Acetic acid/acetate formation processes

As expected, some sequences also occur where a domino anionic/pericyclic process is followed by another bond-forming reaction. An example of this is an anionic/per-icyclic/anionic sequence such as the domino iminium ion formation/aza-Cope/ imino aldol (Mannich) process, which has often been used in organic synthesis, especially to construct the pyrrolidine framework. The group of Brummond [450] has recently used this approach to synthesize the core structure 2-885 of the immunosuppressant FR 901483 (2-886) [451] (Scheme 2.197). The process is most likely initiated by the acid-catalyzed formation of the iminium ion 2-882. There follows an aza-Cope rearrangement to produce 2-883, which cyclizes under formation of the aldehyde 2-884. As this compound is rather unstable, it was transformed into the stable acetal 2-885. The proposed intermediate 2-880 is quite unusual as it does not obey Bredf s rule. Recently, this approach was used successfully for a formal total synthesis of FR 901483 2-886 [452]. 

An efficient chemical process for closing a diphenylamine is that using palladium(II) acetate (2 mol for substrates carrying electron-withdrawing groups) in acetic acid-methanesulfonic acid. Carbazole formation has been achieved with alkyl-, halo-, nitro-, and carboxyl-substituted diphenylamines. 1-Chlorocarbazole and carbazol-l-yl carboxylic acid as examples were efficiently prepared. - This is probably the best method now available for cyclizing diphenylamines. 

The most widely used buffers in this category are acetate, formate, citrate, and succinate. This group is useful in the pH range 3 to 6, a region that offers few other buffer choices. All of these acids are natural metabolites, so they may interfere with the biological processes under investigation. Also, citrate and succinate may interfere by binding metal ions (Fe3+, Ca2+, Zn2+ Mg2+, etc.). Formate buffers are especially useful because they are volatile and can be removed by evaporation under reduced pressure. 

Addition of primary amines to carbonyl groups has been the subject of extensive study, notably by Jencks and co-workers.91 The most striking feature of these reactions is the characteristic maximum in the graph of reaction rate as a function of pH.92 Figure 8.10 illustrates the observations for the reaction of hydroxylamine with acetone. It is also found that the sensitivity of rate to acid catalysis,93 and to substituent effects,94 is different on either side of the maximum in the pH-rate curve. These phenomena may be understood in terms of the two-step nature of the reaction. In acetal formation, we saw in Section 8.3 that the second step is rate-limiting in the overall process, and it is relatively easy to study the two steps separately here, the rates of the two steps are much more closely balanced, so that one or the other is rate-determining depending on the pH. 

Izawa et have reported the photochemical alcoholysis of the acetamide derivatives (122) to afford the esters (123). Related to this is the photochemical alcoholysis of ft),a>,ft)-tribromoacetophenone yielding benzoyl formate. Irradiation of the cycloalkanones (124) in methanol at 300 nm affords high yields of the corresponding dimethylacetals. The authors discount an acid-catalysed process even though the presence of sodium acetate suppresses the acetal formation. 

The cleavage of THP-ethers is performed under mild acidic conditions. Typically, HOAC-H2O (4 1) at 45 °C is used. If water is not suitable or the solubility of the substrate is too low, an acetal exchange process using methanol or ethanol as solvent, with catalytic amounts of acid (often p-toluenesulfonic acid, or its pyridinium salt), can be performed. For the formation of Z-13 this kind of acetal exchange process was chosen. 

The formation of vinyl acetate from ethylene was first reported by Moiseev et al. (31), The compound was obtained by reaction of ethylene with PdCl2 in an acetic acid solution containing sodium acetate. Whether in this medium vinyl acetate formation occurs via the monomeric [PdCla C2H4] TT-complex, postulated as intermediate in the Wacker acetaldehyde process, or via the dimer (C2H4 PdCl2)2, previously described by... 

For acetal formation, besides the methods discussed thus far, further possibilities exist. For instance, oxidation/reduction methods could be envisaged. Thus, as described,the acceptor can be oxidized to the acid. Anomeric 0-acylation is a convenient process (see Section 1.2.4), requiring in the final step reduction of the acyloxy group to the alkoxy group, which could be performed in simple cases with borane. Similarly the donor could oxidized to the lactone stage (and derivatives) and then acceptor addition and reduction would lead to the desired product. However, the efficiency of such methods has to be evaluated on the basis of the existing methodology. 

These polymers are removed at the place of highest concentration, the catalyst solution. The main task of suitable processes is to achieve a rhodium-free separation. This task was solved by differently conceived liquid/liquid extractions [87-91]. With a shift of the production ratio toward acetic acid, the formation of polymers decreases. 

Thus, reactions (6) - (8) explain chemical mechanism of acetic acid chain formation at CA photolysis and agree with kinetic scheme of the process. According to the scheme, length of the chain of acetic acid formation depends on light intensity and at intensities, being used, it is 10-30 units. 

Triphenylpyrylium undergoes exchange at the 3- and 5-positions in hot deuterioacetic acid, but the process probably involves, not protonation of the pyrylium cation, but formation of an equilibrium concentration of an adduct, with acetate added to C-2, allowing enol ether protonation and thus exchange. ... 

JV-Oxide rearrangements in heteroaromatic compounds are frequently induced by either photochemical or acid anhydride initiated processes, and usually involve formation of a C—O bond at the carbon a to the original N-oxide. 60 One of the few -rearrangements is observed by treating 1,3-dimethylpyrido[2,3-r/Jpyrimidine-2,4(l //,3//)-dione 8-oxide (1 see Section 7.2.2.1.1.5.1.) with acetic acid/acetic anhydride at 90 CC, which gives 6-acetoxy-l,3-dimethylpyrido[2,3-t/]pyrimidine-2,4(l//,3//)-dione (2). When the reaction is run at reflux temperature in trifluoroacctic anhydride/trifluoroacetic acid, a 46% yield each of the a- and -product, 3 and 4, respectively, is obtained.310... 

Hu and coworkers [126, 127] firstly developed an in situ precipitation approach to fabrication of 3D ordered chitosan rods with a structure of concentric circles through the formation process illustrated in Fig. 11. Briefly, the chitosan/acetic acid solution is filled into a bag made of chitosan membrane, and then immersed into 5% NaOH aqueous solution. When OH ions from the outside solution permeate into... 

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