Synthesis of the first deprotected indigo N-glycosides (blue sugars) by reductive glycosylation of dehydroindigo

Article abstract of DOI:10.1016/j.tetlet.2006.06.021

The first deprotected indigo N-glycosides (blue sugars) have been prepared by reaction of dehydroindigo with in situ generated rhamnosyl, glucosyl and mannosyl iodide.

Full text of DOI:10.1016/j.tetlet.2006.06.021

Tetrahedron Letters 47 (2006) 5741–5745
Synthesis of the first deprotected indigo N-glycosides
blue sugars) by reductive glycosylation of dehydroindigo
(
a,
a,b
c
d
*
Martin Hein, Nguyen Thi Bich Phuong, Dirk Michalik, Helmar G o¨ rls,
e
a,c,
*
Michael Lalk and Peter Langer
a
Institut f u¨ r Chemie, Universit a¨ t Rostock, Albert-Einstein-Strasse 3a, D-18059 Rostock, Germany
b
Institut f u¨ r Biochemie, Universit a¨ t Greifswald, Soldmannstr. 16, D-17487 Greifswald, Germany
Leibniz-Institut f u¨ r Katalyse e.Van der Universit a¨ t Rostock, Albert-Einstein-Strasse 29a, D-18059 Rostock, Germany
c
d
Institut f u¨ r Anorganische und Analytische Chemie, Universit a¨ t Jena, August-Bebel-Str. 2, D-07740 Jena, Germany
e
Institut f u¨ r Pharmazie, Universit a¨ t Greifswald, Ludwig-Jahn-Str. 17, D-17487 Greifswald, Germany
Received 26 January 2006; revised 24 May 2006; accepted 2 June 2006
Available online 22 June 2006
Abstract—The first deprotected indigo N-glycosides (blue sugars) have been prepared by reaction of dehydroindigo with in situ gen-
erated rhamnosyl, glucosyl and mannosyl iodide.
ꢀ
2006 Elsevier Ltd. All rights reserved.
Glycosylated indole derivatives, such as the prominent
human tumour cell lines. The akashines represent the
first indigo derivatives isolated from nature so far, except
from the well-known purpur (6,6 -dibromoindigo) and
indolo[2,3-a]carbazole glycosides staurosporine, K-
1
,2
0
2
52d, rebeccamycin and the tjipanazoles,
represent
3
promising anticancer agents. Furthermore, several
indirubine derivatives (isomeric to corresponding indigo
from two other brominated indigos. Recently, we have
reported the first synthesis of an indigo glycoside—a
pivaloyl protected rhamnoside—by O-glycosylation of
N-benzylindigo and subsequent rearrangement of the
4
derivatives) show anti-proliferative activity. In this case
also the parental compound, the indirubine itself, is ac-
tive. A few years ago, Laatsch and co-workers reported
the isolation of the first N-glycosides of indigo deriva-
6
O- into the desired N-glycoside. However, the success
of the key step of this approach, the O!N rearrange-
ment, proved to be severely dependent on the type of
carbohydrate moiety and protective groups. In fact, the
application of this strategy to glycosyl donors other than
rhamnosyl, and to protective groups other than pivaloyl
proved to be unsuccessful so far. In addition, all attempts
to remove the pivaloyl protective groups failed. Herein,
we report what is, to the best of our knowledge, the first
synthesis of deprotected indigo N-glycosides. The synthe-
sis was achieved based on a new synthetic strategy—the
addition of a glycosyl iodide to dehydroindigo. Notably,
this strategy allows the mono-glycosylation of indigo
without the need of a nitrogen protective group.
