Synthetic studies on callipeltins: Stereoselective syntheses of (3S,4R)-3,4-dimethyl-L-pyroglutamic acid and fmoc-D-allothreonine from serine derivatives

Article abstract of DOI:10.3987/COM-09-11834

The non-proteinogenic amino acids contained in callipeltins, (3S, 4R)-3, 4-dimefhyl-L-pyroglutamic acid and D-allothreonine, were synthesized from D- or L-serine. The stereoselective synthesis of two methyl groups of (3S, 4R)-3, 4-dimethyl-L-pyroglutamic acid was accomplished by diastereoselective hydrogenation and alkylation. Kinetic epimerization of the C-4 methyl substituent followed by Boc-deprotection with 10% TFA gave the desired (3S, 4R)-3, 4-dimethyl-L-pyroglutamic acid as a single isomer.

Full text of DOI:10.3987/COM-09-11834

HETEROCYCLES, Vol. 81, No. 1, 2010
79
HETEROCYCLES, Vol. 81, No. 1, 2010, pp. 79 - 89. © The Japan Institute of Heterocyclic Chemistry
Received, 18th September, 2009, Accepted, 22nd October, 2009, Published online, 22nd October, 2009
DOI: 10.3987/COM-09-11834
SYNTHETIC STUDIES ON CALLIPELTINS: STEREOSELECTIVE
SYNTHESES OF (3S,4R)-3,4-DIMETHYL-L-PYROGLUTAMIC ACID
AND FMOC-D-ALLOTHREONINE FROM SERINE DERIVATIVES
a,b
c
b
b
Hiroyuki Konno,* Yoko Takebayashi, Kazuto Nosaka, and Kenichi Akaji
a
Department of Chemistry and Chemical Engineering, Graduate School of Science
and Technology, Yamagata University, Yonezawa, Yamagata 992-8510, Japan
b
Department of Chemistry, Graduate School of Medical Science, Kyoto
c
Prefectural University of Medicine, Kita-ku, Kyoto 603-8334, Japan Department
of Biological Science and Technology, Faculty of Engineering, University of
Tokushima, Tokushima 880-8501, Japan
*
E-mail address: konno@yz.yamagata-u.ac.jp
Abstract – The non-proteinogenic amino acids contained in callipeltins, (3S,
R)-3, 4-dimethyl-L-pyroglutamic acid and D-allothreonine, were synthesized
from D- or L-serine. The stereoselective synthesis of two methyl groups of (3S,
R)-3, 4-dimethyl-L-pyroglutamic acid was accomplished by diastereoselective
4
4
hydrogenation and alkylation. Kinetic epimerization of the C-4 methyl substituent
followed by Boc-deprotection with 10% TFA gave the desired (3S, 4R)-3,
4
-dimethyl-L-pyroglutamic acid as a single isomer.
INTRODUCTION
1
2
Callipeltin A (1) and B (2) are cyclic depsipeptides isolated from a shallow water sponge of the genus
Callipelta by Zampella et al. Callipeltins have antiviral and antifungal properties, and callipeltin A (1) is
the first natural peptide found to act against HIV. Callipeltin A (1) was also found to be a selective and
3
powerful inhibitor of cardiac interest as a regulator of myocardial contractility. Similar biological
4
activities were reported for cyclic depsipeptides isolated from other marine sponges. Because of the
unique structure containing unusual amino acid residues and biological properties of these natural
5
products, intensive efforts have been directed toward their synthesis including the solid phase synthesis
6
of callipeltin B (2). Although the total synthesis of callipeltin A (1) has not yet been achieved, several
7
groups have generated the unusual amino acids contained in callipeltins. We also reported the synthesis
8
and identification of the absolute configuration of ꢀ-methoxytyrosine in callipeltin E to be 2R, 3R.

80
HETEROCYCLES, Vol. 81, No. 1, 2010
As part of our efforts toward the total synthesis and elucidation of the structure-activity relationship of
callipeltin B (2), we report herein the synthesis of two unusual amino acids contained in callipeltin B (2),
(3S,4R)-3,4-dimethyl-L-pyroglutamic acid (3) and Fmoc-D-allothreonine (4), from a serine derivative.
9
10
Two other groups, Lipton’s group and Hamada’s group, have achieved the synthesis of 3 using an
intermediate derived from L-pyroglutamic acid, independently. The key step, the diastereoselective
induction of methyl groups, was achieved by Micheal-type addition and subsequent enolate alkylation. In
the present study, serine derivatives, a versatile synthon used in the synthesis of a variety of unusual
amino acids, was employed as the starting compound for the synthesis of both 3 and 4. For the total
synthesis of callipeltin A (1), 3,4-dimethyl-L-glutamine has been briefly transformed from 3.
H N
2
O
OH
OH
NH
OH
O
O
O
H
N
H
N
O
O
O
H N
N
H
N
H
N
H
NH
N NH2
N
H
N
H
NH
2
NH OH
O
HN
O
O
HN
O
N
H
NH2
O
O
H
O
O
O
O
NH
O
NH
HN
N
O
N
O
O
O
O
MeO
N
MeO
N
N
H
OH
N
H
H N
O
H N
O
2
2
OH
OH
callipeltin A (1)
callipeltin B (2)
O
O
OH O
OH
H N
OH
OH
2
O
N
H
NH2
NH2
O
D
-allothreonine (4)
3,4-dimethyl-glutamine
3,4-dimethyl-pyroglutamic acid (3)
Figure 1
RESULTS AND DISCUSSION
Our method of generating 3 and 4 is outlined in Scheme 1. As a common starting compound, protected
serine derivatives (5) prepared from D- or L-serine were selected. For the diastereoselective introduction
1
1
of methyl groups, Hanessian’s 1, 2-asymmetric induction reaction was employed to give anti-product 6.
