- Email: [email protected]
Synthesis of cell wall-active lipopeptide diastereomers
Tetrahedron Letters 53 (2012) 115–118
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Synthesis of cell wall-active lipopeptide diastereomers P. Naresh ⇑, B. Jagadeesh ⇑ Centre for Nuclear Magnetic Resonance, Indian Institute of Chemical Technology, Council of Scientific and Industrial Research, Hyderabad 500 007, India
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 1 August 2011 Revised 14 October 2011 Accepted 16 October 2011 Available online 20 October 2011
The first synthesis of cell wall-active lipopeptide diastereomers from easily accessible starting materials has been reported. The synthetic strategy involves Jacobsen resolution, TEMPO/BAIB & 2C-Wittig (one pot reaction), and peptide couplings. Ó 2011 Published by Elsevier Ltd.
Keywords: Cell wall activity Antifungal Jacobsen’s kinetic resolution NMR analysis
The major challenge faced by most antifungal compounds in their mode of action is to differentiate plant and/or animal cells and fungi. In this regard, the fungal cell wall can act as a selective target since it is absent in their hosts. Although plant cells contain
a cell wall, its composition is different than that present in fungi.1 Hence there has been renewed interest on the identification of new cell wall targets. Several antibiotics are reported to inhibit fungal cell wall synthesis, for example, polyoxins and neopolyoxins OH
OH
O
HO N H
C 9 H19
H N O
O N H
H N
O N H
O
O
H N
1/2 OH
O
OH OMOM
HO
O OH
C9 H19
H N
BOCHN O
O
3
OBn O
(S)-5
O 4
OBn O
H N
N H
OH ( R )-5
6
Scheme 1. Restrosynthetic analysis of ½.
⇑ Corresponding authors. E-mail addresses: [email protected] (P. Naresh), [email protected] (B. Jagadeesh). 0040-4039/$ - see front matter Ó 2011 Published by Elsevier Ltd. doi:10.1016/j.tetlet.2011.10.081
O N H
H N O
O O
116
P. Naresh, B. Jagadeesh / Tetrahedron Letters 53 (2012) 115–118
OH
OBn
O
C9 H19
OBn
(S)-5
C 9H 19
OMOM
(R)-8
OMOM
O f, g
C9 H 19
O
C 9H 19
OEt
OH (R)-3
(R)-9
O
OH
(R)-7
d, e
and limited availability of isolated lipopeptide (1/2), makes this compound an attractive target for the synthesis. Retrosynthetic analysis (Scheme 1) of lipopeptide (1/2) reveals that it could be easily assembled by coupling the aliphatic fatty acid unit 3 with the hexapeptide fragment 4. The key fragment, MOM-protected HMA 3 could be prepared from epoxide 5 and that in turn could be easily prepared from homo allyl alcohol 6. It was envisioned that the other crucial hexapeptide 4 can be prepared from the usual solution phase peptide coupling of the constituent a-amino acids. Synthesis of the (R)-enantiomer of the MOM-protected HMA, (R)-3, began with the chiral epoxide (S)-5 which was obtained in 41% yield (99% ee) by hydrolytic kinetic resolution6 (HKR) of (±)-5 in the presence of (S,S)-Jacobsen’s catalyst. The regioselective opening of the epoxide (S)-5 using octanyl magnesium bromide in THF at 40 °C afforded the required alcohol (R)-7 in 92% yield. Followed by MOM protection of the 2° alcoholic group and benzyl deprotection afforded compound (S)-8 in 90% yield. Oxidation of (S)-8 using TEMPO/BAIB followed by Wittig reaction with Ph3P = CHCOOEt in DCM afforded the olefinic compound (S)-9 in 80% yield.7 Hydrogenation of (S)-9 (H2, Pd/C) followed by saponification of the ester functionality using lithium hydroxide in (3:1:1) THF–MeOH–H2O at 0 °C to room temperature has furnished the required MOM-protected (R)-HMA (R)-311 in 85% yield (Scheme 2). The MOM-protected (S)-HMA (S)-312 was prepared in an identical manner starting from the chiral epoxide (R)-5, which was obtained in 41% yield (99% ee) upon HKR6 of (±)-5 in the presence of (R,R)-Jacobsen’s catalyst (Scheme 2). With both enantiomers being available, the further synthesis of both the corresponding isomers of the lipopeptide (1 and 2), has been taken up in the following schemes (Schemes 3 and 4). The hexapeptide fragment 4 was synthesized from the commercially available protected (L and D)-amino acids. The condensation of Boc-L-Thr-OH and D-alanine methyl ester using EDCI and HOBt as coupling reagents afforded dipeptide 11 in 85% yield. The saponification of the ester functionality in LiOH gave the free acid, which was coupled with L-alanine methyl ester under similar conditions
OMOM b, c
a
OMOM
OBn
O
C9 H19
OH (S)-3
(R)-5
Scheme 2. Synthesis of MOM protected (R)- & (S)-HMA. Reagents and conditions: (a) C8H17MgBr, CuI, THF, 40 °C to rt, 3 h, 92%; (b) MOM–Cl, DIPEA, DMAP, CH2Cl2, 0 °C to rt, 12 h, 95%; (c) H2, 10% Pd/C, CH3OH, rt, 12 h, 95%; (d) TEMPO, BAIB, DCM, 0 °C to rt, 1 h; (e) PPh3 = CH2COOEt, rt, 0.5 h, 80%; (f) H2, 10% Pd/C, CH3OH, rt, 2 h, 86%; (g) LiOH, THF–CH3OH–H2O (3:1:1), 0 °C to rt, 2 h, 85%.
