Synthesis of the LeuTrp component of the celogentin family of cyclic peptides through a CH activationcross-coupling strategy

Article abstract of DOI:10.1071/CH10033

A bioinspired approach to the central leucine(C3)-tryptophan(C6) cross-linked moiety present in the celogentin family of cyclic peptide natural products was achieved. The key transformation was enabled through a palladium-catalyzed C-H activationcross-coupling of leucine quinoline amide and 6-iodotryptophan derivatives. X-Ray crystallographic analysis of a β-(indol-6-yl)-leucine derivative confirms the stereochemistry of the cross-linked adduct matches that of the natural products. The method enables the preparation of the Leu-Trp adduct as a single stereoisomer from l-leucine and l-tryptophan.

Full text of DOI:10.1071/CH10033

Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2010, 63, 438–444
www.publish.csiro.au/journals/ajc
Synthesis of the Leu–Trp Component of the Celogentin Family
of Cyclic PeptidesThrough a C–H Activation–Cross-Coupling
Strategy
Barbara T. Y. Li,A Jonathan M. White,A and Craig A. HuttonA,B
ASchool of Chemistry and Bio21 Molecular Science and Biotechnology Institute,
University of Melbourne, Vic. 3010, Australia.
BCorresponding author. Email: chutton@unimelb.edu.au
A bioinspired approach to the central leucine(C3)–tryptophan(C6) cross-linked moiety present in the celogentin family
of cyclic peptide natural products was achieved. The key transformation was enabled through a palladium-catalyzed
C–H activation–cross-coupling of leucine quinoline amide and 6-iodotryptophan derivatives. X-Ray crystallographic
analysis of a β-(indol-6-yl)-leucine derivative confirms the stereochemistry of the cross-linked adduct matches that of the
natural products. The method enables the preparation of the Leu–Trp adduct as a single stereoisomer from l-leucine and
l-tryptophan.
Manuscript received: 19 January 2010.
Manuscript accepted: 25 February 2010.
Introduction
together with the known and closely related moroidin.[3] Many of
the celogentins show tubulin inhibitory activity; the most potent
of these is celogentin C 1 with an IC50 value of 0.8 µM (Fig. 1).
Celogentin C 1 is a bicyclic octapeptide containing a central
functionalized tryptophan moiety. The C6 position of the tryp-
tophan (Trp5) indole ring is directly coupled to the β-carbon of
The seeds of the cockscomb plant, Celosia argentea, have been
used as a traditional medicine in Japan and China for the treat-
ment of eye and liver diseases.[1] Kobayashi and coworkers
isolated, from the seeds of the cockscomb plant, a group of
closely related peptides that were named the celogentins,[2]
HN
Val4
Pro6
NH2
Val4
O
HN
HN
O
CO2H
O
O
NH
6
O
Ile3
N
O
Arg7
NH
Leu3
O
HN
HN
NH
O
O
O
NH
HN
N
H
N
O
N
N
H
N
CO2H
H
H
Trp5
O
N
H
N
N
His8
Leu2
H
Trp5
PyroGlu1
O
Leu2
PyroGlu1
Celogentin C 1
Stephanotic acid 2
NH2
NH
H
N
Arg6
Arg6
Val4
R
Val/Ile4
O
H
N
NH2
NH
O
O
O
O
O
O
O
Gly7
O
N
NH
HN
HN
Leu3
NH
H
NH
Leu3
HN
HN
NH
NH
6
CO2H
HN
His8
N
O
O
O
N
N
CO2H
O
H
N
N
H
N
His7
N
H
N
2
H
N
N
H
O
H
Trp5
O
Trp5
Leu2
Leu2
PyroGlu1
PyroGlu1
Moroidin 3 R ꢀ H
Celogentin G 4 R ꢀ Me
Celogentin A 5
Fig. 1. Representative members of the celogentin family of cyclic peptides.
© CSIRO 2010 10.1071/CH10033
0004-9425/10/030438

Synthesis of the Leu–Trp Component of the Celogentin Family
439
the leucine residue (Leu2) to form the western-ring. From the
same tryptophan core, the C2 carbon is linked to the histidine
residue (His8) through a C–N bond to form the eastern-ring.
Stephanotic acid 2, the simplest member of the celogentin
family, is a macrocyclic pentapeptide that was isolated by
Yoshikawa and coworkers from the stem of Stephanotis flori-
bunda in 2000.[4] Stephanotic acid contains the same leucinyl
(C3)–tryptophan (C6) cross-link present in the celogentins. The
synthesis of stephanotic acid methyl ester was reported by
Moody and coworkers in 2006.[5] Moody constructed the key
leucinyl–tryptophan cross-link through a non-selective hydro-
genation of a β-indolyl-α,β-dehydroleucine derivative, followed
by elaboration of the indole moiety to a tryptophan residue.
Campagne and coworkers subsequently reported conditions
that enabled a catalytic asymmetric hydrogenation of a closely
related substrate, with good stereoselectivity.[6] Castle recently
published the total synthesis of celogentin C 1,[7] using an inter-
molecular Knoevenagel reaction followed by radical conjugate
addition methodology to generate the key Leu–Trp adduct, albeit
as a mixture of all four diastereomers.
We have been investigating several routes toward the stereo-
selective preparation of the key Leu–Trp component of stephan-
otic and the celogentins. Previous work within our group initially
centred on conjugated addition of an isopropyl anion to gener-
ate the leucine–tryptophan motif,[8] analogous to Castle’s radical
conjugate addition methodology. However, despite modest suc-
cess with model systems, functional group intolerance on the
fully functionalized system led us to abandon this route.[9]
We have recently been exploring a C–H activation strategy
to construct the key leucine–tryptophan linkage. The inspira-
tion for this approach came from the reports of C–H activation
of N-heterocyclic amide derivatives by Daugulis and Corey.
