Total synthesis of the proposed structure of LL15G256γ

Article abstract of DOI:10.1055/s-2008-1042759

The first total synthesis of a molecule possessing the stereochemistry proposed for LL15G256γ is described. The structure synthesized appears to be different from that of the marine natural product. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1042759

LETTER
569
Total Synthesis of the Proposed Structure of LL15G256g
Synthesis of
a
P
h
roposed LL1
u
5G256
g
o Li,a Shuo Liang,a Zhengshuang Xu,*a,b Tao Ye*a,b
Laboratory of Chemical Genomics, Peking University Shenzhen Graduate School, University Town of Shenzhen, Xili, Nanshan District,
Shenzhen 518055, P. R. of China
b
Department of Applied Biology & Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
Fax +85222641912; E-mail: bctaoye@inet.polyu.edu.hk
Received 8 November 2007
rine cyclopeptides and cyclodepsipeptides display a wide
spectrum of bioactivities.2 Many of them were used as
new leads in the pharmaceutical industry.3 LL15G256g
(1) was isolated from the marine fungus Hypoxylon oce-
anicum with the proposed stereochemistry4 as shown in
Scheme 1. It was also isolated, named as arthrichitin,5
from the culture broth of Arthrinium phaeospermun.
LL15G256g/arthrichitin has a broad spectrum of activity
against Candida spp., Trychopbyton spp., and several
phytopathogens, mainly through the inhibition of the for-
mation of fungal chitin and glucan synthases.5,6 We have
been interested for some time in marine cyclopeptides and
cyclodepsipeptides, and view their syntheses as a key
route to structural modification and subsequent activity
control.7 Here we report on our studies toward the synthe-
sis of LL15G256g.
Abstract: The first total synthesis of a molecule possessing the ste-
reochemistry proposed for LL15G256g is described. The structure
synthesized appears to be different from that of the marine natural
product.
Key words: antifungal agents, cyclodepsipeptide, LL15G256g, to-
tal synthesis, benzylic oxidation, macrolactamization
The fungal cell wall is a unique organelle, required for the
growth and the maintenance of the osmotic stability of the
cell. The composition of the fungal cell wall varies among
species, but it generally has three polymeric components:
glucan, chitin, and mannoproteins. The subcellular mech-
anisms of their synthesis and assembly have been used as
potential targets to discover new antifungal agents. Echi-
nocandin-based cyclopeptides have served as useful scaf-
folds for the development of inhibitors of glucan synthesis
and a few echinocandin-like antifungal compounds were
approved by the FDA during the last five years.1 The ma-
Structurally, LL15G256g contains two main frameworks:
(a) a stearic acid unit with a polyketide segment, and (b) a
peptidic chain comprised of L-serine, b-ketotryptophan
TMS
O
OTBS
O
O
CbzHN
OH
O
HN
OH
O
NHCbz
HN
O
NH HN
O
7
5
+
HO
+
NH
O
OH
O
O
O
NHCbz
OH
O
LL-15G256γ (1)
DDQ
Ot-Bu
8
6
macrolactamization
O
TMS
+
OTBS
O
O
CbzHN
O
HN
OH
HN
NH
O
NH HN
O
O
O
O
NHCbz
NH
O
O
HO
O
Ot-Bu
O
Ot-Bu
2
3
4
Scheme 1 Retrosynthetic analysis
SYNLETT 2008, No. 4, pp 0569–0574
x
x.
x
x.
2
0
0
8
Advanced online publication: 12.02.2008
DOI: 10.1055/s-2008-1042759; Art ID: W20007ST
© Georg Thieme Verlag Stuttgart · New York

570
S. Li et al.
LETTER
OTBS
OMe
O
OMe
O
OH
Cbz
NH
O
N
H
HN
i
ii
HN
NH
O
NH
O
O
O
NHCbz
NHCbz
Ot-Bu
O
O
Ot-Bu
Ot-Bu
8
9
12
iii
iii
OTBS
OMe
O
O
O
OMe
O
OTBS
O
N
H
H2N
HN
NH
O
HN
NH
O
O
NHCbz
MeO
NHCbz
O
O
11
Ot-Bu
Ot-Bu
10
13
Scheme 2 Benzylic oxidation of tryptophan-containing peptides. Reagents and conditions: i) EDCI, L-Trp methyl ester hydrochloride, DI-
PEA, DMAP, CH2Cl2, 89%; ii) 1. LiOH, THF–H2O–MeOH; 2. 11, EDCI, DIPEA, DMAP, CH2Cl2, 84%; iii) DDQ, THF–H2O (9:1), 50–60%.
