Synthesis of novel analogues of (+)-varitriol via olefin cross-metathesis reaction

Article abstract of DOI:10.1016/j.bmcl.2008.02.062

Novel analogues of (+)-varitriol have been synthesized via olefin cross-metathesis reaction using Grubb's catalyst. Newly synthesized compounds were screened for cytotoxicity and they showed mild activity against RAW264.7 and HT29 cell lines.

Full text of DOI:10.1016/j.bmcl.2008.02.062

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 2351–2354
Synthesis of novel analogues of (+)-varitriol via olefin
cross-metathesis reaction
Lingaiah Nagarapu,* Venkateswarlu Paparaju and Apuri Satyender
Organic Chemistry Division-II, Indian Institute of Chemical Technology (IICT), FCL Ist Floor, Taranaka, Hyderabad,
Andhra Pradhesh 500 607, India
Received 5 February 2008; revised 22 February 2008; accepted 27 February 2008
Available online 4 March 2008
Abstract—Novel analogues of (+)-varitriol have been synthesized via olefin cross-metathesis reaction using Grubb’s catalyst. Newly
synthesized compounds were screened for cytotoxicity and they showed mild activity against RAW264.7 and HT29 cell lines.
ꢀ 2008 Elsevier Ltd. All rights reserved.
Cancer is currently the second common cause of death
in United States, and it is likely to become the most
common in the foreseeable future.1 Hence the develop-
ment and testing of novel and more selective anticancer
drugs have become an important research area. Further
advances in cancer therapy can be brought by chemical
variation of known structural classes since it provides a
means for obtaining improved drugs, increased under-
standing of the biochemistry of the cancer cell and novel
structure–growth inhibitory relationship.
In 2002, Malmstrøm et al.,5 have isolated (+)-varitriol
(1a) (Fig. 1) from a marine derived strain of the fungus
Emericella variecolor collected from the Caribbean
Waters of Mochima Bay. (+)-Varitriol (1a) was tested
in the National Cancer Institute (NCI) 60-cell line
in vitro panel. It showed a more than 100-fold increased
potency (over the mean toxicity) towards the RXF 393
(renal cancer, GI50 = 1.63 · 10ꢀ7 M), T-47D (breast
cancer, GI50 = 2.10 · 10ꢀ7 M) and SNB-75 (CNS can-
cer, GI50 = 2.44 · 10ꢀ7 M) cell lines and lower potency
against DU-145 (prostate cancer, GI50 = 1.10 ·
10ꢀ6 M), HL-60 (TB leukaemia, GI50 = 2.52 · 10ꢀ5 M),
CCRF-CEM (leukaemia, GI50 = 2.60 · 10ꢀ5 M), OV-
CAR-5 (ovarian cancer, GI50 = 6.82 · 10ꢀ5 M), SNB-
19 (CNS cancer, GI50 = 9.13 · 10ꢀ5 M) and COLO 205
(colon cancer, GI50 = 9.59 · 10ꢀ5 M) cell lines. The
combination of potent biological properties and a rela-
tively straightforward molecular structure of (+)-varitri-
ol (1a) has rekindled an interest in us to obtain novel
analogues for detailed SAR studies.
Natural products have a long history2,3 of providing no-
vel, clinically useful anticancer drugs and they have also
served as prototypes for the development of novel ana-
logues of clinical importance. Moreover, National Can-
cer Institute (NCI) has recently reemphasized the
discovery of natural products potential anticancer
drugs.
Among various natural products, marine natural prod-
ucts4 have received increasing attention from chemists
and pharmacologists during the last few decades. Natu-
ral product chemists have probed marine organisms,
while synthetic chemists have targeted these novel struc-
tures for the development of new synthetic methodolo-
gies and strategies.
OMe
OMe
HO
OH
O
O
H3C
CH3
HO OH
HO OH
Keywords: Varitriol; Analogues; Cross-metathesis reaction; Grubb’s
catalyst; Cytotoxicity.
