Divergent synthesis of cytotoxic styryl lactones isolated from Polyalthia crassa. The first total synthesis of crassalactone B

Article abstract of DOI:10.1016/j.tetlet.2010.04.114

The first total synthesis of (+)-crassalactone B (2) and a new syntheses of (+)-crassalactone C (3) has been achieved starting from d-glucose. The natural products 2 and 3 can be selectively accessed by changing the conditions for TBDPS cleavage in the fi

Full text of DOI:10.1016/j.tetlet.2010.04.114

Tetrahedron Letters 51 (2010) 3426–3429
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Divergent synthesis of cytotoxic styryl lactones isolated from Polyalthia
crassa. The first total synthesis of crassalactone B
a
a
b
b
Velimir Popsavin a, , Goran Benedekovic , Mirjana Popsavin , Vesna Kojic , Gordana Bogdanovic
*
´
´
´
a
´
Department of Chemistry, Faculty of Sciences, University of Novi Sad, Trg D. Obradovica 3, 21000 Novi Sad, Serbia
b Oncology Institute of Vojvodina, Institutski put 4, 21204 Sremska Kamenica, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history:
The first total synthesis of (+)-crassalactone B (2) and a new syntheses of (+)-crassalactone C (3) has been
achieved starting from -glucose. The natural products 2 and 3 can be selectively accessed by changing
the conditions for TBDPS cleavage in the final intermediate 16. The synthesized natural products were
evaluated for their cytotoxic activity against a panel of human tumour cell lines.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 28 February 2010
Revised 19 April 2010
Accepted 27 April 2010
Available online 17 May 2010
D
Ethno-botanical uses of several species of the genus Polyalthia
are well known in South East Asia. The extracts from these plants
are traditionally used as a galactagogue, as a remedy for the treat-
ment of abdominal pain, and as an aphrodisiac.1 Chemical constit-
uents of Polyalthia species have been extensively investigated by
different groups; many of these constituents exhibit antimicro-
bial,2 cytotoxic,3 antimalarial,4 and anti-HIV5 activity. A number
of styryl lactones have been recently isolated from the tropical
plant Polyalthia crassa.6 The bioassay-directed separation of the
ethyl acetate soluble extract of the leaves and twigs led to the iso-
lation of the known antitumor agent (+)-goniofufurone (1, Fig. 1),
along with several new cytotoxic lactones including the cinnamate
derivatives of 1: (+)-crassalactones B (2) and C (3). Their structures
were determined on the basis of spectroscopic methods. The abso-
lute configurations of 2 and 3 were established by chemical semi-
synthesis, starting from a natural sample of goniofufurone.
from the protected dialdose 7 or 10 by means of stereoselective
Grignard addition of phenylmagnesium bromide. Synthesis of
aldehydes of type 7 or 10 is visualised from D-glucose by well
established chemical reactions.9 In such a way, the chirality at C-
4, C-5 and C-6 in the target 2 would be directly translated from
D-glucose.
At the outset, we focused on the synthesis of the key building
block 4 starting from commercially available diacetone-
D-glucose
(5, Scheme 2).
Treatment of 5 with cinnamic acid under the conditions shown
in Scheme 2, provided an almost quantitative yield of the expected
cinnamic ester 6. The terminal isopropylidene functionality in 6
was directly converted into an aldehyde group, in a single step,
by treatment with periodic acid in dry ethyl acetate,10 whereupon
the corresponding aldehyde 7 was obtained in 93% yield. Addition
of phenylmagnesium bromide in THF, to a solution of 7 in toluene,
led to a mixture of two epimeric alcohols. The mixture was not
separated but was oxidised with PCC in dichloromethane to give
the corresponding ketone 8 in 34% yield. Stereocontrolled reduc-
tion of the ketone functionality was achieved using the method
We recently disclosed the first total synthesis of (+)-3 by chiral-
ity transfer from
herein the first total synthesis of (+)-crassalactone B (2) and an
alternative synthesis of (+)-crassalactone C (3) starting from -glu-
D
-xylose.7 In continuation of this work, we report
D
cose, along with their effects on the proliferation of some human
malignant cell lines.
