Abstract
Simple and efficient stereo-selective synthesis of exo-5-norbornene-2-carboxylic acid (NBCA) is reported. Preliminary studies on base promoted isomerization of methyl 5-norbornene-2-carboxylate (MNBC) revealed that rapid isomerization was accomplished with sodium tert-butoxide (tBuONa), and the exo-content at the equilibrium was ca. 60%. The hydrolyses of endo-rich MNBC (endo/exo = 80/20) under various conditions were carried out. The exo selectivity for resulting NBCA was improved when the hydrolysis was conducted with equimolar water at room temperature in the presence of the stronger base (tBuONa) (endo/exo: 18/82). Whereas the use of excess amount of water led to rapid and non-selective hydrolysis affording high endo content of the product. The plausible reaction mechanism involving rapid equilibrium of thermodynamic isomerization and kinetically preferred hydrolysis of exo ester is proposed.
Highlights
Simple and efficient stereo-selective synthesis of exo-5-norbornene-2-carboxylic acid (NBCA) is reported
Preliminary studies on base promoted isomerization of methyl 5-norbornene-2-carboxylate (MNBC) revealed that rapid isomerization was accomplished with sodium tert-butoxide, and the exo-content at the equilibrium was ca. 60%
5-Norbornene-2-carboxylic acid and its derivatives are important as intermediates of pharmaceutically and biologically active compounds and monomers for advanced polymeric materials. 2-Substituted norbornene compounds can be conventionally obtained by Diels-Alder cycloaddition between cyclopentadiene and acrylic compounds
Summary
Simple and efficient stereo-selective synthesis of exo-5-norbornene-2-carboxylic acid (NBCA) is reported. As can be seen in the two examples, exo-selective synthesis of norbornene carboxylic acid derivatives is of importance from the practical viewpoint. We reported exo-selective synthesis of 5-norbornene-2-carboxylic acid from endo-rich carboxylate in order to investigate the effect of endo/exo ratio on performance of photoresist prepared from tert-butyl 5norbornene-2-carboxylate [15]. Various experimental parameters includeing reaction solvent, temperature, type of base, and the amount of water were optimized in order to afford simple and efficient synthetic method for exo-norbornene carboxylic acid, and confirm our hypothesis about exo-selective hydrolysis. 5-Norbornene-2-carboxylate (MNBC) and tert-butyl 5norbornene-2-carbocylate (tBNBC) were synthesized via conventional Diels-Alder cycloaddition reaction according to the previous work [15]. Ltd. and used without any further purification unless otherwise mentioned
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