Asymmetric synthesis of orthogonally protected L-threo-β-hydroxyasparagine

Full text of DOI:10.1021/jo000628s

6770
J . Org. Chem. 2000, 65, 6770-6772
Asym m etr ic Syn th esis of Or th ogon a lly
P r otected L-th r eo-â-Hyd r oxya sp a r a gin e
Dale L. Boger*, Richard J . Lee,
Pierre-Yves Bounaud, and Peter Meier
Department of Chemistry and The Skaggs Institute for
Chemical Biology, The Scripps Research Institute, 10550
North Torrey Pines Road, La J olla, California 92037
boger@scripps.edu
Received April 25, 2000
In the course of efforts on the total synthesis of the
novel antimicrobial lipoglycodepsipeptide ramoplanin A2
(1, Figure 1), the preparation of the unnatural amino acid
L-threo-â-hydroxyasparagine (L-threo-â-OH-Asn) was re-
quired as a key subunit.1 The isomers of â-OH-Asn were
originally isolated from human urine and synthesized as
a racemic mixture that was separated by resolution.2
Although there are several reports of the enantioselective
synthesis of isomers of â-hydroxyaspartic acid and eryth-
ro â-hydroxyasparagine, no descriptions of an asymmetric
synthesis of threo â-hydroxyasparagine have been dis-
closed.3 Herein, we describe an effective asymmetric
synthesis of L-threo-FmocNH-â-OH-Asn(Trt)-OBn (2),
suitably protected for incorporation into a projected total
synthesis of ramoplanin A2, via the Sharpless asym-
metric aminohydroxylation (AA) reaction.
F igu r e 1.
Sch em e 1
The approach to the orthogonally protected L-threo-â-
OH-Asn rested on the knowledge that cinnamate esters
are excellent substrates for the Sharpless AA reaction,
that the trans olefin geometry would fix the desired threo
stereochemistry, and that an aromatic group can serve
as an effective masking group for a carboxylic acid.4,5
Thus, the AA reaction on methyl 4-methoxycinnamate
(3) produced the amino alcohol 4 in 64% yield and over
99% ee (Scheme 1).6 Sequential protection of the alcohol
with tert-butyldimethylsilyl triflate (TBDMSOTf), single
step N-Cbz/Boc exchange, and direct aminolysis of the
methyl ester provided 7 in 68% overall yield with no
evidence of epimerization. The latent carboxylic acid was
unmasked by treating 7 with ruthenium tetraoxide
generated in situ and subsequently protected as a benzyl
ester to provide 9 in 57% and 84% yields, respectively.5
(1) Cavalleri, B.; Pagani, H.; Volpe, G.; Selva, E.; Parenti, F. J .
Antibiot. 1984, 37, 309. Pallanza, R.; Berti, M.; Scotti, R.; Randisi, E.;
Arioli, V. J . Antibiot. 1984, 37, 318. Ciabatti, R.; Kettenring, J . K.;
Winters, G.; Tuan, G.; Zerilli, L.; Cavalleri, B. J . Antibiot. 1989, 42,
254. Kettenring, J . K.; Ciabatti, R.; Winters, G.; Tamborini, G.;
Cavalleri, B. J . Antibiot. 1989, 42, 268. Parenti, F.; Ciabatti, R.;
Cavalleri, B.; Kettenring, J . Drugs Exptl. Clin. Res. 1990, 16, 451.
(2) Okai, H.; Izumiya, N. Bull. Chem. Soc. J pn. 1969, 42, 3550.
(3) (a) Hanessian, S.; Vanasse, B. Can. J . Chem. 1993, 71, 1401.
Cho, G. Y.; Ko, S. Y. J . Org. Chem. 1999, 64, 8745. Cardillo, G.;
Gentilucci, L.; Tolomelli, A.; Tomasini, C. Synlett 1999, 1727. Deng,
J .; Hamada, Y.; Shioiri, T. J . Am. Chem. Soc. 1995, 117, 7824. (b) For
erythro â-hydroxyasparagine: Tohdo, K.; Hamada, Y.; Shioiri, T.
Synlett 1994, 247. Sendai, M.; Hashiguchi, S.; Tomimoto, M.; Kish-
imoto, S.; Matsuo, T.; Ochiai, M. Chem. Pharm. Bull. 1985, 33, 3798.