Scheme 2 shows the synthetic route for 3. The protected serine derivative (5) derived from D-serine was
first transformed through hydrolysis of methyl ester and subsequent Weinreb amidation into an amide (7)
in 75% yield. Subsequent alkylation with methyllithium in THF afforded ketone (8) in 53% yield, which
was then treated with triethyl phosphonoacetate to give the ꢀ,ꢁꢂunsaturated ester (9) in 65% yield as E/Z

HETEROCYCLES, Vol. 81, No. 1, 2010
81
mixture (3:1). Diastereoselective hydrogenation of 9 as an E/Z mixture in the presence of 5% Pd/C gave
the saturated ester (6) in a quantitative yield with stereoselection of 95:5. The desired anti-product was
easily separated from the minor syn-product. The result indicates that the stereo-controlled reaction in the
1
1
acyclic system exploiting internal 1, 2-asymmetric induction proceeded effectively. Treatment of the
potassium enolate of 6 with methyl iodide gave the anti-syn product (10) as a single product isolated in
1
2
7
7% yield, which suggests that diasteroselection in the alkylation of ꢀ-alkyl substituted enolate esters is
applicable to the syn-alkylation to yield ꢀ, ꢁ-dimethyl groups in 10. Deprotection of acetonide (10) with a
catalytic amount of p-TsOH gave hydroxyl amine, which cyclized to afford the corresponding ꢂ-lactone
(11) as a single diastereoisomer in 67% yield. The stereostructure of the two methyl groups of 11 was
1
0
further confirmed by conversion to known hydroxyl ꢃ-lactam (4-epi-13) by Boc-deprotection and
subsequent DBU treatment. The trans-product (11) was converted to the desired cis-lactone (4-epi-11) by
kinetic epimerization of a ꢀ-methyl group of ꢂ-lactone (11). Treatment with sodium hexamethyldisilazide
o
in THF at -78 C and stereoselective protonation gave a single diastereoisomer (4-epi-11) in 67% yield, as
1
1
expected: NOE experiments showed the stereoinversion of the ꢀ-position. The 4-epi-11 was then
o
efficiently converted to 13 without epimerization by treatment with 10% TFA/CH Cl at 0 C over 10 h.
2
2
Treatment of 4-epi-11 with neat TFA gave undesired hydroxyl ꢃ-lactam (4-epi-13) having trans-methyl
groups on the quantitative acid-catalyzed epimerization of the ꢀ-methyl group. The same isomerization
was observed by Lipton et al. in the synthesis of Fmoc-protected dimethylglutamine derivative by the
9
a
indirect conversion of Boc to Fmoc group. Finally, the hydroxyl ꢃ-lactam (13) was converted to 3,
1
3
4
-dimethyl-pyroglutamic acid (3) by TEMPO-mediated oxidation in 80% yield. The spectroscopic data
9
for 3 are well consistent with the reported data.
O
O
OH
O
OH
H N
OH
2
OH
O
N
H
NH2
O
NH2
3
4
O
O
O
MeO
Boc
ent-5
OEt
OMe
O
O
O
N
N
N
Boc
Boc
6
5
Scheme 1. Retrosynthetic analysis of 3 and 4

82
HETEROCYCLES, Vol. 81, No. 1, 2010
Transformation of the alcohol group in 4-epi-13 to the desired 3, 4-dimethyl glutamine has already been
1
0,14
completed by Hamada’s group.
O
P
O
O
O
O
O
1
) LiOH
MeOH/H O
OMe
(
EtO)2
OEt
MeLi
OEt
2
N
OMe
2) NH(OMe)Me
O
O
O
O
O
N
Me
N
N
N
NaH, THF
THF
Boc
Boc
Boc
Boc
BOP, Et N
65% (E:Z=3:1)
3
53%
5
7
8
9
75%
O
O
KHMDS
MeI
H /Pd-C
p-TsOH
2
OEt
OEt
OEt
O
HO
O
N
HN
N
THF
AcOEt, MeOH
MeOH
Boc
10
Boc
Boc
1
00% (95:5)
77% (9:1)
6
NOE
H H
O
H H
NaHMDS
1
0% TFA/CH Cl
2
2
O
NHBoc
O
NHBoc
O
NH2
o
6
7%
THF, -78 C
O
1 h
O
67%
4
-epi 11
11
TEMPO
NaOCl, NaClO2
OH
OH
O
O
O
N
H
MeCN, NaH PO buffer
2 4
quant.
