Total synthesis of 7-ethyl-10-hydroxycamptothecin (SN38) and its application to the development of C18-functionalized camptothecin derivatives

Article abstract of DOI:10.1002/chem.201101389

A new chemical synthesis of SN38, the active metabolite of the camptothecin prodrug irinotecan, has been achieved in 12 steps from simple, commercially available starting materials. A mild and efficient FeCl₃-catalyzed Friedlaender condensation was successfully applied to construct the AB ring system. Functionalization of the C ring was accomplished by a vinylogous Mukaiyama reaction of an in situ generated N-acyliminium intermediate with a silyl enol ether. An intramolecular oxa Diels-Alder reaction efficiently constructed the D and E rings in one step. Successive asymmetric dihydroxylation and I₂-based hemiacetal oxidation furnished the stereochemistry of SN38 with high enantiopurity. Utilizing the ABC-ring intermediate and a functionalized silyl enol ether permitted the synthesis of a number of new C18-functionalized SN38 derivatives. Several of the novel SN38 derivatives that bore a C10 methoxy group were found to exhibit comparable or more potent inhibitory activity against the proliferation of cancer cells relative to SN38.

Full text of DOI:10.1002/chem.201101389

DOI: 10.1002/chem.201101389
Total Synthesis of 7-Ethyl-10-hydroxycamptothecin (SN38) and its
Application to the Development of C18-Functionalized Camptothecin
Derivatives
Yuan-Shan Yao,[a, b] Jia-Li Liu,[c] Jie Xi,[a] Bukeyan Miu,[a] Guai-Sai Liu,[a]
Shaozhong Wang,[a] Linghua Meng,*[c] and Zhu-Jun Yao*[a]
Abstract: A new chemical synthesis of
SN38, the active metabolite of the
camptothecin prodrug irinotecan, has
been achieved in 12 steps from simple,
commercially available starting materi-
als. A mild and efficient FeCl3-cata-
lyzed Friedlꢀnder condensation was
successfully applied to construct the
AB ring system. Functionalization of
the C ring was accomplished by a vi-
nylogous Mukaiyama reaction of an in
situ generated N-acyliminium inter-
mediate with a silyl enol ether. An in-
tramolecular oxa Diels–Alder reaction
efficiently constructed the D and E
rings in one step. Successive asymmet-
ric dihydroxylation and I2-based hemia-
cetal oxidation furnished the stereo-
chemistry of SN38 with high enantio-
purity. Utilizing the ABC-ring inter-
mediate and a functionalized silyl enol
ether permitted the synthesis of
a
number of new C18-functionalized
SN38 derivatives. Several of the novel
SN38 derivatives that bore a C10 me-
thoxy group were found to exhibit
comparable or more potent inhibitory
activity against the proliferation of
cancer cells relative to SN38.
Keywords: antitumor
agents
·
camptothecin derivatives · Diels–
Alder reaction · structure–activity
relationships · total synthesis
Introduction
traditional Chinese medicine.[1] The same group reported
the pentacyclic structure of 1 in 1966.[2] Due to the poor sol-
ubility of 1, carboxylate 2 was initially employed in the clini-
cal trials. Unfortunately, such a protocol proved unsuccessful
because of surprisingly severe toxicity.[3] The discovery of
DNA topoisomerase I, in the 1980s, as a cellular target of
the antitumor actions of 1 renewed the extensive worldwide
attentions.[4] Over the past several decades, a considerable
number of total syntheses of 1 and its useful derivatives
have been achieved with various innovative chemical trans-
formations.[5]
CPT and its derivatives are a successful story in modern
pharmaceutical development, especially in the last twenty
years. Based on numerous medicinal chemistry investiga-
tions, a logical map of the structure–activity relationship
(SAR) of the CPT family of alkaloids has been concluded.[6]
In general, the lactone moiety of the E ring is essential for
bioactivity. Substitutions on rings C, D, and E are not well
tolerated. Substitutent variation on the A and B rings has
proven useful to improve biological activity and water solu-
bility, especially at the C7, C9, and C10 positions, as well as
the C11 position. CPT analogues (Scheme 1) topotecan (Hy-
camtin, 1a) and irinotecan (Camptosar, 1b) have been ap-
proved for the treatment of ovarian, colon, and small-cell
lung cancers.[7] Several other derivatives, which include kaar-
enitecin (BNP-1350, 3), gimatecan (ST-1481, 4), rubitecan
(5), and lurtotecan (6), all in different stages of clinical
trials, have also been studied and show great potential of be-
coming antitumor drugs in the near future. In addition, di-
flomotecan (BN-80915, 7),[8] which features a seven-mem-
Camptothecin (CPT, 1, Scheme 1), a pentacyclic anticancer
alkaloid with powerful antitumor activities, was first isolated
by Wall and co-workers in 1958 from the extracts of Camp-
totheca acuminata, which originated in China and is used as
[a] Y.-S. Yao, J. Xi, B. Miu, Dr. G.-S. Liu, Dr. S. Wang,
Prof. Dr. Z.-J. Yao
State Key Laboratory of Coordination Chemistry
Nanjing National Laboratory of Microstructures
Institute of Chemical Biology and Drug Innovation
School of Chemistry and Chemical Engineering
Nanjing University
22 Hankou Road
Nanjing, Jiangsu 210093 (P. R. China)
Fax : (+86)25-83593732
E-mail: yaoz@nju.edu.cn
yaoz@sioc.ac.cn
[b] Y.-S. Yao
Joint Laboratory of Green Chemistry
Department of Chemistry
University of Science and Technology of China
Hefei, Anhui 230026 (P. R. China)
[c] J.-L. Liu, Prof. Dr. L. Meng
Division of Anti-Tumor Pharmacology
State Key Laboratory of Drug Research
Shanghai Institute of Materia Medica
Chinese Academy of Sciences
555 Zuchongchi Road
Shanghai 201203 (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101389.
