Trifluoroacetylation

The reversible blocking of amino groups by trifluoroacetylation provides a means of limiting tryptic digestion to arginyi residues (Goldberger and Anfinsen 1962 Goldberger 1967). 

In a typical procedure, protein (25 mg/ml) is dissolved in water and brought to pH 10 at 25 °C by careful addition of base regulated by a pH-stat. Ethylthioltrifluoroacetate (0.25 ml/25 mg of protein) is added, and the pH is maintained at 10 by the addition of 1 M KOH. After 1 hr, additional ethylthioltrifluoroacetate (0.1 ml/25 mg of protein) is added and the reaction continued for a further 30 min. The pH is then lowered to 5-6 by the cautious addition of 1 M acetic acid. The protein derivative is then precipitated by adding the reaction mixture to 4 volumes 

An alternative isolation procedure may be employed if the protein derivative is partially soluble in the aqueous ethanol. The pH of the reaction mixture is lowered to 8 by the careful addition of 1 M acetic acid. The mixture is filtered, and the filtrate, containing the protein derivative, is dialyzed against 0.01 M potassium phosphate buffer pH 8, at 4°C, followed by exhaustive dialysis against water, and by lyophiliza-tion. 

It should be noted that high concentrations of ethanethiol are formed on the hydrolysis of ethylthiotrifluoroacetate at high pH. Under alkaline conditions, ethanethiol causes both disruption and rearrangement of disulfide bonds. 

A trifluoroacetylation procedure (Levy and Paselk 1973), performed entirely under non-aqueous conditions, appears to eliminate the side-reactions involving disulfide bonds. In this procedure, insulin hydrochloride (24 mg, 4 pmoles) was dissolved in dimethylformamide (5.0 ml) (pre-purified by distillation at 0.2 mm Hg, after refluxing for 2 hr over calcium hydride), and triethylamine (10 pi, 72 pmole), and stirred at 24°C for 5 min. Ethylthioltrifluoroacetate (18.4 mg, 120 pmoles) dissolved in dimethylformamide (1.15 ml) was added to the insulin solution. The reaction was allowed to proceed for 60 min. The product was precipitated by the addition of anhydrous ether (40 ml). The precipitate was isolated by centrifugation, washed with acetone, and ether, and then dried in a dessicator over P2O5 under high vacuum. 

The early investigations [613-615] of N-vinylpyrroles trifluoroacetylation have been undertaken in order to assess the relative activity of two most probable nucleophilic sites (a-position of the pyrrole ring and p-position of the N-vinyl moiety) and to develop preparative methods for the synthesis of novel fluorinated pyrrole derivatives [614]. 

Usually, trifluoroacetyl cation attacks pyrrole ring at the a-position or, when the latter is occupied, at the p-position [616-618]. However, the vinyl group at nitrogen atom, in principle, could significantly affect the reactivity of the pyrrole molecule. The I-effect of the double bond and its competitive conjugation with nitrogen lone electron pair should decrease the aromaticity of the pyrrole cycle. 

There is a body of evidences [93] that in N-vinylindoles, electrophile attacks the vinyl group rather than the pyrrole moiety. The same phenomenon is observed in the course of electrophilic addition of water and alcohols to N-vinylpyrroles (see Sections 2.2.1 and 2.2.2). This observation is compatible with the data on ready 

SCHEME 2.164 Synthesis of di(pyrrol-2-yl)-l,2,4,5-tetrazine or 3,5-di(pyrrol-2-yl) -amino-l,2,4-triazole from pyrrole-2-carbonitrile and hydrazine hydrate. 

N-vinylpyrroles do not show significant alienation of nitrogen lone electron pair from heteroaromatic system, and as a consequence, p-n-conjugation of the N-vinyl group is not entirely realized. This leads to qualitative alterations in nature and reactivity of the N-vinyl moiety as compared with vinyl ethers, sulfides, enamines, and N-vinyl amides, which are readily trifluoroacylated under the conditions described earlier at the vinyl group p-position [620]. In other words, of the two 

This is quite a useful technique which can give a rapid, positive identification of -OH, -NH2, and -NHR groups in cases where deuteration would be of little value. Even though the technique can be a little time-consuming and labour-intensive in terms of sample preparation, it can nonetheless yield results in less time than it would take to acquire definitive 13C data - particularly if your material is limited. 

The top trace shows what happens when the sample is shaken for a few seconds with a few drops of TFAA. The reaction shown in Structure 7.3 occurs. 

The resultant spectrum is clearly very different from the alcohol, as the trifluoroacetic ester function is far more deshielding with respect to the alpha proton than is the -OH group. A downfield shift of 1 ppm can be seen. This clearly distinguishes the alcohol from the analogous chloro compound which would of course give no reaction. 

