First synthesis of indirubin N-glycosides (red sugars)

Article abstract of DOI:10.1016/j.tetlet.2006.07.024

The first indirubin N-glycosides were prepared by reaction of isatine N-glycosides with indoxyl acetate under basic conditions.

Full text of DOI:10.1016/j.tetlet.2006.07.024

Tetrahedron Letters 47 (2006) 6907–6909
First synthesis of indirubin N-glycosides (red sugars)
Stefanie Libnow,a Martin Hein,a,* Dirk Michalikb and Peter Langera,b,
*
aInstitut fu¨r Chemie, Universita¨t Rostock, Albert-Einstein-Str. 3a, D-18059 Rostock, Germany
bLeibniz-Institut fu¨r Katalyse e.V. an der Universita¨t Rostock, Albert-Einstein-Str. 29a, D-18059 Rostock, Germany
Received 30 May 2006; revised 3 July 2006; accepted 6 July 2006
Available online 4 August 2006
Abstract—The first indirubin N-glycosides were prepared by reaction of isatine N-glycosides with indoxyl acetate under basic
conditions.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Glycosylated indole derivatives are of considerable
pharmacological relevance (e.g., cancerostatic stauro-
sporine, K-252d, rebeccamycin and the tjipanazoles).1,2
Recently, we have reported3 the first synthesis of indigo
N-glycosides (blue sugars), which represent analogues of
the naturally occurring akashins.4 In contrast to phar-
macologically inactive parent indigo, the akashins show
a considerable activity against various human tumour
cell lines. Recently, the first isoindigo glycosides were
prepared, which exhibit antiproliferative activity.5
Indirubin, the most important isomer of indigo, and sev-
eral of its derivatives exhibit considerable antiprolifera-
tive activity.6,7 Herein, we report what are, to the best
of our knowledge, the first syntheses of indirubin
N-glycosides.
H3C
HO
O
NHPh
OH
H3C
HO
O
OH
OH
i
ii
OH
OH
2α,β (96%)
1
H3C
AcO
O
NHPh
OAc
H3C
O
O
iii
N
O
AcO
AcO
OAc
OAc
4α,β (63%)
3α,β (85%)
N-(2,3,4-Tri-O-acetyl-a,b-L-rhamnopyranosyl)isatine
(4a,b) was prepared from L-rhamnose.8–12 Simple
indirubins have been previously prepared by reaction
of a methanolic solution of indoxyl acetate with
isatines.13 The reaction of the pure beta anomer 4b with
indoxyl acetate resulted in the formation of the desired
deprotected N-(b-L-rhamnopyranosyl)indirubin 5b in
up to 77% yield (Scheme 1).14 During the optimisation
of this reaction, the use of an excess of sodium carbon-
ate proved to be important to achieve a complete cleav-
age of the acetyl protective groups of the sugar moiety.
O
O
NH
OAc
O
N
O
iv
N
+
OAc
OH
N
H
O
O
H3C
OAc
OH
H3C
OAc
4β
OH
5β (77%)
Scheme 1. Synthesis of N-(b-L-rhamnopyranosyl)indirubin 5b.
Reagents and conditions: (i) PhNH2, EtOH, rt, 12 h; (ii) Ac2O,
pyridine, 0!4 ꢁC, 8–12 h; (iii) oxalyl chloride, AlCl3, 55 ꢁC, 1.5 h; (iv)
Na2CO3, MeOH, rt, 4 h.
Following this procedure, the anomerically pure indiru-
bin glycosides of D-glucose (product 6b), D-ribose (prod-
Keywords: N-Glycosyl anilines; N-Glycosyl isatines; Indirubin;
N-Heterocycles; N-Glycosides.
*
uct 7b), D-galactose (8b), and D-mannose (9b) were
Corresponding authors. Tel.: +49 381 4986410; fax: +49 381
4986412; e-mail: peter.langer@uni-rostock.de
successfully prepared in good yields starting from the
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.07.024

6908
S. Libnow et al. / Tetrahedron Letters 47 (2006) 6907–6909
3. (a) Hein, M.; Michalik, D.; Langer, P. Synthesis 2005, 20,
O
O
3531–3534; (b) Hein, M.; Nguyen, T. B. P.; Michalik, D.;
Go¨rls, H.; Lalk, M.; Langer, P. Tetrahedron Lett. 2006,
47, 5741.