(+)-Varitriol (1a)
(-)-Varitriol (1b)
*
Corresponding author. Tel.: +91 40 27160123; fax: +91 40
27193382; e-mail: nagarapu2@yahoo.co.in
Figure 1.
0960-894X/$ - see front matter ꢀ 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.02.062

2352
L. Nagarapu et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2351–2354
Two research groups have reported different synthetic
routes to (ꢀ)-varitriol (1b): (i) In 2006 Jennings and
Clemens6 reported the total synthesis of (ꢀ)-varitriol
from D-(ꢀ)-ribose utilizing alkene metathesis to link to-
gether the carbohydrate and aromatic moieties of the
molecule. (ii) In 2007 McAllister et al.7 reported a short
and flexible route to (ꢀ)-varitriol by following Ramberg–
Ba¨cklund reaction. Though the proposed route is
operationally straightforward and shorter, the low a/b ste-
reoselectivity is the major drawback. Of the two methods
available for the synthesis of varitriol, alkene metathesis
was found to be more suitable and more convenient for
the synthesis of novel analogues of (+)-varitriol (1a).
synthesized and the spectroscopic features fully sup-
ported the assigned structures.
The scope of the reaction was further extended to carbo-
hydrate-derived olefin (5)11 and its reaction with sty-
renes (2c and 2d) in presence of 5 mol % catalyst gave
cross-coupled products (6a and 6c) (Scheme 2).
The selective formation of E-isomer of the products has
been confirmed from the coupling constant (J) values of
olefinic products. The formation of compounds (6a) and
(6c) was further confirmed by acetylation of hydroxyl
group using Ac2O/Py to obtain acetyl derivatives (6b)
and (6d), characteristic acetyl protons appeared at d
1.92 ppm (s, 3H).
The establishment of olefin metathesis as a powerful
synthetic tool to C@C bond formation is due to the
development of ruthenium-based catalysts and their
derivatives introduced by Grubbs and coworkers.8 More
specifically, cross-metathesis (CM) of simple alkenes has
now become one of the methods of choice to access
substituted alkenes.
This approach was again repeated for the simple and
easily available carbohydrate-derived olefin (7). How-
ever, the reaction of this olefin (7) with both styrenes
(2a and 2d) resulted in the formation of homocoupled
dimers (8 and 9a–b) as products instead of the desired
cross-coupled products (10) (Scheme 3). This homocou-
pling may be attributed to the absence of furanose ring
in the carbohydrate-derived olefin because the presence
of furanose ring may cause steric hindrance for the for-
mation of homocoupled product. The structures of com-
In continuation of our studies9 towards synthesis of no-
vel compounds as useful biologically active compounds,
we report in this communication an efficient synthesis of
novel analogues of (+)-varitriol (1) by utilizing CM reac-
tion. To the best of our knowledge, in the literature
there appears no report for the synthesis and screening
of novel analogues of (+)-varitriol.
1
pounds 4a–e, 6a–d, 8 and 9a–b were deduced from H
13
and C NMR, mass spectral and elemental analysis.12
Newly synthesized compounds 4a–e, 6a and 6c were
screened for cytotoxicity by following a previously re-
ported procedure.13 All the compounds (4a–e, 6a and
6c) were tested towards RAW264.7 (mouse macro-
phage), A549 (human lung carcinoma) and HT29 (hu-
man colon carcinoma) cell lines. They displayed mild
activity against RAW264.7 and HT29 cell lines (Table
1). In RAW264.7 cell line compounds 4a, 4b, 4d, 4e
and 6a showed activity, whereas compounds 4c and 6c
were inactive. Similarly in HT29 cell line for compounds
4a, 4b, 4e, 6a and 6c was active and compounds 4c and
4d did not show any activity.
As a starting point for this study we have selected com-
pound (3)10 as a sugar olefin which is very closely related
in structural aspects to the carbohydrate-derived olefin
of (+)-varitriol (1). Initially compound (3) was allowed
to react with 4-methoxy styrene (2a) in presence of
Grubb’s second-generation catalyst (5 mol %) to afford
desired cross-coupled product (4a) with pronounced
E-selectivity (Scheme 1).