H
HO
Ph
O
A retrosynthetic pathway outlining our strategy for the synthe-
sis of (+)-crassalactone B (2) is shown in Scheme 1. The strategy is
based on the utility of protected furanose 4, comprising the requi-
site 3-O-cinnamoyl and 5-C-phenyl functionalities, as well as four
contiguous stereocentres of definite configuration. It was assumed
that the required [3.3.0] bicyclic lactone core of 2 could be formed
from 4 through a two-step sequence that involved hydrolytic re-
moval of the 1,2-O-isopropylidene protecting group, followed by
hydroxy directed condensation of a protected lactol derivative
with Meldrum’s acid.8 The key intermediate 4 could be prepared
O
O
H
HO
1
H
H
2
O
Ph
H
HO
Ph
O
O
O
O
O
O
O
O
Ph
Ph
H
HO
O
3
* Corresponding author. Tel.: +381 21 485 27 68; fax: +381 21 454 065.
E-mail address: velimir.popsavin@dh.uns.ac.rs (V. Popsavin).
Figure 1. Structures of (+)-goniofufurone (1), (+)-crassalactone
crassalactone C (3).
B
(2) and (+)-
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.04.114

V. Popsavin et al. / Tetrahedron Letters 51 (2010) 3426–3429
3427
H
RO
Ph
O
O
O
HO
Ph
O
HO
HO
O
OH
OH
O
O
O
O
3
O
4
6
5
O
4
O
H
2
O
Ph
4 R = tBuPh2Si
RO
HO
Ph
O
O
7 R = PhCH:CHCO
10 R = H
(+)-crassalactone B (2)
D-glucose
Scheme 1. Retrosynthetic analysis of (+)-crassalactone B.
of Yatagai and Ohnuki for prochiral ketones.11 The reduction of 8
with NaBH4 in the presence of L-tartaric acid gave the required
in 11 was silylated selectively to give the corresponding silyl ether
14 in 97% yield. Further esterification of 14 with cinnamic acid
(DCC, DMAP) gave the key chiral intermediate 4 in 96% yield. Thus,
this six-step sequence (Scheme 3) represents a more convenient
route toward the key intermediate 4, since it provided a consider-
ably higher overall yield (59% from 5) compared to the six-step se-
quence presented in Scheme 2 (31% from 5).
Conversion of 4 into the targets 2 and 3 is outlined in Scheme 4.
Hydrolytic removal of the isopropylidene protecting group in 4
gave the expected lactol 15 (92%), which upon treatment with Mel-
drum’s acid in the presence of triethylamine gave the protected
furano-lactone 16 in 65% yield. We initially carried out the TBDPS
cleavage in 16 using TBAF,15 which surprisingly gave (+)-crassalac-
tone C (3) as the major product (43%), accompanied by a minor
amount of 2 (14%). Both the 1H and 13C NMR data of compound
3 were consistent with naturally occurring (+)-crassalactone C
and its physical properties were in good agreement with those re-
ported in the literature.6,7 Product 3 was presumably formed by a
competitive intramolecular cinnamate migration process in 2, via
the cyclic orthoester intermediate 16a. A similar intramolecular
acyl migration during silyl deprotection with TBAF has been de-
scribed recently in the literature.16 Moreover, treatment of 16 with
SOCl2 in MeOH gave a 56% yield of (+)-crassalactone B (2).
alcohol 9 as the only stereoisomer in 92% yield. The intermediate
9 was thus obtained in 28% overall yield with respect to the start-
ing compound 5. However, when this four-step sequence was car-
ried out without purification of the intermediates 7 and 8 the
desired product 9 was obtained in a somewhat higher overall yield
(34% from 5). To prevent the benzylic hydroxy group in 9 from tak-
ing part in ensuing reactions, it was blocked as a silyl ether by
treatment of 9 with tert-butyldiphenylsilyl chloride to give the
key intermediate 412 in 90% yield (31% overall yield from 5).
