Asymmetric synthesis of erythro-β-hydroxyasparagine

Article abstract of DOI:10.1016/j.tetlet.2008.12.103

erythro-Hydroxyasparagine (eHyAsn, 1) occurs in a number of naturally occurring peptides and proteins. Previous syntheses have relied on enzyme-catalyzed reactions to produce relevant, optically active intermediates. We report herein a completely chemical

Full text of DOI:10.1016/j.tetlet.2008.12.103

Tetrahedron Letters 50 (2009) 1273–1275
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Asymmetric synthesis of erythro-b-hydroxyasparagine
*
Douglas Wong, Carol M. Taylor
Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
erythro-Hydroxyasparagine (eHyAsn, 1) occurs in a number of naturally occurring peptides and proteins.
Previous syntheses have relied on enzyme-catalyzed reactions to produce relevant, optically active inter-
mediates. We report herein a completely chemical synthesis that intercepts Boger’s synthesis of the dia-
stereomer (tHyAsn, 4), utilizing a Sharpless asymmetric aminohydroxylation reaction to introduce the
two stereocenters. Boc-HyAsn(OTBS)-OH (10) was coupled effectively with phenylalanine methyl ester
using EDC/HOBt.
Received 14 November 2008
Revised 18 December 2008
Accepted 22 December 2008
Available online 8 January 2009
Ó 2009 Elsevier Ltd. All rights reserved.
b-Hydroxy-
L
-asparagine was identified as a normal constituent
selective hydrolysis of eHyAsp diamide (3), using leucine
in human urine by Tominaga et al. in 1963.1 Okai and Izumiya sub-
aminopeptidase.2
sequently demonstrated that this metabolite was the erythro
The diastereomeric threo-b-hydroxy-L-asparagine (tHyAsn, 4,
(2S,3R) diastereomer.2 erythro-b-Hydroxy-
L
-asparagine (1, eHyAsn)
Scheme 1) features in ramoplanin and lysobactin, lipoglycodepsi-
peptides that show promise against vancomycin-resistant bacte-
ria.16 Due to the importance of these antibiotics, the synthesis of
tHyAsn has drawn the attention of the Boger17 and Van-
Nieuwenhze18 groups. We felt that we could readily intercept Bo-
ger’s synthesis,17 invert the b-OH, and rapidly produce a useful
eHyAsn building block for peptide synthesis.
has subsequently been identified in an integral membrane protein
precursor to the epidermal growth factor (EGF),3 one of the sub-
components of the complement protein C1s,4 and in antifungal
peptides, including the theonellamides5 and the xylocandins.6
There have been a number of syntheses of b-hydroxyaspartic
acid reported. A couple of approaches have capitalized on the
Sharpless asymmetric dihydroxylation reaction followed by
manipulations to introduce the amino group with retention7 or
inversion8 of configuration. Hanessian and Vanasse used internal
asymmetric induction to hydroxylate aspartic acid derivatives.9
Cardillo et al. similarly introduced an iodide at Cb; the isolated
oxazoline was then hydrolyzed to give free b-hydroxyaspartic
acid.10 Leckta and co-workers described a novel benzoylquinine-
As reported previously,17 the Sharpless asymmetric amin-
ohydroxylation reaction gave a good yield of protected amino
alcohol 6 in excellent enantiomeric excess (Scheme 2). We then
conducted a Mitsunobu reaction with p-nitrobenzoic acid as the
nucleophile.19 The ester was readily removed by azidolysis to
give 7. The hindered alcohol was protected as its TBDMS ether
and the methyl ester converted to the side chain primary
amide,20 giving compound 8.
catalyzed reaction between acid chlorides and
a-chloroglycine to
give various b-substituted aspartic acids.11 If the two carboxylic
acids of b-hydroxyaspartic acid could be differentiated, these syn-
theses might be applicable to b-HyAsn. However, in most cases,
this transformation is not straightforward.