5
tives—the akashines A, B and C (Fig. 1). In contrast
to pharmacologically inactive parent indigo, the aka-
shines show a considerable activity against various
O
H
Cl
N
N
Cl
O
O
HO
HO
CH3
NH2
Akashine A
7
Dehydroindigo (3) was prepared in high yield by reac-
tion of indigo (1) with KMnO in the presence of acetic
4
Figure 1. Akashine A isolated from terrestric Streptomyces.
acid (to give diacetate 2), and subsequent base mediated
elimination of acetic acid (Scheme 1). The addition of
8
9
hydrogen halides, phenols and thiols to dehydroindigo
has been previously studied. Treatment of 3 with hydro-
gen iodide (HI) afforded indigo (1). The formation of the
Keywords: Dehydroindigo; Indigo; N-Heterocycles; Regioselectivity.
*
Corresponding authors. Tel.: +49 381 4986410; fax: +49 381 4986412
P.L.); e-mail: peter.langer@uni-rostock.de
(
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.06.021

5742
M. Hein et al. / Tetrahedron Letters 47 (2006) 5741–5745
O
O
AcO
replacement of the TMS by acetyl groups proved to be
H
N
H
N
important for practical reasons (stability during chro-
matography). The relatively low yield of the product
after the reaction sequence is mainly due to side reac-
tions of the dehydroindigo in the addition step. A possi-
i
N
H
N
H
OAc
O
O
1
2 (88%)
0
ble N -acetylation of the glycosylated indigo during the
acetolysis can be extensively suppressed by tuning the
ii
O
Ac O/pyridine ratio. Higher ratios cause less formation
2
H
N
0
of N -acetylated by-product (violet colour on TLC), but
O
result in other side reactions.
4
iii
N
N
O
O
AcO
AcO
The structures of 5a and 5b were proved by spectro-
scopic methods (Fig. 2). The NMR signals were
N
CH3
O
1
1
assigned by DEPT and two-dimensional H, H COSY,
OAc
1
1
1
13
3
(85%)
H, H NOESY and H, C correlation spectra (HSQC,
5
(22%)
HMBC) recorded with a Bruker AVANCE 500. Indigo-
glycoside 5a, which was measured both as the pure
anomer (a/b > 98:2) and as the anomeric mixture
iv
(
a/b = 2:1), resides as an N-glycoside containing a a-
O
4
H
rhamnosyl moiety which possesses a C conformation.
The structure was independently confirmed by crystal
structure analysis (Fig. 3). The conformation of the
sugar moiety of 5b was determined to be C . NOE-
correlations between the protons H-15 and H-17, and
H-15 and H-19 clearly indicate this conformation.
1
H C
3
N
O
1
2
Me SiO
OSiMe3
N
3
1
O
4
O
HO
HO
Me SiO
OSiMe3
3
CH3
4
OH
(59%)
6
O
4
7
14
11
H
N
9
3
a
3
8
14a
10a
5
6
13
12
Scheme 1. Synthesis of indigo glycoside 6. Reagents and conditions:
OAc
2
(
i) KMnO
4
, AcOH, high-power-stirring (12,000 rot/min), 20 ꢁC, 3–4 h;
Cl ; (b) Me SiI,
18
19 16O
7
a
N
1
10
(ii) pyridine/toluene = 1:2, 70 ꢁC, 1 h; (iii) (a) 4, CH
2
2
3
15
N
O
1
5
1
7
O
Me
OAc
2
0 ꢁC, 30 min; (c) 3, 0 ꢁC, 30 min; (d) n-PrSH, 0 ! 20 ꢁC, 1 h; (e)
AcO 1
6
19
AcO 20
2
0
Ac
2
O/pyridine = 3:1, KHF
2
, 70 ꢁC, 3 h; (iv) NaO-t-Bu (15 mol %),
Me
1
7
18
MeOH, 20 ꢁC, 4 h.
AcO
-L-rhamno 4C1
α
OAc
0
Figure 2. Structure of 5a.
latter can be explained by initial formation of 2,2 -di-
0
iodo-2,2 -bis(indolin-3-one), analogously to the addition
of HCl to 3, and subsequent extrusion of iodine. Based
on this observation, we developed the synthesis of
indigoglycoside 5. The reaction of dehydroindigo (3)
with TMS protected L-rhamnosyl iodide—generated
in situ by conversion of tetra-O-trimethylsilyl-L-
rhamnopyranose (4) with TMSI—and subsequent ace-
tolysis (Ac O/pyridine/KHF ) afforded the N-(2,3,4-
2
2
1
0
tri-O-acetyl-L-rhamnosyl)indigo
5
(a/b = 2:1).
