Core-Labeling (Radio) Synthesis of Phenols

We report a method that enables the fast incorporation of carbon isotopes into the ipso carbon of phenols. Our approach relies on the synthesis of a 1,5-dibromo-1,4-pentadiene precursor, which upon lithium–halogen exchange followed by treatment with carbonate esters results in a formal [5 + 1] cyclization to form the phenol product. Using this strategy, we have prepared 12 1-13C-labeled phenols, show proof-of-concept for the labeling of phenols with carbon-14, and demonstrate phenol synthesis directly from cyclotron-produced [11C]CO2.


General Considerations
Unless otherwise stated, all reactions were performed under a dry nitrogen atmosphere in oven-dried glassware equipped with a magnetic stir bar. Where noted, reactions performed in a glovebox were performed under a nitrogen atmosphere in an MBraun UniLab Pro SP system with residual oxygen and water maintained under 2.0 ppm. Diethyl ether (Et2O), tetrahydrofuran (THF), dichloromethane (CH2Cl2), toluene (PhMe), acetonitrile (MeCN), triethylamine (Et3N), and pentane were dried by passage through a column of activated alumina under an argon atmosphere using a Pure Process Technology solvent purification system. All other organic solvents were dried over activated molecular sieves (4Ǻ) and degassed prior to use. Reaction temperatures are reported relative to the oil bath surrounding the reaction vessel.
Thin-layer chromatography (TLC) was performed on glass-backed plates of 250μm thickness coated with Silica Gel 60 F254 or neutral alumina F254 as noted. Plates were visualized with UV irradiation or staining as noted.
High resolution mass spectra were recorded on either an Agilent 6224 TOF High Resolution Accurate MS with electrospray ionization or an Agilent 7200B QTOF High Resolution Accurate Mass GCMS using an Agilent HP-5MS column with a temperature gradient of 50 °C to 200 °C over 30 minutes and electron ionization. All mass spectra were processed with an Agilent MassHunter Operating System. Nuclear magnetic resonance spectra ( 1 H NMR, 13 C NMR) were recorded with Bruker spectrometers operating at 400 MHz or 500 MHz for 1 H. All NMR spectra were processed with Mestrelab Research MestReNova. Chemical shifts are reported in parts per million (ppm, δ), downfield from tetramethylsilane (TMS, δ = 0.00 ppm) and are referenced to residual solvent (CDCl3, δ = 7.26 ppm ( 1 H) and 77.16 ppm ( 13 C)). Coupling constants are reported in Hertz (Hz). Data for 1 H-NMR spectra are reported as follows: chemical shift (ppm, s = singlet, d = doublet, t = triplet, q = quartet, quin = quintet, dd = doublet of doublets, td = triplet of doublets, ddd = doublet of doublet of doublets, m = multiplet, coupling constant (Hz), and integration).

Synthesis of Phenols
General procedure A for the synthesis of 1-13 C phenols from dibenzyl carbonate-carbonyl- 13

C
In a glovebox under a nitrogen atmosphere, 0.425 mmol (2.0 equiv.) dibromide precursor 3 was added to a scintillation vial and dissolved in 17 mL Et2O. The reaction was cooled to -78 °C using a liquid-nitrogen jacketed cold well, and tert-butyllithium (1.7 M in pentane, 1.0 mL, 1.7 mmol, 8.0 equiv.) was added dropwise. After 30 minutes, the vial was removed from the cold well and the solution was stirred at room temperature for 2 hours. A solution of 5-carbonyl-13 C (52 mg, 0.213 mmol) in 0.1 mL Et2O was then added dropwise, and the mixture was stirred for 5 minutes. The vial was removed from the glovebox while sealed, and a solution of HCl (2M in Et2O, 0.85 mL) and MeOH (0.85 mL) was added while stirring. Solvents were removed by vacuum, and the crude residue was purified by silica flash column chromatography as noted.

General procedure B for the synthesis of phenols from CO2
In a glovebox under a nitrogen atmosphere, 0.425 mmol (2.0 equiv.) dibromide precursor 3 was added to a septum-capped 40 mL scintillation vial and dissolved in 17 mL Et2O. The reaction was cooled to -78 °C using a liquid-nitrogen jacketed cold well, and tert-butyllithium (1.7 M in pentane, 1.0 mL, 1.7 mmol, 8.0 equiv.) was added dropwise. After 30 minutes, the vial was removed from the cold well and the solution was stirred at room temperature for 2 hours. The reaction mixture was then removed from the glovebox while sealed and 1% CO2 in N2 balance gas in a balloon was bubbled into the solution via 20G needle for 10 minutes. Afterwards, a solution of HCl (2M in Et2O, 0.85 mL) and MeOH (0.85 mL) was added while stirring. Solvents were removed by vacuum, and the crude residue was purified by silica flash column chromatography as noted.

CAUTION:
Tert-butyllithium solution in pentane is highly pyrophoric and must be handled with proper air-free technique. All manipulations for this study were performed on the smallest practical scale under a nitrogen atmosphere in a glovebox.

