Natural tetraponerines: A general synthesis and antiproliferative activity

Article abstract of DOI:10.1021/jo500446f

A stereocontrolled general methodology to access all natural tetraponerines from (+)-T1 to (+)-T8 is detailed. Two consecutive indium-mediated aminoallylations with the appropriate enantiomer of chiral N-tert- butylsulfinamide and a thermodynamic control at the aminal stereocenter allow the formation of each natural tetraponerine with excellent stereoselectivity. The use of 4-bromobutanal in the first aminoallylation leads to the formation of 5-6-5 tetraponerines, while 5-bromopentanal is required to build the scaffold of 6-6-5 tetraponerines. A cross-metathesis reaction of the second aminoallylation product with cis-3-hexene is used to elongate the side chain up to 5 carbons so as to prepare the tetraponerines T5 to T8. The anticancer activity of these heavier tetraponerines against four different carcinoma human cell lines is examined, observing a promising cytotoxic activity of (+)-T7 against breast carcinoma cell line MCF-7.

Full text of DOI:10.1021/jo500446f

Article
pubs.acs.org/joc
Natural Tetraponerines: A General Synthesis and Antiproliferative
Activity
Irene Bosque,† Jose C. Gonzalez-Gomez,*,† María Isabel Loza,‡ and Jose Brea‡
́
†Departamento de Química Organ
99, 03080 Alicante, Spain
‡Grupo de Investigacion
Biofarma, USEF Screening Platform, Centro de Investigacion
Cronicas (CIMUS), Universidad de Santiago de Compostela, Avda de Barcelona s/n, Santiago de Compostela, Spain
́ ́
ica, Facultad de Ciencias and Instituto de Síntesis Organica (ISO), Universidad de Alicante, Apdo.
́
́
en Medicina Molecular y Enfermedades
́
S
* Supporting Information
ABSTRACT: A stereocontrolled general methodology to access all natural tetraponerines from (+)-T1 to (+)-T8 is detailed.
Two consecutive indium-mediated aminoallylations with the appropriate enantiomer of chiral N-tert-butylsulfinamide and a
thermodynamic control at the aminal stereocenter allow the formation of each natural tetraponerine with excellent
stereoselectivity. The use of 4-bromobutanal in the first aminoallylation leads to the formation of 5−6−5 tetraponerines, while 5-
bromopentanal is required to build the scaffold of 6−6−5 tetraponerines. A cross-metathesis reaction of the second
aminoallylation product with cis-3-hexene is used to elongate the side chain up to 5 carbons so as to prepare the tetraponerines
T5 to T8. The anticancer activity of these heavier tetraponerines against four different carcinoma human cell lines is examined,
observing a promising cytotoxic activity of (+)-T7 against breast carcinoma cell line MCF-7.
INTRODUCTION
■
Azaheterocycles with nitrogen atoms at the junction of two
rings constitute a large group of natural alkaloids that
commonly exhibit a quinolizidine or an indolizidine framework
and display a wide range of biological activities.1 The
complexity of these compounds is further increased by the
stereochemistry of the pyramidal nitrogen at the ring junction,
with important consequences in their bioactivities.2 Tetrapo-
nerine alkaloids form a unique family of such compounds,
which share a quinolizidine or an indolizidine fragment with
another indolizidine framework. These tricyclic alkaloids,
named tetraponerines T1 to T8, possess a very uncommon
aminal structure in alkaloids3 and can be divided in two groups
depending on the size of the rings: 5−6−5 or 6−6−5. In each
group, the tetraponerines differ in the length of the alkyl chain
located in C5 (propyl or pentyl) and in the configuration of this
stereocenter (Figure 1).
Tetraponerines are secreted by Pseudomyrmecine ants of the
genus Tetraponera, for which more than 80 species can be
found in Asia, Africa, or Australia; T8 is the major tetraponerine
isolated from the New Guinean ant Tetraponera, while T1 and
T2 are very minor naturally occurring products. It was early
recognized that tetraponerines are paralyzing venoms segre-
gated by the host ants against their enemies. Not surprisingly, it
was found that they exhibit important insecticidal and
neurotoxic biological activities.4 Recent studies categorize
them as efficient inhibitors of a range of nAChRs (nicotinic
Figure 1. Structures of natural tetraponerines.
acetylcholine receptors).5 However, despite their interesting
biological profile, the actual mechanisms of interaction with
their corresponding receptors are still unknown. More recently,
cytotoxic activities have also been described for these natural
products as well as for some synthetic analogues containing
long hydrocarbon chains at C5 position.6
In 1987 Braekman and co-workers reported the first isolation
and structural elucidation of these natural products.4 The
structure of T8 was unequivocally assigned by X-ray diffraction
analysis,4 while the structures of the other natural tetraponer-
ines were proposed by comparison of their spectroscopical data,
1
in particular one-dimensional H and 13C NMR spectra, with
those of T8.7 However, structures of T3, T5, T6, and T7 were
Received: February 25, 2014
Published: April 14, 2014
© 2014 American Chemical Society
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mistakenly described at first and corrected then by further
studies involving nuclear magnetic resonance and circular
dicroism.8 More recently, the combined use of infrared
spectroscopy (IR) and mass spectrometry (MS) have also
been described to easily identify each tetraponerine from its
natural source.9 In this study it was observed that
tetraponerines with 5-(S) configuration show more intense
Bohlmann bands in the infrared spectra at lower vibrational
frequencies than the 5-(R) isomers.
The origin of natural tetraponerines has been studied by the
Braekman’s group with T610 and T8,11 and different
biosynthetic routes were proposed depending on the A ring
size. Importantly, upon studies of its biochemical pathway, T8
was submitted to catalytic hydrogenation under acidic
conditions, and the N11−C11a bond was selectively cleaved,
suggesting that the aminal core is in equilibrium with the
corresponding iminium ion.
The first enantioselective synthesis of (+)-T8 was achieved in
1990, providing a firm evidence of the absolute configuration of
this natural alkaloid.12 The singular tricyclic skeleton of these
alkaloids, which holds an aminal moiety, along with their
interesting biological activities have made the tetraponerines
very attractive targets for total synthesis.13 The group of Royer
applied its CN (R,S) strategy14 to the synthesis of all natural
tetraponerines known, being the only general method reported
to prepare this entire family of alkaloids up to date.15
Aware of the lack of detailed structural information on
tetraponerines we have recently examined the conformational
and configurational space of T3 and T4.16 For this study we
fixed the known configurations at C6a (R) and at C5 [(R)- for
T3 and (S)- for T4] and considered all possible configurations
at nitrogen atoms (N4 and N11) as well as at the aminal carbon
center (C11a). The geometry of the resulting 12 configurational
isomers (for each tetraponerine) was optimized by DFT
calculations, and their populations were estimated from the
calculated energies. The first conclusion from these calculations
is that for both T3 and T4, at least 99% of the population is
represented by either its ttc- or ttt- isomer (Figure 2).17 A closer
prefers a cis-configuration at ring C (65% of ttc-T3), while T4 is
clearly dominated by a trans-configuration at the indolizidine
framework (95% of ttt-T4). Importantly, low activation barriers
were found for inversion of T3 and T4 at N4, which allows a
fast equilibration between the two most populated isomers (ttc
and ttt). Interestingly, tetraponerine analogues with a 5−5−6
tricyclic skeleton have recently been prepared in racemic
form.18 The relative stereochemistry of these latter analogues
was assigned as cis (fusion A/B)-transoid-trans (fusion B/C),
according to NOE correlations.
In the present work we describe a detailed account of our
stereoselective total synthesis of all known tetraponerines, from
(+)-T1 to (+)-T8. We also provide the anticancer activity of
C5-pentyl tetraponerines (T5 to T8) against four human cell
lines.
RESULTS AND DISCUSSION
■
In a previous work,16 we described the total synthesis of natural
tetraponerines T3 and T4 using a divergent approach from
simple starting materials: 4-bromobutanal, 5-bromopentanal,
allyl bromide, and both enantiomeric forms of N-tert-
butylsulfinamide.19 As depicted in Scheme 1, the synthesis
started with a 2-allylpiperidine derivative, which was prepared
by aminoallylation of 5-bromopentanal with (R)-tert-butylsulfi-
namide and in situ generated allylindium reagent, followed by
intramolecular N-alkylation.20 Oxidative cleavage of the double
bond gave rise to the corresponding aldehyde 2, which was
submitted to a second aminoallylation where the configuration
at the new stereogenic center in products 3 and 4 was mainly
controlled by the chiral sulfinyl group regardless the chirality of
the aldehyde.21 This issue was crucial to prepare the required
diamines from the same chiral aldehyde, using the appropriate
enantiomer of chiral tert-butylsulfinamide. Acidic deprotection
of the homoallylic diamines, followed by hydrogenation of the
allyl moiety with concomitant hydrogenolysis of the carbamate,
afforded the corresponding free diamines. Without purification,
these intermediates were put into react with 4-bromobutanal to
initiate a cascade of reactions where the aminal formation is
presumably followed by intramolecular alkylation of N4.22 This
route allowed the preparation in good overall yields of
enantiomerically pure tetraponerines (+)-T3 and (+)-T4 with
identical spectral and physical data to the one reported in the
literature for the natural compounds.8 Importantly, the aminal
formation with the right stereochemistry at C11a for both
compounds clearly supports the hypothesis of a thermody-
namic control to afford the most stable aminal possible through
equilibration with an iminium form. This idea was essential in
our synthetic plan and comes from the following observations:
(a) all natural tetraponerines are proposed to exhibit the same
configuration at the aminal carbon center, regardless the
configuration at C5; (b) during the biosynthetic studies of T8,
the C11a−N11 bond was selectively cleaved under hydro-
genation conditions.11 Moreover, our stereochemical analysis of
tetraponerines T3 and T4 by DFT calculations indicated that
both tetraponerines are mainly populated (>99%) by configura-
Figure 2. Main configurational isomers previously found by DFT
calculations of T3 and T4.16
tional isomers (ttt and ttc) with (S)-configuration at C11a
.