and D-glutamic acid, connected by ester and amide link- get molecules. In this connection, we have carried out two
ages. The b-ketotryptophan is rarely encountered and it model studies. Diprotected D-glutamic acid 8, prepared
has only been reported once in the context of janthinocins according to literature procedures,10 was condensed with
B, an antibiotic produced by Janthinobacterium lividum.8 L-tryptophan methyl ester in the presence of EDCI and
LL15G256g contains three sites of functionality which DIPEA to give dipeptide 9 in 89% yield. Saponification of
might require protection for chemical synthesis: one car- dipeptide 9 with lithium hydroxide furnished the neces-
boxylic acid, one ketone, and a primary hydroxyl group. sary carboxylic acid, which was then activated using
Retrosynthetic analysis (Scheme 1) suggested that EDCI and coupled with protected serine 11 to provide
LL15G256g might arise from the benzylic oxidation of tripeptide 12 in 84% yield. Treatment of dipeptide 9 or
the tryptophan fragment of cyclopeptide 2. In macrocyclic tripeptide 12 with DDQ in THF–H2O, afforded the corre-
peptides, ring disconnection is so strategically important sponding b-oxatryptophan derivatives in acceptable yield
that it can ultimately determine the success of a synthesis. (Scheme 2).
Poor disconnections can lead to slow cyclization rates, fa-
cilitating side reactions such as oligomerization and/or
epimerization of the C-terminal residue. Bearing this in
mind, we decided to disconnect the tryptophan residue
The synthesis of the lipophilic side chain commenced
with a-methylation of chiral oxazolidinone 14 to provide
15 as a single diastereomer with R-configuration at the
newly formed center.11 Compound 15 was converted into
from macrocycle 2. The advantage of this strategy is that
aldehyde 16 via a two-step sequence involving reductive
any potential epimerization of the a-stereogenic center of
removal of the chiral auxiliary with lithium borohydride
followed by Swern oxidation of the resulting alcohol
(Scheme 3). Treatment of the aldehyde 16 with the chiral
boron reagent derived from (E)-2-butene, n-butyllithium,
potassium tert-butoxide, and (+)-b-methoxy diisopi-
nocampheylborane in the presence of boron trifluoride
the tryptophan residue would not affect the total synthesis
as that stereogenic center would be labile anyway, once
the final benzylic oxidation of 2 is conducted. For the pro-
tection of the carboxylic acid functionality we chose a
tert-butyl ester protection strategy based on the variety of
conditions available for deprotection and the stability of
this protection group during esterification, amide forma-
tion, and benzylic oxidation conditions. The primary hy-
etherate gave the secondary alcohol 6 in 70% yield and
with excellent diastereoselectivity (dr > 95:5).12
Oxidative cleavage of the alkene unit in 6 using OsO4–
droxyl group shall not interfere with the final
NaIO4 in the presence of 2,6-lutidine produced the alde-
macrocyclization process due to its lesser nucleophilicity
hyde,13 which was subjected to a further mild oxidation
when compared with the amino functionality. Further dis-
connection of the macrolactamization precursor affords
dipeptide 3 and acid 4, which can be assembled through
process to give the corresponding acid 18 in 78% yield
over two steps.14
fragments 5–8 (Scheme 1).
