Synthesis of 10-hydroxycamptothecin: Evaluation of new moderators for the chemoselective reduction of camptothecin

Article abstract of DOI:10.1080/00397911.2010.515356

10-Hydroxycamptothecin is prepared by chemoselective catalytic hydrogenation of the B-ring of camptothecin over PtO₂ with sulfur moderators followed by oxidation using iodobenzenediacetate. New moderators (viz. thioanisole, dimethyl sulfide, diphenyl sulfide, 2-mercapto ethanol), which moderate the hydrogenation of the B- ring of camptothecin, are being explored. Dr. Reddy's Laboratories Ltd.

Full text of DOI:10.1080/00397911.2010.515356

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Synthesis of 10-Hydroxycamptothecin:
Evaluation of New Moderators for
the Chemoselective Reduction of
Camptothecin
N. M. Sekhar a , Yerramilli Anjaneyulu b & Palle V. R. Acharyulu c
a Research and Development, Integrated Product Development, Dr.
Reddy's Laboratories Ltd. , Ranga Reddy District , Andhra Pradesh ,
India
b Ecologic Technologies Pvt. Ltd. , Hyderabad , Andhra Pradesh ,
India
c Institute of Science and Technology, Center for Environmental
Science, J. N. T. University , Hyderabad , Andhra Pradesh , India
Published online: 29 Jun 2011.
To cite this article: N. M. Sekhar , Yerramilli Anjaneyulu & Palle V. R. Acharyulu (2011) Synthesis
of 10-Hydroxycamptothecin: Evaluation of New Moderators for the Chemoselective Reduction of
Camptothecin, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 41:19, 2828-2834, DOI: 10.1080/00397911.2010.515356
To link to this article: http://dx.doi.org/10.1080/00397911.2010.515356
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Synthetic Communications1, 41: 2828–2834, 2011
Copyright # Dr. Reddy’s Laboratories Ltd.
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.515356
SYNTHESIS OF 10-HYDROXYCAMPTOTHECIN:
EVALUATION OF NEW MODERATORS FOR THE
CHEMOSELECTIVE REDUCTION OF CAMPTOTHECIN
N. M. Sekhar,1 Yerramilli Anjaneyulu,2 and
Palle V. R. Acharyulu3
1Research and Development, Integrated Product Development, Dr. Reddy’s
Laboratories Ltd., Ranga Reddy District, Andhra Pradesh, India
2Ecologic Technologies Pvt. Ltd., Hyderabad, Andhra Pradesh, India
3Institute of Science and Technology, Center for Environmental Science,
J. N. T. University, Hyderabad, Andhra Pradesh, India
GRAPHICAL ABSTRACT
Abstract 10-Hydroxycamptothecin is prepared by chemoselective catalytic hydrogenation of the
B-ring of camptothecin over PtO2 with sulfur moderators followed by oxidation using iodoben-
zenediacetate. New moderators (viz. thioanisole, dimethyl sulfide, diphenyl sulfide, 2-mercapto
ethanol), which moderate the hydrogenation of the B- ring of camptothecin, are being explored.
Keywords Camptothecin; DMSO; 10-hydroxycamptothecin; moderator; thioanisole
INTRODUCTION
Camptothecin (1), an alkaloid with efficacy in animal tumor models, was iso-
lated from Chinese tree Camptotheca acuminata by Wall and coworkers in 1966[1]
and from the Indian tree Nothapodytes foetida by Govindachari and Viswanthan.[2]
Received April 30, 2010.
Address correspondence to Palle V. R. Acharyulu, Ecologic Technologies Pvt. Ltd., Plot Nos. 113
& 114, Road No. 5, ALEAP Industrial Area, Pragathinagar, Kukatpally, Hyderabad 500 072, Andhra
Pradesh, India. E-mail: pallevr@gmail.com
2828