N
H
N
H
N
N
4. Maskey, R. P.; Grun-Wollny, I.; Fiebig, H. H.; Laatsch,
¨
O
O
O
O
H. Angew. Chem. 2002, 114, 623–625; Angew. Chem., Int.
Ed. 2002, 41, 597–599.
OH
OH
HO
5. (a) Sassatelli, M.; Saab, E.; Anizon, F.; Prudhomme, M.;
Moreau, P. Tetrahedron Lett. 2004, 45, 4827–4830; (b)
Sassatelli, M.; Bouchikhi, F.; Messaoudi, S.; Anizon, F.;
Debiton, E.; Barthomeuf, C.; Prudhomme, M.; Moreau,
P. Eur. J. Med. Chem. 2006, 41, 88–100.
7β (81%)
6β (70%)
OH
OH
HO
HO
O
O
6. Moon, M. J.; Lee, S. K.; Lee, J.-W.; Song, W. K.; Kim, S.
W.; Kim, J. I.; Cho, C.; Choi, S. J.; Kim, Y.-C. Bioorg.
Med. Chem. 2006, 14, 237–246.
N
H
N
H
N
N
7. Indirubin, the red shade of indigo. In Life in Progress;
Meijer, L., Guyard, N., Skaltsounis, L. A., Eisenbrand,
G., Ed.; Station Biologique, Roscoff, in press.
8. Chavis, C.; De Gourcy, C.; Dumont, F.; Imbach, J.-L.
Carbohydr. Res. 1983, 113, 1–20.
O
O
O
O
OH
OH
OH
OH
HO
HO
HO
HO
9β (59%)
8β (73%)
9. Douglas, J. G.; Honeyman, J. J. Chem. Soc. 1955, 3674–
3680.
10. Preobrazhenskaya, M. N.; Yartseva, I. V.; Ektova, L. V.
Nucleic Acid Chem. 1978, 2, 725–727.
11. Yartseva, I. V.; Ektova, L. V.; Preobrazhenskaya, M. N.
Bioorg. Khim. 1975, 1, 189–194.
Scheme 2. Synthesis of N-(b-D-glucopyranosyl)indirubin 6b, N-(b-D-
ribopyranosyl)indirubin 7b, N-(b-D-galactopyranosyl)indirubin 8b and
N-(b-D-mannopyranosyl) indirubin 9b; the yields, based on the
corresponding N-b-glycosyl isatine, are given in brackets.
12. Synthesis of isatine glycosides: To a stirred solution of the
acetylated rhamnosyl aniline in oxalyl chloride (about
10 equiv), an equivalent amount of anhydrous aluminium
chloride was added. The mixture was stirred for 1.5 h at
55 ꢁC (TLC control). After cooling to 0 ꢁC, ice water was
added to the stirred solution. The mixture was extracted
with EtOAc (3·). The combined organic layers were washed
with a saturated aqueous solution of sodium bicarbonate
and with water, dried (sodium sulfate) and filtered. The
filtrate was concentrated in vacuo and the residue was
purified by column chromatography to give 4a,b. Starting
with 3a,b (1.24 g, 3.39 mmol, b/a = 2.5:1), 4b (320 mg,
23%) and 4a,b (570 mg, 40%, b/a = 3:1) were isolated after
column chromatography (heptane/EtOAc = 3:1).
corresponding N-glycosyl isatines (Scheme 2). In case of
L-rhamnose, both anomeric glycosyl isatines could be
isolated after cyclisation of 3a,b with oxalyl chloride.
