Synthesis of 1-indolyl substituted β-carboline natural products and discovery of antimalarial and cytotoxic activities

Article abstract of DOI:10.1016/j.tet.2014.05.068

A series of 1-indolyl substituted β-carbolines including the natural products hyrtiosulawesine, pityriacitrin and pityriacitrin B were prepared via Pictet-Spengler condensation - oxidation strategy from the corresponding indolyl-acetaldehydes and substituted tryptamines. Efforts to prepare the C-1 methylene-linked β-carboline analogues for structure-activity relationship studies were unsuccessful. Biological evaluation revealed two analogues (5 and 41) to exhibit weak inhibition of phospholipase A₂ (IC₅₀ 171 and 131 μM, respectively), two to act as antioxidants (3 and 43), and 12 analogues with activity towards a chloroquine-resistant strain (FcB1) of Plasmodium falciparum (IC₅₀ 1.0-23 μM). Testing against a panel of 60 human tumour cell lines revealed a general lack of cytotoxic effect for most of the compounds with the exception of β-carboline 42 exhibiting modest antileukaemic activity towards the HL-60(TB) cell line (LC₅₀ 4.2 μM). In addition, two novel structures (30 and 32) resulting from aldol condensation followed by Pictet-Spengler cyclisation displayed cytotoxicity with pronounced subpanel specificities towards colon cancer (COLO 205 and HCC-2998) cell lines.

Full text of DOI:10.1016/j.tet.2014.05.068

Tetrahedron xxx (2014) 1e11
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of 1-indolyl substituted
b-carboline natural products and
discovery of antimalarial and cytotoxic activities
,
a
b
b
Lydia P.P. Liew a , Jessica M. Fleming , Arlette Longeon , Elisabeth Mouray ,
*
Isabelle Florent b, Marie-Lise Bourguet-Kondracki b, Brent R. Copp a
a School of Chemical Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand
Laboratoire Molecules de Communication et Adaptation des Micro-organismes, UMR 7245 CNRS, Museum National d'Histoire Naturelle, 57 rue Cuvier,
75005 Paris, France
b
ꢀ
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 1-indolyl substituted
b-carbolines including the natural products hyrtiosulawesine, pity-
Received 11 February 2014
Received in revised form 9 May 2014
Accepted 19 May 2014
Available online xxx
riacitrin and pityriacitrin B were prepared via PicteteSpengler condensationdoxidation strategy from
the corresponding indolyl-acetaldehydes and substituted tryptamines. Efforts to prepare the C-1
methylene-linked
Biological evaluation revealed two analogues (5 and 41) to exhibit weak inhibition of phospholipase A2
(IC50 171 and 131 M, respectively), two to act as antioxidants (3 and 43), and 12 analogues with activity
towards a chloroquine-resistant strain (FcB1) of Plasmodium falciparum (IC50 1.0e23 M). Testing against
a panel of 60 human tumour cell lines revealed a general lack of cytotoxic effect for most of the com-
pounds with the exception of -carboline 42 exhibiting modest antileukaemic activity towards the HL-
60(TB) cell line (LC50 4.2 M). In addition, two novel structures (30 and 32) resulting from aldol con-
b-carboline analogues for structureeactivity relationship studies were unsuccessful.
m
Keywords:
m
b
-Carboline
PicteteSpengler reaction
Hyrtiosulawesine
Pityriacitrin
b
m
densation followed by PicteteSpengler cyclisation displayed cytotoxicity with pronounced subpanel
specificities towards colon cancer (COLO 205 and HCC-2998) cell lines.
Ó 2014 Elsevier Ltd. All rights reserved.
Eudistomin U
Antimalarial
1. Introduction
Alkaloids containing the b-carboline skeleton are ubiquitous in
Nature having been found in animals, plants, marine organisms and
even in humans.1,2 In addition to the three different oxidation
states: 1,2,3,4-tetrahydro-
-carboline, this class of compounds can also be substituted at any
of the nine positions along the tricyclic core structure giving rise to
a large structural diversity. Substitution at the C-1 position of a
carboline structure is the most common, as C-1 is the only atom in
the -carboline ring system that does not belong to tryptophan, the
b-carboline, 3,4-dihydro-b-carboline and
b
b
-
b
amino acid precursor for the biosynthesis of this class of
compounds.3
A relatively rare subset of b-carboline alkaloids contain a 1-acyl-
linked heterocycle (Fig. 1). For example, hyrtiomanzamine (1),4
dragmacidonamine B (2)5 and hyrtiosulawesine (3)6 were isolated
from marine sponges while pityriacitrin (4) was isolated from ex-
tracts of the marine bacterium Paracoccus sp.7 Both 4 and pity-
riacitrin B (5) were isolated from the yeast Malassezia furfur.8,9
Fig. 1. Structures of hyrtiomanzamine (1), dragmacidonamine B (2), pityriacitrin (4),
pityriacitrin B (5), hyrtiosulawesine (3), gesashidine A (6) and dragmacidonamine A
(7).
Closely related deoxy analogues gesashidine A (6)10 and dragma-
cidonamine A (7)5 are representative of an even more selective
* Corresponding author. Tel.: þ64 9 373 7599x86843; fax: þ64 9 373 7502;
e-mail addresses: l.liew@auckland.ac.nz (L.P.P. Liew), b.copp@auckland.ac.nz (B.R.
Copp).
group of
b-carboline alkaloids that contain a methylene-linked
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.05.068
Please cite this article in press as: Liew, L. P.P.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.05.068

2
L.P.P. Liew et al. / Tetrahedron xxx (2014) 1e11
heterocycle positioned at C-1. One could speculate that the 1-acyl
group arises from a subsequent oxidation step at the methylene
group in a possible biosynthetic pathway.
The marine
b-carboline alkaloid hyrtiosulawesine has been
previously reported as a PLA2 inhibitor11 and so represents an in-
teresting anti-inflammatory lead compound. It was envisaged that
the 1-acyl natural product as well as a series of analogues, including
deoxy derivatives, could be synthesised and evaluated for struc-
tureeactivity relationships. Alkaloids belonging to the b-carboline
family typically exhibit wide ranging biological activities including
effects on the central nervous system (e.g., harmine and harman),2
cytotoxicity towards cancer cell lines (e.g., eudistomidins, shishiji-
micins)12,13 and antimalarial (opacalines, manzamines)14,15 prop-
erties. As the antimalarial and antitumour effects of b-carboline
alkaloids are well documented, all analogues were also evaluated
against Plasmodium falciparum and human tumour cell lines.
During the course of the present study, Zhang et al. reported
syntheses of 3e5 using a PicteteSpengler reaction between trypt-
amine or tryptophan and substituted indole-3-glyoxals to construct
the C-80 keto-
toxicity data towards three human tumour cell lines. Herein we
report our alternative route for the syntheses of 1-acylindole-
carbolines pityriacitrin (4), pityriacitrin B (5), hyrtiosulawesine (3)
and investigation of the structureeactivity relationships (SAR) of
these and model compounds against PLA2, malaria, human tumour
cell line cytotoxicity and as anti-oxidants.
b
-carboline skeleton.16,17 They also reported cyto-
Scheme 2. Synthesis of pityriacitrin (4), pityriacitrin B (5) and hyrtiosulawesine (3).
Reagents and conditions: (i) 5 or 10% HOAc/H2O, reflux, 1.5e5 h, 37e77%; (ii) DDQ,
CH2Cl2/H2O, rt, 30e90 min, 11e37%; (iii) LiOH, THF/H2O (3:1), reflux, 12 h, 83%; (iv)
BBr3, CH2Cl2, N2, ꢁ78 ꢀC, 40 min, rt, 3 h, 87%.
b
-
conditions were trialed in an effort to achieve one of our SAR ob-
jectives, the conversion to the pyridine ring oxidation state without
concomitant oxidation of the doubly benzylic methylene group.
This selective oxidation has precedence, with Diker et al. reporting
the use of 10% palladium on carbon in xylenes.24 As shown in Table
1, we could not repeat the literature oxidation (entry 1), and fur-
thermore no reagent or condition was found that could achieve the
selective oxidation, either returning unreacted starting material or
affording the oxidised products 23 and pityriacitrin (4). The spec-
troscopic data observed for 4 were in complete agreement with the
literature.9,17
2. Results and discussion
2.1. Chemistry
We explored the use of the two most prevalently used methods
to construct the
b-carboline scaffold, namely the Bisch-
lereNapieralski and PicteteSpengler condensation reactions.18e20
As a model for the former reaction, amide 8,21 prepared by
PyBOP-mediated coupling of tryptamine with phenylacetic acid,
was subjected to reaction with POCl3 in toluene to afford 3,4-
dihydro-b-carboline 9 in 83% yield (Scheme 1). Oxidation of 9
with DDQ gave product mixtures of the 1-acyl product 10 and deoxy
product 11 with variable yields in different solvents (CH2Cl2 and
THF), while oxidation with KMnO4 gave 10 (27%) and 1-benzyl-3,4-
Table 1
Oxidation conditions examined
dihydro-b-carboline (12) (17%). Extension of this model reaction to
the corresponding indole analogue N-(2-(1H-indol-3-yl)ethyl)-2-
(1H-indol-3-yl)acetamide22 was unfortunately unsuccessful, with
the amide not undergoing POCl3-mediated ring closure.
Entry
1
Reagent and conditions
Results
5% Pd/C (19% wt of THBC),
xylenes, reflux, 4 h
No reaction
2
DDQ (4.0 equiv), CH2Cl2
(anhydrous), rt, 15 min
Unidentified decomposition
products
3
4
5
6
IBX (2.5 equiv), DMSO, rt, 5 h
KMnO4 (1.4 equiv), DMF, 0 ꢀCert
MnO2 (3.5 equiv), CH2Cl2, rt, 24 h
23 (trace) and 4 (trace)
23 (28%) and 4 (trace)
No reaction
Scheme 1. Reagents and conditions: (i) POCl3, toluene, 120 ꢀC, 30 min, 83%; (ii) DDQ,
CH2Cl2, N2, rt, 70 min, 1%, 26% (over two steps); (iii) DDQ, THF, N2, rt, 15 min, 5%, 10%
(over two steps); (iv) KMnO4, DMF, 0 ꢀC, 1 h, rt, 30 min, 27%, 17% (over two steps).
S
8 (4.0 equiv), DMSO, reflux, 24 h
4 (7%)
Switching to the PicteteSpengler condensation, reaction of
Similarly,
reflux in MeOH before subjection to acidic conditions, which gave
1,2,3,4-tetrahydro- -carboline 19 in 44% yield. Subsequent oxida-
tion using DDQ in CH2Cl2 afforded -carboline 21 (11%) and
Please cite this article in press as: Liew, L. P.P.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.05.068
L
-tryptophan methyl ester (14) and 16 were heated to
tryptamine (13) with N-Boc-indole-3-acetaldehyde (16) under
acidic conditions (5% HOAc)23 afforded 1,2,3,4-tetrahydro-
b
-car-
b
boline 18 in 37% yield (Scheme 2). With 18 in hand, a variety of
b

L.P.P. Liew et al. / Tetrahedron xxx (2014) 1e11
3
hydrolysis with LiOH in THF/H2O gave pityriacitrin B (5) in 83%
yield. The spectroscopic data observed for 21 again agreed with
literature values.8,17
H-130), dC 131.7 (C-130)] coupled to an adjacent methylene [dH
3.49e3.38 (2H, m, H2-120), dC 26.0 (C-120)]. HMBC data established
connectivity between these fragments, while NOESY correlations
observed between H-130 (dH 6.16) and H-1 (dH 4.96) and NH-9 (dH
7.70) established the olefin geometry as E.
Synthesis of the third target 1-acylindole
b-carboline natural
product, hyrtiosulawesine (3) required tert-butoxycarbonyl-5-
methoxyindole-3-acetaldehyde (17), which was prepared from 5-
methoxyindole (Scheme 3). Reaction of 5-methoxyindole (24)
with oxalyl chloride in anhydrous ether at 0 ꢀC for 1 h followed by
stirring at rt for 1 h gave 5-methoxyindole-3-glyoxyloyl chloride,
which was immediately reduced with lithium aluminium hydride
in THF to afford indole-ethyl-alcohol 25 in 76% yield over two steps.
A sequence of acetylation (to give 26), tert-butoxycarbonyl pro-
tection (to give 27) and ester hydrolysis afforded alcohol 28, which
smoothly underwent oxidation with IBX in DMSO to afford alde-
hyde 17.
The unusual structure of 30 prompted further investigation.
Reaction of tryptamine with phenylacetaldehyde in toluene, fol-
lowed by stirring in CH2Cl2/TFA afforded 1-benzyl-1,2,3,4-
tetrahydro-
b
-carboline25 (31, 14%) and a diphenyl analogue of 30
(i.e., 32) as the major product (22%) (Fig. 2). Similarity in 1H and 13
C
chemical shifts observed for 32 and 30, in addition to scalar and
dipolar couplings observed in 2D data secured the structure of 32.
Fig. 2. Structures of 31, 32 and 33.
