Stereoselective total synthesis of a novel regiomer of herbarumin i and its cytotoxic and antimicrobial activities

Article abstract of DOI:10.1016/j.bmcl.2013.11.010

Stereoselective synthesis of a novel regiomer of the natural nonenolide, herbarumin I has been accomplished. The synthesis involves the coupling of the alcohol and acid fragments of the molecule using Yamaguchi protocol followed by intramolecular ring closing metathesis. The cytotoxic and antimicrobial properties of the synthetic regiomer have been studied.

Full text of DOI:10.1016/j.bmcl.2013.11.010

Bioorganic & Medicinal Chemistry Letters 24 (2014) 325–327
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Stereoselective total synthesis of a novel regiomer of herbarumin I
and its cytotoxic and antimicrobial activities q
Paramesh Jangili a, Jajula Kashanna a, C. Ganesh Kumar b, Y. Poornachandra b, Biswanath Das a,
⇑
a Natural Product Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500607, India
b Chemical Biology Laboratory, CSIR-Indian Institute of Chemical Technology, Hyderabad 500607, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Stereoselective synthesis of a novel regiomer of the natural nonenolide, herbarumin I has been
accomplished. The synthesis involves the coupling of the alcohol and acid fragments of the molecule
using Yamaguchi protocol followed by intramolecular ring closing metathesis. The cytotoxic and antimi-
crobial properties of the synthetic regiomer have been studied.
Received 2 August 2013
Revised 29 October 2013
Accepted 7 November 2013
Available online 19 November 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Herbarumin I
Regiomer
Total synthesis
Cytotoxicity
Antimicrobial activity
Various nonenolides (10-membered ring containing macro-
lides) with interesting structural features have been discovered
from natural source and were found to exhibit important biological
properties. Thus they have attracted much attention to the chem-
ists and also the biologists.1 Herbarumin I (1), a nonenolide, was
isolated from the fungus Phoma herbarum (Sphaeropsidaceae).2
The compound contains two hydroxyl (both in b-configuration)
at C-7 and C-8 positions. The bioevaluation of the compound re-
vealed that it possessed significant phytotoxic activity. The synthe-
sis of the compound by different groups have been accomplished.3
In continuation of our work4 on the construction of naturally
occurring bioactive molecules we have synthesized a novel regio-
mer (2) of herbarumin I (Fig. 1). Compound 2 bears two hydroxyl
groups with b-configuration at C-2 and C-8 positions. We have also
studied the cytotoxicity and antimicrobial properties of the mole-
cule. Herein, we discussed its synthesis and bioactivities.
as TBDPS ether 8 by reaction with TBDPSCl in the presence of imid-
azole. Compound 8 was treated with aqueous 80% TFA to generate
two free hydroxyl groups in 9. The diol 9 on treatment with Bu2-
SnO, TsCl and Et3N followed by reaction with methanolic K2CO3
was converted into the epoxide 10.6 The epoxide ring of 10 was
then opened with Mg and EtBr using CuI to afford the desired
alcohol 3.
For synthesis of the acid fragment 4 the epoxide 6 was
subjected to hydrolytic kinetic resolution7 by using Jacobsen’s
(S,S)-(Salen) Co(III) complex and AcOH to produce the diol 11 (ee
96%) (Scheme 3). The primary hydroxyl group of 11 was protected
as TBS ether 12 by treatment with TBDMSCl and imidazole and the
free hydroxyl group of 12 was protected as MOM ether 13 by reac-
tion with MOMBr and EtN(i-Pr)2. Compound 13 was then treated
with TBAF to furnish the primary alcohol 14 which was oxidized
with BAIB ([bis(acetoxy)iodo]benzene) and TEMPO in MeCN–H2O
(2:1) to generate the required acid fragmant 4.8
The coupling of the fragments 3 and 4 was carried out by treat-
ment with 2,4,6-trichlorobenzoyl chloride, Et3N and DMAP under
Yamaguchi protocol9 to furnish the ester 15. The ester 15 was then
subjected to intramolecular ring closing metathesis reaction by
using Grubbs’ 2nd generation catalyst to afford the cyclic ester
16.10 Finally, the compound 16 on treatment with TBAF followed
by reaction with CeCl3ꢀ7H2O produced the target regiomer 2.
