O-N intramolecular alkoxycarbonyl migration of typical protective groups in hydroxyamino acids

Article abstract of DOI:10.1021/jo0525579

O-N Intramolecular alkoxycarbonyl (carbonate-carbamate) migration was found to occur as a common reaction of hydroxyamino acids under mild basic aqueous conditions with no formation of side products. Carbonate protective groups migrate to produce amino-pr

Full text of DOI:10.1021/jo0525579

a useful tool for organic chemists. For example, it can be used
to shorten a synthesis of some target molecule in a similar
manner as the acyl migration reaction did.10-15
Recently, in a study on “chemical pharmaceutics”, we
reported the first successful application of O-N intramolecular
alkoxycarbonyl migration28 in the development of water-soluble
taxoid prodrugs (isotaxoids),29 which are O-alkoxycarbonyl
isoforms at a characteristic R-hydroxy-â-amino acid (phenyli-
soserine) moiety, and released parent anti-cancer agents with
no side-product formation under physiological conditions.29
O-N Intramolecular Alkoxycarbonyl Migration
of Typical Protective Groups in Hydroxyamino
Acids
Mariusz Skwarczynski, Youhei Sohma, Mayo Noguchi,
Tooru Kimura, Yoshio Hayashi,* and Yoshiaki Kiso*
Department of Medicinal Chemistry, Center for Frontier
Research in Medicinal Science, 21st COE Program, Kyoto
Pharmaceutical UniVersity, Yamashina-ku, Kyoto 607-8412,
Japan
(2) Meutermans, W. D. F.; Golding, S. W.; Bourne, G. T.; Miranda, L.
P.; Dooley, M. J.; Alewood, P. F.; Smythe, M. L. J. Am. Chem. Soc. 1999,
121, 9790-9796.
kiso@mb.kyoto-phu.ac.jp
(3) Sheppard, R. J. Pept. Sci. 2003, 9, 545-552.
(4) Sohma, Y.; Sasaki, M.; Ziora, Z.; Takahashi, N.; Kimura, T.; Hayashi,
Y.; Kiso, Y. Peptides, Peptide ReVolution: Genomics, Proteomics &
Therapeutics; Kluwer Academic: The Netherlands, 2003; pp 67-68.
(5) Sohma, Y.; Sasaki, M.; Hayashi, Y.; Kimura, T.; Kiso, Y. Chem.
Commun. 2004, 124, 124-125.
ReceiVed December 12, 2005
(6) Sohma, Y.; Sasaki, M.; Hayashi, Y.; Kimura, T.; Kiso, Y. Tetrahedron
Lett. 2004, 45, 5965-5968. Sohma, Y.; Hayashi, Y.; Kimura, M.;
Chiyomori, Y.; Taniguchi, A.; Sasaki, M.; Kimura, T.; Kiso, Y. J. Peptide
Sci. 2005, 11, 441-451. Sohma, Y.; Chiyomori, Y.; Kimura, M.; Fukao,
F.; Taniguchi, A.; Hayashi, Y.; Kimura, T.; Kiso, Y. Bioorg. Med. Chem.
2005, 13, 6167-6174.
(7) Sohma, Y.; Hayashi, Y.; Skwarczynski, M.; Hamada, Y.; Sasaki, M.;
Kimura, T.; Kiso, Y. Biopolymers 2004, 76, 344-356.
(8) Taniguchi, A.; Sohma, Y.; Kimura, M.; Okada, T.; Ikeda, K.; Hayashi,
Y.; Kimura, T.; Hirota, S.; Matsuzaki, K.; Kiso, Y. J. Am. Chem. Soc. 2006,
128, 696-697.
O-N Intramolecular alkoxycarbonyl (carbonate-carbamate)
migration was found to occur as a common reaction of
hydroxyamino acids under mild basic aqueous conditions
with no formation of side products. Carbonate protective
groups migrate to produce amino-protected carbamate de-
rivatives of hydroxyamino acids with high efficiency and
purity.
(9) Carpino, L. A.; Krause, E.; Sferdean, C. D.; Schuemann, M.; Fabian,
H.; Bienert, M.; Beyermann, M. Tetrahedron Lett. 2004, 45, 7519-7523.
(10) Owens, T. D.; Semple, J. E. Org. Lett. 2001, 3, 3301-3304.
