Synthesis of 5-[4,5-13C2]- and 5-[1,5-13C2]aminolevulinic acid

Article abstract of DOI:10.1002/jlcr.583

5-[4,5-¹³C₂]- and 5-[1,5-¹³C₂]Aminolevulinic acid (ALA) have been synthesized by the Gabriel condensation of potassium phthalimide with ethyl bromo [1,2-¹³C₂]acetate (derived from [1,2-¹³C₂]acetic acid) or ethyl bromo[2-¹³C]-acetate (derived from sodium [2-¹³C]acetate), followed by conversion to the chloride, coupling reaction with 2-ethoxycarbonylethylzinc iodide derived from ethyl 3-iodopropionate or 2-methoxy[¹³C]carbonylethylzinc iodide derived from methyl 3-iodo[1-¹³C]propionate (generated from potassium [¹³C]cyanide), and hydrolysis. Copyright

Full text of DOI:10.1002/jlcr.583

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2002; 45: 569–576.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.583
Research Article
Synthesis of 5-[4,5-13C2]- and
5-[1,5-13C2]aminolevulinic acid
Katsumi Iida, Shinji Tokiwa, Toshihiro Ishii and Masahiro Kajiwara*
Department of Medicinal Chemistry, Meiji Pharmaceutical University,
2-522-1 Noshio, Kiyose-shi, Tokyo 204-8588, Japan
Summary
5-[4,5-13C2]- and 5-[1,5-13C2]Aminolevulinic acid (ALA) have been synthesized
by the Gabriel condensation of potassium phthalimide with ethyl bromo
[1,2-13C2]acetate (derived from [1,2-13C2]acetic acid) or ethyl bromo[2-13C]-
acetate (derived from sodium [2-13C]acetate), followed by conversion to the
chloride, coupling reaction with 2-ethoxycarbonylethylzinc iodide derived from
ethyl 3-iodopropionate or 2-methoxy[13C]carbonylethylzinc iodide derived
from methyl 3-iodo[1-13C]propionate (generated from potassium [13C]cyanide),
and hydrolysis. Copyright # 2002 John Wiley & Sons, Ltd.
Key Words: 5-[4,5-13C2]aminolevulinic acid; 5-[1,5-13C2]aminolevulinic acid;
[1,2-13C2]acetic acid; sodium [2-13C]acetate; potassium [13C]cyanide
Introduction
We have developed various methods for the regioselective synthesis of
singly 13C-labeled 5-aminolevulinic acid (ALA),1,2 and have used
13C-ALA thus synthesized for studies on the biosynthesis of porphyr-
ins.3–5 We present here a synthesis of 13C-double-labeled ALA,
[4,5-13C2]- and [1,5-13C2]ALA, using [1,2-13C2]acetic acid, sodium
[2-13C]acetate and potassium [13C]cyanide as isotope sources by
modification of the methods described in our previous paper2 reported
by Campbell et al.6
*Correspondence to: Professor M. Kajiwara, Department of Medicinal Chemistry, Meiji
Pharmaceutical University, 2-522-1 Noshio, Kiyose-shi, Tokyo 204-8588, Japan
Copyright # 2002 John Wiley & Sons, Ltd.
Received 11 December 2001
Revised 12 February 2002
Accepted 25 February 2002

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K. IIDA ET AL.
Results and discussion
Synthesis of 2-phthalimido[1,2-13C2]acetic acid (8) and
2-phthalimido[2-13C]acetic acid (9)
As shown in Scheme 1, bromination of [1,2-13C2]acetic acid (1) with
bromine catalyzed by red phosphorus, and esterification with ethanol
gave ethyl bromo[1,2-13C2]acetate (3) in 44% yield. Bromination of
sodium [2-13C]acetate (2) with benzoyl bromide, benzoic acid and acetyl
bromide, followed by bromination of the resulting [2-13C]acetyl
bromide with bromine and esterification with ethanol gave ethyl
bromo[2-13C]acetate (4) in 69% yield. The Gabriel condensation of
potassium phthalimide (5) and ethyl bromo[1,2-13C2]acetate (3) or
ethyl bromo[2-13C]acetate (4) in dry dimethylformamide gave ethyl
2-phthalimido[1,2-13C2]acetate (6) or ethyl 2-phthalimido[2-13C]acetate
(7), respectively, in 77% yield. The hydrolysis of ethyl 2-phthalimi-
do[1,2-13C2]acetate (6) or ethyl 2-phthalimido[2-13C]acetate (7) in acetic
acid and 6 M hydrochloric acid gave 2-phthalimido[1,2-13C2]acetic acid
(8) or 2-phthalimido[2-13C]acetic acid (9), respectively, in 81% yield.