Daugulis and coworkers reported that 8-aminoquinolinyl amides
enable the regioselective Pd-catalyzed C–H activation and cross-
coupling at the β-position.[10] Corey extended this protocol to
N-phthaloyl-l-leucine quinolinyl amide, generating β-arylated
leucine derivatives with high regio- and stereo-selectivity.[11]
Accordingly, we initiated a program to investigate the applica-
tion of this protocol toward the synthesis stephanotic acid and the
celogentins.Very recently, Feng and Chen reported the total syn-
thesis of celogentin C employing a nearly identical strategy.[12]
Given the report of Feng and Chen, we herein disclose the
findings of our work in the C–H activation route to the key
leucine–tryptophan component of these cyclic peptide natural
products.
iodoindole 7 was protected with a Boc group. To our delight, the
protected indole 9 underwent the C–H activation reaction to give
adduct 10 in excellent yield.The leucinyl–indole adduct 10 crys-
tallized from toluene, and proved amenable to analysis by X-ray
crystallography.[13] Single-crystal X-ray diffraction analysis of
10 revealed the adduct possessed the same relative configuration
as present in the natural products 1–5 (Fig. 2).
With this pleasing result in hand, we next turned to the
application of the palladium catalyzed C–H activation with pro-
tected 6-iodotryptophan 15. Nitration of l-tryptophan 11 with
freshly prepared fuming nitric acid, according to the procedure
of Moriya and coworkers,[14] provided the desired 6-nitro-l-
tryptophan 12 (Scheme 2). Esterification of the nitrotrypto-
phan 12 with methanolic hydrogen chloride and subsequent
treatment with di-tert-butyl dicarbonate gave the fully protected
nitrotryptophan 13 in 80% yield over two steps. Reduction of
the protected nitrotryptophan 13 with zinc powder in acetic acid
afforded aminotryptophan 14 in excellent yield.
Conversion of the amine 14 into the corresponding iodide
under standard Sandmeyer conditions gave the iodotrypto-
phan 15 and the inseparable reduced byproduct 16 in a 71:29
ratio. Several strategies to suppress the formation of the unde-
sired reduced by-product 16 were initially largely unsuccess-
ful; careful monitoring of the internal reaction temperature,
N
6, Pd(OAc)2, AgOAc,
O
NH
N
H
PhMe, 110°C, 36h,
I
N
H
PhthN
23%
7
O
8
PhthN
N
Boc2O,
DMAP
97%
H
N
6
N
O
NH
6, Pd(OAc)2, AgOAc,
N
Boc
I
PhMe, 110°C, 2 d,
N
PhthN
81%
9
Boc
10
Scheme 1.
C24
C18
C23
C27
C19
C17
C22
O5
Results and Discussion
N3
C25
C15
C12
Our intitial studies toward the C–H activation route to the
leucine–tryptophan crosslink involved the coupling of leucine
quinolinyl amide 6 with 6-iodoindole 7. In the presence of
Pd(OAc)2 and AgOAc, a low yield (23%) of the arylated prod-
uct 8 was obtained after 36 h, albeit as a single stereoisomer
(Scheme 1). Prolonged heating for 6 days gave the coupled
product 8 in an improved yield of 42%.The reaction was also per-
formed under microwave irradiation, which gave a 25% yield of
8 after 1 h irradiation at 110◦C. However, due to problems aris-
ing from a high loading of the palladium catalyst and the silver
salt, microwave conditions were not explored further.
C26
C14
C32
N2
C11
C13
N4
C28
C31
C30
C10
O4
O3
C7
C34
C33
C29
N1
C9
C6
C5
O2
C36
C8
O1
C35
C1
C3
The low yield and prolonged reaction times led us to spec-
ulate that the nitrogen of the indole 7 could be competing
with the quinolinylamide group for coordination to the pal-
ladium, thereby inhibiting the reaction. To test this postulate,
C4
C2
Fig. 2. ORTEP diagram of leucine–indole adduct 10.

440
B. T. Y. Li, J. M. White, and C. A. Hutton
lowering the reaction temperature to −5◦C, and use of addi-
tionalequivalentsofsodiumnitritewereinvestigatedwithoutany
improvement. Zhu and coworkers have reported that significant
reduction occurs in the preparation of a similar iodotryptophan
derivative under standard Sandmeyer conditions.[15] Eventu-
ally, the use of para-toluenesulfonic acid in place of dilute
hydrochloric acid, according to a procedure by Knochel and
coworkers,[16] gave the iodotryptophan 15 in reproducible yield,
and importantly, free of any reduced byproduct 16 (Scheme 2).
With both the iodotryptophan 15 and leucine quinoline
amide 6 in hand, coupling of the two partners in the presence
of palladium(ii) acetate was performed. Optimization studies of
the C–H activation–cross-coupling reaction were undertaken on
small scale as summarized in Table 1. Corey’s method employs
four equivalents of liquid aryl iodide, without additional solvent,
at 110◦C. Accordingly, initial experiments were conducted neat,
using two equivalents of the iodotryptophan 15, and we were
delighted to obtain the leucinyl–tryptophan adduct 17 in 34%
yield along with recovered amide 6 (10%) and iodotryptophan
15, together with reduced compound 16 (Table 1, entry 1). The
1H NMR spectrum of the product 17 showed the adduct was pro-
duced as a single diastereomer. Use of toluene solvent to ensure
homogeneity, together with use of four equivalents of iodotryp-
tophan 15, improved conversion to 61% (entry 2). Reducing the
amount of 15 to three equivalents improved conversion to 89%,
with further reduction to two equivalents giving a comparable
result (81% conversion, entries 3 and 4). The use of increased
amounts of silver(i) acetate decreased conversion slightly, from
81% to 70% (entry 6). The use of alternative oxidants (includ-
ing CuO, CuCl, oxone, and AgOTf) mostly resulted in lower
conversion (entries 9–15). Notably, however, both Ag2CO3 and,
importantly, Cu(OAc)2 gave the coupled adduct with compa-
rable yields to that obtained using AgOAc (entries 5 and 8).
Complete conversion was eventually obtained by adding a fresh
aliquot of palladium(ii) acetate after 12 h (entries 7 and 8). When
conditions analogous to entry 7 were applied to a preparative
scale, coupled adduct 17 was produced together with the depro-
tected adduct 18, which presumably arises from thermolysis
of the N-indolyl-Boc group, in 74% overall yield (Scheme 3).
