Diverted organic synthesis (DOS): Accessing a new, natural product inspired, neurotrophically active scaffold through an intramolecular Pauson-Khand reaction

Article abstract of DOI:10.1039/c2ob26107c

Drawing inspiration from the impressive neurotrophic activity exhibited by the natural product paecilomycine A, we have designed a new natural product-like scaffold employing an intramolecular Pauson-Khand reaction. Several compounds based on the new designer scaffold exhibited promising neurotrophic activity and are worthy of further biological evaluation. Our findings also highlight the importance of a DOS strategy in creating useful therapeutical leads.

Full text of DOI:10.1039/c2ob26107c

View Online / Journal Homepage
Organic &
Biomolecular
Chemistry
Accepted Manuscript
This is an Accepted Manuscript, which has been through the RSC Publishing peer
Organic &
review process and has been accepted for publication.
Biomolecular
Accepted Manuscripts are published online shortly after acceptance, which is prior
to technical editing, formatting and proof reading. This free service from RSC
Publishing allows authors to make their results available to the community, in
citable form, before publication of the edited article. This Accepted Manuscript will
be replaced by the edited and formatted Advance Article as soon as this is available.
Chemistry
www.rsc.org/obc
Volume 8 | Number 3 | 7 February 2010 | Pages 481–716
To cite this manuscript please use its permanent Digital Object Identifier (DOI®),
which is identical for all formats of publication.
More information about Accepted Manuscripts can be found in the
Information for Authors.
Please note that technical editing may introduce minor changes to the text and/or
graphics contained in the manuscript submitted by the author(s) which may alter
content, and that the standard Terms & Conditions and the ethical guidelines
that apply to the journal are still applicable. In no event shall the RSC be held
responsible for any errors or omissions in these Accepted Manuscript manuscripts or
any consequences arising from the use of any information contained in them.
ISSN 1477-0520
PERSPECTIVE
FULL PAPER
Laurel K. Mydock and
AlexeiV. Demchenko
Shuji Ikeda et al.
Hybridization-sensitive fluorescent
DNA probe with self-avoidance ability
Mechanism of chemical O-glycosylation:
from early studies to recent discoveries
1477-0520(2010)8:3;1-H
www.rsc.org/obc
Registered Charity Number 207890

Organic & Biomolecular Chemistry
Page 1 ofJ4ournal Name
Dynamic Article Links ►
View Online
Cite this: DOI: 10.1039/c0xx00000x
www.rsc.org/xxxxxx
ARTICLE TYPE
Diverted organic synthesis (DOS): Accessing a new, natural product
inspired, neurotrophically active scaffold through intramolecular
Pauson-Khand reaction
Goverdhan Mehta,*a Ramesh Samineni,b Pabbaraja Srihari,*b R. Gajendra Reddy,c Sumana
Chakravarty.c
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
products. As part of this endeavour, we5 and others6 have recently
Drawing inspiration from the impressive neurotrophic activity
reported a synthesis of paecilomycine A 1, a tricothecane derived
50 sesquiterpenoid natural product, shown to enhance and promote
impressive (10nM concentration) neurite outgrowth in PC-12
cells.7 In our synthesis of paecilomycine A and analogues, an
intramolecular Pauson-Khand reaction (IPKR) between tethered
vicinal alkyne and alkene side arms was the pivotal step5 (2 → 3
55 and 4 → 5) to generate its core architecture, Scheme 1.
exhibited by the natural product paecilomycine A, we have
10 designed a new natural product-like scaffold employing
intramolecular Pauson-Khand reaction. Several compounds
based on the new designer scaffold exhibited promising
neurotrophic activity and are worthy of further biological
evaluation. Our findings also highlight the importance of DOS
15 strategy in creating useful therapeutical leads.
H
H
O
H
O
Diverted organic synthesis (DOS) and diverted total synthesis
(DTS) are powerful strategies for creating chemical diversity and
for navigation through unexplored chemical space.1 DOS and
DTS are not mutually exclusive terms and encompass an arena
20 that enables access to complex, non-obvious, high-pedigree
scaffolds for therapeutic profiling. While DOS is an opportunistic
tactic that may derive inspiration from natural product synthesis
efforts to generate new molecular ensembles, DTS is essentially a
modality to access edited and reformed variants of a promising
25 natural product lead structure, through an advanced intermediate
in its synthesis, without compromising the integrity of natural
product core architecture. However, in essence, both DOS and
DTS are adaptive and productive off-shoots of a total synthesis
endeavour.