produced by Streptomyces are inhibitors of chitin synthetase,2 while neopeptins are cyclic lipopeptides isolated from Streptomyces that inhibit mannoprotein and b-1,3-glucan synthetases.3 Echinocandins are a class of fungicidal cell wall-active lipopeptides that are specific inhibitors of b-(1,3)-D-glucan synthesis.4 In addition to the selectivity, the structure of such antifungal molecules should be synthetically accessible to develop more potent analogues. In this context, the main component of cell wall-active lipopeptides (1/2) isolated from the Pochonia bulbinosa, culture 38G272, reported by Frank E. Koehn5, appears to be an enticing target molecule. The lipopeptide (1/2) has a synthetically accessible moiety, which contains a linear hexapeptide (L-Thr, D-Ala, two residues of L-Ala, D-Tyr, and L-Val) with a d-hydroxy myristic acid (HMA) amide substituted N-terminus, together with the biological activity HO O N H
O
HO O
a
OH
O
O
N H
HO O
O
H N
H N
b, c O
OMe
N H
O
O N H
O
OR O
11
10
R = Me (12) b R = H (12a) H N
O
O d
OH
H N
O
O
O N H
O 13
14
OH
H N
e, f
O
RHN O
O
OH
O N H
O O OH
R = Boc (15) e R = H (15a) OH HO 12a
H N
g, 15a RHN O
O N H
H N O
O N H
H N
O O
O
R = Boc (4) e R = H (4a)
Scheme 3. Synthesis of hexapeptide 4. Reagents and conditions: (a) EDCI, HOBt, CH2Cl2, then HClNH2D-Ala-OMe, DIPEA, 0 °C to rt, 12 h, 85%; (b) LiOH, THF–MeOH–H2O (3:1:1), 0 °C to rt, 1 h; (c) EDCI, HOBt, CH2Cl2, then HClNH2Ala-OMe, DIPEA, 0 °C to rt, 12 h, 70%; (d) EDCI, HOBt, CH2Cl2, then HClNH2Val-OMe, DIPEA, 0 °C to rt, 12 h, 70%; (e) TFA, CH2Cl2, 0 °C to rt, 1 h; (f) Boc-Ala-OH, then EDCI, HOBt, CH2Cl2, 0 °C to rt, 12 h, 69%; (g) EDCI, HOBt, CH2Cl2, 0 °C to rt, 12 h, 50%.
117
P. Naresh, B. Jagadeesh / Tetrahedron Letters 53 (2012) 115–118
OH
OH C 9 H19
(R )-3/(S)-3
HO O
(R)
N H
O
H N
N H
O
H N
O N H
O
H N
O OH
O
1
a b, c
OH
OH C 9H 19
HO O
(S)
N H
O
H N
N H
O
H N O
O N H
H N
O OH
O
2
Scheme 4. Synthesis of lipopeptide isomers 1 and 2. Reagents and conditions: (a) EDCI, HOBt, DMF, then 4a, DIPEA, 0 °C to rt, 12 h, 62%; (b) CH3OH–HCl (2:1), 0 °C to rt 2 h. (c) LiOH, THF–CH3OH–H2O (3:1:1), 0 °C to rt, 2 h, 95%.
as above to give tripeptide 12 in 70% yield. The saponification of the ester functionality in LiOH gave the free acid 12a. The condensation of Boc-D-Try-OH 13 and L-valine methyl ester using EDCI and HOBt as coupling reagents afforded dipeptide 14 in 70% yield. Boc deprotection of 14 with TFA in CH2Cl2 followed by coupling with Boc-L-Ala-OH under similar conditions as above gave tripeptide 15 in 69% yield. Boc deprotection of 15 with TFA in CH2Cl2 gave amine 15a, which was coupled with Boc-L-Thr-D-Ala-L-AlaOH 12a under standard peptide coupling conditions as mentioned above to give hexapeptide fragment 4 in 50% yield.10 Boc deprotection of 4 with TFA in CH2Cl2 gave amine 4a (Scheme 3). The final step of the synthesis of lipopeptide, 1/2, involves a coupling of hexapeptide amine 4a to both the enantiomers, of MOM-protected HMA 3. This was achieved using EDCI/HOBT coupling reagent in DMF with 62% yield. Deprotection of MOM group of both the coupled products using MeOH/11 N HCl (2:1, v/v) at 0 °C to room temperature followed by saponification of the ester functionality with LiOH furnished both the HMA epimers8,9of cell wall-active lipopeptide in 95% yield. We attempted to find out the unknown absolute local configuration of the natural product, by comparing its NMR data5 with that of the isomers, 1 and 2. However, the attempt was not successful as the 1H chemical shifts of all the three data sets did not differ appreciably (Dd <0.1 ppm).13 We attribute the observations to the fast dynamic processes of the highly flexible myristic acid chain. On the other hand, the optical rotations of lipopeptide isomers 1 & 2 were found to be 10.88 & 5.79, respectively, which, however could not be readily compared with the natural analogue,5 due to the non-availability of data. In conclusion, the first synthesis of (23R) and (23S) diastereomers of the cell wall-active lipopeptide has been reported. An attempt has been made to gain an insight into the absolute stereochemistry of the natural product. The preparation of analogues (replacing a-amino acids with unnatural b-amino acids or with L-His) and their biological study is under progress. Acknowledgments P.N is grateful to the CSIR, New Delhi, for research fellowship. The authors thank Dr. B. Venkateshwar Rao, J. Prasad Rao, B. Chandrasekhar and J. Shashidhar, Organic Chemistry division, and Dr. C. Ganesh Kumar, Chemical Biology, IICT for fruitful discussions.