Treatment with di-tert-butyl dicarbonate to reintroduce the Boc
group gave an isolated yield of 58% for the leucinyl–tryptophan
adduct 17.
We next investigated the cleavage of the quinolinyl amide
group, and encountered the same problem faced by Feng and
Chen.[12] Model studies showed that amide 6 could be converted
in quantitative yield to imide 19, with subsequent treatment with
lithium hydroperoxide generating carboxylic acid 20 in excel-
lent yield (Scheme 4). However, all attempts to prepare the
corresponding N-Boc imide from the Leu–Trp adduct 17 were
unsuccessful. Using conditions analogous to those employed for
model system 6 resulted in recovery of the starting material 17.
Treatment of amide 17 with excess di-tert-butyl dicarbonate and
triethylamine at room temperature or at reflux resulted in signifi-
cant decomposition. Mass spectrometric and 1H NMR analysis
indicated hydrolysis of the phthaloyl group had occurred with
subsequent Boc-protection of the amide and carbamate nitro-
gens, consistent with formation of 22 (Fig. 3). No evidence for
CO2H
CO2Me
H2N
BocHN
(i) MeOH, HCl
(ii) Boc2O, NEt3,
DMAP
N
H
N
R
O2N
Boc
80%
13
11 R ꢀ H
HNO3,
AcOH
Zn, AcOH
93%
12 R ꢀ NO2
CO2Me
CO2Me
BocHN
BocHN
NaNO2, KI
acid
N
H2N
N
I
Boc
Boc
14
15
CO2Me
5% aq HCl, 15:16 ꢀ 71:29
p-TsOH, 15:16 ꢀ 100:0
BocHN
ꢁ
N
Boc
16
Scheme 2. Preparation of iodotryptophan 15.
Table 1. Optimization of C–H activation–coupling to give 17
Reaction conditions: leucine quinoline amide 6 (1 equiv.), iodotryptophan 15 (2–4 equiv.),
Pd(OAc)2 (0.2 equiv.,A 0.13 equiv. added at 12 h), oxidant, solvent, 110◦C (B130◦C), 18 h.
AgTfa, silver(i) trifluoroacetate; AgOTf, silver(i) triflate; BQ, benzoquinone
Entry
15 (Equiv.)
Oxidant
(Equiv.)
Solvent
% Conv. 6 → 17
1
2
3
4
5
6
7A
8A
9
10
11
12
13
14
15
2
4
3
2
2
2
3
3
2
2
2
2
2
2
2
AgOAc
AgOAc
AgOAc
AgOAc
Ag2CO3
AgOAc
AgOAc
Cu(OAc)2
Cu2O
CuO
CuCl
Oxone®
AgTfa
1.5
1.5
1.5
1.5
1.5
3
Neat
34
61
89
81
73
70
100
100
21
23
0
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
DMSOB
2.4
3
1.5
1.5
1.5
1.5
1.5
1.5
2 + 0.5
0
0
0
0
AgOTf
Ag2CO3 + BQ

Synthesis of the Leu–Trp Component of the Celogentin Family
441
CO2Me
NHBoc
CO2Me
N
N
BocHN
O
NH
O
R
NH
ꢁ
N
PhthN
N
I
Boc
Boc
6
15
17 R ꢀ PhthN
23 R ꢀ NH2
Pd(OAc)2, oxidant,
PhMe, 110°C
H2NNH2 82%
CbzCl,
72%
NaHCO3
24 R ꢀ CbzNH
NHBoc
N
Scheme 5.
CO2Me
O
NH
the indolinyl and tryptophan adducts, 10 and 17. Chen circum-
vented this steric congestion through conversion of the bulky
phthalimide group to an azide, which ultimately enabled imide
formation, hydrolysis, and subsequent elaboration to celogentin
C.[12] We are currently investigating a modified quinolinyl sys-
tem designed to be cleaved under milder conditions, and will
report the results of this investigation in due course.
N
R
PhthN
17 R ꢀ Boc
18 R ꢀ H
Boc2O,
NEt3, DMAP
Scheme 3. Preparation of Leu–Trp adduct 17.
CO2H
Experimental
N-(Quinolin-8-yl) (2S,3R)-Nα-Phthaloyl-β-(indol-6-yl)-
leucinamide 8
PhthN
N
N
Boc2O
DMAP
LiOH
H2O2
O
NH
O
NBoc
20, 99%
In a microwave reaction vessel, a suspension of N-phthaloyl-
l-leucine quinoline amide 6 (99.8 mg, 0.26 mmol), 6-iodo-
1H-indole[17] 7 (234 mg, 0.96 mmol), palladium(ii) acetate
(11.9 mg, 0.053 mmol), and copper(i) acetate (70.2 mg,
0.39 mmol) in dry toluene (1 mL) was heated at 160◦C for
1 h in a microwave reactor. The crude mixture was filtered
through a pad of Celite, rinsed with dichloromethane, and the
filtrate was concentrated under reduced pressure. Purification
of the crude product by chromatography on silica (1:3 ethyl
acetate/petroleum spirits) gave the title compound 8 (31.8 mg,
25%) as a light brown solid; RF 0.14 (1:3 ethyl acetate/petroleum
spirits); [α]D19 −2.60 (c 0.98, CHCl3). νmax/cm−1 3346 (br),
3271, 2961, 2926, 1776, 1710, 1675, 1525, 1487, 1467, 1425,
1384, 1354, 1325, 908. δH (500 MHz, CDCl3) 9.93 (1H, br s),
8.54 (1H, dd, J 6.5, 2.5), 8.34 (1H, br s), 7.95 (2H, m), 7.78 (2H,
m), 7.65 (1H, d, J 8.2), 7.54 (1H, s), 7.31–7.28 (2H, m), 7.23
(1H, m), 7.19 (1H, dd, J 8.2, 4.3), 6.57 (1H, m), 5.80 (1H, d,
J 12.5), 4.30 (1H, dd, J 12.5, 3.5), 2.06 (1H, m), 0.86 (3H,
d, J 7.0), 0.77 (3H, d, J 7.0). δC (125 MHz, CDCl3) 168.8,
166.7, 147.6, 138.3, 136.5, 135.6, 134.3, 134.1, 132.0, 130.2,
127.5, 126.9, 124.4, 123.7, 121.6, 121.2, 120.8, 116.9, 102.5,
58.1, 48.9, 29.7, 21.7, 16.4. LRMS (ESI+): m/z 503 (100%,
[M + H]+), 359 (10, [M − C9H8N2]+); HRMS (ESI+): m/z
calc. for C31H27N4O3: 503.2078. Found: 503.2078.