Synthesis of methyl 3-iodo[1-13C]propionate (14)
As shown in Scheme 2, cyanation of ethylene chlorohydrin (10) by
potassium [13C]cyanide (11) gave ethylene [13C]cyanohydrin (12) in 87%
yield. The hydrolysis and iodination of ethylene [13C]cyanohydrin (12)
with 55% hydroiodic acid gave 3-iodo[1-13C]propionic acid (13) in 85%
yield. The methyl esterification of 3-iodo[1-13C]propionic acid (13) with
diazomethane gave methyl 3-iodo[1-13C]propionate (14) in 96% yield.
Scheme 1.
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571
Scheme 2.
Scheme 3.
Synthesis of [4,5-13C2]ALA hydrochloride (20) and [1,5-13C2]ALA
hydrochloride (21)
As shown in Scheme 3, the coupling reaction2,6–8 of 2-phthalimi-
do[1,2-13C2]acetyl chloride (15) (derived from 2-phthalimido[1,2-13C2]-
acetic acid (8) and thionyl chloride) and 2-ethoxycarbonylethylzinc
iodide (derived from ethyl 3-iodopropionate (17)2,6 treated with
zinc–copper couple) catalyzed by tetrakis(triphenylphosphine)
palladium (0) in dry toluene and dry N,N-dimethylacetamide gave
ethyl 5-phthalimido[4,5-13C2]levulinate (18) in 92% yield. The same
coupling reaction of compounds derived from 2-phthalimido[2-13C]-
acetic acid (9) and methyl 3-iodo[1-13C]propionate (14) was carried
out to give methyl 5-phthalimido[1,5-13C2]levulinate (19). Finally,
hydrolysis of ethyl 5-phthalimido[4,5-13C2]levulinate (18) or methyl
5-phthalimido[1,5-13C2]levulinate (19) in 6 M hydrochloric acid gave
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[4,5-13C2]ALA hydrochloride (20) or [1,5-13C2]ALA hydrochloride (21),
respectively, in 93% yield.
Experimental
Materials
[1,2-13C2]Acetic acid (90 at% 13C) and sodium [2-13C]acetate (98 at%
13C) were purchased from CIL, Inc., Potassium [13C]cyanide (90 at%
13C) was purchased from Isotec, Inc., All other chemicals were of
analytical grade.
Instruments
Melting point determinations were carried on a Yanaco micro-melting
point apparatus, Model MP; values are uncorrected. IR spectra were
recorded on a Jasco VALOR-III FT-IR spectrometer. 1H-NMR
(400 MHz) and 13C-NMR (100 MHz) spectra were recorded on a Jeol
GSX-400 spectrometer. EI- and FAB-MS spectra were obtained on a
Jeol DX302 spectrometer.
Ethyl bromo[1,2-13C2]acetate (3)
Bromine (7.2 ml, 75.4 mmol) was added dropwise to [1,2-13C2]acetic
acid (1) (1.0 g, 16.1 mmol) and dry red phosphorus (0.20 g, 64.6 mmol),
and the mixture was heated at 1208C for 5 h. After cooling,
excess bromine was blown off with argon gas. To the residue, dry
EtOH (5.0 ml, 85.2 mmol) was added at À788C, and the whole
was stirred at room temperature for 12 h. The reaction was neutra-
lized with saturated NaHCO3 and the mixture was extracted with
ether. The combined extracts were washed with water, dried over
anhydrous MgSO4, and evaporated. Distillation of the crude
product gave ethyl bromo[1,2-13C2]acetate (3) (1.21 g, 44%), b.p.
