Synthesis of deuterated dextromethorphan derivatives

Dextromethorphan is a widely used NMDA receptor antagonist and sodium channel blocker. Deuterated dextromethorphan and dextrorphan were synthesized via the N-desmethyl-dextromethorphan intermediate for pharmacokinetic study


Introduction
Dextromethorphan 1 is widely used as a cough suppressant. 1 It is an antagonist of the N-methyl-D-aspartate (NMDA) receptor and an agonist at sigma receptors. 1Dextromethorphan 1 showed anticonvulsant activity and neuroprotective effect in cerebral ischemia. 2It was studied for the treatment of pain and Parkinson's disease. 3The main metabolite of 1 is dextrorphan 1 2, which is also an antagonist of the NMDA receptor.It was studied as a neuroprotective agent in the management of stroke. 4extromethorphan is often used as a component of drug combinations.Recently Richter patented the finding that 1 increased the effectiveness of the voltage gated sodium channel blocker tolperisone in animal models of various central nervous system disorders. 5The mechanism of this effect is not quite clear, but possibly the serotonin uptake blocking effect of 1 plays a role.

Results and Discussion
For pharmacokinetic studies we needed deuterated dextromethorphan and dextrorphan derivatives with three or more deuterium atoms.The isotopic enrichment and purity were required to be over 95% and 98%, respectively.The CD 3 O-derivative of dextromethorphan 3 is a known compound, but its synthesis was not published. 6Through N-desmethyl-dextromethorphan 4 intermediate we planned to prepare the N-CD 3 -derivative of dextromethorphan 5 which can be an intermediate of N-CD 3 -dextrorphan 6 in the following synthetic route (Scheme 1).N-Demethylation is an important question in the chemistry of alkaloids e.g.morphine alkaloids and morphinane derivatives.Several methods for it are known.The traditional method is the von Braun reaction 7 which is not suggested because of the toxicity of the reagent (cyanogen bromide).Photochemical N-demethylation is a quite special reaction with moderate yield. 8The yield of the sodium sulfide or potassium thioacetate method is not high enough and undesired side reactions could be noticed. 9N-demethylation of the tertiary amine alkaloids e.g.certain opiate alkaloids using the non-classical Polonovski reaction resulted in the product in moderate yield only. 10 Nowadays the chloroformate esters (methyl, 11 ethyl, [12][13][14] phenyl, 13,14 benzyl, 14 etc.) are preferred rather than the above mentioned reagents.We applied Peet's method 15 to Ndemethylation under slightly modified conditions (Scheme 2).The crude trichloroethyl carbomate derivative 7 contained a small amount of starting material 1, which can cause an impurity of the end-product.Purification by column chromatography resulted in the trichloroethyl carbomate in high quality (HPLC: >99% purity).The trichloroethoxy carbonyl group was removed by zinc in acetic acid.From the isolated zinc tetraacetate salt 8 the free base N-desmethyl-dextromethorphan 4 was gained smoothly by treatment with sodium hydroxide.Methylation of N-desmethyl-dextromethorphan 4 with iodomethane-d 3 was the crucial step in our synthetic work.At first the usual methods were tested. 16,17Using potassium hydroxide in dimethylsulfoxide the conversion was moderate: 70-72 %.Modifying the conditions (sodium hydroxide, toluene, tetrabutylammonium bromide, potassium carbonate) one compound (Mw: 285) was obtained with 60% conversion.In the presence of potassium carbonate in dipolaraprotic solvent, e.g.acetonitrile or DMF, the conversion was between 93-95 % according to GC-MS results.The quaternary salt was also obtained which is disadvantageous because it can be easily transformed into a rearranged product. 18In the presence of sodium hydride 19 in tetrahydrofuran the conversion of the methylation was more than 95%.
The most common reagents for O-demethylation are boron tribromide or hydrogen bromide.Both methods were studied.The latter transformed easily the N-CD 3 -derivative of dextromethorphan into the corresponding N-CD 3 -dextrorphan.
Scheme 2 summarizes our synthetic pathway.
The quality of our N-CD 3 -derivative of dextromethorphan 5 and N-CD 3 -dextrorphan 6 was checked by GC-MS, HPLC, NMR and MS measurements.The purity of the obtained N-CD 3dextromethorphan was >99% by HPLC, and 97-98% by GC-MS.The GC-MS technique allowed to detect the N-CH, N-CH 2 , and N-CH 3 contamination too.
The NMR spectrum of dextromethorphan hydrobromide showed a complex diastereomeric mixture because of α,β-protonation, which makes it difficult to determine possible N-CH 3 , N-CH 2 D and N-CHD 2 contamination.Figure 2 shows the 1 H NMR spectrum of the commercially available (Sigma-Aldrich) dextromethorphan hydrobromide.It is more advantageous to study the spectrum of the corresponding base, which is less ambiguous.Figure 3 shows the 1 H NMR spectrum of dextrorphan base 2 prepared from dextromethorphan 1 hydrobromide monohydrate (Sigma) by treatment with hydrogen bromide followed with aqueous ammonia.The N-methyl peak can be seen at ~ 2.3 ppm in DMSO-d 6 .A simpler way to obtain the basic forms of these compounds is to dissolve them in a proper basic solvent or solvent mixture.Figure 4 shows the NMR spectrum of our N-CD 3dextromethorphan HCl salt in pyridine-d 6 : D 2 O = 1:2 mixture (Figure 4 lower spectrum).When adding dextromethorphan to the sample, the peak at ~ 2.9 ppm increased (Figure 4 upper spectrum), which belongs to the N-CH 3 group.Integration of this peak showed that the N-CH 3 contamination is smaller than 5%.Our N-CD 3 -dextrorphan samples were studied in a similar way.Adding dextrorphan hydrogen chloride salt to the sample in the NMR tube, N-CH 3 signal at ~2.3 ppm increased (Figure 5 above spectrum).After integration of this peak in the spectrum of our N-CD 3dextrorphan sample it became evident that the rest of the N-CH 3 protons was < 5% (Figure 5 upper spectrum).The FAB MS spectrum of the commercially available dextromethorphan 1 hydrobromide monohydrate (Sigma D2351) is characterized by an intense protonated molecular ion at m/z MH + = 272 and M-H + = 270 (~8%).The M-2H + = 269 peak could not be detected (Figure 6).The FAB mass spectrum of the stable isotope labeled analog N-CD 3 -dextromethorphan hydrochloride salt was dominated by peaks m/z M 1 H + = 275 and M 1 -H + = 273 due to the mass differences (Figure 7).The M 2 H + = 272 contamination was ~2%.The EI spectrum of N-CD 3 -dextromethorphan 5 base was characterized by an intense molecular ion peak M 1 =274, which was accompanied by the molecular ion peak of the N-CH 3derivative as a contamination M 2 =271 (~2-3%; Figure 9).In the case of N-CD 3 -dextrorphan 6 analogue the quantity of the dextrorphan 2 contamination was established in a similar way by MS spectroscopy.

