Pyridine acetic acid esters exchange

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 rearrangement of the 5-cyano-4,5-dihydro-l//-azepine (93) to furo[2,3-6]pyridine (95 Scheme 9) with sodium nitrate in glacial acetic acid or with silver nitrate in aqueous ethanol proceeds by initial protonation at either C-3 or C-6 followed by hydrolysis to the cyanooctanedione ester (94). By carrying out the rearrangement with an acid ion exchange resin it is now possible to isolate the dione ester (77CJC4061). Likewise, the hydrolysis of the tetrahydro-2-benzazepine (96) to an 0 -(anilinoalkyl)benzophenone is an example of proton attack at the /3 -carbon of the enamine system (77JA5045). 

The Bagley group later developed a mild, single-step variant of the reaction, wherein acetic acid or Amberlyst 15 ion-exchange resin was used to promote cyclodehydration at a lower temperature (50 C) to give 2,3,6-tri-and 2,3,4,6-tetra-substituted pyridines 181 with alkyl, aryl, heteroaromatic, heteroatom, and ester substituents in moderate to excellent yields. ... 

Certain solvents should be avoided with alumina or silica gel, especially with the acidic, basic, and highly active forms. For instance, with any of these adsorbents, acetone dimerizes via an aldol condensation to give diacetone alcohol. Mixtures of esters transesterify (exchange their alcoholic portions) when ethyl acetate or an alcohol is the eluent. Finally, the most active solvents (pyridine, methanol, water, and acetic acid) dissolve and elute some of the adsorbent itself. Generally, try to avoid going to solvents more polar than diethyl ether or methylene chloride in the eluent series (Table 19.2). 

A procedure by using enamino ester and alkynone as the substrates was developed as well [2]. 2,3,4,6-Tetrasubstituted pyridines were prepared in a single step. Various acids, such as acetic acid, Amberlyst 15 ion exchange resin, zinc(II) bromide or ytterbium(III) triflate, can be applied as promoter for the cycUzation step of the Michael addition adduct. 4-(3-Oxoalkyl)isoxazoles found could be applied as starting material for pyridine synthesis as well [3],... 

Although free fatty acids are not esterified under the basic conditions described above, methyl esters can be prepared by exchange with N,N-dimethylformamide dimethyl acetal in the presence of pyridine [913]. Similarly, methyl iodide reacts with sodium or potassium salts of fatty acids in the presence of a polar aprotic solvent such as dimethylacetamide to form methyl esters [41,186]. 

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