References and notes 1. 2. 3. 4. 5. 6.
7. 8.
9.
10.
Carpita, N.; Mccann, M.; Griffing, L. R. Plant Cell 1996, 8, 1451. Isono, K.; Suzuki, S. Heterocycles 1979, 13, 333. Ubukata, M.; Uramoto, M.; Uzawa, J.; Isono, K. Agric. Biol. Chem. 1986, 50, 357. (a) Hector, R. F. Clin. Microbiol. Rev. 1993, 6, 1; (b) Herrera, R. J. Antonie van Leewenhoek 1991, 60, 73. Koehn, F. E.; Kirsch, D. R.; Feng, X.; Janso, J.; Young, M. J. Nat. Prod. 2008, 71, 2045. (a) 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; (b) Yadav, J. S.; Premalatha, K.; Harshavardhan, S. J.; Subba Reddy, B. V. Tetrahedron Lett. 2008, 49, 6765; (c) Chennakesava Reddy, B.; Meshram, H. M. Tetrahedron Lett. 2010, 51, 4020. Vatele, J. M. Tetrahedron Lett. 2006, 47, 715. Analytical and spectral data of compound 1: ½a22 D 10.8 (c 1.05, CH3OH); IR (neat): mmax 3280, 2925, 2855, 1639, 1518, 1456, 1234, 1171 and 828 cm1; 1H NMR (600 MHz, DMSO-d6, 303 K): d 9.21 (s, 1H), 8.05 (d, J = 8.6 Hz, 1H), 8.01 (d, J = 7.5 Hz, 1H), 7.94 (d, J = 8.6 Hz, 1H), 7.90 (d, J = 7.3 Hz, 1H), 7.87 (d, J = 7.3 Hz, 1H), 7.73 (d, J = 7.6 Hz, 1H), 7.01 (d, J = 8.3 Hz, 2H), 6.60 (d, J = 8.3 Hz, 2H), 4.54 (m, 1H), 4.18–4.25 (m, 3H), 4.08–4.12 (m, 2H), 3.93 (m, 1H), 2.89 (dd, J = 4.5, 13.7 Hz, 1H), 2.62 (dd, J = 10.0, 13.7 Hz, 1H), 2.15–2.18 (m, 2H), 2.01 (m, 1H), 1.60 (m, 1H), 1.47 (m, 1H), 1.20–1.30 (m, 20H), 1.19 (d, J = 7.1 Hz, 3H), 1.15 (d, J = 7.1 Hz, 3H), 1.01 (d, J = 6.6 Hz, 3H), 1.0 (d, J = 7.3 Hz, 3H), 0.83 (t, J = 6.8 Hz, 3H), 0.82 (d, J = 6.7 Hz, 3H), 0.81 (d, J = 6.7 Hz, 3H); 13C NMR (150 MHz, DMSOd6, 303 K): d 172.7, 172.7, 171.7, 171.7, 171.4, 171.1, 170.1, 155.7, 130.0, 127.6, 114.7, 69.3, 66.5, 58.7, 57.1, 53.9, 48.2, 48.0, 37.4, 37.1, 36.6, 35.2, 31.2, 30.0, 29.1, 29.0, 28.9, 28.6, 28.5, 25.1, 22.0, 21.6, 19.6, 19.0, 18.0, 17.9, 17.9, 17.7, 13.8; Melting point: 145–150 °C; HRMS (ESI): calcd for C41H68 N6O11 Na [M+Na]+ = 843.4843, found: 843.4869. Analytical and spectral data of compound 2: ½a22 D 5.7 (c 0.56, CH3OH); IR (neat): mmax 3280, 2925, 2855, 1639, 1518, 1456, 1234, 1171 and 828 cm1; 1H NMR (600 MHz, DMSO-d6, 303 K): d 9.17 (s, 1H), 8.12 (br.s, 1H), 7.99–8.06 (br.s, 2H), 7.96 (d, J = 7.4 Hz, 1H), 7.88–7.93 (br.s, 1H), 7.02 (d, J = 8.1 Hz, 2H), 6.61 (d, J = 8.1 Hz, 2H), 4.54 (m, 1H), 4.28 (m, 1H), 4.19–4.25 (m, 3H), 4.13 (dd, J = 7.8, 4.4 Hz, 1H), 4.09 (dd, J = 8.0, 6.0 Hz, 1H), 3.93 (m, 1H), 2.89 (dd, J = 4.5, 13.6 Hz, 1H), 2.62 (dd, J = 10.3, 13.6 Hz, 1H), 2.20 (m, 1H), 2.15 (m, 1H), 2.03 (m, 1H), 1.60 (m, 1H), 1.48 (m, 1H), 1.20–1.30 (m, 20H), 