Use of this reagent is however, somewhat limited. You can only use it in solvents which don t react with it (D4-methanol, and D2O are obviously out of the question), or contain a lot of water, i.e., D6-DMSO. Another slight drawback is that the cleaving of the anhydride liberates trifluoroacetic acid, 

---

Formation - with trifluoroacetic anhydride or trifluoroacetyl chloride Cleavage - K2CO3, MeOH... 

Trifluoroacetamides are more stable toward nucleophiles than the corresponding esters and are easily formed from trifluoroacetic anhydride and the amine. The trifluoroacetyl group (Tfac) is slowly cleaved by aqueous or methanolic HQ, NH, or Ba(OH)2 solutions as well as by NaBHj in methanol (M.L. Wolfrom, 1967). 

A -(2 2-Diethoxyethyl)anilines are potential precursors of 2,3-unsubstituted indoles. A fair yield of 1-methylindole was obtained by cyclization of N-inethyl-M-(2,2-diethoxyethyl)aniline with BFj, but the procedure failed for indole itself[2], Nordlander and co-workers alkylated anilines with bromo-acetaldehyde diethyl acetal and then converted the products to N-trifliioro-acetyl derivatives[3]. These could be cyclized to l-(trifluoroacetyl)indoles in a mixture of trifluoroacetic acid and trifluoroacetic anhydride. Sundberg and... 

Nonvolatile analytes must be chemically converted to a volatile derivative before analysis. For example, amino acids are not sufficiently volatile to analyze directly by gas chromatography. Reacting an amino acid with 1-butanol and acetyl chloride produces an esterfied amino acid. Subsequent treatment with trifluoroacetic acid gives the amino acid s volatile N-trifluoroacetyl- -butyl ester derivative. 

Diamide Chiral Separations. The first chiral stationary phase for gas chromatography was reported by GH-Av and co-workers in 1966 (113) and was based on A/-trifluoroacetyl (A/-TFA) L-isoleucine lauryl ester coated on an inert packing material. It was used to resolve the tritiuoroacetylated derivatives of amino acids. Related chiral selectors used by other workers included -dodecanoyl-L-valine-/-butylamide and... 

Fig. 13. Enantiomeric separations of monohalohydrocarbons on a 2,6-0-dipentyl-3-0-trifluoroacetyl-y-cyclodexttin coated capillary column (10 m, 0.25...

Trifluoroethanol was first prepared by the catalytic reduction of trifluoroacetic anhydride [407-25-0] (58). Other methods iaclude the catalytic hydrogeaatioa of trifluoroacetamide [354-38-1] (59), the lithium aluminum hydride reductioa of trifluoroacetyl chloride [354-32-5] (60) or of trifluoroacetic acid or its esters (61,62), and the acetolysis of 2-chloro-l,l,l-trifluoroethane [75-88-7] followed by hydrolysis (60). More recently, the hydrogenation of... 

The synthesis and the quantitative gas chromatographic analysis of stable, yet volatile, A/-trifluoroacetyl- -butyl esters of amino acids has been estabhshed (124). An extensive review of subsequent advances ia gas chromatographic iastmmentation has been provided (125). 

The need for pyrazoles substituted with the trifluoroacetyl group led to the reaction of ethoxyvinyl ether with trifluoroacetic anhydride, yielding 4-ethoxy-l,l,l-trifluoro-3-buten-2-one (38) this further reacted with aldehyde / fZ-butyUiydrazones, and after cyclization at room temperature under mildly acidic conditions the pyrazoles were obtained in satisfactory yields (eq. 7). Further treatment with H2SO4 removed the tert-huty group, thus providing an opportunity for further derivatization at N. ... 

Although the same theoretical studies indicate very small energy differences between the syn and anti conformers of the 3-carbaldehydes of furan, thiophene and pyrrole with a slight preference for the syn conformer, in chloroform solution the furan- and thiophene-3-carbaldehydes adopt the anti conformers to the extent of 100 and 80% respectively (82X3245). However, A-substituted 3-(trifluoroacetyl)pyrroles exist in solution as mixtures of rotational isomers (80JCR(S)42). 

The reactivity of five-membered rings with one heteroatom to electrophilic reagents has been quantitatively compared in a variety of substitution reactions. Table 2 shows the rates of substitution compared to thiophene for formylation by phosgene and iV,AT-dimethylfor-mamide, acetylation by acetic anhydride and tin(IV) chloride, and trifluoroacetylation with trifluoroacetic anhydride (71AHC(13)235). 

The reactivity sequence furan > tellurophene > selenophene > thiophene is thus the same for all three reactions and is in the reverse order of the aromaticities of the ring systems assessed by a number of different criteria. The relative rate for the trifluoroacetylation of pyrrole is 5.3 x lo . It is interesting to note that AT-methylpyrrole is approximately twice as reactive to trifluoroacetylation as pyrrole itself. The enhanced reactivity of pyrrole compared with the other monocyclic systems is also demonstrated by the relative rates of bromination of the 2-methoxycarbonyl derivatives, which gave the reactivity sequence pyrrole>furan > selenophene > thiophene, and by the rate data on the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3] (35) with a further selection of heteroaromatic substrates (Scheme 5). The comparative rates of reaction from this substitution were 2-methylindole == AT-methylindole>indole > pyrrole > furan > thiophene (73CC540). 