The spectral (1H and 13C NMR and MS) properties of the synthe-
sized compound 2 are in good agreement with its structure.11 The
1H NMR spectrum of 2 indicated the presence of two olefinic
protons and three other protons attached with the carbons linked
The retrosynthetic analysis (Scheme 1) indicates that the regio-
mer 2 can be synthesized from the alcohol fragment 3 and the acid
fragment 4. The alcohol 3 can in turn, be prepared from the pro-
tected
The present synthesis was initiated by treatment of the pro-
tected -mannitol 5 [(R)-2,3-cyclohexylideneglyceraldehyde] with
D-mannitol (5) and the acid 4 from 1,2-epoxy 5-hexene (6).
D
allyl bromide, zinc and aqueous NH4Cl at 0 °C to produce the alco-
hol 7 (ee 97%) (Scheme 2).5 The hydroxyl group of 7 was protected
q
Part 71 in the series, ‘Synthetic studies on natural products’.
⇑
Corresponding author. Tel./fax: +91 40 7160512.
E-mail address: biswanathdas@yahoo.com (B. Das).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.11.010

326
P. Jangili et al. / Bioorg. Med. Chem. Lett. 24 (2014) 325–327
OMOM
6
OR'
HO
HO
O
a
e
5
HO
RO
O
R = R' = H
11
12 R = TBS, R' = H
6
b
c
O
O
9
1
HO
OH
13 R = TBS, R' = MOM
14 R = H, R' = MOM
d
HO
O
O
OMOM
O
4
2
1
Scheme 3. Synthesis of the acid fragment 4. Reagents and conditions: (a) (S,S)-
(salen) CoIII OAc (0.5 mol %), distd H2O (0.45 equiv), 0 °C, 24 h, 46% (ee 96%); (b)
TBDMSCl, imidazole, THF, rt, 96%; (c) MOMBr, EtN(i-Pr)2, CH2Cl2, 45 °C, 100%; (d)
TBAF, THF, 0 °C, 100%; (e) [bis(acetoxy)iodo]benzene (BAIB), 2,2,6,6-tetra-meth-
ylpiperidin-1-yl oxide (TEMPO), MeCN/H2O (2:1), rt, 3 h; 84%.
Figure 1. Structures of herbarumin I (1) and its regiomer (2).
O
HO
OH
O
O
b
a
O
RO
OR'
3
4
TBDPSO
OMOM
O
O
2
R = TBDPS, R' = MOM
R = R' = H
16
2
15
c
Scheme 4. Coupling of the fragments 3 and 4. Reagents and conditions: (a) 2,4,6-
trichloro benzoyl chloride, Et3N, DMAP, THF, rt, 98%; (b) Grubbs 2nd generation
catalyst, CH2Cl2, 50 °C; (c) (i) TBAF, THF and (ii) CeCl3ꢀ7H2O, MeCN, reflux, 2–3 h,
44% (over two steps).
OH
HO
TBDPSO
OMOM
O
3
4
HO
HO
O
OH
O
O
O
O
H
17
O
5
6
Figure 2. Structure of herbarumin II.
Scheme 1. Retrosynthetic analysis of compound 2.
membered lactone at the last two steps (Scheme 4) by isomeriza-
tion can be ruled out. Moreover, H-2 (d 4.19) appeared in the 1H
NMR spectrum of 2 as a doublet of doublet with the coupling con-
stants of 9.0 and 2.0 Hz. Almost similar coupling constants have
OH
O
O
c
O
O
a
OH
H
OTBDPS
been observed for the H-2 (with
a-configuration) in herbarumin
OR
R = H
R = TBDPS
O
5
9
II (17) (Fig. 2), a compound of related structure.13 If H-2 of 2 were
with b-configuration one of the coupling constant (9.0 Hz) should
be smaller as in the case of pinolidoxin where H-2 bears b-config-
uration.3b In the NOESY experiment H-2 (d 4.19) showed correla-
tion with H-8 (d 3.78) but not with H-9. This observation also
suggested that H-2 and H-8 are in b-configuration while H-9 in
7
8
b
OH
OTBDPS
O
d
e
OH
TBDPSO
OTBDPS
10
3
a-configuration. Thus it is evident that at the last two steps
(Scheme 4) the epimerization at the C-2 did not occur and the
synthesis of the target molecule (2) has successfully been
accomplished.
