Trifluoroacetic acid Subject

Intramolecular Friedel-Crafts reactions can sometimes compete with organosil-icon hydride reductions of benzylic-type alcohols to cause formation of undesired products. An example is the attempted reduction of alcohol 26 to the corresponding hydrocarbon. When 26 is treated with triethylsilane in trifluoroacetic acid at room temperature for 15 hours, a mixture of the two fluorene isomers 27 and 28 is obtained in a combined yield of 45%. None of the hydrocarbon structurally related to the substrate alcohol 26 is obtained.171 Whether this problem could be circumvented by running the reduction at a lower temperature or with a different acid remains subject to experimentation. 

The diastereoselectivity of the reduction of a-substiluted ketones has been the subject of much investigation. The reagent combination of trifluoroacetic acid and dimethylphenylsilane is an effective method for the synthesis of erythro isomers of 2-amino alcohols, 1,2-diols, and 3-hydroxyalkanoic acid derivatives.86,87,276,375 Quite often the selectivity for formation of the erythro isomer over the threo isomer of a given pair is >99 1. Examples where high erythro preference is found in the products are shown below (Eqs. 218-220).276 Similar but complementary results are obtained with R3SiH/TBAF, where the threo isomer product... 

Recoveries of Galacturonic Acid Subjected to Treatment with 2 M Trifluoroacetic Acid"... 

In addition to phosphate and halide anion binding, carboxylate chelation by sapphyrin macrocycles has been the subject of recent investigation. To date, two crystal structures have been solved. A 2 1 complex formed between diprotonated sapphyrin 3 and trifluoroacetic acid shows that the oxyanions are chelated above and below the sapphyrin plane (Figure 10). ° Greater complexity of organi-... 

Trifluoroethanol [5n] This solvent is strongly acidic and the major impurities are water and trifluoroacetic acid. In its purification, it is dried over potassium carbonate anhydride for a night and then subjected to fractional distillation (reflux ratio 10 1) under atmospheric pressure, the middle fraction being collected. 

Among these groups (siloxy, amino, benzotriazolyl, thiolate, cyanide) the methoxy group has been the subject of a large number of studies. To generate the azomethine ylid, essentialy three reagents have been utilized silyl triflate (or iodide), trifluoroacetic acid and lithium fluoride (sometime combined with sonication). Thermal rearrangement has also proven to be efficient. 

Radical cations that are produced by electrochemical oxidation are not stable in solvents with appreciable base character. This results because such radicals are subject to attack by available nucleophiles, and solvents that contain donor electron pairs are good nucleophiles. Cation radicals are most stable in solvents that are good Lewis acids and show negligible basic properties. Some of the solvent systems that have been employed to stabilize electrochemically produced cation radicals include nitromethane and nitrobenzene,21 dichloro-methane,22 trifluoroacetic acid-dichloromethane (1 9),23 nitromethane-AlCl3,24 and AlCl3-NaCl (1 l).25 Organic chemists should be familiar with the stabilization of carbonium ions by superacid media.26 These media usually contain fluorosulfuric acid, or mixtures of fluorosulfuric acid with antimony pen-tachloride and sulfur dioxide, and are potent solvents for the production and stabilization of organic cations. 

The silver(I)-mediated electrocyclic ring opening of halocyclopropanes has been used to induce extensive skeletal rearrangements in gcm-dibromospiropentanes, providing rapid construction of naphthalenes and/or indenes (Scheme4.21 ).34 A variety of Lewis acids, Brpnsted acids, and solvent effects were carefully examined before optimal conditions were identified. It was found that subjection of spirocycle 60 to silver acetate in trifluoroacetic acid afforded rearrangement products 61 and 62 in moderate to good yields. The proposed mechanism of the reaction is illustrated in Scheme 4.21. 

All of these solvents (neat) and all possible 1 1 mixtures (19X19) were tested for their ability to completely solubilize each of the 5 peptides at a level of 1 mg/ml. Each neat solvent and 1 1 mixture was subjected to 5 independent variations no additives, added 1% trifluoroacetic acid (TEA), added 2% pyridine, added 1% TEA + 5% water, added 2% pyridine + 5% water. Each solvent system was tested for its ability to solubilize the peptides both upon prolonged incubation (48 hrs) at room temperature and also when warmed on a hot water bath (2 minutes, 90 degrees). A number of less systematic tests were conducted involving mixtures including the following solvents formic acid, TEA, N-methylpyrollidinone (NMP), hexafluorobenzene (HEB). 

A f/ireo-selective siloxonium (aldol-like) pathway II was favored when BF3 OEt2 was used as catalyst (Sch. 3) [11]. The reaction of benzaldehyde by quenching after 5 min resulted in 48 % yield of the final cyclic products 3 (1 8 cisitrans ratio) and 46 % yield of the Mukaiyama-like aldol products 5 (1 2 threolerythro ratio). When either threo or erythro adduct was re-subjected to trifluoroacetic acid media, each underwent conversion to the corresponding y-pyrones 3. 

Biological oxidation of a 2-aminoimidazole gives poor yields (<38%), and none at all with l-alkyl-2-aminoimidazoles. Nor will oxidation with peroxy-trifluoroacetic acid work It is, however, satisfactory for the oxidation of 4-aminoiniidazoles (which are usually rather unstable compounds). ITie most common way of making 2-nitroi midazoles is from the diazonium fluoro-borates subjected to the Gattermann reaction (see Section 7.3). Yields vary from 20 to 50% [6, 7], and again are dependent on the availability of the 2-aminoimidazoles (see Section 8.2.2). 

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