10
H
IPKR
16
O
9
13
11
14
OH
8
6
RO
12
RO
7
5
2
3
O
2
3
4
15
OH
H
H
O
O
O
Paecilomycine A (1)
H
IPKR
RO
RO
O
5
4
Scheme 1: IPKR based strategy for the synthesis of 1
Impressed by the exceptional neurotrophic activity exhibited by
1, we felt encouraged to initially generate5a diversity around its
60 core structure and then to extend the key IPKR strategy to create
a new bioactive scaffold. In this context, we recognized the
presence of 2-oxa-spiro[5.5]undecane segment (see highlighted
portion) embedded within paecilomycine A 1 framework.
Notably, 2-oxa-spiro[5.5]undecane sub-structure is also present in
65 many important natural products of contemporary therapeutic
30 One of the emerging healthcare challenges posed by the aging
world population is the maintenance of cognitive functions and
the ability to treat neurodegenerative disorders like dementia and
Alzheimer’s.2 In this quest, chemical entities that can repair and
regenerate neurons hold the key. Nature does provide such a
35 mechanism in the form of peptide based neurotrophins like NGF,
BDNF, NTF3 etc but their action and role needs to be sustained,
stimulated and augmented. In recent years, several small
molecule natural products (SMNP’s) have been found to promote
neurite growth and synaptic plasticity by up-regulating the
40 activity of neurotrophins.3 These observations besides providing
useful leads for developing therapeutic agents for slowing down
neurodegeneration and treating related disorders, underscore the
pressing need for synthetic chemists’ intervention through DOS
and DTS to design new scaffolds based on natural product leads
45 and to amplify natural product diversity.1,3
interest like maoecrystal
V A
6,8 spirotenuipesine 7,9,
rumphellclovane 8,10 ternifolide 1011 and jangomolide 1112 as part
of their interlaced molecular architecture (Figure 1). Taking a cue
from this observation, we decided to design a new scaffold that
70 incorporates
a
2-oxa-spiro[5.5]undecane fragment in
a
‘paecilomycine A 1’ like structural ambiance. We describe here a
short, straightforward access to a 2-oxa-spiro[5.5]undecane based
conceptualization 9 and observe that several derivatives based on
this scaffold exhibit impressive neurotrophic activity.
75 Generation of the new scaffold 9 was envisaged through an
adaptation of the key IPKR steps (2 → 3 and 4 → 5) in our
synthesis of the paecilomycine A. It was envisioned that by
employing a variant 12 of 2, with tethered enyne moieties
For some time now, our group4 has been actively engaged in the
synthesis of complex, diverse and neurotrophically active natural
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

Organic & Biomolecular Chemistry
Page 2 of 4
View Online
occupying geminal positions, on IPKR should deliver
‘paecilomycine A-like’ 13 that embodies 2-oxa-
the spiro-fused tricyclic hydroxy-enone 23, Scheme 5. Two
40 derivatives of 23 were prepared stereoselectively through Luche
reduction to tricyclic diol 25 and nucleophilic epoxidation to
epoxide 26 for bioassay purposes, Scheme 6.
spiro[5.5]undecane fragment, Scheme 2. From synthetic strategy
considerations, precursor 12 was derivable by simply ‘swapping’
the propargyl and OTBS groups in 2. Thus, accessing 12 was our
first objective and a route to it was devised by creating an early
diversion within our synthesis of the natural product 1.
5
Reagents and conditions: (a) NaH, HMPA, THF, 3-(chloromethoxy)prop-
o
o
45 1-yne, TBAI, -10 C, 3 h, 74%; (b) CeCl3.7H2O, NaBH4, MeOH, -15 C,
15 min, 82%; (c) TBSCl, imidazole, CH2Cl2, 8 h, 84%; (d) DIBAL-H, 0
oC, 1 h, CH2Cl2, 93%; (e) IBX, DMSO, THF, rt, 2 h, 90%; (f) BuOK,
t
PPh3CH2I, THF, 0 oC, 15 min, 69%; (g) i) Co2(CO)8, CH2Cl2, 1 h; ii)
NMO, CH2Cl2, 4 h, 75%.
50
Scheme 3: Synthesis of tricyclic spiro-fused scaffold 20
Figure 1: Few natural products that contain spiro scaffold 9
10
Scheme 2: key step for generating the new spiro-fused scaffold
Ethyl 4-methyl-2-oxocyclohex-3-enecarboxylate 14, previously
deployed5a in our earlier foray towards paecilomycine A synthesis
Figure 2: ORTEP diagram of 20
was
subjected
to
propargyloxy-methylation
with
15 (propargyloxy)methyl chloride13 to furnish 15, Scheme 3. Luche
reduction14 in 15 was stereoselective and furnished α-hydroxy
compound 16 in which the hydroxyl group was protected as TBS
ether 17. Ester reduction in 17 with DIBAL-H and IBX oxidation
furnished the aldehyde 18. Wittig olefination in 18 was
20 uneventful and furnished the required IPKR precursor enyne 19.