N
H
(
pH 6.6)
3
13
80%
Scheme 2. Synthetic route for 3
Then, D-allothreonine was prepared from the protected L-serine derivative (ent-5) by the route shown in
Scheme 3. First, the alcohol derivative (14) was prepared from L-serine in 24% overall yield (6 steps)
1
5
according to a published procedure. Protection of a hydroxyl group of 14 with MEMCl/iPr NEt followed
2
by deprotection of acetonide with a catalytic amount of p-TsOH afforded the corresponding alcohol (15)
in 59% yield. Oxidation of the alcohol with Jones reagent gave D-allothreonine derivative (16)
quantitatively. Deprotection of the Boc group followed by Fmoc protection afforded the desired
Fmoc-D-allothreonine (4). The overall yield of 4 was 57% in 5 steps from 14.
In conclusion, two unusual amino acid derivatives, (3S, 4R)-3, 4-dimethyl-L-pyroglutamic acid (3) and
Fmoc-D-allothreonine (4), were synthesized stereoselectively starting from a serine derivative. In the
synthesis of 3, the stepwise diastereoselective introduction of methyl groups proceeded to give the desired
o
compound as a single isomer. In addition, deprotection of the Boc group by 10% TFA/CH Cl at 0 C
2
2
effectively suppressed the acid-catalyzed epimerization of the ꢀ-methyl group. The synthesis of other
components of callipeltins and total synthesis on a solid support are now actively underway.

HETEROCYCLES, Vol. 81, No. 1, 2010
83
O
OH
1) MEMCl
OMEM
CrO , H SO
3 2 4
Hu et al.
iPr NEt
2
MeO
Boc
O
O
N
N
2
) p-TsOH
MeOH
OH acetone/H O
2
Boc
NHBoc
99%
ent-5
14
15
59%
O
OH
MEMO
O
1) TFA, H O
2
OH
OH
2
) Fmoc-OSu
8%
NHFmoc
NHBoc
9
4
1
6
Scheme 3. Synthesis of N-Fmoc-D-alloThreonine (4)
EXPERIMENTAL
General. All manipulations were conducted under an inert atmosphere (N ). All solvents were of reagent
2
grade. THF was distilled from sodium and benzophenone ketyl. CH Cl was distilled from CaH . All
2
2
2
commercial reagents were of the highest purity available. Analytical TLC was performed on silica gel
(
60F-254, plates 0.25 mm). Column chromatography was carried out on Wakogel 60 (particle size,
1
13
0
.063-0.200 mm). H (300 MHz) and C NMR (75 MHz) spectra were recorded on a Bruker AM-300.
1
Chemical shifts are expressed in ppm relative to TMS (0 ppm) or CHCl (7.28 ppm for H and 77.0 ppm
3
1
3
for C). IR spectra were obtained on a HORIBA FREEXACT-II FT-710 spectrometer. Optical rotations
were recorded on a HORIBA SEPA-300 polarimeter at the sodium D line. Low-resolution mass spectra
(
LRMS) and high-resolution mass spectra (HRMS) were obtained on either a JEOL JMS-HX-211A or a
JMS-HX-110A (EI or FAB), or a JMS-T100LC (ESI).
(
R)- N,O-Isopropylidenyl-2-(N-tert-butyloxycarbonylamino)-3-oxo-propanamide-N,O-dimethyl-
hydroxyamine (7). To a solution of 5 (10.0 g, 38.6 mmol) in MeOH (100 mL) was added LiOHꢀH O
2
(
455 mg, 77.2 mmol) and the mixture was stirred for 4 h at room temperature. H O and Et O were then
2 2
added, and the organic layer was dried over MgSO and concentrated in vacuo. To a solution of crude
4
product in CH Cl (100 mL) were added Et N (7.81 mL, 77.2 mmol), BOP (20.0 g, 45.1 mmol), and
2
2
3
NH(OMe)MeꢀHCl (4.52 g, 46.3 mmol) and the mixture was stirred for 3 h at room temperature. 1N HCl
pH=4) and Et O were added and the organic layer was washed with sat. aq. NaHCO , dried over MgSO ,
(
2
3
4
and evaporated in vacuo. The product was purified with silica gel column chromatography
2
3
o
(
hexane/AcOEt=2:1) to the give amide 7 (8.29 g, 28.8 mmol, 75%) as a colorless oil. [ꢀ]D +31.5 (c 1.0,
-
1
1
CHCl ). IR (film) ꢁ max cm : 2978, 1707, 1685, 1390, 1101, 999, 849, 769. H NMR (CDCl ) ꢂ: 1.42
3
3
(4.5H, s), 1.50 (4.5H, s), 1.53 (1.5H, s), 1.58 (1.5H, s), 1.69 (1.5H, s), 1.71 (1.5H, s), 3.22 (3H, s), 3.71

84
HETEROCYCLES, Vol. 81, No. 1, 2010
(1.5H, s), 3.75 (1.5H, s), 3.93 (0.5H, dd, J=9.0, 3.6 Hz), 3.98 (0.5H, dd, J=9.3, 3.0 Hz), 4.19 (0.5H, dd,
J=9.0, 7.2 Hz), 4.20 (0.5H, dd, J=9.0, 7.5 Hz), 4.73 (0.5H, dd, J=7.5, 3.6 Hz), 4.81 (0.5H, dd, J=7.2, 3.0
1
3
Hz). C NMR (CDCl ) ꢀ: 24.6, 24.7, 25.5, 25.7, 28.35, 28.42, 32.6, 36.7, 57.7, 57.9, 61.2, 65.9, 66.1,
3
+
8
0.0, 80.5, 94.4, 95.0, 151.3, 152.2, 170.6, 171.4. HRFABMS (M+H) calcd for C H N O : 289.1763,
1
3
25
2
5
found: 289.1747.