An alternative and more efficient route to the intermediate 4 is
outlined in Scheme 3. This sequence started with treatment of 5
with periodic acid in dry ethyl acetate. The resulting crude alde-
hyde 10 was not purified, but was immediately treated with
PhMgBr to give a 75% combined yield of known9,13 alcohols 11
and 12 in the ratio 3:2. Efforts to improve the epimeric ratio in fa-
vour of 11 possessing the correct stereochemistry for 2 were
unsuccessful. However access to 11 was possible by selective
DDQ-oxidation of the benzylic hydroxy function in 1214 followed
by reduction of the resulting prochiral ketone 13 with NaBH4/L-tar-
taric acid. This procedure provided the requisite intermediate 11 in
64% overall yield (four steps from 5). The benzylic hydroxy group
O
O
O
O
O
O
O
O
O
O
a
a
O
O
O
O
O
O
O
O
O
O
O
HO
HO
HO
Ph
5
6
5
10
b
b
O
O
O
O
OH
OH
Ph
O
O
c
O
O
O
Ph
Ph
O
O
O
O
O
O
HO
HO
O
O
O
Ph
Ph
8
11
7
12
d
d
c
e
OH
OTBDPS
O
O
OTBDPS
O
O
Ph
Ph
O
O
O
e
Ph
Ph
O
O
O
O
O
O
O
RO
HO
O
O
O
Ph
Ph
14 R = H
4 R = PhCH:CHCO
f
13
9
4
Scheme 2. Reagents and conditions: (a) PhCH=CHCO2H, DCC, DMAP, CH2Cl2, rt,
20 h, 98%; (b) H5IO6, EtOAc, rt, 1.5 h, 93%; (c) (i) PhMgBr, THF, PhMe, 0 °C, 3 h; (ii)
PCC, CH2Cl2, reflux, 4 h, or Ac2O, DMSO, rt, 24 h, 32% over two steps; (d) NaBH4, THF,
Scheme 3. Reagents and conditions: (a) H5IO6, EtOAc, rt, 4 h; (b) PhMgBr, THF,
PhMe, 0 °C, 5 h, 46% of 11, 29% of 12; (c) DDQ, CH2Cl2, H2O, rt, 72 h, 79%; (d) NaBH4,
THF,
L
-tartaric acid, ꢀ7 °C?rt, 24 h, 78% of 11, 7% of 12; (e) tBuPh2SiCl, imidazole,
L
-tartaric acid, ꢀ7 °C?rt, 4 h, 92%; (e) tBuPh2SiCl, imidazole, CH2Cl2, rt, 47 h, 90%.
CH2Cl2, rt, 48 h, 97%; (f) PhCH=CHCO2H, DCC, DMAP, CH2Cl2, rt, 50 h, 96%.

3428
V. Popsavin et al. / Tetrahedron Letters 51 (2010) 3426–3429
All three naturally occurring styryl lactones 1–3 retained the
OTBDPS
O
OTBDPS
O
selectivity between the normal human cells (MRC-5) and tumour
cell lines. These results are consistent with previous findings that
the cytotoxicity of styryl lactones is specific to neoplastic cells
since only negligible effects of these natural products on normal
cells were observed.19 Interestingly, only (+)-crassalactone C (3)
has the same growth inhibition pattern as the parent compound
1. As with (+)-goniofufurone (1), compound 3 was inactive against
promyelocytic leukaemia (HL-60), but showed moderate to weak
antiproliferative activity towards K562, MCF-7, HeLa and Jurkat
malignant cells. In contrast to 1 and 3, (+)-crassalactone B (2)
exhibited significant antiproliferative effects towards all the tested
tumour cell lines, including notable activity against HL-60 cells.
