Synthesis of cell wall-active lipopeptide diastereomers

Article abstract of DOI:10.1016/j.tetlet.2011.10.081

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.

Full text of DOI:10.1016/j.tetlet.2011.10.081

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:
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.
Received 1 August 2011
Revised 14 October 2011
Accepted 16 October 2011
Available online 20 October 2011
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
HO
OH
O
O
O
O
H
N
H
N
H
N
1/2
N
H
N
H
C9H19
N
H
OH
O
O
O
OH
O
HO
OMOM
O
O
O
H
H
N
H
N
N
BOCHN
N
N
H
O
C9H19
OH
H
O
O
O
3
4
OBn
OBn
O
O
OH
(
)-5
R
6
(S)-5
Scheme 1. Restrosynthetic analysis of ½.
⇑
Corresponding authors.
E-mail addresses: naresh7077@yahoo.co.in (P. Naresh), bj@iict.res.in (B. Jagadeesh).
0040-4039/$ - see front matter Ó 2011 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2011.10.081

116
P. Naresh, B. Jagadeesh / Tetrahedron Letters 53 (2012) 115–118
OH
OMOM
and limited availability of isolated lipopeptide (1/2), makes this
compound an attractive target for the synthesis.
OBn
b, c
a
O
C9H19
OBn
C9H19
OH
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
(S)-5
(R)-7
(R)-8
OMOM
O
OMOM
O
f, g
d, e
C9H19
OH
C9H19
OEt
(R)-3
(R)-9
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 open-
ing 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 Hydroge-
nation 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-pro-
tected (R)-HMA (R)-311 in 85% yield (Scheme 2). The MOM-pro-
tected (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).
OMOM
O
OBn
O
C9H19
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 Streptomy-
ces that inhibit mannoprotein and b-1,3-glucan synthetases.3 Ech-
inocandins 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 po-
tent analogues. In this context, the main component of cell wall-ac-
tive lipopeptides (1/2) isolated from the Pochonia bulbinosa,
culture 38G272, reported by Frank E. Koehn5, appears to be an
enticing target molecule.
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- -Thr-OH and -alanine methyl ester using EDCI and HOBt as
L
D
The lipopeptide (1/2) has a synthetically accessible moiety,
coupling reagents afforded dipeptide 11 in 85% yield. The saponifi-
cation of the ester functionality in LiOH gave the free acid, which
which contains a linear hexapeptide (
-Ala, -Tyr, and -Val) with a d-hydroxy myristic acid (HMA)
amide substituted N-terminus, together with the biological activity
L-Thr, D-Ala, two residues of
L
D
L
was coupled with
L-alanine methyl ester under similar conditions
HO
HO
HO
O
O
O
O
O
H
N
H
N
a
b, c
OR
OH
O
N
H
N
H
O
N
H
O
N
H
OMe
O
O
O
O
O
11
10
R = Me (12)
b
R = H (12a)
O
O
H
N
H
N
H
N
O
d
e, f
O
O
O
OH
N
H
RHN
N
H
O
O
O
O
O
13
14
OH
OH
OH
R = Boc (15)
R = H (15a)
e
OH
O
HO
O
O
H
H
H
N
g, 15a
N
N
12a
RHN
N
H
N
H
O
O
O
O
R = Boc (4)
R = H (4a)
e
Scheme 3. Synthesis of hexapeptide 4. Reagents and conditions: (a) EDCI, HOBt, CH2Cl2, then HClꢁNH2D-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 HClꢁNH2Ala-OMe, DIPEA, 0 °C to rt, 12 h, 70%; (d) EDCI, HOBt, CH2Cl2, then HClꢁNH2Val-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%.