The formation of compound (4a) was characterized from
1H NMR by the appearance of characteristic aromatic
protons at d 7.23 ppm and d 6.79 ppm (2d, 4H,
J = 9.0 Hz) and olefin protons at d 5.99 ppm ðdd; H-20;
Among the tested compounds, compounds 4b and 4a
exhibited more activity towards RAW264.7 and HT29
cell lines, respectively. All the compounds (4a–e, 6a
and 6c) were found to be inactive against A549 (human
lung carcinoma) cell line.
0
0
0
0
J1 ;2 ¼ 15:8 Hz; J2 ;3 ¼ 8:0 HzÞ and at
d 6.47 ppm
ðd; H-10; J1 ;2 ¼ 15:8 HzÞ. The coupling constant (J) val-
ues of olefinic protons confirmed the selective forma-
tion of E-isomer of product (4a). In EI-MS of
compound (4a) the characteristic [M+] appears at 306.
In this olefin cross-metathesis reaction, undesired homo-
coupled products are also expected, however, the forma-
tion of such by-products has not been observed.
0
0
In conclusion, analogues of cytotoxic varitriol were syn-
thesized (by utilizing CM reaction), characterized and
screened for cytotoxicity against RAW264.7, A549 and
HT29 cell lines. All the compounds showed mild activity
and compounds 4a and 4b displayed more activity among
the screened compounds. Though the compounds
showed mild activity, the synthesis and screening of more
analogues of varitriol, both protected and unprotected,
would give scope for further work in this area.
After establishing the method for the synthesis of
(+)-varitriol analogue (4a), we have explored the applica-
bility of this methodology, for the general synthesis of
other novel analogues by using variously substituted sty-
renes (2b–e). Thus, reaction of compound (2b) and sugar
olefin (3) in the presence of catalyst (5 mol %) resulted in
1
the formation of compound (4b). In H NMR of com-
pound (4b), characteristic olefinic protons resonated at
Acknowledgments
d 6.01 ppm ðdd; H-20; J2 ;3 ¼ 8:4 Hz; J1 ;2 ¼ 15:7 HzÞ
0
0
0
0
and at d 6.42 ppm ðd; H-H-10; J1 ;2 ¼ 15:7 HzÞ. Similarly
The authors thank the Head, Organic Chemistry Divi-
sion-II, and Director, IICT for their support. P.V. and
0
0
other varitriol like compounds 4c, 4d and 4e were also

L. Nagarapu et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2351–2354
2353
1'
R
O
OCH3
O
Catalyst (5 mol%)
CH2Cl2 , Reflux
OCH3
6'
H
4'
H
O
O
R
O
O
2a-e
4a-e
3
a, R = 4-OCH3
b, R = 3-OCH3-4,5-OCH2O-
c, R = 3-OCH3
d, R = 3,5-(OCH3)2
e, R = 4-OH-3-OCH3
N
Catalyst =
N
Cl
Cl
Ru
PCy3
Ph
Scheme 1.
O
R
Catalyst (5 mol%)
O
O
O
H
H
R1
O
CH2Cl2 , Reflux
HO
R
O
6a, R = 3-OCH3 ; R1 = OH
2c, 2d
5
Ac2O, Py, r.t
6b, R = 3-OCH3 ; R1 = OAc
6c, R = 3,5-(OCH3)2 ; R1 = OH
6d, R = 3,5-(OCH3)2 ; R1 = OAc
Ac2O, Py, r.t
Scheme 2.
R
O
O
O
O
R
9a, R = 4-OCH3
9d, R = 3,5-(OCH3)2
8
Catalyst (5 mol%)
CH2Cl2 , Reflux
O
O
R
2a, 2d
7
R
O
O
10
Scheme 3.
Table 1. Cytotoxicity values GI50 (molar) for 4a–e, 6a and 6c
A.S. thank CSIR, New Delhi, for financial assistance.
Authors also thank Dr P. Rama Krishna, Pharmacol-
ogy Division, IICT.