(11) Tamamura, H.; Hori, T.; Otaka, A.; Fujii, N. J. Chem. Soc., Perkin
Trans. 1 2002, 5, 577-580.
(12) Tamamura, H.; Kato, T.; Otaka, A.; Fujii, N. Org. Biomol. Chem.
2003, 1, 2468-2473.
(13) Basso, A.; Banfi, L.; Riva, R.; Piaggio, P.; Guanti, G. Tetrahedron
Lett. 2003, 44, 2367-2370.
(14) Horikawa, M.; Nakajima, T.; Ohfune, Y. Synlett 1998, 6, 609-
610.
Introduction
(15) Oaksmith, J. M.; Peters, U.; Ganem, B. J. Am. Chem. Soc. 2004,
126, 13606-13607.
O-N Intramolecular acyl migration is a well-known rear-
rangement reaction that has been applied in peptide ligation,1
the synthesis of “difficult” peptides,2-9 peptidomimetics,10-15
and lactams,16,17 as well as in water-soluble prodrug strate-
gies7,18-22 and the design of irreversible enzyme inhibitors.23
This type of reaction is also observed in a variety of natural
products,24 and S-N intramolecular acyl migration has recently
been extensively explored.1,25 This suggests that the concept of
the O-N intramolecular acyl migration can be effectively
applied to similar rearrangement reactions. We focused on one
such reaction, O-N intramolecular alkoxycarbonyl (carbonate-
carbamate) migration. From an extensive literature search, it is
assumed that the reports for this migration are rare and that
migration occurred only under forcing conditions.26 On the other
hand, the partial hydrolysis of carbonate20 or the side production
of oxazolidinone27 has been reported under milder reaction
conditions. Herein, we demonstrated that carbonate protective
groups can undergo an O-N intramolecular alkoxycarbonyl
migration reaction to produce amino-protected carbamate de-
rivatives. Therefore, this atom-economical reaction can provide
(16) Botti, P.; Pallin, T. D.; Tam, J. P. J. Am. Chem. Soc. 1996, 118,
10018-10024.
(17) Derrer, S.; Feeder, N.; Teat, S. J.; Davies, J. E.; Holmes, A. B.
Tetrahedron Lett. 1998, 39, 9309-9312.
(18) Damen, E. W. P.; Nevalainen, T. J.; van den Bergh, T. J. M.; de
Groot, F. M. H.; Scheeren, H. W. Bioorg. Med. Chem. 2002, 10, 71-77.
(19) Hayashi, Y.; Skwarczynski, M.; Hamada, Y.; Sohma, Y.; Kimura,
T.; Kiso, Y. J. Med. Chem. 2003, 46, 3782-3784.
(20) Skwarczynski, M.; Sohma, Y.; Kimura, M.; Hayashi, Y.; Kimura,
T.; Kiso, Y. Bioorg. Med. Chem. Lett. 2003, 13, 4441-4444.
(21) Hamada, Y.; Matsumoto, H.; Yamaguchi, S.; Kimura, T.; Hayashi,
Y.; Kiso, Y. Bioorg. Med. Chem. 2004, 12, 159-170 and references therein.
(22) Mutter, M.; Chandravarkar, A.; Boyat, C.; Lopez, J.; Dos Santos,
S.; Mandal, B.; Mimna, R.; Murat, K.; Patiny, L.; Saucede, L.; Tuchscherer,
G. Angew. Chem., Int. Ed. 2004, 43, 4172-4178.
(23) White, E. H.; Chen, Y. Biochemistry 1995, 34, 15123-15133.
(24) Garcia-Rubio, S.; Meinwald, J. J. Org. Chem. 2001, 66, 1082-
1096 and references therein.
(25) For example, see: Offer, J.; Boddy, C. N. C.; Dawson, P. E. J. Am.
Chem. Soc. 2002, 124, 4642-4646. Clive, D. L. J.; Hisaindee, S.; Coltart,
D. M. J. Org. Chem. 2003, 68, 9247-9254. Duhee, B.; Neeraj, C.; Kent,
S. B. H. J. Am. Chem. Soc. 2004, 126, 1377-1383.
(26) Kato, S.; Morie, T. J. Heterocycl. Chem. 1996, 33, 1171-1178.
(27) Kazmierski, W. M.; Bevans, P.; Furfine, E.; Spaltenstein, A.; Yang,
H. Bioorg. Med. Chem. Lett. 2003, 13, 2523-2526.
(28) Formerly, this reaction was incorrectly named as an O-N intramo-
lecular acyloxy migration, see references 20, 27, and 29.
