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 b
Natural Product Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500607, India Chemical Biology Laboratory, CSIR-Indian Institute of Chemical Technology, Hyderabad 500607, India
a r t i c l e
i n f o
Article history: Received 2 August 2013 Revised 29 October 2013 Accepted 7 November 2013 Available online 19 November 2013
a b s t r a c t 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. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Herbarumin I Regiomer Total synthesis Cytotoxicity Antimicrobial activity
Various nonenolides (10-membered ring containing macrolides) 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 chemists 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 revealed that it possessed significant phytotoxic activity. The synthesis 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 regiomer (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 molecule. Herein, we discussed its synthesis and bioactivities. The retrosynthetic analysis (Scheme 1) indicates that the regiomer 2 can be synthesized from the alcohol fragment 3 and the acid fragment 4. The alcohol 3 can in turn, be prepared from the protected D-mannitol (5) and the acid 4 from 1,2-epoxy 5-hexene (6). The present synthesis was initiated by treatment of the protected D-mannitol 5 [(R)-2,3-cyclohexylideneglyceraldehyde] with allyl bromide, zinc and aqueous NH4Cl at 0 °C to produce the alcohol 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: [email protected] (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
as TBDPS ether 8 by reaction with TBDPSCl in the presence of imidazole. Compound 8 was treated with aqueous 80% TFA to generate two free hydroxyl groups in 9. The diol 9 on treatment with Bu2SnO, 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 reaction 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 treatment 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 CeCl37H2O produced the target regiomer 2. The spectral (1H and 13C NMR and MS) properties of the synthesized compound 2 are in good agreement with its structure.11 The 1 H NMR spectrum of 2 indicated the presence of two olefinic protons and three other protons attached with the carbons linked
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P. Jangili et al. / Bioorg. Med. Chem. Lett. 24 (2014) 325–327
6
HO
O
5 O
9
HO
6
O
1
a
HO
b
OH
c d
O
O
OMOM
OR'
e
RO
HO
11 R = R' = H 12 R = TBS, R ' = H
O
13 R = TBS, R ' = MOM 14 R = H, R' = MOM
HO
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-methylpiperidin-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
3
4
a
O
TBDPSO
b
OMOM
2
O
OH
HO
c
16 R = TBDPS, R ' = MOM 2 R = R' = H
Scheme 4. Coupling of the fragments 3 and 4. Reagents and conditions: (a) 2,4,6trichloro benzoyl chloride, Et3N, DMAP, THF, rt, 98%; (b) Grubbs 2nd generation catalyst, CH2Cl2, 50 °C; (c) (i) TBAF, THF and (ii) CeCl37H2O, MeCN, reflux, 2–3 h, 44% (over two steps).
OMOM O
3
OR' O
15
TBDPSO
RO
O
4 HO O
HO O H O
O
OH O
O
5
17
6
Figure 2. Structure of herbarumin II. Scheme 1. Retrosynthetic analysis of compound 2.
O H O
O
O
a
OR
5
7 R=H 8 R = TBDPS
b
d
c
O
O OTBDPS 10
e
OH OH OTBDPS 9
OH
OH
TBDPSO OTBDPS 3
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 sequence (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
membered lactone at the last two steps (Scheme 4) by isomerization 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 constants of 9.0 and 2.0 Hz. Almost similar coupling constants have been observed for the H-2 (with a-configuration) in herbarumin 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-configuration.3b In the NOESY experiment H-2 (d 4.19) showed correlation 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 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 cytotoxicity against five cancerous cell lines: A-549 (human alveolar adenocarcinoma), MDA-MB-231 (human breast adenocarcinoma), Table 1 Cytotoxicity of compound 2 IC50 values in lM
Cell line
A549 MDA-MB-231 DU 145 Hep G2 COLO205
Compound 2
Doxorubicin
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 Table 2 Antimicrobial activity of compound 2 MIC values in lg/mL
Test pathogen
S. aureus MTCC 96 B. subtilis MTCC 121 E. coli MTCC 739 P. aeruginosa MTCC 2453 K. planticola MTCC 530
Compound 2
Ciprofloxacin (control)
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. 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). The compound (2) showed impressive activity against S. aureus, and good activity against B. subtilis and K. planticola. In conclusion, we have described the stereoselective total synthesis of a novel regiomer of the natural nonenolide, herbarumin I. The synthesis consists of Yamaguchi esterification and intramolecular ring closing metathesis as the key steps. The cytotoxic and antimicrobial properties of the synthetic molecule have been evaluated. Acknowledgement The authors thank CSIR and UGC, New Delhi for financial assistance. References and notes 1. Sun, P.; Lu, S.; Ree, T. V.; Krohn, K.; Hi, L.; Zhang, W. Curr. Med. Chem. 2012, 19, 3417.
327
2. Rivero-Cruz, J. F.; Garcia-Aguirre, G.; Cerda-Garcia-Rojas, C. M.; Mata, R. Tetrahedron 2000, 56, 5337. 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. 9. Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull. Chem. Soc. Jpn. 1979, 52, 1989. 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). ½a24 (c 0.2, D 25:4 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. 1 (R)-2-(Methoxymethoxy)hex-5-enoic acid (4). ½a24 D þ 47:0 (c 0.1, CHCl3); H 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) 1 hex-5-enoate (15). ½a24 D 6:1 (c 0.2, CHCl3); H NMR (200 MHz, CDCl3): d 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 1 (2): ½a24 D 15:0 (c 0.2, CHCl3); H 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. 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.