Recent reports by Jung and coworkers indicated that it is pos-
sible to oxidatively degrade a p-methoxyphenyl aromatic ring in
There have been two approaches directed specifically toward
eHyAsn (1) (Scheme 1). Sendai et al. combined aspects of two ear-
lier reports:12 their starting material was (ꢀ)-trans-epoxysuccinic
acid (2), from the fermentation of glucose by Aspergillus fumiga-
tus.13 The epoxide was ammonolysed to give erythro-b-hydroxy-
HOOC
O
S
H2N
COOH
OH
HOOC
2
H NOC
R
2
H2N
CONH2
OH
erythro-HyAsn
L-aspartic acid and then transformed to 1 according to Singerman
(1)
and Liwschitz.14 This approach was also adopted by Tohdo,
Hamada, and Shioiri to produce a building block for their theonel-
lamide F synthesis.15 The second route to eHyAsn involved a regio-
H2NOC
3
S
H2N
COOH
OH
COOMe
H2NOC
S
threo-HyAsn
5
MeO
(4)
* Corresponding author. Tel.: +1 225 578 4751; fax: +1 225 578 3458.
E-mail address: cmtaylor@lsu.edu (C.M. Taylor).
Scheme 1. Previous approaches to hydroxyasparagines.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.12.103

1274
D. Wong, C. M. Taylor / Tetrahedron Letters 50 (2009) 1273–1275
NHCbz
COOMe
BnOCONH2
1. DIAD, PPh3, THF
p-NO2-C6H4-COOH
nPrOH, aq. NaOH
COOMe
PMP = p-methoxyphenyl
PMP
PMP
2. NaN3, MeOH
(66%; 2 steps)
(DHQD)2PHAL
K2OsO2(OH)4
tBuOCl
59% yield
99% e.e.
OH
5
6
NHBoc
CONH2
NHCbz
CONH2
NHCbz
COOMe
1. TBDMSOTf
lutidine, CH2Cl2
H2, Pd/C
Boc2O, MeOH
PMP
PMP
PMP
2. NH3, MeOH
2 weeks
(83%; 2 steps)
OTBDMS
OTBDMS
OH
(98%)
8
9
7
NHBoc
R
NHBoc
H
N
HCl.Phe.OMe
EDC, Et3N
RuCl3.3H2O
MeOOC
Ph
CONH2
OTBDMS
NaIO4, NaHCO3
HOOC
10
H2O/MeCN/EtOAc
(8:1:1)
O
OTBDMS
HOBt, THF
11 R = CONH2 (50%; 2 steps)
12 R = CN
Scheme 2. Synthesis of an eHyAsn building block and incorporation into dipeptide 11.
Acknowledgments
O
O
O
X
COO
X
O
The authors thank the National Science Foundation (CH-
O
O
0809143) for support of this work. We are also grateful to Professor
Dale L. Boger and Dr. Richard Lee for advice on the oxidative deg-
radation of the aromatic ring.
C
N
N
NH
NH2
H
H
R3N
R3N
isoaspartimide
References and notes
Scheme 3. Mechanism of b-cyanoalanine formation.
1. Tominaga, F.; Hiwai, C.; Maekawa, T.; Yoshida, H. J. Biochem. Jpn. 1963, 53, 227–
230.
2. Okai, H.; Izumiya, N. Bull. Chem. Soc. Jpn. 1969, 42, 3550–3555.
3. Valcarace, C.; Björk, I.; Stenflo, J. Eur. J. Biochem. 1999, 260, 200–207.
4. Luo, C.; Thielens, N. M.; Gagnon, J.; Gal, P.; Sarvari, M.; Tseng, Y.; Tosi, M.;
Zavodszky, P.; Arlaud, G. J.; Schumaker, V. N. Biochemistry 1992, 31, 4254–4262.
5. (a) Matsunaga, S.; Fusetani, N.; Hashimoto, K.; Wälchli, M. J. Am. Chem. Soc.
1989, 111, 2582–2588; (b) Bewley, C. A.; Faulkner, D. J. J. Org. Chem. 1994, 59,
4852; (c) Bewley, C. A.; Faulkner, D. J. J. Org. Chem. 1995, 60, 2644; (d)
Matsunaga, S.; Fusetani, N. J. Org. Chem. 1995, 60, 1177–1181; (e) Schmidt, E.