858C (53 mmHg); 1H-NMR (CDCl3) d: 1.31 (t, 3 H, J=7.2 Hz),
3.83 (dd, 2H, J=153.2 Hz, 2J=4.6 Hz), 4.24 (dq, 2H, J=7.2 Hz,
3J=3.1 Hz); 13C-NMR (CDCl3) d: 25.9 (d, J=64.5 Hz), 167.2
(d, J=64.5 Hz); FT-IR (neat) cmÀ1: 1690; EI-MS m/z (rel. int. %):
169 (M+, 10).
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Ethyl bromo[2-13C]acetate (4)
Benzoyl bromide (7.2 ml, 61.1 mmol) was added dropwise to sodium
[2-13C]acetate (2) (1.0 g, 12.2 mmol) and benzoic acid (1.32 g,
10.8 mmol), and the mixture was heated at 1208C for 5 h. The
resulting [2-13C]acetyl bromide was distilled at 808C under 28 mmHg.
Bromine (3.0 ml, 31.4 mmol) was added dropwise to [2-13C]
acetyl bromide at À788C, and the mixture was stirred at room
temperature for 5 min, and then refluxed at 608C for 5 h. It was allowed
to cool to room temperature, then excess bromine was blown off
with argon gas. To the residue, dry EtOH (3.0 ml, 51.1 mmol) was
added at À788C, and the whole stirred at room temperature for
12 h. The reaction mixture was neutralized with saturated NaHCO3
and extracted with ether. The combined extracts were washed with
water, dried over anhydrous MgSO4 and evaporated. Distillation of
the crude product gave ethyl bromo[2-13C]acetate (4) (1.41 g, 69%),
1
b.p. 858C (56 mmHg); H-NMR (CDCl3) d: 1.31 (t, 3H, J=7.2 Hz),
3.83 (d, 2H, J=142.2 Hz), 4.24 (q, 2 H, J=7.2 Hz); 13C-NMR
(CDCl3) d: 25.9; FT-IR (neat) cmÀ1: 1735; EI-MS m/z (rel. int. %):
168 (M+, 5).
Ethyl 2-phthalimido[1,2-13C2]acetate (6) and ethyl
2-phthalimido[2- 13C]acetate (7)
A solution of ethyl bromo[1,2-13C2]acetate (3) (497.0 mg, 3.0 mmol) and
potassium phthalimide (5) (500.0 mg, 2.7 mmol) in dry dimethylforma-
mide (7 ml) was heated at 908C for 3 h. The reaction was neutralized
with 0.1 M NaOH and the mixture was extracted with CHCl3. The
combined extracts were washed with water, dried over anhydrous
MgSO4, and evaporated. The residue was recrystallized from EtOH
to give ethyl 2-phthalimido[1,2-13C2]acetate (6) (537.2 mg, 77%), m.p.
109–1108C; 1H-NMR (CDCl3) d: 1.29 (t, 3 H, J=7.2 Hz), 4.23 (dq, 2H,
J=7.2 Hz, 3J=3.1 Hz), 4.44 (dd, 2 H, J=142.2 Hz, 2J=6.2 Hz),
7.74–7.90 (m, 4 H); 13C-NMR (CDCl3) d: 38.9 (d, J=63.1 Hz), 167.2
(d, J=63.1 Hz); FT-IR (KBr) cmÀ1: 1470, 1740; EI-MS m/z (rel. int.
%): 276 (M+, 17).
Similar procedures using ethyl bromo[2-13C]acetate (4) in place of
ethyl bromo[1,2-13C2]acetate (3) afforded ethyl 2-phthalimido[2-13C]
acetate (7), m.p. 109–1118C; 1H-NMR (CDCl3) d: 1.29 (t, 3H,
J=7.2 Hz), 4.23 (q, 2H, J=7.2 Hz), 4.44 (d, 2H, J=142.2 Hz),
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7.74–7.90 (m, 4H); 13C-NMR (CDCl3) d: 39.0; FT-IR (KBr) cmÀ1
1470, 1720; EI-MS m/z (rel. int. %): 275 (M+, 15).