(4) Li, G.; Angert, H. H.; Sharpless, K. B. Angew Chem., Int. Ed.
Engl. 1996, 35, 2813.
The silyl protecting group was then removed with
Bu4NF to provide 10 in 95% yield.
The unprotected carboxamide of an asparagine residue
is known to undergo side reactions during peptide
coupling, i.e., intramolecular cyclization upon activation
of the carboxylic acid leading to a succinimide byproduct,
or dehydration to a nitrile. To prevent the formation of
such byproducts, additional modifications to 10 were
prepared which contain a suitable protecting group such
as triphenylmethyl or trityl (Trt). The Boc protecting
group was removed quantitatively and the resulting
amine salt was reacted with 9-fluorenylmethyl chloro-
(5) Carlsen, P. H. J .; Katsuki, T.; Martin, V. S.; Sharpless, K. B. J .
Org. Chem. 1981, 46, 3936. Nunez, M. T.; Martin, V. S. J . Org. Chem.
1990, 55, 1928.
(6) The ee was established by HPLC on a Chiracel OD column (0.46
× 25 cm, 10% i-PrOH/hexane, flow rate ) 0.6 mL/min): tR((2S,3R)-4)
) 35.7 min, tR((2R,3S)-4) ) 39.5 min.
10.1021/jo000628s CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/13/2000

Notes
J . Org. Chem., Vol. 65, No. 20, 2000 6771
25 °C for 6 h. The catalyst was removed by filtration through
Celite, and the filtrate was concentrated in vacuo. Flash chro-
matography (SiO2, 30% EtOAc-hexane) provided 6 as a white
formate (FmocCl) to give the Fmoc-protected â-OH-Asn
11 in 78% yield. The acidic conditions developed by Sieber
and co-workers to introduce the trityl group were modi-
fied to minimize the formation of an acetate byproduct
due to the use of acetic anhydride as dehydrating agent.7
Thus, compound 11 was treated with trityl alcohol and
acetic anhydride under acidic conditions to provide trityl-
protected residue 2 in 64% yield.
In summary, the asymmetric synthesis of L-threo-
FmocNH-â-OH-Asn(Trt)-OBn (2) and several related
protected asparagine residues (8-11) was accomplished
from methyl 4-methoxycinnamate via the Sharpless
asymmetric aminohydroxylation reaction. With three
orthogonal protecting groups, asparagine 2 is an attrac-
tive building block for further synthetic endeavors.
Efforts on the incorporation of 2 into the total synthesis
of ramoplanin A2 are in progress and will be disclosed
in due course.
solid (2.20 g, 95%): mp 63-64 °C; [R]23 -1.6 (c 1.9, CHCl3); 1H
D
NMR (CDCl3, 500 MHz) δ 7.17 (d, 2H, J ) 8.8 Hz), 6.83 (d, 2H,
J ) 8.8 Hz), 5.47 (d, 1H, J ) 8.8 Hz), 5.10 (d, 1H, J ) 8.8 Hz),
4.32 (s, 1H), 3.77 (s, 3H), 3.73 (s, 3H), 1.39 (s, 9H), 0.75 (s, 9H),
-0.18 (s, 3H), -0.33 (s, 3H); 13C NMR (CDCl3, 125 MHz) δ 172.2,
159.4, 155.6, 132.2, 128.0, 114.1, 80.0, 76.2, 57.0, 55.7, 52.6, 28.7,
27.8, 25.9, 18.6, -5.2, -5.5; MALDI-FTMS (DHB) m/z 462.2280
(M + Na+, C22H37NO6Si requires 462.2288). Anal. Calcd for
C22H37NO6Si: C, 60.11; H, 8.48; N, 3.19. Found: C, 60.10; H,
8.31; N, 3.10.