SYNTHESIS OF 10-HYDROXYCAMPTOTHECIN
2829
Many camptothecin derivatives were prepared, and their biological activities
were successfully studied and reported.[3–11] Among the camptothecin derivatives,
10-hydroxycamptothecin (2) is a key intermediate for the preparation of drugs,
viz. irinotecan (3) and topotecan (4), which are used for the treatment of different
types of cancer, such as colon cancer, ovarian cancer, cervical cancer, and small cell
lung cancer.
10-Hydroxycamptothecin (2) gained importance in 1985 when it was reported
that it acts as an inhibitor of DNA topoisomerase.[11–14] Subsequently, it has been a
target for synthesis by numerous research groups because of its impressive biological
activity and paucity of naturally derived material. There are several synthetic
schemes reported in literature[11,14–16] for the synthesis of 10-hydroxycamptothecin
(2), and many of them are not suitable for commercial synthesis. Jeffery et al.[14] have
reported an efficient conversion of camptothecin (1) to 10-hydroxycamptothecin (2)
through chemoselective catalytic hydrogenation of the B-ring of camptothecin (1)
over PtO2 with dimethylsulfoxide (DMSO)[14,15] as moderator followed by oxidation
with iodosobenzenediacetate (Scheme 1).
As a part of research program on camptothecin derivatives, we explored the
synthesis of 10-hydroxycamptothecin using DMSO as moderator. However, the
hydrogenation reaction was found to be reasonable on a small scale but suffered
from inconsistency with regard to both yield and quality on a large scale. This
could be due to inconsistent conversion of DMSO to dimethylsulfide (DMS) during
the course of hydrogenation.[17–23] Further investigation motivated us to explore a
better moderator to bring consistency in the formation of the product. Herein, we
report the use of new moderators, viz. thioanisole, dimethylsulfide, diphenylsulfide,
and 2-mercaptoethanol in the reduction of the B-ring of camptothecin for consistent
reaction profile. The advantage has been demonstrated in the commercial appli-
cation to irinotecan (3)[24] and topotecan (4).[25]
Scheme 1. Synthesis of 10-hydroxycamptiothecin by using DMSO as moderator.

2830
N. M. SEKHAR, Y. ANJANEYULU, AND P. V. R. ACHARYULU
RESULTS AND DISCUSSION
Preparation of 10-hydroxycamptothecin (2) was carried out in two steps:
catalytic hydrogenation of camptothecin (1) to 1,2,6,7-tetrahydrocamptothecin
(1A) using PtO2 under hydrogen gas with DMSO as moderator,[14,15] followed by
oxidation with iodozobenzenediacetate (Scheme 1).
The inconsistency in the progress of the reaction may be attributed to these
factors:
1. Often the reaction was not initiated.
2. Presence of a significant amount of starting material even after prolonged
reaction time
3. Over-reduction of ring A along with the presence of a significant amount of
starting material.
Based on the understanding of the mechanism and effect of sulfur poisoning on
the catalyst, we suspected that the moderator could be the reason for the above pro-
blems. Also, it is known in the literature that sulfur is the actual moderator that poi-
sons the catalyst and causes selective partial reduction of B-ring. Selective reduction
depends on several parameters, such as type of catalyst, stirring, and temperature.
Thus, attempts were taken with representative sulfides to control the poisoning
effect and to have a robust and economical process for the preparation of 10-hydro-
xycamptothecin. When sulfur compounds, viz. thioanisole, dimethylsulfide, diphe-
nylsulfide, and 2-mercaptoethanol were used as moderators, better consistency in
yield and quality of the product were observed.
The preparation of 10-hydroxycamptothecin (2) from camptothecin (1) using
new moderators is represented in Scheme 2. The intermediate 1,2,6,7-tetrahydro-
camptothecin (1A) was not isolated as it is unstable, air sensitive, and converts back
to camptothecin (1).[14]
All these sulfide poisoners resulted in good conversion of camptothecin (1) to
10-hydroxycamptothecin (2) with good purity and yield (Table 1). As depicted in
Table 1, the reaction in DMSO was incomplete with 30% of starting material
remaining. Other moderators (thioanisole, dimethylsulfide, diphenylsulfide,
2-mercaptoethanol) gave good conversion of the camptothecin (<2%).
Although all sulfide poisoners, mentioned in Table 1, gave encouraging results
with regard to conversion of the starting material and purity of the product, one sul-
fide moderator had been chosen for further optimization. The sulfide 2-meracpto
ethanol was not considered further because of its toxic data, while DMS was rejected
Scheme 2. Synthesis of 10-hydroxycamptiothecin using new moderators. Moderator (M) ¼ thioanisole,
dimethylsulfide, diphenylsulfide, and 2-mercaptoethanol.