Prompted by these results we decided to implement the
appropriate modifications to use the same general strategy in
the synthesis of all known natural tetraponerines. As depicted in
Scheme 2a, we recognized that the allyl group in the
intermediates 3 and 4 provides a handle to enlarge the
hydrocarbonated side-chain up to 5 carbon atoms by cross-
metathesis with an appropriate partner. Therefore, tetraponer-
examination of these two configurational isomers shows that,
despite the different fusions at the indolizidine frameworks, the
same (S)-configuration at the aminal carbon center is observed.
Remarkably, this configuration at C11a is observed not only for
natural T3 and T4, but also for all the other natural
tetraponerines. On the other hand, tetraponerine T3 slightly
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Scheme 1. Synthesis of Tetraponerines (+)-T3 and (+)-T4
Scheme 2. Modifications for the Synthesis of the Other Tetraponerines
Scheme 3. Synthesis of (+)-T7 and (+)-T8
ine T7 could be obtained from intermediate 3, and following
the same route, compound 4 could be carried forward to the
synthesis of T8. The rest of natural tetraponerines (T1, T2, T5,
and T6) have a 5-membered ring A. Accordingly, we decided to
use 4-bromobutanal in our aminoallylation-cyclization sequence
to prepare the key 2-allylpyrrolidine intermediate that could be
submitted to the same reaction sequence followed for the
synthesis of 6−6−5 tetraponerines (Scheme 2b).
Synthesis of Tetraponerines (+)-T7 and (+)-T8. Cross-
metathesis is a very useful synthetic tool in the preparation of
bioactive compounds from alkene intermediates. In this
context, we23 and others24 have demonstrated that sulfinamines
are tolerated by Grubbs second generation catalyst particularly
when Ti(Oi-Pr)4 is used as additive to accelerate the reaction.
With these precedents we decided to use cis-3-hexene as cross
metathesis partner25 to elongate the side-chain up to 5 carbons
of intermediates 3 and 4, en route to the preparation of T7 and
T8, respectively. We were pleased to observe that the reaction
took place smoothly in CH2Cl2 at 40 °C with both protected
diamines, obtaining the expected compounds in good yields
(Scheme 3).26 After this single modification, the reaction
sequence proceeded as for compounds T3 and T4, obtaining
good overall yields and excellent diasteroselectivities (>99:1 dr
by GC) for both tetraponerines T7 and T8. NMR spectra were
compared with the data of T3 and T4, finding the same signal
pattern distinctive of these 6−6−5 skeletons. More impor-
tantly, the physical and spectral data of the synthesized (+)-T7
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and (+)-T8 were in full agreement with reported values for
these naturally occurring compounds.13b,c
overcomes the influences of the chiral aldehyde moiety.
Consequently, protected diamines 11 and 12 could be obtained
in good isolated yields and excellent diastereoselectivities after
column chromatography (>99:1 dr by HPLC). In this case the
match-mismatched effect was stronger than with piperidine-
derivative substrates, achieving a 90% yield of diasteromerically
pure 11 after purification (94:6 dr before chromatography) and
a 69% yield of 12 isolated as a single isomer after purification
(only 78:22 dr before chromatography). Contrary to what was
observed for the piperidine aldehyde 2, in this case the matched
product was obtained when (S)-N-tert-butylsulfinamide was
used. Importantly, we were able to confirm the absolute
configuration of diamine derivative 12 by single crystal X-ray
analysis,27 and it was found to be consistent with the
stereochemistry predicted by our working model for the
aminoallylation step.20 Finally, the syntheses of (+)-T1 and
(+)-T2 were completed after acidic deprotection, catalytic
hydrogenation, and reaction of both free diamines with 4-
bromobutanal. The GC−MS data obtained shows essentially
one peak with a fragmentation pattern that perfectly matched
Synthesis of Tetraponerines (+)-T1, (+)-T2, (+)-T5, and
(+)-T6. Our next goal was to extend this methodology to the
synthesis of the remaining natural 5−6−5-tetraponerines (T1,
T2, T5, and T6), which differ from the others in the size of ring
A. It is worth saying that this extension is not straightforward,
and our DFT configurational analysis was only performed for
tetraponerines T3 and T4, with a 6−6−5 pattern. The
subgroup of tetraponerines with two indolizidine moieties
fused in the core has a more flexible scaffold, and the
contribution of several configurational isomers could compli-
cate the stereochemical landscape of these compounds. This
issue has been so far overlooked in the literature.
Our synthesis began with the indium-mediated amino-
allylation of 4-bromobutanal with (RS)-tert-butylsulfinamide,
followed by cyclization with KHMDS at 0 °C. The
corresponding 2-allylpyrrolidine 8 was obtained in good
diastereoselectivity and good overall yield after only one
purification step (Scheme 4).
9
1
with the one reported for T1 and T2. Moreover, the H and
13C NMR of the obtained T2 also shows a good correlation
with the data described in the literature.15 However, the NMR
spectral data obtained for T1 shows more signals than the ones
described in the literature for this tetraponerine (a very minor
component in the natural source).28 Contrary to what has been
observed for the other tetraponerines prepared, this worst NMR
correlation could be explained if different isomers of T1 with
different configurations at C11a are under equilibrium.
Scheme 4. Synthesis of 2-Allylpyrrolidine Intermediate 8
Finally, tetraponerines T5 and T6 were prepared by
including in the reaction sequence the cross-metathesis step
of diamine derivatives 11 of 12, respectively, with cis-3-hexene
(Scheme 6). After acidic treatment and catalytic hydrogenation,
the obtained free diamines were treated, as in other cases, with
4-bromobutanal to obtain the corresponding tetraponerines in
moderate overall yields. Once again, the GC−MS of
compounds T5 and T6,9 as well as the NMR data for
T6,13b,29 were in good agreement with the literature. However,
the NMR data obtained for compound T5 showed more signals
Compound 8 was a common intermediate in the synthesis of
all 5−6−5 tetraponerines. Eventually, the nitrogen protecting
group was exchange in a one-pot procedure by Cbz to
accomplish the oxidative cleavage of the terminal double bond
under modified Johnson−Lemieux conditions (Scheme 5). The
obtained aldehyde 10 was submitted to a second amino-
allylation reaction with (R)- or (S)-N-tert-butylsulfinamides.
Gratifyingly, as occurred in the case of the 6−6−5-
tetraponerines, the stereocontrol exerted by the sulfinyl group
Scheme 5. Synthesis of Tetraponerines (+)-T1 and (+)-T2
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vibrational frequencies, and more intense compared to the
odd-numbered tetraponerines (T1, T3, T5 and T7). It is
reasonable to consider that the configurational analyses
performed for T3 and T4 by DFT calculations16 can be
extrapolated to T7 and T8. Consequently, the even-numbered
tetraponerines (T4 and T8) must be mainly populated by a ttt-
isomer, while the ttc is the major isomer for T3 and T7. As
depicted in Figure 2, ttt-T4 and ttt-T8 exhibit two nitrogen
atoms with electron pairs antiperiplanar to 5 C−H bonds,
whereas ttc-isomers exhibit only one N−lone pair antiperiplanar
to 3 C−H bonds. Consequently, the Bohlmann bands pattern
of the 6−6−5-tetraponerines can be rationalized in terms of the
facility for hyperconjugation of nitrogen lone pairs with
antiperiplanar C−H bonds. Extrapolation of this explanation
to the 5−6−5-tetraponerines is not straightforward, especially
to the light of our NMR data for T1 and T5. However, the
same pattern is observed for the Bohlmann bands, and their use
to predict the configuration at C5 of tetraponerines is still valid.