A quantitative study has been made on the effect of a methyl group in the 2-position of five-membered heteroaromatic compounds on the reactivity of position 5 in the formylation and trifluoroacetylation reaction. The order of sensitivity to the activating effect of the substituent is furan > tellurophene >selenophene = thiophene (77AHC(2l)ll9). 

Oxazolium, anhydro-2-m-bromophenyl-5-hydroxy-3-methyl-4-trifluoroacetyl-X-ray, 6, 181 (75JCS(P2)1280)... 

Oxazolo[3,2-a]pyridinium, anhydro-2-hydroxy- C NMR, 6, 652 <70JCS(C)1485) Oxazolo[3,2-a]pyridinium, anhydro-2-hydroxy-3-trifluoroacetyl-IR, Is, 653 <70CPB1233)... 

Carbazole, 2-hydroxy-reactions with citral, 4, 235 Carbazole, 2-hydroxy-9-methyl-synthesis, 4, 294 Carbazole, N-hydroxymethyl-as metabolite of carbazole, 1, 230 Carbazole, N-isopropyl-PE spectroscopy, 4, 190 Carbazole, A7-methyl- N NMR, 4, 175 X-ray spectroscopy, 4, 163 Carbazole, 1-nitro-synthesis, 4, 282 Carbazole, tetrahydro-dehydrogenation, 4, 282, 312 synthesis, 4, 107, 337, 353 Carbazole, 1,2,3,4-tetrahydro-reduction, 4, 255, 256 synthesis, 4, 312, 325, 352 Carbazole, 1,2,3,4-tetrahydro-1 -oxo-synthesis, 4, 337 Carbazole, 9-trifluoroacetyl-synthesis, 4, 218 Carbazole, vinyl-polymers, 1, 275, 301 Carbazole, 9-vinyl-copolymer... 

Oxazolium hydroxide, anhydro-2-m-bromophenyl-5-hydroxy-3-methyl-4-trifluoroacetyl-X-ray structure, 6, 180 Oxazolium hydroxide, anhydro-4-hydroxy-cycloaddition reactions, 6, 208 IR spectra, 6, 186 mesoionic structures, 6, 179 tautomerism, 6, 185 reactions, 6, 206-211 synthesis, 6, 225... 

AcOH, HBr, 10°, 10 min, 70% yield. Phthaloyl or trifluoroacetyl groups on amino acids are stable to these conditions benzyloxycarbonyl (Cbz) or /-butoxycarbonyl (BOC) groups are cleaved. 

Either mechanism explains why trifluoroacetylation of the nucleophile does not occur. Protonation of the anhydride would occur selectively at the more electron-rich carbonyl oxygen, rather than at the carbonyl flanked by the very electron-withdrawing trifluor-omethyl groiq). Similarly, cleavage of the imsymmetrical anhydride would occur to give the more stable acyhum ion. The trifluoroacetylium ion would be less stable. 

A very convenient synthetic procedure for nitration involves the mixing of a nitrate salt with trifluoroacetic anhydride. This presumably generates trifluoroacetyl nitrate. 

Bromination can also be carried out using solutions of acetyl hypobromite or trifluoroacetyl hypobromite. Acetyl hypobromite is considered to be the active halogen-ating species in solutions of hypobromous acid in acetic acid ... 

Chlorine monofluoride can be tamed by reacting it with cyanunc fluoride to form the trimer (CF2NC1)3 The trimer reacts with 1,1-dichloro 2,2,2 tri fluoroethanesulfenyl chloride to form a mixture of 1 chloro-l,2,2,2-tetra-fluoroethanesulfenyl chlonde and perfluoroethanesulfenyl chloride In the same Way, tnfluoroacetyl bromide yields trifluoroacetyl fluonde[14] (equation 16)... 

Electrophilic nitrosation of the carbanion generated from the reaction of an organic base with a strong organic acid, such as a-hydrohexafluoroisobutyronitnle [2], a hydrohexafluoroisobutyric acid or its acid chloride [2], or a hydrotetra fluoroethanesulfonyl fluoride [4], yields the corresponding a-nitroso compound as the major product (equations 2 and 3) The a-hydrohexafluoroisobutyric acid or acid chloride reacts with excess trifluoroacetyl nitrite in dimethylformamide to afford the O substituted oxime [3] (equation 4)... 

In the presence of potassium fluoride in dimethylformamide, trifluoroacetyl nitrite converts perfluoroisobutylene to tris(trifluoromethyl)nitrosomethane via the carbanion generated by the nucleophilic attack of fluoride ion on perfluoro-isobutylene [5] (equation 5)... 

---