Stereoselective total synthesis of the E-isomer of putative lucentamycin A

Article abstract of DOI:10.1016/j.tetlet.2012.08.092

A synthesis of the E-isomer of the proposed structure of the novel tripeptide, lucentamycin A, was performed in an attempt to define the correct stereochemistry of this natural product. The synthetic route developed employs a stereoselective Rh-catalyzed reductive cyclization process to generate the key pyrrolidine residue in the target and a stereospecific inversion of the Z-olefin geometry to form desired E-isomer. Subsequent amide coupling reactions afforded the desired E-isomer of putative lucentamycin A. A comparison of the NMR data of synthetic E-1a with that of the naturally occurring lucentamycin A demonstrated that they are not identical substances and the E-1a was found to display no anti-proliferative activity on the colon cancer cell line HCT-116 in contrast to natural lucentamycin A.

Full text of DOI:10.1016/j.tetlet.2012.08.092

Tetrahedron Letters 53 (2012) 5895–5898
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Stereoselective total synthesis of the E-isomer of putative lucentamycin A
⇑
Khalid B. Selim, Baeck Kyoung Lee, Taebo Sim
Future Convergence Research Division, Korea Institute of Science and Technology, PO Box 131, Cheongryang, Seoul 130-650, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A synthesis of the E-isomer of the proposed structure of the novel tripeptide, lucentamycin A, was per-
formed in an attempt to define the correct stereochemistry of this natural product. The synthetic route
developed employs a stereoselective Rh-catalyzed reductive cyclization process to generate the key pyr-
rolidine residue in the target and a stereospecific inversion of the Z-olefin geometry to form desired E-iso-
mer. Subsequent amide coupling reactions afforded the desired E-isomer of putative lucentamycin A. A
comparison of the NMR data of synthetic E-1a with that of the naturally occurring lucentamycin A dem-
onstrated that they are not identical substances and the E-1a was found to display no anti-proliferative
activity on the colon cancer cell line HCT-116 in contrast to natural lucentamycin A.
Received 3 June 2012
Revised 16 August 2012
Accepted 22 August 2012
Available online 30 August 2012
Keywords:
E-Lucentamycin A
Olefin geometry
Reductive cyclization
Natural product
Chemical elucidation
Ó 2012 Elsevier Ltd. All rights reserved.
Lucentamycins A–D are naturally occurring tripeptides isolated
from the fermentation broth of a marine-derived actinomycete
Nocardiopsis lucentensis by Fenical in 2007.1 Among the four pep-
tides, lucentamycin A was found to have significant in vitro cyto-
toxicity against HCT-116 human colon carcinoma with an IC50
preparative routes used for the syntheses of 1a–d employed
stereoselective Rh-catalyzed reductive cyclization reactions5,6
using (S)- and (R)-BINAP. These processes afforded four key opti-
cally active intermediates, 3a, 3b, 5a, and 5b, which served as
building blocks for the four lucentamycin A isomers (Scheme 1).
Prior to our report, Del Valle and his coworkers described7 ester
enolate-Claisen rearrangement based syntheses of the same four
stereoisomers. Importantly, the data accumulated for the four syn-
thetic substances did not match that reported for naturally occur-
ring lucentamycin A, showing that the configurations of the
pyrrolidine stereogenic centers are not the source of the structural
mis-assignment of the natural product.
value of 0.20 lM. Owing to its high anti-proliferative activity
against colon cancer cells, Lucentamycin A has received great
attention. The gross structure and stereochemistry of this tripep-
tide, assigned as 1a (Fig. 1) by using the NMR spectroscopic analy-
sis and Marfey degradation, features a cis disubstituted exocyclic Z-
alkene containing pyrrolidine core, and N-acylated homoarginine,
and C-terminal leucine moieties.
The intriguing structural motif and biological properties of
lucentamycin A prompted synthetic studies by Lindsley et al.2 that
culminated in the preparation of the proposed 8-epi-lucentamycin
A (1d) instead of the natural product 1a. Concurrently to this effort,
Del Valle et al.3 carried out the first total synthesis of the putative
natural product but observed that NMR spectroscopic data of the
synthetic compound did not match that reported for naturally
occurring lucentamycin A. This finding suggested that the absolute
stereochemistry of the proline residue in the putative structure of
lucentamycin A (1a) needed revision.