1.19 (d, J = 7.1 Hz, 3H), 1.15 (d, J = 7.1 Hz, 3H), 1.01 (m, 6H), 0.85 (t, J = 6.8 Hz, 3H), 0.82 (d, J = 6.7 Hz, 3H), 0.81 (d, J = 6.7 Hz, 3H); 13C NMR (150 MHz, DMSO-d6, 303 K): d 173.1, 172.8, 171.8, 171.8, 171.5, 171.1, 170.3, 155.7, 130.1, 127.8, 114.7, 69.3, 66.6, 58.9, 57.5, 54.1, 48.3, 48.1, 37.2, 36.7, 35.3, 31.3, 30.2, 29.3, 29.1, 29.0, 28.7, 24.6, 22.1, 21.1, 19.7, 18.9, 18.2, 18.14, 18.1, 17.9, 14.0; Melting point: 138–143 °C; HRMS (ESI): calcd for C41H68N6O11 Na [M+Na]+ = 843.4843, found: 843.4829. Analytical and spectral data of compound 4: ½a22 D 24.9 (c 0.7, CH3OH); IR (neat): mmax 3295, 2973, 2931, 1644, 1515, 1239 and 1163 cm1; 1H NMR (600 MHz, DMSO-d6, 303 K): d 9.14 (s, 1H), 8.29 (d, J = 8.34 Hz, 1H), 8.05 (d, J = 8.6 Hz, 1H), 7.97 (d, J = 7.4 Hz, 1H), 7.88–7.93 (m, 2H), 7.02 (d, J = 8.35 Hz, 2H), 6.6 (d, J = 8.46 Hz, 2H), 6.41 (d, J = 8.20 Hz, 1H), 4.85 (d, J = 5.45 Hz, 1H), 4.58 (m, 1H), 4.19–4.28 (m, 3H), 4.17 (dd, J = 6.6, 8.34 Hz, 1H), 3.89 (m, 1H), 3.85 (m, 1H), 3.64 (s, 3H), 2.87 (dd, J = 4.57, 13.70 Hz, 1H), 2.61 (dd, J = 10.1, 13.70 Hz, 1H), 2.0 (m, 1H), 1.38 (s, 9H), 1.19 (d, J = 7.10 Hz, 3H), 1.14 (d, J = 7.10 Hz, 3H), 1.01 (d, J = 6.15 Hz, 3H), 0.99 (d, J = 7.0 Hz, 3H), 0.82 (t, J = 6.65 Hz, 6H); 13C NMR (150 MHz, DMSO-d6, 303 K): d 172.0, 171.8, 171.8, 171.5, 171.4, 170.2, 155.8, 155.4, 130.2, 127.6, 114.7, 78.4, 66.9, 60.1, 57.3, 53.8, 51.8, 48.3, 48.0, 47.9, 37.6, 35.8, 30.1, 28.1, 19.7, 18.9, 18.2, 18.2, 18.0; Melting
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P. Naresh, B. Jagadeesh / Tetrahedron Letters 53 (2012) 115–118
point: 210–220 °C; HRMS (ESI): calcd for C33H52N6O11 Na [M+Na]+ = 731.3591, found: 731.3609. 11. Analytical and spectral data of compound (R)-3: ½a22 D 7.2 (c 1.4, CHCl3); IR (neat): mmax 2925, 2855, 1709, 1460, 1149, 1036 and 919 cm1; 1H NMR (300 MHz, CDCl3, 298 K): 4.65 (d, J = 1.7 Hz, 2H), 3.55 (m, 1H), 3.38 (s, 3H), 2.38 (t, J = 7.4 Hz, 2H), 1.64–1.77 (m, 2H), 1.47–1.59 (m, 4H), 1.27 (m, 14H), 0.88 (t, J = 6.8 Hz, 3H); 13C NMR (75 MHz, CDCl3, 298 K): d 179.4, 95.3, 77.0, 55.5, 34.1,
33.9, 33.5, 31.8, 29.7, 29.6, 29.5, 29.2, 25.2, 22.6, 20.4, 14.0; ESIMS: C15H29O4 Na [M+Na]+ = 311. 12. Analytical and spectral data of compound (S)-3: ½a22 D 6.6 (c 2.19, CHCl3); All other data are identical to that of (R)-3. 13. Chemical shift assignment data along with 1D and 2D NMR spectra of both the diastereomers, 1 and 2, can be found in supporting information. 1H and 13C NMR spectra of the intermediates are also provided in supporting information.