:
2-Phthalimido[1,2-13C2]acetic acid (8) and 2-phthalimido[2-13C]
acetic acid (9)
A solution of ethyl 2-phthalimido[1,2-13C2]acetate (6) (537.2 mg,
2.3 mmol) in acetic acid (6 ml) and 6 M HCl (2 ml) was heated at 908C
for 1.5 h. The reaction mixture was diluted with water, and then
lyophilized. The residue was recrystallized from water to give
2-phthalimido[1,2-13C2]acetic acid (8) (382.6 mg, 81%), m.p.
189–1958C; 1H-NMR (CDCl3) d: 4.49 (dd, 2H, J=142.2 Hz,
2J=5.9 Hz), 7.74–7.90 (m, 4H); 13C-NMR (CDCl3) d: 38.5 (d,
J=1.6 Hz), 171.1 (d, J=61.6 Hz); FT-IR (KBr) cmÀ1: 1470, 1770; EI-
MS m/z (rel. int. %): 207 (M+, 4).
Similar treatment of ethyl 2-phthalimido[2-13C]acetate (7) afforded
2-phthalimido[2-13C]acetic acid (9), m.p. 193–1958C; 1H-NMR (CDCl3)
d: 4.50 (d, 2 H, J=142.2 Hz), 7.74–7.90 (m, 4 H); 13C-NMR (CDCl3)
d: 38.5; FT-IR (KBr) cmÀ1: 1465, 1760; EI-MS m/z (rel. int. %): 206
(M+, 15).
Ethylene [13C]cyanohydrin (12)
Ethylene chlorohydrin (10) (6.6 ml, 98.4 mmol) was added to a
suspension of potassium [13C]cyanide (11) (3.05 g, 46.1 mmol) in dry
MeOH (5 ml), and the mixture was heated at 808C for 4 h. It was
allowed to cool to room temperature, and the crystals were removed by
filtration. The filtrate was evaporated. Distillation of the crude product
gave ethylene [13C]cyanohydrin (12) (2.88 g, 87%), b.p. 1158C (18 mm
Hg); 1H-NMR (CDCl3) d: 2.61 (dt, 2H, J=6.2 Hz, 2J=9.5 Hz), 3.87 (dt,
3
2 H, J=6.2 Hz, J=6.2 Hz); 13C-NMR (CDCl3) d: 118.2.
3-Iodo[1-13C]propionic acid (13)
A solution of ethylene [13C]cyanohydrin (12) (751.1 mg, 10.6 mmol) in
55% HI (10.0 ml, 43.0 mmol) was refluxed at 1308C for 2 h. The reaction
mixture was diluted with water and extracted with ether. The combined
extracts were washed with brine, dried over anhydrous MgSO4, and
evaporated. Chromatography of the crude product on silica gel with
ethyl acetate:hexane (1:1) gave 3-iodo[1-13C]propionic acid (13) (1.80 g,
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1
2
85%), H-NMR (CDCl3) d: 3.06 (dt, 2H, J=7.2 Hz, J=7.2 Hz), 3.32
(dt, 2 H, J=7.2 Hz, J=4.9 Hz); 13C-NMR (CDCl3) d: 117.5.
3
Methyl 3-iodo[1-13C]propionate (14)
A solution of diazomethane in ether was added dropwise to a solution
of 3-iodo[1-13C]propionic acid (13) (1.80 g, 9.0 mmol) in ether (10 ml)
at 08C, until the color of the reaction solution become yellow.
Evaporation afforded a crude product, which was distilled to give
methyl 3-iodo[1-13C]propionate (14) (1.85 g, 96%), b.p. 708C (7 mmHg);
1H-NMR (CDCl3) d: 2.99 (dt, 2H, J=7.2 Hz, 2J=7.2 Hz), 3.33 (dt, 2H,
3
3
J=7.2 Hz, J=4.9 Hz), 3.73 (d, 3 H, J=3.9 Hz); 13C-NMR (CDCl3)
d: 171.5.
Ethyl 5-phthalimido[4,5-13C2]levulinate (18) and methyl
5-phthalimido[1,5-13C2]levulinate (19)
A mixture of 2-phthalimido[1,2-13C2]acetic acid (8) (1.01 g, 4.9 mmol)
and thionyl chloride (6.0 ml, 82.3 mmol) was refluxed at 808C for 4 h.