Experimental Section
General Procedures. 1 H NMR spectra (499.97MHz) were recorded on a Varian Inova-500 spectrometer at 30°C using an HCN indirect detection triple resonance probe.Deuterated solvents 99.95 atom % were purchased from Merck Gmbh ® .Proton chemical shifts are referenced to the residual solvent signal (δ DMSO = 2.50 ppm, δ C5D5N = 7.19 ppm).Four scans were recorded and 20 second relaxation delay was applied to allow proper quantitation via the NMR integrals.Mass spectra were measured on a Finnigan MAT 95 XP instrument.A Fisons MD 800 instrument for GC-MS and a Thermo Separation Products instrument (UV 3000HR, P4000, AS1000, SN4000) for HPLC analysis were used.

N-CD 3 -Dextromethorphan (5)
To the solution of the above prepared N-desmethyl-dextromethorphan 4 (3.1 g, 12.04 mmoles) in 265 ml of tetrahydrofuran 10.1 g of sodium hydride (60 % dispersion in mineral oil) was added.After 20 minutes stirring 1 ml (2.329 g= 16.067 mmoles) of iodomethane-d 3 was dropped into the reaction mixture.The reaction was controlled by TLC.After 1 hour stirring at rt. the reaction mixture was poured into 180 ml of distilled water, which was extracted three times with 100 ml of diethyl ether.The organic layer was separated, dried (sodium sulfate), and evaporated in vacuum.The residue was crystallized in n-hexane.(Yield: 1.7 g, 6.195

Figure 8 .
Figure 8. FAB MS spectra of the N-CD 3 -dextromethorphan base.