Chemistry of the ginkgolides. Part 10: Access to 14C-labelled ginkgolide A

Article abstract of DOI:10.1016/S0040-4020(00)00230-1

In four reaction steps, ginkgolide A (1) was synthesized from the enol- acetate 3, which is available in six steps from 1. The key step was the react on of the α-epoxy-acetate 4 with the lithium enolate of methyl propionate to give the compounds 5 and 6. ¹⁴C-Labelled ginkgolide A, which is of interest for pharmacological studies, can be synthesized using the ¹⁴C-labelled methyl propionate. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4020(00)00230-1

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 3173±3176
1
14
Chemistry of the Ginkgolides. Part 10: Access to C-Labelled
Ginkgolide A
*
Klaus Weinges, Hartmut Schick, Hermann Irngartinger and Thomas Oeser
Organisch-Chemisches Institut der Universit aÈ t Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany
Received 1 February 2000; accepted 13 March 2000
AbstractÐIn four reaction steps, ginkgolide A (1) was synthesized from the enol-acetate 3, which is available in six steps from 1. The key
step was the reaction of the a-epoxy-acetate 4 with the lithium enolate of methyl propionate to give the compounds 5 and 6. C-Labelled
1
4
1
ginkgolide A, which is of interest for pharmacological studies, can be synthesized using the C-labelled methyl propionate. q 2000 Elsevier
4
Science Ltd. All rights reserved.
Introduction
Results and Discussion
1
4
Ginkgolides are contained in a series of medicines. C-
Labelled ginkgolides are needed since they enable to
follow the metabolism and to carry out pharmacological
studies. Therefore we developed initially a simple path
with inactive compounds for the synthesis of ginkgolide
A using ginkgolide A (1) as starting material by
partially degrading and subsequently reconstructing it.
The ketone 2, which we have already reported and which
can be obtained from 1 in good yield in ®ve steps (I. H O
2
elimination, II. catalytical hydrogenation, III. esteri®cation
with diazomethane, IV. methoxymethylation, V. ozoniza-
tion) was transformed into the enol-acetate 3 by reaction
2
,3
4
with acetic anhydride/pyridine. Reaction of 3 with 3-
chloroperbenzoic acid yields the a-epoxy-acetate 4. Both
steps proceed in more than 80% yield. As determined by
X-ray analysis, the oxygen atom of the epoxy group of
1
4
This is the way C-labelled ginkgolide A is presently
being synthesized.
6
Figure 1. Molecular structure of 4.
1
Keywords: ginkgolide A; C-labelling; enolacetate; epoxidation.
4
*
Corresponding author. Tel.: 149-6221-548425; fax: 149-6221-544205; e-mail: hartmut.schick@urz.uni-heidelberg.de
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00230-1

3
174
K. Weinges et al. / Tetrahedron 56 (2000) 3173±3176
Scheme 1.
compound 4 is oriented in the direction of the neighboring
lactone ring (Fig. 1).
(ICN Biomedicals). Optical rotations were measured with a
Perkin±Elmer 241 polarimeter. The NMR spectra were
recorded on Bruker instruments (AC 200 and AC 300).
Mass spectra were taken on a Jeol JMS 700 instrument.
It is well known that the ring opening of a-epoxy ethers
always takes place between the oxygen of the epoxide and
5
the C-atom where the alkoxy group is bound. Accordingly,
3-Acetoxy-2,3-epoxy-10-methoxymethoxy-14,15,16-tri-
norginkgolide (4). In a 100 ml round bottom ¯ask 2 g
(4.73 mmol) of 3 was dissolved in 25 ml of benzene and
under stirring 1.5 g (6 mmol) 3-chloroperbenzoic acid (m-
Cl-PBA) (70%) was added. After 2 h, 1 g of m-Cl-PBA was
added again. After 24 h, a precipitate of 3-chlorobenzoic
acid was formed. For work up 10 ml of an aqueous solution
of Na S O was added and stirred for 10 min. The mixture
the reaction of a-epoxyacetate 4 with the lithium enolate of
methyl propionate yielded 5 and 6 in a ratio of about 1:1.
These compounds were separated by column chromato-
graphy and transformed into their 2-O-mesyl derivatives
5
a and 6a. The yield of 5 and 6 was only ca. 41%, and
could not be improved. After boiling 5a in dioxane/HCl
4-epi-ginkgolide A (7) was obtained together with a
1
2
2
5
small amount of anhydroginkgolide A, whereas 6a
produced the natural ginkgolide A (1) exclusively. We
were able to increase the yield of 1 by epimerization of 7
was transferred into a separatory funnel and the round
bottom ¯ask was irrigated several times with ethyl acetate.
