Experimental

Materials and Methods

Melting points were taken on a Mel-Temp apparatus and are uncorrected. ¹H-NMR spectra were recorded on a Chemagnetics A-200 MHz spectrometer at Purdue University or on the 200-MHz UC Berkeley Chemistry Department NMR. Chemical shifts are reported in delta values (parts per million) relative to an internal reference of Me₄Si. Abbreviations used in the NMR analysis include the following: bs, broad singlet; d, doublet; dd, doublet of doublets; m, multiplet; p, pentet; and s, singlet. Only one NMR analysis is reported for each pair of enantiomers, as the spectra of optical antipodes were virtually identical. Mass spectral analysis was performed on a Finnegan 2000 spectrometer. Optical rotations were recorded using a Perkin-Elmer 241 polarimeter. Microanalyses were performed at the Purdue Microanalysis Laboratory or at the University of California, Berkeley, Chemistry Department Microanalysis Laboratory, and all values were within 0.4% of the calculated composition.

High concentration, no-carrier-added (purportedly 17 Ci/mg iodide) sodium ¹²⁵I-iodide in 0.1M NaOH was purchased from New England Nuclear Corporation. No-carrier-added sodium ¹²³I-iodide in dilute NaOH was purchased from AECL, Canada and Crocker Nuclear Laboratory, University of California, Davis. High performance liquid chromatography (HPLC) was performed with a Waters Associates 590 pump and U6K injector with a Waters Model 450 UV detector (254 nm) and a NaI(Tl) detector in series for absorbance and radioactivity measurements. Two HPLC systems were utilized and kept completely separate so that high specific activity products could be collected on one of the two. Quantitation of the UV and radioactivity peaks was accomplished with a Spectra-Physics Model 4270 integrator. HPLC separations were carried out using Waters C₁₈ reverse phase columns eluted with methanol/water mixtures buffered with 0.2% triethylamine and conc. phosphoric acid to bring the pH to 7.6.

Cold Chemical Syntheses

(R)-1-(2,5-dimethoxyphenyl)-2-trifluoroacetamidopropane ((R)-2)

(R)-2,5-dimethoxyamphetamine ((R)-1) (2.00g, 10.24 mmol) prepared by the method of Nichols et al.³ was stirred into dry benzene (200 mL) under a nitrogen atmosphere. Trifluoroacetic anhydride (10.75g, 51.20 mmol) was then added all at once and the reaction was stirred at room temperature for 30 min. After reflux for an additional 30 min, the solvent was removed in vacuo and the resulting solid was crystallized from ethyl acetate/hexanes to yield feathery white crystals: 2.75g (92.3%); mp 120°C; [α]_D +12.27° (c, 1, CHCl₃).

(S)-1-(2,5-dimethoxyphenyl)-2-trifluoroacetamidopropane ((S)-2)

(S)-2,5-dimethoxyamphetamine (S-1) treated in an identical manner produced feathery white crystals: 2.67g (89.6%); mp 120°C; [α]_D -11.65° (c, 1, CHCl₃).

(R)-1-(2,5-dimethoxy-4-iodophenyl)-2-trifluoroacetamidopropane [(R)-3]

Iodine monochloride (0.290g, 1.716 mmol) was added to 5 mL of glacial acetic acid with stirring under an argon atmosphere. R-2 (0.500g, 1.716 mmol) was transferred to the flask in 5 mL of hot glacial acetic acid, and the reaction was stirred at room temperature for 1 h. After heating at 60°C for 1 h, the mixture was cooled, flooded with H₂O (75 mL), extracted with CHCl₃ (3 x 25 mL), and the pooled extracts were washed with 5% NaHCO₃ (50 mL), and H₂O (50 mL). The organic extract was dried (Na₂SO₄) and the solvent was removed by rotary evaporation. The resulting solid was crystallized from ethyl acetate, affording a fine white crystalline product: 0.610g (85.2%); mp 192-193°C; [α]_D +23.15° (c, 1, CHCl₃).

(S)-1-(2,5-dimethoxy-4-iodophenyl)-2-trifluoroacetamidopropane ((S)-3)

(S)-2 treated in an identical manner afforded white crystalline product: 0.576g (80.4%); mp 191-192°C; [α]_D -22.90° (c, 1, CHCl₃).

(R)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ((R)-4)

To (R)-3 (0.200g, 0.479 mmol) was added 20 mL of 2-propanol and 1 mL of aqueous 2N KOH under an argon atmosphere. This was allowed to stir at room temperature for 4 h, and the solvent was then removed by rotary evaporation. The resultant slurry was dissolved in 3N NaOH (25 mL) and extracted with CHCl₃ (3 x 25 mL). The CHCl₃ extracts were pooled and the amine was extracted with 3N HCl (2 x 25 mL). The pooled acidic aqueous extract was then made strongly basic by addition of 5N NaOH and was re-extracted with CHCl₃ (3x25 mL). The pooled organic extracts were dried (Na₂SO₄) and the solvent was removed by rotary evaporation. Drying under high vacuum afforded a white solid: 0.149g (96.7%); mp 96-97°C. This free base (0.132g, 0.411 mmol) was then dissolved in absolute ethanol and acidified with conc. HCl. The solvent was removed in vacuo and the residual solid was crystallized from 2-propanol/diethyl ether to yield a white crystalline hydrochloride salt: 0.121g (82.3%); mp 232°C; [α]_D -11.23° (c, 1, H₂O) (lit.¹³ mp 218-219°C; [α] -12.0°).

(S)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ((S)-4)

An exact replication of the above method using (S)-3 afforded the white solid free base: 0.149g (96.7%), mp 96-97°C, and a white crystalline hydrochloride salt: 0.120g (80.5% from the base), mp 232°C; [α]_D +11.47° (c, 1, H₂O), (lit.⁴ mp 224-225°C; [α] +12.6°).

