Synthesis of tryptophan N-glucoside

Article abstract of DOI:10.1016/j.tetlet.2003.10.190

The naturally occurring L-tryptophan N-glucoside was synthesized using 2-O-pivaloylated glucosyl trichloroacetimidate, which gave β-N ^In-glucosides. From 2-O-acetylated donors only tryptophan-1-yl- ethylidene compounds (amide acetals) were obtained. The employment of α-azido L-tryptophan benzyl ester facilitated purification and deprotection and improved the yields of the glycosylation step.

Full text of DOI:10.1016/j.tetlet.2003.10.190

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 295–297
Synthesis of tryptophan N-glucoside
*
€
Melanie Schnabel, Birgit Rompp, Daniel Ruckdeschel and Carlo Unverzagt
Bioorganische Chemie, Geb€aude NWI, Universit€at Bayreuth, 95440 Bayreuth, Germany
Received 3 September 2003; revised 22 October 2003; accepted 29 October 2003
This work is dedicated to Prof. W. Steglich on the occasion of his 70th birthday
Abstract—The naturally occurring L-tryptophan N-glucoside was synthesized using 2-O-pivaloylated glucosyl trichloroacetimidate,
which gave b-NIn-glucosides. From 2-O-acetylated donors only tryptophan-1-yl-ethylidene compounds (amide acetals) were
obtained. The employment of a-azido L-tryptophan benzyl ester facilitated purification and deprotection and improved the yields of
the glycosylation step.
ꢀ 2003 Elsevier Ltd. All rights reserved.
The amino acid tryptophan shows many biological
functions. Tryptophan and its derivatives are of great
interest due to their complex metabolism involving
important metabolites and related diseases.1 Besides the
many examples of known tryptophan-containing natu-
ral products, a growing number of recently discovered
C or N-linked tryptophan glycoconjugates has attracted
hexoses4a and tryptophan. Cyclic peptides have been
reported to react at the indole nitrogen of tryptophan
using either a glycal7a or a galactosyl bromide.7b
N-Glycosyl-indoles have been obtained via the glyco-
sylation of substituted indoles8a;b or indolines followed
by oxidation to the corresponding N-glycosyl-indole.9
interest. The C-mannosyl-
L
-tryptophan 12a;b was
We report a chemical synthesis for tryptophan N-
glucoside 2 suitable for preparing the amounts required
for biological studies. Initially, we envisioned the
formation of the b-linked tryptophan-N-glucoside from
tetra-acetyl-glucosylimidate 4 and Cbz–Trp–OBzl 5.10
The glycosylation was initiated with boron trifluoride
etherate as an activator and led to the unexpected
tryptophan amide acetal 7, which can be viewed as an
aza analogue of a glycosyl orthoester (Fig. 2). Only one
stereoisomer was found, which was arbitrarily assigned.
The isolation of stable indole-1-yl-ethylidene glycosides
has been reported in the literature.8 To determine if the
formation of orthoester-like indolyl glycosides was
related to the protecting groups of the amino acid
moiety, the azido-protected tryptophan 6 was employed.
This compound was synthesized from tryptophan benzyl
ester10a in 68% yield following a procedure introduced
by Vasella et al.11 However, even in the presence of the
strong activator TMSOTf the reaction of the azide 6
with donor 4 only gave amide acetal 8,12 suggesting a
low tendency for this compound to rearrange to the
desired N-glycoside.
initially discovered in human RNase and several glyco-
proteins of the complement system.2c Fruits and
other foods were found to be the source of glyco-tetra-
hydro-b-carboline-3-carboxylic acids and various
L
-tryptophan-N-glycosides with pentoses as the sugar
L
part.3 -Tryptophan N-glucoside 24a (Fig. 1) was pre-
viously discovered as a novel tryptophan metabolite in
fruits and foods formed by an enzymatic process.4b The
N-acylated derivative 3 was isolated from the flowers of
Pueraria Lobata, a drug used in traditional Chinese
medicine.5
There are only a few examples of synthetic tryptophan
N-glycosides. In 1985 an N-glucosylated Na-acetyl-
D,L-
tryptophan was obtained from Na-acetyl-
L-tryptophan
and glucose in low yield.6 Herderich et al. found ana-
lytical amounts of N- and C-glycosylated tryptophan
derivatives among glyco-tetrahydro-b-carbolines in
acid-catalyzed condensations between pentoses3 or
Keywords: Tryptophan N-glucoside; Tryptophan; Glycosylation.