(29) Skwarczynski, M.; Sohma, Y.; Noguchi, M.; Kimura, M.; Hayashi,
Y.; Hamada, Y.; Kimura, T.; Kiso, Y. J. Med. Chem. 2005, 48, 2655-
2666.
* Corresponding authors. Phone: +81-75-595-4635. Fax: +81-75-591-9900.
(1) For review, see: Coltart, D. M. Tetrahedron 2000, 56, 3449-3491.
Tam, J. P.; Xu, J.; Eom, K. D. Biopolymers 2001, 60, 194-205.
10.1021/jo0525579 CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/21/2006
2542
J. Org. Chem. 2006, 71, 2542-2545

TABLE 1. O-N Intramolecular Alkoxycarbonyl Migration in Pns
Derivatives
SCHEME 1. Possible Types of Carbonate Conversion in
r-Hydroxy-â-amino Acid Residue under Mild Basic
Conditions
substrate
product
R
time (min)
yielda (%)
1a
1b
1c
1d
2a
2b
2c
2d
C2H5
9-fluorenylmethyl
CH2CCl3
60
960
5
99
99
99
99
CH2C6H5
60
a The yield established by HPLC was quantitative, as one pure product
was detected.
SCHEME 2. O-N Intramolecular Alkoxycarbonyl
Migration in Natural Amino Acid Derivatives
Hence, this atom-economical reaction proceeding in aqueous
solvent systems is also recognized as a green-chemistry reaction.
However, the application of this reaction in synthetic organic
chemistry under mild basic conditions has not yet been
investigated. R-Hydroxy-â-amino acid derivatives were selected
for this study on the basis of our previous observation in the
prodrug study.
Results and Discussion
bamates 2a-d was observed (Table 1 and Figure 1S from
Supporting Information). Neither hydrolysis nor formation of
side products including oxazolidinone was detected. The forma-
tion of 2a-d was also confirmed by mass spectroscopy. The
result with no hydrolysis of the Fmoc group in 1b was quite
surprising considering its instability under basic conditions.32
However, this is consistent with our previous assumption that
carbonate hydrolysis under mild basic conditions is limited only
to the Boc group or its related structures.29
An R-hydroxy-â-amino acid, phenylnorstatine (Pns, (2R,3S)-
3-amino-2-hydroxy-4-phenylbutanoic acid), is a well-known
hydroxymethylcarbonyl isostere used for a variety of potent
peptidomimetic inhibitors for biologically important enzymes.30
We selected this amino acid to understand the possible reactions
occurring in O-alkoxycarbonyl compounds under mild basic
aqueous conditions (pH 7.4): (a) O-N intramolecular alkoxy-
carbonyl migration (2),20,27,29 (b) oxazolidinone formation
(3),27,31 and (c) simple hydrolysis (4;20 Scheme 1). Subsequently,
we synthesized compounds 1a-d (see Experimental Section
and Supporting Information). These model compounds have
typical alkoxycarbonyl residues, that is, ethyloxycarbonyl,
9-fluorenylmethoxycarbonyl (Fmoc), 2,2,2-trichloroethoxycar-
bonyl (Troc), and benzyloxycarbonyl (Z) groups, which are
widely used for the protection of both hydroxyl and amine
groups (Table 1). All expected products, 2a-d, 3, and 4, were
also synthesized independently by standard methods (see
Experimental Section and Supporting Information). Compounds
1a-d were dissolved in phosphate buffered saline (PBS, pH
7.4) and stirred at room temperature, and the reaction was
monitored and analyzed by HPLC. In all cases, 1a-d were
completely consumed, and the quantitative formation of car-
The isolation yield of the desired product (2a) was also
measured according to the procedure shown in Experimental
Section. As expected, compound 3 or 4 was not detected, and
carbamate 2a was isolated as a pure compound with 99% yield.