In an effort to optimise the yield of formation of 32, (E)-2,4-
diphenylbut-2-enal (33)26 was prepared and reacted with trypt-
amine using the standard reaction conditions, affording 32 in 30%
yield. The formation of 30 and 32 is somewhat at odds with a pre-
vious report that reaction of tryptamine with propionaldehyde in
CH2Cl2/TFA affords no ring closure and a product derived from
homo-aldol condensation and imine formation.27 A proposed route
to the formation of 32, via an enamine-mediated aldol condensa-
tion, is presented in Scheme S1.
Scheme 3. Preparation of tert-butoxycarbonyl-5-methoxyindole-3-acetaldehyde (17).
Reagents and conditions: (i) (a) oxalyl chloride, diethyl ether, N2, 0 ꢀC, 1 h, rt, 1 h, 87%;
(b) LAH, THF, N2, reflux, 5 h, 87%, (ii) acetic anhydride, pyridine, N2, rt, 16 h, 83%; (iii)
di-tert-butyl dicarbonate, DMAP, CH2Cl2, N2, rt, 19 h, quant.; (iv) 1 M K2CO3, MeOH/H2O
(1:1), rt, 16 h, 84%; (v) IBX, DMSO, N2, rt, 2 h, 81%.
PicteteSpengler coupling of commercially available 5-
methoxytryptamine (15) with aldehyde 17 afforded the corre-
To complete the SAR study, a series of decarbonylated analogues
modelled on the ascidian metabolite eudistomin U (39)28,29 were
also synthesised via a PicteteSpengler reaction between the ap-
propriate tryptamine or tryptophan and either indole-3-
carboxaldehyde (34) or the 5-methoxy analogue 35 (Scheme 5). It
was found that harsher reaction conditions were essential to drive
these particular reactions, with initial imine formation requiring
heating at reflux in toluene or MeOH, before solvent removal and
addition of CH2Cl2/TFA to achieve ring closure. In this manner, the
reaction of tryptamine with indole-3-carboxaldehyde afforded
sponding 1,2,3,4-tetrahydro-b-carboline 20 in 77% yield. Sub-
sequent steps of oxidation (DDQ in CH2Cl2/H2O) to give 22 (37%)
and demethylation, using BBr3 in CH2Cl2, afforded hyrtiosulawesine
(3, 87% yield).16
Changing the solvent used in the PicteteSpengler reaction be-
tween tryptamine and N-Boc-indole-3-acetaldehyde (16) to tolu-
ene, followed by CH2Cl2/TFA cyclisation, afforded two products 29
(6%) and 30 (15%) (Scheme 4), while use of CH2Cl2 as the solvent led
to the same two products in higher yields of 46% and 39%, re-
spectively (see Experimental). HRESIMS analysis of the major re-
action product established a molecular formula of C25H28N3O2,
1,2,3,4-tetrahydro-b-carboline 36 (36%) and subsequent oxidation
[MþH]þ consistent with the presence of a 1,2,3,4-tetrahydro-
b-
with DDQ in THF gave eudistomin U (39, 40%) as previously re-
ported by Massiot et al.29 Application of similar methodology using
carboline skeleton linked via a methylene group to a Boc-protected
indole. Detailed analysis of the 1D and 2D NMR data confirmed the
structure of 29. HRESIMS analysis of the second product of the
reaction (30) identified a pseudomolecular ion at m/z 643.3256
(requires C40H43N4O4 [MþH]þ) suggesting the presence of a 1,2,3,4-
L-tryptophan methyl ester or 5-methoxytryptamine as the amine
component with either 34 or 35 afforded 1,2,3,4-tetrahydro-
b
-
carboline 37 and 38, and upon subjecting to oxidation gave ester 40
and methylether 42. This was followed by deprotection steps to
tetrahydro-b-carboline group and two N-protected tert-butylox-
ycarbonyl indole moieties. Inspection of NMR data confirmed the
presence of these three moieties, in addition to a fourth 1H-spin
system comprised of a tri-substituted olefin [dH 6.16 (1H, t, J¼7.2 Hz,
Scheme 5. Synthesis of eudistomin U (39) and analogues. Reagents and conditions: (i)
13, 14 or 15, toluene or MeOH reflux, 1e72 h, then CH2Cl2, TFA, rt, 16e24 h, 36e76%; (ii)
DDQ, THF or CH2Cl2, rt, 50 mine24 h, 24e40%; (iii) LiOH, THF/H2O (3:1), reflux, 13 h,
96%; (iv) BBr3, CH2Cl2, N2, ꢁ78 ꢀC, 2 h, then rt, 2 h, 31%.
Scheme 4. PicteteSpengler reaction of N-Boc-indole-3-acetaldehyde (16) and trypt-
amine. Reagents and conditions: (i) tryptamine, toluene, N2, reflux, 3 h; (ii) CH2Cl2,
TFA, N2, rt, 12 h, 46%, 39%.
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4
L.P.P. Liew et al. / Tetrahedron xxx (2014) 1e11
obtain acid 41 and phenol 43 (Scheme 5). In the case of DDQ oxi-
dation of 1,2,3,4-tetrahydro- -carboline 37, a 3,4-dihydro- -car-
analogue 41 being considered weak inhibitors (IC50 171
131 M, respectively).
mM and
b
b
m
boline reaction intermediate (44) was also isolated from the
product mixture (Fig. 3).
2.2.2. Antimalarial activity. Using previously reported protocols,31
the library was evaluated for antimalarial activity against a chloro-
quine-resistant strain (FcB1) of P. falciparum (Table 2). In the case of
fully oxidised
moderately active (IC50 1.3
precursor 22 (IC50 1.0 M) and the corresponding decarbonyl
eudistomin U analogues 42 (IC50 4.7 M) and 43 (IC50 11.0 M) also
b-carbolines, hyrtiosulawesine (3) was found to be
mM) with the methylated synthetic
m
m
m
exhibiting activity. The data indicates that more enhanced activity
is observed for C-5 and C-60 hydroxylated or methoxylated ana-
logues (cf. pityriacitrin (4), IC50 >32
as in 21 (IC50 23.3 M), 5 (IC50 >28
(IC50 >31 M), is detrimental to antimalarial activity. The bioassay
results observed for 1,2,3,4-tetrahydro- -carboline analogues sug-
m
m
M), and that C-3 substitution,
m
M), 40 (IC50 >29 M) and 41
m
m
Fig. 3. Structure of 3,4-dihydro-b-carboline reaction intermediate (44).
b
gest a different structureeantimalarial relationship. Amongst pre-
2.2. Biological activities
cursors to pityriacitrin, pityriacitrin B, hyrtiosulawesine and
eudistomin U tested, the 1,2,3,4-tetrahydro-b-carboline analogues
Most of the compounds were assayed for anti-phospholipase A2
activity, for antimalarial activity against a chloroquine-resistant
strain of P. falciparum, for cytotoxicity against a panel of human
cancer cell lines at the NCI and for antioxidant activity (Table 2).
(18, 19, 29 and 36) all exhibited either comparable or enhanced
antimalarial activity versus the corresponding fully aromatic
carboline structure.
b-
2.2.3. Cytotoxic activity. The library of analogues was also evalu-
ated for cytotoxicity by the NCI in their in vitro disease-oriented
primary antitumour screen (Table 2). The preliminary one-dose
Table 2
Biological results of compounds tested for anti-phospholipase A2 activity, for anti-
malarial activity against a chloroquine-resistant strain of Plasmodium falciparum, for
cytotoxicity against human tumour cell line (COLO 205) at the NCI and for antioxi-
dant activity
(10
compounds were relatively inactive, with mean cell growth close to
þ100%. Three analogues, -carboline 42 and 1,2,3,4-tetrahydro-
m
M) testing regimen at the NCI32 identified that most of the
b
b-
COLO 205c
Mean cell growth (%)
ORACFL
d
Compound
number
PLA2
IC50
Pf (FcB1)
IC50
M)b
carbolines 30 and 32 were identified as exhibiting some measure of
cytotoxic activity, with subpanel specificities towards leukaemia
and ovarian cancer (for 42) and colon cancer (COLO 205 and HCC-
2998) cell lines for both 30 and 32 (see Supplementary data). Al-
though less active, 29 exhibited similar colon cancer subpanel
specificity to that observed for 30 (data not shown). Further 5-dose
testing of 42 and 30 established modest antileukaemic activity of
(m
M)a
(m
3
4
5
>1166
>1286
171ꢂ6
>1471
>1550
320ꢂ30
>1114
>1084
>1078
>998
1.3ꢂ0.2
>32*
>28*
79.2
88.6
93.4
86.5
91.6
96.3
95.0
97.8
97.6
76.7
32.3
53.2
87.8
98.6
85.3
67.9
73.1
97.1
26.2
96.4
65.6
6.2ꢂ1.4
10
11
18
19
21
22
29
30
32
36
37
38
39
40
41
42
43
44
>37*
32.2ꢂ1.6
9.6ꢂ1.6
17.2ꢂ0.6
23.3ꢂ0.7
1.0ꢂ0.4
8.5ꢂ3.1
>16*
42 towards the HL-60(TB) cell line (LC50 4.2
relatively specific antiproliferative activity towards the COLO
205 cell line with LC50 2.7 M. The selective anti-colon tumour cell
mM), while 30 exhibited
m
>622
line activity observed for 30 and 32 is particularly exciting, war-
ranting further research.
nte
nt
958ꢂ41
5.1ꢂ0.4
13.6ꢂ8.4
8.0ꢂ1.8
14.4ꢂ4.9
>29*
>930
494ꢂ35
>1413
>1173
131ꢂ9
>657
2.2.4. Antioxidant activity. Evaluation of the compounds in a qual-
itative DPPH TLC assay33 identified two, hyrtiosulawesine (3) and
43 with antioxidant properties. Their antioxidant activity were
subsequently measured in a quantitative ORAC assay33 against
ascorbic acid. The relative ORAC values were expressed as Trolox
equivalents with both compounds displaying around 6-fold more
potent antioxidant activity than Trolox (ORAC value of 6.23 and
5.66, respectively). In the same assay, both compounds displayed
significant activity over vitamin C (positive control, ORAC value of
0.48). The antioxidant activity displayed here could be attributed to
the presence of the phenolic hydroxyl group; in contrast, the car-
bonyl group does not seem to be essential for antioxidant activity.
>31*
4.7ꢂ1.3
11.0ꢂ1.4
>29*
929ꢂ10
5.7ꢂ0.9
239ꢂ10
a
Apis mellifera bee venom PLA2 IC50 values are presented as the meanꢂSEM
(n¼2). Manoalide was used as a positive control (IC50 0.5ꢂ0.05
m
M).30
b
Plasmodium falciparum IC50 values are presented as the meanꢂSEM (n¼4), ex-
cept for values marked with an asterisk for which n ¼ 2. Chloroquine was the
positive control (IC50¼61.8ꢂ6.2 nM).30
c
Details of the testing of compounds for antitumour activity at the NCI/NIH are
available online.32
d
The ORAC assay was only carried for compounds with a positive result in the
qualitative DPPH assay. Vitamin C was used as a positive control (ORACFL value
2.2.5. Conclusions. In summary, we have reported the use of the
PicteteSpengler reaction between substituted tryptamines and
indole-3-acetaldehyde derivatives to gain entry to the bioactive 1-
0.48ꢂ0.19).33
e
nt: not tested.
acylindole natural products pityriacitrin, pityriacitrin
B and
2.2.1. Anti-phospholipase A2 activity. Hyrtiosulawesine (3) has
previously been reported to inhibit Crotalus adamentus (snake)
hyrtiosulawesine. We were unfortunately unable to realise one of
our original aims of specific oxidation of the C-80-methylene-
venom PLA2 with IC50 14
m
M.11 In the present study,30 no activity
1,2,3,4-tetrahydro-b-carboline intermediates to enable evaluation
was observed for the natural product against bee venom (Apis
mellifera) enzyme (Table 2, entry 1). Most of the remaining test
compounds were also inactive, with the two most active com-
pounds, carboxylic acids pityriacitrin B (5) and eudistomin U
of the influence of the oxo group on observed bioactivity. While
poor activity was observed towards the phospholipase A2 target
enzyme, pronounced activity was observed towards a drug re-
sistant strain of P. falciparum, identifying O-methyl or methoxylated
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L.P.P. Liew et al. / Tetrahedron xxx (2014) 1e11
5
analogues of the marine natural products hyrtiosulawesine and
eudistomin U as new leads for antimalarial drug development.
Although b-carboline alkaloids are well known to exhibit DNA in-
tercalation properties and hence whole cell cytotoxicity, the library
of analogues were typically only weakly cytotoxic towards a panel
of human tumour cell lines. The discovery however of selective
cytotoxicity for 1,2,3,4-b-carbolines 30 and 32 against COLO 205
and HCC-2998 colon tumour cell lines is particularly encouraging
and will be the focus of future studies.