Compound
RAW264.7 cell line (M)
HT29 cell line (M)
4a
4b
4c
4d
4e
6a
6c
6.96 · 10ꢀ4
4.62 · 10ꢀ4
Inactive
5.41 · 10ꢀ4
8.35 · 10ꢀ4
6.71 · 10ꢀ4
Inactive
4.70 · 10ꢀ4
13.97 · 10ꢀ4
Inactive
References and notes
Inactive
13.22 · 10ꢀ4
17.53 · 10ꢀ4
20.03 · 10ꢀ4
1. Varmus, H. Science 2006, 312, 1162.
2. Dourous, J. D.; Suffness, M. Cancer Chemother. Pharma-
col. 1978, 1, 91.

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L. Nagarapu et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2351–2354
3. Cassady, J. M.; Dourous, J. D. Anticancer agents based on
natural products models. In Medicinal Chemistry; Aca-
demic Press, 1980; Vol. 16.
4. Sims, J. J.; Rose, A. F.; Izac, R. R.. In Marine Natural
Products, Chemical and Biological Perspectives; Scheuer,
P. J., Ed.; Academic press: New York, 1978; Vol. 2, p
297.
5. Malmstrøm, J.; Christophersen, C.; Barrero, A. F.;
Oltra, J. E.; Justicia, J.; Rosales, A. J. Nat. Prod. 2002,
65, 364.
6. Clemens, R. T.; Jennings, M. P. Chem. Commun. 2006,
2720.
7. McAllister, G. D.; Robinson, J. E.; Taylor, R. J. K.
Tetrahedron 2007, 63, 12123.
8. Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R.
H. J. Am. Chem. Soc. 2003, 125, 11360.
9. Nagarapu, L.; Satyender, A.; Rajashaker, B.; Srinivas, K.;
Rupa rani, P.; Radhika, K.; Subhashini, G. Bioorg. Med.
Chem. Lett. 2008, 18, 1167.
10. Ugarkar, B. G.; DaRe, J. M.; Kopcho, J. J.; Browne, C.
E.; Schanzer, J. M.; Wiesner, J. B.; Erion, M. D. J. Med.
Chem. 2000, 43, 2883.
EI-MS: m/z = 350 [M +]. Anal. Calcd for C18H22O7: C,
61.71; H, 6.33. Found: C, 61.02; H, 6.28.
1
Compound 4c: Yield: 65%, [a]D ꢀ6.4ꢁ (c 0.70, CHCl3), H
NMR (CDCl3, 300 MHz): d 1.30, 1.49 (2s, 6H, 2· CH3),
3.37 (s, 3H, OCH3), 3.81 (s, 3H, Ar-OCH3), 4.60–4.78 (m,
3H, H-50, H-40, H-30), 4.95 (s, 1H, H-60), 6.15 ðdd; 1H;
H-20; J1 ;2 ¼ 15:8 Hz;J2 ;3 ¼ 8:6 HzÞ, 6.53 ðd; 1H; H-10;
0
0
0
0
0
0
J2 ;3 ¼ 15:8 HzÞ, 6.72–6.95 (m, 3H, Ar-H), 7.18 (m, 1H,
Ar-H). 13C NMR (CDCl3, 75 MHz): d 24.9, 26.4, 29.6,
54.5, 55.1, 84.6, 85.5, 88.0, 109.2, 112.0, 112.3, 113.3,
119.1, 128.9, 129.4, 132.5, 137.8, 159.7. Anal. Calcd for
C17H22O5: C, 66.65; H, 7.24. Found: C, 66.68; H, 7.28.