* Corresponding author. Tel.: +49-921-552670; fax: +49-921-555365;
e-mail: carlo.unverzagt@uni-bayreuth.de
To avoid the formation of the orthoester-like com-
pounds, replacement of the 2-acetyl moiety of donor 4
0040-4039/$ - see front matter ꢀ 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.10.190

296
M. Schnabel et al. / Tetrahedron Letters 45 (2004) 295–297
H2N
COOH
RHN
COOH
HO
OH
OH
O
O
OH
HO
HO
N
N
H
OH
OH
2 R = H
3 R = Caproyl
1
Figure 1. Structures of tryptophan C-mannoside 1 and tryptophan N-glucosides 2 and 3.
R
COOBzl
OAc
OAc
a, b
O
O
AcO
AcO
AcO
AcO
AcO
O
O
O
CCl3
NH
R
N
H
H3C
N
COOBzl
4
5 R = NH-Cbz
6 R = N3
7 R = NH-Cbz
8 R = N3
Figure 2. Reagents and conditions: (a) 4 + 5, 0.14 equiv BF3ÆEt2O, CH2Cl2, 0 ꢁC, 42%; (b) 4+6, 0.1 equiv TMSOTf, CH2Cl2, )10 ꢁC, 26%.
with the bulky 2-pivaloyl residue as introduced by Kunz
and Harreus13 was planned. This protecting group has
been described as giving N-glycosylated indoles.8b
etherate gave the desired tryptophanyl-N-glucoside 12 in
17% yield (Fig. 3). Using TMSOTf as an activator gave
better yields (27%) but in both cases the product 12
could not be purified completely by flash chromatogra-
phy. Thus, the a-azido-tryptophan-benzyl ester 6 was
examined as an acceptor. The reaction of azide 6, gly-
cosyl donor 11 and boron trifluoride etherate as an
activator furnished the N-glucosylated tryptophan 13,16
which could easily be purified by flash chromatography
(23% yield). When TMSOTf was used for activation the
yield could be raised to 43%.
The required trichloroacetimidate building block 11
(Fig. 3) was synthesized from tetra-acetyl-glucose 9.14
After pivaloylation of OH-2 the anomeric acetate was
cleaved with hydrazine acetate and the intermediate
hemiacetal was converted to the imidate 11 in a DBU-
catalyzed reaction with trichloroacetonitrile,15 giving an
overall yield of 78% over the two steps.
The glycosylation of tryptophan derivative 5 with the
pivaloylated donor 11 activated by boron trifluoride
The deprotection of tryptophan N-glucoside 13 was
accomplished via a two step procedure. Catalytic
OAc
OAc
O
OAc
a
b
O
O
AcO
AcO
AcO
AcO
AcO
AcO
PivO
HO
PivO
O
CCl3
NH
OAc
OAc
9
10
11
COOBzl
H2N
R
COOH
c or d
e or f
OAc
O
OH
11 + 5
11 + 6
g, h
O
AcO
HO
HO
N
N
AcO
PivO
OH
12 R = NH-Cbz
13 R = N3
2
Figure 3. Reagents and conditions: (a) 5 equiv PivCl, py, CHCl3, rt, 91%; (b) (i) 1.5 equiv H2N–NH3OAc, DMF, rt; (ii) 0.1 equiv DBU, 5 equiv
CCl3CN, 0 ꢁC, 78% (over two steps); (c) 1.2 equiv BF3ÆEt2O, CH2Cl2, )10 ꢁC, 17%; (d) 0.1 equiv TMSOTf, CH2Cl2, )10 ꢁC, 27%; (e) 0.6 equiv
BF3ÆEt2O, CH2Cl2, )10 ꢁC, 23%; (f) 0.1 equiv TMSOTf, CH2Cl2, )15 ꢁC, 43%; (g) PdO–H2O, H2, MeOH, rt, 42%; (h) MeNH2 40% in H2O, rt, 77%.

M. Schnabel et al. / Tetrahedron Letters 45 (2004) 295–297
297
10. (a) Arai, J.; Muramatsu, J. J. Org. Chem. 1983, 48, 121–
123; (b) Guillaume, H. A.; Perich, J. W.; Johns, R. B.;
Tregear, G. W. J. Org. Chem. 1989, 54, 1664–1668.
11. Vasella, A.; Witzig, C.; Chiara, J.-L.; Martin-Lomas, M.
Helv. Chim. Acta 1991, 74, 2073–2077.
hydrogenation over palladium oxide-hydrate removed
the benzyl ester and reduced the azido group simulta-
neously. Subsequently, complete deacetylation was car-
ried out with 40% aqueous methylamine.17 After
purification using size exclusion chromatography
(Superdex 30) the target molecule 2 was obtained in 77%
yield. NMR spectra were recorded in the same solvent as
reported in the literature and showed excellent accor-
dance.4a;18 In conjunction with the optical rotation the
structural assignment of the isolated compound was
confirmed by total synthesis.
