Topiramate heterocyclic analogues: synthesis and spectroscopic characterization

Herein we describe the synthesis of a set of heterocyclic analogues of topiramate as possible therapeutic agents for epileptic seizure treatment. The new compounds and their precursors were characterized by physical, as well as spectroscopic techniques. We verified that the preferential conformation in solution of the carbohydrate ring is twist-boat. The stereochemistry of the new asymmetric centre, generated during the heterocyclizations, was proposed using NOESY experiment. These novel compounds could help to elucidate the mechanism of action of the commercial drug used for this disease.


Introduction
Anticonvulsants are the primary drugs used for the treatment of epileptic disorders. 1A wide diversity of chemical structures, including heterocycles, exhibits anticonvulsant activity. 2Bruce E. Maryanoff et al., 3 reported the synthesis and biological activity studies of an anticonvulsant called "topiramate", a sulfamate monosaccharide derivative (Figure 1), which is used for epilepsy treatment.][14] Beyond these medical applications, in recent years cognitive effects in low-dose topiramate treatments have been reported. 15On the other hand, Kockelmann et al., 16 reported that the withdrawal of this drug caused significant improvement in frontal lobe associated measures like verbal fluency and working memory.Very recently, Beer et al., 17 described a case of a fatal topiramate poisoning, so it will be necessary to establish the potential lethal topiramate concentrations.
Given the multiple etiologies of epilepsy, the limited understanding of the mechanisms behind this disorder and the adverse effects of the topiramate, the search for less toxic, more efficient agents for the treatment of seizure disorders is an ongoing endeavor.In this context, the aim of this work was to synthesize novel derivatives of topiramate where the C-1 of fructose was used to build heterocycles.We also proposed the preferential conformational in solution of the carbohydrate moiety from measured coupling constants of 1 H NMR spectra and determined the configuration of the new stereocentres generated during the heterocyclization reactions.

Results and Discussion
To carry out this project we chose some heterocycles which were obtained by functionalizing C-1 of 2,3:4,5-di-O-isopropylidene--D-fructopyranose 1.One of these heterocycles was tetrazole, which is well known as a bioisoster of carboxylic acids 18 and Hallberg et al. reported the use of acylsulfonamide and tetrazole as isosteric groups. 19Since the mechanism of action of topiramate is unknown, taking into account the diversity of structures with anticonvulsivant properties, we synthesized a set other heterocyclic derivatives with potential biological interest. 20In Scheme 1 we show the total synthetic pathway from compound 1 as the starting material.
Maryanoff and col.reported the same conformation for the carbohydrate moiety of topiramate, and they concluded that this conformation is very important for the pharmacologic activities. 3In our cases, we observed that the presence of heterocyclic rings does not affect the carbohydrate conformation, so these new compounds are very promising for future biological applications.
In a previous work, 24 we studied the sensitivity to steric hindrance on the oxo-and thioheterocyclization reactions to synthesize spiro heterocycles.We found that the heterocyclizations of pyranose and furanose thiosemicarbazones are more sensitive to a crowded environment, so a single thiadiazoline ring was obtained.Cyclization of the corresponding benzoylhydrazone under standard conditions (110 °C) demonstrated no diastereoselectivity, although the application of a more energetic procedure (reflux) gave only one isomer.
The standard procedure performed on compound 8, gave a crude reaction mixture which showed the presence of compound 9a and 9b (4:1 ratio) under spectroscopic analysis, meanwhile the reaction carried out at reflux shows exclusively one isomer 9a.In order to determine the configuration of the new stereocentre we performed a NOESY experiment on the isolated single isomer.This allowed us to propose the S configuration for the main compound.Figure 2 shows the spectroscopic correlations.For this reaction we did not expect any diastereoselection, because the cyclization takes place on an exocyclic plane carbon, but the relative selectivity observed for cyclization of 8 at 110 °C was unexpected.Because of this we calculated the minimized geometries of the two possible diastereoisomers of 8, and found that the anti form was the less energetic.The conformation of the more stable rotamer is shown in Figure 3.It can be observed that the isopropylidene group, to a certain extent, shields one side of the imine bond inhibiting the approach of the oxygen.If oxygen attacks by the less sterically hindered side, the main compound resulting from the heterocyclization reaction must be the S isomer, which is consistent with the configuration obtained from NOESY spectrum.The application of standard procedure on compound 10, gave a crude reaction mixture which showed the presence of both diastereoisomers 12a and 12b, and found that the relationship between both isomers did not change when a higher temperature was used (62:38 or 61:39 ratio, respectively).The NOESY experiment carried out on the mixture let us to propose the R configuration for the main compound (Figure 4).We tried to determine the reason of the slight preference for the R isomer and we found that the conformation of the lower energy rotamer for anti configuration of seem to be reactive on both sides.However, a little steric hindrance on one side can be observed (Figure 5).This attack generates a slight preference by the R isomer.A new group of heterocyclic analogues of topiramate were efficiently synthesized from 2,3:4,5di-O-isopropylidene--D-fructopyrnose.These new derivatives were characterized physically and spectroscopically using homo and heteronuclear techniques.From the conformational analysis we can observe that in all cases the sugar ring has the required twist-boat conformation to show biological activity.Therefore, we suppose that this set of compounds can help elucidate the mechanism of action of this drug of clinical use.

Experimental Section
General.Synthesis of compounds 2-12 were carried out using all solvents and reagents as purchased, without further purification.Analytical TLC was conducted on Silica Gel 60G (Merck) on precoated plates and visualization was made by UV light and ethanol/sulfuric acid (10:1) or cerium molybdate followed by heating.Flash-chromatographic separations were performed on Silica Gel 60 G (Merck).Elemental analysis was performed on an Exeter Analytical CE-440 elemental analyzer.Optical rotations were recorded at 20 °C on a Perkin Elmer 343 polarimeter, and melting points were uncorrected. 1H, 13 C NMR spectra were recorded in deuterochloroform on a Bruker AC-200 spectrometer, operating at 200, 50 MHz respectively; or a Bruker AMX-500 spectrometer, operating at 500, 125 MHz respectively.Assignments of the 1 H and 13 C NMR spectra were confirmed with the aid of two dimensional techniques 1 H, 13 C (COSY, HSQC).Chemical shifts () are reported in parts per million downfield from tetramethyl silane as internal standard.Minimum energy structures were founded with HyperChem 8.0.3, using MM+ force field.

Heterocyclization of compound (10)
Compound 10 (0.394 g, 1.19 mmol) was dissolved in pyridine (3 mL) and then acetic anhydride (3 mL) was added with stirring.The mixture was heated at 110 °C for 2 h and the reaction was stopped and processed in the same way than for compound 4. The crude residue was disaggregated with water and a greenish-yellow precipitate was obtained.Crystallization from ethanol yields product 12 (0.272 g, 0.65 mmol) as a mixture of two diastereoisomers 12a and 12b in a 5:2 relationship, determined by 1 H NMR Yield 55%; mp 244-248 °C.Anal.Calcd.for C15H22N3O6S: C, 49.

Figure 3 .
Figure 3. Less energetic conformer for anti isomer of compound and its main cyclization product, compound 9a.

Figure 5 .
Figure 5. Less energetic conformer for anti isomer of compound 10 and its main cyclization product, compound 12a.