With the acid 18 and the alcohol 6 in hand, we turned our
attention to the synthesis of intermediate 4. The secondary
alcohol of 6 was first linked to diprotected serine 5. How-
ever, we found the a-stereogenic center of serine was very
easily racemized whenever we used the protocol of
Benzylic oxidation of indole-containing systems with var-
ious oxidizing agents, including DDQ, has been investi-
gated.9 However, we were interested to see whether this
was applicable to different substrates, especially our tar-
Synlett 2008, No. 4, 569–574 © Thieme Stuttgart · New York

LETTER
Synthesis of Proposed LL15G256g
571
O
O
O
O
i
ii, iii
O
H
N
O
N
O
14
15
Bn
16
Bn
iv
Ipc
Ipc
OH OH
v, vi
B
OH
O
17
18
6
Scheme 3 Synthesis of alcohol 6 and acid 18. Reagents and conditions: i) NaHMDS, MeI, –78 °C, THF, 82%; ii) LiBH4, THF–MeOH, 95%;
iii) (COCl)2, DMSO, DIPEA, CH2Cl2, 98%; iv) 17, 16, BF3⋅OEt2, –100 °C, THF, 70% (dr > 95:5); v) OsO4–NaIO4, 2,6-lutidine, dioxane–H2O;
vi) NaClO2, NaH2PO4, MeSO2NH2, Na2HPO4, pH 7.0, 78% (two steps).
OTBS
H
PG
N
PG
coupling
reagents
NH
OTBS
OH
HO
O
O
+
O
5 PG = Cbz
6
22 PG = Cbz
19 PG = Fmoc
20 PG = Fmoc
Scheme 4 Ester bond formation toward fragments 20 or 22. Reagents and conditions: DCC, DMAP, DMAP·HCl, CH2Cl2 or Yamaguchi
reagent, DIPEA, CH2Cl2 (<10%).
O
O
O
N
O
Cbz
ref. 17
i
O
HO
OH
OH
O
N
NH2
Cbz
21
ii, iii
OTBS
O
OTBS
CbzHN
CbzHN
O
iv, v
O
O
O
HO
4
22
Scheme 5 Synthesis of advanced intermediate 4. Reagents and conditions: i) 1. (COCl)2, DMF, CH2Cl2; 2. 6, Et3N, DMAP, CH2Cl2, 85% (con-
formational isomers 2.2:1); ii) TsOH, MeOH, 98%; iii) TBSCl, Et3N, DMAP, CH2Cl2, 94%; iv) OsO4–NaIO4, 2,6-lutidine, dioxane–H2O; v)
NaClO2, NaH2PO4, MeSO2NH2, Na2HPO4, pH 7.0, 81% (two steps).
Keck15 or Yamaguchi,16 and the yield of 22 was rather low obtained smoothly and no racemization was found which
(<10%). Keck’s procedure gave a 1:1 mixture of epimers was confirmed by transformation of 21 into 22. Ester 22
while Yamaguchi’s approach gave a 3:2 ratio. Using appeared as a single diastereoisomer as determined by
Fmoc-protected serine 19 as the coupling partner made no NMR analysis. The alkene unit in 22 was then oxidized to
improvements (Scheme 4).
the corresponding acid 4 by the methods mentioned
above, and the yield was satisfactory (Scheme 5).
To overcome the epimerization problem, Garner’s protec-
tive protocol17 for serine was applied. Compound 21 was
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572
S. Li et al.
LETTER
TMS
TMS
O
OH
O
O
i
ii,iii
O
HN
NH
O
O
HN
NHCbz
HN
NHCbz
NHCbz
O
7
Ot-Bu
3
Scheme 6 Synthesis of dipeptide 3. Reagents and conditions: i) DCC, DMAP, TMSCH2CH2OH, THF, 90%; ii) H2, Pd/C (10%), MeOH,
98%; iii) 8, DIPCI, HOBt, Et3N, DMAP, CH2Cl2, 87%.
TMS
O
HN
O
Cbz
HN
O
OTBS
O
NH
HN
i, ii
O
O
NH
NHCbz
3
t-BuO
O
TMS
t-BuO
NH
O
O
O
O
23
iii, iv, v
O
O
O
HN
OH
HN
OH
vi, vii
O
NH HN
O
NH HN
O
t-BuO
NH
O
O
NH
O
O
OH
O
O
O
1
2
Scheme 7 Completion of the total synthesis of the proposed structure of LL15G256g. Reagents and conditions: i) H2, Pd/C (10%), MeOH, 98%;
ii) 4, Mukaiyama reagent, Et3N, CH2Cl2, 92%; iii) TBAF, THF; iv) H2, Pd/C (10%), MeOH; v) DEPC, DIPEA, DMF, 40% (three steps); vi)
DDQ, THF–H2O (9:1), 69%; vii) TFA, CH2Cl2, 98%.