ꢁ
100%
PhthN
PhthN
N
NHBoc
21, 82%
19
6
Scheme 4. Model deprotection of quinolinyl amide.
NBoc2
N
CO2Me
O
NH
O
N
N
Boc
Boc
CO2H
22
Fig. 3.
imide formation at the quinolinyl amide nitrogen was observed
under any conditions.
With the phthaloyl group being unstable to these conditions,
we switched it for the less base-labile Cbz group. Removal of the
phthaloyl group of 17 by hydrazinolysis gave the free amine 23,
which upon treatment with benzyl chloroformate afforded the
Cbz-protected adduct 24 in good yield (Scheme 5). However,
attempted imide formation was again unsuccessful, with quan-
titative recovery of starting material 24. Introduction of the less
bulky N-acetyl group was also unsuccessful.
The lack of conversion of amides 17 and 24 to the corre-
sponding N-Boc imides is striking when compared with the
success of the model system 6. However, analysis of the X-ray
structure of 10 may provide an explanation. The X-ray struc-
ture (Fig. 2) shows that the N-Boc-indolyl group and one of the
phthaloyl carbonyl groups are shielding the amide nitrogen, and
as such the severe steric hindrance prevents reaction at this site in
N-tert-Butoxycarbonyl-6-iodo-1H-indole 9
To a solution of 6-iodo-1H-indole[17] 7 (988 mg, 4.06 mmol)
in dry acetonitrile (5 mL) was added di-tert-butyl dicarbonate
(1.08 g, 4.96 mmol) and 4-(dimethylamino)pyridine (50.1 mg,
0.41 mmol). The resulting mixture was stirred at room temper-
ature for 15 min. The solvent was concentrated under reduced
pressure and the crude product was purified by chromatography
on silica (1:8 ethyl acetate/petroleum spirits) to give the title
compound 9 (1.31 g, 97%) as an off-white solid; RF 0.45 (1:8
ethyl acetate/petroleum spirits); mp 72–73◦C. νmax/cm−1 2978,
1734, 1429, 1370, 1339, 1249, 1207, 1154, 1125, 1084, 809.
δH (500 MHz, CDCl3) 8.57 (1H, br s), 7.53–7.51 (2H, m), 7.31

442
B. T. Y. Li, J. M. White, and C. A. Hutton
(1H, d, J 8.0), 6.52 (1H, d, J 3.5), 1.68 (9H, s). δC (125 MHz,
CDCl3) 149.3, 136.2, 131.5, 129.8, 126.1, 124.2, 122.3, 107.1,
88.6, 84.2, 28.1. LRMS (EI, 70 eV): m/z 343 (8%, [M+•]),
287 (26, [M − C3H9]+), 243 (36, [M − C5H9O2]+), 115 (20,
[M − C5H9IO2]+), 57 (100).
111.9, 85.3, 80.2, 53.6, 52.5, 28.3, 28.1, 27.8. LRMS(ESI+):m/z
486(29%, [M + Na]+), 464(18, [M + H]+). HRMS(ESI−):m/z
calc. for C22H28N3O8: 462.1882. Found: 462.1873.
Nα,Nind-Bis-tert-butoxycarbonyl-6-amino-L-tryptophan
Methyl Ester 14
N-(Quinolin-8-yl) (2S,3R)-Nα-Phthaloyl-β-(N-tert-
butoxycarbonyl(indol-6-yl))leucinamide 10
To a solution of protected nitrotryptophan 13 (4.86 g, 10.4 mmol)
indrydichloromethane(150 mL)wasaddedzincpowder(45.1 g,
689 mmol). The resulting mixture was cooled to 0◦C and acetic
acid (9.6 mL, 168 mmol) added dropwise. The reaction mixture
was allowed to warm to room temperature and stirred for 2 h.
The reaction mixture was filtered through a pad of Celite and
washed with dichloromethane. The filtrate was washed with
saturated aqueous sodium hydrogen carbonate (2 × 200 mL),
water (200 mL), and brine (200 mL), dried with magnesium
sulfate, then concentrated under reduced pressure. Purification
of the crude product by chromatography on silica (1:1 ethyl
acetate/petroleum spirits) gave the title compound 14 (4.21 g,
93%)asalightyellowfoam;RF 0.36(1:1ethylacetate/petroleum
spirits); mp 82–84◦C. [α]D24 +48.1 (c 1.02 in CHCl3). νmax/cm−1
3369 (br), 2978, 1713, 1625, 1496, 1449, 1384, 1368, 1279,
1255, 1154, 1084. δH (500 MHz, CDCl3) 7.51 (1H, br s), 7.23
(1H, d, J 8.3), 6.62 (1H, dd, J 8.3, 2.0), 7.15 (1H, s), 5.09 (1H,
d, J 7.0), 4.61 (1H, m), 3.73 (2H, br s), 3.68 (3H, s), 3.19–
3.09 (2H, m), 1.64 (9H, s), 1.44 (9H, s). δC (100 MHz, CDCl3)
172.4, 155.1, 149.7, 144.3, 136.8, 123.0, 121.7, 119.4, 115.1,
111.9, 101.4, 83.2, 79.8, 53.6, 52.2, 28.3, 28.2, 27.8. LRMS
(ESI+): m/z 434 (100%, [M + H+]). HRMS (ESI+): m/z calc.
for C22H32N3O6: 434.2286. Found: 434.2285.