Biological Assays. The biological activities of tetraponer-
ines have been only briefly examined, probably because of their
poor availability either by isolation from their natural source or
by chemical synthesis. Among the few studies accomplished, an
assessment of the anticancer activity of some natural
tetraponerines and their analogues against HT-29 cell line
(colorectal carcinoma), showed that the larger hydrocarbonated
chain at the C5, the lower IC50 values were obtained.6 With this
precedent we decided to evaluate the cytoxicity of pentyl-side-
chain tetraponerines ((+)-T5 to (+)-T8) at 4 different human
carcinoma cell lines from different origins (A2780 from ovary
carcinoma, NCI-H460 from lung, Caco-2 from colon, and
MCF-7 from breast). In order to predict the therapeutic range,
we also screened these compounds over a nontumoral cell line
(MRC-5, lung fibroblasts). At a first step, all the compounds
were assayed at 100 μM, and for those compounds showing a
decrease in cell viability higher than 50%, a concentration−
response curve was assayed in order to evaluate their cytotoxic
potency (IC50). Unfortunately, in most of the cases the cellular
growth inhibition was not enough to calculate the IC50 value;
nevertheless, (+)-T7 showed promising results against A2780,
MCF-7 (cellular growth inhibition >50%), being more efficient
against MCF-7 cancer cells. However, (+)-T7 also showed a
good percentage of inhibition at MRC-5 cell lines, which could
lead to nonspecific toxic effects (Table 1).
Scheme 6. Synthesis of Tetraponerines (+)-T5 and (+)-T6
than the previously reported.30 Since the same disagreement
was found for T1 we hypothesized that these 5−6−5
tetraponerines with (S)-configuration at C5 are actually a
mixture of different aminal isomers in equilibrium with
comparable energies. However, this hypothesis was not further
confirmed.
The Bolhmann Bands in IR Spectra of Tetraponerines.
A brief comment is in order here related to the use of the
Bohlmann bands to assign the configuration at C5 of
tetraponerines.31 The relevant region of the IR spectra obtained
for the synthesized tetraponerines are shown in Figure 3. The
The inhibition/concentration curves were obtained for
(+)-T7 against the cell lines where the cellular growth
inhibition was above 50% in order to calculate the IC50 values
(Table 2).33 For A2780 and MRC-5 cell lines, the obtained
IC50 values were above 100 μM, and solubility problems did
not allow achieving the maximum inhibitory concentration.
However, an acceptable value of IC50 (45
obtained against MCF-7 cancer cells.
3 μM) was
Figure 3. Bohlmann bands in the IR spectra of tetraponerines.
It is worth mentioning that in previous studies it was
concluded that the cytotoxicity of tetraponerine analogues
against HT29 cancer cells was independent of the configuration
at C5.6 In this study we have found that the antiproliferative
activity of (+)-T7 is consistently bigger than that of (+)-T8
against another three different human cell lines.
presence of bands for C−H stretching at relatively low
vibrational frequencies (2780−2810 cm−1) can be observed
for the IR spectra obtained for the synthesized tetraponerines.
The origin of these so-called Bohlmann bands is not accurately
known but has been related to the lengthening of C−Haxial
bonds because of the hyperconjugation of the adjacent nitrogen
lone pair with the antiperiplanar antibonding C−H orbitals (n
→ σ* C−H).32
CONCLUSION
■
In summary, the iterative use of stereoselective aminoallylation
of aldehydes with chiral N-tert-butylsulfinamide and in situ
generated allyl indium reagent provides a convenient access to
all natural tetraponerines from easily available starting materials.
As reported,9 the even-numbered tetraponerines (T2, T4,
T6, and T8) with (S)-configuration at C5 show a more severe
deviation on these C−H bands, which appear at lower
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spectrophotometer equipped with an ATR component; wavenumbers
are given in cm−1. Mass spectra (EI) were obtained at 70 eV, and
fragment ions in m/z with relative intensities (%) in parentheses.
HRMS analyses were carried out using the electron impact (EI) mode
at 70 eV or by Q-TOF using electrospray ionization (ESI) mode. GC
analyses were obtained with an HP-5 column (30 m × 0.25 mm, ID ×
0.25 μm) and an EI (70 EV) detector. The temperature program was
as follows: hold at 60 °C for 3 min, ramp from 60 to 270 °C at 15 °C/
min, hold at 270 °C for 10 min. 1H NMR spectra were recorded at 300
Table 1. Cellular Growth Inhibition Percentage of (+)-T5,
(+)-T6, (+)-T7, and (+)-T8 at 100 μM against Human Cell
a
Lines
cell line
b
c
c
c
d
compound
NCI-H460
Caco-2
A2780
MCF-7
MRC-5
CDDP
(+)-T5
(+)-T6
(+)-T7
(+)-T8
68
29
40
48
21
58
20
28
41
12
96
40
30
62
49
89
49
27
82
49
90
43
26
55
35
1
or 400 MHz for H NMR and 75 or 100 MHz for 13C NMR, using
CDCl3 or C6D6 as the solvent and TMS as an internal Standard (0.00
ppm). The data is being reported as s = singlet, d = doublet, t = triplet,
m = multiplet or unresolved, br s = broad signal, coupling constant(s)
in Hz, integration. 13C NMR spectra were recorded with 1H-
decoupling at 100 MHz and referenced to CDCl3 at 77.16 ppm and
to C6D6 at 127.00 ppm. DEPT-135 experiments were performed to
assign CH, CH2 and CH3.
Biological Procedures. Procedures used for biological assays are
described in the Supporting Information.
Experimental Procedures and Full Characterization Data for
Previously Reported Compounds. Procedure as well as complete
characterization data for compounds 1,20a 2, 3, 4, T3, and T4 were
previously described by our group.16
a
Cell lines: NCI-H460, lung carcinoma; Caco-2, colon carcinoma;
A2780, ovary carcinoma; MCF-7, breast carcinoma; MRC-5, human
b
lung fibroblast cells. 48 h cellular growth inhibition (%) as
c
determined by MTT. 96 h cellular growth inhibition (%) as
d
determined by MTT. 7 d cellular growth inhibition (%) as
determined by MTT. MTT = (3-[4,5-dimethylthiazol-2-yl]-2,5-
diphenyltetrazolium bromide). CDDP (cis-diaminedichloroplatinum-
(II)) was used as control.
Table 2. Assessment of Cytotoxicity (IC50, μM) of (+)-T7
a
against Human Cell Lines
(2R,2′R,SS)-(N-Benzyloxycarbonyl)-2-[(2′-tert-butylsulfinamide)-
4′-heptenyl]piperidine (5). The corresponding homoallylamine 3
(464 mg, 1.16 mmol) was placed in four different dry Schlenck tubes
under Argon atmosphere (116 mg each).34 In each flask, the
homoallylamine was disolved in dry CH2Cl2 (5 mL). Then Ti(Oi-
Pr)4 (27 μL, 0.09 mmol), cis-3-hexene (143 μL, 1.16 mmol), and
Grubbs-II catalyst (7 mg, 0.009 mmol) were added sequentially. The
reaction mixtures were heated at 40 °C for 6 h. After that time, all
reactions were collected together, and the solvent was removed under
reduced pressure. The residue obtained was purified by column
chromatography (3:2 hexane/EtOAc). The expected product 5 was
obtained as a colorless wax (391 mg, 79%). Rotamers are present:
cell line
b
b
c
compound
A2780
MCF-7
8.18 0.56
45
MRC-5
CDDP
0.86 0.01
>100
5.72 0.14
>100
(+)-T7
3
a
Cell lines: A2780, ovary carcinoma; MCF-7, breast carcinoma; MRC-
b
5, human lung fibroblast cells. Determined after 96 h using the MTT
method. Determided after 7 d using the MTT method. MTT = (3-
c
[4,5-dimethylthiazol-2-yl]-2,5- diphenyltetrazolium bromide). CDDP
(cis-diaminedichloroplatinum(II)) was used as control.
[α]20 + 50.5 (c 1.01, CHCl3); Rf 0.21 (1:1 hexane/EtOAc); IR ν
D
3242, 2935, 2867, 1677, 1421, 1352, 1260, 1173, 1142, 1064, 730
cm−1; 1H NMR (300 MHz, CDCl3) δ 7.43−7.30 (m, 5H), 5.62−5.42
(m, 1H), 5.37−5.26 (m, 1H), 5.14 (t, J = 12.5 Hz, 2H), 4.91−4.33 (m,
2H), 4.05 (br d, J = 12.9 Hz, 1H), 2.91 (br s, 1H), 2.77 (t, J = 12.4 Hz,
1H), 2.39 (br s, 1H), 2.35−2.19 (m, 1H), 2.11−1.89 (m, 3H), 1.82−
1.33 (m, 7H), 1.20 (s, 9H), 1.02−0.94 (t, J = 7.4 Hz, 3H); 13C NMR
(101 MHz, CDCl3) δ 155.8 (C), 136.9 (C), 135.4 (CH), 128.5 (CH),
127.9 (CH), 127.8 (CH), 125.4 (CH), 67.1 (CH2), 55.6 (C), 53.4
(CH3), 47.6 (CH), 39.3 (CH2), 39.2 (CH2), 36.2 (CH2), 29.3 (CH2),
25.7 (CH2), 25.6 (CH2), 22.8 (CH), 19.2 (CH2), 13.9 (CH3); GC tR
= 20.0 min; LRMS (EI) m/z (%) 378 (M+ − C4H8, 0.1), 328 (3), 241
(8), 218 (31), 175 (8), 174 (58), 92 (8), 91 (100); HRMS (ESI) calcd
for C24H39N2O3S (M+ + 1) 435.2681, found 435.2670.