Careful examination of the 1H NMR spectroscopic properties of
synthetic 1b–d revealed that significant differences exist between
the chemical shifts of the pyrrolidine ring protons relative to those
of natural lucentamycin A. In addition, the closeness of the
H
N
H
N
H
N
H
N
16
NH2
NH
NH2
NH
Ph
Ph
(S)
(S)
20
11
9
O
O
O
O
O
N
O
O
N
(E)
12
∗
8
(Z)
Recently, we described the total syntheses of the stereoisomers
(1a–d) corresponding to the four possible absolute configurations
of the two contiguous C2 and C3 stereocenters in the pyrrolidine
core of putative lucentamycin A. Our hope was that either the pre-
viously uncharacterized 8S,9S (1b) or 8R,9S (1c) stereoisomers
(Fig. 1) would be the naturally occurring lucentamycin A.4 The
∗
(R)
(S)
13
O
NH
NH
(S)
(S)
1
HO
HO
2
3
1a (8S,9R)
1b (8S,9S)
1c (8R,9S)
1d (8R,9R)
E-1a
⇑
Corresponding author. Tel.: +82 2 958 6347; fax: +82 2 958 5189.
E-mail address: tbsim@kist.re.kr (T. Sim).
Figure 1. Proposed structures of putative lucentamycin A (1a), its isomers (1b–d),
and E-isomer of 1a (E-1a).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.08.092

5896
K. B. Selim et al. / Tetrahedron Letters 53 (2012) 5895–5898
O
O
O
O
(S)-BINAP
(R)-BINAP
N
N
O
N
O
O
N
(R)
O
[Rh(cod)2]BF4
H2, DCE
[Rh(cod)2]BF4
H2, DCE
(S)
(S)
(S )
(R)
(R)
2a
3a (trans/cis = >99:1)
2b
3b (trans/cis = >99:1)
Boc
HO
Boc
HO
(R)-BINAP
(S)-BINAP
Boc
HO
Boc
HO
N
N
N
N
(R)
[Rh(cod)2]BF4
H2, DCE
[Rh(cod)2]BF4
H2, DCE
(S)
(S)
(R)
(R)
(S)
4a
5a
4b
5b
(cis/trans = 5:1)
(cis/trans = 5:1)
Scheme 1. Stereoselective synthesis of pyrrolidine isomers 3a,b and 5a,b via Rh-catalyzed reductive cyclization processes.
respective chemical shifts of H8 (4.36 vs 4.22 ppm), H9 (3.18 vs
3.21 ppm), H11a (4.58 vs 4.50 ppm), and H11b (4.44 vs
4.40 ppm) protons in the spectra of synthetic and isolated 1a, sug-
gest that the stereochemistry of the pyrrolidine ring in natural
lucentamycin A is most likely 8S,9R, as was originally proposed.4,7
When all of the current information is taken into account, it is rea-
sonable to propose that mis-assignment of the structure of the nat-
ural product might be a consequence of a difference in the
stereochemistry of the pyrrolidine exocyclic ethylidene moiety
and/or the two other amino acid residues. To address one aspect
of this issue, we have carried out a synthesis of the E-isomer of
the (8S,9R)-stereoisomer (E-1a, Fig. 1) of putative lucentamycin A
in order to determine if it is identical to naturally occurring
substance.
As illustrated in a retrosynthetic manner in Scheme 2, the key
steps in the synthesis of E-1a include a stereoselective construction
of the cis-pyrrolidine core via a BINAP directed Rh-catalyzed reduc-
tive cyclization reaction of the optically active 1,6-enyne 4a and
stereochemical inversion of the ethylidene group in the Z-isomer
5a to form (2S,3R,E)-(4-ethylidene-3-methylpyrrolidin-2-yl)meth-
anol (6). The preparative route began with the transformation of
(2S)-enyne 4a to cis-(2S,3R)-pyrrolidine 5a preferentially (5:1 dr,
92%, Scheme 2) utilizing reductive cyclization reaction promoted
by the Rh-(R)-BINAP complex in 1,2-dichloroethane at room tem-
perature under a hydrogen atmosphere.4 Boc-deprotection of 5a
using HCl–dioxane, followed by oxazolidinone ring formation with
carbonyldiimidazole in the presence of triethylamine afforded the
bicyclic-pyrrolidine 7 in 60% yield and a 5:1 cis:trans ratio. These
diastereomers were readily separated by using flash column
chromatography.