(±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane
(Racemic 4 via Chloramine-T oxidant with iodide as the limiting reagent):

To racemic 2,5-dimethoxyamphetamine⁹ (195 mg, 1.0 mmol) in 30 mL of 2.0M H₃PO₄ was added KI (33 mg, 0.20 mmol) and chloramine-T (100 mg, 0.44 mmol). The reaction was stirred at room temperature for 25 min and quenched with sodium metabisulfite. The mixture was made basic with conc. NaOH and extracted with CH₂Cl₂ (3x25 mL), and the solvent was removed by rotary evaporation. Racemic 4 was separated from excess starting material by preparative HPLC on a Waters 19x150mm C₁₈ column eluted with methanol/water (45/65), and the solvent was removed on a rotary evaporator. The hydrochloride salt was recrystallized from 2-propanol/ether: 40 mg (62% based on KI); mp 227-230°C.

Radioiodination procedures

The effects of differing solvents, oxidizing agents and temperature upon the radioiodination yield were investigated. The general radioiodination procedure was as follows: to 6 µmol of precursor in a Reacti-Vial (Pierce Chemical Company) equipped with a magnetic stirrer there was added solvent, radioiodine in a minimum volume of aqueous base, and oxidant; aliquots of the reaction mixture at various times after addition of the oxidant were quenched with excess sodium metabisulfite (MBS); and the kinetics of the radioiodination were determined by radio-HPLC. These procedures helped to identify the best labelling routes which were then utilized for the larger scale radiosyntheses of DOI.

Radiosynthesis of 2,5-Dimethoxy-4-[¹²⁵I or ¹²³I]-Iodophenylisopropylamine

Method A. Via the N-trifluoroacetyl-(R)- or (S)-2,5-dimethoxyphenylisopropylamine.

To 300 µL of trifluoroacetic acid (TFA) there was added 1.8 mg (6 µmol) of (R)- or (S)-1-(2,5-dimethoxyphenyl)-2-trifluoroacetamidopropane ((R)-2 or (S)-2). Solutions of no-carrier-added ¹²⁵I-iodide or ¹²³I-iodide in 0.1N NaOH (5-50 µL) were added to the vial followed by 0.5 mg (2 µmoles) dichloramine-T (TCI Tokyo Kasei Organic Chemicals) in 20 µL TFA. The vial was sealed and the reaction mixture stirred at room temperature.

Aliquots of the reaction mixture were quenched with excess MBS at various times after the addition of dichloramine-T (DCT) to follow the progress of the reaction. The mixture was analyzed by reverse phase radio-HPLC utilizing a buffered methanol/water (60/40) eluent. The reaction mixture was quenched with 2 mg (10 µmoles) MBS when the radio-HPLC indicated greater than 90% incorporation of the radioiodine (approximately one hour). The solvent was evaporated at 50°C under a gentle stream of N2 and 300 µL of 2-propanol and 50 µL of 2N KOH were added. The pH of the solution was checked and additional KOH added if necessary to make the pH greater than 12. The vial was sealed and the mixture stirred at 50°C until deblocking was complete (>99% at ca. 30 min). Approximately 300 µL of water were added to the vial and the solution was filtered through a 0.45 µm Millipore filter prior to HPLC fraction collecting. A C₁₈ analytical column was eluted with a buffered methanol/water solution (35/65), and the radioactive peak containing radioiodinated (R)- or (S)-DOI was collected. The solvent was removed by vigorously blowing N₂ over the solution while heating in a 90°C water bath. When the product was contained in approximately 1 mL of solution, it was reinjected onto the C₁₈ column with UV monitoring to determine its specific activity and was fraction collected as before. The solvent was completely evaporated and the product taken up in an ethanol/water (50/50) solution and stored at -20°C.

Method B. Via the N-phthalimide protected (±)-2,5-dimethoxyphenylisopropylamine.

To 300 µL of TFA there was added 1.9 mg (6 µmoles) of racemic (±)-1-(2,5-dimethoxyphenyl)-2-phthalimidopropane ((+)-5) prepared as previously described¹⁴. To this solution was added 5-50 µL of ¹²⁵I- or ¹²³I-iodide in 0.1N NaOH followed by 0.5 mg DCT in 20 µL TFA. The vial was sealed and the reaction mixture stirred at room temperature. The reaction was followed as before with radio-HPLC and quenched with MBS when the reaction yield exceeded 90% (1-2 h). The TFA was evaporated with a gentle stream of N₂, and 300 µL butanol and 100 µL hydrazine hydrate were added. The vial was sealed and the mixture was heated to 110°C in an oil bath for approximately 5 min. An aliquot of the reaction mixture was analyzed by radio-HPLC to ensure that complete (>99%) deblocking had occurred. The mixture was taken up in a syringe and filtered through a 0.45 µm Millipore filter before chromatographing twice.

Method C. Via the unprotected (R)- or (S)-2,5-dimethoxyphenylisopropylamine.

To 300 µL of 2.0M phosphoric acid there was added 1.2 mg (6 µmoles) of (R)- or (S)-1-(2,5-dimethoxyphenyl)-2-aminopropane ((R)-1 or (S)-1) prepared as previously described³. To this solution was added 5-50 µL of ¹²⁵I- or ¹²³I-iodide in 0.1N NaOH followed by 0.4 mg (2 µmoles) of chloramine-T (CAT). The vial was sealed and the reaction mixture stirred at room temperature while following the progress of the reaction with radio-HPLC. The reaction was quenched with MBS when the reaction yield exceeded 90% (ca. 10 min). The mixture was made basic by the addition of NaOH and chromatographed twice with HPLC.