The aqueous layer was separated and the organic layer was
2
to 1, which we have already reported (Scheme 1).
washed twice with a solution of NaHCO and with brine and
3
was dried with Na SO . After removal of the solvent the
2
4
Concerning stereoselectivity, the following statements can
be made: by the X-ray structure analysis of 4 it was proven
that the 3-chloroperbenzoic acid attacks the double bond of
residue was crystallized from a small quantity of acetone/
ethanol. Colorless crystals (quantitative yield), mp 145±
20
589
1468C (decomp.; R ˆ0.55 (toluene/acteone, 8:2); [a]
ˆ
f
2
0
78
20
546
13
3
at the C-3 atom only from the re-side. In the next reaction
22.1; [a]5 ˆ22.4; [a] ˆ23.3 (cˆ2, acetonitrile);
C
1
step the attack of the nucleophile comes from the side oppo-
site to the epoxide oxygen, generating the cis-diols 5 and 6,
which have the same con®guration of C-3 as the natural
ginkgolides. By the transformation of 5a into 14-epi-gink-
golide (7) and of 6a into natural ginkgolide A (1) the
con®guration of C-14 in 5 and 6 were established.
NMR: Table 2; H NMR (300 MHz, [D ]DMSO): dˆ6.05
6
(s, 1H, H ), 5.06 (s, 1H, H ), 5.06 (d, 1H, two spin system,
I
J
OCH O) and 4.93 (d, 1H, Jˆ6 Hz, two spin system,
2
OCH O), 4.70 (d, 1H, J ˆ3.7 Hz, H ), 4.60 (s , 1H,
2
B/D
B
br
H ), 3.35 (s, 3H, OCH ), 2.15 (s, 3H, CH CO), 2.1±2.6
3
H
3
(HC,D,E,F,G), 1.04 (s, 9H, tert-butyl). MS (FAB, positive-ion
mode): [M1H] (C H O ) calcd. 439.16043, found
1
2
1
27 10
1
4
39.1612; [M1Na] (C H O Na) calcd. 461.1426,
21 26 10
Experimental
found 461.1491. C21H O10 (438.4) calcd. C, 57.53, H,
26
5
.98; found C, 57.45, H, 5.99.
General
X-Ray analysis of 4
Melting points were determined by a B uÈ chi melting point
apparatus and are not corrected. For TLC sheets `Polygram
Sil G/UV254' (Macherey±Nagel) were used and for
developing spray reagent hydroxylamine/iron(III)-chloride
Table 1 contains the crystallographic data and details of the
re®nement procedure. The re¯ections were collected with a
Nonius-CAD4-diffractometer (Mo Ka-radiation, graphite
monochromator, v22Q-scan). Intensities were corrected
7
according to Stahl. CC was carried out with `Silica 32-63'

K. Weinges et al. / Tetrahedron 56 (2000) 3173±3176
3175
Table 1. Crystallographic data of 4
were calculated. The re®nement was carried out with
shelxl-97. Disorder was found at O11 and C24 (for
9
Compound
4
atomic numbering see Fig. 1) with a multiplicity of 66.7
and 33.3%.
1
0
Empirical formula
Molecular mass [g/mol]
Crystal size [mm]
Crystal color
C
21
H
438.4
26
O
10
0.5£0.45£0.45
(
2R,3R,10R,14R)-2,3-Dihydroxy-10-methoxymethoxy-
ginkgolide-15-acid methyl ester (5) and (2R,3R,10R,
4S)-2,3-dihydroxy-10-methoxymethoxy-ginkgolide-15-
Colorless
Irregular
Crystal shape
Space group
1
1 1 1
P2 2 2
Ê
a [A]
9.205(3)
10.359(4)
22.076(7)
2105(1)
1.38
acid methyl ester (6). 5.65 ml (40 mmol) diisopropylamine
in 30 ml THF was submitted in a three necked round bottom
¯ask (200 ml) equipped with a 50 ml and a 100 ml dropping
funnel (one with a supply for argon the other with a drying
tube) and a septum. 3.85 ml (40 mmol) methyl propionate in
30 ml THF was put into one dropping funnel, into the other
Ê
b [A]
Ê
c [A]
3
V [A ]
Ê
3
D
Z
calc [Mg/m ]
4
928
293
0/12
0/13
0/29
F(000)
Temperature [K]
3
2
.5 g (8 mmol) of 4 dissolved in 50 ml THF. Under argon at
70 to 2788C 25 ml (40 mmol) n-butyllithium (1.6 M in
n-hexane) was added slowly with a syringe through the
h
min/hmax
min/kmax
min/lmax
k
l
Q range [8]
]
2.2±28.0
0.11
94.7
septum under stirring. After 30 min and cooling to
2
21
m [mm
1008C the solution of methyl propionate was added slowly
T
T
min [%]
max [%]
drop by drop. After another 30 min the solution of 4 was
also added slowly. After stirring for 1 h the cooling bath was
removed and the mixture acidi®ed with 2 M HCl. The THF
was removed extensively in vacuo and the mixture was
extracted with ethyl acetate. The organic layer was washed
95.2
Re¯. collected
Re¯. unique
Re¯. observed [I.2s(I)]
Variables
2874
2874
2003
303
,0.01
0.041
0.115
1.03
(
D/s)max
with sat. solution of NaHCO and NaCl and dried with
3
R
R
Na SO . The solvent was removed and the preparation sepa-
4
w
2
S (Gof)
rated by CC (silica gel, toluene/actetone, 9:1). Aside from
the ®rst eluate, which contained methyl-(2-methyl-3-oxo)-
pentanoate, two isomeric products (5 and 6) were isolated.
