Synthesis and absolute configuration of 9-β-D-glucosyloxy-camptothecin, a new gluco camptothecin isolated from Ophiorrhiza pumila regenerated plants

Article abstract of DOI:10.1039/a708263k

9-β-D-Glucosyloxycamptothecin, which has been obtained from the regenerated plantlets of Ophiorrhiza pumila, has been synthesized and its structure and the absolute configuration established.

Full text of DOI:10.1039/a708263k

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Synthesis and absolute configuration of 9-â-D-glucosyloxy-
camptothecin, a new gluco camptothecin isolated from Ophiorrhiza
pumila regenerated plants
Mariko Kitajima, Mio Nakamura, Akihito Watanabe, Hiromitsu Takayama
and Norio Aimi*
Faculty of Pharmaceutical Sciences, Chiba University, 1–33, Yayoi-cho, Inage-ku, Chiba 263,
Japan
9-â-D-Glucosyloxycamptothecin, which has been obtained
from the regenerated plantlets of Ophiorrhiza pumila,
has been synthesized and its structure and the absolute
configuration established.
Introduction
Ophiorrhiza pumila Champ. (Rubiaceae) which grows on the
Amami and Ryukyu Islands, Japan, produces a remarkable
antitumor alkaloid, camptothecin 11 and related alkaloids.2a,b
R1
9
N
O
O
N
O
R2
O
1 R1 = R2 = H : Camptothecin
2 R1 = Glc, R2 = H : 9-β-D-Glucosyloxycamptothecin
3 R1 = Glc(OAc)4, R2 = Ac
During our chemical investigation of O. pumila,2 we have
recently succeeded in regenerating O. pumila from callus cul-
tures. From the regenerated plantlets,
a new hydroxy-
camptothecin glucoside 2 was obtained together with camp-
tothecin 1 and related alkaloids.2f The new metabolite was
characterized as the acetate and the structure was elucidated as
9-β-glucosyloxycamptothecin pentaacetate 3 based on spectro-
scopic studies.2f This compound was the second example of
naturally occurring glycosidic camptothecins. We concluded
that the position of the glucosyloxy group was at C-9 after an
extensive NMR study, and the absolute stereochemistry at C-20
was most probably S as is the case with other natural camp-
tothecins based on a CD spectral comparison (Fig. 1).2f The
absolute configuration of the sugar, glucose, in 3 was assumed
to be  as is encountered in almost all natural glucosides. The
validity of the proposed structure including the absolute con-
figuration at the chiral centers was unambiguously proved by
the chiral total synthesis, which is described here.
Fig. 1 Comparison of CD spectra for compounds 2 and 3
OR1
N
O
O
O
CHO
R2
O
R
O
4 R1 = H, R2 = NO2
5 R1 = Glc(OAc)4, R2 = NO2
6 R1 = Glc(OAc)4, R2 = NH2
7 R = H
8 R = Ac
Results and discussion
The Friedländer condensation strategy3 was adopted for the
construction of the entire molecule. For this purpose, the
A-ring moiety and CDE-ring counterpart were separately pre-
pared. First, 6-glucosyloxy-2-aminobenzaldehyde 6, corre-
sponding to the A-ring part of 2, was prepared from the
known phenol 44 as follows. The phenol 4 was condensed with
tetra-O-acetyl bromoglucose in the presence of K2CO3 in acet-
one under reflux to give the 2-glucosyloxy-6-nitrobenzaldehyde
5 in 59% yield.5 The nitro group in 5 was selectively reduced by
hydrogenation (H2, PtO2) to afford segment 6. In the 1H NMR
spectrum, the two protons of the amino group were observed as
a broad singlet at δ 6.40. The CDE-ring moiety 8 was prepared
from known (S)-73c {[α]D20 ϩ116.4 (c 0.51, CHCl3)} through
J. Chem. Soc., Perkin Trans. 1, 1998
389