The most potent antiproliferative activity of 2 was recorded in
the Jurkat cell line, whereupon this molecule exhibited 146- and
136-fold stronger activities when compared to 1 and 3, respec-
tively. Molecule 2 also showed remarkable antiproliferative activ-
ities toward K562, MCF-7 and HeLa cells, being 5- to 17-fold,
more potent than (+)-goniofufurone (1), and 9- to 13-fold more po-
tent than (+)-crassalactone C (3).
a
Ph
Ph
OH
OH
Ph
O
O
O
O
O
O
Ph
15
4
b
OTBDPS
O
Ph
H
H
Ph
Ph
O
c
O
H
O
O
O
H
O
HO
O
Ph
16
16a
d
In summary, we have developed the first total synthesis of (+)-
crassalactone B (2) and an alternative synthesis of (+)-crassalac-
tone C (3) by chirality transfer from
D-glucose, whereby the C-2,
OH
O
C-3 and C-4 stereocenters of -glucose have been translated into
D
H
O
Ph
O
Ph
the C-4, C-5 and C-6 positions of the targets. Selective access to
either 2 or 3 was accomplished by simply changing the conditions
for TBDPS cleavage in the final intermediate 16. The main charac-
teristic of this approach is its generality and flexibility. It enables
the preparation of a variety of (+)-crassalactone B and C analogues
H
O
O
Ph
O
O
H
O
HO
O
H
O
Ph
2
3
by changing the O-acyl functionality at the C-3 position of D-glu-
cose. In vitro antiproliferative activities of 2 and 3 against a num-
ber of human tumour cell lines were recorded and compared with
those observed for (+)-goniofufurone. The obtained biological data
revealed that (+)-crassalactone B showed superior activity against
all the tested malignant cell lines, and was devoid of any significant
cytotoxicity against normal MRC-5 cells. Based upon these results,
we believe that this compound may serve as a convenient lead in
the synthesis of more potent and selective antitumor agents.
Scheme 4. Reagents and conditions: (a) TFA/H2O (9:1), CH2Cl2, 0 °C, 0.5 h, 92%; (b)
Meldrum’s acid, Et3N, DMF, 47 °C, 63 h, 65%; (c) TBAF, AcOH, THF, rt, 144 h, 43% of 3,
14% of 2; (d) SOCl2, MeOH, rt, 6 h, 56%.
Although our optical rotation value is greater than the reported va-
lue for (+)-crassalactone B {lit.6
½
a 2D0
ꢁ
+8.0 (c 0.5, EtOH); this work:
½ ꢁ
a 2D0
+45.7 (c 0.5, EtOH)}, the melting point and NMR data17 of syn-
thetic sample 2 were in full agreement with those reported in the
literature.6 Thus, the first synthesis of natural product 2 was suc-
cessfully accomplished in nine steps with an overall yield of
Acknowledgement
19.8% starting from commercially available D-glucose derivative
5. The new synthesis of crassalactone C (3) proceeded in nine linear
steps, with 15.2% overall yield from the same starting compound 5.
Financial support from the Ministry of Science and Technologi-
cal Development of the Republic of Serbia (Grant No. 142005) is
gratefully acknowledged.
The preceding preparation of 3 was accomplished starting from D-
xylose in 7.8% overall yield over ten linear steps.7
References and notes
Both (+)-crassalactones B (2) and C (3) were evaluated for their
antitumor activity against human myelogenous leukaemia (K562),
promyelocytic leukaemia (HL-60), T cell leukaemia (Jurkat), breast
adenocarcinoma (MCF-7), cervix carcinoma (HeLa), as well as a
normal human cell line (foetal lung fibroblasts, MRC-5). In vitro
cytotoxicity was evaluated after 48 h cell treatment by the MTT as-
say.18 The results, including the data for the reference compound
(+)-goniofufurone (1), are given in Table 1.
1. (a) Kanokmedhakul, S.; Kanokmedhakul, K.; Ohtani, I. I.; Isobe, M.
Phytochemistry 1998, 47, 131–133; (b) Connolly, J. D.; Haque, E.; Kadir, A. A.
Phytochemistry 1996, 43, 295–297.
2. (a) Faizi, S.; Khan, R. A.; Azher, S.; Khan, S. A.; Tauseef, S.; Ahmad, A. Planta Med.
2003, 69, 350–355; (b) Murthy, M. M.; Subramanyam, M.; Bindu, M. H.;
Annapurna, J. Fitoterapia 2005, 76, 336–339.
3. (a) Wu, Y. C.; Duh, C. Y.; Wang, S. K.; Chen, K. S.; Yang, T. H. J. Nat. Prod. 1990, 53,
1327–1331; (b) Zhao, G.; Jung, J. H.; Smith, D. L.; Wood, K. V.; McLaughlin, J. L.