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Synthesis of cell wall-active lipopeptide diastereomers P. Naresh ⇑, B. Jagadeesh ⇑ Centre for Nuclear Magnetic Resonance, Indian Institute of Chemical Technology, Council of Scientific and Industrial Research, Hyderabad 500 007, India
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 1 August 2011 Revised 14 October 2011 Accepted 16 October 2011 Available online 20 October 2011
The first synthesis of cell wall-active lipopeptide diastereomers from easily accessible starting materials has been reported. The synthetic strategy involves Jacobsen resolution, TEMPO/BAIB & 2C-Wittig (one pot reaction), and peptide couplings. Ó 2011 Published by Elsevier Ltd.
Keywords: Cell wall activity Antifungal Jacobsen’s kinetic resolution NMR analysis
The major challenge faced by most antifungal compounds in their mode of action is to differentiate plant and/or animal cells and fungi. In this regard, the fungal cell wall can act as a selective target since it is absent in their hosts. Although plant cells contain
a cell wall, its composition is different than that present in fungi.1 Hence there has been renewed interest on the identification of new cell wall targets. Several antibiotics are reported to inhibit fungal cell wall synthesis, for example, polyoxins and neopolyoxins OH
OH
O
HO N H
C 9 H19
H N O
O N H
H N
O N H
O
O
H N
1/2 OH
O
OH OMOM
HO
O OH
C9 H19
H N
BOCHN O
O
3
OBn O
(S)-5
O 4
OBn O
H N
N H
OH ( R )-5
6
Scheme 1. Restrosynthetic analysis of ½.
⇑ Corresponding authors. E-mail addresses: [email protected] (P. Naresh), [email protected] (B. Jagadeesh). 0040-4039/$ - see front matter Ó 2011 Published by Elsevier Ltd. doi:10.1016/j.tetlet.2011.10.081
O N H
H N O
O O
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P. Naresh, B. Jagadeesh / Tetrahedron Letters 53 (2012) 115–118
OH
OBn
O
C9 H19
OBn
(S)-5
C 9H 19
OMOM
(R)-8
OMOM
O f, g
C9 H 19
O
C 9H 19
OEt
OH (R)-3
(R)-9
O
OH
(R)-7
d, e
and limited availability of isolated lipopeptide (1/2), makes this compound an attractive target for the synthesis. Retrosynthetic analysis (Scheme 1) of lipopeptide (1/2) reveals that it could be easily assembled by coupling the aliphatic fatty acid unit 3 with the hexapeptide fragment 4. The key fragment, MOM-protected HMA 3 could be prepared from epoxide 5 and that in turn could be easily prepared from homo allyl alcohol 6. It was envisioned that the other crucial hexapeptide 4 can be prepared from the usual solution phase peptide coupling of the constituent a-amino acids. Synthesis of the (R)-enantiomer of the MOM-protected HMA, (R)-3, began with the chiral epoxide (S)-5 which was obtained in 41% yield (99% ee) by hydrolytic kinetic resolution6 (HKR) of (±)-5 in the presence of (S,S)-Jacobsen’s catalyst. The regioselective opening of the epoxide (S)-5 using octanyl magnesium bromide in THF at 40 °C afforded the required alcohol (R)-7 in 92% yield. Followed by MOM protection of the 2° alcoholic group and benzyl deprotection afforded compound (S)-8 in 90% yield. Oxidation of (S)-8 using TEMPO/BAIB followed by Wittig reaction with Ph3P = CHCOOEt in DCM afforded the olefinic compound (S)-9 in 80% yield.7 Hydrogenation of (S)-9 (H2, Pd/C) followed by saponification of the ester functionality using lithium hydroxide in (3:1:1) THF–MeOH–H2O at 0 °C to room temperature has furnished the required MOM-protected (R)-HMA (R)-311 in 85% yield (Scheme 2). The MOM-protected (S)-HMA (S)-312 was prepared in an identical manner starting from the chiral epoxide (R)-5, which was obtained in 41% yield (99% ee) upon HKR6 of (±)-5 in the presence of (R,R)-Jacobsen’s catalyst (Scheme 2). With both enantiomers being available, the further synthesis of both the corresponding isomers of the lipopeptide (1 and 2), has been taken up in the following schemes (Schemes 3 and 4). The hexapeptide fragment 4 was synthesized from the commercially available protected (L and D)-amino acids. The condensation of Boc-L-Thr-OH and D-alanine methyl ester using EDCI and HOBt as coupling reagents afforded dipeptide 11 in 85% yield. The saponification of the ester functionality in LiOH gave the free acid, which was coupled with L-alanine methyl ester under similar conditions
OMOM b, c
a
OMOM
OBn
O
C9 H19
OH (S)-3
(R)-5
Scheme 2. Synthesis of MOM protected (R)- & (S)-HMA. Reagents and conditions: (a) C8H17MgBr, CuI, THF, 40 °C to rt, 3 h, 92%; (b) MOM–Cl, DIPEA, DMAP, CH2Cl2, 0 °C to rt, 12 h, 95%; (c) H2, 10% Pd/C, CH3OH, rt, 12 h, 95%; (d) TEMPO, BAIB, DCM, 0 °C to rt, 1 h; (e) PPh3 = CH2COOEt, rt, 0.5 h, 80%; (f) H2, 10% Pd/C, CH3OH, rt, 2 h, 86%; (g) LiOH, THF–CH3OH–H2O (3:1:1), 0 °C to rt, 2 h, 85%.