2
3
Ê
(
(
Dr)max [e A
Dr)min [e A
]
3
0.20
20.15
2
Ê
]
for Lorentz and polarization effects. The structure was
solved by direct methods (shelxs-97 ). The structural para-
meters of the non-hydrogen atoms were re®ned aniso-
5: R ˆ0.43 (Toluene/acetone, 8:2), 0.8 g (20.7%) of color-
f
8
20
less crystals, mp 218±2198C (decomp.); [a] ˆ219.8;
5
89
2
0
20
13
[a]5 ˆ220.7; [a] ˆ223.8 (cˆ2, acetonitrile);
C
78
546
1
tropically according to
a
full-matrix least-squares
technique (F ). The parameters of the hydrogen atoms
NMR: Table 2. H NMR (300 MHz, [D ]DMSO): dˆ5.91
6
2
(s, 1H, H ), 5.22 (d, 1H, J
ˆ5.6 Hz, 2-OH), 5.12 (s,
H, H ), 5.04 (s, 1H, 3-OH), 5.04 (d, 1H, two spin system)
I
J
2-OH/HB
1
1
Table 2. C NMR data
3
and 4.96 (d, 1H, two spin system, Jˆ5.9 Hz, OCH O), 4.69
2
a
3
b
4
b
5
b
5a
a
6
b
(s , 1H, H ), 4.4 (m , 1H, H ), 3.57 (s, 3H, COOCH ), 3.31
c
C-
6a
br
H
B
3
(
(m , 1H, H ), 2.2±1.5 (H
s, 3H, OCH ), 2.90 (q, 1H, J
ˆ7.3 Hz, H ), 2.83
3
HA/16-CH3
A
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
1
2
3
4
5
6
7
8
35.60
121.73
141.74
98.26
68.03
87.91
35.47
49.11
66.71
73.76
109.33
±
33.87
67.57
85.39
95.04
69.69
87.10
35.30
49.02
68.08
73.59
109.43
±
37.09
73.69
80.57
101.52
66.80
88.06
35.56
48.91
65.72
73.69
108.29
42.10
12.55
31.87
34.53
81.55
81.30
100.11
67.04
87.70
35.70
48.95
66.30
73.78
108.34
42.08
11.78
31.97
38.14
78.87
81.11
34.91
81.32
80.54
), 1.23 (d, 3H, J
16-Me/
c
C
D,E,F,G
ˆ
7.3 Hz, 16-CH ), 1.02 (s, 9H, tert-butyl). C H O
3 23 32 11
484.5) calcd. C, 57.02; H, 6.66; found C, 57.04, H, 6.71.
