Concise synthesis of (2R,4R)-monatin

Article abstract of DOI:10.1248/cpb.c16-00358

Monatin, 4-hydroxy-4-(3-indolylmethyl)-glutamic acid, is a naturally occurring sweet amino acid. The (2R,4R)-monatin isomer has been found to be the sweetest among its four stereoisomers. A concise and efficient synthesis of (2R,4R)-monatin was accomplished by the alkylation of (4R)-N-tert-butoxycarbonyl (tBoc)-4-tert-butyldimethylsilyoxy-D-pyroglutamic acid methyl ester with tert-butyl 3-(bromomethyl)-1H-indole-1-carboxylate to give (4R)-N-tBoc-4-tert-butyldimethylsilyloxy-4-(N-tBoc-3-indolylmethyl)-D-pyroglutamic acid methyl ester, i.e., the lactam form of (2R,4R)-monatin with protecting groups. This was followed by the hydrolysis of the lactam ring and deprotection. The 4-hydroxyl D-pyroglutamic acid derivative was demonstrated to be a suitable precursor for the efficient preparation of (2R,4R)-monatin in high optical purity because the alkylation proceeded in regioselective and stereoselective manners at C4 to form appropriate asymmetric tetra-substituted carbon center; the resulting alkylated pyroglutamic acid derivative was then easily converted into the linear form of monatin.

Full text of DOI:10.1248/cpb.c16-00358

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Vol. 64, No. 8
Note
Concise Synthesis of (2R,4R)-Monatin
Yusuke Amino
Institute for Innovation, Ajinomoto Co., Inc.; 1–1 Suzuki-cho, Kawasaki-ku, Kawasaki 210–0801, Japan.
Received April 25, 2016; accepted May 16, 2016
Monatin, 4-hydroxy-4-(3-indolylmethyl)-glutamic acid, is a naturally occurring sweet amino acid. The
(2R,4R)-monatin isomer has been found to be the sweetest among its four stereoisomers. A concise and
efficient synthesis of (2R,4R)-monatin was accomplished by the alkylation of (4R)-N-tert-butoxycarbonyl
(tBoc)-4-tert-butyldimethylsilyoxy-D-pyroglutamic acid methyl ester with tert-butyl 3-(bromomethyl)-1H-
indole-1-carboxylate to give (4R)-N-tBoc-4-tert-butyldimethylsilyloxy-4-(N-tBoc-3-indolylmethyl)-D-pyroglu-
tamic acid methyl ester, i.e., the lactam form of (2R,4R)-monatin with protecting groups. This was followed
by the hydrolysis of the lactam ring and deprotection. The 4-hydroxyl D-pyroglutamic acid derivative was
demonstrated to be a suitable precursor for the efficient preparation of (2R,4R)-monatin in high optical pu-
rity because the alkylation proceeded in regioselective and stereoselective manners at C4 to form appropriate
asymmetric tetra-substituted carbon center; the resulting alkylated pyroglutamic acid derivative was then
easily converted into the linear form of monatin.
Key words monatin; sweet amino acid; pyroglutamic acid; alkylation; tetra-substituted carbon center
Monatin is a naturally occurring amino acid derivative iso- reacted with benzyl bromides, exclusively yielding the trans
lated from the bark of Schlerochiton ilicifolius roots, a plant isomer.15) Bassoli et al. also reported the same stereoselec-
native to northwestern Transvaal in South Africa. Monatin has tivity in the synthesis of a monatin derivative that lacked the
four stereoisomers because of its two asymmetric centers at 4-hydroxyl group.16)
C2 and C4. Vleggaar et al. reported the structure of natural
Oliveira and Coelho12) examined a similar stereospecific
monatin as (2S,4S)-4-hydroxy-4-(3-indolylmethyl)-glutamic synthesis of (2S,4S)-monatin via the oxidation and alkylation
acid and its sweetness as 1200–1400-fold more intense than of an enolate originating from (S)-pyroglutaminol derivative.