To understand why the known oxazolidinone formation was
not observed, carbonates 1a-d were treated with K2CO3 in
methanol, according to a reported procedure.31 However, the
formation was not detected, and carbamates 2a-d were obtained
as the exclusive products. Therefore, two natural amino acid
carbonate derivatives, 5a and 5b, were synthesized (see
Experimental Section and Supporting Information) and treated
under the same aqueous condition described for compound 1a
(Scheme 2). Even in these cases, O-N intramolecular alkoxy-
carbonyl migration predominantly formed products 6a and 6b
with isolated yields of 98 and 89%, respectively, and oxazoli-
dinone formation or carbonate hydrolysis was not detected. The
lower yield of compound 6b was due to the hydrolysis of the
unstable benzyl ester under mild basic aqueous conditions.
However, hydrolysis of the benzyl ester was not connected with
the O-N intramolecular alkoxycarbonyl migration reaction. This
suggests that O-N intramolecular alkoxycarbonyl migration is
a common and exclusive reaction for O-acyl-R-hydroxy-â-amino
acids and â-hydroxy-R-amino acids. Thus, our findings in O-N
(30) For example, see: Iizuka, K.; Kamijo, T.; Harada, H.; Akahane,
K.; Kubota, T.; Umeyama, H.; Ishida, T.; Kiso, Y. J. Med. Chem. 1990,
33, 2707-2714. Mimoto, T.; Imai, J.; Tanaka, S.; Hattori, N.; Takahashi,
O.; Kisanuki, S.; Nagano, Y.; Shintani, M.; Hayashi, H.; Sakikawa, H.;
Akaji, K.; Kiso, Y. Chem. Pharm. Bull. 1991, 39, 2465-2467. Nezami,
A.; Luque, I.; Kimura, T.; Kiso, Y.; Freire, E. Biochemistry 2002, 41, 2273-
2280. Brynda, J.; Rezacova, P.; Fabry, M.; Horejsi, M.; Stouracova, R.;
Sedlacek, J.; Soucek, M.; Hradilek, M.; Lepsik, M.; Konvalinka, J. J. Med.
Chem. 2004, 47, 2030-2036. Kimura, T.; Shuto, D.; Hamada, Y.; Igawa,
N.; Kasai, S.; Liu, P.; Hidaka, K.; Hamada, T.; Hayashi, Y.; Kiso, Y. Bioorg.
Med. Chem. Lett. 2005, 15, 211-215.
(31) Zhu, M.; Qiu, Z.; Hiel, G. P.; Sieburth, S. McN. J. Org. Chem.
2002, 67, 3487-3493.
(32) Green, T. W., Wuts, P. G. M. In ProtectiVe Groups in Organic
Synthesis; John Wiley and Sons: New York; 1999; p 507.
J. Org. Chem, Vol. 71, No. 6, 2006 2543

3
(d, J(H,H) ) 2.4 Hz, 1H, CH), 4.00-3.94 (m, 2H, CH2), 3.67 (s,
intramolecular migration of common protective groups provide
a highly promising procedure for peptide chemists, also in the
context of green chemistry. This reaction can also be directly
applied to carbohydrate chemistry, because the migration of
protective groups is an important issue in the regioselective
modifications of sugars.33
2
3
3H, CH3), 2.92, 2.81 (2 dd, J(H,H) ) 13.6 Hz, J(H,H) ) 7.8, 7.9
Hz, 2H, CH2), 1.16 (t, 3J(H,H) ) 7.1 Hz, 3H, CH3). 13C NMR (CD3-
OD, 100 MHz): δc 175.1, 158.5, 139.5, 130.4, 129.5, 127.6, 72.0,
61.9, 56.7, 52.6, 38.8, 14.9. HRMS (FAB+) calcd for C14H20NO5
[M+ + H], 282.1341; found, 282.1335. Purity was higher than 99%
(HPLC analysis at 230 nm).
In conclusion, we demonstrated herein that O-N intramo-
lecular alkoxycarbonyl (carbonate-carbamate) migration com-
monly proceeded under pure aqueous mild basic conditions (pH
7.4). In particular, carbonate groups can migrate to produce
carbamate derivatives of hydroxyamino acids with high ef-
ficiency and purity, without byproduct formation (hydrolysis
of the ester bond is not a byproduct of this migration). This
finding offers a useful tool for organic chemists, as typical and
widely used protective groups can undergo this reaction.