10462
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 10462 – 10469

FULL PAPER
Alder reaction (for the DE ring-system formation) success-
fully served as the key steps.[5c] Retrosynthetic analysis of
our second synthesis of 1 and 10-OH CPT (1d) is outlined
in Scheme 2.
Scheme 1. Camptothecin (1), its carboxylate form, and representative
clinically useful derivatives.
bered lactone (homocamptothecin, hCPT) was found to be
much more stable than natural CPT in maintaining the lac-
tone functionality during clinical treatment.
For the excellent biological activities and increasing scar-
city of natural sources, efforts to acquire 1 and its deriva-
tives by chemical synthesis have emerged since the first
total synthesis of rac-1 by Stork and Schultz in 1971.[9] How-
ever, most established syntheses are impractical for large-
scale development because of their long routes, low overall
yields, high costs, and the involvement of harsh conditions.
Therefore, continuous efforts towards an economic, effi-
cient, and easy-to-operate synthesis of 1 are still of extreme
importance today. Recently, our laboratory disclosed two
syntheses of 1, which could also generally apply to other
members in this class of natural alkaloids, as well as their
medicinal analogues.[5d,10] To improve the efficiency of the
construction of the unique pentacyclic core of the CPT alka-
loids, the Diels–Alder reaction and its variants—with the
Scheme 2. Retrosynthesis of camptothecin (1), 10-hydroxycamptothecin
(1d), and SN38 (1c).
To the best of our knowledge, 1a and 1b, the two clinical-
ly used drugs, are both commercially produced from natural
CPT by semi-syntheses, and only a few total syntheses have
been reported.[12] To expand the applications of our newly
established methodologies, we decided to explore a new
chemical synthesis of SN38 (1c, Scheme 1), the bioactive
form of anticancer drug 1b, from readily available starting
materials. Herein, we report the results of our total synthesis
of 1c, as well as its application to the development of new
cytotoxic C18-functionalized CPT derivatives.
À
À
power to form multiple C C and C X bonds in a single re-
action—have been employed as useful protocols in our de-
velopment of efficient total syntheses. In our first route, a
mild cascade oxodiphosphonium-promoted intramolecular
aza Diels–Alder reaction and subsequent eliminative aroma-
tization was successfully applied to construct the common
ABCDE-ring skeleton of the CPT alkaloids in excellent
yields. Unfortunately, the involvement of multiple air- and
moisture-sensitive organometallic reagents in the synthesis
of the D/E ring precursor is very disadvantageous for labo-
ratory scaleup and future industrial applications.[11] To over-
come this shortage, we developed a second straightforward
route, in which a pyrrolidine-catalyzed room-temperature
cascade reaction (for construction of the AB ring system)
and an inverse-electron-demand intramolecular oxa Diels–
Results and Discussion
Synthesis of the A and B rings of 1c: In accordance with
our previous total synthesis (Scheme 2), the AB ring system
of 1c could be synthesized through a similar Michael addi-
tion/aldol condensation cascade annulation,[5c,12] by replace-
ment of aldehyde 12a with easily prepared ketone 12b.[13a]
Unfortunately, reaction of ketone 12b with unsaturated al-
dehyde 13 under the pyrrolidine/benzoic acid catalytic con-
ditions[5c] was unsatisfactory; little product was detected and
most of the starting materials remained. Further attempts to
optimize the conditions, which included variation of the cat-
alyst, reaction media, reaction time, and temperature, all
Chem. Eur. J. 2011, 17, 10462 – 10469
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10463

Z.-J. Yao, L. Meng et al.
failed. Clearly, ketone 12b is chemically inert under the
above conditions.
Friedlꢀnder condensation has been employed in several
previous syntheses of CPT derivatives with a substituted
quinoline (AB ring) moiety.[14] Under Wuꢂs conditions,[15] re-
action of ethyl 4-(benzyloxy)-3-oxobutanoate (15)[16] and
12b was easily accomplished with anhydrous FeCl3
(10 mol%) in the open air at room temperature to afford
the desired quinoline product 16 in 93% yield (Scheme 3).