The formation of the tricyclic aminal scaffold was achieved by
reaction of the corresponding free enantioenriched diamines
with 4-bromobutanal, with concomitant N4 alkylation. The
configuration at the aminal carbon center was thermodynami-
cally controlled with excellent selectivity in the synthesis of any
natural tetraponerine, except for (+)-T1 and (+)-T5.
Interestingly, this methodology allows the access not only to
the eight natural tetraponerines, but also to their eight
enantiomers, by only changing the (R)-tert-butylsulfinamide
for the also commercially available (S)-enantiomer in the first
aminoallylation step. Importantly, the cross metathesis step
could be extended to the use of other alkenes, being possible
the synthesis of a wide range of tetraponerine analogues for
their biological evaluation. The antiproliferative activity of
(+)-T5 to (+)-T8 against four different carcinoma cell lines
show that the configuration at C5 could be important in their
potency. Compound (+)-T7 showed a promising cytotoxic
profile at breast carcinoma cell line MCF-7 when compared
with its effect at nontumoral cell lines. With these results we
think that further evaluation of tetraponerines (+)-T7 and
(+)-T8, or analogues with the same scaffold, against other
cancer cell lines is pertinent in future works.
(2R,2′S,RS)-(N-Benzyloxycarbonyl)-2-[(2′-tert-butylsulfinamide)-
4′-heptenyl]piperidine (6). The expected product 6 was obtained
from 4 (337 mg, 0.83 mmol) as a colorless wax (299 mg, 83%)
following the same procedure described for 5. Rotamers are present:
[α]20 − 17.6 (c 1.05, CHCl3); Rf 0.21 (1:1 hexane/EtOAc); IR ν
D
1
3239, 2932, 2867, 1692, 1422, 1259, 1172, 1065, 1053, 697 cm−1; H
NMR (300 MHz, CDCl3) δ 7.42−7.30 (m, 5H), 5.70−5.47 (m, 1H),
5.41−5.24 (m, 1H), 5.20−5.03 (m, 2H), 4.48 (s, 1H), 4.06 (d, J = 12.0
Hz, 1H), 3.22 (dd, J = 12.4, 6.1 Hz, 1H), 2.89 (t, J = 13.1 Hz, 1H),
2.40 (s, 1H), 2.29 (s, 1H), 2.11−1.91 (m, 3H), 1.88−1.53 (m, 7H),
1.50−1.32 (m, 1H), 1.17 (s, 9H), 0.97 (t, J = 7.5 Hz, 3H); 13C NMR
(101 MHz, CDCl3) δ 155.4 (C), 137.4 (C), 136.9 (CH), 128.6 (CH),
128.6 (CH), 128.0 (CH), 127.9 (CH), 67.1 (CH2), 55.9 (C), 52.9
(CH), 47.8 (CH), 39.5 (CH2), 39.3 (CH2), 34.5 (CH2), 27.9 (CH2),
25.8 (CH2), 25.6 (CH2), 22.7 (CH3), 18.9 (CH2), 14.1 (CH3); GC tR
= 19.9 min; LRMS (EI) m/z (%) 378 (M+ − C4H8, 0.1), 328 (3), 241
(8), 218 (31), 175 (8), 174 (58), 92 (8), 91 (100); HRMS (ESI) calcd
for C24H39N2O3S (M+ + 1) 435.2681, found 435.2671.
EXPERIMENTAL SECTION
■
General Remarks. TLC was performed on silica gel 60 F254, using
aluminum plates and visualized with phosphomolybdic acid (PMA)
stain. Flash chromatography was carried out on handpacked columns
of silica gel 60 (230−400 mesh). Melting points are uncorrected.
Optical rotations were measured using a polarimeter with a thermally
jacketted 5 cm cell at approximately 20 °C, and concentrations (c) are
given in g/100 mL. Infrared analysis was performed with a
Tetraponerine T7. To a solution of compound 5 (300 mg, 0.765
mmol) in THF (2 mL) was added aqueous 6 M HCl (383 μL, 2.30
mmol) at 0 °C under Ar. The reaction mixture was stirred for 1 h
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while reaching 23 °C. Aqueous 2 M NaOH (3 mL) was added to the
reaction mixture, and the free amine was extracted with EtOAc (3 ×
10 mL) and washed with brine (1 × 10 mL). The organics were dried
over MgSO4, filtered, and concentrated under reduced pressure. The
residue was then dissolved in dry MeOH (15 mL), and Pd/C 10%
(256 mg) was added to the mixture. The suspension was shacked
under hydrogen atmosphere (4 atm) for 12 h at 23 °C and filtered
through Celite, and the obtained solution was concentrated under
reduced pressure. The residue (free diamine) was then dissolved in dry
CH2Cl2 (8 mL), and K2CO3 (317 mg, 2.30 mmol) was added,
followed by 4-bromobutanal35 (207 mg, 1.377 mmol). The reaction
mixture was stirred at 23 °C for 4 h, after which time inorganic salts
were removed by filtration. The filtrate was washed twice with aqueous
NaHCO3, followed by brine, and then dried over MgSO4. The
organics were concentrated under reduced pressure, and the residue
was purified by column chromatography (96:4:0.05 CH2Cl2/ MeOH/
20% NH4OH) to provide the desired product as an oil (102 mg, 54%):
[α]20D + 30 (c 0.82, CHCl3), [lit.13b [α]20D + 29.5 (c 2.2, CHCl3)]; Rf
0.50 (9:1 CH2Cl2/MeOH); IR ν 2954, 2926, 2855, 2803, 1455, 1389,
1346, 1156, 1129, 1118 cm−1; 1H NMR (300 MHz, C6D6) δ 3.03 (dd,
J = 4.4, 2.7 Hz, 1H), 2.90 (dd, J = 14.6, 7.5 Hz, 1H), 2.57−2.46 (m,
3H), 1.83−1.73 (m, 1H), 1.68 (dd, J = 12.5, 5.2 Hz, 1H), 1.58−1.20
(m, 9H), 1.16−0.90 (m, 10H), 0.87−0.81 (m, 1H), 0.64 (t, J = 6.7 Hz,
3H); 13C NMR (101 MHz, CDCl3) δ 75.5 (CH), 56.7 (CH), 53.3
(CH), 50.9 (CH2), 50.6 (CH2), 34.1 (CH2), 32.5 (CH2), 32.2 (CH2),
31.0 (CH2), 30.5 (CH2), 27.3 (CH2), 26.4 (CH2), 25.1 (CH2), 23.2
(CH2), 22.2 (CH2), 14.4 (CH3); GC tR = 13.8 min; LRMS (EI) m/z
(%) 251 (M + 1, 6), 250 (M+, 44), 249 (82), 207 (19), 194 (14), 193
(100), 180 (14), 179 (10), 165 (6), 152 (34), 151 (11), 138 (10), 110
(12), 96 (30), 84 (10), 55 (6); HRMS (ESI) calcd for C16H31N2 (M+
+ 1) 251.2487, found 251.2486.
(101 MHz, CDCl3) δ 133.8 (CH), 119.5 (CH2), 56.2 (C), 54.5 (CH),
40.7 (CH2), 33.7 (CH2), 33.6 (CH2), 29.0 (CH2), 22.8 (CH3); GC tR
= 14.1 min; LRMS (EI) m/z (%) 200 (40), 199 (100), 198 (39), 197
(95), 118 (15), 91 (13), 70 (80), 68 (16), 57 (66).
(2R,Rs)-2-Allyl-(N-tert-butylsulfinyl)pyrrolidine (8). A titrated sol-
ution of KHMDS in THF (7.9 mL, 1.03 M, 8.136 mmol) was added
via syringe to a cold solution (0 °C) of crude 7 (1.600 g, 5.424 mmol)
in dry THF (13.8 mL). The reaction mixture was stirred for 1.5 h at 0
°C under Ar, quenched with saturated NH4Cl solution, and allowed to
reach room temperature. The aqueous phase was extracted with
EtOAc (3 times), and the combined extracts were washed with brine,
dried over MgSO4, and concentrated under reduced pressure. The
residue was purified by column chromatography (80:20 hexane/
EtOAc) to provide the product as a colorless oil (1.008 g, 86%, 97:3 dr
1
20
according to the H NMR): [α]D − 17.1 (c 1.038, CHCl3); Rf 0.30
(7:3 hexane/EtOAc); IR ν 3076, 2658, 2871, 1639, 1474, 1456, 1361,
1065, 994, 911 cm−1; H NMR (300 MHz, CDCl3) δ 5.82−5.66 (m,
1
1H), 5.11−5.01 (m, 2H), 3.74 (ddd, J = 10.1, 6.6, 3.7 Hz, 1H), 3.51−
3.43 (m, 1H), 3.14 (dt, J = 10.7, 7.0 Hz, 1H), 2.52−2.43 (m, 1H), 2.17
(ddd, J = 14.0, 9.4, 8.2 Hz, 1H), 1.87−1.73 (m, 3H), 1.71−1.60 (m,
1H), 1.19 (s, 9H); 13C NMR (101 MHz, CDCl3) δ 135.3 (CH), 117.3
(CH2), 58.1 (C), 57.5 (CH), 48.6 (CH2), 39.0 (CH2), 30.4 (CH2),
24.2 (CH2), 23.7 (CH3); GC tR = 11.9 min; LRMS (EI) m/z (%) 215
(M+, 0.1), 159 (16), 118 (100), 117 (29), 70 (10), 57 (13); HRMS
(EI) calcd for C11H21NOS 215.1344, found 215.1348.