(4S,5R)-4-Benzyl-5-[(methoxy)carbonyl]-1,3-oxazolidin-2-
one, 3.36 (2R,3S)-3-Amino-2-hydroxy-4-phenylbutanoic acid methyl
ester hydrochloric acid salt, 4‚HCl (75.0 mg, 305 µmol), was
dissolved in anhydrous THF (3 mL), and Et3N (51 µL, 366 µmol)
was added followed by 1,1′-carbonyldiimidazole (74 mg, 475 µmol)
at 0 °C.37 The cloudy reaction mixture was stirred overnight at room
temperature, diluted with AcOEt, and washed consecutively with
10% citric acid, NaHCO3, and brine. The organic layer was dried
over MgSO4, and the solvent was removed under reduced pressure.
The crude product was applied to preparative HPLC, which was
eluted with a linear gradient of 20-60% CH3CN in 0.1% aqueous
TFA over 40 min at a flow rate of 5 mL/min. The desired fraction
was collected and lyophilized to give 3 (32.7 mg, 139 µmol, 46%).
1H NMR (CDCl3, 400 MHz): δ 7.38-7.20 (m, 5H, CH), 5.58 (br
s, 1H, NH), 4.72 (d, 3J(H,H) ) 4.6 Hz, 1H, CH), 4.15-4.11 (m, 1H,
Experimental Section
(2R,3S)-3-Amino-2-[(ethoxycarbonyl)oxy]-4-phenylbutanoic
Acid Methyl Ester Hydrochloride, 1a. Ethoxycarbonyl chloride
(10 µL, 103 µmol) was added to a stirring solution of (2R,3S)-2-
hydroxy-3-[(tert-butoxycarbonyl)amino]-4-phenylbutanoic acid meth-
yl ester34 (16.0 mg, 51.6 µmol) in dry CH2Cl2 (1 mL) and dry
pyridine (1 mL), and the mixture was stirred under an argon
atmosphere at room temperature for 1 h. The reaction mixture was
then diluted with AcOEt and successively washed with water, 1M
hydrochloric acid (two times), and brine. The organic layer was
dried over MgSO4, and the solvent was removed under reduced
pressure. The resulting oil was dissolved in 4 M HCl in dioxane (1
mL) with anisole (11 µL, 103 µmol), and the reaction mixture was
stirred for 30 min at room temperature. The organic solvent was
evaporated under reduced pressure, and the reaction mixture was
directly applied to preparative HPLC, which was eluted with a linear
gradient of 10-40% CH3CN in 12 mM aqueous HCl over 60 min
at a flow rate of 5 mL/min. The desired fraction was collected and
lyophilized to give a white powder of 1a as an HCl salt (12.0 mg,
37.8 µmol, 73%). 1H NMR (CD3OD, 400 MHz): δ 7.41-7.27 (m,
5H, CH), 4.89 (d, 3J(H,H) ) 2.5 Hz, 1H, CH), 4.29 (q, 3J(H,H) ) 7.1
Hz, 2H, CH2), 4.08 (ddd, 3J(H,H) ) 2.5, 6.7, 9.0 Hz, 1H, CH), 3.79
(s, 3H, CH3), 3.08, 3.01 (2 dd, 2J(H,H) ) 13.8 Hz, 3J(H,H) ) 6.7, 9.0
Hz, 2H, CH2), 1.34 (t, 3J(H,H) ) 7.1 Hz, 3H, CH3). 13C NMR (CD3-
OD, 100 MHz): δc 168.8, 155.1, 135.7, 130.43, 130.38, 129.0,
73.4, 66.6, 54.0, 53.8, 36.7, 14.5. HRMS (FAB+) calcd for C14H20-
NO5 [M+ + H], 282.1341; found, 282.1347. Purity was higher than
99% (HPLC analysis at 230 nm).
(2R,3S)-3-[(Ethoxycarbonyl)amino]-2-hydroxy-4-phenylbu-
tanoic Acid Methyl Ester, 2a.35 (2R,3S)-3-Amino-2-hydroxy-4-
phenylbutanoic acid methyl ester hydrochloric acid salt, 4‚HCl (20.0
mg, 81.4 µmol), was dissolved in CH2Cl2 (0.5 mL), and a saturated
solution of NaHCO3 (0.5 mL) was added. Ethoxycarbonyl chloride
(9.7 µL, 102 µmol) was then added with vigorous stirring. After 1
h, the next portion of ethoxycarbonyl chloride (9.7 µL, 102 µmol)
was added, and after an hour, the reaction mixture was diluted with
diethyl ether and washed with water, NaHCO3, water, 1 M
hydrochloric acid, water, and brine. The organic layer was dried
over MgSO4, and the solvent was removed under reduced pressure
to give 2a (22 mg, 78.2 µmol, 96%). 1H NMR (CD3OD, 400
MHz): δ 7.30-7.18 (m, 5H, CH), 4.20-4.16 (m, 1H, CH), 4.09
2
3
CH), 3.81 (s, 3H, CH3), 3.07, 2.90 (2dd, J(H,H) ) 13.6 Hz, J(H,H)
) 5.0, 8.5 Hz, 2H, CH2), 2.47 (br s, 1H, OH). HRMS (FAB+)
calcd for C12H14NO4 [M+ + H], 236.0923; found, 236.0928.