HA
(
101.66 100.28
67.21
88.27
35.49
48.99
65.85
73.69
67.34
87.91
35.53
48.84
65.44
73.67
6:R ˆ0.35 (Toluene/acetone, 8:2), 0.8 g (20.7%) of color-
f
2
5
13
0
89
less crystals, mp 227±2298C (decomp.), [a] ˆ13.6;
20
78
20
546
[
a]5 ˆ 13.7; [a] ˆ13.9 (cˆ2, acetonitrile). C NMR:
9
1
Table 2. H NMR (200 MHz, [D ]DMSO): dˆ5.97 (s, 1H,
10
12
14
16
17
18
19
20
11
13
15
6
108.26 108.20
H ), 5.38 (d, 1H, J
J
2-OH/HB
ˆ6.6 Hz, 2-OH), 5.16 (s,, 1H, H ),
I
42.87
12.61
31.86
42.22
12.87
31.87
5.11 (d, 1H, two spin system) and 4.95 (d, 1H, two spin
system, OCH O), 4.73 (s , 1H, H ), 4.41 (s, 1H, 3-OH),
±
31.85
±
31.85
2
br
H
4
OCH ), 2.94 (m , 1H, H ), 2.72 (q, 1H, J
.19 (m , 1H, H ), 3.53 (s, 3H, COOCH ), 3.35 (s, 3H,
3
c
B
ˆ7.1 Hz,
28.75
28.75
28.82
28.88
28.80
28.78
3
c
C
HA/16-Me
H ), 2.1±1.5 (H
A
1.02 (s, 9H, tert-butyl). C H O (484.5) calcd. C, 57.07;
H, 6.69; found C, 57.04, H, 6.71.
), 1.24 (d, 3H, Jˆ7.1 Hz, 16-CH ),
D,E,F,G
3
169.44
167.74
±
172.52
95.67
56.52
20.47
±
169.77
166.81
±
172.52
95.69
56.52
20.53
±
171.07
172.25
173.25
±
95.70
56.55
±
171.17
171.82
171.93
±
95.90
56.68
±
171.50 170.94
172.21 171.80
173.82 173.52
2
3
32 11
CH CO
OCH
CH
CH
COOCH
CH SO
3
±
±
(2R,3R,10R,14R)-2-Mesyl-3-hydroxy-10-methoxymethoxy-
ginkgolide-15-acid methyl ester (5a) and (2R,3R,10R,14S)-
2
O
OCH
CO
95.75
56.55
±
51.28
±
95.85
56.59
±
51.57
37.38
3
3
2
2
-mesyl-3-hydroxy-10-methoxymethoxy-ginkgolide-15-
3
51.32
±
51.72
37.59
acid methyl ester (6a). 0.97 g (2 mmol) 5 or 6, respectively,
was dissolved in 15 ml dichloromethane and at 2108C
0.42 ml (3 mmol) triethylamine and 0.21 ml (2.7 mmol)
mesylchloride was added under stirring. The cooling bath
3
3
±
±
a
d values, 50.32 MHz, [D ]DMSO).
(d values, 75.47 MHz, [D ]DMSO).
6
b
6

3
176
K. Weinges et al. / Tetrahedron 56 (2000) 3173±3176
was removed and the stirring was continued for 2 h at room
temperature. Since the starting material was not visible
anymore on the TLC, the solution was washed sequentially
4 ml of 2.5 N HCl was added and re¯uxed for 5 days. The
solution was evaporated in vacuo to dryness, ethanol was
added twice and evaporated again. The residue was crystal-
lized from a small quantity of ethanol.
with 1 N HCl, NaHCO solution and brine and dried over
3
Na SO . After removal of the solvent the compound was
2
4
crystallized from acetone/toluene.
1: Colorless crystals; R ˆ0.35 (toluene/acetone, 7:3). All
f
analytical data were identical with those of a sample of
natural ginkgolide A.
2
,11
5a: Colorless crystals (yield: 94%), mp 162±1638C (decomp.);
R ˆ0.45 (toluene/acetone, 8:2), R ˆ0.38 (CH Cl /acetone,
1
acetonitrile). C NMR: Table 2. H NMR (300 MHz,
f
f
2
2
2
0
20
578
20
546
9
5:5); [a] ˆ239.2; [a] ˆ240.9; [a] ˆ248.4 (cˆ2,
Note: In the meantime the synthesis has been carried out
with C-labelled methyl propionate by the ®rm `BlyChem'
589
3
1
14
[
5
5
D ]DMSO): dˆ6.17, (s, 1H, 3-OH), 5.92 (s, 1H, H ),
(Billingham, UK). Thus detailed pharmacological research
of ginkgolides is possible.
6
J
.28 (dd, 1H, Jˆ7.7 Hz, Jˆ10.3 Hz, H ), 5.14 (s, 1H, H ),
B
I
.01 (d, 1H, two spin system) and 4.93 (d, 1H, two spin
system OCH O), 4.85 (s , 1H, H ), 3.59 (s, 3H,
H
2
br
COOCH ), 3.35 (s, 3H, OCH ), 3.18 (s1m , 4H, SCH
3
Acknowledgements
3
c
3
and H ), 2.97 (q, 1H, J
C
ˆ7.2 Hz, H ), 2.49 (m ,
HA/16-Me
A
c
solvent and H ), 2.2±1.5 (H
7
), 1.28 (d, 3H, J
.2 Hz, 16-CH ), 1.01 (s, 9H, tert-butyl). C H O S
ˆ
We thank Mrs U. Wiesinger for X-ray data collection and
Dr H. Jaggy, Firma Dr Wilmar Schwabe, Karlsruhe,
Germany, for providing us with ginkgolide-15-acid.