(R)-2-Allyl-(N-benzyloxycarbonyl)pyrrolidine (9). An aqueous 6 M
solution of HCl (2 mL, 12.00 mmol) was added to a solution of 8
(860 mg, 4.0 mmol) in dry THF (4 mL) at 0 °C under Ar. The
solution was allowed to reach 23 °C and was stirred for 2 h. After
being cooled again to 0 °C, an aqueous 2 M solution of NaOH (14
mL, 27.00 mmol) was carefully added, and the resulting mixture was
stirred at the same temperature. After 5 min, a solution of
benzyloxicarbonyl chloride (670 μL, 4.70 mmol) in CH2Cl2 (12
mL) was added to the stirring solution. The resulting mixture was then
allowed to reach 23 °C and stirred for 3 h. The reaction mixture was
extracted with EtOAc (3 times), and the combined organic layers were
washed with H2O, dried over MgSO4, filtered, and concentrated under
reduced pressure. The residue was purified by column chromatograpy
(95:5 hexane/EtOAc) to provide the desired product 9 as a colorless
Tetraponerine T8. Tetraponerine T8 was obtained from 6 (205 mg,
0.49 mmol) as a yellow oil (74 mg, 60%) following the same
procedure described for T7: [α]20 + 99 (c 0.70, CHCl3), [lit.13b
D
[α]20 + 101 (c 2.0, CHCl3)]; Rf 0.40 (9:1 CH2Cl2/MeOH); IR ν
D
2928, 2857, 2790, 2775, 1631, 1454, 1377, 1338, 1188, 1156, 1035
1
cm−1; H NMR (300 MHz, C6D6) δ 3.26 (td, J = 8.1, 2.2 Hz, 1H),
2.98−2.91 (m, 1H), 2.42 (t, J = 6.8 Hz, 1H), 2.23 (ddd, J = 11.0, 7.3,
3.7 Hz, 1H), 2.19−2.08 (m, 1H), 1.91−1.66 (m, 8H), 1.65−1.24 (m,
14H), 1.01 (t, J = 6.9 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 85.6
(CH), 62.7 (CH), 61.4 (CH), 51.5 (CH2), 49.0 (CH2), 38.0 (CH2),
34.6 (CH2), 32.9 (CH2), 32.8 (CH2), 29.7 (CH2), 26.2 (CH2), 25.2
(CH2), 25.1 (CH2), 23.1 (CH2), 20.2 (CH2), 14.3 (CH3); GC tR =
14.2 min; LRMS (EI) m/z (%) 251 (M + 1, 6), 250 (M+, 45), 249
(100), 207 (19), 194 (13), 193 (95), 180 (20), 179 (11), 165 (12),
152 (45), 151 (15), 138 (12), 137 (10), 110 (14), 96 (30), 84 (16), 55
(9); HRMS (ESI) calcd for C16H31N2 (M+ + 1) 251.2487, found
251.2493.
20
oil (831 mg, 85%). Rotamers are present: [α]D + 53.1 (c 0.617,
CHCl3); Rf 0.60 (7:3 hexane/EtOAc); IR ν 3035, 2972, 2876, 1696,
1
1639, 1446, 1407, 1355, 1100 cm−1; H NMR (300 MHz, CDCl3) δ
7.41−7.27 (m, 5H), 5.83−5.64 (m, 1H), 5.21−4.96 (m, 4H), 3.92 (br
s, 1H), 3.52−3.35 (m, 2H), 2.51 (br d, J = 54.4 Hz, 1H), 2.15 (td, J =
14.8, 8.1 Hz, 1H), 1.96−1.70 (m, 4H); 13C NMR (101 MHz, CDCl3)
δ 155.1 (C), 154.9 (C), 137.3 (C), 137.1 (C), 135.2 (CH), 135.0
(CH), 128.6 (CH), 127.9 (CH), 117.4 (CH2), 66.8 (CH2), 66.6
(CH2), 57.4 (CH), 56.9 (CH), 47.0 (CH2), 46.7 (CH2), 39.1 (CH2),
38.2 (CH2), 30.2 (CH2), 29.3 (CH2), 23.8 (CH2), 23.0 (CH2); GC tR
= 14.6 min; LRMS (EI) m/z (%) 245 (M+, 0.1), 204 (26), 160 (28),
91 (100); HRMS (EI) calcd for C15H19NO2 245.1416, found
245.1427.
(4R,RS)-N-(tert-Butylsulfinyl)-7-bromohept-1-en-4-amine (7). To a
dry flask were added (RS)-N-tert-butylsulfinamide (829 mg, 6.79
mmol) followed by indium powder (968 g, 8.489 mmol) under Ar.
Then a solution of 4-bromobutyraldehyde (1.121 g, 7.470 mmol) in
dry THF (15.9 mL) and Ti(OEt)4 (3.1 mL, 13.582 mmol) were added
successively, and the reaction mixture was stirred under Ar for 1 h at
23 °C. At this time allyl bromide (897 μL, 10.186 mmol) was added to
the mixture, and the reaction was allowed to reach 60 °C and stirred at
that temperature for 5 h. The mixture was allowed to reach room
temperature and was carefully added over a stirring mixture of 4:1
EtOAc/brine (100 mL). The resulted white suspension was filtered
through a short pad of Celite, washed with EtOAc, and organics were
concentrated in vacuo. The resulted suspension was diluted in 4:1
EtOAc/hexane (50 mL) and filtered again through Celite. Organics
were concentrated to afford the expected compound 7 (1.783 g, 89%,
95:5 dr according to 1H NMR) as a colorless wax, pure enough to be
used in the next step. To provide the spectroscopy data, a sample of
homoallylamine 7 was purified by column chromatography: Rf 0.13
(R)-(N-Benzyloxycarbonyl)-2-(2-oxoethyl)pyrrolidine (10). To a
solution of 9 (790 mg, 3.224 mmol) in 1,4-dioxane:H2O (3:1, 30 mL)
were successively added 2,6-lutidine (751 μL, 6.45 mmol), NaIO4 (2.8
g, 12.90 mmol) and a solution of OsO4 in t-BuOH (2.5% wt in t-
BuOH, 317 μL). The mixture was stirred at 23 °C for 1.5 h before
being quenched with water (20 mL). The mixture was extracted with
EtOAc (3 × 10 mL), and the collected organic layers were washed
with H2O, dried over MgSO4, filtered, and concentrated under
reduced pressure. The residue was purified by column chromatograpy
(4:1 hexane/EtOAc) to provide the pure product as a colorless oil
(639 mg, 80%). Rotamers are present: [α]D20 + 40.4 (c 1.44, CHCl3);
Rf 0.24 (7:3 hexane/EtOAc); IR ν 3031, 2956, 2879, 2726, 1718, 1692,
1
1497, 1448, 1409, 1356, 1100 cm−1; H NMR (300 MHz, CDCl3) δ
9.80 (br s, 0.6H), 9.67 (br s, 0.4H), 7.44−7.29 (m, 5H), 5.24−5.01
(m, 2H), 4.32 (dt, J = 12.1, 5.9 Hz, 1H), 3.58−3.27 (m, 2H), 2.99 (dd,
J = 16.5, 3.1 Hz, 0.6H), 2.83 (br d, J = 16.8 Hz, 0.4H), 2.53 (d, J = 7.6
Hz, 0.6H), 2.48 (d, J = 7.6 Hz, 0.4H), 2.14 (dq, J = 12.5, 7.9 Hz, 1H),
1.92−1.81 (m, 2H), 1.74−1.61 (m, 1H); 13C NMR (101 MHz,
1
(7:3 hexane/EtOAc); H NMR (300 MHz, CDCl3) δ 5.79 (ddt, J =
19.6, 9.4, 7.2 Hz, 1H), 5.20 (d, J = 0.7 Hz, 1H), 5.16 (d, J = 3.6 Hz,
1H), 3.41 (t, J = 6.6 Hz, 3H), 3.27 (d, J = 6.2 Hz, 1H), 2.55−2.25 (m,
2H), 2.03−1.82 (m, 2H), 1.82−1.56 (m, 2H), 1.22 (s, 9H); 13C NMR
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CDCl3) δ 200.9 (CH), 200.6 (CH), 155.0 (C), 154.7 (C), 136.9 (C),
136.6 (C), 128.6 (CH), 128.3 (CH), 128.1 (CH), 128.0 (CH), 67.1
(CH2), 66.9 (CH2), 53.1 (CH), 52.3 (CH), 49.4 (CH2), 48.7 (CH2),
46.9 (CH2), 46.5 (CH2), 32.1 (CH2), 31.3 (CH2), 23.9 (CH2), 23.1
(CH2); GC tR = 15.4 min; LRMS (EI) m/z (%) 247 (M+, 0.6), 219
(10), 160 (10), 156 (10), 112 (10), 91 (100), 65 (10); HRMS (EI)
calcd for C14H17NO3 247.1208, found 247.1213.