The synthetic compound (2) was examined for in vitro cytotox-
icity against five cancerous cell lines: A-549 (human alveolar
adenocarcinoma), MDA-MB-231 (human breast adenocarcinoma),
Scheme 2. Synthesis of the alcohol fragment 3. Reagents and conditions: (a) allyl
bromide, Zn, aq NH4Cl, 0 °C, 4 h, 91%; (b) TBDPSCl, imidazole, CH2Cl2, 0 °C – rt, 88%.
(c) aqueous 80% TFA, CH2Cl2, 0 °C–rt, 81%; (d) (i) Bu2SnO, TsCl, Et3N, rt, 30 min; (ii)
K2CO3, MeOH, rt,1 h, 75% (over two steps); (e) ethylmagnesiumbromide, CuI, ꢁ50 °C
to rt, 85%.
with oxygen together with three sets of methylene protons (six
protons) and a propyl group as the side chain. The synthetic se-
quence (Scheme 4) also revealed that the compound 2 contains a
propyl side chain. The 13C NMR spectrum of 2 showed nine carbon
signals: one lactone carbonyl carbon, two olefinic methine carbons,
three oxymethine carbons and three methylene carbons along with
three other carbon signals for the propyl group. These spectral data
clearly suggested12 that 2 is a 10 membered macrolactone with
one double bond, two hydroxyl groups and a propyl group. In the
HMBC spectrum of 2 H-9 (d 4.79) showed correlation with the
carbonyl group at C-1 (d 175.6). Thus the formation of a nine
Table 1
Cytotoxicity of compound 2
Cell line
IC50 values in lM
Compound 2
Doxorubicin
A549
MDA-MB-231
DU 145
Hep G2
COLO205
8.1
6.2
8.9
9.1
5.1
0.7
0.6
0.6
0.6
0.5

P. Jangili et al. / Bioorg. Med. Chem. Lett. 24 (2014) 325–327
327
Table 2
2. Rivero-Cruz, J. F.; Garcia-Aguirre, G.; Cerda-Garcia-Rojas, C. M.; Mata, R.
Tetrahedron 2000, 56, 5337.
Antimicrobial activity of compound 2
3. (a) Fürstner, A.; Radkowski, K. Chem. Commun. 2001, 671; (b) Fürstner, A.;
Radkowski, K.; Wirtz, C.; Goddard, R.; Lehmann, C. W.; Mynott, R. J. Am. Chem.
Soc. 2002, 124, 7061; (c) Sabino, A. A.; Pilli, R. A. Tetrahedron Lett. 2002, 43,
2819; (d) Nagaiah, K.; Sreenu, D.; Rao, R. S.; Yadav, J. S. Tetrahedron Lett. 2007,
48, 7173; (e) Selvam, J. J. P.; Rajesh, K.; Suresh, V.; Chanti Babu, D.;
Venkateswarlu, Y. Tetrahedron: Asymmetry 2009, 20, 1115; (f) Radha Krishna,
P.; Venkata Ramana, D. J. Org. Chem. 2012, 77, 674.
4. (a) Kashanna, J.; Paramesh, J.; Kumar, R. A.; Das, B. Helv. Chim. Acta 2012, 95,
1666; (b) Sudhakar, C.; Reddy, P. R.; Kumar, C. G.; Sujitha, P.; Das, B. Eur. J. Org.
Chem. 2012, 1253; (c) Kumar, J. N.; Das, B. Tetrahedron Lett. 2013, 54, 3865; (d)
Reddy, C. R.; Veeranjaneyulu, B.; Nagendra, S.; Das, B. Helv. Chim. Acta 2013, 96,
505; (e) Reddy, P. R.; Sudhakar, C.; Kumar, J. N.; Das, B. Helv. Chim. Acta 2013,
96, 289.