produced by Streptomyces are inhibitors of chitin synthetase,2 while neopeptins are cyclic lipopeptides isolated from Streptomyces that inhibit mannoprotein and b-1,3-glucan synthetases.3 Echinocandins are a class of fungicidal cell wall-active lipopeptides that are specific inhibitors of b-(1,3)-D-glucan synthesis.4 In addition to the selectivity, the structure of such antifungal molecules should be synthetically accessible to develop more potent analogues. In this context, the main component of cell wall-active lipopeptides (1/2) isolated from the Pochonia bulbinosa, culture 38G272, reported by Frank E. Koehn5, appears to be an enticing target molecule. The lipopeptide (1/2) has a synthetically accessible moiety, which contains a linear hexapeptide (L-Thr, D-Ala, two residues of L-Ala, D-Tyr, and L-Val) with a d-hydroxy myristic acid (HMA) amide substituted N-terminus, together with the biological activity HO O N H
O
HO O
a
OH
O
O
N H
HO O
O
H N
H N
b, c O
OMe
N H
O
O N H
O
OR O
11
10
R = Me (12) b R = H (12a) H N
O
O d
OH
H N
O
O
O N H
O 13
14
OH
H N
e, f
O
RHN O
O
OH
O N H
O O OH
R = Boc (15) e R = H (15a) OH HO 12a
H N
g, 15a RHN O
O N H
H N O
O N H
H N
O O
O
R = Boc (4) e R = H (4a)
Scheme 3. Synthesis of hexapeptide 4. Reagents and conditions: (a) EDCI, HOBt, CH2Cl2, then HClNH2D-Ala-OMe, DIPEA, 0 °C to rt, 12 h, 85%; (b) LiOH, THF–MeOH–H2O (3:1:1), 0 °C to rt, 1 h; (c) EDCI, HOBt, CH2Cl2, then HClNH2Ala-OMe, DIPEA, 0 °C to rt, 12 h, 70%; (d) EDCI, HOBt, CH2Cl2, then HClNH2Val-OMe, DIPEA, 0 °C to rt, 12 h, 70%; (e) TFA, CH2Cl2, 0 °C to rt, 1 h; (f) Boc-Ala-OH, then EDCI, HOBt, CH2Cl2, 0 °C to rt, 12 h, 69%; (g) EDCI, HOBt, CH2Cl2, 0 °C to rt, 12 h, 50%.
117
P. Naresh, B. Jagadeesh / Tetrahedron Letters 53 (2012) 115–118
OH
OH C 9 H19
(R )-3/(S)-3
HO O
(R)
N H
O
H N
N H
O
H N
O N H
O
H N
O OH
O
1
a b, c
OH
OH C 9H 19
HO O
(S)
N H
O
H N
N H
O
H N O
O N H
H N
O OH
O
2
Scheme 4. Synthesis of lipopeptide isomers 1 and 2. Reagents and conditions: (a) EDCI, HOBt, DMF, then 4a, DIPEA, 0 °C to rt, 12 h, 62%; (b) CH3OH–HCl (2:1), 0 °C to rt 2 h. (c) LiOH, THF–CH3OH–H2O (3:1:1), 0 °C to rt, 2 h, 95%.
as above to give tripeptide 12 in 70% yield. The saponification of the ester functionality in LiOH gave the free acid 12a. The condensation of Boc-D-Try-OH 13 and L-valine methyl ester using EDCI and HOBt as coupling reagents afforded dipeptide 14 in 70% yield. Boc deprotection of 14 with TFA in CH2Cl2 followed by coupling with Boc-L-Ala-OH under similar conditions as above gave tripeptide 15 in 69% yield. Boc deprotection of 15 with TFA in CH2Cl2 gave amine 15a, which was coupled with Boc-L-Thr-D-Ala-L-AlaOH 12a under standard peptide coupling conditions as mentioned above to give hexapeptide fragment 4 in 50% yield.10 Boc deprotection of 4 with TFA in CH2Cl2 gave amine 4a (Scheme 3). The final step of the synthesis of lipopeptide, 1/2, involves a coupling of hexapeptide amine 4a to both the enantiomers, of MOM-protected HMA 3. This was achieved using EDCI/HOBT coupling reagent in DMF with 62% yield. Deprotection of MOM group of both the coupled products using MeOH/11 N HCl (2:1, v/v) at 0 °C to room temperature followed by saponification of the ester functionality with LiOH furnished both the HMA epimers8,9of cell wall-active lipopeptide in 95% yield. We attempted to find out the unknown absolute local configuration of the natural product, by comparing its NMR data5 with that of the isomers, 1 and 2. However, the attempt was not successful as the 1H chemical shifts of all the three data sets did not differ appreciably (Dd <0.1 ppm).13 We attribute the observations to the fast dynamic processes of the highly flexible myristic acid chain. On the other hand, the optical rotations of lipopeptide isomers 1 & 2 were found to be 10.88 & 5.79, respectively, which, however could not be readily compared with the natural analogue,5 due to the non-availability of data. In conclusion, the first synthesis of (23R) and (23S) diastereomers of the cell wall-active lipopeptide has been reported. An attempt has been made to gain an insight into the absolute stereochemistry of the natural product. The preparation of analogues (replacing a-amino acids with unnatural b-amino acids or with L-His) and their biological study is under progress. Acknowledgments P.N is grateful to the CSIR, New Delhi, for research fellowship. The authors thank Dr. B. Venkateshwar Rao, J. Prasad Rao, B. Chandrasekhar and J. Shashidhar, Organic Chemistry division, and Dr. C. Ganesh Kumar, Chemical Biology, IICT for fruitful discussions.