(1R,2S)-[2-[(ter t-Bu tyld im eth ylsilyl)oxy]-2-ca r ba m oyl-1-
(4-m eth oxyp h en yl)eth yl]ca r ba m ic Acid ter t-Bu tyl Ester
(7). A sample of 6 (2.20 g, 5.0 mmol) was dissolved into CH3OH
(25 mL), and NH3 was bubbled through the CH3OH solution at
0 °C until saturation. The tube was sealed and stirred at 25 °C
for 7-10 days. The volume of the reaction mixture was reduced
to half under reduced pressure and directly subjected to flash
chromatography (SiO2, 25% EtOAc-hexane) which provided 7
as a white solid (1.46 g, 69%; 69-77%): mp 79-80 °C; [R]23
D
-9.1 (c 0.75, CHCl3); 1H NMR (CDCl3, 500 MHz) δ 7.18 (d, 2H,
J ) 8.5 Hz), 6.81 (d, 2H, J ) 8.5 Hz), 6.19 (s, 1H), 6.02 (d, 1H,
J ) 8.8 Hz), 5.43 (s, 1H), 4.93 (dd, 1H, J ) 9.2, 3.0 Hz), 4.29 (d,
1H, J ) 3.3 Hz), 3.77 (s, 3H), 1.41 (s, 9H), 0.89 (s, 9H), 0.01 (s,
3H), -0.02 (s, 3H); 13C NMR (CDCl3, 125 MHz) δ 175.0, 159.4,
155.3, 131.0, 128.6, 113.9, 80.0, 76.1, 56.9, 55.7, 28.8, 26.2, 18.5,
-4.7, -5.1; MALDI-FTMS (DHB) m/z 447.2291 (M + Na+,
C21H36N2O5Si requires 447.2291). Anal. Calcd for C21H36N2O5-
Si: C, 59.40; H, 8.55; N, 6.60. Found: C, 59.24; H, 8.29; N, 6.39.
L-th r eo-BocNH-â-OTBDMS-Asn (8). A solution of NaIO4
(7.0 g, 33 mmol) in CH3CN/H2O (15/24 mL) was treated with a
solution of 7 (1.00 g, 2.35 mmol) in CCl4 (15 mL) followed by
RuCl3‚3H2O (10 mg, 0.04 mmol) and NaHCO3 (50 mg). The
reaction mixture was stirred vigorously at 25 °C for 24 h. The
reaction mixture was extracted into saturated aqueous NaHCO3
and washed with CH2Cl2. The aqueous layer was acidified with
the addition of 10% aqueous HCl to pH 2-3 in an ice-bath and
extracted with EtOAc several times. The organic layer was dried
(Na2SO4) and concentrated in vacuo to provide 8 as a white solid
Exp er im en ta l Section
Meth yl (2S,3R)-3-[(Ben zyloxycar bon yl)am in o]-2-h ydr oxy-
3-(4-m eth oxyp h en yl) p r op ion a te (4). Benzyl carbamate (1.88
g, 12.4 mmol) was dissolved in 14 mL of n-PrOH. A freshly
prepared solution of NaOH (0.488 g, 12.2 mmol) in 22 mL of
H2O was added to this stirred solution, followed by a freshly
prepared solution of tert-butyl hypochlorite (1.324 g, 12.2 mmol)
and a solution of (DHQD)2PHAL (160 mg, 0.2 mmol) in 8 mL of
n-PrOH. The reaction vessel was immersed in a room-temper-
ature water bath and stirred for a few minutes. Methyl 4-meth-
oxycinnamate 3 (1.00 g, 5.2 mmol) was added, followed by
K2OsO2(OH)4 (14.7 mg, 0.04 mmol). The reaction mixture was
stirred for 1 h at 0 °C, and the reaction mixture was homoge-
neous at this point. The homogeneous mixture was stirred at 0
°C for an additional 1 h by which it transformed into a pale
yellow slurry. The crystalline precipitate was isolated by filtra-
tion. One wash with ice-cold EtOH-H2O (1:1, 5 mL) yielded 4
as a white solid (1.20 g, 64%, >99% ee):6 mp 114-115 °C; [R]23
D
(485 mg, 57%): mp >250 °C (dec); [R]23 -142 (c 0.65, CHCl3);
D
-5.3 (c 0.94, CHCl3); 1H NMR (CDCl3, 500 MHz) δ 7.33-7.28
(m, 7H), 6.86 (d, 2H, J ) 8.8 Hz), 5.58 (d, 1H, J ) 8.8 Hz), 5.19
(d, 1H, J ) 8.8 Hz), 5.08 (d, 1H, J ) 12.5 Hz), 5.04 (d, 1H, J )
12.5 Hz), 4.44 (s, 1H), 3.79 (s, 6H); 13C NMR (CDCl3, 125 MHz)
δ 159.3, 136.3, 130.9, 128.5, 128.2, 128.1, 128.0, 114.1, 73.5, 67.0,
56.0, 55.3, 53.1; MALDI-FTMS (DHB) m/z 382.1269 (M + Na+,
C19H21NO6 requires 382.1267). Anal. Calcd for C19H21NO6: C,
63.50; H, 5.89; N, 3.90. Found: C, 63.44; H, 5.92; N, 3.85.
