Pyridinium salt amine 2518 Transformation

Amination of AT-alkylpyridinium salts with amide ions, which in principle should be easier than the reaction with the parent pyridine, has been little studied. The main reason for this is that solvent selection is difficult. Metal amides are only soluble in liquid ammonia (with which pyridinium salts react easily, vide infra), and pyridinium salts are soluble in solvents that are not suitable for use with metal amides. The A/ -methylacridinium cation undergoes direct imination to give (153) in 35% yield by treatment with potassium amide and iron (III) nitrate in liquid ammonia. Two other products (154) and (155) are also formed, probably by hydrolysis and subsequent disproportionation (Scheme 90). One might question whether sodamide is necessary to the above transformation in light of the fact that quin-olinium, isoquinolinium and certain pyridinium ions give cr-complexes (156), (157) and (158) in liquid ammonia alone at 0 °C (73JOC1949). 

The preparation of conjugated dienes from pyridines is exemplified by the transformation of 2-picoline into the sex pheromone (669) of Lobesia botrana, a major pest of vineyards (Scheme 154) (80TL67). Thus, the lithio salt of 2-picoline was alkylated by 2-(5-chloropentyl-oxy)tetrahydropyran, the resulting pyridine (665) N-methylated, and the pyridinium salt reduced by sodium borohydride. Quaternization of the 1,2,3,6-tetrahydropyridine (666) and Hofmann elimination gave the (7 , 9Z)-undecadien-l-ol (667) as the sole isomer. Protection of the alcohol and treatment of the corresponding ammonium salt (668) of the amine with lithium dimethylcuprate gave pure (669) after hydrolysis, acetylation and HPLC purification. 

PL, unlike other lactones, undergoes polymerization with weakly nucleophilic initiators such as metal carboxylates, tertiary amines, phosphines, and a variety of other initiators [81-83]. This is primarily due to the high ring-strain in the four-membered ring. Pyridine and other tertiary amines initiate the anionic polymerization via a betaine that rapidly transforms into a pyridinium salt of acrylic acid. In order to minimize the chain transfer reactions, the polymerization is performed at a temperature between 0 and 10 °C (Scheme 9). 

Pyridinium salts corresponding to 2,4,6-trisubstituted pyridines, which must be prepared by reacting a primary amine with 2,4,6-trisubstituted pyrylium perchlorate (see section 8.1.2.2) are attacked by a variety of nucleophiles with transfer of the N-substituent to the attacking reagent and as such are convenient alkylating agents, and, recalling that the precursor to the pyridinium salt is the primary amine, the sequence also represents the overall transformation of a primary amine into a variety of derivatives. 

Molina et al. have continued to publish their variation on the use of pyridinium salts for functional-group transformation. In a method which is analogous to that previously reported (Vol. 5, p. 193), arylmethyl thicxiyanates are now available from amines via 4,6-diphenyl-2-methylthiopyridinium thiocyanates (Scheme 62). ... 

As well as alkoxy- and aryloxy-groups, the corresponding sulphur functions have been replaced in the amination of pyridine l-oxides s and quaternary pyridinium salts Examples with pyridines are rare" . An interesting intramolecular amination occurs in the transformation of 2-pyridyl-2 -(3 -aminopyridyl)thioether into 3-(2 -pyridylamino)pyridine-2-thiol 09 l-Alkylpyrid-2>thiones, warmed with alcoholic ammonia and mercuric oxide, give l-alkylpyrid-2-one-imines ii. 

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