SYNTHESIS OF 10-HYDROXYCAMPTOTHECIN
2831
Table 1. Effect of moderators on product quality
Reaction temperature
(ꢀC)=pressure (psi)
HPLC
purity (%)
Unreacted starting Yield
material (%)
Entry Catalyst
Moderator
DMSO
DMS
(%)
1
2
3
4
5
6
PtO2
PtO2
PtO2
PtO2
PtO2
PtO2
60–65=60–65
60–65=60–65
60–65=60–65
60–65=60–65
60–65=60–65
60–65=60–65
25
30
40
64
67
65
63
59
98.1
98.7
98.5
97.9
97.8
1.6
1.1
1.3
1.7
1.8
Thioanisole
Thioanisole
2-Mercaptoethanol
Diphenylsulfide
for its volatility and flammability. Thioanisole has been chosen for further study and
optimization.
Further studies on the hydrogenation with thioanisole resulted in a highly
robust transformation with very high reproducibility levels and purity of the pro-
duct. The reaction has been scaled up to multikilograms with ease and control.
Because of the poor solubility of 2, purification was quite difficult. Hence, a
purification process for crude 10-hydroxycamptothecin (2) was established using
N,N-dimethylformamide and methanol to achieve the desired purity required for
further stages in the synthesis of active pharmaceutical ingredients (APIs). This
has been demonstrated in the formal synthesis of irinotecan hydrochloride (3) and
toptecan hydrochloride (4).
CONCLUSION
We developed an improved, cost-effective, and robust scalable process for
10-hydroxycamptothecin starting from naturally occurring camptothecin and
commercially available new sulfide moderators (dimethyl sulfide, diphenyl sulfide,
2-mercapto ethanol), and preferably with thioanisole and commercial solvents. The
established procedure was carried out on a pilot scale successfully, which increased
the yield and purity of 10-hydroxycamptothecin. Hence, further studies on the appli-
cation of prepoisoned catalyst with moderators will be reported in due course. The
utility of 10-hydroxycamptothecin has been demonstrated in the synthesis of irinote-
can and topotecan in achieving the quality required for preparation of dosage forms.
EXPERIMENTAL
Caution
Camptothecin has been established as a significant clastogenic agent, causing
chromosomal aberrations. Consequently, it and all structurally related compounds
must be considered potential mutagens and potential reproductive hazardous for
both males and females. Appropriate precautions (use of respirator, gloves, fume
hood) must be taken while handling these compounds.
General
All solvents and reagents were obtained from commercial sources and used
without further treatment or purification. The NMR experiments were recorded