NH-10), 9.40 (1H, d, J¼3.1 Hz, H-20), 8.62e8.60 (1H, m, H-40), 8.58
(1H, d, J¼4.8 Hz, H-3), 8.28-8.25 (2H, m, H-4 and H-5), 7.78 (1H, d,
J¼7.8 Hz, H-8), 7.62 (1H, ddd, J¼7.8, 7.8, 1.0 Hz, H-7), 7.60e7.56 (1H,
m, H-70), 7.36e7.29 (3H, m, H-6, H-50 and H-60); 13C NMR (CD3CN,
100 MHz) d
189.4 (C-80), 142.4 (C-8a), 139.4 (C-1), 138.7 (C-20), 138.4
(C-3), 137.0 (C-9a), 136.9 (C-70a), 132.1 (C-4a), 129.9 (C-7), 128.4 (C-
30a), 124.2 (C-60), 123.3 (C-50), 123.1 (C-40), 122.6 (C-5), 121.7 (C-4b),
121.2 (C-6), 118.8 (C-4), 115.8 (C-30), 113.4 (C-8), 112.9 (C-70);
(þ)-HRESIMS m/z 312.1135 [MþH]þ (calcd for C20H14N3O,
312.1131). The spectroscopic data were consistent with literature
values.9,17
3. Experimental
3.1. General experimental procedures
3.1.3. Pityriacitrin B (5).8,17 Pityriacitrin B methyl ester (21) (5.0 mg,
0.001 mmol) was dissolved in THF (1.5 mL). LiOH (6.0 mg,
0.012 mmol) was dissolved into H2O (0.5 mL) and added to the
reaction mixture. The reaction mixture was heated to reflux under
N2 for 12 h. The pH of the cooled reaction mixture was adjusted
with 1 M HCl solution until yellow solids appeared. The yellow
solids were filtered and washed with H2O (2.5 mL), MeOH (0.5 mL)
and ether (2.5 mL) successively in 0.5 mL portions. The yellow
solids were dried under vacuum to yield pityriacitrin B (5) (4.0 mg,
83% yield). Mp 285 ꢀC (lit.8 >300 ꢀC); Rf (5% MeOH/CH2Cl2) 0.30; IR
Details of instrumental methods and general experimental
procedures34 have been reported previously. NMR chemical shifts
were calibrated to residual solvent signals (DMSO-d6: dH 2.50, dC
39.52; CD3OD: dH 3.31, dC 49.00; CD3CN: dH 1.94, dC 1.32, 118.26;
CDCl3: dH 7.26, dC 77.16)35 and structural assignments were aided by
x
COSY, edited-HSQC and HMBC (optimised for JCH¼8.33 Hz)
experiments.
tert-Butyl 3-(2-oxoethyl)-1H-indole-1-carboxylate (16),36
L-
tryptophan methyl ester$HCl (14),37 eudistomin U (39),29 5-
nmax (ATR) 3226, 2923, 1702, 1596, 733 cmꢁ1
;
1H NMR (CD3OD,
methoxy-1H-indole-3-carboxaldehyde (35)38 and 1-(1H-indol-3-
400 MHz)
d
9.64 (1H, s, H-20), 9.02 (1H, s, H-4), 8.63 (1H, m, H-40),
yl)-1,2,3,4-tetrahydro-
ture methods.
b
-carboline (36),29 were prepared by litera-
8.30 (1H, d, J¼7.8 Hz, H-5), 7.76 (1H, d, J¼7.8 Hz, H-8), 7.62 (1H, dd,
J¼7.8, 7.8 Hz, H-7), 7.52e7.49 (1H, m, H-70), 7.36 (1H, dd, J¼7.8,
7.8 Hz, H-6), 7.28e7.26 (2H, m, H-50 and H-60); 13C NMR (CD3OD,
3.1.1. Hyrtiosulawesine (3).6,16 A solution of 6,50-dimethyl hyrtio-
sulawesine (22) (4 mg, 0.01 mmol) in CH2Cl2 (3 mL) cooled to
100 MHz) d
189.3 (C-80), 171.4 (C-10), 143.5 (C-1 or C-3), 143.3 (C-
8a), 140.4 (C-1 or C-3), 140.2 (C-20), 138.9 (C-70a), 137.7 (C-9a), 133.1
(C-4a), 130.2 (C-7), 128.9 (C-30a), 124.2 (C-60), 123.4 (C-40), 123.2 (C-
50), 122.7 (C-5), 122.4 (C-4b), 121.8 (C-6), 120.0 (C-4), 116.2 (C-30),
113.7 (C-8), 112.8 (C-70); (þ)-HRESIMS m/z 356.1007 [MþH]þ (calcd
for C21H14N3O3, 356.1030). The spectroscopic data were consistent
with literature values.8
ꢁ78 ꢀC and was added BBr3 (305
mL, 3.23 mmol) under N2. The
reaction was allowed to stir at ꢁ78 ꢀC for 40 min then warmed to
rt with stirring under N2 continued for 3 h. MeOH (4 mL) was added
to quench the remaining BBr3. Solvent was removed under pressure
to give a crude material, which was diluted with H2O (50 mL) and
extracted with EtOAc (50 mL). The aqueous layer was extracted
with EtOAc (5ꢃ30 mL). The organic fractions were combined, dried
(MgSO4) and solvent removed in vacuo. The crude reaction product
was purified by silica gel column chromatography (40% EtOAc/n-
hexanes) to afford 3 as a yellow solid (4.0 mg, 87% yield). Mp 300 ꢀC
(lit.16 362e363 ꢀC); Rf (50% EtOAc/n-hexanes) 0.26; IR nmax (ATR)
3402, 3201, 1603, 1493, 1132, 739 cmꢁ1; 1H NMR (CD3OD, 400 MHz)
1H NMR (DMSO-d6, 400 MHz)
d 12.38 (1H, br s, NH-9),12.26 (1H,
br s, NH-10), 9.75 (1H, d, J¼2.9 Hz, H-2), 9.14 (1H, s, H-4), 8.62e8.59
(1H, m, H-40), 8.46 (1H, d, J¼7.8 Hz, H-5), 7.90 (1H, d, J¼7.8 Hz, H-8),
7.64 (1H, dd, J¼7.8, 7.8 Hz, H-7), 7.58e7.55 (1H, m, H-70), 7.36 (1H,
dd, J¼7.8, 7.8 Hz, H-6), 7.33e7.27 (2H, m, H-50 and H-60); 13C NMR
(DMSO-d6, 100 MHz) d
186.4 (C-80), 166.9 (C-10), 142.1 (C-8a), 138.5
(C-20), 137.1 (C-1 or C-3), 136.1 (C-1 or C-3), 135.9 (C-9a or C-70a),
135.8 (C-9a or C-70a), 131.3 (C-4a), 129.1 (C-7), 127.3 (C-30a), 123.0
(C-60), 122.2 (C-5), 122.1 (C-50), 121.7 (C-40), 120.7 (C-6), 120.5 (C-
4b), 119.8 (C-4), 114.2 (C-30), 113.4 (C-8), 112.3 (C-70). The spectro-
scopic data were consistent with literature values.17
d
8.88 (1H, s, H-20), 8.42 (1H, d, J¼5.0 Hz, H-3), 8.14 (1H, d, J¼5.0 Hz,
H-4), 8.02 (1H, d, J¼2.4 Hz, H-40), 7.57 (1H, d, J¼2.4 Hz, H-5), 7.54
(1H, d, J¼8.8 Hz, H-8), 7.32 (1H, d, J¼8.8 Hz, H-70), 7.14 (1H, dd,
J¼8.8, 2.4 Hz, H-7), 6.82 (1H, dd, J¼8.8, 2.4 Hz, H-60); 13C NMR
(CD3OD, 100 MHz) d
189.9 (C-80), 154.4 (C-50), 152.6 (C-6), 140.3 (C-
1), 139.1 (C-20), 137.5 (C-9a), 137.4 (C-8a), 137.3 (C-3), 132.5 (C-4a),
132.3 (C-70a), 129.8 (C-30a), 122.6 (C-4b), 119.8 (C-7), 118.5 (C-4),
116.0 (C-30), 113.9 (C-8), 113.8 (C-60), 113.3 (C-70), 108.0 (C-40), 106.8
(C-5); (þ)-HRESIMS m/z 344.1037 [MþH]þ (calcd for C20H14N3O3,
344.1030). The spectroscopic data were consistent with literature
values.6,16
3.1.4. 1-Benzyl-3,4-dihydro-
(2-(1H-indol-3-yl)ethyl)-2-phenylacetamide
0.18 mmol) stirring in toluene (10 mL) was added POCl3 (167
b
-carboline (9). To a suspension of N-
(8) (50 mg,
L,
m
1.80 mmol). The reaction mixture was heated to reflux at 120 ꢀC
under N2 for 30 min. After cooling to rt, the reaction mixture was
poured over ice. The mixture was poured over saturated NaHCO3
solution (100 mL) and extracted with CH2Cl2 (100 mL). The CH2Cl2
fraction was washed with H2O (2ꢃ100 mL), dried (MgSO4) and
concentrated to afford 9 as the hydrochloride salt39 (orange oil,
44 mg, 83% yield).
3.1.2. Pityriacitrin (4).9,17 To a solution of 1-(3-methyl-1H-indole)-
1,2,3,4-tetrahydro-
(10 mL) was added H2O (100
b
-carboline (18) (35 mg, 0.12 mmol) in CH2Cl2
L). DDQ (132 mg, 0.58 mmol) was
m
added to the reaction mixture with vigorous stirring. The reaction
was allowed to stir under N2 for 45 min. The reaction mixture was
poured over 1 M KOH solution (50 mL) and extracted with CH2Cl2
(40 mL). The organic fraction was washed once more with 1 M KOH
solution (50 mL), dried (MgSO4) and concentrated in vacuo. The
crude reaction product was purified by silica gel column chroma-
tography (CH2Cl2) to afford 4 as a bright yellow solid (5.5 mg, 15%
yield). Mp 241e242 ꢀC (lit.9 227e230 ꢀC); Rf (CH2Cl2) 0.18; IR nmax
Rf (10% MeOH/CH2Cl2) 0.55; IR nmax (ATR) 3027, 2827, 1628,1555,
1275, 731 cmꢁ1; 1H NMR (CDCl3, 400 MHz)
d 11.67 (1H, br s, NH-9),
11.51 (1H, br s, NH-2), 7.60 (1H, d, J¼8.0 Hz, H-5), 7.51 (1H, d,
J¼8.2 Hz, H-8), 7.46 (2H, d, J¼7.3 Hz, H-20 and H-60), 7.38 (1H, ddd,
J¼8.0, 8.0, 0.7 Hz, H-6), 7.25e7.21 (2H, m, H-30 and H-50), 7.18e7.11
(2H, m, H-7 and H-40), 4.37 (2H, s, H2-70), 3.86 (2H, ddd, J¼8.7, 8.7,
2.8 Hz, H2-3), 3.09 (2H, t, J¼8.7 Hz, H2-4); 13C NMR (CDCl3,
100 MHz) d
168.3 (C-1), 142.4 (C-8a), 132.4 (C-10), 129.8 (C-7), 129.6
(ATR) 3418, 3226, 2923, 1598, 1510, 1492, 1468 cmꢁ1
;
1H NMR
(C-30 and C-50), 129.5 (C-20 and C-60), 128.3 (C-40), 125.6 (C-4a),
125.4 (C-9a), 124.5 (C-4b), 122.3 (C-6), 121.5 (C-5), 114.4 (C-8), 42.7
(CD3CN, 400 MHz)
d
10.97 (1H, br s, NH-9), 10.09 (1H, br d, J¼3.1 Hz,
Please cite this article in press as: Liew, L. P.P.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.05.068

6
L.P.P. Liew et al. / Tetrahedron xxx (2014) 1e11
(C-3), 38.2 (C-70), 19.5 (C-4); (þ)-HRESIMS m/z 261.1392 [MþH]þ
3.1.6. tert-Butyl-5-methoxy-3-(2-oxoethyl)-1H-indole-1-carboxylate
(17). IBX (384 mg, 1.37 mmol) was dissolved in DMSO (1.0 mL) with
stirring at rt under N2 for 50 min. tert-Butyl 3-(2-hydroxyethyl)-5-
methoxy-1H-indole-1-carboxylate (28) (200 mg, 0.69 mmol) in
DMSO (1.6 mL) was added to the reaction mixture and allowed to
stir for 2 h at rt under N2. H2O (10 mL) was added to the reaction,
and the solids were removed by filtration and washed with CH2Cl2
(50 mL). The filtrate was transferred into a separating funnel and
washed with H2O (2ꢃ50 mL), dried (MgSO4) and concentrated in
vacuo. The crude material was purified by silica gel column chro-
matography (CH2Cl2) to afford 17 as an unstable pale yellow oil
(153 mg, 81% yield). Rf (CH2Cl2) 0.60; IR nmax (ATR) 2978, 1722, 1376,
(calcd for C18H17N2, 261.1386).
