Biomimetic synthesis and biological evaluation of aplidiopsamine A

Article abstract of DOI:10.1021/ol3024665

The first total synthesis of Aplidiopsamine A, a rare 3H-pyrrolo[2,3-c] quinoline alkaloid from the Aplidiopsis confluata, has been achieved following the proposed biosynthesis. This biomimetic synthesis requires only five steps and proceeds in 20.8% overall yield. Biological evaluation across large panels of discrete molecular targets identified that Aplidiopsamine A is a highly selective PDE4 inhibitor, a target for numerous CNS disorders.

Full text of DOI:10.1021/ol3024665

ORGANIC
LETTERS
2012
Vol. 14, No. 22
5808–5810
Biomimetic Synthesis and Biological
Evaluation of Aplidiopsamine A
Joseph D. Panarese and Craig W. Lindsley*
Departments of Chemistry and Pharmacology, Vanderbilt University, Nashville,
Tennessee 37232, United States
craig.lindsley@vanderbilt.edu
Received September 6, 2012
ABSTRACT
The first total synthesis of Aplidiopsamine A, a rare 3H-pyrrolo[2,3-c]quinoline alkaloid from the Aplidiopsis confluata, has been achieved
following the proposed biosynthesis. This biomimetic synthesis requires only five steps and proceeds in 20.8% overall yield. Biological evaluation
across large panels of discrete molecular targets identified that Aplidiopsamine A is a highly selective PDE4 inhibitor, a target for numerous CNS
disorders.
The tricyclic 3H-pyrrolo[2,3-c]quinoline ring system is a
rare moiety found only in a few marine natural products.1,2
The first members of this structural class were the mar-
inoquinolines AꢀF (1ꢀ5), isolated from multiple species
of bacteria and found to possess antibacterial, antifungal,
and acetylcholinesterase activities (Figure 1).1,2 Several
synthetic approaches to marinoquinolines AꢀC and E
(1ꢀ3, 5) have been reported, relying on either a key
MorgenꢀWalls or PictetꢀSpengler reaction.3,4 Recently,
aplidiopsamine A (6) was isolated from the Australian
ascidian, Aplidiopsis confluata, and represents the first
example of the 3H-pyrrolo[2,3-c]quinoline moiety linked
to an adenine via a methylene bridge, a rare nonglycoside
adenine conjugate. Thus, 6 represents only the second
report of a marine organism producing the 3H-pyrrolo-
[2,3-c]quinoline ring system; importantly, 6 was found to
be a potent antimalarial agent, without toxicity to healthy
cells, further providing support for synthesis.5
Figure 1. Structures of the marinoquinolines AꢀF (1ꢀ5) and
aplidiopsamine A (6), marine natural products possessing the
rare 3H-pyrrolo[2,3-c]quinoline ring system (shown in red).
(1) Sangoi, Y.; Sakuleko, O.; Yuenyongsawad, S.; Kanjana-opas, A.;
Ingkaninan, K.; Plubrukan, A.; Suwanborirux, K. Mar. Drugs 2008, 6,
578–586.
(2) Okanya, P. W.; Mohr, K. I.; Gerth, K.; Jansen, R.; Muller, R.
J. Nat. Prod. 2011, 74, 603–608.
(3) Ni, L.; Li, Z.; Wu, F.; Xu, J.; Wu, X.; Kong, L.; Yao, H.
Tetrahedron Lett. 2012, 53, 1271–1274.
(4) Schwalm, C. S.; Correia, C. R. D. Tetrahedron Lett. 2012, 53,
4836–4840.
Carroll and co-workers developed a convergent biosyn-
thetic proposal for 6 from adenine 7 and tryptophan 10
(Scheme 1). Here, 9 is generated from the reaction of
adenine and hydroxyacetic acid. Pyrrole 11, itself a natural
product isolated from R. thailandica, was postulated to be
a dead-end product in the biosynthesis of pyrrolnitrin from
tryptophan. Condensation of 9 and 11 would ultimately
(5) Carroll, A. J.; Duffy, S.; Avery, V. M. J. Org. Chem. 2010, 75,
8291–8294.
r
10.1021/ol3024665
Published on Web 10/29/2012
2012 American Chemical Society