12. Compound 8: ESI-MS: C32H34N4O11 Mr (calcd) 650.22,
21
M (found) 673.18 (M+Naþ); ½aŠ )1.1 (c 1.0, dichlorom-
D
ethane); 1H NMR (360MHz, [ d6]-DMSO): d 7.65 (d,
J6;7 ¼ 8:2 Hz, 1H, H-7), 7.54 (d, J4;5 ¼ 7:8 Hz, 1H, H-4),
7.33–7.18 (m, 7H, Ph, H-2, H-6), 7.09 (dd, J4;5 ¼ 7:8 Hz,
J5;6 ¼ 7:3 Hz, 1H, H-5), 5.78 (d, J1 ;2 ¼ 5:7 Hz, 1H, H-10),
5.14–5.10(m, 3H, CH 2–Ph, H-30), 4.84–4.81 (m, 1H, H-40),
4.63–4.59 (m, 1H, H-a), 4.21–4.08 (m, 4H, H-20, H-50, H-
60), 3.18 (dd, Jgem ¼ 14:8 Hz, Jvic ¼ 5:7 Hz, 1H, H-ba), 3.13
(dd, Jgem ¼ 14:8 Hz, Jvic ¼ 7:6 Hz, 1H, H-bb), 2.10, 2.05,
1.97, 1.84 (4s, 12H, CH3); 13C NMR (90MHz, [ d6]-
DMSO): d 170.2, 169.8, 169.5, 168.9 (4C@O), 135.3 (Cq,
Ph), 134.0(C-7a), 129.1 (C-3a), 128.6–127.9 (CH, Ph),
124.1 (C-2), 122.3 (C-6), 119.9 (C-5), 118.7 (C-4), 112.4
(C00), 112.3 (C-7), 110.0 (C-3), 96.8 (C-10, JC;H ¼ 186:7 Hz),
72.2 (C-20), 68.9 (C-30), 67.7 (C-40), 66.8 (CH2–Ph), 66.5
(C-50), 62.8 (C-60), 61.3 (C-a), 26.9 (C-b), 23.8 (CH3), 20.7,
20.6, 20.5 (CH3, OAc).
0
0
In conclusion we have developed a strategy to obtain the
natural product N-glucosyl-tryptophan 2 by chemical
synthesis. Key steps involve the introduction of a
2-pivaloyl moiety to suppress the formation of the
tryptophan-1-yl amide acetals and the use of an a-azido
tryptophan derivative for improved yields. This chemi-
cal synthesis can provide sufficient amounts of N-glu-
cosyl-tryptophan to conduct biological studies.
13. Kunz, H.; Harreus, A. Liebigs Ann. Chem. 1982, 41–48.
14. Helferich, B.; Zirner, J. Chem. Ber. 1962, 95, 2604–2611.
15. Schmidt, R. R.; Kinzy, W. Adv. Carbohydr. Chem.
Biochem. 1994, 50, 21–123.
Acknowledgements
16. Compound 13: ESI-MS: C35H40N4O11 Mr (calcd) 692.27,
24
M (found) 715.31 (M+Naþ); ½aŠ )20.0 (c 0.4, dichloro-
We are grateful to Degussa AG for a generous donation
of chemicals.
D
methane); IR (KBr) m ¼ 2108:6 cmꢀ1 azide; 1H NMR
(360MHz, [ d6]-DMSO): d 7.63 (d, J6;7 ¼ 8:4 Hz, 1H, H-
7), 7.53 (d, J4;5 ¼ 7:7 Hz, 1H, H-4), 7.40–7.32 (m, 5H, Ph),
7.27 (s, 1H, H-2), 7.19 (dd, J6;7 ¼ 8:4 Hz, J5;6 ¼ 7:6 Hz, 1H,
H-6), 7.06 (dd, J4;5 ¼ 7:7 Hz, J5;6 ¼ 7:6 Hz, 1H, H-5), 6.22
(d, J1;2 ¼ 8:6 Hz, 1H, H-10), 5.59–5.48 (m, 2H, H-20, H-30),
5.25–5.15 (m, 3H, CH2–Ph, H-40), 4.54–4.50(m, 1H, H- a),
4.33–4.29 (m, 1H, H-50), 4.15–4.02 (m, 2H, H-6a0, H-6b0),
3.18 (dd, Jgem ¼ 14:9 Hz, Jvic ¼ 5:2 Hz, 1H, H-ba), 3.07 (dd,
Jgem ¼ 14:9 Hz, Jvic ¼ 8:0Hz, 1H, H- bb), 2.04, 1.96, 1.93,
(3 · s, total 9H, Ac), 0.67 (s, 9H, Piv); 13C NMR (90MHz,
[d6]-DMSO): d 175.5 (COOBzl), 170.1, 169.7, 169.5, 169.4
(4CO), 136.3 (C-7a), 135.4 (Cq, Ph), 128.5, 128.2, 128.1
(CH, Ph), 127.7 (C-3a), 124.2 (C-2), 122.1 (C-6), 120.0 (C-
5), 118.7 (C-4), 110.8 (C-3), 110.4 (C-7), 81.1 (C-10), 73.0
References and Notes
1. Sainio, E. L.; Pulkki, K.; Young, S. N. Amino Acids 1996,
10, 21–47.