Scheme 4. Completion of the total synthesis of SN38 (1c).
position of the acetate during purification by silica gel chro-
matography, a one-pot procedure was adopted for these two
steps. After treatment of the crude acetate product with
BF3·Et2O in CH2Cl2 at À788C for 30 min, silyl enol ether
25[5c] was slowly added at the same temperature. The expect-
ed product 26 was afforded in 69% yield, 94% based on re-
covered starting material (brsm).
Scheme 3. Synthesis of tricyclic compound 23.
Formation of the C ring of 1c: Ester 16 was reduced to alco-
hol 17 with LiAlH4 in diethyl ether at 08C in 84% yield
(Scheme 3). A number of oxidants were examined for the
subsequent oxidation of benzyl (Bn) alcohol 17. Dess–
Martin periodinane (DMP) was found to be the most effi-
cient and afforded benzaldehyde 18 in 90% yield. Oxime
formation and hydrogenation gave amine 20, as expected.
Notably, a catalytic amount of concentrated HCl is required
for the hydrogenation of 19 (1 atm at 308C). Selective N-
acylation of amino alcohol 20 with (E)-3-ethoxyacryloyl
chloride (21)[5c] was carried out in DMF to give amide 22 in
86% yield. Hemiaminal 23 was obtained in 85% yield after
treatment of 22 with freshly prepared acidic MnO2 in
CH2Cl2 at room temperature, along with a minor over-oxi-
dized byproduct 24 (5% yield).
The crucial inverse-electron-demand intramolecular oxa
Diels–Alder reaction of 26 was accomplished in mesitylene
at 160–1708C in a sealed tube. In this case, a single diaste-
reoisomer 27b was provided in 85% yield. The relative ste-
reochemistry of 27b was determined by NMR spectroscopy.
Next, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) de-
hydrogenation was carried out in 1,4-dioxane at room tem-
perature in the presence of a catalytic amount of acetic acid
(AcOH).[17] The crude product was then treated with Et3SiH
and BF3·Et2O[18] in dichloromethane at room temperature to
give 28 in 75% yield (2 steps). Sharpless asymmetric dihy-
droxylation[5d] and subsequent I2/CaCO3-based hemiacetal
oxidation successfully converted the cyclic enol ether 28 to
CPT 29 in 57% yield (80% brsm) and 92% enantiomeric
excess (ee), determined by chiral HPLC. Demethylation of
29 was completed with 48% HBr in AcOH[13b] in a sealed
tube to afford 1c in 80% yield. The physical data for 1c
were in good agreement with those previously reported.[13d]
Completion of the total synthesis of 1c: With the tricyclic
hemiaminoacetal 23 in hand, elaboration of the D and E
rings of 1c was continued (Scheme 4). C-Alkylation of 23
was achieved by a vinylogous Mukaiyama aldol reaction in
a two-step procedure. The hemiacetal was first converted
into the corresponding acetate, which served as a precursor
to the active N-acyliminium intermediate. To avoid decom-
Design of new CPT derivatives: As mentioned, natural CPT
is somewhat unsuccessful in clinical studies due to its non-
specific toxicity and poor water solubility. To improve the
10464
www.chemeurj.org
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 10462 – 10469

C18-Functionalized Camptothecin Derivatives
FULL PAPER
drawbacks associated with the natural alkaloid, a large
number of CPT derivatives have been developed
(Scheme 1). The most common synthetic strategies are
direct modifications of the natural structure of CPT on sev-
eral chemically available positions. However, such positions
are very limited in the skeleton of 1. For example, it is im-
possible to introduce additional functionality into the termi-
nal of the ethyl group of 1 (C18, see Scheme 1). To demon-
strate the innovative ability of the above newly established
total synthesis of 1c, we focused on the enantioselective syn-
thesis of new C18-functionalized derivatives.
Scheme 6. Synthesis of silyl enol ether 33.
Modification of the ethyl group situated on the E ring of
1 is rather difficult. Jew and co-workers have synthesized
several simply functionalized CPT analogues in racemic
form.[19] By using our strategy, this task would be easier
(Scheme 5). Replacement of silyl enol ether 25 with pre-
functionalized equivalent 33 (Scheme 5, R1 =OAc) in the
Scheme 7. Synthesis of pentacyclic enol ether 40.
Scheme 5. Retrosynthetic analysis of newly designed derivatives of 1.
meric products, 39a and 39b, in 89% combined yield (39a/
39b 1:6). The relative stereochemistries of 39a and 39b
were confirmed by NMR spectroscopy and comparison with
our previous data for similar compounds.[5c] Subsequent
DDQ dehydrogenation and Et3SiH reduction afforded the
pentacyclic compound 40 in 70% yield (over two steps).