(
(
-
R)-N,O-Isopropylidenyl-4-(N-tert-butyloxycarbonylamino)-3-oxo-2-butanone (8). To a solution of 7
4.15 g, 14.4 mmol) in THF (100 mL) was added MeLi (1.0 M in THF solution; 25.0 mL, 25.0 mmol) at
78 ꢀ and the mixture was stirred for 1 h. H O and Et O were then added and the organic layer was
2
2
washed with sat. aq. NaHCO , dried over MgSO , and evaporated in vacuo. The product was purified
3
4
with silica gel column chromatography (hexane/AcOEt=2:1) to give the ketone 8 (1.80 g, 7.69 mmol,
2
3
o
-1
5
1
3%) as a colorless oil. [ꢁ]D +48.0 (c 1.0, CHCl ). IR (film) ꢂ max cm : 2979, 1732, 1705, 1392, 1250,
3
1
176, 1092, 850, 769. H NMR (CDCl ) ꢀ: 1.21 (3H, s), 1.30 (9H, s), 1.46 (3H, s), 1.98 (3H, s), 3.73 (1H,
3
1
3
brs), 3.93 (1H, brs), 4.16 (1H, brs). C NMR (CDCl ) ꢀ: 23.3, 24.3, 25.0, 25.5, 25.9, 27.8, 64.7, 65.0,
3
+
7
9.9, 80.0, 93.9, 94.6, 150.9, 151.9, 205.7. HRFABMS (M+H) calcd for C H NO : 244.1549, found:
1
2
22
4
2
44.1570.
(S) Ethyl N,O-isopropylidenyl-4-(N-tert-butyloxycarbonylamino)-3-methyl-5-oxo-2-pentenoate (9).
To a solution of triethyl phosphonoacetate (2.49 mL, 11.1 mmol) in THF (20 mL) was added NaH (444
mg, 11.1 mmol) at 0 ꢀ. The mixture was stirred for 1 h and then the ketone 8 (1.80 g, 7.41 mmol) was
added dropwise. After 24 h of stirring, sat. aq. NH Cl and AcOEt were added, and the organic layer was
4
washed with brine, dried over MgSO , and evaporated in vacuo. The product was purified with silica gel
4
column chromatography (hexane/AcOEt=4:1) to give the ethyl ester 9 (1.50 g, 4.79 mmol, 65%) as a
2
3
o
-1
colorless oil. [ꢁ]D +10.4 (c 1.0, CHCl ). IR (film) ꢂ max cm : 2979, 1700, 1376, 1243, 1213, 1161,
3
1
1
092, 1058, 852. H NMR (CDCl ) ꢀ: 1.27 (3H, m), 1.39 (3H, brs), 1.52 (9H, brs), 1.68 (3H, brs), 2.14
3
(3H, s), 3.75 (1H, dd, J=9.3, 3.3 Hz), 4.13 (1H, dd, J=9.3, 7.2 Hz), 4.15 (2H, m), 4.32 (1H, dd, J=7.2, 3.3
1
3
Hz), 5.77 (1H, s). C NMR (CDCl ) ꢀ: 14.0, 14.7, 23.1, 25.5, 28.0, 29.4, 63.8, 67.1, 68.9, 79.8, 94.6,
3
+
1
15.8, 151.6, 156.6. HRFABMS (M+H) calcd for C H NO : 314.1967, found: 314.1951.
1
6
28
5
(3S,4S) Ethyl N,O-isopropylidenyl-4-(N-tert-butyloxycarbonylamino)-3-methyl-5-oxo-pentanoate (6).
To a solution of ethyl ester 9 (1.50 g, 4.81 mmol) in MeOH/AcOEt (1:1; 20 mL) was added Pd-C (50 mg)
at room temperature. The mixture was stirred for 12 h in a hydrogen atmosphere. It was then filtered on a
Celite pad and concentrated in vacuo. The crude product was purified with silica gel column
chromatography (hexane/AcOEt=4:1) to give the ester 6 (1.52 g, 4.81 mmol, 100%) as a colorless oil.

HETEROCYCLES, Vol. 81, No. 1, 2010
85
2
3
o
-1
[
ꢀ]D +6.3 (c 1.0, CHCl ). IR (film) ꢁ max cm : 2979, 1736, 1697, 1458, 1375, 1257, 1176, 1084, 854.
3
1
H NMR (CDCl ) ꢂ: 0.87 (3H, d, J=5.4 Hz), 1.18 (3H, td, J=6.9, 1.8 Hz), 1.40 (9H, m), 1.41 (3H, brs),
3
1
.53 (3H, brs), 2.05 (1H, m), 2.43 (2H, d, J=13.2 Hz), 3.73 (2H, m), 3.83 (1H, m), 4.06 (2H, qd, J=6.9,
1
3
1
1
.5 Hz). C NMR (CDCl ) ꢂ: 14.1, 16.4, 26.1, 28.2, 32.5, 32.8, 36.4, 60.0, 61.0, 64.1, 79.8, 94.0, 152.4,
