Pentafluorophenyl acetate

The formation of ethyl cyano(pentafluorophenyl)acetate illustrates the intermolecular nucleophilic displacement of fluoride ion from an aromatic ring by a stabilized carbanion. The reaction proceeds readily as a result of the activation imparted by the electron-withdrawing fluorine atoms. The selective hydrolysis of a cyano ester to a nitrile has been described. (Pentafluorophenyl)acetonitrile has also been prepared by cyanide displacement on (pentafluorophenyl)methyl halides. However, this direct displacement is always aecompanied by an undesirable side reaetion to yield 15-20% of 2,3-bis(pentafluoro-phenyl)propionitrile. 

Pentafluorophenyl acetate is a highly selective acetylating reagent, useful for acetylations at hydroxyl and amino groups under mild conditions When applied to the acetylation of amino alcohols, it gives selective formation of H acetyl derivatives at room temperature and M,0 diacetylated products under moderate heating in the presence of triethylamine [149]... 

Ethyl frfl/is-l,3-butadiene-l-carbamate, 59, 5 Ethyl butyrylacetate, 55, 73, 75 Ethyl carbamate, 57, 95 Ethyl chloroformate, 59, 2 Ethyl cyanoacetate, 55, 58, 60 57,80 ETHYL 1-CYANO-l-METHYLCYCLO-HEXANECARBOXYLATE, 55, 57 Ethyl 7-2-cyano-6-octenoate, 55,57 Ethyl cyano(pentafluorophenyl)acetate,... 

Ethyl bromide, 57, 66, 58, 1, 2, 4 Ethyl carbamate, 57, 95 Ethyl cyanoacetate, 57, 80 Ethyl cyano(pentafluorophenyl)acetate,... 

The catalytic activity of the oxoisoindolium salt 54 and 55 was compared to that of trityl tetrakis[pentafluorophenyl]borate salts in the addition reaction of enol acetate to benzaldehyde and glycosylation reaction (Scheme 59) [151, 152]. 

Since the appearance of CHEC-II(1996), no entirely new approaches to pyridazino[4,3-t]pyridazines have been reported. However, the cyclization of furanylidene acetates described previously has been extended to permit the preparation of ort o-quinonoid systems, as illustrated in Equation (3) <1990JPS123>. A further variation on the bicyclization of a bis(hydrazone) has been reported more recently (Scheme 4), involving nucleophilic displacement of fluoride from a pentafluorophenyl group <2005MC252>. 

To a solution of (8-pentafluorophenyloxycarbonylmethyl-6//, 12//-5,11 -methano-dibenzo[ / [1, 5]dibenzocin-2-yl)-acetic acid pentafluorophenyl ester 28 (0.541 g, 0.807 mmol) in THF (8.00 mL) was added NH4OH (1.60 mL, 13.2 mmol). The reaction mixture was stirred for 1 h at room temperature followed by addition of sat. NaHCC>3 (45.0 mL). The reaction mixture was continued stirring for another 20 min followed by filtration to separate the precipitated product, which was dried to give 0.233 g of 29 as a white solid in 86 % yield. 

Bis(pentafluorophenyl) tin dibromide effects the Mukaiyama aldol reaction of ketene silyl acetal with ketones, but promotes no reaction with acetals under the same conditions. On the other hand, reaction of silyl enol ether derived from acetophenone leads to the opposite outcome, giving acetal aldolate exclusively. This protocol can be applied to a bifunctional substrate (Equation (105)). Keto acetal is exposed to a mixture of different types of enol silyl ethers, in which each nucleophile reacts chemoselectively to give a sole product.271... 

Thus, two types of active esters are of interest those formed from an acid and a substituted phenol (12-15) and those formed from an acid and a substituted hydroxylamine (16-19). Both types are reactive by virtue of the electron-withdrawing properties of the OR moiety in 2. The level of activation of the substituted phenyl esters varies directly with the electronic effect going from 4-nitrophenyl to 2,4,5-trichlorophenyl, pentachlorophenyl, and pentafluorophenyl, which corresponds with the increasing acidity of the phenols. A diminution in the rate of aminolysis is caused by the presence of a substituent in the ortho position of the ring.f l An additional phenomenon contributes to the reactivity of the esters formed from substituted hydroxylamines, namely anchimeric assistance. Since the anoinolysis of active esters is a bimolecular reaction, it is dependent on concentration and can be forced to completion by an excess of one of the reactants. Aminolysis is also characterized by a pronounced dependence on the polarity of the solvent in particular for the esters formed from substituted phenols, the half-life of a 2,4,5-trichlorophenyl ester in the presence of benzylamine being one hundred times less in dimethylformamide than in benzene. Furthermore, aminolysis is catalyzed by mild acid such as acetic acid. The rate of anoinolysis is slowed if the side chain of the active ester contains a P-methyl substituent. 

HPLC analysis of taxanes is achieved almost exclusively in reversed phase mode on various stationary phases. The normal-phase HPLC mode has been applied in very limited cases and resulted in broad peaks and long analysis times (retention times of 45 min for paclitaxel and 38 min for cephalomannine). The namre of the sample is the main criterion for the choice of the stationary phase. Analysis of plant material is performed mostly on phenyl, biphenyl, and pentafluorophenyl materials, but silica-based cyano, Cig, and Cg materials have been used as well. C18 phases are the most common material utilized in pharmacokinetic studies. Mobile phases typically consist of mixtures of methanol, acetonitrile, and water or buffer (mostly ammonium acetate). Detection is performed by UV, mostly in the low region of 225 -230 nm. Taxanes give similar UV spectra with a minimum at 210-215 nm and a maximum at 225-232 nm. Therefore, detection is performed, preferably at 227-228 nm. Dual/multiple UV detection is performed in both low and upper regions, e.g. 227 and 273 nm 230 and 280 nm 227, 254, and 270 nm, etc. (Fig. 2). 

Identification of active library components is done by micro sequencing (peptide libraries) or by incorporation and recognition of the tag associated with each bead (nonpeptide libraries). An example of this latter approach, known as encoded combinatorial synthesis [19], may use the dialkylamine tagging system shown in Fig. 7. The amine group of the tag addition site is acylated with iminodi-acetic anhydride to yield a pentafluorophenyl ester derivative. The active ester is reacted with a binary mixture of secondary alkylamines, leading to a tag polymer. 

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