References and notes 1. 2. 3. 4. 5. 6.
7. 8.
9.
10.
Carpita, N.; Mccann, M.; Griffing, L. R. Plant Cell 1996, 8, 1451. Isono, K.; Suzuki, S. Heterocycles 1979, 13, 333. Ubukata, M.; Uramoto, M.; Uzawa, J.; Isono, K. Agric. Biol. Chem. 1986, 50, 357. (a) Hector, R. F. Clin. Microbiol. Rev. 1993, 6, 1; (b) Herrera, R. J. Antonie van Leewenhoek 1991, 60, 73. Koehn, F. E.; Kirsch, D. R.; Feng, X.; Janso, J.; Young, M. J. Nat. Prod. 2008, 71, 2045. (a) 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; (b) Yadav, J. S.; Premalatha, K.; Harshavardhan, S. J.; Subba Reddy, B. V. Tetrahedron Lett. 2008, 49, 6765; (c) Chennakesava Reddy, B.; Meshram, H. M. Tetrahedron Lett. 2010, 51, 4020. Vatele, J. M. Tetrahedron Lett. 2006, 47, 715. Analytical and spectral data of compound 1: ½a22 D 10.8 (c 1.05, CH3OH); IR (neat): mmax 3280, 2925, 2855, 1639, 1518, 1456, 1234, 1171 and 828 cm1; 1H NMR (600 MHz, DMSO-d6, 303 K): d 9.21 (s, 1H), 8.05 (d, J = 8.6 Hz, 1H), 8.01 (d, J = 7.5 Hz, 1H), 7.94 (d, J = 8.6 Hz, 1H), 7.90 (d, J = 7.3 Hz, 1H), 7.87 (d, J = 7.3 Hz, 1H), 7.73 (d, J = 7.6 Hz, 1H), 7.01 (d, J = 8.3 Hz, 2H), 6.60 (d, J = 8.3 Hz, 2H), 4.54 (m, 1H), 4.18–4.25 (m, 3H), 4.08–4.12 (m, 2H), 3.93 (m, 1H), 2.89 (dd, J = 4.5, 13.7 Hz, 1H), 2.62 (dd, J = 10.0, 13.7 Hz, 1H), 2.15–2.18 (m, 2H), 2.01 (m, 1H), 1.60 (m, 1H), 1.47 (m, 1H), 1.20–1.30 (m, 20H), 1.19 (d, J = 7.1 Hz, 3H), 1.15 (d, J = 7.1 Hz, 3H), 1.01 (d, J = 6.6 Hz, 3H), 1.0 (d, J = 7.3 Hz, 3H), 0.83 (t, J = 6.8 Hz, 3H), 0.82 (d, J = 6.7 Hz, 3H), 0.81 (d, J = 6.7 Hz, 3H); 13C NMR (150 MHz, DMSOd6, 303 K): d 172.7, 172.7, 171.7, 171.7, 171.4, 171.1, 170.1, 155.7, 130.0, 127.6, 114.7, 69.3, 66.5, 58.7, 57.1, 53.9, 48.2, 48.0, 37.4, 37.1, 36.6, 35.2, 31.2, 30.0, 29.1, 29.0, 28.9, 28.6, 28.5, 25.1, 22.0, 21.6, 19.6, 19.0, 18.0, 17.9, 17.9, 17.7, 13.8; Melting point: 145–150 °C; HRMS (ESI): calcd for C41H68 N6O11 Na [M+Na]+ = 843.4843, found: 843.4869. Analytical and spectral data of compound 2: ½a22 D 5.7 (c 0.56, CH3OH); IR (neat): mmax 3280, 2925, 2855, 1639, 1518, 1456, 1234, 1171 and 828 cm1; 1H NMR (600 MHz, DMSO-d6, 303 K): d 9.17 (s, 1H), 8.12 (br.s, 1H), 7.99–8.06 (br.s, 2H), 7.96 (d, J = 7.4 Hz, 1H), 7.88–7.93 (br.s, 1H), 7.02 (d, J = 8.1 Hz, 2H), 6.61 (d, J = 8.1 Hz, 2H), 4.54 (m, 1H), 4.28 (m, 1H), 4.19–4.25 (m, 3H), 4.13 (dd, J = 7.8, 4.4 Hz, 1H), 4.09 (dd, J = 8.0, 6.0 Hz, 1H), 3.93 (m, 1H), 2.89 (dd, J = 4.5, 13.6 Hz, 1H), 2.62 (dd, J = 10.3, 13.6 Hz, 1H), 2.20 (m, 1H), 2.15 (m, 1H), 2.03 (m, 1H), 1.60 (m, 1H), 1.48 (m, 1H), 1.20–1.30 (m, 20H), 