3.1.5. 1-Phenylmethanone-
(11) and 1-benzyl-3,4-dihydro-
(26 mg, 0.09 mmol) stirring in toluene (10 mL) was added POCl3
(87 L, 0.93 mmol). The reaction mixture was heated to reflux at
b
-carboline (10),40 1-benzyl-
-carboline (12). To a suspension of 8
b-carboline
b
m
120 ꢀC under N2 for 30 min. After cooling to rt, the reaction mixture
was poured over ice. The mixture was poured over saturated
NaHCO3 solution (50 mL), extracted with CH2Cl2 (2ꢃ50 mL), dried
(MgSO4) and concentrated to obtain 9, which was dissolved into
CH2Cl2 (10 mL). DDQ (85 mg, 0.37 mmol) was added to the reaction
mixture, which was allowed to stir at rt under N2 for 70 min. The
reaction was poured into 1 M KOH solution (50 mL) and extracted
with CH2Cl2 (3ꢃ30 mL). The organic fractions were combined, dried
(MgSO4) and concentrated to obtain a brown crude material, which
was immediately purified by silica gel column chromatography
(CH2Cl2e1% MeOH/CH2Cl2) to afford 10 as a yellow solid (1.0 mg, 4%
yield over two steps) and 11 as an orange-brown oil (6.3 mg, 26%
yield over two steps).
1076, 765 cmꢁ1; 1H NMR (CDCl3, 400 MHz)
d 9.77 (1H, s, H-9), 8.03
(1H, br d, J¼7.3 Hz, H-7), 7.55 (1H, s, H-2), 6.96 (1H, d, J¼7.3 Hz, H-
6), 6.88 (1H, s, H-4), 3.85 (3H, s, OMe-14), 3.73 (2H, s, H2-8), 1.66
(9H, s, 3H3-13); 13C NMR (CDCl3, 100 MHz)
d 198.5 (C-9),156.1 (C-5),
149.5 (C-10),131.0 (C-3a),130.2 (C-7a),125.4 (C-2),116.2 (C-7),113.5
(C-6), 110.7 (C-3), 101.4 (C-4), 83.7 (C-12), 55.7 (C-14), 40.1 (C-8),
28.2 (C-13).
An alternative, higher yielding route to 10 is as follows: To
a solution of 9, prepared from 8 (88 mg, 0.32 mmol) as previously
described, in DMF (2 mL) was added KMnO4 (105 mg, 0.66 mmol)
and the reaction stirred at 0 ꢀC for 1 h. The reaction mixture was
allowed warm to rt over 30 min. The resulting mixture was diluted
with water (30 mL) and extracted with DCM (3ꢃ50 mL). The or-
ganic fractions were combined, dried (MgSO4) and concentrated to
obtain a crude material, which was immediately purified by silica
gel column chromatography (CH2Cl2) to afford 10 as a yellow solid
3.1.7. 1-(3-Methyl-1H-indole)-1,2,3,4-tetrahydro-b-carboline
(18).23 tert-Butyl 3-(2-oxoethyl)-1H-indole-1-carboxylate (16)
(83 mg, 0.32 mmol) was weighed into a round bottom flask as
a neat oil. Tryptamine (13) (31 mg, 0.19 mmol) and tryptamine$HCl
(38 mg, 0.19 mmol) were weighed into a separate vial. 5% HOAc
solution (20 mL) was added to the tryptamine mixture. This mix-
ture was transferred to the round bottom flask as a suspension and
set to reflux at 100 ꢀC under N2 for 2 h with vigorous stirring. The
reaction mixture was allowed to cool to rt, then to 0 ꢀC with stirring
for another 30 min. Water (30 mL) was added to the reaction
mixture, and extracted with EtOAc (3ꢃ50 mL). The organic solvent
fractions were combined, dried (MgSO4) and solvent removed in
vacuo. The crude reaction product was purified by silica gel column
chromatography (6e10% MeOH/CH2Cl2) to afford 18 as a brown-
orange oil (36 mg, 37% yield). Rf (10% MeOH/CH2Cl2) 0.44; IR nmax
(ATR) 3399, 2915, 1730, 1560, 1452, 1151, 738 cmꢁ1; 1H NMR (CDCl3,
(23 mg, 27% yield over two steps) and 1-benzyl-3,4-dihydro-b-
carboline (12) as a yellow oil (12 mg, 14% yield over two steps).
Compound 10: mp 132e133 ꢀC (lit.40 135e138 ꢀC); Rf (CH2Cl2)
0.71; IR nmax (ATR) 3430, 3056, 1640, 1619 cmꢁ1
d6, 400 MHz)
;
1H NMR (DMSO-
12.07 (1H, br s, NH-9), 8.53 (1H, d, J¼5.0 Hz, H-3),
d
8.45 (1H, d, J¼5.0 Hz, H-4), 8.33 (1H, dd, J¼7.5, 0.8 Hz, H-5),
8.21e8.18 (2H, m, H-20 and H-60), 7.83 (1H, d, J¼8.0 Hz, H-8),
7.69e7.55 (4H, m, H-7, H-30, H-40 and H-50), 7.32 (1H, ddd, J¼7.5, 7.5,
400 MHz) d
8.35 (1H, br s, NH-10), 7.94 (1H, br s, NH-9), 7.62 (1H, d,
0.9 Hz, H-6); 13C NMR (DMSO-d6, 100 MHz)
d
193.9 (C-70), 141.7 (C-
J¼7.6 Hz, H-40), 7.45 (1H, d, J¼7.6 Hz, H-5), 7.36 (1H, d, J¼8.4 Hz, H-
70), 7.22e7.17 (2H, m, H-8 and H-60), 7.12e7.05 (3H, m, H-6, H-7 and
H-50), 6.96 (1H, s, H-20), 4.58 (1H, t, J¼6.8 Hz, H-1), 3.37e3.23 (3H,
m, H2-3A and H2-80), 3.05e2.98 (1H, m, H2-3B), 2.84e2.70 (2H, m,
8a), 137.5 (C-10), 137.2 (C-3), 136.3 (C-1), 135.8 (C-9a), 132.2 (C-40),
131.0 (C-4a), 130.8 (C-20 and C-60), 129.0 (C-7), 127.9 (C-30 and C-50),
121.8 (C-5), 120.2 (C-6), 120.1 (C-4b), 118.9 (C-4), 113.0 (C-8);
(þ)-HRESIMS m/z 273.1008 [MþH]þ (calcd for C18H13N2O,
273.1022). The spectroscopic data were consistent with literature
values.40
H2-4); 13C NMR (CDCl3, 100 MHz) 136.5 (C-70a), 135.8 (C-8a), 134.8
d
(C-9a), 127.4 (C-3a), 127.1 (C-4b), 123.3 (C-20), 122.6 (C-60), 121.8 (C-
7),119.9 (C-50), 119.5 (C-6),119.0 (C-40), 118.3 (C-5),111.6 (C-70), 111.4
(C-30), 111.0 (C-8), 108.9 (C-4a), 52.8 (C-1), 42.2 (C-3), 30.7 (C-80),
21.8 (C-4); (þ)-HRESIMS m/z 302.1637 [MþH]þ (calcd for C20H20N3,
302.1652). The spectroscopic data were consistent with literature
values.23
Compound 11: Rf (5% MeOH/CH2Cl2) 0.74; IR nmax (ATR) 3061,
1626, 1495 cmꢁ1; 1H NMR (CDCl3, 300 MHz)
H-3), 8.09 (1H, d, J¼7.9 Hz, H-5), 7.87 (1H, d, J¼5.4 Hz, H-4), 7.48
(1H, ddd, J¼8.3, 8.3, 0.7 Hz, H-7), 7.38e7.23 (7H, m, H-6, H-8, H-20,
H-30, H-40, H-50 and H-60), 4.55 (2H, s, H2-70); 13C NMR (CDCl3,
d
8.43 (1H, d, J¼5.4 Hz,
100 MHz)
d
143.6 (C-1), 140.6 (C-8a), 138.1 (C-3), 137.9 (C-10), 134.6
3.1.8. 1-(1H-Indol-3-yl)methyl-3-methylcarboxylate-1,2,3,4-
tetrahydro-b-carboline (19). L-Tryptophan methyl ester$HCl (14)
(C-9a), 129.9 (C-4a), 129.1 (C-30 and C-50), 129.0 (C-20 and C-60),
128.8 (C-7), 127.1 (C-40), 121.9 (C-4b), 121.7 (C-5), 120.5 (C-6), 113.þ7
(C-4), 111.9 (C-8), 41.3 (C-70); (þ)-HRESIMS m/z 259.1235 [MþH]
(calcd for C18H15N2, 259.1230).
(57 mg, 0.22 mmol) and tert-butyl 3-(2-oxoethyl)-1H-indole-1-
carboxylate (16) (58 mg, 0.22 mmol) were dissolved in anhydrous
MeOH (4 mL) and allowed to stir under N2 at rt for 3 h. MeOH was
removed before redissolving the crude material into AcOH/H2O
(1:10) (11 mL) and set to stir under N2 at 50 ꢀC for 1.5 h. The reaction
mixture was then heated to reflux for 4 h. After cooling to rt, the
reaction mixture was diluted with H2O (50 mL) and extracted with
EtOAc (3ꢃ50 mL). The organic fractions were combined, dried
(MgSO4) and solvent removed in vacuo. The crude reaction product
was purified by silica gel column chromatography (2% MeOH/
CH2Cl2) to afford 19 [a mixture of diastereomers (1:0.9)] as a yellow
oil (35 mg, 44% yield). Rf (5% MeOH/CH2Cl2) 0.28; IR nmax (ATR)
3403, 2918, 1728, 1455, 738 cmꢁ1; Major diastereomer: 1H NMR
Compound 12: Rf (50% n-hexanes/CH2Cl2) 0.45; IR nmax (ATR)
3439, 3056, 2926, 2831, 1655, 1538, 1189, 1173, 954, 878, 741,
689 cmꢁ1 1H NMR (CDCl3, 400 MHz)
; d 9.42 (1H, br s, NH-9),
8.19e8.17 (2H, m, H-20 and H-60), 7.63e7.58 (2H, m, H-5 and H-
40), 7.51e7.47 (2H, m, H-30 and H-50), 7.42 (1H, ddd, J¼7.4, 0.8,
0.8 Hz, H-8), 7.31 (1H, ddd, J¼7.4, 7.4, 1.0 Hz, H-7), 7.16 (1H, ddd,
J¼7.4, 7.4, 0.8 Hz, H-6), 4.20e4.15 (2H, m, H2-3), 3.05e3.01 (2H, m,
H2-4); 13C NMR (CDCl3, 100 MHz)
d
193.5 (C-70), 156.0 (C-1), 137.1
(C-8a), 135.5 (C-10), 133.6 (C-40), 131.2 (C-20 and C-60), 128.4 (C-30
and C-50), 126.8 (C-9a), 125.3 (C-7), 124.9 (C-4b), 120.5 (C-6), 120.1
(C-5), 118.2 (C-4a), 112.4 (C-8), 49.4 (C-3), 19.2 (C-4); (þ)-HRESIMS
m/z 275.1182 [MþH]þ (calcd for C18H15N2O, 275.1179).