The synthesis of dipeptide 3 was carried out by a similar The precursor was exposed, under high-dilution condi-
route as described for the synthesis of dipeptide 9. Protec- tions, to a variety of coupling reagents including bis(2-
tion of the carboxylic acid of N-Cbz-tryptophan was oxo-3-oxazolidinyl)phosphinyl chloride (BOP-Cl), 3-(di-
achieved with 2-(trimethylsilyl)ethanol under Keck’s pro- ethoxyphosphorylloxy)-1,2,3-benzotriazin-4-(3H)-one
cedure to afford 7 in 90% yield. The Cbz protecting group (DEPBT); diethyl cyanophosphonate (DEPC), diphenyl
in 7 was then removed by hydrogenolysis and the free phosphoryl azide (DPPA); Mukaiyama’s reagent, O-(7-
amino group of the resulting product was condensed with azabenzotriazol-1-yl)-N,N,N¢,N¢-tetramethyluronium
diprotected D-glutamic acid 8, under conditions employed hexafluorophosphate/1-hydroxybenzotriazole (HATU/
earlier for the synthesis of 9, to provide dipeptide 3 in 87% HOBt), EDCI/HOBt, and pentafluorophenol/EDCI (PFP/
yield from 7 (Scheme 6).
EDCI), in different solvents (CH2Cl2, DMF, MeCN). The
results from these studies revealed that the acyclic precur-
sor provided the best yield for the desired macrocycle 2
when it was treated with DEPC18 as coupling reagent and
DMF as solvent. The cyclodesipeptide 219 was submitted
to benzylic oxidation with the identical conditions ex-
plored in our model studies. To our delight, the desired
benzylic oxidized product was obtained in 69% yield. Re-
moval of the tert-butyl ester by treatment of the oxidative
product with trifluoroacetic acid provided synthetic
LL15G256g (1) in excellent yield.20
With the synthesis of the key subunits 3 and 4 completed,
the assembling of the acyclic precursor 23 was investigat-
ed (Scheme 7). Removal of the Cbz protecting group in
dipeptide 3 by hydrogenolysis yielded the corresponding
amine which was then condensed with acid 4 by Mukaiya-
ma’s reagent to furnish the linear depsipeptide 23 in ex-
cellent yield. The acyclic precursor for the
macrolactamization reaction was efficiently prepared by
treatment of 23 with TBAF to remove both 2-(trimethyl-
silyl)ethyl ester and TBS protecting groups, and then fur-
ther deprotection of the Cbz group by hydrogenolysis.
Synlett 2008, No. 4, 569–574 © Thieme Stuttgart · New York

LETTER
Synthesis of Proposed LL15G256g
573
(9) (a) Oikawa, Y. I.; Yonemitsu, O. Heterocycles 1976, 5, 233.
(b) Oikawa, Y. I.; Yonemitsu, O. J. Org. Chem. 1977, 42,
1213. (c) For the benzylic oxidation of tryptophan
derivatives with DDQ, see: Bagley, M. C.; Hind, S. L.;
Moody, C. J. Tetrahedron Lett. 2000, 41, 6897.
(10) (a) Van Heerbeek, R.; Kamer, P. C. J.; Van Leeuwen, P. N.
M. W.; Reek, J. N. H. Org. Biomol. Chem. 2006, 4, 211.
(b) Ley, S. V.; Smith, S. C.; Woodward, P. R. Tetrahedron
Lett. 1988, 29, 5829.
(11) (a) Evans, D. A.; Ennis, M. D.; Mathre, D. J. J. Am. Chem.
Soc. 1982, 104, 1737. (b) For a review on asymmetric
synthesis using chiral oxazolidinones, see: Ager, D. J.;
Prakash, I.; Schaad, D. R. Aldrichimica Acta 1997, 30, 312.
(12) (a) Brown, H. C.; Bhat, K. S. J. Am. Chem. Soc. 1986, 108,
293. (b) Brown, H. C.; Bhat, K. S.; Randad, R. S. J. Org.
Chem. 1989, 54, 1570.