1.19 (d, J = 7.1 Hz, 3H), 1.15 (d, J = 7.1 Hz, 3H), 1.01 (m, 6H), 0.85 (t, J = 6.8 Hz, 3H), 0.82 (d, J = 6.7 Hz, 3H), 0.81 (d, J = 6.7 Hz, 3H); 13C NMR (150 MHz, DMSO-d6, 303 K): d 173.1, 172.8, 171.8, 171.8, 171.5, 171.1, 170.3, 155.7, 130.1, 127.8, 114.7, 69.3, 66.6, 58.9, 57.5, 54.1, 48.3, 48.1, 37.2, 36.7, 35.3, 31.3, 30.2, 29.3, 29.1, 29.0, 28.7, 24.6, 22.1, 21.1, 19.7, 18.9, 18.2, 18.14, 18.1, 17.9, 14.0; Melting point: 138–143 °C; HRMS (ESI): calcd for C41H68N6O11 Na [M+Na]+ = 843.4843, found: 843.4829. Analytical and spectral data of compound 4: ½a22 D 24.9 (c 0.7, CH3OH); IR (neat): mmax 3295, 2973, 2931, 1644, 1515, 1239 and 1163 cm1; 1H NMR (600 MHz, DMSO-d6, 303 K): d 9.14 (s, 1H), 8.29 (d, J = 8.34 Hz, 1H), 8.05 (d, J = 8.6 Hz, 1H), 7.97 (d, J = 7.4 Hz, 1H), 7.88–7.93 (m, 2H), 7.02 (d, J = 8.35 Hz, 2H), 6.6 (d, J = 8.46 Hz, 2H), 6.41 (d, J = 8.20 Hz, 1H), 4.85 (d, J = 5.45 Hz, 1H), 4.58 (m, 1H), 4.19–4.28 (m, 3H), 4.17 (dd, J = 6.6, 8.34 Hz, 1H), 3.89 (m, 1H), 3.85 (m, 1H), 3.64 (s, 3H), 2.87 (dd, J = 4.57, 13.70 Hz, 1H), 2.61 (dd, J = 10.1, 13.70 Hz, 1H), 2.0 (m, 1H), 1.38 (s, 9H), 1.19 (d, J = 7.10 Hz, 3H), 1.14 (d, J = 7.10 Hz, 3H), 1.01 (d, J = 6.15 Hz, 3H), 0.99 (d, J = 7.0 Hz, 3H), 0.82 (t, J = 6.65 Hz, 6H); 13C NMR (150 MHz, DMSO-d6, 303 K): d 172.0, 171.8, 171.8, 171.5, 171.4, 170.2, 155.8, 155.4, 130.2, 127.6, 114.7, 78.4, 66.9, 60.1, 57.3, 53.8, 51.8, 48.3, 48.0, 47.9, 37.6, 35.8, 30.1, 28.1, 19.7, 18.9, 18.2, 18.2, 18.0; Melting
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P. Naresh, B. Jagadeesh / Tetrahedron Letters 53 (2012) 115–118
point: 210–220 °C; HRMS (ESI): calcd for C33H52N6O11 Na [M+Na]+ = 731.3591, found: 731.3609. 11. Analytical and spectral data of compound (R)-3: ½a22 D 7.2 (c 1.4, CHCl3); IR (neat): mmax 2925, 2855, 1709, 1460, 1149, 1036 and 919 cm1; 1H NMR (300 MHz, CDCl3, 298 K): 4.65 (d, J = 1.7 Hz, 2H), 3.55 (m, 1H), 3.38 (s, 3H), 2.38 (t, J = 7.4 Hz, 2H), 1.64–1.77 (m, 2H), 1.47–1.59 (m, 4H), 1.27 (m, 14H), 0.88 (t, J = 6.8 Hz, 3H); 13C NMR (75 MHz, CDCl3, 298 K): d 179.4, 95.3, 77.0, 55.5, 34.1,
33.9, 33.5, 31.8, 29.7, 29.6, 29.5, 29.2, 25.2, 22.6, 20.4, 14.0; ESIMS: C15H29O4 Na [M+Na]+ = 311. 12. Analytical and spectral data of compound (S)-3: ½a22 D 6.6 (c 2.19, CHCl3); All other data are identical to that of (R)-3. 13. Chemical shift assignment data along with 1D and 2D NMR spectra of both the diastereomers, 1 and 2, can be found in supporting information. 1H and 13C NMR spectra of the intermediates are also provided in supporting information.