(CDCl3, 400 MHz) d
8.21 (1H, br s, NH-10), 7.75 (1H, br s, NH-9), 7.65
(1H, d, J¼8.0 Hz, H-5), 7.51e7.46 (1H, m, H-60), 7.39 (1H, d, J¼3.5 Hz,
Please cite this article in press as: Liew, L. P.P.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.05.068

L.P.P. Liew et al. / Tetrahedron xxx (2014) 1e11
7
H2-8), 7.26e7.21 (1H, m, H-7), 7.18e7.06 (4H, m, H-6, H-40, H-50 and
H-70), 7.06 (1H, d, J¼2.4 Hz, H-20), 4.61 (1H, tt, J¼6.7, 2.0 Hz, H-1),
3.83e3.80 (1H, m, H-3), 3.79 (3H, s, OMe-11), 3.32e3.21 (2H, m, H2-
80), 3.19e3.12 (1H, m, H2-4A), 2.86 (1H, ddd, J¼15.0, 11.4, 2.5 Hz, H2-
(2H, m, H-50 and H-60), 4.03 (3H, s, OMe-11); 13C NMR (DMSO-d6,
100 MHz)
186.3 (C-80), 165.6 (C-10), 142.1 (C-8a), 138.4 (C-20),
d
137.3 (C-1), 136.0 (C-9a), 135.9 (C-70a), 134.8 (C-3), 131.3 (C-4a),
129.2 (C-7), 127.3 (C-30a), 123.0 (C-60), 122.2 (C-50), 122.1 (C-5), 121.7
(C-40), 120.8 (C-6), 120.5 (C-4b), 119.9 (C-4), 114.1 (C-30), 113.5 (C-8),
112.3 (C-70), 52.4 (C-11); (þ)-HRESIMS m/z 370.1187 [MþH]þ (calcd
for C22H16N3O3, 370.1186). The spectroscopic data were consistent
with literature values.17
4B); 13C NMR (CDCl3, 100 MHz)
d 173.7 (C-10), 136.6 (C-8a), 135.9
(C-70a), 135.4 (C-9a), 127.3 (C-30a), 127.0 (C-4b), 123.1 (C-20), 122.7
(C-7), 121.9 (C-40), 119.9 (C-6), 119.7 (C-50), 119.0 (C-5), 118.2 (C-60),
111.8 (C-30), 111.6 (C-8), 111.0 (C-70), 108.3 (C-4a), 56.7 (C-3), 53.0 (C-
1), 52.4 (C-11), 31.5 (C-80), 26.1 (C-4); Minor diastereomer: 1H NMR
(CDCl3, 400 MHz)
d
8.17 (1H, br s, NH-10), 7.63 (1H, d, J¼8.0 Hz, H-5),
3.1.11. 6,50-Dimethyl hyrtiosulawesine (22).16 To a solution of 1-(5-
7.51e7.46 (2H, m, NH-9 and H-60), 7.41 (1H, d, J¼3.5 Hz, H-8),
7.26e7.21 (1H, m, H-7), 7.18e7.06 (4H, m, H-6, H-40, H-50 and H-70),
6.99 (1H, d, J¼2.3 Hz, H-20), 4.69 (1H, t, J¼7.1 Hz, H-1), 4.07 (1H, dd,
J¼7.2, 7.2 Hz, H-3), 3.72 (3H, s, OMe-11), 3.32e3.21 (2H, m, H2-80),
3.19e3.12 (1H, m, H2-4A), 3.03 (1H, ddd, J¼15.3, 7.2, 1.3 Hz, H2-4B);
methoxy-1H-indol-3-yl)methyl-6-methoxy-1,2,3,4-tetrahydro-
carboline (20) (29 mg, 0.08 mmol) in CH2Cl2 (5 mL) was added H2O
(50 L). DDQ (91 mg, 0.40 mmol) was added to the reaction mixture
b-
m
with vigorous stirring. The reaction was allowed to stir under N2 for
1.5 h. The reaction mixture was poured into 1 M KOH (50 mL) and
extracted with additional CH2Cl2 (30 mL). The organic fraction was
washed with 1 M KOH solution (2ꢃ50 mL), dried (MgSO4) and
concentrated in vacuo. The crude reaction product was purified by
silica gel column chromatography (CH2Cl2) to afford 22 as a bright
yellow solid (11.0 mg, 37% yield). Mp 220e221 ꢀC (lit.16
220e221 ꢀC); Rf (2% MeOH/CH2Cl2) 0.45; IR nmax (ATR) 3568,
13C NMR (CDCl3, 100 MHz) 174.2 (C-10), 136.5 (C-8a),135.8 (C-70a),
d
135.4 (C-9a), 127.4 (C-30a), 127.0 (C-4b), 123.0 (C-20), 122.6 (C-7),
121.8 (C-40), 119.9 (C-6), 119.5 (C-50), 119.0 (C-5), 118.2 (C-60), 112.3
(C-30), 111.5 (C-8), 110.9 (C-70), 107.2 (C-4a), 52.9 (C-3), 52.3 (C-11),
50.8 (C-1), 32.2 (C-80), 25.1 (C-4); (þ)-HRESIMS m/z 360.1694
[MþH]þ (calcd for C22H22N3O2, 360.1707).
3424, 3157, 1603, 1491 cmꢁ1; 1H NMR (DMSO-d6, 400 MHz)
d 11.99
3.1.9. 1-(5-Methoxy-1H-indol-3-yl)methyl-6-methoxy-1,2,3,4-
tetrahydro-b-carboline (20). Aldehyde (17) (49 mg, 0.17 mmol) and
(1H, s, NH-10), 11.86 (1H, s, NH-9), 9.24 (1H, d, J¼3.2 Hz, H-20), 8.52
(1H, d, J¼4.8 Hz, H-3), 8.37 (1H, d, J¼4.8 Hz, H-4), 8.11 (1H, d,
J¼2.3 Hz, H-40), 7.85 (1H, d, J¼2.2 Hz, H-5), 7.75 (1H, d, J¼8.8 Hz, H-
8), 7.46 (1H, d, J¼8.7 Hz, H-70), 7.23 (1H, dd, J¼8.8, 2.2 Hz, H-7), 6.91
(1H, dd, J¼8.7, 2.3 Hz, H-60), 3.88 (3H, s, OMe-10), 3.86 (3H, s, OMe-
5-methoxytryptamine hydrochloride (46 mg, 0.20 mmol) were
weighed into a round bottom flask. Acetic acid (0.5 mL) and H2O
(10 mL) was added and the reaction was purged with N2. The re-
action mixture was set to reflux with vigorous stirring for 5 h. Re-
action mixture was allowed to cool to rt, and then to 0 ꢀC with
stirring for another 30 min. The reaction mixture was slowly added
to a saturated solution of NaHCO3 (50 mL) and extracted with EtOAc
(3ꢃ30 mL). The organic solvent fractions were combined, dried
(MgSO4) and solvent removed in vacuo. The crude reaction product
was purified by silica gel column chromatography (4e10% MeOH/
CH2Cl2) to afford 20 as an orange-brown oil (47 mg, 77% yield). Rf
(10% MeOH/CH2Cl2) 0.48; IR nmax (ATR) 3402, 1624, 1483,
80); 13C NMR (DMSO-d6, 100 MHz)
d
187.3 (C-90), 155.7 (C-50), 153.8
(C-6),138.5 (C-1),137.9 (C-20),136.5 (C-8a),136.3 (C-3),135.5 (C-9a),
130.7 (C-4a and C-70a), 128.1 (C-30a), 120.4 (C-4b), 118.6 (C-7), 118.0
(C-4), 114.2 (C-30), 113.9 (C-8), 113.0 (C-70), 112.8 (C-60), 103.5 (C-5),
103.4 (C-40), 55.7 (C-10), 55.3 (C-80); (þ)-HRESIMS m/z 372.1341
[MþH]þ (calcd for C22H18N3O3, 372.1343). The spectroscopic data
were consistent with literature values.16
3.1.12. 1-(3-Methanone-1H-indole)-3,4-dihydro-
b-carboline
1212 cmꢁ1; 1H NMR (CDCl3, 400 MHz)
d
8.08 (1H, br s, NH-10), 7.60
(23). To a solution of 1,2,3,4-tetrahydro- -carboline 18 (24 mg,
b
(1H, br s, NH-9), 7.28 (1H, d, J¼8.8 Hz, H-70), 7.08 (1H, d, J¼8.8 Hz, H-
8), 7.01 (1H, d, J¼2.4 Hz, H-40), 6.99 (1H, d, J¼2.2 Hz, H-20), 6.93 (1H,
d, J¼2.4 Hz, H-5), 6.89 (1H, dd, J¼8.8, 2.4 Hz, H-60), 6.77 (1H, dd,
J¼8.8, 2.4 Hz, H-7), 4.49 (1H, t, J¼6.8 Hz, H-1), 3.84 (3H, s, OMe-10),
3.77 (3H, s, OMe-80), 3.37 (1H, dt, J¼12.4, 4.3 Hz, H2-3A), 3.27e3.16
(2H, m, H2-90), 3.08e3.01 (1H, m, H2-3B), 2.78e2.66 (2H, m, H2-4);
0.08 mmol) in DMF (1 mL), which was cooled to 0 ꢀC, was added
KMnO4 (18 mg, 0.11 mmol). The reaction mixture was stirred at this
temperature for 1 h, and warmed to rt in another 2 h. The resulting
mixture was diluted with water (50 mL) and extracted with EtOAc
(2ꢃ20 mL). The organic fractions were combined, dried (MgSO4)
and concentrated in vacuo. The crude residue was purified by silica
gel column chromatography (CH2Cl2e5% EtOAc/CH2Cl2) to afford
23 as a bright yellow solid (7.0 mg, 28%) and 4 in trace amounts.
Mp 203e204 ꢀC; Rf (10% MeOH/CH2Cl2) 0.81; IR nmax (ATR) 3458,
3044, 2924, 2830, 1626, 1609, 1581, 1497, 1442, 742 cmꢁ1; 1H NMR
13C NMR (CDCl3, 100 MHz)
d
154.4 (C-50), 154.2 (C-6), 137.1 (C-9a),
131.6 (C-70a), 130.8 (C-8a), 127.9 (C-30a or C-4b), 127.8 (C-30a or C-
4b), 123.8 (C-20), 113.0 (C-60), 112.3 (C-70), 111.9 (C-30), 111.5 (C-8),
111.4 (C-7), 109.3 (C-4a), 100.7 (C-40), 100.5 (C-5), 56.1 (C-10), 56.0
(C-80), 52.9 (C-1), 42.9 (C-3), 31.4 (C-90), 22.8 (C-4); (þ)-HRESIMS m/
z 362.1860 [MþH]þ (calcd for C22H24N3O2, 362.1863).
(DMSO-d6, 400 MHz) d
12.16 (1H, br s, NH-10), 11.21 (1H, br s, NH-9),
8.53 (1H, s, H-20), 8.41e8.38 (1H, m, H-40), 7.59 (1H, d, J¼7.8 Hz, H-
5), 7.57e7.55 (1H, m, H-70), 7.53 (1H, d, J¼7.8 Hz, H-8), 7.31e7.26
(2H, m, H-50 and H-60), 7.20 (1H, dd, J¼7.8, 7.8 Hz, H-7), 7.05 (1H, dd,
J¼7.8, 7.8 Hz, H-6), 4.04 (2H, t, J¼8.6 Hz, H2-3), 2.94 (2H, t, J¼8.6 Hz,
3.1.10. Pityriacitrin B methyl ester (21).17 To a solution of 1-(1H-
indol-3-yl)methyl-3-methylcarboxylate-1,2,3,4-tetrahydro-
boline (19) (17 mg, 0.05 mmol) in CH2Cl2 (5 mL) with vigorous
stirring was added H2O (50 L). DDQ (43 mg, 0.19 mmol) was added
b-car-
H2-4); 13C NMR (DMSO-d6, 100 MHz)
d
186.6 (C-80), 157.5 (C-1),
m
138.3 (C-20), 137.2 (C-8a), 136.5 (C-70a), 126.3 (C-9a and C-30a), 124.3
(C-4b), 123.9 (C-7), 123.2 (C-60), 122.3 (C-50), 121.4 (C-40), 119.4 (C-5
and C-6), 116.3 (C-4a), 113.3 (C-30), 113.1 (C-8), 112.5 (C-70), 48.2 (C-
3), 18.6 (C-4); HRESIMS m/z 314.1293 [MþH]þ (calcd for C20H16N3O,
314.1288).
and the reaction mixture and allowed to stir under N2 for 30 min.
The reaction mixture was poured over 1 M KOH solution (50 mL)
and extracted with CH2Cl2 (3ꢃ50 mL). The combined organic
fractions were dried (MgSO4), concentrated in vacuo and purified
by silica gel column chromatography (CH2Cl2) to afford 21 as
a
yellow solid (2.0 mg, 11% yield). Mp 259e260 ꢀC (lit.17
3.1.13. 2-(5-Methoxy-1H-indol-3-yl)ethanol (25). To a solution of 5-
methoxyindole (24) (1.00 g, 6.79 mmol) in ether (20 mL) at 0 ꢀC was
added oxalyl chloride (690 mL, 8.15 mmol) in a dropwise manner
251e252 ꢀC); Rf (2% MeOH/CH2Cl2) 0.68; IR nmax (ATR) 3356,
3336, 1704, 1422, 743 cmꢁ1; 1H NMR (DMSO-d6, 400 MHz)
12.43
d
(1H, br s, NH-9), 12.25 (1H, br s, NH-10), 9.66 (1H, d, J¼2.0 Hz, H-20),
9.15 (1H, s, H-4), 8.62e8.58 (1H, m, H-40), 8.48 (1H, d, J¼7.9 Hz, H-
5), 7.90 (1H, d, J¼7.9 Hz, H-8), 7.65 (1H, ddd, J¼7.9, 7.9, 1.0 Hz, H-7),
7.61e7.56 (1H, m, H-70), 7.36 (1H, dd, J¼7.9, 7.9 Hz, H-6), 7.33e7.27
over 5 min. The reaction mixture was allowed to stir at 0 ꢀC for 1 h
and then allowed to warm to rt in another 1 h with stirring. The
orange precipitate was filtered and washed with cold diethyl ether
(100 mL). The product was dried in vacuo to yield a fine orange
Please cite this article in press as: Liew, L. P.P.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.05.068

8
L.P.P. Liew et al. / Tetrahedron xxx (2014) 1e11
powder (1.40 g, 87% yield), which was used in the next step without
further purification.