Mukaiyama reaction with intermediate 23 would be a
straightforward way to introduce an additional hydroxyl
group to the terminal of the ethyl group at the C18 position
of 1c. This newly introduced hydroxyl group could be an ex-
cellent starting point to introduce other diverse functionali-
ties at the C18 position of the CPT skeleton.
Asymmetric dihydroxylation: Asymmetric dihydroxylation
of cyclic enol ether 40 and subsequent iodine oxidation were
applied to establish the desired stereochemistry of CPT de-
rivative 41 (Scheme 8). Unfortunately, it was found that con-
version of olefin 40 into the corresponding diol terminated
at an early stage of reaction and only a small amount of
starting material 40 was consumed. Further observation in-
dicated that the subsequent I2/CaCO3 oxidation proceeded
very well. Use of CH2Cl2 as a cosolvent led to an accelerat-
ed and improved dihydroxylation reaction, and gave the de-
sired product 41 in 50% yield (78% brsm). It was difficult
to optimize a set of satisfactory conditions to directly ana-
lyze sample 41 by chiral HPLC. After removal of the acetyl
group of 41 under reflux conditions in ethanol, the diol 42
was obtained in 80% yield. Interestingly, a minor ethyl
Synthesis of the pentacyclic precursor 40: Silyl enol ether 33
was prepared from the inexpensive g-lactone 34 in four
steps (Scheme 6). Notably, the amount of trimethylsilyl tri-
fluoromethanesulfonate (TMSOTf) had to be controlled at
0.85–0.90 equiv when aldehyde 37 was converted into 33.
Use of excess TMSOTf could easily cause a reverse transfor-
mation during the workup procedure.
With compounds 23 and 33 in hand, Mukaiyama alkyla-
tion on the C ring was attempted under the standard condi-
tions (Scheme 7). To our delight, vinylogous Mukaiyama re-
action proceeded smoothly and provided the desired prod-
uct 38 in 59–70% yield (88–94% brsm). The subsequent in-
tramolecular oxa Diels–Alder reaction was conducted in a
sealed tube at 160–1708C and gave two separable diastereo-
Chem. Eur. J. 2011, 17, 10462 – 10469
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10465

Z.-J. Yao, L. Meng et al.
(MOM), tert-butyldimethylsilyl
(TBS), and tert-butyldiphenyl-
silyl (TBDPS) ethers 46a–c, re-
spectively, were obtained in 92–
95% yield (Scheme 9).
Substrates 40 and 46a–c were
then examined under various
dihydroxylation conditions, by
altering the loadings of ligand,
osmium oxidant, and sulfona-
mide, or by varying the reaction
temperature (Table 1). As men-
tioned above, dihydroxylation
of acetate 40 under the stan-
dard conditions (at 08C) afford-
ed an 82% ee of diol 42 (deter-
Scheme 8. Sharpless asymmetric dihydroxylation of cyclic enol ether 40.
ether byproduct 43 (7% yield) was also isolated. If metha-
nol was used as the solvent, methyl ether 44 was obtained in
5% yield. Chiral HPLC analysis of 42 was successful, how-
ever, the optical purity of 82% ee was unsatisfactory for our
work in the further development of new derivatives for bio-
logical assessment. Additional efforts were required for the
asymmetric dihydroxylation of 40.
Sharpless conditions—(DHQD)2–PYR (catalytic ligand),
K2OsO2(OH)4 (catalytic oxidant), CH3SO2NH2 (sulfonamide
additive)—have been commonly employed for the asymmet-
ric dihydroxylation of various alkenes.[20] Generally, the
loadings of ligand and osmium catalyst are crucial for the
enantioselectivity. Less than 1 mol% catalytic osmium oxi-
dant and less than 2 mol% ligand are usually adequate
amounts to achieve ideal results. In addition, methanesulfo-
namide is a very efficient additive to accelerate the reaction.
The reaction time can be
Scheme 9. Preparation of dihydroxylation substrates 46a–c.
greatly shortened in the pres-
Table 1. Examination of the Sharpless asymmetric dihydroxylation.
ence of sulfonamide (1 equiv).
In a wide range of olefin sub-
strates, the reaction tempera-
ture also affects the optical
purity of the products. In
other words, the results of the
reaction are sensitive to multi-
ple factors in the dihydroxyla-
tion conditions and fine opti-
mizations are usually needed
case-by-case. Based on our
above results, several other
substrates were considered for
further
examination.