5. Chatterjee, S.; Kanojia, S. V.; Chattopadhyay, S.; Sharma, A. Tetrahedron:
Asymmetry 2011, 22, 367.
6. Sharma, G. V. M.; Manohar, V. Tetrahedron: Asymmetry 2012, 23, 252.
7. Schaus, S. E.; Brandes, B. D.; Larrow, J. F.; Tokunaga, M.; Hansen, K. B.; Gould, A.
E.; Furrow, M. E.; Jacobsen, E. N. J. Am. Chem. Soc. 2002, 124, 1307.
8. Epp, J. B.; Widlanski, T. S. J. Org. Chem. 1999, 64, 293.
Test pathogen
MIC values in lg/mL
Compound 2
Ciprofloxacin (control)
S. aureus MTCC 96
B. subtilis MTCC 121
E. coli MTCC 739
P. aeruginosa MTCC 2453
K. planticola MTCC 530
7.8
15.6
—
—
15.6
0.9
0.9
0.9
0.9
0.9
DU145 (human prostate adenocarcinoma) HepG2 (human liver
carcinoma), and COLO205 (human colon carcinoma). Doxorubicin
was used as the positive control. The MTT assay (according to
the method of Mosmann14) was utilized to evaluate the activity.
The IC50 values (50% inhibitory concentration) were calculated
from the plotted absorbance data for the dose–response curves.
9. Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull. Chem. Soc. Jpn.
1979, 52, 1989.
IC50 value (in lM) for each cell line was determined as the average
of two independent experiments (Table 1). The results showed that
synthetic compound (2) exhibited significant cytotoxic activity
against all of the five cell lines. However, its activity is higher
against COLO205 and MDA-MB-231 cell lines.
The antimicrobial activity of synthetic compound (2) was also
tested against several bacterial organisms: Staphylococcus aureus
(MTCC 96), Bacillus subtilis (MTCC 121), Escherichia coli (MTCC
739), Klebsiella planticola (MTCC 530) and Pseudomonas aeruginosa
(MTCC 2453). Ciprofloxacin was used as the positive control for
bacterial strains and microtiter broth dilution method was applied
to determine the antimicrobial activity.15 The minimum inhibitory
concentration (MIC) value for each pathogen was evaluated after
four individual observations (Table 2).
10. Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953.
11. The spectral properties of selected compounds are given below.
(4R,5S)-5-(tert-Butyldiphenylsilyloxy)oct-7-en-4-ol (3).
½ ꢂ ꢁ 25:4 (c 0.2,
a 2D4
CHCl3); 1H NMR (200 MHz, CDCl3): d 7.71–7.63 (4H, m), 7.49–7.32 (6H, m),
5.68 (1H, m), 4.98–3.89 (2H, m), 3.75 (1H, m), 3.53 (1H, m), 2.29–2.17 (2H, m),
1.31–1.21 (4H, m), 1.08 (9H, s), 0.81 (3H, t, J = 7.0 Hz); 13C NMR (50 MHz,
CDCl3): d 136.0, 135.1, 134.0, 129.9, 127.8, 116.9, 76.1, 74.0, 36.2, 33.9, 27.1,
20.2, 20.0, 13.9; ESIMS: m/z 405 [M+Na]+. Anal. Calcd for C24H34O2Si: C, 75.34;
H, 8.36. Found: C, 75.36; H, 8.34.
(R)-2-(Methoxymethoxy)hex-5-enoic acid (4). ½a D24
ꢂ
þ 47:0 (c 0.1, CHCl3); 1H NMR
(200 MHz, CDCl3): d 9.48 (1H, brs), 5.79 (1H, m), 5.12–4.96 (2H, m), 4.70 (2H, q,
J = 7.0 Hz), 4.18 (1H, t, J = 7.0 Hz), 3.42 (3H, s), 1.99–1.82 (2H, m), 1.32–1.17
(2H, m); 13C NMR (50 MHz, CDCl3): d 178.0, 136.8, 115.2, 90.6, 74.9, 55.4, 31.3,
29.6; ESIMS: m/z 197 [M+Na]+. Anal. Calcd for C8H14O4: C, 55.16; H, 8.10.