Although the mass and UV spectra of synthetic 1 and
those of the natural product were identical, both the H
1
NMR and 13C NMR spectra obtained for synthetic 1 were
inconsistent with those published for natural LL15G256g.
Furthermore, HPLC analysis revealed that the retention
time for the synthetic sample and the natural product in
two different solvent systems (with either acetonitrile or
methanol as modifier) were different.21 In addition, the
data for the biological activities of the synthetic sample
were inconsistent with that published for natural
LL15G256g. We were confident that our synthesis pro-
duced the structure of LL15G256g with proposed stereo-
chemistry. The analytic data suggested that the synthetic
1 was a diastereomer of the natural product. The synthesis
of analogues of LL15G256g is currently being conducted
within our laboratory. This work will be reported in due
course.
(13) Yu, W.; Mei, Y.; Kang, Y.; Hua, Z.; Jin, Z. Org. Lett. 2004,
6, 3217.
(14) (a) Bal, B. S.; Childers, W. E. Jr.; Pinnick, H. W.
Tetrahedron 1981, 37, 2091. (b) Lindgren, B. O.; Nilsson,
T. Acta Chem. Scand. 1973, 27, 888.
(15) Boden, E. P.; Keck, G. E. J. Org. Chem. 1985, 50, 2394.
(16) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi,
M. Bull. Chem. Soc. Jpn. 1979, 52, 1989.
(17) McKillop, A.; Taylor, R. J. K.; Watson, R. J.; Lewis, N.
Synthesis 1994, 31.
Acknowledgment
We are indebted to Dr. Gerhard Schlingmann (Wyeth Research,
New York) for conducting the HPLC comparison between the syn-
thetic sample and the naturally occurring LL15G256g and for
giving us helpful advice. We acknowledge financial support from
the Hong Kong Research Grants Council (Project: PolyU 5015/
03P). ZSX would like to thank the support from GDSF (Guangdong
province Scientific Foundation, 04002986).
(18) (a) Yamada, S.; Kasai, Y.; Shioiri, T. Tetrahedron Lett.
1973, 1595. (b) Takuma, S.; Hamada, Y.; Shioiri, T. Chem.
Pharm. Bull. 1982, 30, 3147. (c) Jeremic, T.; Linden, A.;
Heimgartner, H. Helv. Chim. Acta 2004, 87, 3056.
(19) Cyclodepsipeptide 2: [a]D25 –3.6 (c 1.0, MeOH). 1H NMR
(500 MHz, CD3OD): d = 7.56 (1 H, d, J = 7.9 Hz), 7.32 (1
H, d, J = 8.2 Hz), 7.18 (1 H, s), 7.09 (1 H, dd, J = 7.8, 7.8
Hz,), 7.01 (1 H, dd, J = 7.8, 7.1 Hz), 4.96 (1 H, dd, J = 9.2,
7.2 Hz), 4.62 (1 H, dd, J = 8.8, 5.0 Hz), 4.45 (1 H, dd,
J = 7.8, 5.7 Hz), 3.98 (1 H, t, J = 7.4 Hz,), 3.54–3.46 (2 H,
m), 3.26 (1 H, dd, J = 15.1, 5.3 Hz,), 3.26 (1 H, dd, J = 15.1,
8.9 Hz), 2.82 (1 H, dd, J = 9.7, 7.3 Hz), 2.20–2.17 (2 H, m),
2.04–1.95 (2 H, m), 1.85–1.80 (1 H, m), 1.43 (9 H, s), 1.31
(14 H, br s), 1.17 (3 H, d, J = 7.2 Hz), 0.90 (3 H, d, J = 6.9
Hz), 0.88 (3 H, t, J = 7.2 Hz) ppm. 13C NMR (125 MHz,
CD3OD): d = 176.6, 174.2, 173.9, 172.8, 169.2, 138.1,
128.7, 124.3, 122.6, 119.9, 119.2, 112.4, 110.7, 81.8, 80.4,
62.6, 57.3, 56.9, 54.2, 44.7, 36.1, 35.1, 33.0, 32.5, 30.8, 30.6,
30.4, 28.4 (3 C), 28.3, 27.8, 25.3, 23.7, 15.5, 14.4, 14.1 ppm.