€
€
2. (a) Hofsteenge, J.; Muller, D. R.; de Beer, T.; Loffler, A.;
Richter, W. J.; Vliegenthart, J. F. G. Biochemistry 1994,
€
33, 13524–13530; (b) Gade, G.; Kellner, R.; Rinehart, K.
L.; Proefke, M. L. Biochem. Biophys. Res. Commun. 1992,
189, 1303–1309; (c) Hofsteenge, J.; Blommers, M.; Hess,
D.; Furmanek, A.; Miroshnichenko, O. J. Biol. Chem.
1999, 274, 32786–32794.
0
(C-50), 72.5 (C-30), 69.7 (C-20), 68.0(C-4 ), 66.9 (CH2–Ph),
3. Diem, S.; Albert, J.; Herderich, M. Eur. Food Res.
Technol. 2001, 213, 439–447.
62.2 (C-60), 61.5 (C-a), 37.9 (Cq, Piv), 26.6
(C-b), 26.1 (CH3, Piv), 20.5, 20.4, 20.1 (CH3, Ac).
17. Griffin, B. E.; Jarman, M.; Reese, C. B. Tetrahedron 1968,
24, 639–662.
4. (a) Gutsche, B.; Grun, C.; Scheutzow, D.; Herderich, M.
Biochem. J. 1999, 343, 11–19; (b) Diem, S.; Bergmann, J.;
Herderich, M. J. Agric. Food Chem. 2000, 48, 4913–4917.
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1988, 36, 4171–4173.
6. Nyhammar, T.; Pernemalm, P.-A. Food Chem. 1985, 17,
289–296.
7. (a) Kessler, H.; Michael, K.; Kottenhahn, M. Liebigs Ann.
Chem. 1994, 811–816; (b) Itazaki, H.; Fujiwara, T.; Sato,
A.; Kawamaru, Y.; Matsumoto, K. 1995, JP7109299;
Chem. Abstr. 1995, 701969.
8. (a) Sokolova, T. N.; Shevchenko, V. E.; Preobrazhens-
kaya, M. N. Carbohydr. Res. 1980, 83, 249–261; (b)
Buchanan, J. G.; Stoddart, J.; Wightman, R. H. J. Chem.
Soc., Perkin Trans. 1 1994, 1417–1426.
9. Preobrazhenskaya, M. N.; Vigdorchik, M. M.; Suvorov,
N. N. Tetrahedron 1967, 23, 4653–4660, and references
cited therein.
18. Compound 2: ESI-MS: C17H22N2O7 Mr (calcd) 366.1427,
21
M (found) 367.1507 (M+H)þ; ½aŠ )19.0( c 0.2, water);
D
25
D
1
lit.4b: ½aŠ )20.3 (c 0.2, water); H NMR (360MHz, [ d4]-
MeOH+1% TFA): d 7.62 (d, J4;5 ¼ 7:8 Hz, 1H, H-4), 7.58
(d, J6;7 ¼ 8:2 Hz, 1H, H-7), 7.36 (s, 1H, H-2), 7.23
(dd, J6;7 ¼ 8:2 Hz, J5;6 ¼7:1 Hz, 1H, H-6), 7.14
(dd, J4;5 ¼ 7:8 Hz, J5;6 ¼ 7:1 Hz, 1H, H-5), 5.51
(d, J1 ;2 ¼ 9:1 Hz, 1H, H-10), 4.28 (dd, J8a;9 ¼ 8:6 Hz,
J8b;9 ¼ 4:5 Hz, 1H, H-9), 3.92–3.85 (m, 2H, H-20, H-6a0),
3.67–3.46 (m, 5H, H-30, H-40, H-50, H-6b0, H-8a), 3.33–
3.27 under MeOD signal (m, 1H, H-8b); 13C NMR
(90MHz, [ d4]-MeOH + 1% TFA): d 171.5 (COOH), 138.8
(C-7a), 129.1 (C-3a), 125.7 (C-2), 123.6 (C-6), 121.3 (C-5),
119.3 (C-4), 111.1 (C-7), 109.6 (C-3), 86.4 (C-10), 80.3
(C-50), 78.6 (C-30), 74.0(C-2 0), 71.2 (C-40), 62.3 (C-60), 54.1
(C-9), 27.3 (C-8).
0
0