Found: C, 55.12; H, 8.12.
(R)-((4R,5S)-5-(tert-Butyldiphenylsilyloxy)oct-7-en-4-yl)
2-(methoxymethoxy)
hex-5-enoate (15). ½a D24
ꢂ
ꢁ 6:1 (c 0.2, CHCl3); 1H NMR (200 MHz, CDCl3): d
The compound (2) showed impressive activity against S. aureus,
and good activity against B. subtilis and K. planticola.
7.71–7.62 (4H, m), 7.48–7.31 (6H, m), 5.75 (1H, m), 5.52 (1H, m), 5.09–4.99
(3H, m), 4.91–4.81 (2H, m), 4.64 (2H, q, J = 12.0 Hz), 4.04 (1H, t, J = 7.0 Hz), 3.75
(1H, m), 3.39 (3H, s), 2.28–2.08 (4H, m), 1.79–1.70 (2H, m), 1.59 (1H, m), 1.32–
1.13 (3H, m), 1.02 (9H, s), 0.83 (3H, t, J = 7.0 Hz); 13C NMR (50 MHz, CDCl3): d
172.2, 137.6, 136,2, 133.9, 133.3, 130.0, 127.8, 117.5, 115.2, 96.8, 76.9, 75.0,
74.1, 56.1, 38.1, 32.0, 31.5, 29.9, 27.2, 19.2, 18.7, 14.1; ESIMS: m/z 561 [M+Na]+.
Anal. Calcd for C32H46O5Si: C, 71.33; H, 8.61. Found: C, 71.38; H, 8.65.
(3R,9S,10R,E)-3,9-Dihydroxy-10-propyl-3,4,5,8,9,10-hexahydro-2H-oxecin-2-one
In conclusion, we have described the stereoselective total syn-
thesis of a novel regiomer of the natural nonenolide, herbarumin
I. The synthesis consists of Yamaguchi esterification and intramo-
lecular ring closing metathesis as the key steps. The cytotoxic
and antimicrobial properties of the synthetic molecule have been
evaluated.
(2): ½a 2D4
ꢂ
ꢁ 15:0 (c 0.2, CHCl3); 1H NMR (200 MHz, CDCl3): d 5.54 (1H, ddd,
J = 16.0, 10.0, 2.0 Hz), 5.45 (1H, m), 4.79 (1H, m), 4.19 (1H, dd, J = 9.0, 2.0 Hz),
3.78 (1H, m), 2.50 (1H, m), 2.41 (1H, m), 2.38–2.30 (2H, m), 2.15 (1H, m), 2.01
(1H, m), 1.66–1.52 (2H, m), 1.51–1.42 (2H, m), 0.93 (3H, t, J = 7.0 Hz); 13C NMR
(50 MHz, CDCl3): d 175.6, 134.3, 124.8, 79.1, 72.5, 68.9, 37.7, 35.1, 32.0, 27.6,
18.9, 14.0; ESIMS: m/z 251 [M+Na]+. Anal. Calcd for C12H20O4: C, 63.14; H, 8.83.
Found: C, 63.12; H, 8.85.
Acknowledgement
The authors thank CSIR and UGC, New Delhi for financial
assistance.
12. Rukachaisirikul, V.; Pramjit, S.; Pakawatchai, C.; Isaka, M.; Supothina, S. J. Nat.
Prod. 2004, 67, 1953.
13. Diez, E.; Dixon, D. J.; Ley, S. V.; Polaru, A.; Rodriguez, F. Synlett 2003, 1186.
14. Mosmann, T. J. Immunol. Methods 1983, 65, 55.
15. Das, B.; Shinde, D. B.; Kanth, B. S.; Kamle, A.; Kumar, C. G. Eur. J. Med. Chem.
2011, 46, 3124.
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