sucrose.1) We previously reported that all isomers exhibit a The alkylation of their derivative proceeded in a regioselective
sweet taste; however, (2S,4S)-monatin was found to be the manner because it has a single active hydrogen atom at C4. In
least sweet isomer, whereas the other three isomers, especially addition, the alkylation of their derivative at C4 asymmetric
(2R,4R)-monatin, were found to be intensely sweet (1700 center was proceeded from the less hindered face of the lac-
times at 10% sucrose equivalent).2,3)
tam ring opposite from the bulky tert-butyldimethylsilyloxy-
A major problem in the synthesis of (2R,4R)-monatin is methyl substituent at C2. However, the transformation of the
the highly diastereoselective formation of a tetra-substituted hydroxymethyl group to a carboxylic acid was required in the
carbon center at C4. Various chemical synthetic methods have final stage of their total synthesis.12)
been reported for the formation of the (2S,4S)-isomer, some of
Merino et al. reported the synthesis of (4R)-N-tBoc-4-tert-
which produce a mixture of stereoisomers,4–7) whereas others butyldimethylsilyoxy-D-pyroglutamic acid methyl ester (6),
describe stereoselective syntheses.8–13) In addition, some pro- a possible precursor for the synthesis of the lactam form of
cedures require several steps after the construction of asym- (2R,4R)-monatin.17) Zhang et al. also reported the synthesis of
metric tetra-substituted carbon center to complete the total (4S)-N-tBoc-4-tert-butyldimethylsilyoxy-L-pyroglutamates, an
synthesis.
antipode of (6).18) However, the alkylation of the lithium eno-
A few groups have reported the chemoenzymatic synthesis late of these compounds has not been examined. As such, we
of stereoisomers of monatin, in which the stereogenic center at examined the utilization of (6) for the synthesis of (2R,4R)-
C4 was introduced by an enantiospecific enzymatic hydrolysis monatin.19)
of the chemically synthesized racemic ester derivatives using
a protease.3,14) Since the racemization at C4 of these ester
Results and Discussion
derivatives is impossible, these methods are not efficient for
The synthetic sequence using commercially available cis-
obtaining a single stereoisomer of monatin.
4-hydroxy-D-proline (2) as a starting material was performed
The lactam form of monatin, i.e., the pyroglutamic acid following Zhang et al.18) (Chart 1). The conversion of 2 to
derivative, is an attractive intermediate in the retrosynthetic (4R)-N-tBoc-cis-4-hydroxy-D-proline methyl ester (4) (81%
analysis of monatin. Indeed, Nakamura et al. and Tamura yield, two steps) was followed by the protection of the alco-
et al. observed the formation of the lactam form of monatin hol moiety with tert-butyldimethylsilyl (TBDMS) group to
during their total synthesis.8,13) A concise method to obtain give (4R)-N-tBoc-cis-4-(tert-butyldimethylsilyloxy)-D-proline
4-substituted glutamic acid is the alkylation of the lithium methyl ester (5) in 94% yield.20,21) The oxidation of 5 using
enolate derived from N-protected pyroglutamic acid esters, ruthenium oxide and periodate (NaIO4) gave 6 in 88% yield.22)
followed by the cleavage of the lactam ring. Ezquerra et al.
A modification of Christiansen et al. was followed to gener-
reported that lithium enolates of N-tert-butoxycarbonyl (tBoc)- ate tert-butyl 3-(bromomethyl)-1H-indole-1-carboxylate (10)
protected pyroglutamic acid alkyl ester stereospecifically starting from indole-3-carbaldehyde (7)23) (Chart 2). The usual
e-mail: yusuke_amino@ajinomoto.com
© 2016 The Pharmaceutical Society of Japan

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Chem. Pharm. Bull.
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Reagents: (a) HCl–MeOH; (b) (Boc)2O, Et3N–CH2Cl2, 81% for 2 steps; (c) TBDMSCl, imidazole–DMF, 94%; (d) RuO2–xH2O, NaIO4–AcOEt, H2O, 88%.
Chart 1. Synthesis of Compound 6
Reagents: (a) (Boc)2O, DMAP–CH3CN, 97%; (b) NaBH4–EtOH, 99%; (c) CBr4, PPh3–CH2Cl2, 88%.
Chart 2. Synthesis of Compound 10
Reagents: (a) LiHMDS–THF, −78°C; (b) 10, 72%; (c) LiOH–iPrOH, THF, H2O; (d) formic acid, HCl–dioxane; (e) NaOH–H2O, Δ; (f) Amberlite IR 120B AG, 49% for
4 steps.