To a solution of leucine quinoline amide 6 (41.1 mg, 0.16 mmol),
N-Boc-6-iodoindole 9 (101 mg, 0.29 mmol) in dry toluene
(3 mL) was added palladium(ii) acetate (6.3 mg, 28.0 µmol)
and silver(i) acetate (36.3 mg, 0.22 mmol). The reaction mixture
was heated at 110◦C for 24 h, then a second portion of palla-
dium(ii) acetate (10.5 mg, 47 µmol) and dry toluene (3 mL) were
added and the mixture was heated at 110◦C for a further 16 h.
The mixture was filtered through a pad of Celite, washed with
dichloromethane and the filtrate was concentrated under reduced
pressure. Purification of the crude product by chromatography
on silica (1:4 ethyl acetate/petroleum spirits) and recrystalliza-
tion from toluene gave the title compound 10 (51.9 mg, 81%) as
colourless prisms; RF 0.12 (1:4 ethyl acetate/petroleum spirits);
mp 186–188◦C. [α]D19 −8.94 (c 1.36 in CHCl3). νmax/cm−1 3291
(br), 2960, 2925, 2854, 1715, 1680, 1525, 1382, 1370, 1333,
1252, 1151, 1131, 1123. δH (500 MHz, CDCl3) 9.97 (1H, br s),
8.53 (1H, d, J 7.0), 8.32 (1H, br s), 8.23 (1H, d, J 2.5), 7.98 (1H,
dd, J 13.8, 7.0), 7.94 (2H, m), 7.77 (2H, m), 7.65 (1H, br s), 7.54
(1H, d, J 13.8), 7.36–7.31 (2H, m), 7.27 (1H, m), 6.57 (1H, d, J
4.0), 5.76 (1H, d, J 12.4), 4.36 (1H, dd, J 12.4, 3.2), 2.07 (1H, m),
1.72 (9H, s), 0.88 (3H, d, J 7.0), 0.80 (3H, d, J 7.0). δC (125 MHz,
CDCl3) 168.6, 166.4, 147.7, 135.7, 134.3, 134.2, 132.6, 132.0,
127.6, 127.0, 126.1, 123.6, 121.6, 121.2, 120.9, 116.8, 107.1,
58.1, 48.9, 29.5, 28.3, 21.7, 16.3. LRMS (ESI+): m/z 625 (100%,
[M + Na]+), 603 (57, [M + H]+). HRMS (ESI+): m/z calc. for
C36H34N4O5Na: 625.2421. Found: 625.2420.
Nα,Nind-Bis-tert-butoxycarbonyl-6-iodo-L-tryptophan
Methyl Ester 15
A solution of amine 14 (916 mg, 2.11 mmol) in acetonitrile
(8.5 mL) was cooled to 10◦C followed by addition of para-
toluene sulfonic acid monohydrate (1.22 g, 6.40 mmol). After
5 min, a solution of sodium nitrite (326 mg, 4.72 mmol) and
potassium iodide (893 mg, 5.38 mmol) in water (1.3 mL) was
added dropwise and the mixture was stirred at 10◦C for a fur-
ther 10 min then warmed to room temperature and stirred a
further 2 h. The crude mixture was added to an ice–water mix-
ture (150 mL) and extracted with ethyl acetate (3 × 100 mL).
The combined organic extracts were washed with brine (30 mL),
dried with magnesium sulfate, and concentrated under reduced
pressure. Purification of the crude product by column chro-
matography (1:4 ethyl acetate/petroleum spirits) gave the title
compound 15 (747 mg, 65%) as a light brown foam; RF 0.33
(1:4 ethyl acetate/petroleum spirits); mp 62–64◦C. [α]D23 +16.4
(c 0.50, CHCl3). νmax/cm−1 3363 (br), 2979, 1736, 1500, 1430,
1369, 1333, 1283, 1253, 1157, 1086. δH (500 MHz, CDCl3)
8.53 (1H, br s), 7.51 (1H, d, J 8.5), 7.30 (1H, s), 7.22 (1H,
dd, J 8.5, 3.3), 5.11 (1H, br s), 4.62 (1H, m), 3.68 (3H, s),
3.24–3.10 (2H, m), 1.66 (9H, s), 1.42 (9H, s). δC (125 MHz,
CDCl3) 172.1, 155.0, 149.1, 136.3, 131.4, 129.9, 124.4, 124.3,
120.4, 115.0, 88.9, 84.2, 80.0, 53.6, 52.3, 28.3, 28.1, 27.7. LRMS
(ESI+): m/z 567 (100%, [M + Na]+). HRMS (ESI+): m/z calc.
for C22H29IN2O6Na: 567.0963. Found: 567.0961.