The stage was now set for the key intramolecular Pauson-Khand
reaction and exposure of 19 to Co2(CO)8 furnished the spiro-fused
tricyclic enone 20 as a single diastereomer in a decent 75% yield,
Scheme 3. Stereostructure of 20 was secured through single
25 crystal X-ray structure determination and it was interesting to
note that intramolecular [2+2+1]-cycloaddition (IPKR) occurred
preferentially from the -face. An ORTEP diagram of 20 is
displayed in the Figure 2. To generate variegated functionalities
on the core structure, we carried out Luche reduction14 in 20 to
30 deliver stereoselectively the allylic alcohol 21 (stereochemistry at
newly generated centre was secured from NOESY analysis). An
epoxide 22 could be readily prepared from 20 via nucleophilic
epoxidation,15 Scheme 4. However, TBS deprotection in 20 to
deliver hydroxyl-enone 23 proved to be unexpectedly
35 problematic and an alternative way was sought. It was observed
that TBS deprotection in the IPKR precursor enyne 19 was no
issue at all and the resulting 24 could be induced to undergo
stereoselective intramolecular Pauson-Khand reaction to furnish
Reagents and conditions: (a) CeCl3.7H2O, NaBH4, MeOH, -15 oC, 15
55 min, 78%; (b) H2O2, 6 N NaOH, MeOH, 0 oC, 2 h, 83%;
Scheme 4: Functional group modifications on 20
Reagents and conditions: (a) TBAF, THF, rt, 12 h, 92%; (b) i) Co2(CO)8,
CH2Cl2, 1 h; ii) NMO, CH2Cl2, 6 h, 79%.
60
Scheme 5: Synthesis of hydroxy-enone 23
2
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 3 of 4
Organic & Biomolecular Chemistry
View Online
Reagents and conditions: (a) CeCl3.7H2O, NaBH4, MeOH, -15 oC, 15
min, 74%; (b) H2O2, 6 N NaOH, MeOH, 0 oC, 2 h, 78%.
Scheme 6: Functional group modifications on 23
5
Biological evaluation
With six new compounds 20, 21, 22, 23, 25 and 26, of secured
stereostructures and based on the novel scaffold 9 in hand,
attention was turned towards profiling their CNS activity using
Figure 3: Bar representation of the neurite outgrowth of differentiated
45 Neuro2a cells by the more active compounds 20, 23, 25, 26 and
comparison with control and NGF and honokiol
10 Neuro-2a cell lines.16,17 Interestingly, all the six compounds 20-
23, 25 and 26, embodying the 2-oxa-spiro[5.5]undecane
fragment, were found to be neuroprotective in standard MTT
assay and Trypan blue assay for cell viability.16,18,19 Encouraged
by this preliminary observation, we further examined the ability
15 of our compounds to stimulate neurite outgrowth in Neuro2a cells
following standard cell culture method.16,18 Cells were grown
accordingly and then exposed to the compounds (20, 21, 22, 23,
25 and 26) along with NGF (positive control) and natural product
honokiol (a standard reference for neurotrophic activity) at
20 different concentrations for 48 hours. All compounds were
diluted in DMSO and its concentration in the culture medium
was not more than 0.1%, a concentration threshold at which
DMSO did not affect cell growth or death. Cells were incubated
for ~ 48 hours and analyzed for neurite outgrowth under a
25 microscope using ImageJ software.
Figure 4: Immunocytochemical images stained with beta III tubulin
showing optimum changes in neurite outgrowth of diffentiated Neuro2a
50 cells with control, 23 and 25, respectively.
Conclusions
Our quest for novel structures that can exhibit neurotrophic
activity, inspired by the natural product paecilomycine A, has led
Average Neurite Length
per Neuron(µM)
Percent change
from control
to the design of
a
new scaffold based on 2-oxa-
Treatment
55 spiro[5.5]undecane framework. The newly conceptualized
scaffold and many of its derivatives have been accessed through a
concise IPKR based strategy and attendant functional group
modifications. It was gratifying to note that all the compounds
based on the 2-oxa-spiro[5.5]undecane scaffold were neuro-
60 protective and exhibited noteworthy neurotrophic activity,
warranting further developmental efforts in the area.