Planta Med. 1991, 57, 380–383; (c) Ma, X.; Lee, I. S.; Chai, H. B.; Zaw, K.;
Farnsworth, N. R.; Soejarto, D. D.; Cordell, G. A.; Pezzuto, J. M.; Kinghorn, A. D.
Phytochemistry 1994, 37, 1659–1662; (d) Chen, C. Y.; Chang, F. R.; Shih, Y. C.;
Hsieh, T. J.; Chia, Y.-C.; Tseng, H. Y.; Chen, H. C.; Chen, S. J.; Hsu, M. C.; Wu, Y. C.
J. Nat. Prod. 2000, 63, 1475–1478.
4. Kanokmedhakul, S.; Kanokmedhakul, K.; Yodbuddee, D.; Phonkerd, N. J. Nat.
Prod. 2003, 66, 616–619.
5. (a) Li, H. Y.; Sun, N. J.; Kashiwada, Y.; Sun, L.; Snider, J. V.; Cosentino, L. M.; Lee,
K. H. J. Nat. Prod. 1993, 56, 1130–1133; (b) Tuchinda, P.; Pohmakotr, M.;
Reutrakul, V.; Thanyachareon, W.; Sophasan, S.; Yoosook, C.; Santisuk, T.;
Pezzuto, J. M. Planta Med. 2001, 67, 572–575.
Table 1
In vitro cytotoxicity of 1, 2 and 3
a
Compound
IC50
Jurkat
(lM)
K562
HL-60
MCF-7
HeLa
MRC-5
1
2
3
7.29
1.57
13.78
>100
5.46
>100
65.87
0.45
61.24
25.31
1.22
15.26
10.36
2.12
27.36
>100
>100
>100
6. Tuchinda, P.; Munyoo, B.; Pohmakotr, M.; Thinapong, P.; Sophasan, S.; Santisuk,
T.; Reutrakul, V. J. Nat. Prod. 2006, 69, 1728–1733.
´
´
´
7. (a) Popsavin, V.; Benedekovic, G.; Sreco, B.; Popsavin, M.; Francuz, J.; Kojic, V.;
Bogdanovic´, G. Org. Lett. 2007, 9, 4235–4238; (b) Popsavin, V.; Benedekovic´, G.;
Srec´o, B.; Francuz, J.; Popsavin, M.; Kojic´, V.; Bogdanovic´, G.; Divjakovic´, V.
Tetrahedron 2009, 65, 10596–10607.
a
IC50 is the concentration of compound required to inhibit cell growth by 50%
compared to an untreated control. The values are means of three independent
experiments performed in quadruplicate. Coefficients of variation were <10%.

V. Popsavin et al. / Tetrahedron Letters 51 (2010) 3426–3429
3429
8. For a recent review on applications of Meldrum’s acid in the synthesis of
natural products, see: Ivanov, A. S. Chem. Soc. Rev. 2008, 37, 789–811.
9. Gracza, T.; Szolcsányi, P. Molecules 2000, 5, 1386–1398.
10. Reddy, L. V. R.; Swamy, G. N.; Shaw, A. K. Tetrahedron: Asymmetry 2008, 19,
1372–1375.
15. Mori, M.; Somada, A.; Oida, S. Chem. Pharm. Bull. 2000, 48, 716–728.
16. (a) Nishiyama, T.; Kusumoto, Y.; Okumura, K.; Hara, K.; Kusaba, S.; Hirata, K.;
Kamiya, Y.; Isobe, M.; Nakano, K.; Kotsuki, H.; Ichikawa, Y. Chem. Eur. J. 2010,
16, 600–610; (b) Chevallier, O. P.; Migaud, M. E. Beilstein J. Org. Chem. 2006, 2.
article no. 14.