OMe-17), 2.99 (2H, td, J¼7.2, 0.8 Hz, H2-8), 2.06 (3H, s, H3-12), 1.66
(9H, s, 3H3-16); 13C NMR (CDCl3, 100 MHz)
171.1 (C-11), 156.0 (C-
d
To lithium aluminium hydride (997 mg, 26.28 mmol) in a round
bottom flask under N2 cooled to 0 ꢀC was slowly added THF (30 mL)
with gentle stirring. 2-(5-Methoxy-1H-indol-3-yl)-2-oxoacetyl
chloride (1.39 g, 5.84 mmol) in THF (30 mL) was added dropwise
to the reaction mixture at 0 ꢀC. The reaction mixture was then
heated to reflux for 5 h. The reaction mixture was cooled to 0 ꢀC
before slow addition of EtOAc to quench any remaining lithium
aluminium hydride. When fizzing had ceased, a solution of sodium
potassium tartrate tetrahydrate (9.00 g, 31.90 mmol) in H2O
(100 mL) was added to the reaction mixture. EtOAc (100 mL) was
added to extract the organic material and the mixture was allowed
to stir slowly at rt until a clear layer (EtOAc) appeared on top. The
EtOAc layer was washed with H2O (3ꢃ100 mL), dried (MgSO4) and
concentrated in vacuo to give a green oil. The crude reaction
product was purified by silica gel column chromatography (30%
EtOAc/n-hexanes) to afford 25 as a pale yellow oil (970 mg, 87%
yield). Rf (50% EtOAc/n-hexanes) 0.37; IR nmax (ATR) 3406, 3330,
5), 149.8 (C-13), 131.4 (C-3a), 130.2 (C-7a), 124.1 (C-2), 116.6 (C-3),
116.1 (C-7), 113.1 (C-6), 101.8 (C-4), 83.5 (C-15), 63.8 (C-9), 55.8 (C-
17), 28.3 (C-16), 24.7 (C-8), 21.1 (C-12); (þ)-HRESIMS m/z 356.1471
[MþNa]þ (calcd for C18H23NNaO5, 356.1468).
3.1.16. tert-Butyl 3-(2-hydroxyethyl)-5-methoxy-1H-indole-1-
carboxylate (28). To a solution of tert-butyl 3-(2-acetoxyethyl)-5-
methoxy-1H-indole-1-carboxylate (27) (1.40 g, 4.19 mmol) dis-
solved in MeOH (40 mL) was added K2CO3 (11.58 g, 83.80 mmol) in
H2O (40 mL). The concentration of K2CO3 in solution was approxi-
mately 1.0 M. The reaction mixture was allowed to stir at
rt overnight (16 h). MeOH was removed under reduced pressure
and the remaining H2O fraction was extracted CH2Cl2 (3ꢃ100 mL).
The CH2Cl2 fractions were combined, dried (MgSO4) and concen-
trated in vacuo to give 28 as a colourless oil (1.02 g, 84% yield). Rf
(CH2Cl2) 0.10; IR nmax (ATR) 3557, 2920, 1699, 1480, 761 cmꢁ1
NMR (CDCl3, 400 MHz)
;
1H
d
8.01 (1H, br d, J¼7.8 Hz, H-7), 7.44 (1H, br
2937, 1484, 1211, 1027, 793 cmꢁ1; 1H NMR (CDCl3, 400 MHz)
d
8.08
s, H-2), 6.98 (1H, d, J¼2.6 Hz, H-4), 6.93 (1H, dd, J¼7.8, 2.6 Hz, H-6),
3.92 (2H, td, J¼6.3, 6.0 Hz, H2-9), 3.86 (3H, s, OMe-14), 2.93 (2H, td,
J¼6.3, 0.8 Hz, H2-8), 1.65 (9H, s, H3-13), 1.56 (1H, t, J¼6.0 Hz, OH-9);
(1H, br s, NH-1), 7.21 (1H, d, J¼8.8 Hz, H-7), 7.04 (1H, d, J¼2.4 Hz, H-
4), 6.99 (1H, d, J¼2.4 Hz, H-2), 6.86 (1H, dd, J¼8.8, 2.4 Hz, H-6), 3.88
(2H, td, J¼6.1, 5.1 Hz, H2-9), 3.84 (3H, s, OMe-10), 2.98 (2H, t,
J¼6.1 Hz, H2-8), 1.77 (1H, br t, J¼5.1 Hz, OH-9); 13C NMR (CDCl3,
13C NMR (CDCl3, 100 MHz)
d 156.0 (C-5), 149.8 (C-10), 131.4 (C-3a),
130.5 (C-7a), 124.4 (C-2), 116.9 (C-3), 116.2 (C-7), 113.1 (C-6), 102.0
(C-4), 83.5 (C-12), 62.1 (C-9), 55.9 (C-14), 28.6 (C-8), 28.4 (C-13);
(þ)-HRESIMS m/z 292.1537 [MþH]þ (calcd for C16H22NO4,
292.1543).
100 MHz) d 154.1 (C-5), 131.7 (C-7a), 127.9 (C-3a), 123.5 (C-2), 112.4
(C-6), 112.1 (C-7), 112.0 (C-3), 100.8 (C-4), 62.7 (C-9), 56.1 (C-10),
28.8 (C-8); (þ)-HRESIMS m/z 192.1019 [MþH]þ (calcd for
C
11H14NO2, 192.1019). The 1H NMR data were in agreement with
literature values.41
3.1.17. 1-(1H-tert-Butyloxycarbonyl-indol-3-yl)methyl-1,2,3,4-
tetrahydro-
b-carboline (29)and 1-(1,3-bis(1H-tert-butyloxycarbonyl-
3.1.14. 2-(5-Methoxy-1H-indol-3-yl)ethyl acetate (26). To a solution
of 2-(5-methoxy-1H-indol-3-yl)ethanol (25) (970 mg, 5.07 mmol)
dissolved in pyridine (10 mL) was added acetic anhydride (956 mL,
indol-3-yl)prop-1-ene)-1,2,3,4-tetrahydro-
b
-carboline (30). tert-Bu-
tyl 3-(2-oxoethyl)-1H-indole-1-carboxylate (16) (130 mg,
0.50 mmol) and tryptamine (120 mg, 0.75 mmol) were stirred in
toluene (20 mL) and heated to reflux for 3 h. Toluene was removed
10.14 mmol) in a dropwise manner. The reaction mixture was
purged with N2 and allowed to stir at rt overnight (16 h). The re-
action mixture was poured into H2O (50 mL) and extracted with
CH2Cl2 (50 mL). The organic fraction was washed with H2O
(2ꢃ50 mL), dried (MgSO4) and concentrated in vacuo. The
remaining pyridine was removed under vacuum to give a pale
green crude oil. The product was purified by silica gel column
chromatography (20% EtOAc/n-hexanes) to afford 26 as white
crystals (980 mg, 83% yield). Mp 60 ꢀC; Rf (50% EtOAc/n-hexanes)
and the residue was dissolved in CH2Cl2 (30 mL). TFA (186
2.51 mmol) was added and the resulting mixture was allowed to
stir at rt overnight (13 h). TEA (486 L, 3.51 mmol) was slowly
mL,
m
added to the reaction. The reaction mixture was poured into H2O
(100 mL) and extracted with additional CH2Cl2 (60 mL). The organic
fraction was washed once more with H2O (100 mL). The organic
layer was dried (MgSO4) and concentrated in vacuo. The crude
product was purified by silica gel column chromatography eluting
in 2% MeOH/CH2Cl2. The first product 30 was recrystallised from
MeOH as a white solid (48 mg, 15% yield). The mother liquor was
concentrated and purified by silica gel column chromatography (2%
MeOH/CH2Cl2), which gave a second product 29 as a yellow-orange
oil (11 mg, 6% yield).
Alternatively, using the same procedure but substituting CH2Cl2
as the solvent, the reaction of tert-butyl 3-(2-oxoethyl)-1H-indole-
1-carboxylate (16) (70 mg, 0.27 mmol) and tryptamine (48 mg,
0.30 mmol) afforded 29 as a yellow-orange oil (50 mg, 46% yield)
and 30 as a white solid (34 mg, 39% yield).
;
0.36; IR nmax (ATR) 3361, 3002, 1724, 1488 cmꢁ1 1H NMR (CDCl3,
400 MHz)
d
8.05 (1H, br s, NH-1), 7.21 (1H, d, J¼8.8 Hz, H-7), 7.07
(1H, d, J¼2.4 Hz, H-4), 6.98 (1H, d, J¼2.4 Hz, H-2), 6.85 (1H, dd,
J¼8.8, 2.4 Hz, H-6), 4.34 (2H, t, J¼7.2 Hz, H2-9), 3.86 (3H, s, OMe-13),
3.05 (2H, td, J¼7.2, 0.8 Hz, H2-8), 2.05 (3H, s, H3-12); 13C NMR
(CDCl3, 100 MHz)
d
171.3 (C-11), 154.1 (C-5), 131.5 (C-7a), 127.9 (C-
3a), 123.0 (C-2), 112.4 (C-6), 112.0 (C-7), 111.7 (C-3), 100.7 (C-4), 64.7
(C-9), 56.0 (C-13), 24.9 (C-8), 21.2 (C-12); (þ)-HRESIMS m/z
234.1120 [MþH]þ (calcd for C13H16NO3, 234.1125).
3.1.15. tert-Butyl 3-(2-acetoxyethyl)-5-methoxy-1H-indole-1-
carboxylate (27). 2-(5-Methoxy-1H-indol-3-yl)ethyl acetate (26)
(918 mg, 3.94 mmol), di-tert-butyl dicarbonate (1.29 g, 5.90 mmol)
and DMAP (48 mg, 0.39 mmol) were dissolved in CH2Cl2 (30 mL)
and allowed to stir at rt under N2 overnight (19 h). The reaction
mixture was poured over H2O (50 mL) and extracted with CH2Cl2
(3ꢃ50 mL). The organic fractions were combined, dried (MgSO4)
and concentrated in vacuo to give the crude material as a clear
orange oil. The product was purified by silica gel column chroma-
tography (CH2Cl2) to give 27 as a colourless oil (1.43 g, quantitative
yield). Rf (CH2Cl2) 0.68; IR nmax (ATR) 2958, 1733, 1716, 1367, 1247,
Compound 29: Rf (10% MeOH/CH2Cl2) 0.54; IR nmax (ATR) 2931,
1728, 1451, 1368, 1152, 738 cmꢁ1; 1H NMR (CDCl3, 400 MHz)
d 8.19
(1H, br d, J¼7.6 Hz, H-70), 7.86 (1H, br s, NH-9), 7.56 (1H, d, J¼7.8 Hz,
H-40), 7.50e7.47 (2H, m, H-5 and H-20), 7.35 (1H, ddd, J¼7.6, 7.6,
1.1 Hz, H-60), 7.26e7.22 (2H, m, H-8 and H-50), 7.14e7.06 (2H, m, H-6
and H-7), 4.49e4.46 (1H, m, H-1), 3.33 (1H, dt, J¼12.0, 4.4 Hz, H2-
3A), 3.23 (1H, dd, J¼14.4, 6.0 Hz, H2-120A), 3.09e2.99 (2H, m, H2-3B
and H2-120B), 2.84e2.70 (2H, m, H2-4), 1.64 (9H, s, H3-110); 13C NMR
(CDCl3, 100 MHz) d
149.7 (C-80), 135.7 (C-70a and C-9a),135.5 (C-8a),
130.4 (C-30a), 127.4 (C-4b), 124.9 (C-60), 124.1 (C-20), 122.8 (C-50),
121.7 (C-7), 119.5 (C-6), 119.1 (C-40), 118.3 (C-5), 116.9 (C-30), 115.6
(C-70), 110.9 (C-8), 109.5 (C-4a), 83.9 (C-100), 52.4 (C-1), 42.6 (C-3),
31.1 (C-120), 28.3 (C-110), 22.6 (C-4); (þ)-HRESIMS m/z 402.2165
[MþH]þ (calcd for C25H28N3O2, 402.2176).
1160, 764 cmꢁ1 1H NMR (CDCl3, 400 MHz)
; d 8.00 (1H, br d,
J¼7.0 Hz, H-7), 7.41 (1H, br s, H-2), 7.01 (1H, d, J¼2.5 Hz, H-4), 6.93
(1H, dd, J¼7.0, 2.5 Hz, H-6), 4.34 (2H, t, J¼7.2 Hz, H2-9), 3.87 (3H, s,
Please cite this article in press as: Liew, L. P.P.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.05.068

L.P.P. Liew et al. / Tetrahedron xxx (2014) 1e11
9
Compound 30: mp 194e195 ꢀC; Rf (10% MeOH/CH2Cl2) 0.68; IR
nmax (ATR) 3152, 2984, 1740, 1721, 1453, 1366, 1152, 746 cmꢁ1 1H
NMR (CDCl3, 400 MHz)
8.18e8.13 (2H, m, H-70 and H-210), 7.70
(142 mg, 0.72 mmol). The mixture was heated at 130 ꢀC under
a nitrogen atmosphere for 3 h. The toluene was then removed in
vacuo and the crude material was re-dissolved in dry CH2Cl2
(10 mL) and TFA (0.296 mL, 3.98 mmol). This mixture was stirred
overnight under a nitrogen atmosphere at ambient temperature.
The reaction was then quenched with NEt3 (0.77 mL, 5.57 mmol)
and the mixture was diluted to 100 mL with CH2Cl2. The organic
layer was washed with water (2ꢃ100 mL), dried over MgSO4 and
the solvent removed in vacuo. Purification by silica gel column
chromatography (MeOH/CH2Cl2) gave the product 32 as a yellow oil
(87 mg, 30%). The product exhibited identical spectroscopic data to
those observed earlier.
