Esterification pyridinium salt

Several total syntheses of antirhine (11) and 18,19-dihydroantirhine (14) have been developed during the last decade. Wenkert et al. (136) employed a facile route to ( )-18,19-dihydroantirhine, using lactone 196 as a key building block. Base-catalyzed condensation of methyl 4-methylnicotinate (193) with methyl oxalate, followed by hydrolysis, oxidative decarboxylation with alkaline hydrogen peroxide, and final esterification, resulted in methyl 4-(methoxycar-bonylmethyl)nicotinate (194). Condensation of 194 with acetaldehyde and subsequent reduction afforded nicotinic ester derivative 195, which was reduced with lithium aluminum hydride, and the diol product obtained was oxidized with manganese dioxide to yield the desired lactone 196. Alkylation of 196 with tryptophyl bromide (197) resulted in a pyridinium salt whose catalytic reduction... 

Musso et al. (87,97) have developed an elegant synthesis of muscaflavin dimethyl ester (61) (Scheme 8). It is conceptionally similar to the synthesis of decarboxybetalains described before. The synthesis starts with conversion of pyridylalanine derivative 86 to the dimethyl ester 87 by esterification and consecutive N-protection with the base-labile p-toluenesulfonylethoxycarbonyl group. For cleavage of the pyridine ring, 87 was transformed into the A -methoxy-pyridinium salt (88) which was then reacted with pyrrolidine in tetrahydrofuran... 

Polymer supported pyridinium salts, such as Mukaiyama reagent, have proven very useful synthetic tools in the preparation of 2-oxazoline libraries 05JC0688> and in automatable esterification reactions <05TL2817>. Yamamoto et al. have examined the use of polymer supported boronopyridinium salts for the preparation of amides and esters in good to excellent yield <050L5043,05TL5047>. 

Esterification. The reaction of carboxylic acids and alcohols in the presence of 1.2 eq. of this pyridinium salt and 2.4 eq. of tri-n-butylamine affords esters... 

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