Acids a-fluorocarboxylic

HF-pyridine reagent, however, can also be used for the hy-drofluorination of alkenes,7 alkynes,7 cyclopropanes,7 and diazo compounds,8 the halofluonnation of alkenes,9 the preparation of fluoroformates from carbamates,10 the preparation of a-fluorocarboxylic acids from a-amino acids,11 and as a deprotecting reagent in peptide chemistry.12 Examples of the hydrofluonnation of alkenes with HF-pyridine are given in Table II. 

Takeuchi and co-workers5 have recently prepared a number of a-fluorocarboxylic acid derivatives 1 and 2 and it is very interesting that they emphasized that no explosion occurred in more than 200 fluorinations performed 17 however, they recommend the use of perchloryl fluoride diluted with nitrogen and synthesized in the laboratory. 

Direct conversion of aldehydes 14 to a-fluorocarboxylic acids 16 by generating dichloroepoxides 15 in situ in the presence of a fluoride ion has been skillfully effected.169... 

Fluorination of alkenes. The reagent adds in the Markownikofl fashion to alkenes to yield alkyl fluorides (35-90% yield). It converts secondary and tertiary alcohols into the corresponding alkyl fluorides. In situ diazotization followed by dediazotization converts a-amino acids into a-fluorocarboxylic acids (38-98% yield). The reagent converts acid chlorides and anhydrides into acid fluorides. ... 

By a similar process, amino acids have been converted to a-fluorocarboxylic acids by fluorodediazotisation processes [234] and, generally, retention of configuration is observed due to neighbouring group participation of the adjacent carboxyl group (Figure 3.47 and Table 3.8). 

Electrolytic coupling of two molecules of a fluorocarboxylic acid accompanied by decarboxylation, according to Kolbe, is a general method for the preparation of suitable fluoro derivatives and the early literature on this has been dealt with in Houben-Weyl, Vol. 5/3.24... 

ETHYNYLCARBINOLS Lithium acetylide. a-FLUOROCARBOXYLIC ACIDS Polyhydrogen fluoride-Pyridine. a-FLUORO KETONES Polyhydrogen fluoride-Pyridine. 

Further evidence for the -oxidation of (a fluorocarboxylic acids in vivo... 

Oxazolidine 198 was produced from condensation of (R)-phenylg-lycinol and fluoral ethyl hemiacetal as a 62 38 tram/cis diastereomeric mixture. The enantiopure trans-198 was obtained through an epimeriza-tion with p-toluenesulfonic acid that realized the selective precipitation of the desired diastereoisomer. The repetition of this crystallization-induced dynamic resolution (CIDR) protocol on mother liquors allowed an almost complete conversion of the initial mixture into the desired trans-198. This chiral oxazolidine was used as chiral auxiliary for the synthesis of a-fluorocarboxylic acids and P-fluoroalcohols in a one-pot procedure that also allowed its recovery without loss of optical purity (13JOC3487). 

Catalytic enantioselective fluorination of aryl acetyl thiazolidi-nones with NFSi was reported with the unique combination system of Ni(II)-BINAP/Et3SiOTf 2,6-lutidine (eq 23). Such a reaction allows the preparation of a-fluorocarboxylic acid derivatives that have not been as thoroughly explored as activated /3-keto esters due to the difficulty for generation of metal enolates under in situ catalytic conditions. ... 

Another advance in this field is the catalytic enantioselective a-fiuorination of acid chlorides (eq 28). Here, NFSi is an effective electrophilic fiuorination reagent in the presence of a trifunctional catalytic system. This is a practical method that produces a-fluorocarboxylic acid derivatives. A third catalyst, a Lewis acidic lithium salt, could be introduced to amplify yields of aliphatic products, primarily through activation of the NFSi. 

Synthesis from a-Fluorocarboxylic Acids and Their Derivatives... 

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