Nα,Nind-Bis-tert-butoxycarbonyl-6-nitro-L-tryptophan
Methyl Ester 13
A solution of 6-nitrotryptophan[14] 12 (380 mg, 1.5 mmol) in
dry methanol (10 mL) was cooled to 0◦C followed by dropwise
addition of thionyl chloride (500 µL, 6.9 mmol). The resulting
mixture was heated at reflux for 17 h. The solvent was concen-
trated under reduced pressure to give the crude 6-nitrotryptophan
methyl ester hydrochloride as yellow crystals, which was used
in the next step without further purification. δH (400 MHz,
CD3OD) 11.50 (1H, br s), 8.38 (1H, d, J 2.0), 8.00 (1H, d, J 8.8,
2.0), 7.69 (1H, d, J 8.8), 7.57 (1H, s), 4.38 (1H, t, J 6.4), 3.80
(3H, s), 3.51–3.38 (2H, m). The crude methyl ester hydrochlo-
ride was suspended in dry acetonitrile (10 mL), cooled to 0◦C,
then dry triethylamine (0.50 mL) was added slowly, followed by
addition of di-tert-butyl dicarbonate (837 mg, 3.84 mmol) and
4-(dimethylamino)pyridine (37.0 mg, 0.30 mmol). The reaction
mixture was stirred at room temperature for 3.5 h then concen-
trated under reduced pressure. The crude product was purified
by chromatography on silica (1:1 diethyl ether/petroleum spirits)
to give the title compound 13 (603 mg, 85%) as yellow crystals;
RF 0.23 (1:1 diethyl ether/petroleum spirits); mp 68–70◦C. [α]D23
+58.5 (c 1.04, CHCl3). νmax/cm−1 3392 (br), 2980, 1741, 1715,
1520, 1370, 1341, 1255, 1156, 1088. δH (500 MHz, CDCl3) 9.06
(1H, br s), 8.13 (1H, dd, J 8.5, 1.5), 7.65 (1H, s), 7.58 (1H, d, J
8.5), 5.15 (1H, d, J 7.5), 4.65 (1H, m), 3.70 (3H, s), 3.34–3.17
(2H, m), 1.73 (9H, s), 1.57 (9H, s). δC (125 MHz, CDCl3) 171.9,
155.0, 148.7, 145.2, 135.3, 134.1, 129.1, 119.0, 117.9, 115.3,
Nα,Nind-Bis-tert-butoxycarbonyl-6-[(1R,2S)-2-(1,3-
dioxoisoindolin-2-yl)-1-isopropyl-3-oxo-3-(quinolin-
8-ylamino)propyl]-L-tryptophan Methyl Ester 17
To a solution of leucine quinoline amide 6 (165 mg, 0.43 mmol)
and iodotryptophan 15 (694 mg, 1.27 mmol) in dry toluene

Synthesis of the Leu–Trp Component of the Celogentin Family
443
(3.3 mL) was added palladium(ii) acetate (30.9 mg, 0.14 mmol)
and silver(i) acetate (174 mg, 1.04 mmol). The resulting mixture
was heated at 110◦C for 24 h, after which a second por-
tion of palladium(ii) acetate (13.4 mg, 59.7 µmol) was added
and the mixture was heated at 110◦C for a further 24 h. The
crude mixture was filtered through a pad of Celite, rinsed
with dichloromethane, and the filtrate was concentrated under
reduced pressure. Purification of the crude product by chro-
matography on silica (1:2 ethyl acetate/petroleum spirits) gave
the title product 17 (120 mg, 35%) as a light yellow foam; RF
0.17 (1:2 ethyl acetate/petroleum spirits); mp 123–125◦C. [α]D19
+16.0 (c 1.00 in CHCl3). νmax/cm−1 3291 (br), 2971, 2931,
1714, 1526, 1487, 1369, 1326, 1257, 1157, 1089. δH (500 MHz,
CDCl3) 10.06 (1H, br s), 8.52 (1H, dd, J 7.8, 1.5), 8.33 (1H,
br s), 8.28 (1H, br s), 7.99 (1H, dd, J 7.8, 1.5), 7.93 (2H, m),
7.77 (2H, m), 7.42 (2H, br s), 7.36–7.30 (4H, m), 5.73 (1H,
d, J 12.5), 5.04 (1H, d, J 7.5), 4.60 (1H, m), 4.39 (1H, dd,
J 12.5, 2.5), 3.48 (3H, s), 3.24–3.15 (2H, m), 2.06 (1H, m),
1.72 (9H, s), 1.39 (9H, s), 0.88 (3H, d, J 6.8), 0.80 (3H, d, J
6.8). δC (100 MHz, CDCl3) 172.2, 168.5, 166.3, 155.0, 147.8,
138.4, 135.8, 135.5, 134.2, 132.9, 131.9, 130.2, 127.6, 127.0,
124.2, 123.7, 121.6, 121.4, 118.8, 116.8, 114.9, 84.0, 79.9,
58.2, 53.6, 52.2, 48.7, 29.7, 29.3, 28.3, 27.6, 21.6, 16.3. LRMS
(ESI+): m/z 804 (100%, [M + H]+). HRMS (ESI+): m/z calc.
for C45H49N5O9Na: 826.3423. Found: 826.3425.
25.4, 23.4, 21.2. LRMS (ESI+): m/z 510 (100%, [M + Na]+),
216 (17, [M − C15H15N2O3]+). HRMS (ESI+): m/z calc. for
C28H29N3O5Na: 510.1999. Found: 510.1999.
Cleavage of N-tert-Butoxycarbonyl-N-(quinolin-8-yl)
leucinamide
A
solution of imide 19 (48.2 mg, 0.10 mmol) in 3:1
tetrahydrofuran/water (2.0 mL) was cooled to 0◦C then 30%
hydrogen peroxide (100 µL, 0.88 mmol) and lithium hydroxide
monohydrate (9.8 mg, 0.24 mmol) were added and stirred at
0◦C for 2.5 h. The reaction was quenched at 0◦C with 1.5 M
aqueous sodium thiosulfate (0.7 mL) and the solvent was con-
centrated under reduced pressure. The residue was washed with
dichloromethane (3 × 10 mL) then the aqueous phase was acid-
ified to pH 2 with 10% aqueous hydrochloric acid and extracted
with ethyl acetate (3 × 10 mL). The organic extracts were dried
with magnesium sulfate and concentrated under reduced pres-
sure to give N-phthaloyl-l-leucine 20 as an off-white foam
(26 mg, 99%). Concentration of the dichloromethane washings
under reduced pressure followed by chromatography of the
residue on silica (1:9 ethyl acetate/petroleum spirits) gave the
N-Boc-aminoquinoline 21 (19.8 mg, 82%) as a light yellow oil.