3
+
72.9. HRFABMS (M+H) calcd for C H NO : 316.2124, found: 316.2126.
1
6
30
5
(
2S,3S,4S) Ethyl N,O-isopropylidenyl-4-(N-tert-butyloxycarbonylamino)-2,3-dimethyl-5-oxo-
pentanoate (10). To a solution of the ester 6 (1.52 g, 4.82 mmol) in THF (20 mL) was added NaHMDS
1.0 M in THF solution; 7.82 mL, 7.82 mmol) at -78 ꢀ and the mixture was stirred for 30 min. Methyl
(
iodide (0.675 ml, 10.8 mmol) was added, the solution was stirred for 2 h, sat. aq. NH Cl and AcOEt were
4
added, and the organic layer was washed with brine, dried over MgSO , and evaporated in vacuo. The
4
product was purified with silica gel column chromatography (hexane/AcOEt=4:1) to give the ethyl ester
2
9
o
-1
1
0 (1.22 g, 3.70 mmol, 77%) as a colorless oil. [ꢀ]D +3.0 (c 1.0, CHCl ). IR (film) ꢁ max cm : 2979,
3
1
1
734, 1697, 1456, 1383, 1255, 1176, 1086, 860. H NMR (CDCl ) ꢂ: 0.93 (3H, d, J=7.2 Hz), 1.13 (3H, d,
3
J=7.2 Hz), 1.26 (3H, t, J=6.7 Hz), 1.50 (9H, s), 1.52 (3H, s), 1.60 (3H, s), 2.34 (1H, brs), 2.52 (1H, m),
1
3
3
.87 (3H, m), 4.15 (2H, q, J=6.9 Hz). C NMR (CDCl ) ꢂ: 10.4, 11.5, 14.2, 14.3, 16.1, 22.9, 24.5, 26.7,
3
2
7.2, 28.3, 28.4, 32.6, 38.0, 40.5, 40.9, 59.9, 60.1, 60.2, 64.2, 66.6, 79.9, 93.8, 94.2, 152.4, 153.3, 175.7.
+
HRFABMS (M+H) calcd for C H NO : 330.2280, found: 330.2289.
1
7
32
5
(3S,4S,5S)-N-(tert-Butyloxycarbonyl)-5-amino-3,4-dimethyl-pyran-2-one (11). To a solution of the
ethyl ester 10 (300 mg, 0.911 mmol) in MeOH (5 mL) was added p-TsOH (10 mg) at room temperature.
After the solution had been stirred for 4 h, H O and AcOEt were added and the organic layer was washed
2
with brine, dried over MgSO , and evaporated in vacuo. The product was purified with silica gel column
4
chromatography (hexane/AcOEt=4:1) to give the lactone 11 (149 mg, 0.612 mmol, 67%) as a colorless
2
7
o
-1
oil. [ꢀ]D -13.2 (c 1.1, CHCl ). IR (film) ꢁ max cm : 3356, 2978, 1740, 1712, 1522, 1367, 1248, 1171.
3
1
H NMR (CDCl ) ꢂ: 1.22 (3H, d, J=6.9 Hz), 1.27 (3H, d, J=6.9 Hz), 1.45 (9H, s), 1.55 (1H, m), 2.21 (1H,
3
dq, J=11.1, 6.9 Hz), 3.68 (1H, brs), 4.15 (1H, dd, J=12.0, 4.2 Hz), 4.33 (1H, dd, J=12.0, 3.6 Hz), 4.94
1
3
(
1H, m). C NMR (CDCl ) ꢂ: 13.9, 18.2, 28.1, 36.7, 40.0, 51.4, 51.8, 68.8, 79.6, 155.1, 174.5.
3
+
HRFABMS (M+H) calcd for C H NO : 244.1549, found: 244.1530.
1
2
22
4
(3S,4S,5S)-5-Hydroxymethyl-3,4-dimethyl-pyrrolidin-2-one (4-epi-13). To a solution of the lactone 11
(
20 mg, 0.0821 mmol) in CH Cl (1.0 mL) was added TFA (0.10 mL) at room temperature. After 1 h of
2
2
stirring, the solvent was removed in vacuo. To a solution of crude product in THF (2 mL) was added
DBU (5 μL, 0.0328 mmol) and then the mixture was stirred for 3 h at room temperature. To the mixture

86
HETEROCYCLES, Vol. 81, No. 1, 2010
were added H O and AcOEt. The organic layer was washed with brine, dried over MgSO , and
2
4
evaporated in vacuo. The product was purified with silica gel column chromatography
CHCl /MeOH=19:1) to give the lactam 4-epi-13 (12 mg, 0.0821 mmol, 100%) as a colorless solid.
(
3
o
23
o
-1
1
Mp=81-84 C. [ꢀ] -2.0 (c 1.0, CHCl ). IR (film) ꢁ max cm : 3384, 2992, 2981, 1685, 1460, 1063. H
D
3
NMR (CDCl ) ꢂ: 1.12 (6H, brs), 2.16 (2H, brs), 3.58 (2H, m), 3.72 (1H, m), 4.71 (1H, brs), 7.51 (1H, brs).
3
1
3
+
C NMR (CDCl ) ꢂ: 13.1, 13.9, 39.8, 42.6, 57.3, 62.0, 181.6. HRFABMS (M+H) calcd for C H NO :
3
7
14
2
1
44.1025, found: 144.1031.