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 b
Natural Product Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500607, India Chemical Biology Laboratory, CSIR-Indian Institute of Chemical Technology, Hyderabad 500607, India
a r t i c l e
i n f o
Article history: Received 2 August 2013 Revised 29 October 2013 Accepted 7 November 2013 Available online 19 November 2013
a b s t r a c t 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. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Herbarumin I Regiomer Total synthesis Cytotoxicity Antimicrobial activity
Various nonenolides (10-membered ring containing macrolides) 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 chemists 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 revealed that it possessed significant phytotoxic activity. The synthesis 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 regiomer (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 molecule. Herein, we discussed its synthesis and bioactivities. The retrosynthetic analysis (Scheme 1) indicates that the regiomer 2 can be synthesized from the alcohol fragment 3 and the acid fragment 4. The alcohol 3 can in turn, be prepared from the protected D-mannitol (5) and the acid 4 from 1,2-epoxy 5-hexene (6). The present synthesis was initiated by treatment of the protected D-mannitol 5 [(R)-2,3-cyclohexylideneglyceraldehyde] with allyl bromide, zinc and aqueous NH4Cl at 0 °C to produce the alcohol 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: [email protected] (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
as TBDPS ether 8 by reaction with TBDPSCl in the presence of imidazole. Compound 8 was treated with aqueous 80% TFA to generate two free hydroxyl groups in 9. The diol 9 on treatment with Bu2SnO, 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 reaction 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 treatment 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 CeCl37H2O produced the target regiomer 2. The spectral (1H and 13C NMR and MS) properties of the synthesized compound 2 are in good agreement with its structure.11 The 1 H NMR spectrum of 2 indicated the presence of two olefinic protons and three other protons attached with the carbons linked
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P. Jangili et al. / Bioorg. Med. Chem. Lett. 24 (2014) 325–327
6
HO
O
5 O
9
HO
6
O
1
a
HO
b
OH
c d
O
O
OMOM
OR'
e
RO
HO
11 R = R' = H 12 R = TBS, R ' = H
O
13 R = TBS, R ' = MOM 14 R = H, R' = MOM
HO
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-methylpiperidin-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
3
4
a
O
TBDPSO
b
OMOM
2
O
OH
HO
c
16 R = TBDPS, R ' = MOM 2 R = R' = H
Scheme 4. Coupling of the fragments 3 and 4. Reagents and conditions: (a) 2,4,6trichloro benzoyl chloride, Et3N, DMAP, THF, rt, 98%; (b) Grubbs 2nd generation catalyst, CH2Cl2, 50 °C; (c) (i) TBAF, THF and (ii) CeCl37H2O, MeCN, reflux, 2–3 h, 44% (over two steps).
OMOM O
3
OR' O
15
TBDPSO
RO
O
4 HO O
HO O H O
O
OH O
O
5
17
6
Figure 2. Structure of herbarumin II. Scheme 1. Retrosynthetic analysis of compound 2.
O H O
O
O
a
OR
5
7 R=H 8 R = TBDPS
b
d
c
O
O OTBDPS 10
e
OH OH OTBDPS 9
OH
OH
TBDPSO OTBDPS 3
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 sequence (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
membered lactone at the last two steps (Scheme 4) by isomerization 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 constants of 9.0 and 2.0 Hz. Almost similar coupling constants have been observed for the H-2 (with a-configuration) in herbarumin 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-configuration.3b In the NOESY experiment H-2 (d 4.19) showed correlation 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 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 cytotoxicity against five cancerous cell lines: A-549 (human alveolar adenocarcinoma), MDA-MB-231 (human breast adenocarcinoma), Table 1 Cytotoxicity of compound 2 IC50 values in lM
Cell line
A549 MDA-MB-231 DU 145 Hep G2 COLO205
Compound 2
Doxorubicin
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 Table 2 Antimicrobial activity of compound 2 MIC values in lg/mL
Test pathogen
S. aureus MTCC 96 B. subtilis MTCC 121 E. coli MTCC 739 P. aeruginosa MTCC 2453 K. planticola MTCC 530
Compound 2
Ciprofloxacin (control)
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. 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). The compound (2) showed impressive activity against S. aureus, and good activity against B. subtilis and K. planticola. In conclusion, we have described the stereoselective total synthesis of a novel regiomer of the natural nonenolide, herbarumin I. The synthesis consists of Yamaguchi esterification and intramolecular ring closing metathesis as the key steps. The cytotoxic and antimicrobial properties of the synthetic molecule have been evaluated. Acknowledgement The authors thank CSIR and UGC, New Delhi for financial assistance. References and notes 1. Sun, P.; Lu, S.; Ree, T. V.; Krohn, K.; Hi, L.; Zhang, W. Curr. Med. Chem. 2012, 19, 3417.
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2. Rivero-Cruz, J. F.; Garcia-Aguirre, G.; Cerda-Garcia-Rojas, C. M.; Mata, R. Tetrahedron 2000, 56, 5337. 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. 9. Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull. Chem. Soc. Jpn. 1979, 52, 1989. 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). ½a24 (c 0.2, D 25:4 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. 1 (R)-2-(Methoxymethoxy)hex-5-enoic acid (4). ½a24 D þ 47:0 (c 0.1, CHCl3); H 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) 1 hex-5-enoate (15). ½a24 D 6:1 (c 0.2, CHCl3); H NMR (200 MHz, CDCl3): d 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 1 (2): ½a24 D 15:0 (c 0.2, CHCl3); H 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. 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.