Meth yl (2S,3R)-3-[(Ben zyloxyca r bon yl)a m in o]-2-[(ter t-
bu tyldim eth ylsilyl)oxy]-3-(4-m eth oxyph en yl)pr opion ate (5).
A solution of 4 (1.00 g, 2.78 mmol) in CH2Cl2 (10 mL) at 0 °C
was treated with 2,6-lutidine (0.97 mL, 8.35 mmol) followed by
TBDMSOTf (0.77 mL, 3.34 mmol). The reaction mixture was
stirred at 0 °C for 2.5 h. The reaction mixture was diluted with
EtOAc (20 mL) and washed sequentially with 10% aqueous HCl
(10 mL), H2O (15 mL), and saturated aqueous NaCl (10 mL).
The organic layer was dried (Na2SO4), filtered, and concentrated
in vacuo. Flash chromatography (SiO2, 25% EtOAc-hexane)
provided 5 as a clear oil (1.25 g, 95%): [R]23D -0.9 (c 1.1, CHCl3);
1H NMR (CDCl3, 500 MHz) δ 7.34 (s, 5H), 7.19 (d, 2H, J ) 8.8
Hz), 6.85 (d, 2H, J ) 8.8 Hz), 5.75 (d, 1H, J ) 8.8 Hz), 5.16 (d,
1H, J ) 8.8 Hz), 5.06 (s, 2H), 4.34 (s, 1H), 3.78 (s, 3H), 3.71 (s,
3H), 0.75 (s, 9H), -0.16 (s, 3H), -0.32 (s, 3H); 13C NMR (CDCl3,
125 MHz) δ 171.7, 159.1, 155.6, 136.4, 131.9, 131.4, 128.5, 128.2,
127.6, 113.8, 75.7, 66.9, 57.1, 55.3, 52.2, 25.5, 18.2, -5.6;
MALDI-FTMS (DHB) m/z 496.2123 (M + Na+, C25H35NO6Si
requires 496.2131). Anal. Calcd for C25H35NO6Si: C, 63.40; H,
7.45; N, 2.96. Found: C, 63.69; H, 7.71; N, 3.06.
1H NMR (CD3OD, 500 MHz) δ 4.60 (d, 1H, J ) 2.2 Hz), 4.33 (d,
1H, J ) 2.21), 1.42 (s, 9H), 0.93 (s, 9H), 0.12 (s, 6H); 13C NMR
(CD3OD, 125 MHz) δ 176.0, 173.1, 157.8, 81.0, 75.4, 58.3, 28.6,
26.2, 19.1, -5.0, -5.2; IR (film) vmax 2930, 1720, 1502 cm-1
;
MALDI-FTMS (DHB) m/z 385.1782 (M + Na+, C15H30N2O6Si
requires 385.1765).