Chart 3. Synthesis of (2R,4R)-Monatin (1)
tBoc protection method ((tBoc)2O, N,N-dimethylaminopyridine butyldimethylsilyloxy-4-(N-tBoc-3-indolylmethyl)-D-pyroglu-
(DMAP), CH3CN) was applied to 7 to obtain tert-butyl 3-for- tamic acid methyl ester (11) in 72% yield after purification
myl-1H-indole-1-carboxylate (8), followed by the reduction of (Chart 3). There was slight improvement in the yield with the
8 with NaBH4 to give tert-butyl 3-(hydroxymethyl)-1H-indole- use of N,N′-dimethylpropyleneurea as a co-solvent. As expect-
1-carboxylate (9) in 96% total yield.24)
ed from the reaction of the analogue lacking the 4-protected
The synthesis of 10 required careful purification at the hydroxyl group,15,16) the alkylation of 6 by 10 proceeded in a
final stage because of its instability. Thus, the bromination stereoselective manner. The stereochemistry at C4 of 11 was
of 9 was performed using carbon tetrabromide (CBr4) and confirmed after the transformation of 11 to the linear form
triphenylphosphine (PPh3) to generate phosphorus tribromide of monatin. The relative trans configuration at the ring junc-
in situ in CH2Cl2 at −20°C, followed by the removal of triphe- tions (C2, C4) of 11 obtained by this alkylation coincided with
nylphosphine oxide by continuous crystallization to give 10 in the relative configuration of C2 and C4 of (2R,4R)-monatin.
88% yield, which was stored in a refrigerator and used as soon Therefore, the alkylation at C4 of 6 proceeded in a stereose-
as possible.
Next, regioselectivity and stereoselectivity in the alkylation ic acid ring opposite from the methoxycarbonyl group at C2.12)
of the lithium enolate of 6 were examined. Reaction sites for As the electrophile of this reaction, tert-butyl 3-
lective manner from the less hindered face of the pyroglutam-
the alkylation of 6, i.e., C2 or C4, as well as the stereoselec- (chloromethyl)-1H-indole-1-carboxylate,25) easily obtainable
tivity in the alkylation of the enolate of 6 were of particular from gramine in two steps without purification, was used in-
interest. Eventually, compound 6 was selectively enolized at stead of 10; however, no coupling product was obtained, prob-
C4 using 1.2eq of lithium hexamethyldisilazide (LHMDS) in ably because of its low reactivity. N-tBoc-gramine methiodide
tetrahydrofuran (THF) at −78°C, and the enolate form was was prepared immediately before use from N-tBoc-gramine
then reacted with 1.0eq of 10 to furnish (4R)-N-tBoc-4-tert- and methyl iodide following the preparation of N-triisopro-

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Vol. 64, No. 8 (2016)
Fig. 1. Chiral HPLC Chromatograms of all Diastereoisomers of (2R,4R)-Monatin (1) (a), the Stereoselective Synthesis of (2R,4R)-Monatin (1) (b), All
Diastereoisomers of Compound 17 (c), and the Stereoselective Synthesis of Compound 17 (d)
Chromatogram (a) and (b): A: (2R,4S)-1; B: (2R,4R)-1; C: (2S,4R)-1; D: (2S,4S)-1. Chromatogram (c) and (d): A′: (2R,4S)-17; B′: (2S,4R)-17; C′: (2R,4R)-17; D′: (2S,4S)-
17.
Reagents: (a) LiHMDS–THF, −78°C; (b) benzyl bromide, 53%; (c) LiOH–iPrOH, THF, H2O; (d) formic acid, HCl–dioxane; (e) NaOH–H2O; (f) Amberlite IR 120B AG,
57% for 4 steps.
Chart 4. Synthesis of Compound 17
pylsilyl-gramine methiodide reported by Iwao and Motoi26) bered ring compounds under acidic conditions. Indeed, during
The reaction of this compound gave a complex mixture; our previous study on the stability of monatin, we found
however, the desired product was obtained in about 10% yield that monatin and its lactone were in equilibrium in strongly
after careful purification by silica gel chromatography.
acidic aqueous solutions, which gradually converted to lactam.