lead to aplidiopsamine A, via a BischlerꢀNapieralski-like
cyclization and dehydration sequence.5
Scheme 2. Biomimetic Total Synthesis of Aplidiopsamine (6)
Scheme 1. Proposed Biosynthesis of Aplidiopsamine (6)
Therefore, we elected to pursue a biomimetic approach
for the first total synthesis of aplidiopsamine A and
attempt the condensation of a suitable synthetic surrogate
of 9 and pyrrole 11 to access 12. Interestingly, Correia
synthesized 11, en route to marinoquinolines AꢀC and E,
from advanced materials in five synthetic steps in 47.5%
overall yield.4 We envisioned a more expedited route and
were gratified that a two-step sequence from commercial
reagents produced 11. In the event, aniline 14 smoothly
underwent a Suzuki coupling with boronate ester 13 to
deliver TIPS protected pyrrole 15, which upon basic
hydrolysis generated 11 in 96% yield over the two steps
(Scheme 2).6 Acylation with R-bromo acetyl bromide
provided 16 in 86% yield. Selective N-alkylation at the
9- versus 7-position of adenine to afford 12 was achieved
via a hard deprotonation with NaH in 56% yield. Softer
approaches with cesium carbonate resulted in 2:1 regio-
siomeric mixtures and poor conversion. With 12 in hand,
we were now posied to perform the biomimetic condensa-
tion via a BischlerꢀNapieralski-type cyclization. Classical
variants of this cyclization/dehydration sequence7 employ
TFA, POCl3, or other Lewis acids; however, in the pre-
sence of the adenine moiety, these conditions either failed
or led to intractable gums or poor conversion (less than
10% yields). Ultimately, we found that 4 M HCl/dioxanes
under microwave irradiation (130 °C, 10 min) facilitated
the reaction sequence to deliver, for the first time, apli-
diopsamine A in 45% yield.6 Overall, the biomimetic
synthesis of 6 required five steps and proceeded in 20.8%
overall yield, an ideal route to prepare unnatural analogs.
The synthetic 6 exhibited physical and spectroscopic data
identical to those of the natural aplidiopsamine A.5,6
With large quantities of 6 in hand, we elected to further
profile 6 against a larger panel of discrete molecular targets
of therapeutic significance beyond antimalarial activity, as
6 was shown to not be cytotoxic. Indeed, we have pre-
viously elucidated intriguing activities for a number of
marine alkloids at CNS targets with unprecedented selec-
tivities among highly conserved receptor families.8ꢀ11 As
6 possesses the basic pharmacophore (H-bond donor/
acceptor triad) of many known ATP-competitive kinase
inhibitors,12 we profiled synthetic 6 in a KINOMEscan
panel against 97 kinases (both wild-type and mutants) at a
10 μM concentration.6,13 Despite the presence of a known
pharmacophore,12 6 was uniformly inactive; however, this
can also be viewed as possessing very clean ancillary
pharmacology. In parallel, we also profiled 6 in a Lead
Profiling screen (a radio ligand binding panel at a 10 μM
concentration) against 68 GPCRs, ion channels, and
transporters.14 Once again, 6 displayed very clean ancillary
pharmacology, possessing activity at only two targets:
5-HT2B (62% @ 10 μM) and PDE4/Rolipram (74% @
10μM). Whenfullconcentrationꢀresponse curves(CRCs)
were obtained for 6, the5-HT2B Ki was weak at ∼10 μM, but
6 proved to be a moderately potent PDE4 inhibitor (Ki =
1.2 μM, IC50 = 3.3 μM).6,14 PDE4 is a high profile target
for antidepressant, antipsychotic, and neuroprotective drug
(8) Kennedy, J. P.; Brogan, J. T.; Lindsley, C. W. J. Nat. Prod. 2008,
71, 1783–1788.
(9) Aldrich, L. N.; Stoops, S. L.; Crews, B. C.; Marnett, L. M.;
Lindsley, C. W. Bioorg. Med. Chem. Lett. 2010, 20, 5207–5211.
(10) Brogan, J. T.; Stoops, S. L.; Crews, B. C.; Marnett, L. M.;
Lindsley, C. W. ACS Chem. Neurosci. 2011, 2, 633–639.
(11) Brogan, J. T.; Stoops, S. L.; Lindsley, C. W. ACS Chem.
Neurosci. 2012, 3, 658–664.
(12) Lewis, J. A.; Lebois, E. P.; Lindsley, C. W. Curr. Opin. Chem.
Biol. 2008, 12, 269–279.
(13) For information of the KIONMEscan panel assay, see: www.
discoverx.com
(14) For information on the Lead Profiling screen, see: www.ricerca.
com.
(15) Pages, L.; Gavalda, A.; Lehner, M. D. Expert Opin. Ther. Pat.
2009, 19, 1501–1519.
(6) See Supporting Information for full details.
(7) (a) Kennedy, J. P.; Breininger, M. L.; Lindsley, C. W. Tetrahedron
Lett. 2009, 50, 7067–7069. (b) Capilla, A. S.; Romero, M.; Pujol, M. D.;
Caignard, D. H.; Renard, P. Tetrahedron 2001, 57, 8297–8303.
(16) Burgin, A. B.; Magnusson, O. T.; Witte, P.; Bjornsson, J. M.;
Thorsteindottir, S.; Hage, T.; Kiselyov, A. S.; Stewart, L. J.; Gurnet,
M. E. Nat. Biotechnol. 2010, 28, 63–70.
Org. Lett., Vol. 14, No. 22, 2012
5809