An
analytically pure sample of the a anomer was isolated
by repeated crystallisation from MeOH. The yield of
the glycosylation reaction is relatively low, due to loss
of material during chromatography and formation of
side-products (bis-glycosylation and desilylation). Treat-
ment of a MeOH solution of 5 with NaO-t-Bu (5–
1
5 mol %) afforded the desired deprotected indigo glyco-
1
1
side 6 (a/b = 2:1). The use of catalytic amounts of
NaO-t-Bu proved to be important, since employment
of stoichiometric amounts or the use of other reagents
(
e.g., K CO , MeOH) resulted in decomposition.
2 3
The formation of 5 can be explained by addition of TMS
protected L-rhamnosyl iodide (generated in situ by treat-
ment of the rhamnose derivative 4 with Me SiI) to dehy-
3
droindigo, addition of nPrSH, extrusion of iodine and
dipropyl disulfide and subsequent acetolysis (by addi-
tion of acetic anhydride, pyridine and KHF ). The
Figure 3. Crystal structure of 5a.
2

M. Hein et al. / Tetrahedron Letters 47 (2006) 5741–5745
5743
O
Acknowledgement
O
N
4
7
H
14
3
a
3
8
14a
10a
20
5
6
N
13
Financial support from the Ministry of Education of
Vietnam (scholarship for T.B.P.N.) is gratefully
acknowledged.
2
9
N
1
10
12
7
a
N
O
i
1
1
O
3
7
15
O
AcO 1
6
19
Me SiO
OSiMe3
OSiMe3
3
1
7
18
AcO
OAc
Me SiO
OAc
References and notes
3
O
Me SiO
8 (20%)
-D-gluco
3
1. Review: Gribble, G.; Berthel, S. In Studies in Natural
Products Chemistry; Elsevier Science: New York, 1993;
Vol. 12, pp 365–409.
α
Scheme 2. Synthesis of the acetylated N-(a-D-glucosyl)indigo 8.
Reagents and conditions: (i) (a) 7, CH Cl ; (b) Me SiI 20 ꢁC, 30 min;
c) 3, 0 ꢁC, 30 min; (d) nPrSH, 0 ! 20 ꢁC, 1 h; (e) Ac O/pyridine = 3:1,
KHF , 70 ꢁC, 3 h.
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; 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.
2
2
3
(
2
2
The reaction of 3 with TMS protected D-glucose 7—car-
ried out following the procedure given for the synthesis of
3. Yamashita, Y.; Fujii, N.; Murkata, C.; Ashizawa, T.;
5
—afforded the desired anomerically pure N-(2,3,4,6-
Okabe, M.; Nakano, H. Biochemistry 1992, 31, 12069.
1
3
4
5
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.
. Maskey, R. P.; Gr u¨ n-Wollny, I.; Fiebig, H. H.; Laatsch,
H. Angew. Chem. 2002, 114, 623; Angew. Chem., Int. Ed.
tetra-O-acetyl-a-D-glucopyranosyl)indigo 8 (Scheme 2).
The structure of 8 was established by NMR spectro-
scopy. In the NOESY, spectrum relevant cross peaks were
found for H-15 with H-20a,b (confirming the a-configu-
ration) and for H-15 with H-18 (confirming a boat-like
conformation). Notably, the direct reaction of acetyl pro-
tected a-D-glucopyranosyl bromide with dehydroindigo
was unsuccessful. The reaction of 3 with TMS protected
D-mannose 9, subsequent acetolysis and deprotection
2
002, 41, 597.
. Hein, M.; Michalik, D.; Langer, P. Synthesis 2005, 20,
531.