W.; Bewley, C. A.; Faulkner, D. J. J. Org. Chem. 1998, 63, 1254–1258.
6. Bisacchi, G. S.; Hockstein, D. R.; Koster, W. H.; Parker, W. L.; Rathnum, M. L.;
Unger, S. E. J. Antibiot. 1987, 40, 1520–1529.
the presence of a benzyl carbamate.21 We tried unsuccessfully to
unmask the -COOH from compound 8 and resorted to switch-
ing carbamate protecting groups. The oxidative degradation of
compound had to be conducted on reasonable scale
(500 mg or more) with a mechanical stirrer. In addition to the
issues discussed for conducting the reaction, careful control of
pH during workup and exhaustive extraction of the aqueous
layer are vital. The optical rotation, while somewhat larger than
that reported by Tohdo et al.,15 was the same sign and of similar
magnitude.
We next sought to form a dipeptide, relevant to theonella-
mide C,5d and to test whether or not the side chain amide would
require protection during this amide bond formation. Tohdo
et al. had coupled 10 with a p-bromo-phenylalanine-b-alanine
dipeptide ester using diethyl-phosphorylcyanide (DEPC, DMF,
DIEA) in high yield.15 The groups of both Boger and Van-
Nieuwenhze protected the side chain functionality as a trityla-
mide. The potential side reaction of the unprotected primary
amide, upon activation of the carboxylic acid in 10, is isoaspar-
timide formation, followed by dehydration to give b-cyanoala-
nine derivative 12 (Scheme 3).22 Indeed, this was the major
product isolated on coupling with BOP/DIEA. As has been de-
scribed elsewhere,23 the most effective conditions to circumvent
this side reaction involve a carbodiimide-mediated coupling and
inclusion of hydroxybenzotriazole (HOBt) in the reaction mix-
ture. The role of the latter additive is to serve as a superior pro-
ton donor, relative to the NH of the isoaspartimide. Thus,
formation of the BtOꢀ anion occurs in preference to dehydration
to generate a nitrile.
a
9
a
7. Cho, G. Y.; Ko, S. Y. J. Org. Chem. 1999, 64, 8745–8747.
8. Deng, J.; Hamada, Y.; Shioiri, T. J. Am. Chem. Soc. 1995, 117, 7824–7825.
9. Hanessian, S.; Vanasse, B. Can. J. Chem. 1993, 71, 1401–1406.
10. Cardillo, G.; Gentilucci, L.; Tolomelli, A.; Tomasini, C. Synlett 1999, 1727–1730.
11. Hafez, A. M.; Dudding, T.; Wagerle, T. R.; Shah, M. H.; Taggi, A. E.; Lectka, T. J.
Org. Chem. 2003, 68, 5819–5825.
12. Sendai, M.; Hashigughi, S.; Tomimoto, M.; Kishimoto, S.; Matsuo, T.; Ochaiai, M.
Chem. Pharm. Bull. 1985, 33, 3798–3810.
13. Miller, M. W. J. Med. Chem. 1963, 36, 233–237.
14. Singerman, A.; Liwschitz, Y. Tetrahedron Lett. 1968, 46, 4733–4735.
15. (a) Tohdo, K.; Hamada, Y.; Shiori, T. Pept. Chem. 1992, 7–12; (b) Tohdo, K.;
Hamada, Y.; Shioiri, T. Synlett 1994, 247–249.
16. (a) Fulco, P.; Wenzel, R. P. Exp. Rev. Anti-Inf. Ther. 2006, 4, 939–945; (b) Walker,
S.; Chen, L.; Hu, Y.; Rew, Y.; Shin, D.; Boger, D. L. Chem. Rev. 2005, 105, 449–475;
(c) McCafferty, D. G.; Cudic, P.; Frankel, B. A.; Barkallah, S.; Kruger, R. G.; Li, W.
Biopolymers 2002, 66, 261–284.
17. (a) Boger, D. L.; Lee, R. J.; Bounaud, P.-Y.; Meier, P. J. Org. Chem. 2000, 65, 6770–
6772; (b) Jiang, W.; Wanner, J.; Lee, R. J.; Bounaud, P.-Y.; Boger, D. L. J. Am.
