Density functional theory (DFT) calculations at B3LYP/6-31 G (d,p) and B3LYP/6-311+G(d,p) levels for the substituted
pyridine-catalyzed isomerization of monomethyl
maleate revealed that isomerization proceeds via four steps, with the rate-limiting step being
proton transfer from the substituted pyridinium ion to the C=C double bond in INT1. In addition, it was found that the isomerization rate (
maleate to
fumarate) is
solvent dependent. Polar
solvents, such as water, tend to accelerate the isomerization rate, whereas apolar
solvents, such as
chloroform, act to slow down the reaction. A linear correlation was obtained between the isomerization activation energy and the dielectric constant of the
solvent. Furthermore, linearity was achieved when the activation energy was plotted against the pKa value of the catalyst. Substituted-
pyridine derivatives with high pKa values were able to catalyze isomerization more efficiently than those with low pKa values. The calculated relative rates for
prodrugs 1-6 were: 1 (406.7), 2 (7.6×10(6)), 3 (1.0), 4 (20.7), 5 (13.5) and 6 (2.2×10(3)). This result indicates that isomerizations of
prodrugs 1 and 3-5 are expected to be slow and that of
prodrugs 2 and 6 are expected to be relatively fast. Hence,
prodrugs 2 and 3-5 have the potential to be utilized as
prodrugs for the slow release of
monomethylfumarate in the treatment of
psoriasis and
multiple sclerosis.