subsequently treated with methanolic KOH to afford epoxide 10
in quantitative yield.8 Because epoxide ring formation takes place
by inversion of configuration, this epoxide should serve as a pre-
cursor of the targeted E-ethylidene derivative 11. Relatively few
general methods exist for stereocontrolled olefin forming through
removal of oxygen from epoxides and most of the known proce-
dures require the use of expensive reagents and harsh reaction
conditions.9 However, we observed that deoxygenation of epoxide
10 occurred under mild condition using diphosphorous tetraiodide
(P2I4)10 in refluxing pyridine-CH2Cl2 to afford 11 (58% yield, 15%
recovery of 10), which possesses the desired E-ethylidene stereo-
chemistry. Moreover, when this reaction is carried out utilizing
the higher boiling CCl4 instead of CH2Cl2 as solvent, alkene 11 is
produced in 69% yield in 10 h. The resulting alkene 11 was found
to be a single diastereoisomer with E-stereochemistry (see below).
Hydrolysis of oxazolidinone ring in 11 followed by Boc-protec-
tion of the amine moiety afforded the (2S,3R,E)-alcohol 6 in 88%
yield (Scheme 2). The configuration of the ethylidene moiety in 6
was determined by comparing the results of 1D NOE experiments
with the (Z/E)-isomers 7 and 11 (Fig. 2). The, E-isomer 11 displayed
a clear NOE between the methyl protons on the olefin moiety and
the methine proton at C7. This NOE was absent in the spectrum of
the Z-isomer 7. However, a strong NOE was observed between the
methyl protons on the olefin moiety in 7 and the methylene proton
at C5. The stereochemical assignment also gained support from a
comparison of the 13C NMR chemical shifts in the spectra of 7
and 11. The strong shielding effect experienced by the olefinic
methyl group in the E-isomer 11 results in an upfield shift of the
C7 (36.3 ppm) resonance relative to that of Z-isomer
7
(d = 40.4 ppm) and an upfield shift of the C5 (d = 46.2 ppm) reso-
nance of Z-isomer 7 relative to that of E-isomer 11 (d = 48.5 ppm)
was observed. The cis-relative stereochemistry of C7a and C7 in
E-isomer 11 was also associated with a clear NOE correlation be-
tween the protons at C7a and C7.
With the aim of developing a shorter synthetic route, we ex-
plored a sequence for the preparation of the key intermediate 6
that does not require installation and removal of oxazolidinone
ring. Accordingly, protection of the alcohol moiety in 5a (cis/
trans = 5:1) to form the silylated alcohol 12 was accomplished
Inversion of ethylidene group stereochemistry was accom-
plished employing a four step sequence involving dihydroxylation,
mesylation, epoxide ring formation, and deoxygenation. Specifi-
cally, catalytic cis-dihydroxylation of 7 using osmium tetraoxide
and N-methylmorpholine N-oxide at room temperature furnished
the diol 8 as a single stereoisomer in 46% yield. However, when this
reaction is performed at 0 °C the diol 8 is generated in 81% yield.
Mesylation of the secondary hydroxyl group in 8 using MsCl in
the presence of triethylamine at 0 °C gave mesylate 9, which was
H
N
H
N
NH2
Ph
Rh-catalyzed
reductive
NH
O
O
O
O
N
peptide
alkene
Boc
Boc
Boc
inversion
cyclization
coupling
N
N
N
HO
HO
HO
NH
E-1a
HO
6 (2S,3R,E)
5a (2S,3R,Z)
4a
Scheme 2. Retrosynthetic analysis of the E-isomer of proposed lucentamycin A.