11. Yatagai, M.; Ohnuki, T. J. Chem. Soc., Perkin Trans. 1 1990, 1826–1828.
17. Selected data for 2: mp 172–174 °C (Et2O), ½a D20
ꢁ
+45.7 (c 0.5, EtOH); lit.6 mp
12. Selected data for 4 (syrup): ½a D20
ꢁ
+18.6 (c 2.0, CHCl3). 1H NMR (250 MHz, CDCl3):
d 0.93 (s, 9 H, Me3CPh2Si), 1.30 and 1.58 (2 ꢂ s, 3H each, CMe2), 4.57 (d, 1H,
J1,2 = 3.7 Hz, H-2), 4.70 (dd, 1H, J3,4 = 2.6, J4,5 = 8.7 Hz, H-4), 4.95 (d, 1H,
J4,5 = 8.7 Hz, H-5), 5.47 (d, 1H, J3,4 = 2.6 Hz, H-3), 5.77 (d, 1H, J1,2 = 3.7 Hz, H-1),
171–173 °C (EtOH), ½a D20
ꢁ
+8.0 (c 0.5, EtOH). 1H NMR (250 MHz, CDCl3): d 2.59
(d, 1H, J2a,2b = 18.8 Hz, H-2a), 2.74 (dd, 1H, J2a,2b = 18.8, J2b,3 = 5.8 Hz, H-2b),
4.27 (dd, 1H, J5,6 = 2.5, J6,7 = 8.6 Hz, H-6), 4.69 (d, 1H, J6,7 = 8.6 Hz, H-7), 5.00–
5.09 (m, 2H, H-3 and H-4), 5.76 (d, 1H, J5,6 = 2.5 Hz, H-5), 6.55 (d, 1H,
6.15 (d, 1H, J2 ,3 = 16.0 Hz, H-20), 7.08–8.65 (m, 21H, 4 ꢂ Ph and H-30). 13C NMR
(62.9 MHz, CDCl3): d 19.2 (Me3CPh2Si), 26.2 and 26.7 (CMe2 and Me3CPh2Si),
72.6 (C-5), 76.4 (C-3), 82.1 and 82.9 (C-2 and C-4), 104.4 (C-1), 111.9 (CMe2),
117.3 (C-20), 127.0, 127.5, 127.8, 128.0, 128.06, 128.1, 128.9, 129.2, 129.4,
130.5, 132.9, 133.1, 134.0, 135.8, 136.0 and 140.6 (4 ꢂ Ph), 145.2 (C-30), 165.7
(C-10). HRMS (ESI): m/z 657.2651 (M++Na), calcd for C39H42NaO6Si: 657.2643.
13. (a) Inch, T. D. Carbohydr. Res. 1967, 5, 45–52; (b) Bruns, R.; Wernicke, A.; Koll, P.
Tetrahedron 1999, 55, 9793–9800.
J2 ,3 = 15.9 Hz, H-20), 7.30–7.65 (m, 10 H, 2 ꢂ Ph), 7.85 (d, 1H, J2 ,3 = 15.9 Hz, H-
30). 13C NMR (62.9 MHz, CDCl3): d 35.6 (C-2), 70.8 (C-7), 75.0 (C-5), 77.1 (C-3),
83.1 (C-6), 85.3 (C-4), 116.0 (C-20), 126.8, 128.4, 128.6, 129.1, 130.8, 131.1,
133.7 and 140.3 (2 ꢂ Ph), 147.6 (C-30), 166.5 (C-10), 174.8 (C-1). HRMS (ESI): m/
z 403.1143 (M++Na), calcd for C22H20NaO6: 403.1152.
0
0
0
0
0
0
18. Scudiero, D. A.; Shoemaker, R. H.; Paull, K. D.; Monks, A.; Tierney, S.; Nofziger,
T. H.; Currens, M. J.; Seniff, D.; Boyd, M. R. Cancer Res. 1988, 48, 4827–4833.
19. For a recent review on biological activities of styryl lactones and analogues,
see: de Fatima, A.; Modolo, L. V.; Conegero, L. S.; Pilli, R. A.; Ferreira, C. V.; Kohn,
L. K.; de Carvalho, J. E. Curr. Med. Chem. 2006, 13, 3371–3384.
14. Stereoisomers 11 and 12 were separated by flash column chromatography
before oxidation.