;
d
(1H, br s, NH-9), 7.47 (2H, d, J¼7.4 Hz, H-5 and H-180), 7.37e7.31
(4H, m, H-40, H-60, H-160 and H-200), 7.26e7.19 (3H, m, H-8, H-50 and
H-190), 7.13 (1H, ddd, J¼7.3, 7.3,1.3 Hz, H-7), 7.08 (1H, ddd, J¼7.3, 7.3,
1.1 Hz, H-6), 6.88 (1H, br s, H-20), 6.16 (1H, t, J¼7.2 Hz, H-130), 4.96
(1H, s, H-1), 3.49e3.38 (2H, m, H2-120), 3.28 (1H, dt, J¼12.7, 4.1 Hz,
H2-3A), 3.03e2.97 (1H, m, H2-3B), 2.72e2.61 (2H, m, H2-4), 1.67
(9H, s, 3H3-110), 1.47 (9H, s, H3-250); 13C NMR (CDCl3, 100 MHz)
d
149.9 (C-80 or C-220), 149.5 (C-80 or C-220), 135.8 (C-70a), 135.7 (C-
8a),135.3 (C-210a),133.5 (C-9a),133.3 (C-140),131.7 (C-130),130.4 (C-
30a and C-170a), 127.8 (C-4b), 124.8 (C-200), 124.6 (C-40), 124.2 (C-20),
123.1 (C-190), 122.8 (C-160), 122.5 (C-50), 121.9 (C-7), 120.3 (C-180),
119.5 (C-6), 119.3 (C-30), 119.0 (C-60), 118.3 (C-5), 116.6 (C-170), 115.6
(C-70), 115.5 (C-210), 111.1 (C-8), 111.0 (C-4a), 83.8 (C-240), 83.7 (C-
100), 60.8 (C-1), 42.5 (C-3), 28.4 (C-110), 28.1 (C-250), 26.0 (C-120),
22.6 (C-4); (þ)-HRESIMS m/z 643.3256 [MþH]þ (calcd for
3.1.19. (E)-3-Benzyl-2-phenyl-propenal (33). Phenylacetaldehyde
(0.3 mL, 2.56 mmol) and ammonium acetate (198 mg, 2.56 mmol)
were refluxed in a solution of AcOH (1 mL), H2O (5 mL) and EtOAc
(1 mL) overnight under a nitrogen atmosphere. The solution was
washed with EtOAc (2ꢃ30 mL) and the organic layer dried (MgSO4)
and the solvent removed in vacuo. Purification by silica gel column
chromatography (CH2Cl2) gave the product as a yellow oil (248 mg,
44%). Rf (CH2Cl2) 0.94; IR (ATR) nmax 3059, 3027, 1688, 1494,
C
40H43N4O4, 643.3279).
3.1.18. 1H-1-Benzyl-1,2,3,4-tetrahydro-
(1,3-diphenylprop-1-en-1-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]in-
dole (32). To solution of phenylacetaldehyde (0.30 mL,
b
-carboline (31)25 and (E)-1-
695 cmꢁ1; 1H NMR (CDCl3, 400 MHz)
d 9.65 (1H, s, H-10), 7.46e7.42
a
(2H, m, H-13 and H-15), 7.39e7.38 (1H, m, H-14), 7.34e7.32 (2H, m,
H-3 and H-5), 7.29e7.27 (2H, m, H-12 and H-16), 7.24e7.22 (1H, m,
H-4), 7.17e7.15 (2H, m, H-2 and H-6), 6.86 (1H, t, J¼7.7 Hz, H-8),
2.56 mmol) in dry toluene (51.3 mL) was added tryptamine
(370 mg, 2.31 mmol) and tryptamine$HCl (454 mg, 2.31 mmol). The
mixture was then heated to reflux under a nitrogen atmosphere for
3 h. Toluene was removed in vacuo and the crude material was re-
dissolved in dry CH2Cl2 (50 mL) and TFA (0.95 mL, 12.82 mmol).
This was stirred overnight under a nitrogen atmosphere at ambient
temperature. The reaction was quenched with Et3N (5.49 mL,
18.0 mmol) and the mixture was diluted to 100 mL with CH2Cl2. The
organic layer was washed with H2O (2ꢃ100 mL), dried (MgSO4) and
the solvent removed in vacuo. Purification by silica gel column
chromatography (MeOH/CH2Cl2) gave 31 as a yellow oil (94 mg,
14%) and 32 as a yellow oil (206 mg, 22%).
3.69 (2H, d, J¼7.7 Hz, H2-7); 13C NMR (CDCl3, 100 MHz)
d 193.6 (C-
10), 153.5 (C-8), 144.2 (C-9), 138.2 (C-1), 132.3 (C-11), 129.6 (C-12
and C-16),129.0 (C-2 and C-6),128.6 (C-13 and C-15),128.5 (C-3 and
C-5), 128.3 (C-14), 126.9 (C-4), 35.9 (C-7); (þ)-HRESIMS [MþNa]þ
245.0934 (calcd for C16H14ONa, 245.0937). The 1H NMR data agreed
with literature values.26
3.1.20. 1-(1H-Indol-3-yl)-3-methylcarboxylate-1,2,3,4-tetrahydro-
b-
carboline (37). -Tryptophan methyl ester$HCl (14) (100 mg,
L
0.39 mmol) and indole-3-carboxaldehyde (57 mg, 0.39 mmol) were
dissolved in MeOH (10 mL) and heated to reflux under N2 for 72 h.
MeOH was removed and the crude material suspended in CH2Cl2
(10 mL) with stirring under N2. TFA (1 mL) was added to the re-
action mixture and it was allowed to stir under N2 at rt for 23 h. The
reaction mixture was diluted with EtOAc (20 mL) and carefully
added to saturated NaHCO3 solution (100 mL). The aqueous layer
was extracted with EtOAc (3ꢃ100 mL), the organic solvent layers
combined, dried (MgSO4) and concentrated in vacuo. The crude
reaction product was purified by silica gel column chromatography
(1% MeOH/CH2Cl2) to afford [as a mixture of stereoisomers (1:0.7)]
37 as a yellow-orange oil (63 mg, 47% yield). Rf (5% MeOH/CH2Cl2)
0.45; IR nmax (ATR) 3395, 3056, 1725, 1454, 1243, 739 cmꢁ1; Major
Compound 31: Rf (10% MeOH/CH2Cl2) 0.58; IR (ATR) nmax 2941,
1671, 1447, 1187, 1129, 742, 720, 699 cmꢁ1 1H NMR (CDCl3,
;
400 MHz)
d
7.69 (1H, s, NH-9), 7.43 (1H, d, J¼7.6 Hz, H-5), 7.33e7.30
(3H, m, H-30, H-40 and H-50), 7.25e7.22 (2H, m, H-20 and 60),
7.18e7.15 (1H, m, H-8), 7.15e7.11 (1H, m, H-7), 7.10e7.06 (1H, m, H-
6), 4.64 (1H, t, J¼7.6 Hz, H-1), 3.36 (1H, dt, J¼12.6, 5.0 Hz, H2-3A),
3.28 (1H, dd, J¼13.6, 6.3 Hz, H2-70A), 3.16e3.09 (2H, m, H2-3B and
H2-70B), 2.93e2.85 (1H, m, H2-4A), 2.82e2.75 (1H, m, H2-4B); 13C
NMR (CDCl3, 100 MHz)
d
136.5 (C-10), 135.9 (C-8a), 132.0 (C-9a),
129.6 (C-30 and C-50),129.2 (C-20 and C-60),127.6 (C-40),126.6 (C-5a),
122.4 (C-7),119.8 (C-6), 118.4 (C-5),111.2 (C-8),108.4 (C-4a), 53.8 (C-
1), 41.7 (C-3) 40.2 (C-70), 20.6 (C-4); (þ)-HRESIMS [MþH]þ
263.1542 (calcd for C18H19N2, 263.1543). The 1H NMR data were
consistent with literature values.25
stereoisomer: 1H NMR (CDCl3, 300 MHz)
d
8.29 (1H, br s, NH-10),
7.69 (1H, br s, NH-9), 7.59e7.53 (1H, m, H-5), 7.31e7.27 (2H, m, H-40
and H-70), 7.19e7.01 (5H, m, H-6, H-7, H-8, H-20 and H-60),
6.99e6.94 (1H, m, H-50), 5.51 (1H, s, H-1), 4.01 (1H, dd, J¼11.1,
4.3 Hz, H-3), 3.77 (3H, s, OMe-11), 3.24e3.22 (1H, m, H2-4A),
Compound 32: Rf (10% MeOH/CH2Cl2) 0.66; IR (ATR) nmax 2921,
1493, 1446, 737, 699 cmꢁ1; 1H NMR (CDCl3, 400 MHz)
d 7.73 (1H, br
s, NH-9), 7.48 (1H, d, J¼7.5 Hz, H-5), 7.31e7.26 (6H, m, H-8, H-30, H-
50, H-110, H-130, H-150), 7.22e7.18 (1H, m, H-40), 7.16e7.15 (1H, m, H-
7), 7.12e7.11 (2H, m, H-20 and H-60), 7.11e7.09 (1H, m, H-6),
7.08e7.05 (2H, m, H-120 and H-140), 5.95 (1H, t, J¼7.6 Hz, H-80), 4.92
(1H, s, H-1), 3.37e3.35 (2H, m, H2-70), 3.28e3.23 (1H, m, H2-3A),
3.03e2.96 (1H, m, H2-3B), 2.68e2.64 (2H, m, H2-4); 13C NMR
3.15e2.99 (1H, m, H2-4B); 13C NMR (CDCl3, 75 MHz)
d 173.6 (C-10),
136.6 (C-70a), 136.0 (C-8a), 135.4 (C-9a), 127.4 (C-4b), 126.1 (C-30a),
124.1 (C-20), 122.6 (C-60), 121.8 (C-7), 120.3 (C-50), 119.6 (C-6), 119.4
(C-40), 118.2 (C-5), 115.0 (C-30), 111.6 (C-70), 111.2 (C-8), 108.0 (C-4a),
57.3 (C-3), 52.3 (C-11), 50.9 (C-1), 26.0 (C-4); Minor stereoisomer:
(CDCl3, 100 MHz)
d
141.5 (C-100), 140.8 (C-10), 137.7 (C-90), 135.8 (C-
1H NMR (CDCl3, 300 MHz) 8.23 (1H, br s, NH-10), 7.72 (1H, br s,
d
8a), 133.9 (C-9a), 130.1 (C-80), 129.1 (C-20 and C-60), 128.7 (C-30 and
C-50), 128.6 (C-120 and C-140), 128.4 (C-110 and C-150), 127.8 (C-4b),
127.7 (C-40), 126.3 (C-130), 121.8 (C-7), 119.5 (C-6), 118.3 (C-5), 111.0
(C-8), 110.6 (C-4a), 60.2 (C-1), 42.3 (C-3), 35.2 (C-70), 22.6 (C-4);
(þ)-HRESIMS [MþH]þ 365.1999 (calcd for C26H25N2, 365.2012).
Alternative route to 32: To a solution of (E)-3-benzyl-2-phenyl-
propenal (33) (177 mg, 0.80 mmol) in dry toluene (15.9 mL) was
added tryptamine (115 mg, 0.72 mmol) and tryptamine$HCl
NH-9), 7.59e7.53 (1H, m, H-5), 7.48 (1H, d, J¼8.0 Hz, H-40),
7.31e7.27 (1H, m, H-70), 7.19e7.01 (5H, m, H-6, H-7, H-8, H-50 and H-
60), 6.74 (1H, d, J¼2.4 Hz, H-20), 5.71 (1H, s, H-1), 3.95 (1H, dd, J¼7.5,
5.2 Hz, H-3), 3.67 (3H, s, OMe-11), 3.29e3.27 (1H, m, H2-4A),
3.15e2.99 (1H, m, H2-4B); 13C NMR (CDCl3, 75 MHz)
d 174.3 (C-10),
136.7 (C-70a), 136.1 (C-8a), 134.1 (C-9a), 127.2 (C-4b), 126.3 (C-30a),
123.9 (C-20), 122.6 (C-60), 121.8 (C-7), 120.3 (C-50), 119.5 (C-6), 119.1
(C-40), 118.3 (C-5), 116.8 (C-30), 111.5 (C-70), 111.1 (C-8), 107.9 (C-4a),
Please cite this article in press as: Liew, L. P.P.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.05.068

10
L.P.P. Liew et al. / Tetrahedron xxx (2014) 1e11
53.0 (C-3), 52.2 (C-11), 47.5 (C-1), 24.9 (C-4); (þ)-HRESIMS m/z
346.1546 [MþH]þ (calcd for C21H20N3O2, 346.1550).