Compound 4d: Yield: 51%, [a]D ꢀ4.35ꢁ (c 0.39, CHCl3), 1H
NMR (CDCl3, 300 MHz): d 1.31, 1.50 (2s, 6H, 2· CH3),
3.35 (s, 3H, OCH3), 3.90 (2s, merged, 6H, 2· Ar-OCH3),
4.60–4.73 (m, 3H, H-50, H-40, H-30), 4.96 (s, 1H, H-60),
6.02 ðdd; 1H; H-20; J1 ;2 ¼ 15:8 Hz;J2 ;3 ¼ 8:6 HzÞ, 6.48
0
0
0
0
ðd; 1H; H-10; J1 ;2 ¼ 15:8 HzÞ, 6.76 (m, 1H, Ar-H), 6.85
( m, 2H, Ar-H). 13C NMR (CDCl3, 75 MHz): d 24.9, 26.4,
29.6, 54.5, 55.8, 55.8, 84.7, 85.6, 88.3, 109.0, 109.1, 111.1,
112.3, 119.7, 126.6, 129.4, 132.5, 149.0, 149.0. Anal.
Calcd for C18H24O6: C, 64.27; H, 7.19. Found: C, 64.31;
H, 7.11.
0
0
11. Hall, L.; Hough, L.; Pritchard, R. A. J. Chem. Soc. 1961,
1537.
1
Compound 4e: Yield: 58%, [a]Dꢀ8.6ꢁ (c 0.15, CHCl3), H
12. Experimental: 1H NMR spectra were recorded with an
AVANCE 300 Bruker at 300 MHz and Gemini 200 MHz
in CDCl3. Chemical shifts relative to TMS as internal
standard are given as d values in ppm. 13C NMR was
recorded in CDCl3 on Varian (75 Hz) spectrometer. EI-
MS mass spectra were measured at 70 eV (EI).
General procedure for synthesis of compounds 4a–e, 6a and
6c: To a stirred solution of sugar olefin (0.5 mmol) and
substituted styrene (0.5 mmol) in 15 mL of DCM was
added Grubbs second-generation catalyst (5 mol %). The
mixture was allowed to reflux at 40 ꢁC under N2
atmosphere for 18 h. The reaction was then cooled to
room temperature and concentrated under reduced
pressure. Flash chromatography (silica, 10% ethyl acetate
in hexanes) afforded the cross-coupled product.
NMR (CDCl3, 300 MHz): d 1.30, 1.50 (2s, 6H, 2· CH3),
3.25 (s, 3H, OCH3), 3.90 (s, 3H, Ar-OCH3), 4.6–4.72 (m,
3H, H-50, H-40, H-30), 4.95 (s, 1H, H-60), 5.98
ðdd; 1H; H-20; J1 ;2 ¼ 15:8 Hz;J2 ;3 ¼ 9:0 HzÞ, 6.46 ðd; 1H;
0
0
0
0
H-10; J1 ;2 ¼ 15:8 HzÞ, 6.78–6.90 (m, 3H, Ar-H). 13C
NMR (75 MHz, CDCl3): d 25.0, 26.4, 29.6, 54.5, 55.8,
84.7, 85.6, 88.3, 108.5, 109.2, 114.4, 120.3, 126.3, 132.7.
Anal. Calcd for C17H22O6: C, 63.34; H, 6.88. Found: C,
63.41; H, 6.79.
0
0
1
Compound 6a: Yield: 61%, [a]D ꢀ7.0ꢁ (c 0.50, CHCl3), H
NMR (CDCl3, 300 MHz): d 1.33, 1.52 (2s, 6H, 2· CH3),
3.80 (s, 3H, Ar-OCH3), 4.10 (m, 1H, H-40), 4.55 ðd; 1H;
0
H-50; J5 ;6 ¼ 3:0 HzÞ, 4.86 (m, 1H, H-30), 5.95 ðd; 1H; H-6 ;
0
0
J5 ;6 ¼ 3:0 HzÞ, 6.19 ðdd; 1H; H-20; J1 ;2 ¼ 15:863; J2 ;3
¼
0
0
0
0
0
0
5:2 HzÞ, 6.8 ðd; 1H; H-10; J1 ;2 ¼ 15:8 HzÞ, 6.73–6.97 (m,
3H, Ar-H), 7.20 (m, 1H, Ar-H). 13C NMR (75 MHz,
CDCl3): d 26.074, 26.649, 29.570, 55.738, 108.5, 112.2,
119.4, 126.5, 127.9, 128.4, 130.5, 148.5, 149.0. Anal. Calcd
for C16H20O5: C, 65.74; H, 6.90. Found: C, 65.81; H, 6.85.