Control
DMSO
39.70±2.41
42.54±2.35
63.68±2.73
59.59±3.35
53.56±3.10
47.36±3.20
45.07±2.85
58.64±4.24
61.78±2.89
56.19±3.62
-
7.15
NGF(100ng/mL)
Honokiol (1µM)
20 (0.01µM)
21 (0.01µM)
22 (0.01µM)
23 (0.01µM)
25 (0.01µM)
26 (0.01µM)
60.40
50.10
34.91
19.29
13.53
47.70
55.62
41.54
Acknowledgments
Table: Morphometric analysis of neurite outgrowth of differentiated
Neuro2a cells cultured for 48 h in DMSO (0.1%), Compounds (0.01µM).
The tabulated data and bar graphs were expressed as mean±SEM where
n=60and *p<0.01 versus control.
This research was carried out under the Indo-French “joint
laboratory for sustainable chemistry at interfaces”. We thank Dr.
65 Saikat Sen for solving the X-ray crystal structure at the CCD
facility of the University of Hyderabad. RS and RGR thank CSIR
for the award of research fellowship. GM thanks the Government
of India for the award of National Research Professorship and
acknowledges current research support from Eli Lilly and
70 Jubilant-Bhartia Foundations. PSH thanks CSIR, New Delhi for
financial support. SC acknowledges the financial assistance of
Department of Biotechnology (DBT), India, for biology part,
under Ramalingaswami re-entry Fellowship.
30 Table and Figure 3 display increase in neurite length per neuron
(average of 60 neurons for each compound) after 48 hours of
exposure to the test compounds along with the controls.
Gratifyingly, the percentage change in neurite outgrowth
mediated by our compounds was quite comparable to that
35 induced by NGF and honokiol, Table. However, it was quite
remarkable to find that compounds based on our designer
scaffold exhibited activity comparable with honokiol at ~100-
times lower concentration, Figure 3. Significant neurite
outgrowth affected by representative designer compounds 23 and
40 25 on neuro 2a cells vis a vis the control can be seen in Figure 4.
75
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3

Organic & Biomolecular Chemistry
Page 4 of 4
View Online
12. J. Ahmad, K. Wizart, K. M. Shamsuddin, A. Zaman, J. D. Connoly
Phytochemistry 1984, 23, 1269-1270.
13. A. Hasan, P. C. Srivastava J. Med. Chem. 1992, 35, 1435-1439.
14. J. L. Luche J. Am. Chem. Soc. 1978, 100, 2226-2227.
15. M. Miyashita, T. Suzuki, A. Yoshikoshi J. Am. Chem. Soc. 1989,
111, 3728-3734
75 16. M. Yamazaki, K. Chiba J. Health. Sci. 2005, 51, 687-692.
17. M. Yamazaki, K. Chiba, K. Satoh J. Health. Sci. 2008, 54, 638-644.
18. V. P. Kumar, R. G. Reddy, D. D. Vo, S. Chakravarthy, S.
Chandrasekhar, R. Gree Bioorg. Med. Chem. Lett. 2012, 22, 1439-
1444.
Notes and references
70
a School of Chemistry, University of Hyderabad, Hyderabad- 500 046,
India. Fax: : +91 4023012460; Tel: +91 4023010785; E-mail:
gmsc@uohyd.ernet.in, gm@orgchem.iisc.ernet.in (G. Mehta).
b Division of Natural Products Chemistry, CSIR-Indian Institute of
Chemical Technology, Hyderabad-500 607, India.
5
c Chemical biology/ Division of Natural Products Chemistry, CSIR-
Indian Institute of Chemical Technology, Hyderabad-500 607, India.
10 † Electronic Supplementary Information (ESI) available: [Experemental
details and 1H, 13C spectras available here]. See DOI: 10.1039/b000000x/
80 19. Y. P. Wang, Z. F. Wang, Y. C. Zhang, Q. Tian, J. Z. Wang cell
culture 2004, 14, 467-472.
‡ Single crystal X-ray diffraction data were collected on a Bruker AXS
SMART APEX CCD diffractometer at 291
K using graphite
15 monochromated MoKα radiation (λ = 0.7107Å). The data were reduced
by SAINTPLUS; an empirical absorption correction was applied using the
package SADABS and XPREP was used to determine the space group.
The crystal structures were solved by direct methods using SIR92 and
85
90
95
refined by full-matrix least-squares method on F2 using SHELXL97.
