Acyl fluorides acetals

These reactions are most important for the preparation of acyl fluorides. " Acyl chlorides and anhydrides can be converted to acyl fluorides by treatment with polyhydrogen fluoride-pyridine solution" or with liquid HF at — 10°C. Formyl fluoride, which is a stable compound, was prepared by the latter procedure from the mixed anhydride of formic and acetic acids. Acyl fluorides can also be obtained by reaction of acyl chlorides with KF in acetic acid or with DAST. Carboxylic esters and anhydrides can be converted to acyl halides other than fluorides by the inorganic acid halides mentioned in 10-77, as well as with PhsPXa (X = Cl or but this is seldom done. Halide exchange can be carried out in a... 

The described procedure, first applied by Colson and Freden-hagen, is useful for the preparation of a wide variety of acyl fluorides. The yields are normally 80-90%. Some examples of acyl fluorides prepared are listed in Table I. Benzoyl fluoride can also be employed as a convenient source of acetyl fluoride by reaction with acetic acid. ... 

Naked enolates without any complexation can be made from silyl enol ethers using fluoride ion, a very selective nucleophile for silicon 49, and a non-metallic cation, usually a tetra-alkylammonium ion. The commonest reagent is TBAF (TetraButylAmmonium Fluoride Bu4N+ F ). In this way the naked enolate 50 was made. It proved to be acylated with acetic anhydride exclusively at oxygen to give the enol acetate 53 and alkylated with Mel at carbon to give the ketone 51 in 84% isolated yield.18... 

Silyl ketene acetals are acylated enantioselectively by acyl fluorides, with an aryl- pyrrolidino-Meucine-thiourea catalyst, giving useful a ,a -disubstituted... 

As expected, MDL can be easily acylated with acetic anhydride to MDD (conversions of 84—89% and selectivities to MDD of >99.0%) in the presence of nanoscopic, partly hydrox-ylated fluorides (Table 6.4) [82]. 

Recently, the same group reported a highly enantioselective acylation of silyl ketene acetals 298 with acyl fluorides 299 via thiourea and a 4-pyrrolidinopyildine... 

Besides acetophenone, this reaction was also applied to p-chloro- andp-methoxyacetophenone, and even to an aliphatic ketone, acetone (although the yield was stated to be only half as large as that obtained from mesityl oxide, i.e., less than 30%, Dorofeenko and co-workers reported a 45% yield of 2,4,6-trimethylpyrylium perchlorate from acetone, acetic anhydride, and perchloric acid), and is the standard method for preparing pyrylium salts with identical substituents in positions 2 and 4. The acylating agent may be an anhydride in the presence of anhydrous or hydrated ferric chloride, or of boron fluoride, or the acid chloride with ferric chloride.Schneider and co-workers ... 

Polyphosphoric acid is a commonly used catalyst for this reaction however, in some cases a mixture of hydrogen bromide/acetic acid gives better results. Acylation of the S-phenyl-, V-(4-tolyl)- or S-(l-naphthyl)-substituted thiobenzenepyruvic acids 3a-c affords the corresponding dibenzo[A,/]thiepins in satisfactory yields, while reaction of the S-(4-methoxyphenyl) or S-(2-naphthyl) derivatives fails to provide any thiepin.60 The intramolecular Friedel-Crafts acylation of 2-(arylsulfanyl)benzeneacetic acids also yields the corrresponding dibenzothiepins in this case the use of hydrogen fluoride sometimes results in purer products.38 The applicability of this method is restricted to the synthesis of stable bisannulated thiepins. 

Glycosyl fluorides have also been prepared by treatment of per-O-acyl or partially 0-acylated sugars with hydrogen fluoride [liquid HF (for example, see Refs. 38 and 39) or HF in acetic acid or dichloromethane], as exemplified by 2,3,4-tri-O-benzyl-a-D-xylopyranosyl (18), a-D-glucopyranosyl (19), tetra-O-pivaloyl-a-D-glucopyranosyl (20), and 2,3,5-tri-O-acetyl-D-xylofur-anosyl fluorides (21) (see Table 1). Frequently, HF treatment - leads to... 

Additional acylation studies were also reported (24), (26). In the first case it is claimed that acylation of thiophene is achieved by means of HC104 and acetic anhydride affording a 65 % yield of 2-acetylthiophene. In the second paper Levine and coworkers reported that while 2,5-dimethylthiophene could be readily acetylated, 2,5-dichlorothiophene acetylated sluggishly. This is, however, readily explained, since the presence of chlorine atoms on the thiophene ring decreased its reactivity in electrophilic substitution reactions. In the case of methyl substitution, however, the 3 and 4 positions of the ring are activated toward electrophilic substitution by the inductive and hyperconjugative effects. Thus 2,5-dimethylthiophene was successfully acylated by the boron fluoride etherate method in high yield with three aliphatic anhydrides. 

If esters arc perfluorinated the main products are perfluoroalkanes or perfluoroacyl fluorides byproducts are carbon dioxide and oxygen difluoride. The acyl and the alkyl groups in the ester give perfluorocarboxylic acids.41 A comparison of the results obtained from butyl acetate and ethyl butyrate, and from butyl propionate and propyl butyrate, indicates that perfluoro-butanoic acid is obtained in better yield from ethyl and propyl butyrate than from butyl acetate and butyl propionate. Thus, perfluorocarboxylic acids are formed more readily from the acyl than from the alkyl group of the ester. 

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