K. B. Selim et al. / Tetrahedron Letters 53 (2012) 5895–5898
5897
Saponification of methyl ester group in 18, followed by N-Boc
deprotection using 25% TFA provided the E-isomer of putative
lucentamycin A, which was purified by using reverse-phase HPLC
to afford the pure E-1a as a white solid in 50% yield.
H
O
O
O
O
H H
H
46.2
N
48.5
Me
H
H
N
N
N
5
5
7
O
O
O
O
7a
7a
7
40.4
15.7
Me
36.3
12.6
Our expectation that the alkene stereochemically inverted iso-
mer of 1a would be natural lucentamycin A turned out to be incor-
rect. Synthetic E-1a was found to display no anti-proliferative
activity on the colon cancer cell line HCT-116 in contrast to natural
lucentamycin A. Moreover, a comparison of the NMR data of syn-
thetic E-1a with that of naturally occurring lucentamycin A shows
that these substances have remarkably different chemical shifts,
particularly for protons (H2 and H16) at the stereogenic centers
in the leucine and homoarginine residues (see Supplementary
data). The chemical shifts of H2 and H16 in the spectrum of E-1a
are 4.21 and 4.62 ppm, while those of H2 and H16 for natural
lucentamycin A are 3.86 and 4.78 ppm. It is worth noting that
small H2 and H16 chemical shift differences were observed in
the spectra of synthetic 1a and natural lucentamycin A.4,7 Interest-
ingly, little or no chemical shift differences exist for protons on the
pyrrolidine core and ethylidene group; H8 (4.26 for E-1a vs
4.22 ppm for natural lucentamycin A), H9 (3.21 vs 3.21 ppm),
H11a (4.47 vs 4.50 ppm), H11b (4.45 and 4.40 ppm), H12 (5.43
vs 5.38 ppm), and H13 (1.66 vs 1.61 ppm). Based on these observa-
tions, the possibility might exist that structural differences be-
tween synthetic E-1a and natural lucentamycin A reside in the
leucine and homoarginine residues.
H
H
Me
Me
H
H
7
11
E-isomer,
7
11
E-isomer,
Z-isomer,
Z-isomer,
Figure 2. Structures of 7 and 11 showing 1D NOE correlations (left) and its 13C NMR
chemical shifts (right).
(87%) using TBSCl (Scheme 4). Dihydroxylation of 12 utilizing the
conditions described above (Scheme 3) followed by chromato-
graphic separation furnished the vicinal diol 13 as a single diaste-
reomer in 74% yield. Mesylation of the secondary hydroxyl group
in 13 followed by treatment of the resulting mesylate with alco-
holic KOH gave epoxide 14 in 72% yield as a single isomer. Deoxy-
genation was carried out using P2I4 (see above) to generate E-15 in
64% yield, which was subjected to TBS-deprotection using TBAF to
furnish the desired E-alcohol 6 in 84% yield.
The stage was now set for the straightforward synthesis of the
E-isomer of putative lucentamycin A utilizing the route outlined
in Scheme 5. Dess–Martin periodinane oxidation of 6 afforded
the corresponding aldehyde 16 in 94% yield. Pinnic oxidation of
16 using NaClO2 gave the corresponding carboxylic acid, which
was subjected to amide coupling reaction with H-Leu-OMe in the
presence of HATU/DIEA to form dipeptide 17 in 68% yield. Follow-
ing HCl-mediated N-Boc deprotection, an amide coupling reaction
with the N-benzoyl protected arginine derivative was carried out
using HATU/DIEA to yield (66%) the required tripeptide 18.
In summary, the study described above has led to the first total
synthesis of the E-isomer of putative lucentamycin A 1a, which uti-
lizes a stereospecific inversion of the Z-olefin geometry of the pyr-
rolidine core structure. The results of spectroscopic and biological
studies show that the synthetic E-isomer, E-1a and natural lucenta-
mycin A are not identical.