(1H, dd, J¼7.8, 7.8 Hz, H-6), 4.64 (1H, dd, J¼12.1, 6.9 Hz, H-3), 3.75
(3H, s, H3-11), 3.27 (1H, dd, J¼16.2, 6.9 Hz, H2-4A), 3.09 (1H, dd,
J¼16.2, 12.1 Hz, H2-4B); 13C NMR (DMSO-d6, 100 MHz)
d 173.3 (C-
3.1.21. 1-(5-Methoxy-1H-indol-3-yl)-6-methoxy-1,2,3,4-tetrahydro-
10), 154.7 (C-1), 137.7 (C-8a), 136.7 (C-70a), 130.5 (C-20), 127.8 (C-9a),
125.8 (C-30a), 125.0 (C-4b), 124.3 (C-7), 122.7 (C-60), 122.3 (C-40),
120.7 (C-50), 120.0 (C-6), 119.7 (C-5), 115.7 (C-4a), 113.0 (C-8), 111.9
(C-70), 111.4 (C-30), 59.5 (C-3), 52.1 (C-11), 21.9 (C-4); (þ)-HRESIMS
m/z 344.1386 [MþH]þ (calcd for C21H18N3O2, 344.1394).
b-carboline
(38). 5-Methoxy-1H-indole-3-carbaldehyde
(35)
(155 mg, 0.88 mmol) and 5-methoxytryptamine$HCl (200 mg,
0.88 mmol) were heated at reflux in MeOH (10 mL) overnight (24 h)
under N2. The MeOH was then removed under reduced pressure to
give a pale yellow oil. The crude material was suspended in CH2Cl2
(10 mL), TFA (1 mL) added and the solution stirred at rt under N2
overnight (19 h). The reaction mixture was diluted with EtOAc
(10 mL) and added slowly to a saturated solution of NaHCO3
(50 mL). EtOAc (30 mL) was added to extract the product. The
aqueous layer was extracted with EtOAc (2ꢃ50 mL). The organic
layers were combined, dried (MgSO4) and solvent removed in
vacuo. The crude reaction product was purified by silica gel column
chromatography (5% MeOH/CH2Cl2) to afford 38 as a yellow-orange
solid (224 mg, 73% yield). Mp 145e146 ꢀC; Rf (5% MeOH/CH2Cl2)
3.1.23. Eudistomin U 3-carboxylic acid (41).42 To a solution of 1-
(1H-indol-3-yl)-3-methylcarboxylate-b-carboline (40) (13 mg,
0.04 mmol) in THF/H2O (3:1) (4 mL) was added LiOH (16 mg,
0.38 mmol). The reaction mixture was heated to reflux under N2 for
13 h. The pH of the cooled reaction mixture was adjusted with 1 M
HCl solution (a drop) and a yellow precipitate formed. The yellow
solid was filtered, washed successively with H2O (5 mL), MeOH
(3 mL) and ether (5 mL) in 1 mL portions, then dried in vacuo to
afford 41 as a yellow solid (12 mg, 96% yield). Mp 299 ꢀC (lit.42
230 ꢀC decomp.); Rf (10% MeOH/CH2Cl2) 0.52; IR nmax (ATR) 3273,
0.44; IR nmax (ATR) 2832, 1673, 1203 cmꢁ1
;
1H NMR (CDCl3,
400 MHz)
d
8.91 (1H, br s, NH-10), 8.00 (1H, s, NH-9), 7.06 (1H, d,
2919, 1583, 1366, 733 cmꢁ1 1H NMR (DMSO-d6, 400 MHz)
; d 11.87
J¼8.8 Hz, H-70), 6.98 (1H, d, J¼8.8 Hz, H-8), 6.93 (1H, d, J¼2.4 Hz, H-
5), 6.78 (1H, s, H-20), 6.75 (1H, dd, J¼8.8, 2.4 Hz, H-7), 6.69 (1H, dd,
J¼8.8, 2.2 Hz, H-60), 6.47 (1H, d, J¼2.2 Hz, H-40), 5.34 (1H, s, H-1),
3.83 (3H, s, OMe-10), 3.44 (3H, s, OMe-80), 3.23 (1H, dt, J¼12.4,
4.5 Hz, H2-3A), 3.09e3.02 (1H, m, H2-3B), 2.91e2.83 (1H, m, H2-4A),
(1H, br s, NH-10), 11.70 (1H, br s, NH-9), 8.74e8.72 (2H, m, H-4 and
H-40), 8.45 (1H, s, H-20), 8.36 (1H, d, J¼7.8 Hz, H-5), 7.77 (1H, d,
J¼7.8 Hz, H-8), 7.58 (1H, dd, J¼7.8, 7.8 Hz, H-7), 7.54 (1H, d, J¼7.5 Hz,
H-70), 7.31 (1H, dd, J¼7.8, 7.8 Hz, H-6), 7.23 (1H, dd, J¼7.5, 7.5 Hz, H-
60), 7.16 (1H, dd, J¼7.5, 7.5 Hz, H-50); 13C NMR (DMSO-d6, 100 MHz)
2.78e2.74 (1H, m, H2-4B); 13C NMR (CDCl3, 100 MHz)
d
154.2 (C-6
d
167.7 (C-10), 141.3 (C-8a), 139.5 (C-3), 138.2 (C-1), 136.6 (C-70a),
and C-50), 133.3 (C-9a),131.5 (C-70a),131.2 (C-8a),127.4 (C-4b),126.4
(C-30a), 125.9 (C-20), 112.7 (C-60), 112.5 (C-70), 112.2 (C-30), 111.9 (C-7
and C-8), 108.5 (C-4a), 100.6 (C-5), 100.4 (C-40), 56.1 (C-10), 55.6 (C-
80), 50.1 (C-1), 42.4 (C-3), 21.1 (C-4); (þ)-HRESIMS m/z 348.1703
[MþH]þ (calcd for C21H22N3O2, 348.1707).
133.3 (C-9a), 128.5 (C-4a), 128.1 (C-7), 126.6 (C-20), 126.2 (C-30a),
122.6 (C-40), 122.2 (C-60), 121.6 (C-5), 121.5 (C-4b), 120.2 (C-6), 120.1
(C-50), 113.9 (C-4), 112.9 (C-8), 112.7 (C-30), 111.6 (C-70); (þ)-HRE-
SIMS m/z 328.1081 [MþH]þ (calcd for C20H14N3O2, 328.1081). The
spectroscopic data were consistent with literature values.42
3.1.22. 1-(1H-Indol-3-yl)-3-methylcarboxylate-
and 1-(1H-indol-3-yl)-3-methylcarboxylate-3,4-dihydro-
(44). The stereoisomeric mixture of 1-(1H-indol-3-yl)-3-
methylcarboxylate-1,2,3,4-tetrahydro- -carboline (37) (30 mg,
b
-carboline (40)42
-carboline
3.1.24. 50,6-Dimethoxy eudistomin U (42). To a solution of 1-(5-
methoxy-1H-indol-3-yl)-6-methoxy-1,2,3,4-tetrahydro-b-carbo-
b
line (38) (8.0 mg, 0.02 mmol) in THF (1 mL) was added DDQ (26 mg,
0.12 mmol). The reaction mixture was allowed to stir at rt under N2
for 3 h. The reaction mixture was poured into 1 M KOH solution
(50 mL) and extracted with CH2Cl2 (2ꢃ25 mL). The organic fractions
were combined, dried (MgSO4) and solvent removed in vacuo. The
crude reaction product was purified by silica gel flash column
chromatography (2% MeOH/CH2Cl2) to afford 42 as an orange oil
(2.0 mg, 25% yield). Rf (5% MeOH/CH2Cl2) 0.53; IR nmax (ATR) 2925,
b
0.09 mmol) was dissolved in CH2Cl2 (10 mL), and DDQ (79 mg,
0.35 mmol) added. The reaction mixture was purged with N2 and
allowed to stir at rt for 50 min. The reaction mixture was poured
into 1 M KOH solution (50 mL) and extracted with CH2Cl2
(3ꢃ50 mL). The organic solvent fractions were combined, dried
(MgSO4) and the solvent removed in vacuo. The crude reaction
product was purified by silica gel column chromatography (2e10%
MeOH/CH2Cl2) to afford 40 as an orange solid (7.0 mg, 24% yield)
and 44 as a bright yellow oil (12 mg, 40% yield).
1491, 1214 cmꢁ1; 1H NMR (CDCl3, 400 MHz) 9.27 (1H, br s, NH-10),
d
8.51 (1H, d, J¼5.3 Hz, H-3), 8.46 (1H, br s, NH-9), 7.83 (1H, d,
J¼5.3 Hz, H-4), 7.58 (1H, d, J¼2.4 Hz, H-5), 7.50 (1H, s, H-20), 7.48
(1H, d, J¼2.3 Hz, H-40), 7.37 (1H, d, J¼8.8 Hz, H-8), 7.25 (1H, d,
J¼8.0 Hz, H-70), 7.18 (1H, dd, J¼8.8, 2.4 Hz, H-7), 6.89 (1H, dd, J¼8.0,
2.3 Hz, H-60), 3.95 (3H, s, OMe-10), 3.77 (3H, s, OMe-80); 13C NMR
Compound 40: mp 259e260 ꢀC (lit.42 202 ꢀC); Rf (5% MeOH/
CH2Cl2) 0.40; IR nmax (ATR) 3432, 3177, 2921, 1716, 1430, 735 cmꢁ1
;
1H NMR (DMSO-d6, 400 MHz) 11.80 (1H, br s, NH-10), 11.69 (1H, br
d
s, NH-9), 8.90 (1H, d, J¼7.6 Hz, H-40), 8.77 (1H, s, H-4), 8.46 (1H, s, H-
20), 8.39 (1H, d, J¼7.8 Hz, H-5), 7.77 (1H, d, J¼7.8 Hz, H-8), 7.60 (1H,
dd, J¼7.8, 7.8 Hz, H-7), 7.54 (1H, d, J¼7.6 Hz, H-70), 7.33 (1H, dd,
J¼7.8, 7.8 Hz, H-6), 7.25 (1H, dd, J¼7.6, 7.6 Hz, H-60), 7.19 (1H, dd,
J¼7.6, 7.6 Hz, H-50), 3.99 (3H, s, OMe-11); 13C NMR (DMSO-d6,
(CDCl3, 100 MHz) d
154.9 (C-50), 154.3 (C-6), 139.7 (C-1), 138.3 (C-3),
135.2 (C-8a), 134.3 (C-9a), 131.7 (C-70a), 129.0 (C-4a), 126.3 (C-30a),
125.1 (C-20), 122.4 (C-4b), 118.3 (C-7), 114.3 (C-30), 113.2 (C-60), 112.5
(C-8 and C-70), 112.4 (C-4), 103.5 (C-5), 102.2 (C-40), 56.0 (C-10), 55.9
(C-80); (þ)-HRESIMS m/z 344.1379 [MþH]þ (calcd for C21H18N3O2,
344.1394).
100 MHz)
d
166.3 (C-10), 141.1 (C-8a), 140.0 (C-3), 136.5 (C-70a),
136.2 (C-1),133.2 (C-9a),128.2 (C-4a and C-7),126.5 (C-20),126.3 (C-
30a), 122.9 (C-40), 122.3 (C-60), 121.7 (C-5), 121.5 (C-4b), 120.3 (C-6),
120.2 (C-50), 114.3 (C-4), 112.8 (C-8), 112.7 (C-30), 111.5 (C-70), 52.0
(C-11); (þ)-HRESIMS m/z 342.1230 [MþH]þ (calcd for C21H16N3O2,
342.1237). The spectroscopic data were consistent with literature
values.42
3.1.25. 50,6-Dihydroxy eudistomin U (43). To a solution of 50,6-
dimethoxy eudistomin U (42) (7.0 mg, 0.02 mmol) in CH2Cl2
(4 mL) cooled to ꢁ78 ꢀC was added BBr3 (58
mL, 0.61 mmol). The
reaction mixture was allowed to stir under N2 at ꢁ78 ꢀC for 2 h and
then at rt for another 2 h. The reaction mixture was quenched by
slow addition of water (40 mL) at 0 ꢀC and extracted with EtOAc
(5ꢃ30 mL). The organic solvent fractions were combined, dried
(MgSO4) and the solvent removed in vacuo. The crude reaction
product was purified by silica gel column chromatography (EtOAc)
to afford 43 as a bright yellow solid (2.0 mg, 31% yield). Mp 252 ꢀC;
Compound 44: Rf (10% MeOH/CH2Cl2) 0.74; IR nmax (ATR) 3386,
3179, 3059, 2952, 1727, 1435, 742 cmꢁ1 1H NMR (DMSO-d6,
;
400 MHz) d
11.91 (1H, br s, NH-10), 11.43 (1H, br s, NH-9), 8.37 (1H,
d, J¼7.8 Hz, H-40), 8.14 (1H, s, H-20), 7.69 (1H, d, J¼7.8 Hz, H-5), 7.52
(2H, d, J¼7.8 Hz, H-8 and H-70), 7.27 (1H, dd, J¼7.8, 7.8 Hz, H-7), 7.24
(1H, dd, J¼7.8, 7.8 Hz, H-60), 7.17 (1H, dd, J¼7.8, 7.8 Hz, H-50), 7.11
Rf (6% MeOH/EtOAc) 0.33; IR nmax (ATR) 3215, 1195, 801 cmꢁ1 1H
;
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L.P.P. Liew et al. / Tetrahedron xxx (2014) 1e11
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Please cite this article in press as: Liew, L. P.P.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.05.068