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1
acetylation with AcCl (neat) in 85% yield. In the H NMR
Preparation of 9-â-D-glucosyloxycamptothecin 2
spectrum, the acetyl methyl protons were observed at δ 2.16.
Condensation of the two components 6 and 8 in 5% AcOH–
MeOH under reflux gave the camptothecin skeleton 3 in 58%
yield. The synthetic compound 3 was identical with the penta-
A mixture of 3 (4.7 mg, 0.006 mmol) and K2CO3 (8.7 mg, 0.063
mmol) in dry MeOH (3 cm3) was stirred at rt under argon for 2
h. The reaction mixture was subjected to Amberlite IR-120B
eluted with H2O. The water layer was extracted with BunOH
and the organic layer was washed with water, then evaporated.
The residue was purified by SiO2 column chromatography (25%
MeOH–CHCl3) to give 2 (2.7 mg, 80%). m/z (FAB, NBA)
527 (MHϩ) [HRMS (FAB): found MHϩ, 527.1660. Calc. for
C26H27N2O10, 527.1666]; λmax(MeOH)/nm 372 (sh), 359, 319,
304, 262; δH(400 MHz; [2H6]DMSO) 9.01 (1 H, s, H-7), 7.81 (1
H, d, J 7.9, H-12), 7.77 (1 H, dd, J 7.9 and 7.9, H-11), 7.36 (1 H,
d, J 7.9, H-10), 7.34 (1 H, s, H-14), 6.53 (1 H, s, 20-OH), 5.42 (2
H, s, H-17), 5.34 (1 H, d, J 19.8, H-5), 5.26 (1 H, d, J 19.8, H-5),
5.11 (1 H, d, J 7.8, H-1Ј), 4.62 (1 H, dd, J 6.1 and 5.9, 6Ј-OH),
3.73 (1 H, m, H-6Ј), 3.44 (1 H, m, H-6Ј), 3.44 (3 H, m, H-3Ј–5Ј‡),
3.27 (1 H, m, H-2Ј‡), 1.86 (2 H, m, H-19) and 0.87 (3 H, m,
H-18); δC(125 MHz; [2H6]DMSO) 152.8 (C-2), 145.5 (C-3), 50.4
(C-5), 129.2 (C-6), 126.5 (C-7), 120.4 (C-8), 153.0 (C-9), 96.8
(C-10), 130.5 (C-11), 122.4 (C-12), 148.6 (C-13), 110.2 (C-14),
150.0 (C-15), 119.1 (C-16), 65.3 (C-17), 7.8 (C-18), 30.3 (C-19),
72.4 (C-20), 172.5 (C-21), 156.9 (C-22), 101.1 (C-1Ј), 69.7
(C-2Ј)‡, 73.4 (C-3Ј)‡, 76.4 (C-4Ј)‡, 77.3 (C-5Ј)‡ and 60.7
(C-6Ј); CD (c 0.247 mmol dmϪ3, MeOH, 25 ЊC); ∆ε/dm3 molϪ1
cmϪ1 (λ/nm) 0 (389), Ϫ0.42 (354), 0 (318), ϩ0.31 (276), ϩ0.26
(266), ϩ3.44 (234), 0 (216).
1
acetate derived from the natural glucoside (TLC, UV, IR, H
and 13C NMR, CD and mass spectra).2f Thus the structure of
the new compound including the absolute configuration of
the sugar moiety and the C-20 position was established.
Deacetylation of 3 with K2CO3 (10 equiv.) in MeOH at room
temperature gave 9-β--glucosyloxycamptothecin 2 in 80%
yield.
In conclusion, 9-β--glucosyloxycamptothecin, the second
example of naturally occurring camptothecinoid glucosides,
was synthesized in a chiral manner and its absolute configur-
ation was established.
Experimental
Selected data for the A-ring segment 6
Yellow needles, mp 135 ЊC (AcOEt) (Found: C, 53.63; H, 5.51;
N, 3.06. C21H25NO11 requires C, 53.96; H, 5.39; N, 3.00%);