To cleave the lactam ring, compound 11 was treated with Therefore, it is suggested that a part of 14 originated from un-
LiOH in a mixture of isopropyl alcohol (iPrOH), THF, and reacted 11 in the ring-opening step and remaining 14 might be
H2O to give possible intermediate 12, the exact structure of regenerated during deprotection under acidic conditions after
which was not confirmed, but a part of the tBoc group on the ring cleavage by LiOH.
indole ring appeared to be deprotected under basic conditions.
Nakamura et al. reported that the lactam form of monatin
To remove the TBDMS group and remaining tBoc group, 12, underwent ring opening to return to the linear form of mona-
without purification, was treated with a mixture of 4N HCl– tin by hydrolysis with NaOH in refluxing aqueous ethanol for
dioxane and formic acid to give (2R,4R)-monatin (1·HCl); 3h.8) Accordingly, a mixture of 1·HCl, 13·HCl, and 14 was
however, HPLC analysis indicated the existence of a signifi- treated with excess NaOH at 95°C in aqueous ethanol for 3h
cant amount of its lactone (13·HCl) and a small amount (5 to to convert to the linear form of (2R,4R)-monatin sodium salt
10%) of lactam (14). The distinctive structure of monatin is (1 sodium salt). The resulting solution was neutralized and de-
considered to be a cause of the facile formation of five-mem- salted, and then, 1 sodium salt was recovered by precipitation

Vol. 64, No. 8 (2016)
Chem. Pharm. Bull.
1245
in aqueous ethanol in a 49% yield. In chiral HPLC analysis,
(4R)-N-tBoc-4-tert-Butyldimethylsilyloxy-4-(N-tBoc-3-
(2R,4R)- and (2R,4S)-monatin were observed in a ratio of indolylmethyl)-D-pyroglutamic Acid Methyl Ester (11)
98:2 accompanied by a small amount of lactam (14) (Figs. 1a, LHMDS was added (1.7mol/L in THF, 29.10mL, 50.34mmol)
b). Therefore, product 1 was shown to be of high diastereoiso- to a stirred solution of compound 6 (15.67g, 41.95mmol) in
meric purity at the newly created asymmetric center.
anhydrous THF (60mL) under an argon atmosphere at −78°C.
When the order of the original conversion route was re- The resulting solution was stirred at −78°C for 1h. A solu-
versed, i.e., deprotection under acidic conditions followed by tion of compound 10 (13.01g, 41.95mmol) in anhydrous THF
heating at reflux in ethanol with aqueous NaOH for 18h, the (20mL) was added portionwise into the reaction solution; the
desired linear form of monatin was still obtained; however, solution was stirred at −78°C for 25min and then warmed to
compared with the original method, a larger number of impu- room temperature (r.t.) and allowed to stir for 2h. The reac-
rities were detected by HPLC analysis.
tion was quenched with aqueous NH4Cl (50mL) and extracted
Using a similar procedure, unsubstituted or substituted with AcOEt (100mL, thrice). Combined organic layers were
phenyl analogs of (2R,4R)-monatin (17–19) were prepared washed with water (50mL) and brine (50mL), dried over
(Chart 4). Phenyl analog (17) was obtained using benzyl anhydrous MgSO4, and concentrated in vacuo. The residue
bromide as an electrophile in a modest total yield (31% total was purified by column chromatography (silica gel 300g, n-
yield) and good stereoselectivity (>99%, Figs. 1c, d). Because hexane–AcOEt=20:1–4:1) to give 11 as a pale yellow foam
1
of the low electrophilicity of 4-methoxybenzyl chloride and (18.20g, 30.19mmol). The H-NMR spectrum showed a peak
3,5-dimethoxybenzyl chloride, the yields of analogs 18 and marked by a very small amount (several percent) of impurity
19 were unsatisfactory, although excess LHMDS and electro- beside the isomers of the compound.
philes were used. Phenyl analogues 17–19 were accordingly
1H-NMR (CDCl3) δ: 0.14 (3H, s), 0.30 (3H, s), 0.87 (9H,
faintly sweet because the indole moiety is considered to be s), 1.45 (9H, s), 1.66 (9H), 2.09 (1H, dd, J=4.5, 13.6Hz),
indispensable to elicit the strong sweet taste in this series of 2.42 (1H, dd, J=9.2, 13.6Hz), 3.02 (1H, d, J=14.6Hz), 3.19
compounds.