D
E,F,G
16-Me/HA
3
24 34 13
(
562.4) calcd. C, 51.24; H, 6.09; found C, 51.28, H, 6.11.
6a: Colorless crystals (yield: 94%), mp 184±1858C (decomp.);
R ˆ0.45 (toluene/acetone, 8:2), R ˆ0.37 (CH Cl /acetone,
1
acetonitrile). C NMR: Table 2. H NMR (300 MHz,
References
f
f
2
2
2
0
20
578
20
546
9
5:5); [a] ˆ231.5; [a] ˆ232.8; [a] ˆ237.6 (cˆ2,
589
13
1. Weinges, K.; Schick, H.; Irngartinger, H.; Oeser, T. (Part IX).
Liebigs Ann. Chem. 1997, 1755±1756.
2. Weinges, K.; R uÈ mmler, M.; Schick, H.; Schilling, G. Liebigs
Ann. Chem. 1993, 287±291.
[
D ]DMSO): dˆ6.07 (s, 1H, 3-OH), 5.97 (s, 1H, H ), 5.18
6
J
(
s1m, 2H, H and H ), 5.02 (d, 1H, two spin system) and
I B
4
1
.95 (d, 1H, two spin system, Jˆ5.8 Hz, OCH O), 4.90 (s ,
2
br
H, H ), 3.60 (s, 3H, COOCH ), 3.34 (HDO1OCH , and
H 3 3
3. Weinges, K.; Schick, H.; Pritzkow, H. Liebigs Ann. Chem.
1997, 991±993.
H [by H/D exchange]), 2.94 (m , 1H, H ), ù2.56 (m, H )
C
c
A
D
with 2.49 (solvent), 2.2±1.5 (H
), 1.27 (d, 3H, 16-CH ),
4. Weinges, K.; Schick, H.; Irngartinger, H.; Oeser, T. Liebigs
Ann. Chem. 1997, 1607±1609.
5. Stevens, C. L.; Cof®eld, T. H. J. Am. Chem. Soc. 1958, 80,
E,F,G
3
1.02 (s, 9H, tert-butyl). C H O S (562.4) calcd C, 51.24;
H, 6.09; found C, 51.31, H, 6.11.
24 34 13
1
919±1921.
(3R,10R)-3,10-Dihydroxy-14-epiginkgolide (14-epigink-
golide A) (7). 0.5 g (0.9 mmol) 5a was dissolved in 5 ml
6. Keller, E. schakal-97, Universit aÈ t Freiburg, Germany, 1997.
7. Stahl, E. D uÈ nnschichtchromatographie, 2nd ed.; Springer:
Berlin, 1987, p 834.
dioxane, 4 ml of 2.5 N HCl was added and re¯uxed for
5
days. The solution was evaporated to dryness in vacuo,
ethanol was added twice and evaporated again. Since the
residue showed two spots on the TLC, a separation by CC
8. Sheldrick, G. M. shelxs-97, Universit aÈ t G oÈ ttingen, Germany,
1997.
9. Sheldrick, G. M. shelxl-97, Universit aÈ t G oÈ ttingen, Germany,
1997.
(
3
toluene/acetone, 9:1) was necessary. Aside from 7 (10R)-
,14-didehydro-10-hydroxy-ginkgolide (anhydroginkgolide
10. Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publi-
cation no. CCDC-135942. Copies of the data can be obtained free
of charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (Fax: int. code 144(1223)336-033, E-mail:
deposit@ccdc.cam.ac.uk).
A; R ˆ0.53, toluene/acetone, 7:3) was obtained.
f
7: Colorless crystals from ethanol, mp 2558C (decomp.),
R ˆ0.23 (toluene/acetone, 7:3), 0.13 (CH Cl /acetone 9:1).
f
2
2
2
compare lit. for epimerization of 7 to 1 see lit. ]
2
[
(
0
3R,10R)-3,10-Dihydroxy-ginkgolide (ginkgolide A) (1).
.5 g (0.9 mmol) 6a was dissolved in 5 ml of dioxane,
11. Maruyama, M.; Terahara, A.; Itagaki, Y.; Nakanishi, K.
Tetrahedron Lett. 1967, 93, 299±302.