(3R,4S,5S)-N-(tert-Butyloxycarbonyl)-5-amino-3,4-dimethyl-pyran-2-one (4-epi-11). To a solution of
1
1 (462 mg, 1.90 mmol) in THF (10 mL) was added NaHMDS (1.0 M in THF solution; 5.70 mL, 5.70
mmol) at -78 ꢀ and the mixture stirred for 30 min. Sat. aq. NH Cl and AcOEt were then added and the
4
organic layer was washed with brine, dried over MgSO , and evaporated in vacuo. The product was
4
purified with silica gel column chromatography (hexane/AcOEt=2:1) to give the lactone 4-epi-11 (311
2
8
o
-1
mg, 1.28 mmol, 67%) as a colorless oil. [ꢀ]D +13.8 (c 1.1, CHCl ). IR (film) ꢁ max cm : 3238, 2976,
3
1
1
743, 1701, 1281, 1255, 1163, 1101, 855, 792. H NMR (CDCl ) ꢂ: 1.12 (3H, d, J=6.9 Hz), 1.17 (3H, d,
3
J=6.9 Hz), 1.48 (9H, s), 2.08 (1H, ddq, J=10.2, 8.1, 6.9 Hz), 2.23 (1H, dq, J=10.2, 6.9 Hz), 3.72 (1H, ddt,
1
3
J=8.1, 3.6, 1.2 Hz), 3.93 (1H, dd, J=10.8, 8.1 Hz), 4.22 (1H, dd, J=10.8, 3.6 Hz), 5.92 (1H, brs). C
+
NMR (CDCl ) ꢂ: 13.4, 13.8, 27.7, 39.9, 41.7, 53.8, 66.7, 82.8, 153.3, 179.6. HRFABMS (M+H) calcd
3
for C H NO : 244.1549, found: 244.1544.
1
2
22
4
(3R,4S,5S)-5-Hydroxymethyl-3,4-dimethyl-pyrrolidin-2-one (13). To a solution of the lactone 4-epi-11
(
311 mg, 1.28 mmol) in CH Cl (10 mL) was added TFA (1 mL) at room temperature. After 1 h of
2
2
stirring, the solvent was removed in vacuo. The product was purified with silica gel column
chromatography (CHCl /MeOH=19:1) to give the lactam 13 (105 mg, 0.733 mmol, 57%) as a colorless
3
o
23
o
-1
solid. Mp=76-79 C. [ꢀ] +81 (c 0.2, CHCl ). IR (film) ꢁ max cm : 3325, 2962, 1682, 1458, 1267,
D
3
1
1
3
065. H NMR (CDCl ) ꢂ: 1.02 (3H, d, J=7.3Hz), 1.09 (3H, d, J=7.6 Hz), 2.25 (1H, m), 2.54 (1H, m),
3
.33 (1H, m), 3.50 (1H, dd, J=11.2, 7.3 Hz), 3.74 (1H, dd, J=11.2, 2.9 Hz), 4.32 (1H, brs), 6.05 (1H, brs).
1
3
+
C NMR (CDCl ) ꢂ: 10.7, 13.8, 34.6, 39.8, 62.1, 64.6, 181.4. HREIMS (M) calcd for C H NO :
3
7
13
2
1
43.0943, found: 143.0944.
(2S,3S,4R)-3,4-Dimethylpyroglutamic acid (3). To a solution of 13 (105 mg, 0.733 mmol) in MeCN-pH
6
.6 NaH PO buffer (1:1; 10 mL) were added a 10~15% NaOCl solution (12 mL), NaClO (150 mg, 1.65
2
4
2
mmol) and TEMPO (15 mg, 0.1 mmol). The mixture was stirred for 48 h at room temperature. To the
mixture were added 1N HCl and AcOEt. The organic layer was washed with brine, dried over MgSO4,

HETEROCYCLES, Vol. 81, No. 1, 2010
87
and evaporated in vacuo. The product was purified with silica gel column chromatography
CHCl /MeOH=9:1) to give the dimethyl pyroglutamic acid 3 (92 mg, 0.587 mmol, 80%) as a yellow
(
3
2
3
o
-1
1
solid. [ꢀ]D +41 (c 0.1, CHCl ). IR (film) ꢁ max cm : 3262, 2979, 1719, 1654, 1243. H NMR (D O) ꢂ:
3
2
1
3
0
9
1
.89 (3H, d, J=6.6 Hz), 0.97 (3H, d, J=6.2 Hz), 2.53 (2H, m), 3.80 (1H, d, J=3.9 Hz). C NMR (D O) ꢂ:
2
+
.4, 13.7, 38.0, 39.2, 61.4, 176.2, 183.8. HRFABMS (M+H) calcd for C H NO : 158.0817, found:
7
12
3
58.0815.