The
acetyl group of 40 was re-
moved by treatment with
K2CO3/MeOH at room tem-
perature to give alcohol 45 in
85% yield. Alcohol 45 was
elaborated with three differ-
Entry
Substrate
Conditions
ee [%] of 42
1
2
3
4
5
6
7
40
40
40
40
46a
46b
46c
ligand (2 mol%), oxidant (0.4 mol%), additive (1 equiv), 08C
ligand (2 mol%), oxidant (0.4 mol%), additive (1 equiv), RT
ligand (5 mol%), oxidant (1 mol%), additive (2 equiv), 08C
ligand (3 mol%), oxidant (0.5 mol%), additive (2 equiv), 08C
ligand (5 mol%), oxidant (1 mol%), additive (2 equiv), 08C
ligand (5 mol%), oxidant (1 mol%), additive (2 equiv), 08C
ligand (5 mol%), oxidant (1 mol%), additive (2 equiv), 08C
82
71
85
99
71
89
80
ent
hydroxyl
protecting
groups and methoxy methyl
10466
www.chemeurj.org
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 10462 – 10469

C18-Functionalized Camptothecin Derivatives
FULL PAPER
Table 2. New CPT derivatives prepared from the common intermediate
42.
mined after transformation to the lactone with I2/CaCO3
and acid hydrolysis of the acetate group). If the reaction
was carried out at room temperature, a lower enantiopurity
of 42 (71% ee) was obtained. To improve the conversion of
starting material, we further examined the reaction at 08C
by increasing the loadings of catalytic ligand and osmium
oxidant and increasing the amount of the methanesulfona-
mide additive but, unfortunately, little improvement was
achieved (Table 1, entries 1–3). Use of methanesulfonamide
(2 equiv) greatly accelerated the reactions and improved the
conversion of substrate. Up to 90% conversion of starting
material was achieved and the overall yield of 41 was raised
to 63% (over two steps). Under the same conditions, sub-
strates 46a–c were also examined. The reaction of 46b gave
the best result (89% ee, determined by HPLC after TBS re-
moval; Table 1, entry 6), whereas the ee values for the reac-
tions of 46a and 46c were somewhat poor (Table 1, entries 5
and 7). These results indicate that use of methanesulfona-
mide (2 equiv) and a reaction temperature of 08C were im-
portant and that the loadings of ligand and osmium catalyst
could be reduced to a lower level. A combination of ligand
(3 mol%), osmium catalyst (0.5 mol%), and sulfonamide
(2 equiv) was finally found to be the most efficient set of
conditions for the dihydroxylation of 40. The ee of diol 42
could be improved up to 99% (determined by HPLC after
removal of the acetyl group from 41) under the optimized
conditions (Table 1, entry 4).
entry
1
48 or 51
49 or 52
Yield [%][a]
71
48a
48b
49a
2
49b
70
3
4
5
48c
48d
48e
49c
49d
49e
64
75
73
Synthesis of new CPT derivatives: Two new series of CPT
derivatives (oxygen or nitrogen functionalized at the C18
position) were considered for further functionalization of
the newly introduced primary hydroxyl group of enantio-
pure diol 42.
6
51a
52a
75
7
8
9
51b
51c
51d
52b
52c
52d
94
82
85
A number of analogues were synthesized and the results
are summarized in Table 2. Five esters 49a–e were prepared
by selective esterification of the diol 42 with the carboxylic
acids 48a–e, respectively, in the presence of 1-ethyl-3-(3-di-
methylaminopropyl) carbodiimide (EDCI). For example,
ester 49a was prepared in 71% yield by the reaction with
acid 48a (1.1 equiv) in the presence of EDCI (1.5 equiv)
and 4-dimethylaminopyridine (DMAP) (50 mol%) in
CH2Cl2, 08C–RT (Table 2, entry 1). Under the same condi-
tions, reaction of 48b gave the diester (reaction at both the
primary and tertiary alcohols) as a major product, even
when the temperature was lowered to 08C. This situation
was improved when DMF served as the solvent[21] and esters
49b–e were obtained as the major products in good yields.
Nitrogen is a frequently used element in many pharma-
ceutical products. Accordingly, we attempted to introduce
some N-heterocycles into the common precursor 42. After
selective mesylation of the primary alcohol of 42 in DMF, a
number of amines (3 equiv) were subjected in parallel to
mesylate 50 at 60–708C for 8–12 hr (Table 2). Notably, com-
pounds 52i and 52j (Table 2, entries 14 and 15) could be
synthesized from products 52d and 52e (Table 2, entries 9
and 10), respectively, by deprotection of the N-Boc group in
TFA/CH2Cl2 (1:2 v/v) at RT.
10
51e
52e
93
11
12
51 f
51g
52 f
52g
92[b]
80[b]
13
51h
52h
75[b]
14
15
52i
52j
70[c]
78[c]
[a] Isolated yields. [b] Based on recovered starting material. [c] N-Boc
groups of 52d and 52e were removed in TFA/CH2Cl2 (1:2 v/v) at RT to
afford 52i and 52j, respectively.
Chem. Eur. J. 2011, 17, 10462 – 10469
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10467

Z.-J. Yao, L. Meng et al.