(1H, d, J=14.6Hz), 3.71 (3H, s), 4.16 (1H, dd, J=4.5, 9.2Hz),
7.22–7.25 (1H, m), 7.31 (1H, t, J=7.1Hz), 7.49–7.52 (2H, m),
8.17 (1H, brd, J=7.9Hz). ESI-MS m/z: 626.07 (M+Na)+.
(2R,4R)-Monatin (1) Sodium Salt LiOH–H2O (20.14g,
Conclusion
(2R,4R)-Monatin (1) was successfully synthesized with
high optical purity, employing the regioselective and stereo- 480mmol) was added to a solution of 11 (18.0g, 29.86mmol)
selective alkylation of (4R)-N-tBoc-4-tert-butyldimethylsilyl- in a mixed solvent of isopropyl alcohol (50mL), THF (50mL),
oxy-D-pyroglutamic acid methyl ester (6) with tert-butyl and water (100mL) cooled to 0°C. The solution was stirred at
3-(bromomethyl)-1H-indole-1-carboxylate (10) to give (4R)-N- 0°C for 10min, warmed to r.t., and stirred for 16h. The sol-
tBoc-4-tert-butyldimethylsilyloxy-4-(N-tBoc-3-indolylmethyl)- vent was evaporated in vacuo, and the residue was suspended
D-pyroglutamic acid methyl ester (11). This was followed by in water (50mL). The pH of the solution was then adjusted to
the hydrolysis of the lactam ring and deprotection.
approximately 3 with aqueous HCl (2N). The aqueous phase
was extracted with AcOEt (80mL, thrice). Combined organic
layers were washed with brine (50mL), dried over anhydrous
Experimental
1H-NMR spectra were obtained using Brucker Avance 400 MgSO4, and concentrated to give a colorless foam (12, 15.3g).
(400MHz), and electrospray ionization (ESI)-MS spectra were
obtained using Thermo Quest TSQ 700.
A solution of HCl (4N in dioxane, 40mL) was added por-
tionwise to a solution of the abovementioned residue in formic
Analytical conditions for the determination of the optical acid (40mL) cooled to 0°C. The mixture was stirred at 0°C
purity of monatin stereoisomers: Column: Crownpack CR(+) for 5min, warmed to r.t., and stirred for 30min. The mixture
4×150mm. Detection: UV 210nm. Eluent: 1–aqueous perchlo- was concentrated in vacuo, and the residue was triturated and
ric acid (pH 1.9)–12% methanol; 17–aqueous perchloric acid washed by Et2O (30mL) and then with AcOEt (30mL) to give
(pH 1.5). Flow rate: 1.2mL/min. Temperature: 20°C.
a yellow powder (1·HCl, 13·HCl, 14, 13.22g).
(4R)-N-tBoc-4-tert-Butyldimethylsilyloxy-D-pyroglutamic
A solution of the abovementioned residue in a mixed sol-
acid methyl ester (6) was obtained as a colorless oily sub- vent of EtOH (160mL) and aqueous NaOH (2N, 40mL) was
stance in a total yield of 66% from cis-4-hydroxy-D-proline heated at 95°C for 3h, and the insoluble material was removed
(2) in accordance with the method described in the litera- by filtration after cooling to r.t. The filtrate was concentrated
tures.18,19–22) tert-Butyl 3-(bromomethyl)-1H-indole-1-carboxyl- in vacuo. The residue was dissolved in water (100mL), and
ate (10) was obtained as a white solid in a total yield of 85% the solution was successively washed with AcOEt (50mL) and
from indole-3-carbaldehyde (7) in accordance with the method Et2O (50mL). The solution was neutralized by the addition
described in the literatures.23,24)
of Amberlite IR 120B AG (H+), filtered, and concentrated in
(4R)-N-tBoc-4-tert-Butyldimethylsilyloxy-D-pyroglutamic vacuo. The residue was suspended in EtOH (50mL), and the
Acid Methyl Ester (6) 1H-NMR (CDCl3) δ: 0.11 (3H, s), insoluble material (NaCl) was removed by filtration. The fil-
0.16 (3H, s), 0.89 (9H, s), 1.50 (9H, s), 1.99 (1H, dt, J=7.0, trate was concentrated in vacuo, and the residue was crystal-
13.0Hz), 2.57 (1H, dt, J=7.7, 13.0Hz), 3.76 (3H, s), 4.28 lized with aqueous EtOH (95%) at r.t. to give 1 sodium salt as
(1H, t, J=7.4Hz), 4.46 (1H, t, J=7.4Hz). ESI-MS m/z: 374.61 a white crystal (4.64g, 14.71mmol). Analysis was conducted
(M+H)+, 396.59 (M+Na)+.