Nα,Nind-Bis-tert-butoxycarbonyl-6-[(1R,2S)-2-amino-
1-isopropyl-3-oxo-3-(quinolin-8-ylamino)propyl]-
L-tryptophan Methyl Ester 23
Further elution with ethyl acetate gave the Nα-Boc protected
adduct 18 (117 mg, 39%) as a brown foam; RF 0.06 (1:2 ethyl
acetate/petroleum spirits). δH (500 MHz, CDCl3) 10.03 (1H, br
s), 8.51 (1H, dd, J 7.1, 1.6), 8.37 (1H, m), 8.00 (1H, br s), 7.96–
7.91 (3H, m), 7.76 (2H, m), 7.51 (2H, m), 7.33–7.30 (3H, m),
7.26 (1H, m), 7.03 (1H, br s), 5.76 (1H, d, J 12.4), 5.04 (1H, br d,
J 8.0), 4.61 (1H, m), 4.32 (1H, m), 3.48 (3H, s), 3.27 (2H, br d, J
4.0), 2.06 (1H, m), 1.35 (9H, s), 0.85 (3H, d, J 7.0), 0.76 (3H, d,
J 7.0). To a solution of 18 (117 mg, 0.17 mmol) in dry aceto-
nitrile (2 mL) was added di-tert-butyl dicarbonate (53.1 mg,
0.24 mmol) and 4-(dimethylamino)pyridine (4.7 mg, 39 µmol).
The resulting mixture was stirred for 21 h at room temperature,
then the solvent was removed under reduced pressure. Purifi-
cation by column chromatography (1:1 ethyl acetate/petroleum
spirits) gave the title compound 17 (81.1 mg, 61%) as a light
yellow foam.
To a solution of leucinyl–tryptophan coupled adduct 17
(93.1 mg, 0.12 mmol) in ethanol (1.47 mL) was added hydrazine
hydrate (5 µL). The resulting mixture was heated at reflux for
4 h, then cooled, diluted with water (15 mL), extracted with
dichloromethane (3 × 15 mL), and dried with sodium sulfate.
The solvent was removed under reduced pressure and the crude
product was purified by column chromatography (1:1 ethyl
acetate/petroleum spirits) to give amine 23 (64.4 mg, 82%) as
an off-white foam; RF 0.21 (1:1 ethyl acetate/petroleum spirits).
[α]1D9 +105.6 (c 1.49 in CHCl3). νmax/cm−1 3297 (br), 2975,
2930, 1713, 1517, 1485, 1438, 1368, 1325, 1273, 1255, 1157,
1088. δH (500 MHz, CDCl3) 11.25 (1H, br s), 8.83 (1H, dd, J 7.5,
1.3), 8.71 (1H, dd, J 4.0, 1.5), 8.10 (2H, dd, J 8.3, 1.3), 7.52 (1H,
m), 7.47 (1H, m), 7.38 (1H, dd, J 8.3, 4.2), 7.31–7.29 (2H, m),
7.17 (1H, d, J 8.0), 5.07 (1H, br d, J 8.0), 4.57 (1H, m), 3.95
(1H, d, J 4.0), 3.57 (3H, s), 3.15–3.07 (3H, m), 2.63 (1H, m),
1.63 (9H, s), 1.38 (9H, s), 1.18 (3H, d, J 6.5), 0.79 (3H, d, J 6.5).
δC (125 MHz, CDCl3) 172.8, 172.3, 155.0, 149.4, 148.3, 139.0,
138.4, 136.0, 135.6, 134.5, 128.0, 127.2, 123.8, 123.6, 121.4,
121.3, 118.8, 116.4, 115.6, 114.8, 83.5, 79.9, 58.7, 58.3, 53.6,
52.2, 29.7, 29.0, 28.2, 28.2, 27.7, 21.9, 21.5. LRMS (ESI+): m/z
697 (46%, [M + Na]+), 675 (100, [M + H]+). HRMS (ESI+):
m/z calc. for C37H48N5O7: 674.3548. Found: 674.3547.
N-tert-Butoxycarbonyl-N-(quinolin-8-yl)
Nα-phthaloyl-L-leucinamide 19
To a solution of leucine quinoline amide 6 (103 mg, 0.26 mmol)
in dry acetonitrile (3 mL) was added 4-(dimethylamino)pyridine
(34.2 mg, 0.28 mmol) and di-tert-butyl dicarbonate (91.0 mg,
0.42 mmol). The mixture was stirred at room temperature for
1 day then concentrated under reduced pressure. Purification
of the crude mixture by chromatography on silica (2:3 ethyl
acetate/petroleum spirits) gave the title compound 19 (132 mg,
100%) as an off-white solid; RF 0.43 (2:3 ethyl acetate/petroleum
spirits); mp 157–159◦C. [α]D20 −9.85 (c 1.12 in CHCl3).
νmax/cm−1 2960, 1739, 1712, 1382, 1369, 1270, 1255, 1152,
1122, 910. δH (500 MHz, CDCl3) 8.83 (1H, dd, J 4.2, 1.7), 8.13
(1H, dd, J 8.3, 1.7), 7.83 (2H, dd, J 5.5, 3.0), 7.79 (1H, dd, J
8.3, 1.4), 7.69 (2H, dd, J 5.5, 3.0), 7.63 (1H, dd, J 7.4, 1.4),
7.55 (1H, m), 7.36 (1H, dd, J 8.3, 4.2), 6.16 (1H, d, J 7.3), 2.54
(1H, br s), 2.09 (1H, m), 1.60 (1H, ddd, J 10.3, 8.3, 4.2), 1.16
(9H, s), 1.01 (3H, d, J 6.6), 0.97 (3H, d, J 6.6). δC (125 MHz,
CDCl3) 173.4, 167.9, 152.5, 150.3, 144.2, 136.8, 135.8, 133.9,
132.0, 128.8, 128.0, 126.2, 123.3, 121.5, 82.9, 54.1, 37.8, 27.4,
Nα,Nind-Bis-tert-butoxycarbonyl-6-[(1R,2S)-2-
benzyloxycarbonyl-amino-1-isopropyl-3-oxo-3-(quinolin-
8-ylamino)propyl]-L-tryptophan Methyl Ester 24
To a solution of amine 23 (65.5 mg, 0.097 mmol) in dioxane/
water (10:1, 1 mL) was added sodium hydrogen carbonate
(14.5 mg, 0.17 mmol). The mixture was stirred for 15 min then
benzyl chloroformate (16 µL, 0.11 mmol) was added and the
mixture was stirred at room temperature for 20 min. The solvent
was removed under reduced pressure, diluted with water (5 mL),
and the aqueous phase was extracted with dichloromethane
(3 × 15 mL). The combined organic extracts were dried with
sodium sulfate and concentrated under reduced pressure. The

444
B. T. Y. Li, J. M. White, and C. A. Hutton
crude product was purified by column chromatography (1:2
ethyl acetate/petroleum spirits) to give the title compound
24 (77.3 mg, 98%) as an off-white foam; RF 0.26 (1:2 ethyl
acetate/petroleum spirits). [α]1D9 −1.29 (c 1.00 in CHCl3).