(2R,3S)-3-Amino-2-hydroxy-4-phenylbutanoic Acid Methyl
Ester Hydrochloride, 4.38,39 (2R,3S)-2-Hydroxy-3-[(tert-butoxy-
carbonyl)amino]-4-phenylbutanoic acid methyl ester34 (1.0 g, 3.23
mmol) was dissolved in 4 M HCl in dioxane (16 mL) with anisole
(0.7 mL, 6.46 mmol), and the reaction mixture was stirred for 1 h
at room temperature. The organic solvent was evaporated, and the
solid was washed with diethyl ether to give a white powder of 4 as
1
a HCl salt (0.776 g, 3.16 mmol, 98%). H NMR (CD3OD, 400
3
MHz): δ 7.39-7.28 (m, 5H, CH), 4.12 (d, J(H,H) ) 2.5 Hz, 1H,
3
CH), 3.82 (ddd, J(H,H) ) 2.5, 6.4, 9.0 Hz, 2H, CH2), 3.74 (s, 3H,
2
3
CH3), 3.07, 3.00 (2 dd, J(H,H) ) 13.6 Hz, J(H,H) ) 6.6, 9.0 Hz,
2H, CH2). HRMS (FAB+) calcd for C11H16NO3 [M+ + H],
210.1130; found, 210.1135.
O-Ethoxycarbonyl-L-threonine Benzyl Ester Hydrochloride,
5a. Ethoxycarbonyl chloride (17.8 µL, 186 µmol) was added to
the stirring solution of N-(tert-butoxycarbonyl)-L-threonine benzyl
ester40 (28.8 mg, 93.2 µmol) in dry CHCl3 (1 mL) and dry pyridine
(1 mL), and the mixture was stirred under an argon atmosphere at
room temperature for 1 h. Another portion of ethoxycarbonyl
chloride (17.8 µL, 93.2 µmol) was added, and after two more hours,
the reaction mixture was diluted with AcOEt and successively
washed with water, 1 M hydrochloric acid (two times), and brine.
The organic layer was dried over MgSO4, and the solvent was
removed under reduced pressure. The resulting oil was dissolved
in 4 M HCl in dioxane (1.5 mL) with anisole (20 µL, 187 µmol),
and the reaction mixture was stirred for 1 h at room temperature.
The organic solvent was evaporated under reduced pressure, and
the reaction mixture was directly applied to preparative HPLC,
which was eluted with a linear gradient of 10-40% CH3CN in 12
mM aqueous HCl over 60 min at a flow rate of 5 mL/min. The
(36) Juhl, K.; Jorgensen, K. A. J. Am. Chem. Soc. 2002, 124, 2420-
2421.
(37) Oxazolidinone was synthesized according to a known method:
Bunnage, M. E.; Davies, S. G.; Goodwin, C. J.; Ichihara, O. Tetrahedron
1994, 50, 3975-3986.
(33) For example, see: Yamasaki, T.; Kubota, Y.; Tsuchiya, T.;
Umezawa, S. Bull. Chem. Soc. Jpn. 1976, 49, 3190-3192. Yu, H.; Ensley,
H. E. Tetrahedron Lett. 2003, 44, 9363-9366. Xu, F.; Simmons, B.; Savary,
K.; Yang, C.; Reamer, R. A. J. Org. Chem. 2004, 69, 7783-7786.
(34) The compound was purchased from Nippon Kayaku Co., Ltd.
(Tokyo, Japan).
(35) Compounds 2-4 were synthesized as standards for monitoring the
O-N intramolecular alkoxycarbonyl migration reaction in compounds 1a-
d.
