Abstract
The synthesis of fully deuterated amphetamine (phenyl-2-aminopropane-d11) in which 11 deuterium atoms are bonded to carbons and two other highly deuterated analogues is described. Their toxicities and in vivo spontaneous locomotor activities in mice were examined and compared with that of the parent protioamphetamine. A significant reduction in toxicities and a decrease in spontaneous locomotor activity were observed for these highly enriched deuterated analogues, as compared to protioamphetamine.
Amphetamine is a unique drug with respect to the simplicity of its structure and the multiplicity of its biochemical and pharmacological effects. This has made the molecule an ideal target for extensive molecular modification in order to accentuate some of its effects and/or abolish others. For example, Horn and Snyder2 synthesized cis- and trans-2-phenylcyclopropylamines as rigid analogues of amphetamine to examine the steric requirements for its inhibition of neuronal uptake of norepinephrine. Tessel et al.3 studied the structure-activity relationships of meta-substituted N-ethylamphetamines and their spontaneous locomotor activity.
As part of the synthetic design to study the effect of deuteration on drug biotransformations and pharmacological actions, we synthesized the fully and highly enriched deuterated amphetamines (1-3) and examined their in vivo pharmacological effect(s) on spontaneous locomotor activity. To our knowledge, the in vivo effect of deuteration on amphetamine actions has not been reported.
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Isotopically altered drugs have shown widely divergent pharmacological effects. Elison et al.4 reported a reduced analgesic potency and a 24% increase in the basicity of morphine deuterated in the N-methyl group. Foreman and co-workers5 found an isotope effect of 1.9 in the in vitro metabolism of α-deuterated amphetamine 4 with rabbit liver homogenate. This suggests that the deuterioamphetamine is metabolized at a slower rate than its protio form. Deuterium isotope effects in man have been reported for the metabolism of N-alkyl-deuterated and α-deuterated amphetamines6. A 12% increase in the basicity and a decreased lipid solubility were observed. These observations suggested that further deuteration of the amphetamine molecule such as compounds 1-3 might lead to a more pronounced alteration of the physicochemical properties of the drug and, hence, its in vivo pharmacological response. In this paper, we report the synthesis and the in vivo spontaneous locomotor activity of these highly deuterated amphetamines. These activities were compared to other deuterated amphetamines (i.e., 4-6) which have been synthesized previously in this laboratory.7
Chemistry
The synthetic routes to the desired compounds are outlined in Schemes I and II. All compounds are racemic where this is possible. Most of the methods that are described in the literature concerning the synthesis of amphetamine and its derivatives employ the readily available phenylacetone (10) as the starting material. To effect the synthesis of fully deuterated amphetamine 1, it was necessary to prepare the unreported phenyl-2-propanone-d10 (8a). To do this, the method of Tegner8 for the preparation of nondeuterated phenylacetone was employed. The action of lithium metal on methyl-d3 iodide in anhydrous ether furnished methyl-d3-lithium which upon coupling with phenylacetic-d7 acid (7) in anhydrous ether afforded the fully deuterated phenylacetone 8a in good yield. Treatment of this ketone with hydroxylamine hydrochloride in sodium deuterioxide-deuterium oxide medium yielded phenyl-2-propanone-d10 oxime (9a).
Lithium aluminum deuteride reduction of the oxime in anhydrous ether furnished the desired fully deuterated dl-phenyl-2-aminopropane-d11 (1), as depicted in Scheme I.
The dl-phenyl-2-aminopropane-d7 (2) was prepared in a similar manner as described previously. Phenylacetic-d7 acid (7) was coupled with the readily available methyllithium in anhydrous ether to furnish phenyl-2-propanone-d7 (8b). Conversion of this ketone to its oxime 9b, followed by lithium aluminum hydride reduction of the resulting oxime, furnished the desired dl-phenyl-2-aminopropane-d7 (2).
The preparation of dl-phenyl-2-aminopropane-d5 (3) was accomplished by the lithium aluminum deuteride reduction of phenyl-2-propanone-d5 oxime (12) in ether. The oxime 12 was readily prepared by deuterium exchange of the nondeuterated oxime 11 in deuterium oxide, dimethylformamide, and anhydrous sodium carbonate mixture as shown in Scheme II.
It is interesting to note that the deuterium exchange of an oxime has not been reported and, hence, it represents a new and easy approach to the synthesis of amphetamine with complete deuteration at the side chain. For example, phenyl-2-propanone-d10 oxime (9a) can be prepared quantitatively from 9b by the deuterium exchange procedure. This method provides a practical alternative to the synthesis of dl-phenyl-2-aminopropane-d1 (1) described in Scheme I. The tedious in situ preparation of methyl-d3-lithium is avoided and the purity of the product of the coupling reaction between phenylacetic-d7 acid and the readily available methyllithium is improved.
Analogues 5 and 6 were synthesized from benzaldehyde-d1 and will be reported elsewhere.7 α-Deuterated amphetamine 4 was synthesized according to the procedure of Foreman et al.5
Experimental
Phenylacetic-d7 acid, methyl-d3 iodide and lithium aluminum deuteride (LiAlD4) were purchased from Merck Co., Inc. All melting points were determined with a Mel-Temp apparatus and are uncorrected. TLC was performed on Eastman chromagram silica gel sheets with fluorescent indicator, using CHCl3 MeOH (95:5) and CHCl3 MeOH (4:1) as developing solvents.