(2S,3R)-2-N-(tert-Butyloxycarbonyl)-amino-3-methoxyethoxymethoxybutan-1-ol (15). To a solution
of 14 (3.04 g, 12.4 mmol) in CH Cl (50 mL) were added iPr NEt (3.75 g, 37.0 mmol) and MEMCl (2.31
2
2
2
o
g, 28.7 mmol) at 0 C and the mixture stirred for 10 h at room temperature. 1N HCl and CH Cl were then
2
2
added and the organic layer was washed with brine, dried over MgSO , and evaporated in vacuo. The
4
product was purified with silica gel column chromatography (hexane/AcOEt=4:1) to give the MEM ether
1
(
3.98 g, 11.9 mmol, 96%) as a colorless oil. H NMR (CDCl ) ꢂ: 1.17 (3H, d, J=6.6 Hz), 1.49 (9H, s),
3
1
4
.52 (3H, s), 1.58 (3H, s), 3.41 (3H, s), 3.56 (2H, t, J=4.2 Hz), 3.70 (2H, m), 3.76 (1H, m), 3.93 (1H, m),
.08 (1H, dd, J=8.4, 1.8 Hz), 4.11 (1H, m), 4.77 (1H, d, J=6.9 Hz), 4.83 (1H, d, J=6.9 Hz). To a solution
of the MEM ether (3.23 g, 9.67 mmol) in MeOH (50 mL) was added p-TsOH (10 mg) at room
temperature and the mixture stirred for 10 h. Sat. aq. NaHCO and AcOEt were then added and the
3
organic layer was washed with brine, dried over MgSO , and evaporated in vacuo. The product was
4
purified with silica gel column chromatography (hexane/AcOEt=2:1) to give the alcohol 15 (1.78 g, 6.07
2
5
o
-1
mmol, 63%) as a colorless oil. [ꢀ]D -1.3 (c 0.6, CHCl ). IR (film) ꢁ max cm : 3448, 3363, 2978, 2931,
3
1
1
712, 1511, 1365, 1172, 1041, 856. H NMR (CDCl ) ꢂ: 1.19 (3H, d, J=6.3 Hz), 1.45 (9H, s), 3.07 (1H,
3
brs), 3.40 (3H, s), 3.54-3.63 (6H, m), 3.79 (1H, m), 4.11 (1H, q, J=5.7 Hz), 4.64 (1H, d, J=7.2 Hz), 4.77
1
3
(
1H, d, J=7.2 Hz), 4.91 (1H, brs). C NMR (CDCl ) ꢂ: 16.4, 28.3, 56.0, 59.0, 62.6, 67.0, 70.6, 71.6, 79.3,
3
+
9
3.0, 156.3. HRESIMS (M+Na) calcd for C H NO Na: 316.1736, found: 316.1748.
1
3
27
6
(2R,3R)-2-N-(tert-Butyloxycarbonyl)-3-methoxyethoxymethyl-D-allothreonine (16). To a solution of
o
1
5 (1.36 g, 4.64 mmol) in acetone (20 mL) was added Jones reagent (3 mL; 2.67 M solution) at 0 C and
the mixture was stirred for 30 min. 2-Propanol (1 mL) was then added and the mixture was filtered on a
Celite pad and concentated in vacuo. The crude product was treated with 1N HCl and extracted with
AcOEt. The organic layer was dried over MgSO , and concentrated in vacuo. The product was purified
4
with silica gel column chromatography (CHCl /MeOH=9:1) to give 16 (1.30 g, 4.63 mmol, 91%) as a
3
2
3
o
-1
colorless oil. [ꢀ]D +1.4 (c 1.3, CHCl ). IR (film) ꢁ max cm : 3459, 3330, 2978, 2934, 1716, 1506, 1367,
3
1
1
167, 1039, 858. H NMR (CDCl ) ꢂ: 1.21 (3H, d, J=6.3 Hz), 1.42 (9H, s), 3.28 (3H, s), 3.51-3.70 (5H,
3
1
3
m), 4.31 (2H, m), 4.61-4.74 (2H, m), 5.35 (1H, m). C NMR (CDCl ) ꢂ: 16.8, 28.2, 58.0, 58.8, 67.0,
3

88
HETEROCYCLES, Vol. 81, No. 1, 2010
+
7
1.6, 72.9, 79.9, 93.6, 156.1, 173.9. ESIMS (M) calcd for C H NO : 307.16, found: 307.12.
1
3
25
7
(2R,3R)-2-N-(tert-9-Fluorenylmethyloxycarbonyl)-D-allothreonine (4). A solution of 16 (67 mg, 2.19
mmol) in 90% TFA/H O (2 mL) was stirred for 10 h at room temperature and the solvent was removed in
2
vacuo. To a solution of the crude product in H O (2 mL) were added 1N NaOH (0.4 mL) and Fmoc-OSu
2
(
147 mg, 0.436 mmol) in dioxane (1 mL) and then the mixture was stirred for 15 min at room temperature.
1
N HCl was added and the resulting precipitate was filtered to give 4 (73 mg, 0.214 mmol, 98%) as a
1
white powder. All spectra for the synthetic 4 were well consistent with those reported. H NMR (CDCl )
3
ꢀ
7
3
: 1.26 (3H, m), 2.65 (1H, brs), 4.16 (1H, brs), 4.35 (3H, brs), 5.39 (1H, brs), 6.05 (1H, brs), 7.30 (2H, m),
+
.37 (2H, t, J=7.2 Hz), 7.58 (2H, brs), 7.73 (2H, d, J=7.2 Hz). ESIMS (M+Na) calcd for C H NO Na :
1
9
19
5
7
64.12, found: 364.05.
ACKNOWLEDGEMENTS
We thank Dr. Nobutaka Fujii and Dr. Shinya Oishi (Kyoto University) for the measurement of mass
spectra. This work was supported in part by Chisso, Co. Ltd., and a Grant-in-aid for Scientific Research
from the Japan Society for the Promotion of Science (Grant 21689004 to H.K.).