3
7. Kalb, L. Chem. Ber. 1909, 42, 3642.
8. Kalb, L. Chem. Ber. 1912, 45, 2136.
(
NaO-t-Bu, 15 mol %) afforded the anomerically pure
9. Christophersen, C.; Waetjen, F.; Buchardt, O.; Anthoni,
U. Tetrahedron Lett. 1977, 20, 1747.
N-(a-D-mannopyranosyl)indigo 11 (Scheme 3). Like in
the other cases, the indigo rest prefers an equatorial
arrangement at the sugar, which is indicated by a large
vicinal coupling of the sugar protons J
1
0. Synthesis of indigo glycoside 5: To a CH
2 2
Cl solution
(
8 mL) of 1,2,3,4-tetra-O-trimethylsilyl-L-rhamnopyran-
3
ose (4) (925 mg, 2.04 mmol) was added iodotrimethyl-
silane (280 lL, 2.06 mmol) and the mixture was stirred for
= 9.5 Hz.
H-1,H-2
3
0 min at 20 ꢁC. The solution, now containing the glycosyl
iodide, was cooled to 0 ꢁC and dehydroindigo (531 mg,
.04 mmol) was added. After stirring for 30–45 min at
In conclusion, we have reported what are, to the best of
our knowledge, the first syntheses of deprotected N-gly-
cosides of indigo.
2
0 ꢁC, propanethiol (280 lL, 3.09 mmol) was added to the
solution. The solution was warmed to 20 ꢁC and was
stirred for 1 h. The mixture was again cooled to 0 ꢁC and a
solution (8 mL) of NaHCO and Na SO (3.0 g of each
per 100 mL of water) was added. Subsequently, 30–40 mL
of cold ethyl acetate was added. The mixture was filtered
and the aqueous and the organic layers were separated.
The latter was washed with an aqueous solution of
O
N
3
2
3
O
H
N
N
O
3
N
i
O
Me SiO
OSiMe3
3
O
NaHCO
3
/Na
2
SO
3
(1 · 10 mL)
and
with
brine
AcO
AcO
(
2 · 10 mL). The solution was dried (Na
2
SO ), filtered
4
Me SiO
OSiMe3
3
and the filtrate was concentrated in vacuo. The residue
was dissolved in a mixture of pyridine (2 mL) and acetic
anhydride (6 mL). To the solution was added finely
OAc
O
OAc
0 (15%)
-D-manno
Me Si
3
1
O
9
2
powdered KHF (800 mg, 10.25 mmol) and the reaction
α
mixture was stirred at 70 ꢁC until one main spot of blue
colour could be detected by TLC (2–3 h). The pyridine
and acetic anhydride were removed by distillation under
reduced pressure and the residue was purified by column
chromatography (EtOAc/toluene = 1:5 ! 1:2) to give 5
ii
O
H
N
N
O
(
198 mg, 22%, a/b = 2:1) as a blue solid. An analytically
O
HO
HO
pure sample of the a anomer was isolated by repeated
crystallisation from methanol.
OH
1
OH
1 (32%)
Compound 5a: H NMR (500.13 MHz, CDCl ): d 10.87
(
3
3
4
s, 1H, NH); 7.74 (dd, 1H, J4,5 = 7.8 Hz, J4,6 = 1.5 Hz,
3
H-4); 7.72 (d, 1H,
1
J6,7 = 8.2 Hz, H-7); 7.71 (br d, 1H,
3
3
Scheme 3. Synthesis of N-(a-D-mannosyl)indigo 11. Reagents and
conditions: (i) (a) 9, CH Cl ; (b) Me SiI, 20 ꢁC, 30 min; (c) 3, 0 ꢁC,
0 min; (d) nPrSH, 0 ! 20 ꢁC, 1 h; (e) Ac O/pyridine = 3:1, KHF
0 ꢁC, 3 h (ii) NaO-t-Bu (15 mol %), MeOH, THF, 20 ꢁC, 4 h.