(100), 70 (23); HRMS (EI) calcd for C21H32N2O3S − C4H8 336.1508,
found 336.1509.
Tetraponerine T1. Tetraponerine T1 was prepared from 11 (300
mg, 0.765 mmol) and 4-bromobutanal (207 mg, 1.377 mmol)
following the same procedure described above for compound T7,
affording the expected product as an oil (51 mg, 32% from 11).
Configurational isomers must be present:36 [α]D + 14 (c 0.498,
20
CHCl3) [lit.15 [α]20 + 11 (c 0.14, CHCl3)]; Rf 0.15 (95:5 CH2Cl2/
(2R,2′R,SS)-(N-Benzyloxycarbonyl)-2-[2′-(tert-butylsulfinamine)-
4′-pentenyl]pyrrolidine (11). To a dry flask were added (SS)-N-tert-
butylsulfinamide (134 mg, 1.107 mmol) and indium powder (158 mg,
1.38 mmol) under Ar. Then was added a solution of aldehyde 10 (301
mg, 1.218 mmol) in dry THF (2.7 mL) followed by Ti(OEt)4 (500
μL, 2.22 mmol), and the reaction mixture was stirred under Ar for 1 h
at 23 °C. At this time allyl bromide (143 μL, 1.66 mmol) was added to
the mixture, and it was heated to 60 °C for 5 h. The mixture was
allowed to reach 23 °C and was carefully added over a stirring mixture
of 4:1 EtOAc/brine (15 mL). The resulted white suspension was
filtered through a short pad of Celite and washed with EtOAc, and the
organics were concentrated under reduced pressure. According to
HPLC analysis (Chiralcel AD-H column 25 cm × 0.46 cm, isocratic
elution with 95:5 n-hexane/i-PrOH, 1.0 mL/min, UV detection at 217
nm), the crude reaction mixture showed a major diastereoisomer at
16.17 min (94%) and other diastereoisomers at 20.80−23.50 min
(6%). After column chromatography (3:2 hexane/EtOAc), the major
isomer was isolated pure (>99:1 dr according to HPLC) as a colorless
wax product as a unique diastereoisomer after purification by column
chromatography (391 mg, 90%). Rotamers are present: [α]D20 + 83.2
(c 1.220, CHCl3); Rf 0.20 (1:1 hexane/EtOAc); IR ν 3243, 3065,
2957, 2888, 1685, 1452, 1409, 1360, 1099, 1062 cm−1; 1H NMR (300
MHz, CDCl3) δ 7.44−7.28 (m, 5H), 5.80 (dq, J = 10.0, 6.9 Hz, 0.8H),
5.62 (dd, J = 15.8, 8.3 Hz, 0.2H), 5.27−5.00 (m, 5H), 4.34 (dd, J =
13.0, 7.6 Hz, 0.8H), 3.99 (br s, 0.2H), 3.42 (dt, J = 11.4, 8.5 Hz, 2H),
3.20 (qd, J = 10.1, 5.5 Hz, 1H), 2.52 (dt, J = 14.5, 7.4 Hz, 1H), 2.42−
2.27 (m, 1H), 2.09−1.81 (m, 3H), 1.81−1.54 (m, 3H), 1.24 (s, 7H),
1.12 (s, 2H); 13C NMR (101 MHz, CDCl3) δ 156.3 (C), 137.0 (C),
135.6 (CH), 134.1 (CH), 128.6 (CH), 128.0 (CH), 127.8 (CH),
119.0 (CH2), 117.5 (CH2), 67.3 (CH2), 66.9 (CH2), 55.6 (C), 54.9
(CH), 53.3 (CH), 46.6 (CH2), 42.4 (CH2), 41.3 (CH2), 39.8 (CH2),
39.2 (CH2), 31.4 (CH2), 23.8 (CH2), 23.0 (CH3), 22.7 (CH3); GC tR
= 22.7 min; LRMS (EI) m/z (%) 336 (M+, 0.8), 287 (6), 218 (14),
204 (10), 174 (10), 160 (20), 114 (6), 91 (100), 70 (23); HRMS (EI)
calcd for C21H32N2O3S − C4H8 336.1508, found 336.1510.
D
MeOH); IR ν 2954, 2928, 2871, 2792, 2639, 2585, 1457, 1381, 1349,
1191, 1167, 1111 cm−1; 1H NMR (300 MHz, CDCl3) δ 3.92−3.74 (m,
1H), 3.46−3.15 (m, 2H), 3.14−2.88 (m, 2H), 2.77−2.36 (m, 1H),
2.31−1.78 (m, 8H), 1.79−1.16 (m, 7H), 0.92 (t, J = 7.3 Hz, 3H); 13C
NMR (101 MHz, CDCl3) δ 77.0 (CH), 58.4 (CH), 53.7 (CH), 50.4
(CH2), 49.8 (CH2), 33.8 (CH2), 30.8 (CH2), 29.5 (CH2), 29.2 (CH2),
21.3 (CH2), 20.5 (CH2), 20.1 (CH2), 14.3 (CH3); GC tR = 11.6 min;
LRMS (EI) m/z (%) 208 (M+, 46), 207 (100), 180 (12), 179 (91),
165 (10), 138 (51), 137 (19), 124 (23), 110 (17), 96 (64), 70 (29);
HRMS (EI) calcd for C13H24N2 208.1939, found 208.1929.
Tetraponerine T2. Tetraponerine T2 was prepared from 12 (230
mg, 0.587 mmol), and 4-bromobutanal (159 mg, 1.057 mmol)
following the same procedure described above for compound T7,
20
affording the expected product as an oil (42 mg, 35% from 12): [α]D
+ 47 (c 0.232, CHCl3), [lit.15 [α]20 + 36 (c 1.79, CHCl3)]; Rf 0.15
D
(95:5 CH2Cl2/MeOH); IR ν 2955, 2931, 2871, 2787, 2695, 1457,
1
1379, 1362, 1190, 1162, 1099 cm−1; H NMR (300 MHz, C6D6) δ
3.17−2.93 (m, 3H), 2.60−2.51 (m, 1H), 2.43 (td, J = 8.7, 5.1 Hz, 1H),
2.07−1.83 (m, 5H), 1.83−1.60 (m, 4H), 1.60−1.25 (m, 7H), 1.01 (t, J
= 7.1 Hz, 3H); 13C NMR (101 MHz, C6D6) δ 83.0 (CH), 63.8 (CH),
59.1 (CH), 48.6 (CH2), 45.5 (CH2), 36.5 (CH2), 32.7 (CH2), 30.4
(CH2), 28.9 (CH2), 21.1 (CH2), 20.8 (CH2), 19.3 (CH2), 14.6
(CH3); GC tR = 11.9 min; LRMS (EI) m/z (%) 208 (M+, 41), 207
(100), 180 (8), 179 (53), 165 (11), 138 (59), 137 (20), 124 (25), 110
(18), 96 (47), 70 (37); HRMS (EI) calcd for C13H24N2 208.1939,
found 208.1932.
(2R,2′R,SS)-(N-Benzyloxycarbonyl)-2-[(2′-tert-butylsulfinamine)-
4′-heptenyl]pyrrolidine (13). Compound 13 was prepared from 11
(513 mg, 1.31 mmol) following the same procedure described above
for compound 5, affording the corresponding product as a colorless
wax (473 mg, 86%). Rotamers are present: [α]20 + 81.6 (c 1.20,
D
CHCl3); Rf 0.31 (1:1 hexane/EtOAc); IR ν 3248, 3031, 2958, 2931,
1
1685, 1453, 1409, 1359, 1099, 1062, 968, 697 cm−1; H NMR (300
MHz, CDCl3) δ 7.46−7.31 (m, 5H), 5.66−5.48 (m, 1H), 5.47−5.29
(m, 1H), 5.29−5.03 (m, 2H), 4.42−3.89 (m, 1H), 3.56−3.05 (m, 3H),
2.42 (dt, J = 14.1, 7.1 Hz, 1H), 2.34−2.19 (m, 1H), 1.23 (s, 7H), 1.12
(s, 2H), 1.18 (d, J = 34.3 Hz, 9H), 0.96 (t, J = 7.4 Hz, 3H); 13C NMR
(101 MHz, CDCl3) δ 156.1 (C), 137.0 (C), 135.4 (CH), 128.6 (CH),
128.0 (CH), 127.8 (CH), 125.6 (CH), 66.9 (CH2), 55.7 (C), 55.0
(CH), 53.5 (CH), 46.5 (CH2), 42.2 (CH2), 38.0 (CH2), 31.3 (CH2),
25.8 (CH2), 23.8 (CH2), 22.9 (CH3), 22.7 (CH3), 14.3 (CH3), 13.9
(CH3); GC tR = 19.4 min; LRMS (EI) m/z (%) 314 (3), 227 (3), 204
(12), 173 (4), 161 (3), 160 (23), 145 (24), 92 (8), 91 (100), 70 (6),
69 (3), 65 (5); HRMS (ESI) calcd for C23H37N2O3S (M+ + 1)
421.2525, found 421.2521.