2832
N. M. SEKHAR, Y. ANJANEYULU, AND P. V. R. ACHARYULU
on a 400-MHz Varian Mercury Plus Fourier transform (FT)-NMR spectrometer
1
using dimethylsulfoxide (DMSO-d6) solvent. H chemical shifts are reported on the
d scale in parts per million (ppm) relative to tetramethylsilane (TMS) (d 0.00) as inter-
nal standard. The electrospray ionization (ESI) and MS-MS studies were performed
on a triple quadrupole mass spectometer PE Sciex model API 3000 instrument.
Preparation of 1,2,6,7-Tetrahydrocamptothecin Using Thioanisole as
a Moderator
Camptothecin (2 kg, 5.74 mol) was mixed with glacial acetic acid (20 L) con-
taining 60 mL (0.3% v=v) of moderator (thioanisole), and the resulting suspension
was charged into a 50-L stainless steel, high-pressure, stirred autoclave. This was
followed by addition of 670 g of PtO2 (33% w=w), and the reactor was sealed. The
stirred reactor (300 rpm) was pressurized with hydrogen gas to a pressure of
60–65 Psi. This approximate pressure of hydrogen was maintained throughout the
reaction. The reactor was gradually heated to a temperature of about 60–65 ꢀC. After
4.5 h, the reactor was gradually cooled to about 30 ꢀC, and hydrogenation was con-
tinued at that temperature for 16 h. The reactor was then vented to the atmosphere,
and the reaction mass was removed from the reactor. The reactor was washed with
glacial acetic acid (about 6 L); the total volume of acetic acid was filtered to remove
suspended catalyst to obtain 1,2,6,7-tetrahydrocamptothecin as its solution in acetic
acid, which was oxidized directly.
Preparation of 10-Hydroxycamptothecin by Oxidation of
1,2,6,7-Tetrahydrocamptothecin
Water (26 L) was added to the solution of 1,2,6,7-tetrahydrocamptothecin in
26 L of acetic acid, as prepared in example 1. This was followed by addition of iodo-
sobenzene diacetate (4 kg, 12.41 mol) with rapid stirring in one portion. The slurry
turned dark green as nearly all of precipitate dissolved. Over a few minutes, the color
of the slurry faded to yellow as more precipitate formed. The slurry was stirred at
room temperature for 18 h. The mixture was then heated to distill the solvent at
atmospheric pressure. A total of 52 L of a 1:1 solution of acetic acid–water was
periodically added to maintain an approximately constant volume through most
of the distillation; the slurry was eventually concentrated to a final volume of
approximately 300 mL. The initial distillate was cloudy and separated into two
phases upon collection. The heavier phase of the distillate consisted of mostly iodo-
benzene. Distillation was stopped, and the yellow slurry was cooled to 25–30 ꢀC. The
solid was collected by filtration and rinsed with methanol (1.5 L ꢁ 2) and dried at
50 ꢀC under high vaccum to obtain 1.36 kg (yield: 65%) of 10-hydroxycamptothecin
as a fine yellow powder. The purity of the obtained product is >95% as determined
by high-performance liquid chromatography (HPLC) versus a standard sample. The
isolated product contained less than approximately 2% camptothecin.
The obtained product was optionally purified if necessary by dissolving the
wet cake in N,N-dimethylformamide (54 L) at 70–75 ꢀC, transferred to another
vessel containing methanol (25 L), stirred for 6–12 h at 25–30 ꢀC, filtered,
slurred with methanol (40 L) under reflux for 2 h, filtered, and dried at 55–60 ꢀC to

SYNTHESIS OF 10-HYDROXYCAMPTOTHECIN
2833
obtain 1.26 kg (yield: 60%) of yellowish powder with >98% purity by HPLC with
about 1% camptothecin.
Spectral Data
ES-MS: m=z 365.2 (M þ H)þ, 387.2 (M þ Na)þ, 402.3 (M þ K)þ, 729.4
1
(2M þ H)þ, 751.2 (2M þ Na)þ. H NMR (400 MHz, DMSO-d6): d 0.88 (t, J ¼ 7.2,
3H), 1.87 (m, 2H), 5.23 (s, 2H), 5.41 (s, 2H), 6.46 (s, OH), 7.26 (s, 1H), 7.28 (d,
J ¼ 2.4, 1H) 7.42 (dd, J ¼ 9.2, 2.4, 1H), 8.02 (d, J ¼ 9.2, 1H), 8.45 (s, 1H), 10.3 (s,
OH). 13C NMR (400 MHz, DMSO-d6): d 7.83, 29.89, 49.77, 64.87, 72.21, 95.53,
108.53, 117.87, 122.82, 129.05, 129.50, 129.85, 130.53, 142.99, 145.72, 149.33,
149.81, 156.52, 156.71, 172.28.
ACKNOWLEDGMENTS
The authors thank the management of Dr. Reddy’s Laboratories Ltd. for
supporting this work, and support from analytical colleagues is highly appreciated.
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