J11,12 = 7.8 Hz, H-11); 7.53 (ddd, 1H, J6,7 = 8.2 Hz,
3
3
4
J
J
5,6 = 7.2 Hz,
13,14 = 8.2 Hz, J12,13 = 7.3 Hz, J11,13 = 1.2 Hz, H-13);
3
J4,6 = 1.5 Hz, H-6); 7.48 (ddd, 1H,
2
2
3
3 4
3
7
2
2
,
3
7.26 (d, 1H, J15,16 = 9.8 Hz, H-15); 7.10 (‘t’, 1H, J4,5
=

5744
M. Hein et al. / Tetrahedron Letters 47 (2006) 5741–5745
3
3
3
3
4
7
.8 Hz, J5,6 = 7.2 Hz, H-5); 7.00 (d, 1H, J13,14 = 8.2 Hz,
J
11,12 = 7.5 Hz,
J
11,13 = 0.8 Hz, H-11); 7.57 (ddd, 1H,
3
3
3
3
4
H-14); 6.96 (‘t’, 1H, 3J11,12 = 7.8 Hz,
H-12); 5.63 (dd, 1H,
H-16); 5.44 (dt, 1H,
.2 Hz, H-17); 4.95 (dd, 1H,
.2 Hz, H-18); 4.45 (m, 1H, H-19); 2.23 (s, 3H, Ac); 2.15
s, 3H, Ac); 1.86 (d, 3H, J
H, Ac). C NMR (125.8 MHz, CDCl
C-3, C-10); 169.3, 169.3, 169.0 (3 Ac–CO); 151.7 (C-14a);
50.3 (C-7a); 136.4 (C-13); 135.6 (C-6); 126.0; 122.4 (C-2,
C-9); 124.9 (C-11); 124.0 (C-4); 122.9 (C-3a); 122.2 (C-5);
20.9 (C-12); 119.9 (C-10a); 115.9 (C-7); 112.0 (C-14); 78.9
C-15); 74.4 (C-19); 71.6 (C-18); 69.0 (C-17); 65.3 (C-16);
1.0, 20.7, 20.2 (3 Ac–Me); 16.3 (C-20). HRMS (NCI,
a/b = 2:1): m/z calcd for C28
J
12,13 = 7.3 Hz,
J6,7 = 8.3 Hz,
J5,6 = 7.2 Hz,
J
4,6 = 1.4 Hz, H-6); 7.51
3
J
1
= 9.8 Hz, J16,17 = 3.5 Hz,
(m, 1H, H-13); 7.37 (d, 1H, J
‘t’, 1H, J4,5 = 7.7 Hz, J5,6 = 7.2 Hz, J5,7 = 0.8 Hz, H-5);
6.94 (d ‘t’, 1H, J11,12 = J12,13 = 7.5 Hz, J12,14 = 0.8 Hz,
= 8.0 Hz, H-14); 7.10 (d
5,16
13,14
3
3
4
3
3
4
J
16,17 = J17,18 = 3.5 Hz, 3
J
J
17,19
=
=
3
3
3
4
1
1
(
3
(
J
17,18 = 3.5 Hz,
18,19
3
H-12); 6.28 (d, 1H, J15,16 = 9.5 Hz, H-15); 5.11 (d, 1H,
3
3
3
= 7.5 Hz, H-20); 1.68 (s,
J18,OH = 3.1 Hz, OH(18)); 4.93 (d, 1H, J
= 3.5 Hz,
1
9,20
17,OH
1
3
3
3
): d 189.2, 187.0
OH(17)); 4.55 (d, 1H,
J16,OH = 8.4 Hz, OH(16)); 4.20
3
(‘q’, 1H, H-19); 4.10 (d ‘t’, 1H,
J15,16 = 9.5 Hz,
3
3
1
J16,OH = 8.4 Hz, J16,17 = 3.5 Hz, H-16); 3.85 (m, 1H,
3
H-17); 3.67 (br, 1H, H-18); 1.65 (d, 3H, J19,20 = 7.3 Hz,
1
3
1
(
2
Me(20)). C NMR (125.8 MHz, DMSO) d: 188.7 (C-3);
186.3 (C-10); 152.2 (C-14a); 151.5 (C-7a); 135.9 (C-13);
134.9 (C-6); 124.3 (C-9); 124.1 (C-11); 123.6 (C-2);123.3
(C-4); 122.9 (C-3a); 121.6 (C-5); 120.4 (C-12); 119.4 (C-
10a); 117.1 (C-7); 113.2 (C-14); 81.9 (C-15); 75.9 (C-19);
ꢀ
H
26
2
N O
9
([M] ): 534.1644;
found: 534.1622.