νmax/cm−1 3337 (br), 2973, 1720, 1528, 1487, 1440, 1369, 1326,
1272, 1257, 1159, 1089. δH (500 MHz, CDCl3) 10.02 (1H, br s),
8.63 (1H, t, J 4.4), 8.58 (1H, br s), 8.09 (1H, dd, J 8.5, 1.5), 8.01
(1H, br s), 7.45 (2H, d, J 4.4), 7.39–7.37 (2H, m), 7.34–7.26
(4H, m), 7.05 (1H, d, J 8.5), 5.49 (1H, br d, J 8.5), 5.17 (2H,
ABq), 5.08 (1H, t, J 7.8), 5.02 (1H, br d, J 6.5), 4.53 (1H, m),
3.51 (3H, s), 3.13–3.05 (2H, m), 2.98 (1H, m), 2.54 (1H, m),
1.59 (9H, s), 1.39 (9H, s), 1.22 (3H, d, J 5.8), 0.85 (3H, d, J
5.8). δC (125 MHz, CDCl3) 172.2, 169.1, 155.9, 155.0, 149.4,
148.1, 138.4, 136.3, 136.0, 135.5, 135.1, 133.9, 129.7, 128.5,
128.1, 128.0, 127.8, 127.1, 124.0, 123.8, 121.7, 121.5, 118.8,
116.5, 116.0, 114.8, 83.6, 79.9, 67.1, 57.8, 56.6, 53.6, 52.2, 29.7,
29.0, 28.3, 28.1, 27.7, 21.8, 20.4. LRMS (ESI+): m/z 830 (20%,
[M + Na]+), 808 (100, [M + H]+). HRMS (ESI+): m/z calc. for
C45H54N5O9: 808.3916. Found: 808.3915.
[4] K. Yoshikawa, S. Tao, S. Arihara, J. Nat. Prod. 2000, 63, 540.
doi:10.1021/NP990532X
[5] (a) D. J. Bentley, A. M. Z. Slawin, C. J. Moody, Org. Lett. 2006, 8,
1975. doi:10.1021/OL060153C
(b) D. J. Bentley, C. J. Moody, Org. Biomol. Chem. 2004, 2, 3545.
doi:10.1039/B414996C
[6] J. Michaux, P. Retailleau, J.-M. Campagne, Synlett 2008, 1532.
doi:10.1055/S-2008-1078411
[7] (a) B. Ma, B. Banerjee, D. N. Litvinov, L. He, S. L. Castle, J. Am.
Chem. Soc. 2010, 132, 1159. doi:10.1021/JA909870G
(b) B. Ma, D. N. Litvinov, L. He, B. Banerjee, S. L. Castle, Angew.
Chem. Int. Ed. 2009, 48, 6104. doi:10.1002/ANIE.200902425
[8] A. K. L. Yuen, K. A. Jolliffe, C. A. Hutton, Aust. J. Chem. 2006, 59,
819. doi:10.1071/CH06324
[9] Note added in proof: A related conjugate addition strategy to the
Leu–Trp adduct, employing a tryptophan-C6 Grignard reagent and
an Evans’ oxazolidinone auxiliary-appended 4-methyl-2-pentenoate,
was recently reported by Jia and co-workers, ultimately leading to a
total synthesis of celogentin C: see W. Hu, F. Zhang, Z. Xu, Q. Liu,
Y. Cui, Y. Jia, Org. Lett. 2010, 12, 956. doi:10.1021/OL902944F
[10] V. G. Zaitsev, D. Shabashov, O. Daugulis, J. Am. Chem. Soc. 2005,
127, 13154. doi:10.1021/JA054549F
[11] B. V. S. Reddy, L. R. Reddy, E. J. Corey, Org. Lett. 2006, 8, 3391.
doi:10.1021/OL061389J
[12] Y. Feng, G. Chen, Angew. Chem. Int. Ed. 2010, 49, 958. doi:10.1002/
ANIE.200905134
Accessory Publication
1H and 13C NMR spectra for compounds 8, 9, 10, 13, 14, 15,
17, 19, 23, and 24 are available on the Journal’s website.
[13] CCDC deposition number 762457. Crystal data for 10:
[C36H34N4O5]2·[C7H8] M 262.20, T 130.0(2) K, λ 1.5418 Å, Tri-
clinic, space group P1, a 10.6987(7), b 10.930(1), c 15.3299(9) Å,
α 80.202(7)◦, β 74.102(5)◦, γ 85.532(7)◦, V 1698.0(2) Å3, Z 2,
Dc 1.269 Mg m−3, µ(CuKα) 0.682 mm−1, F(000) 686, crystal size
0.52 × 0.15 × 04 mm3. 16115 reflections measured, 8683 indepen-
dent reflections (Rint 0.0441) the final R was 0.0429 [I > 2σ(I)] and
wR(F2) was 0.0920 (all data).
[14] (a) T. Moriya, K. Hagio, N. Yoneda, Bull. Chem. Soc. Jpn. 1975, 48,
2217. doi:10.1246/BCSJ.48.2217
(b) A. S. Osborne, P. Som, J. L. Metcalf, R. S. Phillips, Bioorg. Med.
Chem. Lett. 2008, 18, 5750. doi:10.1016/J.BMCL.2008.09.086
[15] Y. Jia, M. Bois-Choussy, J. Zhu, Org. Lett. 2007, 9, 2401. doi:10.1021/
OL070889P
Acknowledgments
C.A.H. thanks the Australian Research Council for funding.
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