ESI-HRMS: m/z calcd for C37H56N4O8Na [M + Na]+:
707.3996; found: 707.3998.
References and Notes
(1) Turner, M. S.; Drew, R. H.; Perfect, J. R. Expert Opinion on
Emerging Drugs 2006, 11, 231.
(2) (a) Burja, A. M.; Banaigs, B.; Abou-Mansour, E.; Burgess,
J. G.; Wright, P. C. Tetrahedron 2001, 57, 9347.
(b) Gerwick, W. H.; Tan, L. T.; Sitachitta, N. In The
Alkaloids, Vol. 57; Cordell, G. A., Ed.; Academic Press: San
Diego, 2001, 75–184. (c) Luesch, H.; Harrigan, G. G.;
Goetz, G.; Horgen, F. D. Curr. Med. Chem. 2002, 9, 1791.
(3) (a) Proksch, P.; Edrada, R. A.; Ebel, R. Applied
Microbiology and Biotechnology 2002, 59, 125. (b) Frenz,
J. L.; Kohl, A. C.; Kerr, R. G. Expert Opin. Ther. Pat. 2004,
14, 17.
(4) Schlingmann, G.; Milne, L.; Williams, D. R.; Carter, G. T.
J. Antibiot. 1998, 51, 303.
(20) Procedure for the Synthesis of LL15G256g (1)
To a stirred solution of cyclodepsipeptide 2 (30 mg, 0.044
mmol) in 90% THF (5 mL), DDQ (25 mg, 0.11 mmol) was
added at 0 °C. The reaction was allowed to warm to r.t. and
stirred for 6 h. Then, EtOAc (30 mL) was added and the
mixture was washed with sat. aq NaHCO3 (3 × 15 mL) and
brine (15 mL). The organic layer was dried over Na2SO4 and
concentrated. The residue, after being purified by flash
chromatography on silica gel, using MeOH–CH2Cl2 (5:95)
as eluent, provided the desired b-keto product (21.1 mg,
69%). [a]D25 –8.0 (c 0.8, CH2Cl2). IR (KBr): 3421.5, 3282.6,
2927.7, 2854.5, 1697.2, 1651.0, 1508.2, 1419.5, 1280.6,
1157.2, 1068.5, 748.3 cm–1. 1H NMR (500 MHz, DMSO-
d6): d = 12.14 (1 H, s), 8.58 (1 H, d, J = 7.7 Hz), 8.40 (1 H,
s), 8.31 (1 H, d, J = 6.9 Hz), 8.14 (1 H, dd, J = 8.4, 1.6 Hz),
7.50 (1 H, dd, J = 8.2, 1.3 Hz), 7.25–7.18 (3 H, m), 5.77 (1
H, d, J = 8.6 Hz), 5.02 (1 H, dd, J = 6.6, 4.6 Hz), 4.90 (1 H,
d, J = 5.7 Hz), 4.28 (1 H, dd, J = 9.0, 6.3 Hz), 3.73 (1 H, d,
J = 8.4 Hz), 3.40–3.34 (2 H, m), 2.74 (1 H, dd, J = 10.6, 7.1
Hz), 2.18–2.15 (1 H, m), 2.02–1.97 (1 H, m), 1.96–1.93 (1
(5) Vijayakumar, E. K. S.; Roy, K.; Chatterjee, S.; Deshmukh,
S. K.; Ganguli, B. N.; Fehlhaber, H.-W.; Kogler, H. J. Org.
Chem. 1996, 61, 6591.
(6) Albaugh, D.; Albert, G.; Bradford, P.; Cotter, V.; Froyd, J.;
Gaughran, J.; Kirsch, D. R.; Lai, M.; Rehnig, A.; Sieverding,
E.; Silverman, S. J. Antibiot. 1998, 51, 317.
(7) (a) Pang, H. W.; Xu, Z. S.; Chen, Z. Y.; Ye, T. Lett. Org.
Chem. 2005, 2, 699. (b) Peng, Y. G.; Pang, H. W.; Xu, Z. S.;
Ye, T. Lett. Org. Chem. 2005, 2, 703. (c) Peng, Y. G.; Pang,
H. W.; Ye, T. Org. Lett. 2004, 6, 3781. (d) Xu, Z. S.; Chen,
Z. Y.; Ye, T. Tetrahedron: Asymmetry 2004, 15, 355.
(e) Xu, Z. S.; Peng, Y. G.; Ye, T. Org. Lett. 2003, 5, 2821.