(38) Hoover, D. J.; Lefkowitz-Snow, S.; Burgess-Henry, J. L.; Martin,
W. H.; Armento, S. J.; Stock, I. A.; McPherson, R. K.; Genereux, P. E.;
Gibbs, E. M.; Treadway, J. L. J. Med. Chem. 1998, 41, 2934-2938.
(39) Harbeson, S. L.; Abelleira, S. M.; Akiyama, A.; Barrett, R., III;
Carroll, R. M.; Straub, J. A.; Tkacz, J. N.; Wu, C.; Musso, G. F. J. Med.
Chem. 1994, 37, 2918-2929.
(40) Starting material was synthesized according to a published proce-
dure: Jiang, W.; Wanner, J.; Lee, R. J.; Bounaud, P.-Y.; Boger, D. L. J.
Am. Chem. Soc. 2003, 125, 1877-1887.
2544 J. Org. Chem., Vol. 71, No. 6, 2006

desired fraction was collected and lyophilized to give a white
25 °C overnight. The product, 2a, was then extracted with AcOEt.
Carbamate 2a was isolated as a pure compound with 99% yield
(10.3 mg). The spectroscopic data of 2a synthesized by this method
and by the standard method were identical.
1
powder of 5a as a HCl salt (22.4 mg, 70.5 µmol, 76%). H NMR
3
(CD3OD, 400 MHz): δ 7.42-7.33 (m, 5H, CH), 5.32 (dq, J(H,H)
2
) 3.5, 6.6 Hz, 1H, CH), 5.34, 5.23 (2d, J(H,H) ) 12.0 Hz, 2H,
CH2), 4.37 (d, 3J(H,H) ) 3.3 Hz, 1H, CH), 4.14, 4.09 (2dq, 2J(H,H)
)
10.6 Hz, 3J(H,H) ) 7.1 Hz, 2H, CH2), 1.42 (d, 3J(H,H) ) 6.8 Hz, 3H,
CH3), 1.24 (t, 3J(H,H) ) 7.1 Hz, 3H, CH3). 13C NMR (CD3OD, 100
MHz): δc 168.1, 155.0, 136.2, 129.92, 129.88, 129.7, 72.7, 69.6,
65.8, 57.6, 16.9, 14.5. HRMS (FAB+) calcd for C14H20NO5 [M+
+ H], 282.1341; found, 282.1346. Purity was higher than 99%
(HPLC analysis at 230 nm).
Acknowledgment. This research was supported in part by
the “Academic Frontier” Project for Private Universities:
matching fund subsidy from MEXT (Ministry of Education,
Culture, Sports, Science and Technology), the 21st Century
Center of Excellence Program “Development of Drug Discovery
Frontier Integrated from Tradition to Proteome” from MEXT,
and by Grant-in-Aid for Scientific Research on Priority Areas
17035085 from MEXT. M.S. is grateful for the Postdoctoral
Fellowship of JSPS. Y.S. is grateful for the Research Fellowship
of JSPS for Young Scientists. We are grateful to Mr. T. Hamada
and Ms. S. Tsuruoka for the measurement of mass spectra.
Standard Procedure for HPLC Monitoring of O-N Alkoxy-
carbonyl Migration. (2R,3S)-3-[(Ethoxycarbonyl)amino]-2-hy-
droxy-4-phenylbutanoic Acid Methyl Ester, 2a. Compound 1a
was dissolved in PBS (pH 7.4) to give a 0.1 mM solution and stirred
at room temperature, and the reaction was monitored and analyzed
by HPLC in comparison with synthetic 2a, 3, and 4. The production
of only 2a was observed.
Standard Procedure for the Isolation of O-N Alkoxycarbo-
nyl Migration Reaction Product. (2R,3S)-3-[(Ethoxycarbonyl)-
amino]-2-hydroxy-4-phenylbutanoic Acid Methyl Ester, 2a.
Namely, 1a (11.7 mg) was suspended in 3.7 mL of PBS (pH 7.4)
to give a 10 mM solution (this concentration was establish to
prevent a pH change during the process) and vigorously stirred at
Supporting Information Available: Characterization data for
1
compounds 1b-d, 2b-d, 5b, and 6a,b and copies of H and 13C
NMR spectra and HPLC profiles for new compounds 1a-d, 2a-
d, 5a,b and 6a,b. This material is available free of charge via the
Internet at http://pubs.acs.org.
JO0525579
J. Org. Chem, Vol. 71, No. 6, 2006 2545