(2R,2′S,RS)-(N-Benzyloxycarbonyl)-2-[(2′-tert-butylsulfinamine)-
4′-pentenyl]pyrrolidine (12). Compound 12 was prepared from
aldehyde 10 (301 mg, 1.218 mmol) and (RS)-N-tert-butylsulfinamide
(134 mg, 1.107 mmol) following the same procedure described above
for compound 11. According to HPLC analysis (Chiralcel AD-H
column 25 cm × 0.46 cm, isocratic elution with 95:5 n-hexane/i-
PrOH, 1.0 mL/min, UV detection at 217 nm), the crude reaction
mixture showed a major diastereoisomer at 21.4 min (78%) and other
diastereoisomers at 14.4−17.0 min (22%). After column chromatog-
raphy (3:2 hexane/EtOAc), the major isomer was isolated pure (>99:1
dr according to HPLC) as a colorless wax (301 mg, 69%): mp 76.5−
77.1 °C; [α]D20 −31.8 (c 0.929, CHCl3); Rf 0.16 (1:1 hexane/EtOAc);
IR ν 3203, 3075, 2958, 2883, 1689, 1471, 1453, 1407, 1096, 1045,
1032 cm−1; Rotamers are present (as shown in the X-ray at the
(2R,2′S,RS)-(N-Benzyloxycarbonyl)-2-[(2′-tert-butylsulfinamine)-
4′-heptenyl]pyrrolidine (14). Compound 14 was prepared from 12
(358 mg, 0.90 mmol) following the same procedure described above
for compound 5, affording the corresponding product as a colorless
1
wax (295 mg, 78%). Rotamers are present: [α]20 − 20.5 (c 0.99,
Supporting Information) H NMR (300 MHz, CDCl3) δ 7.48−7.32
D
(m, 5H), 5.94−5.58 (m, 1H), 5.27−4.94 (m, 4H), 4.04 (br d, J = 35.6
Hz, 1H), 3.58−3.36 (m, 2.6H), 3.28 (br d, J = 32.9 Hz, 1H), 3.02 (d, J
= 8.4 Hz, 0.4H), 2.58−2.31 (m, 2H), 2.09−1.80 (m, 4H), 1.68 (br d, J
= 5.1 Hz, 1H), 1.51−1.35 (m, 1H), 1.22 (s, 5H), 1.06 (s, 4H); 13C
NMR (101 MHz, CDCl3) δ 154.9 (C), 137.1 (C), 137.0 (C), 134.0
(CH), 133.3 (CH), 128.7 (CH), 128.6 (CH), 128.4 (CH), 128.0
(CH), 127.9 (CH), 119.8 (CH2), 119.2 (CH2), 67.2 (CH2), 66.6
(CH2) 56.4 (C), 56.2 (C), 55.2 (CH), 54.6 (CH), 54.5 (CH), 54.3
(CH), 46.7 (CH2), 46.4 (CH2), 42.2 (CH2), 42.0 (CH2), 40.3 (CH2),
39.5 (CH2), 30.5 (CH2), 30.1 (CH2), 23.9 (CH2), 23.1 (CH2), 22.9
(CH3), 22.7 (CH3); GC tR = 22.9 min; LRMS (EI) m/z (%) 336 (M+,
0.8), 287 (6), 218 (14), 204 (10), 174 (10), 160 (25), 114 (12), 91
CHCl3); Rf 0.30 (1:1 hexane/EtOAc); IR ν 3232, 3030, 2959, 2873,
1
1691, 1454, 1412, 1359, 1102, 1057, 970, 750 cm−1; H NMR (300
MHz, CDCl3) δ 7.41−7.30 (m, 5H), 5.59 (dt, J = 15.2, 6.4 Hz, 1H),
5.47−5.22 (m, 1H), 5.22−4.97 (m, 2H), 4.18−3.87 (m, 1H), 3.59−
3.00 (m, 4H), 2.34 (br d, J = 21.0 Hz, 2H), 2.11−1.92 (m, 4H), 1.92−
1.59 (m, 3H), 1.37 (dd, J = 24.1, 12.3 Hz, 1H), 1.23 (s, 5H), 1.05 (s,
4H), 0.97 (t, J = 7.5 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 154.7
(C), 137.7 (CH), 137.2 (CH), 136.9 (C), 136.8 (C), 128.4 (CH),
128.2 (CH), 127.9 (CH), 127.7 (CH), 123.8 (CH), 123.1 (CH), 67.0
(CH2), 66.5 (CH2), 56.1 (C), 55.9 (C), 55.1 (CH), 54.6 (CH), 54.4
(CH), 46.6 (CH2), 46.2 (CH2), 40.6 (CH2), 40.5 (CH2), 40.2 (CH2),
39.3 (CH2), 30.4 (CH2), 29.9 (CH2), 25.7 (CH2), 23.7 (CH2), 22.8
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(CH3), 22.5 (CH3), 20.7 (CH2), 14.2 (CH3), 13.9 (CH3); GC tR =
19.4 min; LRMS (EI) m/z (%) 314 (3), 227 (3), 207 (3), 204 (11),
179 (3), 174 (4), 173 (4), 161 (3), 160 (23), 145 (9), 92 (8), 91
(100), 70 (6), 65 (4); HRMS (ESI) calcd for C23H37N2O3S (M+ + 1)
421.2525, found 421.2519.
(b) Chemler, S. R. Curr. Bioact. Compd. 2009, 5, 2. (c) Michael, J. P.
Nat. Prod. Rep. 2008, 25, 139−165 and references therein. (d) Wei, L-;
Shi, Q.; Bastow, K. F.; Brossi, A.; Morris-Natschke, S. L.; Nakagawa-
Goto, K.; Wu, T. S.; Pan, S. L.; Teng, C. M.; Lee, K. H. J. Med. Chem.
2007, 50, 3674−3680.
(2) (a) Royer, J.; Bonin, M.; Micouin, L. Chem. Rev. 2004, 104,
2311−2352. (b) Hanessian, S.; Parthasarathy, S.; Mauduit, M.; Payza,
K. J. Med. Chem. 2003, 46, 34−48. (c) Casy, A. F.; Parfitt, R. T. Opioid
Alkaloids; Springer: Heidelberg, 1989; p 465. (d) Shiotani, S.;
Kometani, T.; Iitaka, Y.; Itai, A. J. Med. Chem. 1978, 21, 153−154.
(e) Belleau, B.; Conway, T.; Ahmed, F. R.; Hardy, A. D. J. Med. Chem.
1974, 17, 907−908. (f) Portoghese, P. S. J. Med. Chem. 1965, 8, 609−
616.
(3) Very few alkaloids possess an aminal carbon: Ban, Y.; Kimura, M.;
Oishi, T. Heterocycles 1974, 2, 323−328.
(4) Braekman, J. C.; Daloze, D.; Pasteels, J. M.; Vanhecke, P.;
Declercq, J. P.; Sinnwell, V.; Francke, W. Z. Naturforsch., C: Biochem.,
Biophys., Biol., Virol. 1987, 42, 627−630.
(5) Kem, W. R.; Wildeboer, K.; LeFrancois, S.; Raja, M.; Marszalec,
W.; Braekman, J. C. Cell Mol. Neurobiol. 2004, 24, 535−551.
(6) Rouchaud, A.; Braekman, J. C. Eur. J. Org. Chem. 2009, 2666−
2674.
Tetraponerine T5. Tetraponerine T5 was prepared from 13 (460
mg, 1.10 mmol), and 4-bromobutanal following the same procedure
described above for compound T7, affording the expected product as a
yellow oil (90 mg, 35%). Configurational isomers must be present:36
[α]20 + 14 (c 1.60, CHCl3), [lit.15 [α]20 + 10 (c 0.24, CHCl3)]; Rf
D
D
0.35 (9:1 CH2Cl2/MeOH); IR ν 3414, 2953, 2927, 2857, 2793, 1457,
1384, 1348, 1233, 1162, 1114, 1064 cm−1; 1H NMR (300 MHz,
CDCl3) δ 3.75−3.45 (m, 1H), 3.40−2.64 (m, 4H), 2.62−2.15 (m,
1H), 2.16−1.43 (m, 12H), 1.40−0.98 (m, 7H), 0.89 (dd, J = 9.0, 4.3
Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 59.9 (CH), 58.3 (CH), 53.9
(CH), 50.3 (CH2), 49.5 (CH2), 31.9 (CH2), 31.3 (CH2), 30.6 (CH2),
29.3 (CH2), 29.0 (CH2), 26.9 (CH2), 22.6 (CH2), 21.1 (CH2), 20.0
(CH2), 14.1 (CH3); GC tR = 13.0 min; LRMS (EI) m/z (%) 237 (5),
236 (M+, 42), 235 (100), 193 (13), 180 (12), 179 (81), 166 (15), 165
(6), 152 (11), 138 (27), 137 (12), 124 (10), 110 (21), 96 (43), 70
(18), 55 (7); HRMS (ESI) calcd for C15H29N2 (M+ + 1) 237.2331,
found 237.2324.