1
20 Crystal data for 20: C20H32O3Si• (C6H6), M = 387.6, triclinic, P-1, a =
2
7.5526(11), b = 8.7285(13), c = 17.680(3) Å, α = 85.539(3), β =
82.149(3), γ = 87.533(3)°, V = 1150.4(3) Å3, Z = 2, ρcalcd = 1.119 g/cm3,
8616 reflections measured, 4204 unique (Rint = 0.0718), R1 = 0.1084 and
wR2 = 0.2088 for 2233 observed reflections, CCDC-876473.
25
1.
Several recent reviews capture the flavor and importance of the
DOS-DTS arena: For reviews see: (a) R. M. Wilson, S. J.
Danishefsky J. Org. Chem. 2007, 72, 4293-4305. (b) P. A. Wender,
B. A. Miller Nature 2009, 460, 197-201. (c) R. M. Wilson, S. J.
Danishefsky Angew. Chem. Intl. Ed. 2010, 49, 6032-6056. (d) A. M.
Szpilman, E. M. Carriera Angew. Chem. Intl. Ed. 2010, 49, 9592-
9628. (e) J. J. La Claire Nat. Prod. Rep. 2010, 27, 969-995. (f) A.
Furstner Isr. J. Chem. 2011, 51, 329-345. (g) C. J. O’Connor, H. S.
G. Beckmann, D. R. Spring Chem. Soc. Rev. 2012, 41, 4444-4456.
Reviews: (a) S. L. Miksys, R. F. Tyndale Clinical Pharmacology
and Therapeutics 2010, 88, 427-430. (b) A. Autrey, L. M.
Monteggie Pharmacological Reviews 2012, 64, 238-258.
Review: (a) R. M. Wilson, S. J. Danishefsky Acc. Chem. Res. 2006,
39, 539-549. (b) A. P.-J. Chen, C. C. Muller, H. M. Cooper, C. M.
Williams Tetrahedron 2010, 66, 6842-6850 and references cited
therein.
30
100
35 2.
105
110
115
120
125
130
135
3.
40
4.
For related work from our group, see: (a) G. Mehta, R. Singh
Tetrahedron Lett. 2005, 46, 2079-2082. (b) G. Mehta, R. Singh
Angew. Chem. Int. Ed. 2006, 45, 953-955. (c) G. Mehta, H. M.
Shinde Tetrahedron Lett. 2007, 48, 8297-8300. (d) G. Mehta, P.
Maity Tetrahedron Lett. 2007, 48, 8865-8868. (e) G. Mehta, B. A.
Bhat Tetrahedron Lett. 2009, 50, 2474-2477. (f) G. Mehta, P. Maity
Tetrahedron Lett. 2011, 52, 1749-1752. (g) G. Mehta, P. Maity
Tetrahedron Lett. 2011, 52, 1753-1756. (h) G. Mehta, P. Maity
Tetrahedron Lett. 2011, 52, 5161-5165. (h) G. Mehta, H. M. Shinde,
Tetrahedron Lett. 2012, 53, accepted for publication.
(a) G. Mehta, R. Samineni, P. Srihari Tetrahedron Lett. 2011, 52,
1663-1666. (b) G. Mehta, R. Samineni, P. Srihari Tetrahedron Lett.
2012, 53, 829-832.
S.-J. Min, S. J. Danishefsky Angew. Chem. Int. Ed. 2007, 46, 2199-
2202.
45
50
5.
55 6.
7.
H. Kikuchi, Y. Miyagawa, Y. Sahashi, S. Inatomi, A. Haganuma, N.
Nakahata, Y. Oshima Tetrahedron Lett. 2004, 45, 6225-6228.
S. H. Li, J. Wang, X. M. Niu, Y. H. Shen, H. J. Zhang, H. D. Sun,
M. L. Li, Q. E. Tian, Y. Lu, P. Cao, Q. T. Zheng Org. Lett. 2004, 6,
4327.
8.
60
9.
H. Kikuchi, Y. Miyagawa, Y. Sahashi, S. Inatomi, A. Haganuma, N.
Nakahata, Y. Oshima J. Org. Chem. 2004, 69, 352-356.
10. H.-M. Chung, Y. H. Chen, T.-L. Hwang, L.-F. Chuang, W.-H.
Wang, P. J. Sung Tetrahedron Lett. 2010, 51, 2734-2736.
11. J. Zou, X. Du, G. Pang, Y.-M. Shi, W.-G. Wang, R. Zhan, L.-M.
Kong, X,-N. Li, Y. Li, J.-X. Pu, H.-D. Sun Org. Lett. 2012, 14,
3210-3213.
65
140
4
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]