Phenyl-2-propanone-d10 Oxime (9a)
A solution of phenylacetic-d7 acid (7, 2.0 g, 14 mmol) in anhydrous ether (35 mL) was placed in a clean and dry three-necked round-bottom flask equipped with a magnetic stirrer, a dropping funnel, a gas inlet tube, and a reflux condenser with a drying tube on top. Under nitrogen atmosphere and with rapid stirring an ethereal solution of methyl-d3-lithium [70 mL, freshly prepared from methyl-d3 iodide (5.3 g, 36.5 mmol) and lithium metal (0.6 g, 86.5 mmol) in anhydrous ether] was added dropwise at such a rate sufficient to maintain gentle reflux. After the addition of all the methyl-d3-lithium solution, the mixture was refluxed on a steam bath for 15 min. The solution was allowed to cool to room temperature and 15 mL of water was added slowly. The aqueous layer was removed, the ethereal layer washed three times with water and dried (MgSO4), and the solvent removed under reduced pressure yielding 1.5 g (75%) of phenyl-2-propanone-d10 (8a) as a colorless oil.
A mixture of phenyl-2-propanone-d10 (8a, 1.5 g, 10.4 mmol) and hydroxylamine hydrochloride (0.88 g, 12.7 mmol) in D2O (3.2 mL) was warmed in a water bath maintained at 40°C. With vigorous stirring, 2.4 mL of 10 N NaOD in D2O was added dropwise. After the addition of all of the base, the mixture was stirred at 40°C for 20 min. At this time, an additional amount of hydroxylamine hydrochloride (0.88 g) and 10 N NaOD (2.4 mL) was added in the same manner to ensure completion of the reaction. The mixture was then neutralized by the slow addition of glacial acetic acid (1.5 mL) and was extracted with ether (2×100 mL). The combined ether extract was washed with a saturated solution of calcium chloride, dried (MgSO4), and filtered, and the solvent was removed in vacuo affording 1.26 g of 9a (84% yield) as a colorless oil which thickened on standing. This product was used for subsequent reaction without further purification.
Phenyl-2-propanone-d7 Oxime (9b)
This compound was prepared analogously to 9a beginning with phenylacetic-d7 acid (1.9 g, 13.4 mmol) and methyllithium (18 mL, 32 mmol). Phenyl-2-propanone-d7 (8b) was obtained in 83% yield (1.55g). The desired oxime 9b was then readily prepared in a similar manner as described above from 8b (1.4 g, 9.7 mmol), hydroxylamine (1.64 g, 24 mmol), and 10 N NaOH (4.4 mL) in D2O (3 mL) in 84% yield (1.18 g) and as a viscous oil. This product was used for subsequent reaction without further purification.
Phenyl-2-aminopropane-d11 (1)
A solution of the oxime 9a (1.26 g, 7.9 mmol) in anhydrous ether (60 mL) was added through a dropping funnel to a vigorously stirred suspension of LiAlD4 (0.44 g, 10.5 mmol) in anhydrous ether (15 mL). The mixture was refluxed for 7 h. After cooling, the excess hydride was decomposed by slow addition of water (1.0 mL), followed by the hydrolysis of the LiAl complex with 10% NaOH solution (3 mL) and stirring at room temperature for 30 min. The suspension was then filtered and the precipitate washed with ether (50 mL). The combined filtrate and washings were dried (MgSO4) and filtered, and the solvent was removed in vacuo, affording 0.63 g (50%) of liquid 1. The base was converted to the sulfate by the addition of an exactly equivalent amount of ethereal sulfuric acid to a cool ethereal solution of the base. The salt was recrystallized from dilute aqueous ethanol yielding 0.7 g: mp 310°C dec (lit.14 mp 310-320°C dec for the nondeuterated salt).
Phenyl-2-aminopropane-d7 (2)
The oxime 9b (1.1 g, 6.9 mmol) was reduced in a similar manner with LiAlH4 (1.0 g, 26.3 mmol) in anhydrous ether. The yield was 1.1 g of the free amine 2 and 0.88 g of the purified sulfate salt: mp 310°C dec.
Phenyl-2-propanone-d5 Oxime (12)
A mixture of phenyl-2-propanone oxime (11, 3.0 g, 20 mmol), dimethylformamide (DMF, 10 mL), D2O (10 mL), and anhydrous sodium carbonate (2 g) was refluxed under gentle stirring for 24 h. The mixture was then allowed to cool to room temperature, acidified with 10% aqueous HCl, and extracted with ether (3×50 mL). The combined extracts were washed with H2O (5×60 mL), dried (MgSO4), filtered, and evaporated to dryness under reduced pressure. The deuterium exchange reaction was repeated for an additional 24 h with fresh DMF, D2O, and anhydrous Na2CO3, as before, to ensure complete exchange. Similar workup yielded 2.0 g (97%) of the desired oxime 12.
Phenyl-2-aminopropane-d6 (3)
This compound was prepared in a similar manner described for 1 from phenyl-2-propanone-d5 oxime (12, 2.0 g, 13 mmol) and LiAlD4 (0.62 g, 15 mmol) in anhydrous ether (20 mL). After workup in the usual manner, 1.6 g of the free amine was obtained and converted immediately to its sulfate salt which was recrystallized from dilute aqueous ethanol yielding 0.7 g of pure 3: mp 310°C dec.