Table 3. Preparation of C10-hydroxyl CPT derivatives by O-demethyla-
tion.
mine B (SRB) or cell counting kit-8 (CCK-8) assays against
the proliferation of four cancer cell lines originated from
different tissue types. Initially, we investigated the effects of
1c and its derivatives by treatment of HL60 cells at a con-
centration of 100 nm. It was found that those compounds
which bore a smaller oxygenated functionality at C18 (esters
41, 49c, and 50; alcohol 42; ethers 44, 47a, and 47b) signifi-
cantly inhibited the proliferation of HL60 cells with inhibi-
tory rates similar to those of 1c. In the series of compounds
with terminal nitrogen functionalities, only 52c, which bears
a morphiline moiety, exhibited potent antiproliferative activ-
ity. More surprisingly, the derivatives with a free C10 phenol
hydroxyl showed much less potency relative to their 10-me-
thoxy derivatives. To the best of our knowledge, this is very
unusual and not concluded in the general SAR of CPT and
its derivatives.
Based on these results, we selected eight compounds (41,
42, 44, 47a–c, 50, and 52c) that displayed potent activity in
HL60 cells for further assessment in human colon cancer
(HCT-116), human liver cancer (BEL-7402), and human cer-
vical cancer (Hela) cells. IC50 values were measured by stan-
dard methods and the results are summarized in Table 4.
Relative to the other cell lines, BEL-7402 is less sensitive to
these CPT derivatives. We also found that several of the
newly prepared compounds exhibited more potent antiproli-
ferative activity than 1c. For instance, the IC50 of 44 against
BEL-7402 is about 4 times lower than that of 1c, and 47a
inhibited the proliferation of Hela cells by 60.6% at a con-
centration of 10 nm with an IC50 below 10 nm, a more potent
activity than 1c in Hela cells.
Entry Substrate Conditions[a] (R/substrate)
Product
Yield[b]
[%]
1
52c
48% aq HBr (sealed),
1108C, 5–7 h
complicated[c]
NR[b]
–
2
3
4
52c
49a
49a
TMSI, CH2Cl2, 08C–RT
–
–
–
BBr3, CH2Cl2, 7888C to RT complex[c]
48% aq HBr, N2, reflux,
1 h
clean reac-
tion[d]
5
6
42
R=OH
53a
80
65
52a
53b
53c
53d
53e
7
8
9
52b
52c
52d
80
75
70[e]
10
11
52 f
52g
53 f
53g
85
80
[a] Compounds 53a–g were prepared under the optimized conditions re-
ported in entry 4: 48% aq. HBr, heated at reflux, N2 atmosphere. [b] Iso-
lated yield based on recovered starting material. [c] Detection by TLC
analysis. [d] Only starting material and the desired product were detect-
ed. [e] 52i was also recovered.
Table 4. IC50 values of selected CPT derivatives against the proliferation
of cancer cells.[a,b]
According to the generalized SAR of CPT derivatives,
the free phenol hydroxyl group at the C10 position is impor-
tant for the biological activity. Thus, demethylation of the
C10 methyl ether of the above newly prepared compounds
was attempted. Under the demethylation conditions from
our previous work,[5c] reaction of 52c gave several unidenti-
fied compounds before the starting material was consumed
(Table 3, entry 1). When TMSI in CH2Cl2 at 08C–RT were
the conditions for demethylation[22] no product was generat-
ed and 52c was completely recovered (Table 3, entry 2).
Treatment with a solution of BBr3 in CH2Cl2 is another
known protocol for demethylation of aryl methyl ethers.[23]
However, the reaction of ester 49a with BBr3 at À788C–RT
gave no product and reaction at room temperature afforded
a mixture of the desired product and several byproducts
(Table 3, entry 3). Fortunately, treatment of 49a with a 48%
aqueous solution of HBr at reflux conditions and under a N2
atmosphere[24] provided the desired product, with concomi-
tant hydrolysis of the acetyl group (Table 3, entry 4). Under
these optimized conditions, seven demethylated derivatives
53a–g were prepared (Table 3, entries 5–11).
Compound
IC50 [nm]
HL60
13.5
36.3
25.5
19.7
5.81
61
50.6
26.9
19.7
HCT-116
33
BEL-7402
Hela
204.3
41.9
529.4
11.4
<10
32.2
111.9
262.7
23.6
1c
41
42
44
474.9
352.9
668
104
69.8
45.8
32
160.6
91.3
47.6
94.6
92.9
114
439
280.1
449.8
240.9
47a[c]
47b[d]
49c
50
52c
[a] Antiproliferative activities of the CPT derivatives were measured by
SRB assay in BEL-7402 and HCT-116 cells, or by CCK-8 assay in HL60
and Hela cells after 3 d of incubation. The concentrations required to in-
hibit the cell proliferation by 50% (IC50) were calculated, and data pre-
sented are the average IC50 values from at least three independent tests.
[b] Enantiopurity of the sample=99% ee. [c] Enantiopurity of the
sample=71% ee. [d] Enantiopurity of the sample=89% ee.