In contrast, only traces of a-anomers were detected for
all other sugars. In these cases, isomerically enriched
b-anomers (b/a >5:1) of the corresponding acetylated
N-glycosyl anilines were employed in the cyclisation
reaction. However, the b/a-ratio seems to slightly vary
during the course of the reaction; otherwise, higher
amounts of the a-anomeric N-glycosyl isatines would
have been detected after the reaction. The determination
of the anomeric configuration of the products was car-
ried out by two-dimensional NMR spectroscopy
The
syntheses
of
N-(2,3,4,6-tetra-O-acetyl-b-D-
glucopyranosyl)isatine,10 N-(2,3,4-tri-O-acetyl-b-D-ribo-
1
(1H,1H COSY, H,1H NOESY, 13C,1H COR, HMBC).
pyranosyl)isatine,11 N-(2,3,4,6-tetra-O-acetyl-b-D-galacto-
pyranosyl)isatine
and
N-(2,3,4,6-tetra-O-acetyl-b-D-
In conclusion, we have reported the first syntheses of
deprotected N-glycosides of indirubin (red sugars).
mannopyranosyl)isatine were carried out following the
procedure as described above. In contrast, isomerically
enriched b-anomers (b/a >5:1) were employed as starting
materials. Starting with N-(2,3,4,6-tetra-O-acetyl-a,b-D-
glucopyranosyl)aniline (1.00 g, 2.36 mmol), N-(2,3,4,6-
tetra-O-acetyl-b-D-glucopyranosyl)isatine was isolated
(0.34 g, 30%). N-(2,3,4-Tri-O-acetyl-a,b-D-ribopyrano-
syl)aniline (0.75 g, 2.13 mmol) was transformed into
Acknowledgements
Financial support by the state of Mecklenburg-
Vorpommern (scholarship for S.L.) is gratefully
acknowledged.
N-(2,3,4-tri-O-acetyl-b-D-ribopyranosyl)isatine
(0.18 g,
21%). Starting with N-(2,3,4,6-tetra-O-acetyl-a,b-D-galac-
topyranosyl)aniline (0.80 g, 1.89 mmol), N-(2,3,4,6-tetra-
O-acetyl-b-D-galactopyranosyl)isatine was isolated (0.47 g,
References and notes
52%).
N-(2,3,4,6-Tetra-O-acetyl-a,b-D-mannopyrano-
syl)aniline (1.00 g, 2.36 mmol) was transformed
into N-(2,3,4,6-tetra-O-acetyl-b-D-mannopyranosyl)isatine
(0.24 g, 21%). All products were purified by column
chromatography (heptane/EtOAc = 2:1).
1. Review: Gribble, G.; Berthel, S. In Studies in Natural
Products Chemistry; Elsevier Science: New York, 1993;
Vol. 12, pp 365–409.
13. (a) Russell, G. A.; Kaupp, G. J. Am. Chem. Soc. 1969, 91,
3851–3859; (b) Polychronopoulos, P.; Magiatis, P.;
Skaltsounis, A.-L.; Myrianthopoulos, V.; Mikros, E.;
Tarricone, A.; Musacchio, A.; Roe, S. M.; Pearl, L.;
Leost, M.; Greengard, P.; Meijer, L. J. Med. Chem. 2004,
47, 935–946.
2. Isolation of staurosporin: (a) Omura, S.; Iwai, Y.; Hirano,
A.; Nakagawa, A.; Awaya, J.; Tsuchiya, H.; Takahashi,
Y.; Masuma, R. J. Antibiot. 1977, 30, 275–282; synthesis:
(b) Link, J. T.; Raghavan, S.; Gallant, M.; Danishefsky, S.
J.; Chou, T. C.; Ballas, L. M. J. Am. Chem. Soc. 1996, 118,
2825–2842; pharmacological activity: (c) Yamashita, Y.;
Fujii, N.; Murakata, C.; Ashizawa, T.; Okabe, M.;
Nakano, H. Biochemistry 1992, 31, 12069–12075.
14. Synthesis of indirubin glycosides: To a MeOH solution of
the acetylated glycosyl isatine, indoxyl acetate (1.0 equiv)

S. Libnow et al. / Tetrahedron Letters 47 (2006) 6907–6909
and sodium carbonate (4.0 equiv) were added under argon
6909
6
5
20
atmosphere. The mixture was stirred for 1.5–4 h at 20 ꢁC.
