Pyridinium betaine

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). 

Hydroxypyridine (86, R = H) and its derivatives also belong to the class of heterocyclic enols. In benzene and dioxane, 3-hydroxy-pyridine occurs as the neutral molecule (and not as a betaine).Its reaction with diazomethane, in heterogeneous media, gives a mixture of 3-methoxypyridine (86, R = Me) (10%) and l-methyl-3-hydroxy-pyridinium betaine (87) (30%If tert-butanol is used as a... 

Zwitterionic 2-oxo-l,2,3,4-tetrahydropyrido[l,2-ft]pyridazine 78 was obtained from 6-(3-methoxycarbonylpropionyl)-l,l-dioxo-l,2-thiazine 76 with hydrazine hydrate via pyridinium betaine 77 (99JPR37). 

Despite the lack of success in the attempts at intramolecular cycloaddition with substrates 83 and 91, a moderately promising outcome was observed for the nitroalkene substrate (98, Scheme 1.10c). Heating a dilute solution of oxido-pyridinium betaine 98 in toluene to 120 °C produced a 20 % conversion to a 4 1 mixture of two cycloadducts (110 and 112), in which the major cycloadduct was identified as 110. While initially very encouraging, it became apparent that the dipolar cycloaddition reaction proceeded to no greater than 20 % conversion, an outcome independent of choice of reaction solvent. Further investigation, however, revealed that the reaction had reached thermodynamic equilibrium at 20 % conversion, a fact verified by resubmission of the purified major cycloadduct 110 to the reaction conditions to reestablish the same equilibrium mixture at 20 % conversion. 

Allyl pyridinium betaines 441 isoelectronic with enol betaines 427 likewise reacted with diphenyl cyclopropenone by elimination of pyridine272,213 The product formation, different in aprotic and protic media (phenol 443 in aprotic solvent, A3,5-hexadienoic esters 445 in alcohol solvent), suggested that the diene... 

The versatility of this triafulvene reaction type is demonstrated by the interaction of ally pyridinium betaines 441 and l,2-diphenyl-4,4-diacetyl triafulvene272, which gives rise to fulvenes 565, benzene derivatives 566, or acyclic systems 567 these products are likely to result from an allenic precursor 563 and its isomer 564 originating from a 1,5-H-shift. 

Dimroth and Reichardt defined the Ej parameter on the basis of the solvatochro-mism of the pyridinium betaine 17 as equation 44 ... 

Ex sv A solvent parameter defined as the transition energy for the pyridinium betaine... 

Diels-Alder reactions.1 Nitrosoarenes undergo Diels-Alder reactions at 25° with cis- and/or tra/is-hexadienals 2 to give unstable adducts that can be identified by IR and H-NMR as 3 or the hemiacetals 4. On standing or warming to 40° these primary products rearrange to pyridinium betaines (5) or pyrroloindoles (6) as the... 

Transition energies for the solvatochxomic pyridinium betaine t(30) [recalculated from the literature values given in kcal mol (Reichardt, 1994)]. 

Figure 95 A pyridinium betaine derivative of a phenolic cryptand [122]...

A rather interesting synthesis of a pyridinium ylid was reported by Phillips and Rails 113>. The reaction of pyridine with bromoacetic acid in the presence of benzaldehyde at 120 °C in nitrobenzene solvent gives l-(2-hydroxy-2-phenylethyl)pyridinium bromide in 75% yield. The mechanism suggested for the reaction involves the formation of the the pyridinium betaine which decarboxylates to the ylid. Subsequent reaction of the ylid with benzaldehyde and protonation gives the final product. 

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