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M. Kikuchi, H. Konno / Tetrahedron 69 (2013) 7098e7101
not in progress (entry 2), therefore we attempted the condition
using 60% HClO4 aq in MeCN to afford Fmoc-
-alloThr-OtBu (2) and
Fmoc- -alloThr-OH (6) in 43% and 39% yields instead of 1 (entry 3).
5 M HCl (40 ml) for 2.5 h, the solution was evaporated under re-
duced pressure. To the residue was added cold EtOH (40 ml), NH4Cl
was removed by filtration, and adjusted to pH 6 with Et3N. The
D
D
In practice, the reaction conditions as depicted in entries 4 and 5
gave 2 in 80% and 70% (based on recovered starting material). In
contrast, the HClO4-mediated deprotections of tBu ether with
several additives were not effective (entries 6e8). These results
show chemoselective deprotection of tBu group; SiO2/toluene
precipitated
lected by filtration and washed with cold EtOH. After adding EtOH
(29 ml) to a solution of the crude -alloThr in water (7.3 ml) and then
stirring for 30 min at room temperature, -alloThr was collected by
filtration and washed with cold EtOH to give -alloThr (4) (566 mg,
D-alloThr (735 mg, 6.17 mmol, 56%, 96% de) was col-
D
D
D
condition afford Fmoc-
OtBu (2) is obtained by 60% HClO4 aq/CH2Cl2.12
In conclusion, we described the improved synthesis of
(4). The diastereomeric excess of 4 was achieved at 96% de. In ad-
dition, protection of the side chain and C-terminus of Fmoc-
D
-alloThr(tBu)-OH (1) and Fmoc-
D
-alloThr-
4.74 mmol, 43%, >99% de) as a white powder. Mp: 274e275 ꢀC
(dec); [
a
]
D
28 ꢂ7.5 (c 0.2, H2O). The spectral data were consistent with
D
-alloThr
those of the reported values.7,8 RP-HPLC: 7, tR 8.67 min, 8, tR 8.13 min
(CH3CN gradient; 0e40% in 30 min).
D
-
t
alloThr-OH (6) with the Bu group could be easily conducted by
3.2.3. Fmoc-D-alloThr-OH (6). D-alloThr (4) (738 mg, 6.19 mmol) in
comparison with using 2-methylpropene. It is possible to easily
saturated NaHCO3 aq (20 ml) was added to Fmoc-OSu (2.09 g,
6.19 mmol) in dioxane (20 ml) and then the mixture was stirred for
2 h at room temperature. The reaction mixture was added to Et2O
and 1 M HCl and extracted with AcOEt. The organic layer was
washed with brine, dried over MgSO4, and evaporated under re-
purify Fmoc-
using this method. This protocol has the advantage of the em-
ployment of cheap -Thr. Synthetic study of callipeltins containing
synthetic -alloThr (4) on a solid support is now underway.
D
-alloThr(tBu)-OH (1) and Fmoc-
D
-alloThr-OtBu (2)
L
D
duced pressure to give Fmoc-
a white powder. Mp 107e108 ꢀC; [
(CD3OD, 500 MHz)
D
-alloThr-OH (6) (2.11 g, quant.) as
3. Experimental
3.1. General
a
]
28 ꢂ12.2 (c 1.0, CHCl3); 1H NMR
D
d
: 1.24 (3H, d, J¼7.0 Hz), 4.10 (1H, m), 4.22e4.26
(2H, m), 4.33e4.41 (2H, m), 7.31 (2H, m), 7.39 (2H, t, J¼7.5 Hz), 7.70
(2H, dd, J¼7.0, 4.0 Hz), 7.81 (2H, d, J¼8.0 Hz); 13C NMR (CD3OD,
All solvents were reagent grade. CH2Cl2 was distilled from CaH2.
All commercial reagents were of the highest purity available. An-
alytical TLC was performed on silica gel (Merck Silica gel 60F254).
Column chromatography was carried out on Silica Gel 60 N [Kanto,
125 MHz) d: 18.1, 47.1, 59.9, 66.8, 67.4, 119.6, 124.9, 126.8, 127.5,
141.2, 143.9, 157.4, 172.5; IR nmax (KBr) cmꢂ1: 3403, 3043, 2360,
1710, 1540, 1477, 1072, 1004; ESIeHRMS m/z [MþNa]þ: calcd for
C19H19NO5Na: 364.1161, Found: 364.1145. RP-HPLC: tR 16.71 min
(CH3CN gradient; 10e100% in 30 min).
particle size, (spherical, neutral) 40e100 mm or 63e210 m
m]. 1H
(400 and 500 MHz) and 13C NMR (100 and 125 MHz) spectra were
recorded on either a JNM-ECX400, JNM-ECX500 or Varian INOVA
500. Chemical shifts are expressed in parts per million relative to
TMS (0 ppm), CHCl3 (7.26 ppm for 1H and 77.1 ppm for 13C), H2O
(4.79 ppm for 1H) or CH3OH (3.31 ppm for 1H and 49.0 ppm for 13C).
IR spectra were obtained on an HORIBA FT-710 or FT-720 spec-
trometer. Optical rotations were recorded on a JASCO DIP-371 po-
larimeter at the sodium D line. Low-resolution mass spectra (LRMS)
and high-resolution mass spectra (HRMS) were obtained using ei-
ther a JEOL AccuTOF JMS-T100LC (ESI-MS) or JMS-700 (FABMS).