O
Boc
[Rh(cod)2]BF4 6 mol%
1. 4N HCl, Et2O, rt
OsO4 - NMO
Boc
HO
(R)-BINAP 9 mol%
N
N
N
O
2. CDI, TEA
CH2Cl2, rt
60%
(Z)
(R)
(Z)
(R)
H2O/THF, 0 °C
81%
(S)
HO
(S)
(S)
H2, DCE, rt
92%
4a
5a
7
O
O
O
O
OH
MsCl, Et3N
P2I4
OH
OH
O
OMs
KOH/MeOH
0 °C
N
N
O
N
O
CH2Cl2, 0 °C
Pyridine-CCl4
80 °C
99% (2 steps)
69%
8
9
10
O
O
NaOH
HO
(Boc)2O
Boc
HN
N
N
HO
EtOH/H2O (3:1)
Et3N, CH2Cl2
rt
88% (2 steps)
80 °C
11
6
Scheme 3. Synthesis of E-alcohol 6.
1. MsCl, Et3N
CH2Cl2, 0 °C
OH
OH
Boc
Boc
OsO4 - NMO
TBSCl
N
N
5a
TEA, DMAP
DMF, rt
87%
H2O/THF
74%
2.KOH/MeOH
0 °C
72% (2 steps)
TBSO
TBSO
12
13
P2I4
O
Boc
N
Boc
TBAF
N
6
pyridine-CCl4
80 °C
THF, 0 °C
84%
TBSO
TBSO
64%
15
14
Scheme 4. Alternative synthesis of E-alcohol 6.

5898
K. B. Selim et al. / Tetrahedron Letters 53 (2012) 5895–5898
Boc
1. NaClO2, NaH2PO4
t-BuOH/H2O, rt
1. HCl-dioxane, rt, 7 h
2. Bz-Homoarginine(Boc)2-OH
N
Boc
H
DMP
N
O
6
CH2Cl2, 0 °C
94%
2. Leu-OMe, HATU
DIPEA, THF, rt
68% (2 steps)
MeOOC
NH
HATU, DIPEA, THF, 0 °C
66% (2 steps)
O
16
17
H
N
H
H
H
N
1) LiOH, H2O/THF
rt, 3 h
NHBoc
NBoc
N
NH2
NH
Ph
N
Ph
O
O
O
O
N
O
O
N
2) TFA, CH2Cl2, rt, 16 h
50% (2 steps)
MeOOC
NH
HOOC
NH
18
E-1a
Scheme 5. Synthesis of E-isomer E-1a of putative lucentamycin A.
3. Pal, U.; Ranatunga, S.; Ariyarathna, Y.; Del Valle, J. R. D. Org. Lett. 2009, 11,
5298–5301.
Acknowledgments
4. Ham, Y. J.; Yu, H.; Kim, N. D.; Hah, J.-M.; Selim, K. B.; Choi, H. G.; Sim, T.
Tetrahedron 2012, 68, 1918–1925.
This research was supported by the Korea Institute of Science
and Technology and a Grant (2011-0028676) from the creative/
challenging research program of National Research Foundation of
Korea funded by the Ministry of Education, Science and Technol-
ogy, Republic of Korea.
5. (a) Jang, H.-Y.; Krische, M. J. J. Am. Chem. Soc. 2004, 126, 7875–7880; (b) Jang,
H.-Y.; Hughes, F. W.; Gong, H.; Zhang, J.; Brodbelt, J. S.; Krische, M. J. J. Am.
Chem. Soc. 2005, 127, 6174–6175; (C) Krische, MJ; Jang, H.-Y. Metal-Catalyzed
Reductive Carbocyclization (C@C, C„C, C@O Bonds) In Crabtree, R. H., Mingos,
D. M. P, Eds.; Comprehensive Organometallic Chemistry III; Elsevier: Oxford,
2006; Vol. 10, pp 493–536.
6. For selected reviews on metal-catalyzed enyne cycloisomerization, see: (a)
Aubert, C.; Buisine, O.; Malacria, M. Chem. Rev. 2002, 102, 813–834; (b) Lloyd-
Jones, G. C. Org. Biomol. Chem. 2003, 1, 215–236; (c) Diver, S. T.; Giessert, A. J.
Chem. Rev. 2004, 104, 1317–1382; (d) Bruneau, C. Angew. Chem., Int. Ed. 2005,
44, 2328–2334; (e) Michelet, V.; Toullec, P. Y.; Genêt, J.-P. Angew. Chem., Int. Ed.
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8976.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.08.
092.
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