The excess thionyl chloride was removed in vacuo. The residue was
dissolved in dry toluene (3 ml) and evaporated. This process
was repeated twice, and the resulting 2-phthalimido[1,2-13C2]acetyl
chloride (15) was dissolved in dry toluene (4 ml) under argon. Ethyl
3-iodopropionate (17) (1.66 g, 7.8 mmol) was added to a suspension of
Zn–Cu couple (634.5 mg) in dry toluene (0.8 ml) and dry N,
N-dimethylacetamide (9 ml) under argon, and the mixture was heated
at 608C for 4.5 h. To this suspension, a suspension of Pd(PPh3)4
(153.4 mg, 132.7 mmol) in dry toluene (4 ml) under argon was added, and
the whole was stirred at 608C for 5 min. Then, a solution of 2-
phthalimido[1,2-13C2]acetyl chloride (15) was added, and the whole was
stirred at 608C for 25 min.The reaction was diluted with ethyl acetate,
and the whole was washed with 1 M HCl, saturated NaHCO3 and brine,
dried over anhydrous MgSO4, and evaporated. Chromatography of
the crude product on silica gel with ethyl acetate:hexane (1:1) gave ethyl
5-phthalimido[4,5-13C2]levulinate (18) (1.31 g, 92%), m.p. 69–718C;
1H-NMR (CDCl3) d: 1.26 (t, 2H, J=7.2 Hz), 2.66 (m, 2 H), 2.85
(m, 2H), 4.14 (q, 2H, J=7.2 Hz), 4.56 (dd, 2H, J=139.4 Hz,
2J=4.1 Hz), 7.74–7.90 (m, 4H); 13C-NMR (CDCl3) d: 46.5 (d,
J=41.1 Hz), 200.7 (d, J=41.1 Hz); FT-IR (KBr) cmÀ1: 1410, 1770;
EI-MS m/z (rel. int. %): 291 (M+, 2).
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A similar procedure starting from 2-phthalimido[2-13C]acetic acid
(9) via methyl 3-iodo[1-13C]propionate (14) afforded methyl
5-phthalimido[1,5-13C2]levulinate (19), 1H-NMR (CDCl3) d: 2.66
(m, 2H), 2.85 (m, 2H), 3.70 (d, 2H, 2J=3.9 Hz), 4.56 (d, 2H,
J=139.4 Hz), 7.74–7.90 (m, 4H); 13C-NMR (CDCl3) d: 46.5, 172.5.
[4,5-13C2]ALA hydrochloride (20) and [1,5-13C2]ALA
hydrochloride (21)
A solution of ethyl 5-phthalimido[4,5-13C2]levulinate (18) (1.02 g,
3.5 mmol) in 6 M HCl (5 ml) was heated under reflux for 14 h. The
reaction mixture was cooled to room temperature and filtered. The
filtrate was washed with ether and evaporated. The residue was
recrystallized from EtOH–ether to give [4,5-13C2]ALA hydrochloride
1
(20) (1.86 g, 93%), m.p. 146–1498C; H-NMR (D2O) d: 2.74 (m, 2H),
2
2.91 (m, 2H), 4.15 (dd, 2H, J=143.5 Hz, J=4.1 Hz); 13C-NMR (D2O)
d: 49.9 (d, J=39.6 Hz), 207.0 (d, J=41.1 Hz); FT-IR (KBr) cmÀ1: 1727;
FAB-MS (glycerol) m/z: 134 (MH+-HCl).
Similar treatment of methyl 5-phthalimido[1,5-13C2]levulinate (19)
1
afforded [1,5-13C2]ALA hydrochloride (21), m.p. 155–1578C; H-NMR
(D2O) d: 2.74 (m, 2H), 2.91 (m, 2H), 4.15 (d, 2 H, J=143.5 Hz);
13C-NMR (D2O) d: 49.9, 179.7; FT-IR (KBr) cmÀ1: 1725; FAB-MS
(glycerol) m/z: 134 (MH+-HCl).
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