Chem. Soc. 2003, 125, 1877–1887.
18. Guzman-Martinez, A.; VanNieuwenhze, M. S. Synlett 2007, 1513–1516.
19. Compound 7. p-Nitrobenzoic acid (3.94 g, 23.6 mmol, 2.2 equiv) was added to a
solution of compound 6 (3.86 g, 10.7 mmol, 1.0 equiv) in dry THF (130 mL).
Triphenylphosphine (6.19 g, 23.6 mmol, 2.2 equiv) was added, and then the
mixture was cooled to 0 °C under N2. Diisopropyl azodicarboxylate (4.65 mL,
4.77 g, 23.6 mmol, 2.2 equiv) was slowly added via syringe. Upon completion
of the addition, the ice bath was removed, and the contents were stirred at
room temperature for 2 d. The reaction mixture was concentrated and the
residue dissolved in anhydrous MeOH (130 mL). Sodium azide (3.49 g,
53.6 mmol, 5.0 equiv) was added and the mixture was heated at 45 °C for
3.5 d. The solvent was removed under reduced pressure and the product was
isolated from the residue by flash chromatography (Hex–EtOAc, 3:1) to give 7
The crude acid 10 was subjected directly to the peptide cou-
pling reaction.24 The isolated yield of dipeptide 11 is 50% over
the two steps.
In summary, we have prepared eHyAsn building block 10 in se-
ven steps and 23% overall yield from methyl p-methoxycinnamate.
We have found conditions to form a dipeptide, and demonstrated
that this can be done without protection of the side-chain amide
functionality.
as a pale yellow gum (2.56 g, 66%). Rf 0.32 (1:1 Hex–EtOAc); ½a D25:0
ꢀ20.8 (c 0.8,
ꢁ
CHCl3); 1H NMR (CDCl3, 400 MHz) d 2.92 (d, J = 6.5 Hz, 1H), 3.69 (s, 3H), 3.77 (s,
3H), 4.59 (dd, J = 6.5, 3.3 Hz, 1H), 5.05 (d, J = 12.2 Hz, 1H), 5.11 (d, J = 12.2 Hz,
1H), 5.10–5.13 (m, 1H), 5.83 (d, J = 8.6 Hz, 1H), 6.82 (d, J = 8.5 Hz, 2H), 7.17 (d,
J = 8.5 Hz, 2H), 7.34 (m, 5H); 13C NMR (CDCl3, 100 MHz) d 52.4, 55.1, 56.4, 66.8,
73.1, 113.8, 128.1, 128.3, 128.4, 128.6, 136.2, 155.5, 159.3, 172.1; HRMS (ESI)
calcd for C19H20NO6 (MꢀH)+ 358.1296, obsd 358.1295.

D. Wong, C. M. Taylor / Tetrahedron Letters 50 (2009) 1273–1275
1275
20. Boger and co-workers previously explored and discussed other approaches to
without purification. Rf 0.64 (6:4:1 CHCl3–MeOH–H2O); ½a D24:0
ꢁ
+51.1 (c 1.0,
primary amide formation.17b Ammonolysis, while slow due to steric hindrance,
proved to be the most satisfactory route.
MeOH), lit.15 a 2D4:0 +40.9 (c 1.0, MeOH); 1H NMR (CD3OD, 400 MHz) d 0.15 (s,
½ ꢁ
3H), 0.16 (s, 3H), 0.94 (s, 9H), 1.45 (s, 9H), 3.83 (s, 1H), 4.57 (d, J = 9.7 Hz, 1H),
13C NMR (CD3OD, 100 MHz) d ꢀ5.1, ꢀ4.9, 19.0, 26.2, 28.7, 58.7, 75.3, 81.0,
157.1, 171.7, 175.9; HRMS (ESI) calcd for C15H31N2O6Si (M+H)+ 363.1945, obsd
363.1955. (b) Dipeptide 11. A portion of acid 10 (266 mg, ꢃ0.7 mmol, 1.0 equiv)
was dissolved in anhydrous THF (15 mL) and the resulting solution was cooled
21. (a) Kim, I. S.; Ji, Y. J.; Jung, Y. H. Tetrahedron Lett. 2006, 47, 7289–7293; (b) Kim,
I. S.; Byu, C. B.; Li, Q. R.; Zee, O. P.; Jung, Y. H. Tetrahedron Lett. 2007, 48, 6258–
6261.