Conclusion
A new chemical synthesis of SN38 (1c), the active metabo-
lite of anticancer drug irinotecan (1b), has been accom-
plished in 12 steps from simple commercially available start-
ing materials. A mild FeCl3 catalyzed Friedlꢀnder condensa-
Biological screening: All newly prepared derivatives and
their 10-methoxy analogues were screened by sulforhoda-
10468
www.chemeurj.org
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 10462 – 10469

C18-Functionalized Camptothecin Derivatives
FULL PAPER
1989, 246, 1046; c) S. Sawada, S. Okajima, R. Aiyama, K. Nokata, T.
Furuta, T. Yokokura, E. Sugino, K. Yamaguchi, T. Miyasaka, Chem.
Pharm. Bull. 1991, 19, 1446; d) U. Vanhoefer, A. Harstrick, W. Ach-
terrath, S. Cao, S. Seeber, Y. M. Rustum, J. Clin. Oncol. 2001, 19,
1501.
tion, a Lewis acid catalyzed vinylogous Mukaiyama aldol-
type reaction, an intramolecular oxa Diels–Alder reaction,
and a Sharpless asymmetric dihydroxylation successfully
served as key reactions in the synthesis. With the newly es-
tablished methodologies, a number of new derivatives of 1c
with nitrogen or oxygen functionality at the C18 position
have been synthesized by replacement of the silyl enol ether
25 from the synthesis of 1c with silyl enol ether 33. This is
the first time the enantioselective synthesis of C18-function-
alized CPT derivatives has been achieved. Derivatives 44
and 47a were found to exhibit more potent inhibitory activi-
ty than 1c against the proliferation of cancer cells. In addi-
tion, among these new derivatives, the C10 free phenol com-
pounds are less potent than their 10-methoxy derivatives.
This is unusual in the general SAR of CPT and its deriva-
tives and should be a valuable addition to the present litera-
ture.
[8] For more information, see reviews and papers therein: a) M. S.
Butler, Nat. Prod. Rep. 2005, 22, 162; b) S. M. Butler, Nat. Prod.
Rep. 2008, 25, 475.
[9] G. Stork, A. G. Schultz, J. Am. Chem. Soc. 1971, 93, 4074.
[10] H. B. Zhou, G. S. Liu, Z.-J. Yao, J. Org. Chem. 2007, 72, 6270.
[11] D. Bankston, F. Fang, E. Huie, S. Xie, J. Org. Chem. 1999, 64, 3461.
[12] a) I. Ibrahem, H. Sundꢃn, R. Rios, G. L. Zhao, A. Cꢄrdova, Chimia
2007, 61, 219; b) H. Sundꢃn, R. Rios, I. Ibrahem, G. L. Zhao, L.
Eriksson, A. Cꢄrdova, Adv. Synth. Catal. 2007, 349, 827.
[13] a) W. Shen, C. A. Coburn, W. G. Bornmann, S. J. Danishefsky, J.
Org. Chem. 1993, 58, 611; b) H. Josien, S. B. Ko, D. Bom, D. P.
Curran, Chem. Eur. J. 1998, 4, 67; c) K. Tagami, N. Nakazawa, S.
Sano, Y. Nagao, Heterocycles 2000, 53, 771; d) Y. Luo, H. Y. Gao,
Y. F. Li, W. G. Huang, W. Lu, Z. G. Zhang, Tetrahedron 2006, 62,
2465.
[14] a) M. E. Wall, M. C. Wani, S. M. Nstschke, A. W. Nicholas, J. Med.
Chem. 1986, 29, 1553; b) S. S. Jew, K. D. Ok, H. J. Kim, M. G. Kim,
J. M. Kim, J. M. Hah, Y. S. Cho, Tetrahedron: Asymmetry 1995, 6,
1245; c) K. E. Henegar, S. W. Ashford, T. A. Baughman, J. C. Sih,
R. L. Gu, J. Org. Chem. 1997, 62, 6588.
Acknowledgements
[15] J. Wu, L. Zhang, T. N. Diao, Synlett 2005, 2653.
[16] a) S. Wolfe, S. Ro, Z. Shi, Can. J. Chem. 2001, 79, 1259; b) S. L.
Zheng, W. Y. Yu, C. M. Che, Org. Lett. 2002, 4, 889.
[17] a) S. P. Chavan, K. Pasupathy, M. S. Venkatraman, R. R. Kale, Tetra-
hedron Lett. 2004, 45, 6879; b) K. Mekouar, Y. Gꢃnisson, S. Leue,
A. E. Greene, J. Org. Chem. 2000, 65, 5212.
Financial support from NSFC (No. 21032002), Ministry of Health (No.
2009ZX09501), 973 Program (No. 2010CB833200), and the Fundamental
Research Funds for the Central Universities (No. 1082020502) is greatly
appreciated.
[18] S. W. Elmore, M. J. Coghlan, D. D. Anderson, J. K. Pratt, B. E.