L-th r eo-BocNH-â-OTBDMS-Asn -OBn (9). A solution of 8
(405 mg, 1.15 mmol) in DMF (5.5 mL) at 0 °C was treated with
NaHCO3 (190 mg, 2.25 mmol) and benzyl bromide (0.54 mL, 4.5
mmol). The reaction mixture was stirred at 0 °C for 2 h and
was allowed to warm to 25 °C and stirred for 24 h before 15 mL
of H2O was added at 0 °C. The mixture was extracted with
EtOAc (15 mL) and washed with H2O (2 × 10 mL). The organic
layer was dried (Na2SO4) and concentrated in vacuo. Flash
chromatography (SiO2, 25% EtOAc-hexane) provided 9 as a
white solid (428 mg, 84%): mp 45-47 °C; [R]23 -5.4 (c 1.5,
D
CHCl3); 1H NMR (CD3OD, 500 MHz) δ 7.36 (m, 5H), 6.38 (d,
1H, J ) 9.6 Hz), 5.19 (d, 1H, J ) 12.5 Hz), 5.13 (d, 1H, J ) 12.5
Hz), 4.63 (d, 1H, J ) 2.6 Hz), 4.58 (dd, 1H, J ) 9.5 Hz, 2.2 Hz),
1.41 (s, 9H), 0.90 (s, 9H), 0.08 (s, 3H), 0.03 (s, 3H); 13C NMR
(CD3OD, 125 MHz) δ 175.7, 171.4, 157.8, 136.6, 129.8, 129.6,
129.5, 81.2, 75.0, 68.6, 58.9, 28.6, 26.2, 19.0, -4.9, -5.2; IR (film)
vmax 2930, 1698, 1498, 1472 cm-1; MALDI-FTMS (DHB) m/z
475.2262 (M + Na+, C22H36N2O6Si requires 475.2240). Anal.
Calcd for C22H36N2O6Si: C, 58.38; H, 8.02; N, 6.19. Found: C,
58.59; H, 8.41; N, 5.71.
L-th r eo-BocNH-â-OH-Asn -OBn (10). A solution of 9 (727
mg, 1.61 mmol) in THF (10 mL) at 0 °C was treated with a
premixed solution of 1 M solution of Bu4NF in THF (4.8 mL,
4.8 mmol) and HOAc (0.276 mL, 4.8 mmol). The reaction mixture
was stirred at 0 °C for 30 min, diluted with EtOAc (20 mL), and
washed successively with saturated aqueous NaHCO3 (20 mL)
and saturated aqueous NH4Cl (20 mL). The organic layer was
dried (Na2SO4), filtered, and concentrated in vacuo. Flash
chromatography (SiO2, 75% EtOAc-hexane) provided 10 as a
Meth yl (2S,3R)-3-[(ter t-Bu tyloxycar bon yl)am in o]-2-[(ter t-
bu tyldim eth ylsilyl)oxy]-3-(4-m eth oxyph en yl)pr opion ate (6).
A solution of 5 (2.50 g, 5.28 mmol) and Boc2O (1.33 mL, 5.80
mmol) in CH3OH (100 mL) was treated with 10% Pd-C (50 mg).
The resulting black suspension was stirred under H2 (1 atm) at
(7) Sieber, P.; Riniker, B. Tetrahedron Lett. 1991, 32, 739.

6772 J . Org. Chem., Vol. 65, No. 20, 2000
Notes
white solid (515 mg, 95%): mp 134-135 °C; [R]23 -17 (c 1.5,
HOAc (0.4 mL) at 50 °C was treated successively with concen-
trated sulfuric acid (4 µL, 0.069 mmol) and acetic anhydride (27
µL, 0.288 mmol). The reaction mixture was stirred at 50 °C for
2.5 h and partitioned between EtOAc and saturated aqueous
NaHCO3. The aqueous layer was further extracted with EtOAc,
and the combined organic layers were dried (MgSO4), filtered,
and concentrated in vacuo. Flash chromatography (SiO2, 33%
EtOAc-hexane) afforded 2 as a white solid (52 mg, 64%): mp
D
CHCl3); 1H NMR (CD3OD, 400 MHz) δ 7.39-7.30 (m, 5H), 5.19
(s, 2H), 4.69 (d, 1H, J ) 2.0 Hz), 4.58 (d, 1H, J ) 2.0 Hz), 1.40
(s, 9H); 13C NMR (CD3OD, 125 MHz) δ 178.8, 174.3, 160.4, 139.4,
131.8, 131.5, 131.4, 83.2, 75.0, 70.5, 60.4, 30.9; IR (film) vmax
3346, 2976, 1684, 1561 cm-1; MALDI-FTMS (DHB) m/z 361.1370
(M + Na+, C16H22N2O6 requires 361.1369). Anal. Calcd for
C16H22N2O6: C, 56.80; H, 6.55; N, 8.28. Found: C, 56.44; H, 6.20;
N, 8.12.
