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. It is well-known that Diels-Alder cycloaddiotion is endoselective due to secondary orbital overlap, and the selectivity is enhanced by Lewis acid [1,2]. Polynorbornenes, which can be synthesized by vinyl polymerization or ring-opening metathesis polymerization (ROMP) of norbornene derivatives, show high thermal stability and transparency, and are useful for optical applications such as optical fibers, disk, lens and displays [3,4]. Norbornene derivatives are also useful as monomers or intermediates for photoresist materials [5,6]. Exo-isomer of norbornene carboxylic ester shows higher reactivity in living ROMP than endo-isomer [7]. In the case of photoresist monomer synthesis, endo-isomer provides undesired lactone by intramolecular cycloaddition to result in a low yield of desired photoresist monomer [8]. As can be seen in the two examples, exo-selective synthesis of norbornene carboxylic acid derivatives is of importance from the practical viewpoint.

Some of exo-selective syntheses of norbornene derivatives have been reported. Gouverneur et al. reported asymmetric Diels-Alder reaction by using antibody catalyst [9]. Fraile reported asymmetric Diels-Alder reaction of furan with chiral acrylate [10]. Avenoza and Kawamura reported asymmetric Diels-Alder reaction of cyclopentadiene and chiral dienophile [11,12]. Because of high cost of these reactions, practically pure exo-isomer is obtained by removing of endo-isomer through lactonization [13, 14]. Previously 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 5- norbornene-2-carboxylate [15]. High exo-content and yield of the product cannot be explained only by exoselective hydrolysis as reported by of Niwayama and Hiraga [16]. As the plausible mechanism, we proposed that hydrolysis should be carried out accompanying with endo/exo isomerization.

In this paper, 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.

Figure 1 shows ¹H-NMR spectrum of MNBC synthesized via a conventional Diels-Alder cycloaddition reaction as a starting material. The mole ratio of endo/exo was determined as 80/20. In order to obtain 5-norbornene-2-exo-carboxylic acid (exo-NBCA), isomerization from endo-isomer to exo-isomer is required. As MNBC has an active proton at α-position of carbonyl group, the corresponding carbanion can be generated by elimination of the proton in basic conditions. In general, carbanion has sp³-type trigonal pyramid structure, and low activation barrier for the inversion of this geometry. By utilizing the continuous inversion, the mixture comprising mainly endo-isomer of MNBC can be isomerized to corresponding endo/exo mixture of which ratio is ruled by thermodynamic equilibrium under basic conditions.