Compound 6c: Yield: 53%, [a]D ꢀ68.5ꢁ (c 0.48, CHCl3), 1H
NMR (CDCl3, 300 MHz): d 1.33, 1.52 (2s, 6H, 2· CH3),
3.88 (s, 6H, 2· Ar-OCH3), 4.05–4.15 (m, 1H, H-40), 4.55
0
0
1
Compound 4a: Yield: 66%, [a]D ꢀ1.5ꢁ (c 0.33, CHCl3), H
NMR (CDCl3, 300 MHz): d 1.30, 1.49 (2s, 6H, 2· CH3),
3.33 (s, 3, OCH3), 3.88 (s, 3H, Ar-OCH3), 4.62–4.72 (m,
3H, H-50, H-40, H-30), 4.93 (s, 1H, H-60), 5.99 ðdd; 1H;
H-10; J1 ;2 ¼ 15:8 Hz;J2 ;3 ¼ 8:0 HzÞ, 6.47 ðd; 1H; H-6;
0
0
0
0
0
0
J1 ;2 ¼ 15:8 HzÞ, 6.79 (d, 2H, Ar-H, J = 9.0 Hz), 7.23 (d,
2H, Ar-H, J = 9.0 Hz). 13C NMR (75 MHz, CDCl3): d
25.0, 26.5, 29.5, 54.5, 55.2, 84.7, 85.6, 88.4, 109.1, 113.9,
126.4, 127.7, 132.3. EI-MS: m/z = 306 [M+]. Anal. Calcd
for C17H22O5: C, 66.65; H, 7.24. Found: C, 66.51; H, 7.32.
Compound 4b: Yield: 67%, 1H NMR (CDCl3, 200 MHz): d
1.30, 1.49 (2s, 6H, 2· CH3), 3.32 (s, 3H, Ar-OCH3), 3.91
ðd; 1H; H-50; J5 ;6 ¼ 3:7 HzÞ, 4.83 ðd; 1H; H-30; J2 ;3
¼
0
0
0
0
0
0
5:6 HzÞ, 5.92 ðd; 1H; H-60; J5 ;6 ¼ 3:7 Hz;Þ, 6.05 ðdd; 1H;
H-20; J2 ;3 ¼ 5:6 Hz;J1 ;2 ¼ 15:8 HzÞ, 6.74 ðd; 1H; H-10;
0
0
0
0
0
0
J1 ;2 ¼ 15:8 HzÞ, 6.78 (m, 1H, Ar-H), 6.88 (m, 2H, Ar-
H). 13C NMR (CDCl3, 75 MHz): d 26.0, 26.6, 29.5, 55.7,
55.8, 76.1, 81.0, 84.9, 104.5, 108.8, 110.9, 111.6, 119.6,
129.1, 134.1, 148.9, 149.1. EI-MS: m/z = 322 [M +]. Anal.
Calcd for C17H22O6: C, 63.34; H, 6.88. Found: C, 63.43;
H, 6.89.
(s, 3H, Ar-OCH3), 4.59–4.61 (m, 3H, H-50, H-40, H-30),
0
4.95 (s, 1H, H-60), 6.01 ðdd; 1H; H-2 ; J2 ;3 ¼ 8:4 Hz;
0
0
J1 ;2 ¼ 15:7 HzÞ, 5.95 (s, 2H, CH2), 6.42 ðd; 1H; H-10;
0
0
0
0
J1 ;2 ¼ 15:7 HzÞ, 6.45 (s, 1H, Ar-H), 6.56 (s, 1H, Ar-H).
13C NMR (75 MHz, CDCl3): d 25.0, 26.4, 29.6, 54.5, 56.5,
84.7, 85.6, 88.1, 100.0, 101.4, 107.2, 109.2, 127.4, 132.5.
13. Kan, S.-F.; Huang, W.-J.; Lin, L.-C.; Wang, P. S. Int. J.
Cancer 2004, 110, 641.