7
2.7 (C-17); 72.0 (C-18); 64.0 (C-16); 17.1 (C-20). HRMS
+
O
(EI, a/b = 2:1): m/z calcd for C H N O ([M] ):
2
2
20
2
6
4
H
N
9
14
11
3
a
3
8
14a
10a
5
6
13
12
408.1316; found: 408.1300.
OAc
18
2
19
O
16
7a
N
1
10
O
3
7
15
4
H
N
9
14
11
N
O
3a
8
14a
10a
1
5
5
6
OH
18
1
7
13
12
O
Me
OAc
20
AcO 1
6
2
19
19
20
AcO
α- -rhamno C
16O
OH
Me
7a
N
1
10
15
1
7
18
7
N
O
17
AcO
4
15
Me
20
5
α
L
OAc
1
O
19
HO 16
HO
HO
α-
20
Me
1
1
8
6α
rhamno 4C1
Compound 5b: H NMR (500.13 MHz, CDCl
H, NH); 7.83 (d, 1H, J6,7 = 8.4 Hz, H-7); 7.70–7.67 (m,
3
) d: 10.88 (s,
17
L
-
3
1
OH
2
H, H-4,11); 7.48–7.45 (m, 2H, H-6,13); 7.02 (‘t’, 1H,
3
3
3
1
J4,5 = J = 7.5 Hz, H-5); 6.97 (d, 1H, J
H-14); 6.94 (‘t’, 1H, J11,12 = J12,13 = 7.5 Hz, H-12); 6.79
= 8.2 Hz,
Compound 6b: H NMR (500.13 MHz, DMSO) d: 11.00
5,6
13,14
3
3
3
(s, 1H, NH); 8.07 (d, 1H, J6,7 = 8.5 Hz, H-7); 7.58 (m,
2H, H-4,11); 7.52 (ddd, 1H,
3
3
3
(
d, 1H,
.5 Hz,
J
15,16 = 1.4 Hz, H-15); 5.81 (dd, 1H,
J
16,17
=
J13,14 = 8.1 Hz,
3
3
3
4
3
J
15,16 = 1.4 Hz, H-16); 5.46 (dt, 1H,
J
J
12,13 = 7.2 Hz, J11,13 = 1.3 Hz, H-13); 7.48 (ddd, 1H,
3
3
3 4
J17,18 = 10.0 Hz, J16,17 = 3.5 Hz, H-17); 5.22 (‘t’, 1H,
J
J
6,7 = 8.5 Hz, J5,6 = 7.0 Hz, J4,6 = 1.5 Hz, H-6); 7.36 (d,
3 3
3
3
3
17,18 = 10.0 Hz,
J
18,19 = 9.8 Hz, H-18); 3.88 (dq, 1H,
1H, J13,14 = 8.1 Hz, H-14); 7.02 (ddd, 1H, J4,5 = 7.7 Hz,
3
3
3
4
18,19 = 9.8 Hz, J19,20 = 6.2 Hz, H-19); 2.09 (s, 3H, Ac);
J
J
5,6 = 7.0 Hz,
J5,7 = 0.8 Hz, H-5); 6.95 (‘dt’, 1H,
3
4
1
.97 (s, 3H, Ac); 1.76 (s, 3H, Ac); 1.38 (d, 3H,
11,12 = J12,13 = 7.2 Hz, J12,14 = 0.8 Hz, H-12); 5.86 (d,
3
13
3
3
J19,20 = 6.2 Hz, H-20). C NMR (125.8 MHz, CDCl3)
d: 188.7, 187.7 (C-3, C-10); 170.1, 169.7, 169.6 (3 Ac–CO);
1H, J15,16 = 1.2 Hz, H-15); 5.17 (d, 1H, J16,OH = 4.9 Hz,
3
OH(16)); 4.85 (d, 1H, J17,OH = 5.4 Hz, OH(17)); 4.81 (d,
1H, J18,OH = 5.9 Hz, OH(18)); 4.47 (m, 1H, H-16); 3.55
(m, 1H, H-17); 3.33 (m, 1H, H-18); 3.31 (m, 1H, H-19);
3
1
1
3
1
1
51.7 (C-14a); 150.7 (C-7a); 136.7 (C-13); 134.7 (C-6);
26.4; 121.1 (C-2, C-9); 125.2 (C-11); 123.4 (C-4); 122.0 (C-
a); 121.4 (C-5); 120.9 (C-12); 119.8 (C-10a); 117.6 (C-7);
11.9 (C-14); 85.5 (C-15); 73.4 (C-19); 71.5 (C-16); 70.8 (C-
8); 70.7 (C-17); 20.7, 20.7, 20.5 (3 Ac–Me); 17.7 (C-20).