74-75 °C; [R]23 -14 (c 0.93, CHCl3); 1H NMR (CD3OD, 500
D
L-th r eo-F m ocNH-â-OH-Asn -OBn (11). A solution of 10 (436
mg, 1.29 mmol) in EtOAc (20 mL) was treated with 4 N HCl-
EtOAc (6.5 mL). The reaction mixture was stirred for 50 min,
and the volatiles were removed. The resulting white residue was
dissolved in 1,4-dioxane (20 mL) and H2O (20 mL), and the
solution was treated with NaHCO3 (650 mg, 7.74 mmol) and
FmocCl (435 mg, 1.68 mmol). The reaction mixture was stirred
for 2 h and partitioned between saturated aqueous NaHCO3 (30
mL) and EtOAc (50 mL). The aqueous layer was extracted with
EtOAc (2 × 50 mL), and the combined organic layers were dried
(Na2SO4), filtered, and concentrated in vacuo. Flash chroma-
tography (SiO2, 75% EtOAc-hexane) provided 11 as a white
solid (463 mg, 78%): mp 175-176 °C; [R]23D -13 (c 0.45, CHCl3);
1H NMR (CD3OD, 400 MHz) δ 7.77 (d, 2H, J ) 7.4 Hz), 7.64 (d,
2H, J ) 1.8 Hz), 7.38-7.26 (m, 9H), 5.23 (d, 1H, J ) 12.5 Hz),
5.19 (d, 1H, J ) 12.5 Hz), 4.82 (d, 1H, J ) 1.8 Hz), 4.64 (d, 1H,
MHz) δ 7.78 (d, 1H, J ) 7.7 Hz), 7.77 (d, 1H, J ) 7.7 Hz), 7.66
(d, 1H, J ) 7.7 Hz), 7.61 (d, 1H, J ) 7.7 Hz), 7.38-7.17 (m, 24H),
5.21 (d, 1H, J ) 12.5 Hz), 5.18 (d, 1H, J ) 12.5 Hz), 4.80 (d, 1H,
J ) 2.2 Hz), 4.65 (d, 1H, J ) 2.2 Hz), 4.51 (dd, 1H, J ) 13.6, 9.9
Hz), 4.17 (m, 2H); 13C NMR (CD3OD, 125 MHz) δ 172.2, 171.8,
158.9, 145.7, 145.3, 145.0, 142.6, 142.5, 137.1, 129.9, 129.5, 129.2,
129.0, 128.9, 128.8, 128.7, 128.3, 128.2, 128.1, 126.5, 126.2, 120.9,
120.8, 73.4, 71.5, 68.6, 68.3, 58.6, 48.2; IR (film) vmax 3366, 3060,
1698, 1668, 1495 cm-1; MALDI-FTMS (DHB) m/z 752.2621
(M + Na+, C45H38N2O6 requires 752.2627). Anal. Calcd for
C45H38N2O6: C, 76.90; H, 5.45; N, 3.99. Found: C, 76.77; H, 5.81;
N, 3.68.
Ack n ow led gm en t. We gratefully acknowledge the
financial support of the National Institutes of Health
(CA41101), the Skaggs Institute of Chemical Biology,
and the Swiss National Science Foundation (postdoc-
toral fellowship, P.M.).
J ) 1.8 Hz), 4.35 (dd, 1H, J ) 9.9 Hz, 6.6 Hz), 4.22 (m, 2H); 13
C
NMR (CD3OD, 125 MHz) δ 176.4, 171.7, 145.2, 145.1, 142.5,
142.4, 137.0, 129.5, 129.2, 129.1, 128.8, 128.7, 128.2, 128.2, 126.3,
126.2, 120.1, 72.8, 68.4, 68.3, 58.6, 48.2; IR (film) vmax 3331, 1691,
1653, 1539 cm-1; MALDI-FTMS (DHB) m/z 483.1520 (M + Na+,
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra of 8 are available. This material is available free of
charge via the Internet at http://pubs.acs.org.
C
26H24N2O6 requires 483.1532). Anal. Calcd for C26H24N2O6: C,
67.82; H, 5.25; N, 6.08. Found: C, 67.64; H, 5.00; N, 6.03.
L-th r eo-F m ocNH-â-OH-Asn (Tr t)-OBn (2). A solution of 11
(53 mg, 0.115 mmol) and trityl alcohol (300 mg, 1.15 mmol) in
J O000628S