Acetals reaction with hydrogen fluoride

A further method permits stereoselective conversion of cis-l,2-diol diacetates into acetoxonium derivatives it involves reaction with liquid hydrogen fluoride. Thus, cis-l,2-diacetoxycyclopentane (16) reacts rapidly in liquid hydrogen fluoride to give the ion 15. trans-1,2-Diacetoxycyclopentane (14), on the other hand, is converted" only very slowly (72 hours) into 15. Similarly, cis-l,2-diacetoxycyclohexane (19) is rapidly converted into the ion 18, whereas the corresponding trans derivative does not react.Such acyclic derivatives as 5 react relatively slowly, to give the corresponding ion (2). Pinacol, in acetic anhydride and in the presence of 70% perchloric acid, can be converted into the acetoxonium perchlorate. This reaction is, presumably, comparable to the reaction with hydrogen fluoride. 

A variety of media have been used for the Wallach fluorination reaction anhydrous hydrogen fluoride alone or with cosolvents such as methylene chloride, benzene, or tetrahydrofuran and hydrogen fluoride-pyridine alone or with co solvents such as benzene, glyme, or acetic acid [42,43, 46 50] Solutions of cesium fluoride, tetraethylammonium fluoride, or tetrabutylammonium fluoride in strong acids such as methanesulfonic acid or trifiuoroacetic acid with numerous cosolvents have also been studied [48, 49]... 

Low-temperature fluorination of sodium acetate in suspension gives a mixture of acetyl hypo-fluorite (AcOF, 1, R = Me) and alkyl hypofluorite the crude mixture can be used, in some cases, directly for the introduction of fluorine into organic molecules. The course of the reaction strongly depends on the reaction conditions and, when the fluoride anion is quenched with hydrogen fluoride or water, trifluoroacetyl hypofluorite (1, R = CF3), 3 acetyl hypofluorite (1, R = Me) and long-chain acid fluorites are obtained.4-6 On the other hand, an excess of fluorine can lead to the formation of alkyl fluoroxy compounds 2 or geminal bis(fluoroxy) products 3. 

A convenient method for the chlorofluorination of cyclohexene, with a mixture of lead(IV) acetate and anhydrous hydrogen fluoride in the presence of sodium or copper(II) chloride in 1,1,2-trichlorotrifluoroethane (Halon-113) at room temperature, has been described.22 The major product in this reaction is /ranj-l-chloro-2-fluorocyclohexane obtained in 70% yield. Byproducts are 2-chlorocyclohexanol and l-acetoxy-2-chlorocyclohexane which are obtained in 25 and 5 % yield, respectively. 

More convenient for the formation of aryl fluorides 4 is the decomposition of triazenes with hydrogen fluoride/pyridine (70 30 w/w) mixture, easier to handle than pure hydrogen fluoride. In this case, decomposition is performed at a lower temperature (18-50°C) and acetic acid, for example, is a suitable solvent.299 However, radical side reactions can be more pronounced, especially with bromo-, iodo-, nitro- or 2-methoxy-subslituted substrates. 

A study concerning the yields of fluorination by acid decomposition of piperidinotriazenes from several anilines with hydrogen fluoride (anhyd HF, coned aq HF, or HF, pyridine) in various solvents, such as tetrahydrofuran, benzene, dichloromethane, and acetic acid, was carried out in order to establish the best reaction conditions for the [ F]fluorine labeling of estrogens. Hence, 2 -fluorohexcstrol dimethyl ether (4) can be obtained from triazene 3 using 49% aqueous hydrogen fluoride. ... 

Formyl fluoride is prepared by reaction of acetic-formic anhydride with hydrogen fluoride at atmospheric pressure with continuous removal of formyl fluoride (b.p. 

Certain tetrols may be epimerized by Angyal s 95% acetic acid procedure, but side reactions occur. Epimerization with hydrogen fluoride should also be considered. 

The di-cation 183, evidently a strongly favored species, can also be obtained as the principal product after treating the arabino ester 188 with hydrogen fluoride for 24 hours.The reaction proceeds by way of the ion 189, from which the di-cation 183 is formed by a cis-diacyloxy reaction in which the loss of acetic acid, rather than benzoic acid, is favored. Hydrolysis of the solution gives the anticipated dibenzoate 186. 

Reaction of tiichloroacetaldehyde with hydrogen fluoride in the presence of chromium-based catalysts gives CFs-CHO.HF which, on treatment with an alkoxysilane (RO)4Si or with ROH-SiCU (R = Me, Et, Pr, or n-CsHu), yields the corresponding hemiacetals CF3-CH(OH)OR (69— 92%) the acetal I-----------------------1... 

Excess fluoride ions prevent the reaction through the formation of stable [TiFe] ions. However, by demasking, it is possible to detect titanium in the presence of even large amounts of fluoride (see below). Large eimounts of acetates, nitrates, chlorides, bromides, and colored ions reduce the sensitivity. Chromates, vanadates, and molybdates, should not be present because they too give color reactions with hydrogen peroxide. Iron salts yield... 

Fluoronaphthalene [321-38-0] is prepared from 1-naphthylamine by the Balz-Schiemaim reaction in 52% yield or by diazotization in anhydrous hydrogen fluoride in 82% yield. Electrophilic substitution occurs at the 4-position, eg, nitration with fuming nitric acid in acetic acid gave 88% yield of l-fluoro-4-nitro-naphthalene [341 -92-4]. 

A suspension of 17a,21-dihydroxypregna-4,9(ll)-diene-3,20-dione 21-acetate (0.77 g) and iV-bromoacetamide (0.3 g) in anhydrous methylene dichloride (40 ml) is added over 2-3 min with stirring to a mixture of anhydrous hydrogen fluoride (10.19 g), and anhydrous tetrahydrofuran (18 g) in a polyethylene bottle at —80° (acetone-dry ice). After 1 hr at —80° the reaction mixture is kept for a further 1 hr at 0° and then added cautiously to an excess of an ice-cold solution of sodium carbonate. Extraction with methylene dichloride and crystallization from acetone-hexane furnish 9a-bromo-ll -fluoro-17a,21-dihydroxypregn-4-ene-3,20-dione 21-acetate (0.69 g), mp 205-208°, raised by several crystallizations from acetone-hexane to 215-217° [aju 142° (CHCI3) max 240-242 mju (e 15,500). 

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