3
13
1.19 (d, 3H,
J19,20 = 5.7 Hz, Me(20)).
C
NMR
(125.8 MHz, DMSO) d: 188.4 (C-3); 187.1 (C-10); 152.1
(C-14a); 151.9 (C-7a); 136.5 (C-13); 134.5 (C-6); 125.1 (C-
9
1
); 124.3 (C-11); 123.1 (C-2); 122.3 (C-4); 121.4 (C-3a);
21.1 (C-5); 120.6 (C-12); 119.9 (C-7); 119.3 (C-10a); 113.3
O
3
4
H
N
9
14
3
a
a
8
14a
10a
(C-14); 88.1 (C-15); 75.3 (C-19); 73.4 (C-17); 72.8 (C-16);
71.9 (C-18); 18.3 (C-20).
5
6
13
12
2
20
19
15
N
1
1
0
Me
O
N
7
7
11
AcO
O
17
16
1
5
18
O
O
3
AcO 1
6
AcO
19
4
H
N
9
14
11
20
OAc
rhamno 1
C
4
3a
7a
8
14a
10a
Me
5
6
13
12
1
7
18
2
AcO
5β
β-
L
-
OAc
N
1
10
7
1
5
O
O
1
1. Synthesis of indigo glycoside 6: Acetylated indigo glycoside
(100 mg, 0.187 mmol, anomeric mixture) was dissolved
HO 1
6
19
20
β-L-rhamno
6β
5
Me
1
7
18
in a mixture of THF (2 mL) and of MeOH (6 mL). To the
solution was dropwise added a MeOH solution (1%) of
sodium tert-butoxide and the rate of the deacetylation was
adjusted (TLC-control, ca 5–15 mol %). After complete
conversion (2–4 h), the mixture was cooled to 0 ꢁC and
silica gel was added (3.0 g). The solvent was removed by
distillation at 30 ꢁC under reduced pressure and the
residue was purified by column chromatography
HO
OH
12. CCDC-293346 contains all crystallographic details of this
publication and is available free of charge at www.ccdc.
cam.ac.uk/conts/retrieving.html or can be ordered from
the following address: Cambridge Crystallographic Data
Centre, 12 Union Road, GB-Cambridge CB21EZ; Fax:
+44 1223 336 033; or deposit@ccdc.cam.ac.uk.
13. Synthesis of indigo glycoside 8: The synthesis of 8 was
carried out following the procedure given for the synthesis
of 5. Starting with 7 (454 mg, 0.84 mmol), 8 was isolated
(100 mg, 20%) as a blue solid.
(
2
EtOAc/THF = 1:0 ! 5:1) to give 6 (45 mg, 59%, a/b =
:1) as a blue solid.
