Reactions Involving Acetals

The cleavage of acetals usually involves acidic conditions, incompatible with acid-sensitive substrates. [Ru(TRIPHOS)(MeCN)3][OTf]2 (2) in acetone offers a solution 

Transition metal Lewis acids can be used to effect deacetalization under mild conditions. Thus, 1,3-dioxolanes are hydrolyzed by PdCl2(MeCN)2 in wet acetonitrile but a better procedure is transacetalization of the dioxolanes by the same complex (1-5 %, room temperature in acetone) [6]. A good example of this method is the clean deprotection of an oxolane of a /3-hydroxyketone that is susceptible to elimination (Table 2, entry 1) [20]. TBDPS ethers and epoxides are tolerated [21]. 

Although the focus in the above cited references is on acetal cleavage, it is clear that the same catalyst systems also promote acetal formation. 

The Rh(I) complex also catalyzes aldol reactions between acetals and silyl enol ethers or ketene silyl acetals (Table 5) [44]. 

In the presence of M0CI5, acetals and thioacetals act as electrophiles towards tri-methylstyrylsilanes [45,46], WCle and TiCU can also be used in this reaction but they are less efficient (Sch. 4) [45]. 

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Entries 10 to 14 show reactions involving acetals. Interestingly, Entry 10 shows much-reduced stereoselectivity compared to the corresponding reaction of the aldehyde (The BF3-catalyzed reaction of the aldehyde is reported to be 24 1 in favor of the anti product ref. 80, p. 91). There are no stereochemical issues in Entries 11 or 12. Entry 13, involving two cyclic reactants, gave a 2 1 mixture of stereoisomers. Entry Mis a step in a synthesis directed toward the taxane group of diterpenes. Four stereoisomeric products were produced, including the Z E isomers at the new enone double bond. 

The reaction involving acetic anhydride and water in the hydrolysis is... 

Despite the lower rate in the presence of inhibitor, radioactively labelled palmitate was added to a CHCla-inhibited system. Table IV shows that butyrate was formed at a faster rate than formate, acetate, or propionate. The fact that butyrate was now one of the major end products of palmitate dissimilation indicates that secondary reactions involving acetate and/or propionate were probably serving to remove hydrogen produced during dissimilation since methanogenesis was inhibited in these experiments. This was partially verified by the findings that radioactively labelled acetate was converted to formate and butyrate at faster rates in inhibited than in uninhibited sludge. It is also possible that formation of butyrate indicates some alternative to -oxidation as a dissimilatory reaction. Acetate itself was formed from C02 in the presence or absence... 

Several new aspects of organoalane chemistry have been described this year. Orthoesters react with alanes, prepared from a variety of allyl and propargyl halides, to give the corresponding 0,y-unsaturated acetals (Scheme 45). Interestingly reactions involving acetals rather than ortho esters lead exclusively to the formation of allenic ethers (160). ... 

High-yield aq lations of hindered alcohols are of considerable current interest [ 142 ]. Older methods for such reactions involve acetic anhydride, acetyl chloride, and the use of -butyUithium in the presence of an acid chloride. More contemporary approaches include catalysts such as DMAP [143], BujP [144], and 4-pyrrohdinopyridine (PPY),with DMAP being the most popular. Although both BujP and DMAP provide good yields of acylated alcohols, these catalysts have disadvantages. For example, BU3P has a low flash point and DMAP is fairly toxic. 

It will be observed that the reaction involves two equivalents of the amine and produces, in addition to the substituted amide, an equivalent quantity of the amine hydrochloride. Acetic anhydride, on the other hand, converts the amine quantitatively into the acyl derivative, for example ... 

The solvent used m catalytic hydrogenation is chosen for its ability to dissolve the alkene and is typically ethanol hexane or acetic acid The metal catalysts are insoluble m these solvents (or indeed m any solvent) Two phases the solution and the metal are present and the reaction takes place at the interface between them Reactions involving a substance m one phase with a different substance m a second phase are called het erogeneous reactions... 

About half of the wodd production comes from methanol carbonylation and about one-third from acetaldehyde oxidation. Another tenth of the wodd capacity can be attributed to butane—naphtha Hquid-phase oxidation. Appreciable quantities of acetic acid are recovered from reactions involving peracetic acid. Precise statistics on acetic acid production are compHcated by recycling of acid from cellulose acetate and poly(vinyl alcohol) production. Acetic acid that is by-product from peracetic acid [79-21-0] is normally designated as virgin acid, yet acid from hydrolysis of cellulose acetate or poly(vinyl acetate) is designated recycle acid. Indeterrninate quantities of acetic acid are coproduced with acetic anhydride from coal-based carbon monoxide and unknown amounts are bartered or exchanged between corporations as a device to lessen transport costs. 

The N-oxide function has proved useful for the activation of the pyridine ring, directed toward both nucleophilic and electrophilic attack (see Amine oxides). However, pyridine N-oxides have not been used widely ia iadustrial practice, because reactions involving them almost iavariably produce at least some isomeric by-products, a dding to the cost of purification of the desired isomer. Frequently, attack takes place first at the O-substituent, with subsequent rearrangement iato the ring. For example, 3-picoline N-oxide [1003-73-2] (40) reacts with acetic anhydride to give a mixture of pyridone products ia equal amounts, 5-methyl-2-pyridone [1003-68-5] and 3-methyl-2-pyridone [1003-56-1] (11). 