δH(400 MHz; CDCl3) 10.26 (1 H, s, CHO), 7.20 (1 H, t, J 8.2†,
aromatic-H), 6.40 (2 H, br s, NH2), 6.33 (1 H, d, J 8.2, aromatic-
H), 6.24 (1 H, d, J 8.2, aromatic-H), 5.15 (1 H, d, J 7.3, H-1Ј).
Selected data for the CDE-ring component 8
(Found: C, 58.85; H, 5.25; N, 4.79. C15H15NO6 requires C,
59.01; H, 4.95; N, 4.59%); δH(500 MHz; CDCl3) 6.76 (1 H, s,
aromatic-H), 2.16 (3 H, s, COCH3), 0.92 (3 H, dd, J 7.4 and 7.4,
CH3); CD (c 0.328 mmol dmϪ3, MeOH, 25 ЊC) ∆ε/dm3 molϪ1
cmϪ1 (λ/nm) 0 (377), Ϫ0.42 (360), 0 (335), ϩ0.25 (305), ϩ0.14
(287), ϩ2.78 (262), ϩ2.05 (245), ϩ7.90 (233).
Acknowledgements
Our thanks are due to the Ministry of Education, Science,
Sports and Culture, Japan, for a Grant-in-Aid for Scientific
Research (No. 08457578).
‡ Interchangeable.
Preparation of 9-â-D-glucosyloxycamptothecin tetraacetate 3
AcOH (0.2 cm3) was added to a mixture of the A-ring segment
6 (219 mg, 0.441 mmol) and the CDE-ring component 8 (19
mg, 0.063 mmol) in dry MeOH (4 cm3) and the mixture was
refluxed under argon for 11.5 h. The reaction mixture was
diluted with CHCl3, washed with sat. aqueous NaHCO3 and
then with water, dried (MgSO4) and evaporated. The residue
was purified by MPLC (SiO2, 20% AcOEt–CHCl3) to afford the
camptothecin derivative 3 (26.5 mg, 58%). [HRMS (FAB):
found MHϩ, 737.2167. Calc. for C36H37N2O15, 737.2194];
λmax(MeOH)/nm 371 (sh), 356, 316, 299, 260; νmax/cmϪ1 1752,
1235, 1053; δH(400 MHz; CDCl3) 8.68 (1 H, s, H-7), 7.94 (1 H,
d, J 8.5, H-12), 7.73 (1 H, dd, J 8.5 and 7.9, H-11), 7.21 (1 H, s,
H-14), 7.18 (1 H, d, J 7.9, H-10), 5.68 (1 H, d, J 17.3, H-17),
5.40 (1 H, d, J 17.3, H-17), 5.33 (1 H, d, J 19.8, H-5), 5.26 (1 H,
d, J 19.8, H-5), 5.34 (1 H, d, J 7.8, H-1Ј), 2.28 (1 H, dd, J 13.9
and 7.5, H-19), 2.22 (3 H, s, COMe), 2.14 (1 H, dd, J 13.9 and
7.5, H-19), 2.09, 2.08, 2.07 and 2.06 (each 3 H, s, 4 × COMe)
and 0.98 (3 H, dd, J 7.5 and 7.5, H3-18); δC(125 MHz; CDCl3)
153.0 (C-2), 146.0 (C-3), 50.2 (C-5), 128.5 (C-6), 126.0 (C-7),
121.1 (C-8), 152.2 (C-9), 110.2 (C-10), 130.2 (C-11), 124.3
(C-12), 149.4 (C-13), 96.2 (C-14), 145.8 (C-15), 120.6 (C-16),
67.1 (C-17), 7.5 (C-18), 31.8 (C-19), 75.9 (C-20), 167.5 (C-21),
157.3 (C-22), 99.1 (C-1Ј), 71.0 (C-2Ј), 72.3 (C-3Ј), 68.2 (C-4Ј),
72.3 (C-5Ј), 61.7 (C-6Ј); CD (c 0.054 mmol dmϪ3, MeOH,
25 ЊC) ∆ε/dm3 molϪ1 cmϪ1 (λ/nm) 0 (396), Ϫ1.79 (345), 0 (306),
ϩ0.63 (283), 0 (269), Ϫ1.04 (263), 0 (253), ϩ4.55 (232), ϩ1.58
(217), 0 (210).
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Paper 7/08263K
Received 17th November 1997
Accepted 10th December 1997
† J Values are given in Hz.
390
J. Chem. Soc., Perkin Trans. 1, 1998