Melting points were determined on an AS ONE ATM-02. HPLC was
carried out on a Cosmosil 5C18 AR-II column (4.6ꢁ150 mm or
10ꢁ250 mm), which was eluted with MeCN in 0.1% aqueous TFA
and detected at OD 220 nm.
3.2.4. Fmoc-
D
-alloThr(tBu)-OtBu (5). Fmoc-
D
-alloThr-OH (6) (3.79 g,
11.1 mmol) in AcOtBu (278 ml) was added to 60% HClO4 aq (126
ml,
2.22 mmol) and then the mixture was stirred at room temperature.
After stirring for 13.5 h, saturated NaHCO3 aq and AcOEt were
added to the solution. The organic layer was washed with brine,
dried over MgSO4, and evaporated under reduced pressure. The
product was purified with silica gel column chromatography
(hexane/AcOEt¼8:1 to 4:1) to give Fmoc-
D
-alloThr(tBu)-OtBu (5)
28
(3.95 g, 8.71 mmol, 78%). Mp 94e95 ꢀC; [
1H NMR (CDCl3, 400 MHz)
a
]
ꢂ11.2 (c 1.0, CHCl3);
D
d
: 1.18 (9H, s), 1.22 (3H, d, J¼6.4 Hz), 1.49
(9H, s), 4.00 (1H, m), 4.22e4.41 (4H, m), 5.56 (1H, d, J¼7.8 Hz; NH),
7.33 (2H, t, J¼7.8 Hz), 7.42 (2H, t, J¼7.4 Hz), 7.62 (2H, d, J¼7.4 Hz),
7.77 (2H, d, J¼7.8 Hz); 13C NMR (CDCl3, 125 MHz)
d: 19.7, 28.2, 28.4,
47.3, 59.9, 67.1, 68.6, 74.1, 82.2, 120.0, 125.3, 127.1, 127.7, 141.4, 144.0,
156.0,169.4; IR nmax (film) cmꢂ1: 3440, 3359, 2977,1724,1504,1450,
1365, 1157, 848, 740; ESIeHRMS m/z [MþNa]þ: calcd for
C27H35NO5Na: 476.2413, Found: 476.2392. RP-HPLC: tR 29.56 min
(CH3CN gradient; 10e100% in 30 min).
3.2. Synthesis
3.2.1. Epimerization of
L
-Thr (3). The solution of
L-Thr (3) (5.08 g,
42.6 mmol) in AcOH (244 ml) was added to salicylaldehyde (834
m
l,
8.52 mmol) and the mixture was heated at w70 ꢀC for 7 h. After
evaporation under reduced pressure to remove the solvent, the
residue was added to MeOH/2-PrOH (1:1, 244 ml), stirred for
30 min at room temperature, collected by filtration, and washed
3.2.5. Fmoc-
D
-alloThr(tBu)-OH (1). To
mol) in toluene (11 ml) was
a solution of Fmoc-D-
alloThr(tBu)-OtBu (5) (218 mg, 482
m
added SiO2 (2.34 g), which was then heated to reflux for 30 min.
The mixture was filtrated, washed with 10% MeOH/CH2Cl2, and
evaporated under reduced pressure. To the crude product were
added saturated NaHCO3 aq and AcOEt. The organic layer was
washed with brine, dried over MgSO4, and evaporated under
reduced pressure. The crude product was purified with silica gel
with 2-PrOH. The product was the mixture of
L
-Thr (3) and D-alloThr
(4) in a molar ratio of 1:0.74 (3.08 g, 25.9 mmol, 61%). The 1H NMR
spectral data was consistent with those of the reported values.8
3.2.2. Preparation of
D
-alloThr (4). To the diastereoisomeric mixture
of -Thr (3) and -alloThr (4) (1:0.74, 3.08 g, 25.9 mmol) in AcOH
L
D
column chromatography (CHCl3/MeOH¼99:1) to give Fmoc-
alloThr(tBu)-OH (1) (73.7 mg, 186
mol, 39%) as a colorless solid.
Fmoc-
-alloThr(tBu)-OH (1). Mp 74e75 ꢀC; [
CH2Cl2). RP-HPLC: tR 21.99 min (CH3CN gradient; 10e100%
in 30 min). The spectral data were consistent with those of the
reported values.9
D-
(40 ml) was added acetic anhydride (3.6 ml, 38.7 mmol) dropwise.
After stirring for 2.5 h at 50 ꢀC, the mixture was evaporated under
reduced pressure. To a solution of the residue in water (20 ml) was
added concd NH3 aq (pH 10). After evaporating the mixture under
reduced pressure, cold EtOH (40 ml) was added to the residue. The
m
28
D
a
]
ꢂ15.6 (c 1.0,
D
precipitated Ac- -Thr NH3 (7) was collected by filtration and washed
with a small amount of cold EtOH. After evaporating the filtrate
under reduced pressure and then refluxing a solution of the residue
L
3.2.6. Fmoc-
D
-alloThr-OtBu (2). To
a
solution of Fmoc-
D-
alloThr(tBu)-OtBu (5) (12.9 mg, 28.4
m
mol) in CH2Cl2 (700 l) was
m