by HPLC using a chiral column and revealed the (2R,4R) and
tert-Butyl 3-(Bromomethyl)-1H-indole-1-carboxylate (10) (2R,4S) forms of isomers having an integrated peak ratio of
1H-NMR (CDCl3) δ: 1.66 (9H, s), 4.69 (2H, s), 7.29–7.38 98:2.
(2H, m), 7.44–7.50 (1H, m), 7.64–7.69 (2H, m), 8.13 (1H, d,
J=7.8 Hz).
1H-NMR (D2O) δ: (sodium salt of (2R,4R)-monatin) 2.06
(1H, dd, J=11.6, 15.2Hz), 2.68 (1H, dd, J=2.0, 15.2Hz),

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3.10 (1H, d, J=14.8Hz), 3.30 (1H, d, J=14.8Hz), 3.64 (1H, AcOEt (20mL). The aqueous layer was neutralized with aque-
dd, J=2.0, 11.6Hz), 7.16 (1H, brt, J=8.0Hz), 7.23 (1H, brt, ous NaOH (2N) and Amberlite IR 120B AG (H+) then filtered.
J=8.0Hz), 7.24 (1H, s), 7.50 (1H, d, J=8.0Hz), 7.74 (1H, d, The filtrate was concentrated in vacuo to about one fifth of
J=8.0Hz). 13C-NMR (D2O, 100MHz) δ: 38.1, 41.0, 56.5, 83.1, volume and 20mL of ethanol was added. The resulting crys-
111.9, 114.5, 121.9, 122.1, 124.5, 127.8, 130.7, 138.7, 177.1, tals were collected by filtration and dried under reduced pres-
182.9. ESI-MS m/z: 291 (M−H)−. FAB-MS m/z: 315.0963 sure to give 130mg (0.47mmol) of 17 sodium salt. Analysis
(M+Na) (Calcd for C14H16N2NaO5: 315.0957).
was conducted by HPLC using a chiral column and revealed
(2R,4R)-2-Amino-2,3-dideoxy-4-C-(1H-indol-3-ylmeth- only the (2R,4R) and (2R,4S) forms of isomers having an inte-
yl)pentaric Acid 1,4-Lactone (13, Monatin Lactone) grated peak ratio of 99:1 or higher.
1H-NMR (D2O) δ: 2.38 (1H, dd, J=10.7, 13.6Hz), 2.92 (1H,
1H-NMR (D2O) δ: (sodium salt of (2R,4R)-4-hydroxy-
dd, J=9.8, 13.6Hz), 3.17 (1H, dd, J=9.8, 10.7Hz), 3.37 (2H, d, 4-benzylglutamic acid) 1.95 (1H, dd, J=11.7, 15.3Hz), 2.56
J=2.6Hz), 7.16 (1H, t, J=6.9Hz), 7.18 (1H, t, J=8.2Hz), 7.27 (1H, d, J=15.3Hz), 2.81 (1H, d, J=13.5Hz), 3.07 (1H, d,
(1H, s), 7.46 (1H, d, J=8.0Hz), 7.68 (1H, d, J=8.0Hz). ESI-MS J=13.5Hz), 3.55 (1H, d, J=11.7Hz), 7.19–7.31 (m, 5H). ESI-
m/z: 275.51 (M+H)+.