1
Compound 6a: H NMR (500.13 MHz, DMSO) d: 10.90
s, 1H, NH); 7.82 (d, 1H, J6,7 = 8.3 Hz, H-7); 7.69 (dd,
3
(
1
3
4
H,
J
4,5 = 7.7 Hz,
J4,6 = 1.4 Hz, H-4); 7.62 (dd, 1H,

M. Hein et al. / Tetrahedron Letters 47 (2006) 5741–5745
5745
1
Compound 8: H NMR (500.13 MHz, CDCl
3
): d = 10.91
(125.8 MHz, CDCl
3
): d 188.9, 187.5 (C-3, C-10); 170.5,
3
(
s, 1H, NH); 7.76 (br d, 1H, J6,7 = 8.2 Hz, H-7); 7.68 (br
169.4, 169.1, 168.8 (4 Ac–CO); 151.8 (C-14a); 150.5 (C-
7a); 136.6 (C-13); 134.5 (C-6); 126.3; 121.5 (C-2, C-9);
124.9 (C-11); 123.5 (C-4); 122.9 (C-3a); 121.4 (C-5); 120.9
(C-12); 119.8 (C-10a); 117.3 (C-7); 112.0 (C-14); 80.6 (C-
15); 74.0 (C-19); 69.7 (C-17); 69.4 (C-16); 65.1 (C-18); 61.5
(C-20); 20.9, 20.8, 20.7, 20.5 (4 Ac–Me). HRMS (EI):
m/z calcd for C30
592.1682.
3
d, 1H,
J4,5 = 7.8 Hz, H-4); 7.58 (br d, 1H,
3
3
J
J
J
11,12 = 7.6 Hz, H-11); 7.47 (ddd, 1H,
J
6,7 = 8.2 Hz,
3
3
4
5,6 = 7.2 Hz,
J4,6 = 1.3 Hz, H-6); 7.44 (ddd, 1H,
3
4
13,14 = 8.2 Hz, J12,13 = 7.3 Hz, J11,13 = 1.3 Hz, H-13);
3
7
J
.12 (d, 1H, J15,16 = 2.7 Hz, H-15); 7.02 (‘dt’, 1H,
3
3
4
+
4,5 = 7.8 Hz,3
J5,6 = 7.2 Hz,
J5,7 = 0.9 Hz, H-5); 6.96
H
28
2
N O
11 ([M] ): 592.1688; found:
(
br d, 1H,
J
13,14 = 8.2 Hz, H-14); 6.91 (‘dt’, 1H,
3
3
4
J11,12 = 7.6 Hz, J12,13 = 7.3 Hz, J12,14 = 0.9 Hz, H-12);
3
3
5
.67 (‘t’, 1H, J16,17 = 3.0 Hz, J15,16 = 2.7 Hz, H-16); 5.23
3 3 4
O
(
ddd, 1H,
J17,18 = 4.5 Hz,
J
16,17 = 3.0 Hz,
J
17,19
=
4
7
H
N
9
14
3
a
3
8
14a
10a
3
5
6
13
12
0
.6 Hz, H-17); 5.09 (‘dt’, 1H,
J18,19 = 4.0 Hz,
20a,20b = 12.0 Hz, J19,20a = 7.3 Hz, H-20a); 4.48 (‘ddt’,
19,20a = 7.3 Hz,
17,19 = 0.6 Hz, H-19); 4.36 (dd, 1H, J20a,20b = 12.0 Hz,
J19,20b = 4.5 Hz, H-20b); 2.27 (s, 3H, Ac); 2.16 (s, 3H,
Ac); 2.01 (s, 3H, Ac); 1.68 (s, 3H, Ac). C NMR
J17,18 = 4.5 Hz,
2
3
4
J
16,18
= 0.6 Hz, H-18); 4.67 (dd, 1H,
7
a
N1
10
2
3
J
H,
11
O
3
3
3
15
1
J
J
19,20b = 4.5 Hz,
J18,19 = 4.0 Hz,
O
AcO 16
4
2
19
α-
D-gluco
20
J
3
18
1
7
AcO
OAc
1
3
8
OAc