Various processes involve acetic acid or hydrocarbons as solvents for either acetylation or washing. Normal operation involves the recovery or recycle of acetic acid, any solvent, and the mother Hquor. Other methods of preparing aspirin, which are not of commercial significance, involve acetyl chloride and saHcyHc acid, saHcyHc acid and acetic anhydride with sulfuric acid as the catalyst, reaction of saHcyHc acid and ketene, and the reaction of sodium saHcylate with acetyl chloride or acetic anhydride. 

Three-dimensional potential energy diagrams of the type discussed in connection with the variable E2 transition state theory for elimination reactions can be used to consider structural effects on the reactivity of carbonyl compounds and the tetrahedral intermediates involved in carbonyl-group reactions. Many of these reactions involve the formation or breaking of two separate bonds. This is the case in the first stage of acetal hydrolysis, which involves both a proton transfer and breaking of a C—O bond. The overall reaction might take place in several ways. There are two mechanistic extremes ... 

An apparently related reaction involves iodination in pure methanol in the presence of calcium chloride. The main product is the 21,21-d3-iodo derivative, which on reaction with acetate ion gives, surprisingly, the 21-monoacetate ... 

Citing amine basicity according to the pK of the conjugate acid permits acid-base reactions involving amines to be analyzed according to the usual Brpnsted relationships. For exarnple, we see that amines are converted to anmoniurn ions by acids even as weak as acetic acid ... 

It was found inadvisable to use more than four molecules of form-amide [ (47) when R = H] per molecule of anthranilic acid and the condensation produces best results when the mixture is heated at 120 -130°C for 2 hr followed by further heating at 170°-180 C for 2 hr. Other variants of this reaction involve the use of ammonium o-acylaminobenzoates, anthranilic acid in the presence of nitriles and acetic anhydride, o-acetamidonitrile with acetic anhydride or hydrogen peroxide, anthranilic esters and aliphatic or aromatic amides or amidines, isatoic anhydride with amides or amidines, and anthranilic esters with aryl iminochlorides in acetoned The mechanism proposed by Bogert and Gotthelf has had experimental supporR and is represented in Scheme 12. 

A modification of the Gomberg reaction involves the use of acetate instead of hydroxide ions, the process then being analogous to the decomposition of acylarylnitrosamines. 

Scheeren et al. reported the first enantioselective metal-catalyzed 1,3-dipolar cycloaddition reaction of nitrones with alkenes in 1994 [26]. Their approach involved C,N-diphenylnitrone la and ketene acetals 2, in the presence of the amino acid-derived oxazaborolidinones 3 as the catalyst (Scheme 6.8). This type of boron catalyst has been used successfully for asymmetric Diels-Alder reactions [27, 28]. In this reaction the nitrone is activated, according to the inverse electron-demand, for a 1,3-dipolar cycloaddition with the electron-rich alkene. The reaction is thus controlled by the LUMO inone-HOMOaikene interaction. They found that coordination of the nitrone to the boron Lewis acid strongly accelerated the 1,3-dipolar cycloaddition reaction with ketene acetals. The reactions of la with 2a,b, catalyzed by 20 mol% of oxazaborolidinones such as 3a,b were carried out at -78 °C. In some reactions fair enantioselectivities were induced by the catalysts, thus, 4a was obtained with an optical purity of 74% ee, however, in a low yield. The reaction involving 2b gave the C-3, C-4-cis isomer 4b as the only diastereomer of the product with 62% ee. 

Important applications for titanium have been developed in processes involving acetic acid, malic acid, amines, urea, terephthalic acid, vinyl acetate, and ethylene dichloride. Some of these represent large scale use of the material in the form of pipework, heat exchangers, pumps, valves, and vessels of solid, loose lined, or explosion clad construction. In many of these the requirement for titanium is because of corrosion problems arising from the organic chemicals in the process, the use of seawater or polluted cooling waters, or from complex aggressive catalysts in the reaction. 

Compound A, C I-fygO, undergoes reaction with dilute H2S04 at 25 °C to 3rield a mixture of two alkenes, C Hm- The major alkene product, B, gives only cvclopentanone after ozone treatment followed by reduction with zinc in acetic acid. Write the reactions involved, and identify A and B. 

Balance the half-reaction involved in the oxidation of ethanol to acetic acid. Compare the number of electrons released per mole of ethanol with the number per mole of methanol in the equivalent reaction (73c). How many electrons would be released per mole of propanol in the oxidation to propionic acid ... 

The first report of the use of bromine for the oxidation of sulphoxides appeared in 1966116. Diphenyl sulphone was isolated in 0.5-1% yield when the sulphoxide was treated with bromine in aqueous acetic acid for several hours. The yield was increased to about 5% by quenching the reaction with sodium carbonate. A kinetic study117 of a similar reaction involving dimethyl sulphoxide showed no significant yield improvement but postulated that the mechanism proceeds via an equilibrium step forming a bromosulph-onium type intermediate which reacted slowly with water giving dimethyl sulphone as indicated in equation (35). 

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