Pyridinium formate

Some commonly used buffers, such as sodium and potassium phosphate, are incompatible with ELSD, but there are ready alternatives. For example, ammonium acetate has similar buffering properties to potassium phosphate, and ammonium carbonate, ammonium formate, pyridinium acetate, and pyridinium formate are options for different pH ranges. Typical mobile phase modifiers that do not meet the volatility criteria can be replaced by a wide variety of more volatile alternates. For example, phosphoric acid, commonly used as an acid modifier fo control pH and ionization, can be replaced by trifluoroacetic acid other acids that are sufficiently volatile for use with FLSD include, acetic, carbonic, and formic acids. Triethylamine, commonly used as a base modifier, is compatible with FLSD other base modifiers that can be used are ethylamine, methylamine, and ammonium hydroxide [78]. 

Two enzymatic digestions examined, separation by cation-exchange HPLC (Ionosphere-5C, Chrompack International) with gradient elution using pyridinium formate mobile phase (separation of 30 Se compounds within 60 min) ICP-DRC-MS detection ( Se, Se). 

Fifth method a subsample of 150 mg was ultrasonically extracted with a mixture of methanol/methane (5/2 v/v) (4 x 2.8 mL) and back-extracted in water the extract was evaporated under N2 stream and stored in a glass flask at 4°C in the dark. Separation was by cation exchange LC (Chrompack Ionosphere) in isocratic mode (pyridinium formate). Final detection was by ICP-MS of mass 75. 

Hexanoyl chitosan is an ac lated derivative of chitosan. It was found to be anti-thrombogenic and resistant to hydrolysis by lysozyme. To take advantage of these properties, hexanoyl chitosan was electrospun from its chloroform solution with pyridinium formate added to improve electrospinabibty [188]. The porous fiber mats were proven to be non-toxie for mouse fibroblasts (L929) and able to support the attachment and the proliferation of human keratinocytes and human foreskin fibroblasts [189]. 

Hexanoyl chitosan chloroform Pyridinium formate added to improve electrospinnability. Non-toxicity and cell proliferation support. [189]... 

The preparation of e/n-difluoro compounds by the oxidative fluorodesul-furization ot 1,3-dithiolanes readily proceeds by treatment with a pyridinium polyhydrogen fluoride-Af-halo compound reagent the latter serves as a bromonium ion source [2], l,3-Dibromo-5,5-dimethylhydantoin is the most effective of several At-halo oxidants. It is believed that /V-halo compounds combine with hydrogen fluoride to generate in situ halogen fluorides, the oxidants. Formation of gem-difluorides from dithiolanes derived from ketones is efficient and rapid, even at -78 °C, whereas the reaction of dithiolanes derived from aldehydes requires higher temperature (0 °C) (equation 4). 

Methyl propiolate and pyridine give a rather unstable 2 1 molar adduct which is the 1,2-dihydropyridine (112). The reaction sequence proposed to account for its formation is identical in principle to a similar scheme proposed earlier in the acridine series (Section II,A,2) and is also supported by the observation that the 1-benzoyl-pyridinium cation with the phenylacetylide anion yields (113). ... 

Intramolecular nucleophilic displacement of the bromo group by an azine-nitrogen occurs in the cyclization of A-2-quinaldyl-2-bromo-pyridinium bromide (248) to give the naphthoimidazopyridinium ring system. The reaction of 2-bromopyridine and pyridine 1-oxide yields l-(2-pyridoxy)pyridinium bromide (249) which readily undergoes an intramolecular nucleophilic substitution in which departure of hydrogen as a proton presumably facilitates the formation of 250 by loss of the JV-oxypyridyl moiety. 

From a practical point of view, literature data indicate that it is not necessary in most cases to isolate the title salts prior to their reaction. Tliis can be readily understood since their preparation is nearly quantitative and sulfur dioxide is the sole product evolved during their formation. In fact, it is highly advisable to recourse to a two-step procedure during the reactions between A -(l-chloroalkyl)pyridinium chlorides and amino acids (Section IV,C,6) (97BSB383). 

Reactions between A -(l-chloroalkyl)pyridinium chlorides 33 and amino acids in organic solvents have a low synthetic value because of the low solubility of the amine partner. A special protocol has been designed and tested in order to circumvent this drawback. Soon after the preparation of the salt, an aqueous solution of the amino acid was introduced in the reaction medium and the two-phase system obtained was heated under reflux for several hours. However, this was not too successful because sulfur dioxide, evolved during the preparation of the salt, was converted into sulfite that acted as an 5-nucleophile. As a result, A -(l-sulfonatoalkyl)pyridinium betaines such as 53 were obtained (Section IV,B,3) (97BSB383). To avoid the formation of such betaines, the salts 33 were isolated and reacted with an aqueous solution of L-cysteine (80) to afford thiazolidine-4-carboxylic acids hydrochlorides 81 (60-80% yields). 

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