REFERENCES AND NOTES
1
.
A. Zampella, M. V. D’Auria, L. Gomez-Paloma, A. Casapulla, C. Debitus, and Y. Henin, J. Am.
Chem. Soc., 1996, 118, 6202.
2
.
M. V. D’Auria, A. Zampella, L. Gomez-Paloma, and L. Minare, Tetrahedron, 1996, 52, 9589; A.
Zampella, A. Randazzo, N. Borbone, S. Luciani, L. Trevisi, C. Debitus, and M. V. D’Auria,
Tetrahedron Lett., 2002, 43, 6163; M. V. D’Auria, V. Sepe, R. D’Orsi, F. Rellota, C. Debitus, and A.
Zampella, Tetrahedron, 2007, 63, 131.
3
.
.
L. Trevisi, S. Bova, G. Cargnelli, D. Danieli-Betto, M. Floreani, M. V. D’Auria, and S. Liciani,
Biochem. Biophys. Res. Commun., 2000, 279, 219; L. Trevisi, G. Cargnelli, G. Ceolotte, I.
Papparella, A. Semplicini, A. Zampella, M. V. D’Auria, and S. Luciani, Biochem. Pharmacol., 2004,
6
8, 1331.
4
P. W. Ford, K. R. Gustafson, T. C. McKee, N. Shigematsu, L. K. Maurizi, Pannell, D. E. Williams,
E. D. de Silva, P. Lassota, T. M. Allen, R. Van Soest, R. J. Andersen, and M. R. Boyd, J. Am. Chem.
Soc., 1999, 121, 5899; M. A. Rashid, K. R. Gustafson, L. K. Cartner, N. Shigematsu, L. K. Pannell,
and M. R. Boyd, J. Nat. Prod., 2001, 64, 117; N. Oku, K. R. Gustafson, L. K. Cartner, J. A. Wilson,
N. Shigematsu, S. Hess, L. K. Pannell, M. R. Boyd, and J. B. McMahon, J. Nat. Prod., 2004, 67,
1
407; A. Zampella, V. Sepe, P. Luciano, F. Bellota, M. C. Monti, M. V. D’Auria, T. Jepson, S.

HETEROCYCLES, Vol. 81, No. 1, 2010
89
Petek, M.-T. Adeline, O. Laprevote, A.-M. Aubertin, C. Debitus, C. Poipat, and A. Ahond, J. Org.
Chem., 2008, 73, 5319.
5
.
.
W. Xie, D. Ding, W. Zi, G. Li, and D. Ma, Angew. Chem. Int. Ed., 2008, 47, 2844.
R. Krishnamoorthy, L. D. Vazquez-Serrano, J. A. Tunk, J. A. Kowalski, A. G. Benson, N. T. Breaux,
and M. A. Lipton, J. Am. Chem. Soc., 2006, 128, 15392; R. Krishnamoorthy, B. L. Richardson, and
M. A. Lipton, Bioorg. Med. Chem. Lett., 2007, 17, 5136.
6
7
.
K. A. Parker and Q. Xie, Org. Lett., 2008, 10, 1349; J. Joen, S. K. Hong, J. S. Oh, and Y. G. Kim, J.
Org. Chem., 2006, 71, 3310; D. C. Cranfill, A. I. Morales-Ramos, and M. A. Lipton, Org. Lett.,
2
005, 7, 5881; and references cited therein.
8
.
.
H. Konno, S. Aoyama, K. Nosaka, and K. Akaji, Synthesis, 2007, 3666.
C. Selcuk and M. A. Lipton, J. Org. Chem., 2005, 70, 6218; C. M. Acevedo, E. F. Kogut, and M. A.
Lipton, Tetrahedron, 2001, 57, 6353.
9
1
0. N. Okamoto, O. Hara, K. Makino, and Y. Hamada, Tetrahedron: Asymmetry, 2001, 12, 1353.
1. S. Hanessian, W. Wang, and Y. Gai, Tetrahedron Lett., 1996, 37, 7477; S. Hanessian, Y. Gai, and W.
Wang, Tetrahedron Lett., 1996, 37, 7473.
1
1
2. Y. Morizawa, A. Tasuda, and K. Uchida, Tetrahedron Lett., 1986, 27, 1833.
3. M. M. Zhao, Z. Li, E. Mano, Z. J. Song, and D. M. Tschaen, Organic Syntheses, 2005, 81, 195; J. Z.
Song, M. Zhao, R. Desmond, P. Devine, D. M. Tschaen, R. Tiller, L. Frey, R. Heid, F. Xu, B. Foster,
J. Li, R. Reamer, R. Volante, E. J. Grabowski, U. H. Dolling, P. J. Reider, S. Okada, Y. Kato, and E.
Mano, J. Org. Chem., 1999, 64, 9658.
1
1
1
4. B. Liang, P. J. Carroll, and M. M. Joullié, Org. Lett., 2000, 2, 4157.
5. L. Williams, Z. Zhang, F. Shao, P. J. Carroll, and M. M. Joullié, Tetrahedron, 1996, 36, 11673; L. A.
Paquette, T. M. Mitzel, M. B. Issac, C. F. Crasto, and W. W. Schomer, J. Org. Chem., 1997, 62,
4
293; X. E. Hu, N. K. Kim, and B. Ledoussai, Org. Lett., 2002, 4, 4499.