Tetraponerine T6. Tetraponerine T6 was prepared from 14 (255
mg, 0.61 mmol), and 4-bromobutanal following the same procedure
described above for compound T7, affording the expected product as a
yellow oil (52 g, 36%): [α]20D + 40 (c 0.75, CHCl3), [lit.15 [α]20D + 35
(c 0.31, CHCl3)]; Rf 0.50 (9:1 CH2Cl2/MeOH); IR ν 3395, 2954,
2928, 2858, 2784, 1458, 1378, 1204, 1188, 1161, 1064 cm−1; 1H NMR
(300 MHz, C6D6) δ 3.11−3.01 (m, 1H), 2.94 (td, J = 8.4, 2.5 Hz, 1H),
2.85 (t, J = 5.3 Hz, 1H), 2.48−2.36 (m, 1H), 2.32 (dd, J = 8.4, 5.3 Hz,
1H), 2.00−1.84 (m, 2H), 1.83−1.52 (m, 7H), 1.50−1.19 (m, 11H),
0.91 (t, J = 6.9 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 83.4 (CH),
64.2 (CH), 59.7 (CH), 49.1 (CH2), 45.9 (CH2), 34.7 (CH2), 33.5
(CH2), 32.6 (CH2), 30.6 (CH2), 29.2 (CH2), 25.9 (CH2), 23.1 (CH2),
21.3 (CH2), 20.9 (CH2), 14.4 (CH3); GC tR = 13.3 min; LRMS (EI)
m/z (%) 237 (M + 1, 5), 236 (M+, 40), 235 (100), 207 (5), 193 (15),
180 (8), 179 (54), 166 (17), 165 (10), 152 (13), 151 (10), 138 (38),
137 (14), 124 (12), 123 (5), 110 (14), 97 (7), 96 (35), 70 (24), 68
(7), 55 (6); HRMS (ESI) calcd for C15H29N2 (M+ + 1) 237.2331,
found 237.2333.
(7) Merlin, P.; Braekman, J. C.; Daloze, D.; Pasteels, J. M. J. Chem.
Ecol. 1988, 14, 517−527.
(8) Macours, P.; Braekman, J. C.; Daloze, D.; Pasteels, J. M.
Tetrahedron 1995, 51, 1415−1428.
(9) Garraffo, M. H.; Spande, T. F.; Jain, P.; Kaneko, T.; Jones, T. H.;
Blum, M. S.; Ali, T. M. M.; Snelling, R. R.; Isabell, L. A.; Robertson, H.
G.; Daly, J. W. Rapid Commun. Mass Spectrom. 2001, 15, 1409−1415.
(10) Devijver, C.; Braekman, J. C.; Daloze, D.; Pasteels, J. M. Chem.
Commun. 1997, 661−662.
(11) Renson, B.; Merlin, P.; Daloze, D.; Braekman, J. C.; Roisin, Y.;
Pasteels, J. M. Can. J. Chem. 1994, 72, 105−109.
(12) Yue, C.; Royer, J.; Husson, H.-P. J. Org. Chem. 1990, 55, 1140−
1141.
(13) For selected syntheses of enantioenriched tetraponerines:
(a) Airiau, E.; Girard, N.; Pizzeti, M.; Salvadori, M.; Taddei, M.;
Mann, A. J. Org. Chem. 2010, 75, 8670−8673. (b) Stragies, R.;
Blechert, S. J. Am. Chem. Soc. 2000, 122, 9584−9591. (c) Takahata, H.;
Kubota, M.; Ikota, N. J. Org. Chem. 1999, 64, 8594−8601.
(14) Guerrier, L.; Roger, J.; Grierson, D. S.; Husson, H.-P. J. Am.
Chem. Soc. 1983, 105, 7754−7755.
(15) Yue, C. W.; Gauthier, I.; Royer, J.; Husson, H.-P. J. Org. Chem.
1996, 61, 4949−4954.
(16) Bosque, I.; Gonzalez-Gomez, J. C.; Guijarro, A.; Foubelo, F.;
Yus, M. J. Org. Chem. 2012, 77, 10340−10346.
(17) A definition of the stereochemical descriptors used is in order
here. The first descriptor refers to the configuration of the nitrogen
N11, with its lone pair either cis or trans to H6a. Next, the relative
stereochemistry of the quinolizidine and indolizidine frameworks (i.e.,
the rings AB and BC) was defined as either cisoid- or transoid-. Finally
the indolizidine ring fusion BC was again defined as either cis- or
trans-. Thus, as depicted in the example, the ttc-isomer would
correspond to a trans quinolizidine-transoid-cis indolizidine.
(18) Rouchaud, A.; Braekman, J.-C. Eur. J. Org. Chem. 2011, 2346−
2353.
ASSOCIATED CONTENT
* Supporting Information
■
S
Copies of 1H and 13C NMR spectra for compounds 5−14, T1,
T2, and T5−T8 are provided. GC−MS for T1−T8, HPLC
traces of compounds 11 and 12, crystallographic data (CIF) for
compound 12, as well as cytotoxic data for T5−T8 against four
different carcinoma cell lines are also included. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: josecarlos.gonzalez@ua.es.
■
Notes
The authors declare no competing financial interest.
(19) For a comprehensive review on the application of tert-
butylsulfinamides, see: Ellman, J. A.; Robak, M. T.; Herbage, M. A.
Chem. Rev. 2010, 110, 3600−3740.
ACKNOWLEDGMENTS
We thank the Spanish Ministerio de Ciencia e Innovacion
their financial support (CTQ2011-24165). I.B. acknowledges
the Generalitat Valenciana for a predoctoral fellowship (ACIF/
2011/159). We thank Medalchemy for a gift of (RS)-N-tert-
butylsulfinamide and its enantiomer.
■
́
for
(20) (a) Bosque, I.; Gonzalez-Gomez, J. C.; Foubelo, F.; Yus, M. J.
Org. Chem. 2012, 77, 780−784. (b) Gonzalez-Gomez, J. C.; Medjahdi,
M.; Foubelo, F.; Yus, M. J. Org. Chem. 2010, 75, 6308−6311.
(21) For a review on diastereoselective allylation of imines and its
application to the synthesis of natural products, see: Yus, M.;
Gonzalez-Gomez, J. C.; Foubelo, F. Chem. Rev. 2013, 113, 5595.
(22) Barluenga, J.; Tomas, M.; Kouznetsov, V.; Rubio, E. J. Org.
Chem. 1994, 59, 3699−3700.
REFERENCES
■
(1) (a) Wang, K.; Su, B.; Wang, Z.; Wu, M.; Li, Z.; Hu, Y.; Fan, Z.;
Mi, N.; Wang, Q. J. Agric. Food Chem. 2010, 58, 2703−2709.
3990
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Article
(23) (a) Medjahdi, M.; Gonzalez-Gomez, J. C.; Foubelo, F.; Yus, M.
Heterocycles 2012, 86, 727−734. (b) Gonzalez-Gomez, J. C.; Foubelo,
F.; Yus, M. Synlett 2008, 18, 2777−2780.
(24) Fustero, S.; Monteagudo, S.; Sanchez-Rosello, M.; Flores, S.;
Barrio, P.; del Pozo, C. Chem.Eur. J. 2010, 16, 9835−9845.
(25) It is reported that cis-olefins are more sterically accessible to
ruthenium alkylidene catalysts in the cross-metathesis reaction. See:
Ulman, M.; Grubbs, R. H. Organometallics 1998, 17, 2484−2489.
(26) The presence of rotamers made difficult the determination of
the alkene double bond stereochemistry; however, this issue has no
consequences in the present synthetic approach.
(27) The crystallographic data of this compound (CDCC 971540)
can be found in the Supporting Information.
(28) The NMR data was compared to the one reported in ref 15;
however, to the best of our knowledge, the NMR traces of T1 or T2
are not reported in the literature.
(29) Kim, J. T.; Butt, J.; Gevorgyan, V. J. Org. Chem. 2004, 69, 5638−
5645.
(30) The NMR data was compared to the one reported in ref 15;
however, to the best of our knowledge, the NMR traces of T5 are not
reported in the literature.
(31) These bands are commonly used to elucidate the structure of
alkaloids; see, for example: Gribble, G. W.; Nelson, R. B. J. Org. Chem.
1973, 38, 2831−2834.
(32) Wolfe, S.; Schlegel, B.; Whangbo, M.-H. Can. J. Chem. 1974, 52,
3787−3792.
(33) See inhibition vs concentration curves in the Supporting
Information.
(34) The reactions were run in parallel at the scale of 0.3 mmol
maximum each because we observed lower conversion at higher scale.
(35) 4-Bromobutanal is commercially available, but we prepared it by
DIBAL-H reduction of the corresponding methyl ester.
(36) To ensure that the amine was not partially protonated, the
sample was washed with saturated solution of NaHCO3 before
submitting to NMR experiments. Similar NMR spectra were obtained
in CDCl3 and CD2Cl2 (where traces of HCl are less likely).
3991
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