development; moreover, 6 represents a fundamentally new
PDE4 inhibitor chemotype.15,16
acyclic substrate 12, devoid of the 3H-pyrrolo[2,3-c]-
quinoline ring system. Overall, this suggests that the
tricyclic 3H-pyrrolo[2,3-c]quinoline ring system with a
C9 amino methyl moiety is essential for PDE4 activity,
and this chemotype represents a fundamentally new PDE4
pharmacophore.
With the identification of a therapeutically relevant
target for 6, the need emerged to develop rapid chemistry
to enable the synthesis of unnatural analogs, and ideally,
from a common intermediate. Application of our opti-
mized cyclization/dehydration conditions with the ad-
vanced R-bromo intermediate 16 led directly to the 3H-
pyrrolo[2,3-c]quinoline derivative 17, wherein the reaction
conditions with HCl generated the corresponding chloro-
methyl congener (Scheme 3). Notably, high reaction
temperatures typically employed in BichlerꢀNapieralski
reactions were avoided as the condensation was found to
proceed at room temperature. Compound 17 proved to be
an ideal common intermediate for not only the synthesis of
unnatural analogs of 6, compounds 18aꢀ18c, in high
yields (82ꢀ96%) but also the synthesis of marinoquinoline
A (1) when the nucleophile was a hydride source, LiAlH4.
This approach affords the natural product 1 in only
five steps and in 52% overall yield, representing a highly
expeditious route.3,4
Table 1. Phosphodiesterase PDE4 Activities of 6 and Unnatural
Analogs
rat Roliprama
human PDE4b
compd
Ki (μM)
IC50 (μM)
IC50 (μM)
6
1.2
3.3
6.1
1
>100
25.6
4.7
>100
71.7
13.1
14.0
>100
>100
31.8
72.9
>100
>100
18a
18b
18c
12
5.0
>100
a Wistar rat brain, 1.8 nM [3H] rolipram, 1% DMSO, incubation for
60 min at 4 °C in 50 mM Tris-HCI, pH 7.4. b Human U937 cells, 1.10 μM
[3H]cAMPþcAMP, incubation for 20 min at 25 °C, 50 mM Tris-HCl,
pH 7.5, 5 mM MgCl2, quantitaion of [3H]adenosine.14
Scheme 3. Synthesis of Marinoquinoline A (1) and Unnatural
Analogs of 6 from a Common Intermediate
In summary, we have completed the first total synthesis
of aplidiopsamine A (6) following the proposed biosyn-
thetic pathway in five steps and with a 20.8% overall yield.
Extensive biological profiling identified 6 as a moderately
potent ligand for, and inhibitor of, PDE4, an important
target for CNS drug discovery. These data further argue
for natural products as a source of novel chemotypes to
initiate drug discovery efforts. Furthermore, we developed
chemistry to rapidly access unnatural analogs of 6 and also
synthesized marinoquinoline A (1) in only five steps and in
52% overall yield. The SAR within this novel class of
PDE4 ligands demonstrated that both the tricyclic 3H-
pyrrolo[2,3-c]quinoline ring systemand a C9 aminomethyl
moiety are required for PDE4 activity. Further refine-
ments, DMPK analysis, and additional unnatural analog
synthesis are underway and will be reported in due course.
Acknowledgment. This work was supported, in part, by
the Department of Pharmacology and William K. Warren,
Jr. Funding for the NMR instrumentation was provided in
part by a grant from the NIH (S10 RR019022).
With unnatural analogs of 6 in hand, we evaluated them
against both rat PDE4 (Rolipram) and human PDE4 to
discern structureꢀactivity relationships (SAR). As shown
in Table 1, several of the unnatural analogs possessed
PDE4 actvity but were weaker than 6 (rat Ki’s of 4.6 to
25 μM and IC50’s of 13 to 71 μM). All proved to be weaker
functional antagonists on human PDE4 than rat. Interest-
ingly, marinoquinoline A (1), lacking the amino methyl
moiety at C9, was inactive as was the BischlerꢀNapieralski
Supporting Information Available. Experimental pro-
1
cedures, characterization data, and H and 13C NMR
spectra for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
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Org. Lett., Vol. 14, No. 22, 2012