P. Naresh, B. Jagadeesh / Tetrahedron Letters 53 (2012) 115–118
117
OH
HO
O
O
O
O
OH
H
N
H
N
H
N
(R)
C9H19
N
H
N
H
N
H
OH
O
O
O
1
a
b, c
(
R
)-3/( )-3
S
OH
HO
O
O
O
O
OH
H
N
H
N
H
N
(S)
C9H19
N
H
N
H
N
H
OH
O
O
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 conden-
sation 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
References and notes
1. Carpita, N.; Mccann, M.; Griffing, L. R. Plant Cell 1996, 8, 1451.
2. Isono, K.; Suzuki, S. Heterocycles 1979, 13, 333.
3. Ubukata, M.; Uramoto, M.; Uzawa, J.; Isono, K. Agric. Biol. Chem. 1986, 50, 357.
4. (a) Hector, R. F. Clin. Microbiol. Rev. 1993, 6, 1; (b) Herrera, R. J. Antonie van
Leewenhoek 1991, 60, 73.
with Boc-
tide 15 in 69% yield. Boc deprotection of 15 with TFA in CH2Cl2
gave amine 15a, which was coupled with Boc- -Thr- -Ala- -Ala-
L-Ala-OH under similar conditions as above gave tripep-
5. Koehn, F. E.; Kirsch, D. R.; Feng, X.; Janso, J.; Young, M. J. Nat. Prod. 2008, 71,
2045.
L
D
L
6. (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.
OH 12a under standard peptide coupling conditions as mentioned
above to give hexapeptide fragment 4 in 50% yield.10 Boc deprotec-
tion of 4 with TFA in CH2Cl2 gave amine 4a (Scheme 3).
7. Vatele, J. M. Tetrahedron Lett. 2006, 47, 715.
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 cou-
pling 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.
8. Analytical and spectral data of compound 1: ½a D22
ꢂ
ꢀ10.8 (c 1.05, CH3OH); IR
(neat): m ;
max 3280, 2925, 2855, 1639, 1518, 1456, 1234, 1171 and 828 cmꢀ1 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, DMSO-
d6, 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,
We attempted to find out the unknown absolute local configu-
ration of the natural product, by comparing its NMR data5 with
that of the isomers, 1 and 2. However, the attempt was not suc-
cessful as the 1H chemical shifts of all the three data sets did not
13.8; Melting point: 145–150 °C; HRMS (ESI): calcd for
C41H68 N6O11 Na
[M+Na]+ = 843.4843, found: 843.4869.
differ appreciably (D
d <0.1 ppm).13 We attribute the observations
9. Analytical and spectral data of compound 2: ½a D22
ꢂ
ꢀ5.7 (c 0.56, CH3OH); IR
(neat): m ;
max 3280, 2925, 2855, 1639, 1518, 1456, 1234, 1171 and 828 cmꢀ1 1H
to the fast dynamic processes of the highly flexible myristic acid
chain. On the other hand, the optical rotations of lipopeptide iso-
mers 1 & 2 were found to be ꢀ10.88 & ꢀ5.79, respectively, which,
however could not be readily compared with the natural ana-
logue,5 due to the non-availability of data.
In conclusion, the first synthesis of (23R) and (23S) diastereo-
mers of the cell wall-active lipopeptide has been reported. An at-
tempt has been made to gain an insight into the absolute
stereochemistry of the natural product. The preparation of ana-
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.
logues (replacing
a-amino acids with unnatural b-amino acids or
10. Analytical and spectral data of compound 4: ½a D22
ꢂ
ꢀ24.9 (c 0.7, CH3OH); IR
with L-His) and their biological study is under progress.
(neat): mmax 3295, 2973, 2931, 1644, 1515, 1239 and 1163 cmꢀ1
;
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
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.

118
P. Naresh, B. Jagadeesh / Tetrahedron Letters 53 (2012) 115–118
point: 210–220 °C; HRMS (ESI): calcd for C33H52N6O11 Na [M+Na]+ = 731.3591,
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
found: 731.3609.
[M+Na]+ = 311.
11. Analytical and spectral data of compound (R)-3: ½a D22
ꢂ
ꢀ7.2 (c 1.4, CHCl3); IR
1H NMR
12. Analytical and spectral data of compound (S)-3: ½a D22
ꢂ
6.6 (c 2.19, CHCl3); All
(neat): mmax 2925, 2855, 1709, 1460, 1149, 1036 and 919 cmꢀ1
;
other data are identical to that of (R)-3.
(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,
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.