can be performed directly by the reaction of a 1,2-diol with an a-diketone, trimethyl orthoformate, in methanol in the presence of camphorsulfonic acid. ... 

The resulting alcohols are useful synthetic intermediates. For example, 2-butyne-l,4-diol is a precursor for the production of oxacyclopentane (tetrahydrofuran, one of the solvents most frequently employed for Grignard and organoUthium reagents) by hydrogenation, followed by acid-catalyzed dehydration. 

The hydrolysis of epoxides is a eonvenient method for the preparation of vic-diols. The reaction is catalyzed by acids or bases (see discussion of the mechanism on p. 462). Among acid catalysts the reagent of choice is perchloric acid, since side... 

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