MS m/z: 252.0 (M−H)−. FAB-MS m/z: 252.0862 (M−H)
(2R,4R)-4-Hydroxy-4-(1H-indol-3-ylmethyl)-5-oxo-pro- (Calcd for C12H14NO5: 252.0872).
line (14, Monatin Lactam) 1H-NMR (DMSO-d6) δ: 1.80
(2R,4R)-4-Hydroxy-4-(4-methoxybenzyl)glutamic Acid
(1H, dd, J=6.8, 13.0Hz), 2.41 (1H, dd, J=8.0, 13.0Hz), 2.89 (18) Sodium Salt The same operation described for 17 was
(1H, d, J=14.1Hz), 2.97 (1H, d, J=14.1Hz), 3.45 (1H, dd, performed using 4-methoxybenzyl chloride, and 18 was ob-
J=6.8, 8.0Hz), 5.45 (1H, brs), 6.93–6.98 (1H, m), 7.04 (1H, t, tained in an overall yield of 13.5% as a white solid.
J=7.0Hz), 7.16 (1H, d, J=2.3Hz), 7.32 (1H, d, J=8.0Hz), 7.62
(1H, d, J=8.0Hz), 7.97 (1H, s), 10.88 (1H, brs). ESI-MS m/z: (1H, dd, J=2.5, 14.3Hz), 2.66 (1H, d, J=13.8Hz), 2.90 (1H,
1H-NMR (D2O) δ: 1.53 (1H, dd, J=10.6, 14.3Hz), 2.18
275.06 (M+H)+.
d, J=13.8Hz), 3.14 (1H, dd, J=2.5, 10.6Hz), 3.68 (3H, s),
N-tBoc-(4R)-4-tert-Butyldimethylsilyloxy-4-benzyl-D- 6.79 (2H, m), 7.06 (2H, m). ESI-MS: 305.9 (M+Na)+, 281.8
pyroglutamic Acid Methyl Ester (15) To a stirred solution (M−H)−.
of 6 (1.09g, 3.0mmol) in anhydrous THF (10mL) under an
(2R,4R)-4-Hydroxy-4-(3,5-dimethoxybenzyl)glutamic
argon atmosphere at −78°C was added LHMDS (1.7mol/L Acid (19) Sodium Salt The same operation described for
in THF, 2.1mL, 3.6mmol). The resulting solution was stirred 17 was performed using 3,5-dimethoxybenzyl chloride and 19
at −78°C for 1h. Benzyl bromide (0.38mL, 3.15mmol) was was obtained in an overall yield of 18.5% as a white solid.
added dropwise into the reaction solution, the solution was
1H-NMR (D2O) δ: 1.59 (1H, dd, J=10.7, 14.3Hz), 2.23
stirred at −78°C for 25min then warmed to r.t. and allowed (1H, dd, J=2.4, 14.3Hz), 2.72 (1H, d, J=13.5Hz), 2.98 (1H,
to stir for 1.5h. The reaction was quenched with aqueous d, J=13.5Hz), 3.21 (1H, dd, J=2.4, 10.7Hz), 3.72 (6H, s),
NH4Cl (10mL) and extracted with AcOEt (50mL, twice). The 6.38 (1H, m), 6.43 (2H, m). ESI-MS: 336.2 (M+Na)+, 311.8
combined organic layers were washed with water (30mL) and (M−H)−.
brine (30mL), dried over anhydrous MgSO4, and concentrated
in vacuo. The residue was purified by preparative thin layer
Acknowledgments The author would like to thank his
chromatography (PTLC) to give 15 as a white solid (741mg, colleagues in Ajinomoto Co., Inc., Shinichiro Kubo and Ma-
1
1.60mmol). The H-NMR spectrum showed a single stereo- sakazu Sugiyama, for technical assistance.
isomer.
1H-NMR (CDCl3) δ: 0.13 (s, 3H), 0.29 (s, 3H), 0.86 (s, 9H),
Conflict of Interest The author is an employee of Aji-
1.46 (s, 9H), 1.96 (1H, dd, J=5.9, 13.5Hz), 2.45 (1H, dd, J=8.8, nomoto Co., Inc. and has no further conflicts of interest to
13.5Hz), 2.88 (1H, d, J=13.3Hz), 3.16 (1H, d, J=13.3Hz), 3.71 declare.
(3H, s), 3.80 (1H, dd, J=5.9, 8.8Hz), 7.20–7.31 (5H, m). ESI-
MS m/z: 464.83 (M+H)+, 486.81 (M+Na)+.
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