Green, A. X. Wang, M. A. Stashko, C. W. Lin, C. M. Tyree, J. N.
Miner, P. B. Jacobson, D. M. Wilcox, B. C. Lane, J. Med. Chem.
2001, 44, 4481.
[19] a) S. S. Jew, M. G. Kim, H. J. Kim, E. Y. Rho, Y. S. Cho, J. K. Kim,
K. H. Cha, K. K. Lee, H. J. Han, H. Lee, Bioorg. Med. Chem. Lett.
1996, 6, 849; b) S. S. Jew, M. G. Kim, H. J. Kim, E. Y. Rho, H. G.
Park, J. K. Kim, H. J. Han, H. Lee, Bioorg. Med. Chem. Lett. 1998,
8, 1797.
[20] H. C. Kolb, M. S. VanNieuwenhze, K. B. Sharpless, Chem. Rev. 1994,
94, 2483.
[21] X. D. Pan, R. Han, P. Y. Sun, Bioorg. Med. Chem. Lett. 2003, 13,
3739.
[22] a) S. Chackalamannil, Y. G. Wang, Y. Xia, M. Czarniecki, Tetrahe-
dron Lett. 1995, 36, 5315; b) X. K. Zhu, J. Guan, Z. Y. Xiao, M. Co-
sentino, K. H. Lee, Bioorg. Med. Chem. 2004, 12, 4267.
[23] a) E. H. Vickery, L. F. Pahler, E. J. Eisenbraun, J. Org. Chem. 1979,
44, 4444; b) Z. Ahmed, P. Langer, Tetrahedron 2005, 61, 2055.
[24] a) M. Sugimori, A. Ejima, S. Ohsuki, K. Uoto, I. Mitsui, K. Matsu-
moto, Y. Kawato, M. Yasuoka, K. Sato, H. Tagawa, H. Terasawa, J.
Med. Chem. 1994, 37, 3033; b) M. Sugimori, A. Ejima, S. Ohsuki, K.
Uoto, I. Mitsui, Y. Kawato, Y. Hirota, K. Sato. H. Terasawa, J. Med.
Chem. 1998, 41, 2308.
[1] M. E. Wall in Chronicles of Drug Discovery, Vol. 3 (Ed.: D. Lednic-
er), ACS, Washington, 1993, p. 327.
[2] M. E. Wall, M. C. Wani, C. E. Cook, K. H. Palmer, A. T. McPhail,
G. A. Sim, J. Am. Chem. Soc. 1966, 88, 3888.
[3] a) C. G. Moertel, A. J. Schutt, R. J. Reitemeier, R. G. Hahn, Cancer
Chemother. Rep. 1972, 56, 95; b) J. A. Gottlieb, J. K. Luce, Cancer
Chemother. Rep. 1972, 56, 103.
[4] a) Y. H. Hsiang, R. Hertzberg, S. Hecht, L. F. Liu, J. Biol. Chem.
1985, 260, 14873; b) Y. H. Hsiang, L. F. Liu, M. E. Wall, M. C. Wani,
A. W. Nicholas, G. Manikumar, S. Kirschenbaum, R. Silber, M. Pot-
mesil, Cancer Res. 1989, 49, 4385; c) J. M. Covey, C. Jaxel, K. W.
Kohn, Y. Pommier, Cancer Res. 1989, 49, 5016.
[5] For several recent syntheses of camptothecin, see: a) S. P. Chavan,
A. N. Dhawane, U. R. Kalkote, Tetrahedron Lett. 2010, 51, 3099;
b) L. P. Zhang, Y. Bao, Y. Y. Kuang, F. E. Chen, Helv. Chim. Acta
2008, 91, 2057; c) G. S. Liu, Q. L. Dong, Y. S. Yao, Z. J. Yao, Org.
Lett. 2008, 10, 5393; d) H. B. Zhou, G. S. Liu, Z. J. Yao, Org. Lett.
2007, 9, 2003; e) S. P. Chavan, A. B. Pathak, U. R. Kalkote, Synlett
2007, 2635.
[6] For SAR of camptothecin, see reviews and papers therein: a) C. J.
Thomas, N. J. Rahier, S. M. Hecht, Bioorg. Med. Chem. 2004, 12,
1585; b) R. P. Verma, C. Hansch, Chem. Rev. 2009, 109, 213.
[7] a) T. Kunimoto, K. Nitta, T. Tanaka, N. Uehara, H. Baba, M. Akeu-
chi, T. Yokokura, S. Sawada, T. Miyasaka, M. Mutai, Cancer Res.
1987, 47, 5944; b) B. C. Giovanella, J. S. Stehlin, M. E. Wall, M. C.
Wani, A. W. Nicholas, L. F. Liu, R. Silber, M. Potmesil, Science
Received: May 6, 2011
Published online: August 17, 2011
Chem. Eur. J. 2011, 17, 10462 – 10469
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10469