Optimization of Reaction Conditions Standard conditions for optimization of phenol cyclization from dibenzyl carbonate
In a glovebox under a nitrogen atmosphere, dibromide precursor 3a (26.4 mg, 0.085 mmol, 2.0 equiv.) was added to a two-dram vial and dissolved in 3.2 mL Et2O. The reaction was cooled to -78 °C using a liquidnitrogen jacketed cold well, and tert-butyllithium (1.7 M in pentane, 0.2 mL, 0.34 mmol, 8.0 equiv.) was added dropwise. After 30 minutes, the vial was removed from the cold well and the solution was stirred at room temperature for 2 hours. A solution of dibenzyl carbonate (5, 10.3 mg,0.0425 mmol) in 0.1 mL Et2O was then added dropwise, and the mixture was stirred for 5 minutes. The vial was removed from the glovebox, and a solution of HCl (2M in Et2O, 0.2 mL) and MeOH (0.2 mL) was added while stirring. Solvents were removed by vacuum, and the crude residue was dissolved in 0.50 mL CD3OD and 0.20 mL CDCl3. A standard of 3.3 mg mesitylene was added for quantitation by 1

Carbon-11 Radiochemistry
Optimized radiosynthesis of [1-11 C]4a ([1-11 C]propofol) from cyclotron produced [ 11 C]CO2 The corresponding dilithiate precursor was prepared analogously to general procedure A from dibromide 3a. An 800 µL (20 µmol) aliquot of the reaction mixture was transferred to an oven-dried 2 mL v-vial, equipped with magnetic stir bar, and sealed with a screw-cap PTFE septum and electrical tape. The vial was kept under a static atmosphere of dry nitrogen gas.

HPLC information
Radioactivity was measured via flow radiation detector (Carroll & Ramsey Model 105S). The UV detection wavelength used for HPLC measurements was 210 nm.
Trapping efficiency (TE) was calculated by dividing the radioactivity trapped in the reaction vial by the total amount of radioactivity:

TE = Radioactivity in reaction vial
Total C 11 radioactivity Radiochemical conversion (RCC) was calculated by multiplying the HPLC radiochemical purity by the trapping efficiency:

RCC = HPLC Purity × TE
Isolated radiochemical yield (RCY) was calculated by dividing the radioactivity collected from the HPLC by the total amount of radioactivity in the reaction vial and ascarite and decay-corrected to end of bombardment.

Molar activity measurement
The minimum molar activity (Am) of [1-11 C]4a was estimated by injecting an aliquot of the isolated product into the analytical HPLC. The radioactivity in the aliquot was measured via dose-calibrator before injection and decay-corrected to end of synthesis time (EOS) to determine the activity concentration of the isolated product. Due the low absorptivity of 4a, no significant UV signal was observed for aliquots of the isolated product. A calibration curve was made by injecting a known amount of reference compound to determine the limit of detection under our HPLC conditions of 0.561 nmol/mL.

Representative crude analytical HPLC trace
Top: Radioactivity; Bottom: 210 nm UV

Synthesis of dibenzyl carbonate (5) from sodium carbonate.
To an oven-dried 100 mL Kontes®-topped Schlenk flask equipped with magnetic stir bar were added sodium carbonate (106 mg, 1.0 mmol), 18-crown-6 (1.06 g, 4.0 mmol), cesium chloride (337 mg, 2.0 mmol), benzyl chloride (1.27 g, 10.0 mmol), and N,N-dimethylformamide (1.0 mL) under nitrogen atmosphere and sealed. The reaction mixture was allowed to stir in a 100 °C oil bath for 24 hours. Afterwards, the reaction mixture was added to 50 mL brine, and the crude product was extracted with EtOAc (3 x 25 mL). The combined organic fractions were dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The title compound was isolated as a white solid in 30% yield (73 mg, 0.30 mmol) by silica flash column chromatography. Rf = 0.65 (20% EtOAc in hexanes).

Synthesis of dibenzyl carbonate (5) from barium carbonate.
An oven-dried 20 mL COware two-chamber flask was evacuated and filled with dry N2 gas. On one side of the flask, benzyl alcohol (162 mg, 1.5 mmol), 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU, 381 mg, 2.5 mmol), and 0.1 mL 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) were dissolved in 1.0 mL dibromomethane and equipped with a stir bar. On the other side of the flask was barium carbonate (99 mg, 0.5 mmol), and the entire flask was placed under 1 atm of static N2 gas. Carefully, 300 µL concentrated sulfuric acid was added dropwise to the barium carbonate. Then the flask was placed in a 70 °C oil bath, and the reaction mixture was allowed to stir for 24 hours. Afterwards, the reaction mixture was filtered over a pad of silica and the crude filtrate was concentrated under high vacuum. The title compound was isolated as a white solid in 18% yield (22 mg, 0.09 mmol) by silica flash column chromatography. Rf = 0.65 (20% EtOAc in hexanes). Spectroscopic data as above.