22. (a) Gish, D. T.; Katsoyannis, P. G.; Hess, G. P.; Stedman, R. J. J. Am. Chem. Soc.
1956, 78, 5954; (b) Ressler, C. J. Am. Chem. Soc. 1956, 78, 5956–5957; (c)
Stammer, C. H. J. Org. Chem. 1961, 26, 2556–2560.
23. (a) Mojsov, S.; Mitchell, A. R.; Merrifield, R. B. J. Org. Chem. 1980, 45, 555–560;
(b) Gausepohl, H.; Kraft, M.; Frank, R. W. Int. J. Pept. Prot. Res. 1989, 34, 287–
294.
24. Dipeptide 11. (a) Acid 10. Sodium periodate (5.504 g, 26 mmol, 18.1 equiv) in
H2O (128 mL) was added to a solution of compound 9 (604 mg, 1.4 mmol.
1.0 equiv) in EtOAc–CH3CN (1:1, 32 mL). The resulting solution was stirred
mechanically at rt for 30 min. Ruthenium trichloride trihydrate (60 mg,
0.3 mmol, 20 mol %) was added, followed by NaHCO3 (464 mg, 5.5 mmol,
3.9 equiv). The reaction mixture was stirred mechanically at rt overnight. The
yellow solution was diluted with satd aqueous NaHCO3 (240 mL) and extracted
with CH2Cl2 (160 mL). The organic layer was washed with satd aqueous
NaHCO3 (240 mL) again. The combined aqueous layers were acidified with 10%
aqueous HCl to pH 2.5 at 0 °C and extracted with EtOAc (6 ꢂ 300 mL). The
combined organic layers were dried over MgSO4, filtered, and concentrated to
give compound 10 as a brown foam (376 mg, 73%). This material was used
to 0 °C.
1.0 equiv) was added and the solution was stirred at 0 °C for 15 min.
Triethylamine (204 L, 149 mg, 1.5 mmol, 2.0 equiv) was added and the
L-Phenylalanine methyl ester hydrochloride (158 mg, 0.7 mmol,
l
solution was stirred at 0 °C for 10 min. 1-(3-Dimethylamino-propyl)-3-ethyl-
carbodiimide hydrochloride (148 mg, 0.8 mmol, 1.05 equiv) was added,
followed by HOBt hydrate (149 mg, 1.1 mmol, 1.5 equiv) at 0 °C. The reaction
mixture was allowed to stir for 20 min at 0 °C and then warmed to rt and
stirred overnight. The solvent was removed and the product was isolated from
the residue by flash chromatography (Hex–EtOAc 4:1?Hex–EtOAc 1:1) to give
a colorless foam (262 mg, 68%; 50% over two steps). Rf 0.33 (1:1 Hex–EtOAc);
mp 49–50 °C; ½a D25:0
ꢁ
+48.6 (c 1.0, CHCl3); 1H NMR (CDCl3, 400 MHz) d 0.10 (s,
3H), 0.13 (s, 3H), 0.90 (s, 9H), 1.44 (s, 9H), 3.08 (d, J = 5.4 Hz, 2H), 3.65 (s, 3H),
4.60–4.70 (m, 2H), 4.80 (d, J = 6.5 Hz, 1H), 5.44 (s, 1H), 5.98 (s, 1H), 6.54 (s, 1H),
6.60 (d, J = 7.4 Hz, 1H), 7.12–7.30 (m, 5H); 13C NMR (CDCl3, 100 MHz) d ꢀ5.3,
ꢀ5.1, 18.0, 25.6, 28.3, 37.8, 52.2, 53.5, 57.4, 74.0, 80.1, 127.1, 128.6, 129.3,
135.7, 154.9, 167.5, 171.4, 174.3; HRMS (ESI) calcd for C25H40N3O7Si (MꢀH)+
522.2641, obsd 522.2654.