The yellow to orange colour of the solution changed from
red to violet. The mixture was neutralised with IR 120
(H+), filtered and the filtrate was concentrated under
reduced pressure. Column chromatography of the residue
gave the indirubin glycosides as red to violet solids:
Starting with 4b (300 mg, 0.715 mmol) (reaction time: 4 h),
5b (224 mg, 77%) was isolated by column chromatography
7
Me
4
7a
O
3a
19
17
O
10
15
HO 18
HO
N
3
2
1
16
10a
11
12
9
O
HN
8
14a
OH
14
β-L-rhamno
13
(CHCl3/MeOH, 20:1!10:1). 1H NMR (500.13 MHz,
3
Compound 6b: N-(2,3,4,6-tetra-O-acetyl-b-D-glucopyr-
anosyl)isatine (160 mg, 0.335 mmol) was converted
according to the above mentioned procedure (reaction
duration: 4 h). Column chromatography (CHCl3–MeOH,
20:1!10:1) yielded 6b (100 mg, 70%). Compound 7b: N-
DMSO) d: 10.08 (s, 1H, NH); 8.80 (dd, 1H, J4,5
=
4
3
7.9 Hz, J4,6 = 1.0 Hz, H-4); 7.67 (d, 1H, J11,12 = 7.6 Hz,
3
H-11); 7.64 (d, 1H, J6,7 = 7.9 Hz, H-7); 7.60 (dt, 1H,
3J13,14 = 7.9 Hz, J12,13 = 7.6 Hz, J11,13 = 1.0 Hz, H-13);
3
4
3
7.42 (d, 1H, J13,14 = 7.9 Hz, H-14); 7.23 (dt, 1H,
3J6,7 = 7.9 Hz, J5,6 = 7.5 Hz, J4,6 = 1.0 Hz, H-6); 7.06–
3
4
(2,3,4-tri-O-acetyl-b-D-ribopyranosyl)isatine
(120 mg,
0.296 mmol) was converted according to the above men-
tioned procedure (reaction duration: 1.5 h). Column
chromatography (CHCl3–MeOH, 20:1!10:1) yielded 7b
(95 mg, 81%). Compound 8b: N-(2,3,4,6-tetra-O-acetyl-b-
D-galactopyranosyl)isatine (200 mg, 0.419 mmol) was con-
verted according to the above mentioned procedure
(reaction duration: 3 h). Column chromatography
(CHCl3–MeOH, 20:1!10:1) yielded 8b (130 mg, 73%).
Compound 9b: N-(2,3,4,6-tetra-O-acetyl-b-D-mannopyr-
anosyl)isatine (190 mg, 0.398 mmol) was converted
according to the above mentioned procedure (reaction
duration: 3 h). Column chromatography (CHCl3–MeOH,
20:1!10:1) yielded 9b (100 mg, 59%).
7.02 (m, 2H, H-5,12); 5.62 (s (br), 1H, H-15); 5.12 (d, 1H,
3
3J16,OH = 5.0 Hz, OH(316)); 4.96 (d, 1H, J18,OH = 5.0 Hz,
OH(18)); 4.85 (d, 1H, J17,OH = 6.0 Hz, OH(17)); 3.86 (m,
1H, H-16); 3.50 (m, 1H, H-17); 3.41–3.35 (m, 2H, H-
3
18,19); 1.27 (d, 3H, J19,20 = 5.8 Hz, Me(20)); 13C NMR
(125.8 MHz, DMSO) d: 188.7 (C-10); 168.5 (C-2); 152.4
(C-14a); 140.9 (C-7a); 138.8 (C-9); 137.3 (C-13); 128.5
(C-6); 124.6 (C-11); 123.7 (C-4); 121.6, 121.5 (C-5,12);
120.7 (C-3a); 119.2 (C-10a); 114.9 (C-7); 113.6 (C-14);
105.5 (C-3); 82.3 (C-15); 75.5 (C-18); 73.3 (C-17); 72.0
(C-16); 71.5 (C-19); 18.3 (C-20). HRMS (ESI): m/z calcd
for C22H20N2NaO6 ([M+Na]+): 431.1214; found:
431.1216.