(f) Chen, Z. Y.; Deng, J. G.; Ye, T. ARKIVOC 2003, (vii),
268.
(8) Johnson, J. H.; Tymiak, A. A.; Bolgar, M. S. J. Antibiotics
1990, 43, 920.
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LETTER
H, m), 1.70–1.67 (1 H, m), 1.37 (9 H, s), 1.22 (14 H, br s),
1.35–1.30 (1 H, m), 1.05 (3 H, d, J = 6.7 Hz), 0.84 (3 H, t,
J = 6.8 Hz), 0.83 (3 H, d, J = 6.9 Hz) ppm. 13C NMR (125
MHz, DMSO-d6): d = 186.2, 174.1, 172.4, 170.2, 167.9,
167.5, 137.1, 136.9, 126.1, 123.8, 122.8, 121.6, 113.9,
113.0, 80.1, 77.4, 62.1, 61.6, 56.1, 53.1, 42.9, 34.0, 33.9,
31.9, 31.7, 29.6, 29.4, 29.1, 28.3 (3 C), 27.3, 24.0, 22.5, 15.6,
14.4, 13.7 ppm. ESI-HRMS: m/z calcd for C37H54N4O9Na
[M + Na]+: 721.3788; found: 721.3790. To a solution of the
above b-keto macrocyclic peptide (21 mg, 0.03 mmol) in
anhyd CH2Cl2 (2 mL) was added TFA (1 mL) at 0 °C. The
reaction mixture was allowed to warm to r.t. and stirred for
6 h. The solvent was evaporated under reduced pressure and
the crude product was purified by flash chromatography on
(1 H, d, J = 3.0 Hz), 8.32 (1 H, br s), 8.15 (1 H, d, J = 6.8
Hz), 7.94 (1 H, s), 7.49 (1 H, d, J = 7.3 Hz), 7.49–7.19 (3 H,
m), 5.77 (1 H, d, J = 8.7 Hz), 5.01 (1 H, t, J = 5.5 Hz), 4.95
(1 H, d, J = 10.6 Hz), 4.29 (1 H, dd, J = 8.9 Hz), 3.72 (1 H,
br s), 3.38–3.34 (2 H, m), 2.76–2.71 (1 H, m), 2.08–2.03 (2
H, m), 1.99–1.94 (2 H, m), 1.70–1.65 (1 H, m), 1.34–1.30 (2
H, m), 1.28 (11 H, br s), 1.09–1.04 (1 H, m), 1.05 (3 H, d,
J = 6.9 Hz), 0.83 (3 H, t, J = 7.0 Hz), 0.82 (3 H, d, J = 6.8
Hz) ppm. 13C NMR (125 MHz, CDCl3): d = 185.7, 174.0,
173.5, 173.4, 167.3, 166.9, 136.5, 136.2, 125.5, 123.1,
122.3, 113.3, 112.3, 121.0, 76.7, 61.5, 61.0, 55.5, 52.5, 42.1,
33.4, 33.2, 31.1, 30.0, 29.1, 28.9, 28.7, 26.7, 23.1, 21.9, 15.1,
13.8, 13.1 ppm. ESI-HRMS: m/z calcd for C33H47N4O9 [M +
H]+: 643.3343; found: 643.3334; m/z calcd for
silica gel, using MeOH–CH2Cl2 (5:95 to 10:90) as eluent, to
C33H46N4O9Na [M + Na]+: 665.3162; found: 665.3150.
(21) The HPLC comparison between the synthetic sample and the
naturally occurring LL15G256g were conducted by Dr.
Gerhard Schlingmann (Wyeth Research, New York).
25
provide the desired target molecule (19 mg, 98%). [a]D
–
11.6 (c 1.0, CD3OH). 1H NMR (500 MHz, DMSO-d6): d =
12.16 (1 H, s), 12.06 (1 H, s), 8.60 (1 H, d, J = 8.0 Hz), 8.38
Synlett 2008, No. 4, 569–574 © Thieme Stuttgart · New York

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