Synthesis of Dibromide Precursors
General procedure C for synthesis of dibromo precursors from dialkyne alcohols In a round-bottom flask, a 0.2 M solution of dialkyne alcohol 1 in THF was cooled using an ice bath. Slowly, 4.0 equiv. of sodium bis(2-methoxyethoxy)aluminum hydride (Red-Al® 70 % wt. in toluene) was added to the reaction mixture. After one hour, the flask was removed from the ice bath and heated to 60 °C for 5 hours. The reaction was then cooled to -78 °C using a dry ice/acetone bath, and 4.4 equiv. Nbromosuccinimide was added in small portions. The reaction was allowed to warm up to room temperature while stirring overnight. Upon workup, an equal volume of aqueous Na2S2O3 (1 M) and aqueous Rochelle salt (1 M) were added and the mixture was allowed to stir for 1 hour. The organic layer was separated, and the aqueous layer was extracted with diethyl ether. The combined organic fractions were washed with aqueous Na2S2O3 (1 M), saturated aqueous NaHCO3, and brine. Removal of the solvent in vacuo provided the dark brown oil, which was filtered through a plug of neutral alumina in 1/1 hexanes/ethyl acetate to provide the crude dibromo alcohol 2.
A 0.5 M solution of 2 in CH2Cl2 was cooled to 0 °C using an ice bath. Subsequently, 1.0 equiv. triethylsilane and 2.0 equiv. trifluoroacetic acid were added and the reaction mixture was stirred for 45 minutes. An equal volume of saturated aqueous NaHCO3 was added, and the mixture was stirred vigorously for 15 minutes to quench the remaining acid. The organic layer was separated, and the aqueous layer was extracted twice more with CH2Cl2. The combined organic fractions were washed with brine, dried over anhydrous Na2SO4, and the solvents were removed in vacuo. The crude residue was purified by flash column chromatography as noted. Yields for the dibromide precursor 3 are reported over two steps. These compounds were stored in a -40 °C freezer inside a glovebox until use.
((1Z,4Z)-1,5-dibromo-3-ethylpenta-1,4-diene-1,5-diyl)dibenzene (3j) Synthesized according to general procedure C from 1j (3.30 g, 12.7 mmol). After silica flash column chromatography (Rf = 0.33 (100% hexanes)), the title compound was obtained in an inseparable mixture with impurities as a pale-yellow oil. Isolation of this pure compound was not successful; however, the desired compound can be seen via 1 H NMR and HRMS. The product as obtained was carried forward without further purification. It is used in excess in the subsequent phenol synthesis.

Synthesis of Dialkyne Alcohols
General procedure D for the synthesis of symmetric dialkyne alcohols Symmetric dialkyne alcohols were prepared according to modified literature procedures. 6 A stirred solution of alkyne (25 mmol) in 50 mL THF at -78 °C was treated dropwise with n-butyllithium (2.5 M in hexanes, 10 mL, 25 mmol). Afterwards, 11.9 mmol of methyl formate, acyl chloride, or acid anhydride was added dropwise. The solution was allowed to warm to room temperature and stirred for 1 hour, followed by the addition of 50 mL aqueous sat. ammonium chloride. The layers were separated, and the aqueous layer was extracted diethyl ether (3 x 50 mL). The combined organic layers were washed with brine, dried over anhydrous Na2SO4, filtered, and concentrated. In most cases, the crude organic extract was brought forward without further purification.
To an oven-dried 250 mL Schlenk flask equipped with a magnetic stir bar, methyl formate (480 mg, 8.0 mmol) was dissolved in 15 mL THF and cooled to -78 °C. Each of the prepared lithiate solutions was then added via cannula transfer over the course of 10 minutes: first ((4-methoxyphenyl)ethynyl)lithium, and then (cyclopentylethynyl)lithium. Afterwards, the reaction mixture was allowed to warm to room temperature and stirred for 1 hour, followed by the addition of 50 mL aqueous sat. ammonium chloride. The layers were separated, and the aqueous layer was extracted diethyl ether (3 x 50 mL). The combined organic layers were washed with brine, dried over anhydrous Na2SO4, filtered, and concentrated. Title compound was obtained as a yellow oil in 64% yield (1.31 g, 5.14 mmol) after purification by silica flash column chromatography. Rf = 0.13 (10% EtOAc in hexanes).

Carbon-13 T1 Measurement
A sample of 10 mg 4a-1-13 C in 0.50 mL CDCl3 was prepared. Using a Bruker spectrometer, a 13 C T1 inversion-recovery experiment was programmed in TOPSPIN using the following parameters: Scans were processed in MestReNova with uniform processing parameters. The integral of the labeled carbon was plotted against the variable time delay and the exponential curve was fit to the following equation, where I is the integral value, τ is the variable time delay, and T1 is the calculated longitudinal relaxation time.
The calculated value of T1 was found to be 29.4 s for the labeled carbon of 4a-1-13 C.