Vinylogous Annulation Cascade Toward Stereoselective Synthesis of Highly Functionalized Indanone Derivatives

Article abstract of DOI:10.1002/adsc.201900860

A base promoted vinylogous annulation cascade of alkylidene malononitriles with cyclopentene-1,3-diones with a subsequent reduction sequence has been devised to furnish densely functionalized 3-hydroxyindanone scaffolds in high yields with excellent diastereoselectivity. Unlike preceding approaches that account for the formation of the pentanoid ring, this strategy features construction of the aromatic ring. The protocol is scalable, displays very broad substrates scope including late-stage functionalization of bioactive estrone, applicable to activated coumarin system, and is suitable to access an indenoquinoline derivative. (Figure presented.).

Full text of DOI:10.1002/adsc.201900860

UPDATES
DOI: 10.1002/adsc.201900860
Vinylogous Annulation Cascade Toward Stereoselective Synthesis
of Highly Functionalized Indanone Derivatives
a
a
a,
Vinod Bhajammanavar, Sumitava Mallik, and Mahiuddin Baidya *
a
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, India
E-mail: mbaidya@iitm.ac.in
Manuscript received: July 11, 2019; Revised manuscript received: August 28, 2019;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.201900860
[
4a]
Typically metal catalyzed cyclotrimerization
dehydro Diels-Alder reactions
and
Abstract: A base promoted vinylogous annulation
cascade of alkylidene malononitriles with cyclo-
pentene-1,3-diones with a subsequent reduction
sequence has been devised to furnish densely
functionalized 3-hydroxyindanone scaffolds in high
yields with excellent diastereoselectivity. Unlike
preceding approaches that account for the formation
of the pentanoid ring, this strategy features con-
struction of the aromatic ring. The protocol is
scalable, displays very broad substrates scope
including late-stage functionalization of bioactive
estrone, applicable to activated coumarin system,
and is suitable to access an indenoquinoline deriva-
tive.
[4b]
of well-organized
substrates at elevated temperature were considered for
this purpose.
The principle of vinylogy, as defined by Fuson,
features the transmission of electronic effects of a
specific functional group through a conjugated π-
[5]
system. This concept allows functionalization at a
distant position from the parent functional group and
became a putative strategic manoeuver in the synthetic
[6]
chemist’s repertoire. We envisioned that exploitation
of vinylogous nucleophilic reactivity of alkylidene
malononitriles (1) and subsequent annulation cascade
with cyclopentene-1,3-diones (2) would be a conven-
ient route to access 3-hydroxy indanone frameworks
[7]
(Scheme 1).
The enolate A thus formed from
Keywords: Vinylogous annulation; Cascade reac-
tion; Diastereoselectivity; 3-Hydroxy indanones;
Late-stage functionalization
alkylidene malononitrile 1 in the presence of a suitable
base will undergo regioselective vinylogous Michael
addition followed by cyclization to give intermediate
B. It will then isomerize to intermediate C, which
upon oxidation would produce the annulated stable
Indanones and derivatives thereof, particularly 3- aromatic product 3. A successful chemoselective
hydroxy indanones, represent a class of valuable reduction on 3 could furnish densely substituted 3-
benzofused carbocyclic compounds that are ubiquitous hydroxy indanone.
in various natural products and biologically active
We commenced our investigations following the
molecules including clinical drug candidates (Fig- model reaction of acyclic alkylidene malononitrile 1a
[1]
ure 1). They also serve as important building blocks
to construct different heterocyclic frameworks with
[2]
increased molecular complexity. Consequently, syn-
thetic endeavors toward these high-value scaffolds
remain in the limelight of contemporary organic syn-
thesis. Strategically, 3-hydroxy indanones can be
accessed either through the formation of the pentanoid
ring or via construction of the aromatic ring
(Scheme 1). However, majorities of the existing
synthetic protocols are grounded on strategic construc-
[3]
tion of pentanoid ring. On the contrary, fabrication of
aromatic ring en route to production of 3-hydroxy
indanone derivatives is increasingly challenging and
[4]
Figure 1. Bioactive indanone derivatives.
currently, such practical synthetic routes are scarce.
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Scheme 2. Gram scale reaction.
and Cs CO , gave inferior results (entry 3). Organic
2
3
i
bases such as DBU, Hünig’s base ( Pr NEt), Et N, and
2
3
DABCO were also ineffective for this reaction (en-
try 4). The reaction was very sensitive to the loading
of base; yield decreased drastically with the lowering
Scheme 1. Synthetic planning towards 3-hydroxy indanones.
t
and increasing amount of KO Bu and completely shut
[a]
Table 1. Optimization of reaction conditions.
down in the absence of it (entries 5,6). Though the
t
exact reason of why K OBu is so special in this
annulation cascade is not apparent now, one must note
that the choice of base is very critical as starting
materials, intermediates, and product possess base
sensitive reactive sites which could potentially lead to
cumbersome outcomes. It is important to mention that
the disubstitution in 2a is essential for this annulation
reaction as monosubstituted 2-methyl cyclopentene-
[
b]
entry
deviation from standard condition
yield (%)
1
2
none
82
Toluene/mesitylene/PhCl/THF/
74/70/55/
[
c] [c]
1,3-dione failed to deliver the desired product with the
CH CN instead of DCE
0 /0
3
[
d]
3
4
LiOtBu/NaH/NaOMe/Cs CO
12/25/18/0
10/16/15/0
recovery of both the starting materials (Table 1,
below).
2
3
t
instead of KO Bu
i
[d]
DBU/ Pr NEt/Et N/DABCO
Scale-up was also suitable and the product 3a was
prepared in 1.2 g scale (72% yield, Scheme 2). At this
juncture, we have also detected the formation of
reduced products 1a’, 2a’, 2a’’, and 2a’’’ that favors a
2
3
t
instead of KO Bu
0.2/0.5/1.5 equiv. of KO Bu
without KO Bu
t
5
6
19/48/53
0
t
[8]
hydride-transfer mechanism for the aromatization
step to produce annulated product 3a (also see,
Scheme 6).
Having acquired the optimal conditions, we next
examined the scope of the reaction (Table 2). This
vinylogous annulation reaction was very general for a
broad range of alkylidene malononitriles. Acyclic
alkylidene malononitriles derived from substituted
propiophenones having donating (3b–c, 3f–g) and
withdrawing (3d–e, 3h) substituents in the aromatic
ring uniformly delivered the desired products in very
[
a]
Reaction conditions: 1a (0.22 mmol), 2a (0.36 mmol), N2,
solvent (4 mL), rt, 1.5 h.
Isolated yields.
Decomposition of 1a was observed.
Unreacted 1a and 2a were recovered.
[b]
[c]
[d]
with 2,2-disubstituted cyclopentene-1,3-dione 2a (Ta- high yields. Sensitive functionalities such as allyl (3f),
ble 1). Gratifyingly, when mixture of 1a and 2a was propargyl (3g), and styryl (3l) were undisturbed.
t
exposed to KO Bu in anhydrous dichloroethane (DCE) Alkylidene malononitriles derived from acetophenone
at room temperature, the vinylogous annulation cas- (3i), butyrophenone (3j), deoxybenzoin (3k), and 2-
cade proceeded rapidly with concomitant oxidation acetonaphthone (3m) also effectively participated in
and the desired annulated product 3a was isolated in this reaction to furnish corresponding products in high
8
2% yield (entry 1). Reaction was also effective in yields. Product 3m was also crystallized and X-ray
other aromatic nonpolar solvents such as toluene analysis unambiguously established the structure of the
[9]
(
74%), mesitylene (70%), and chlorobenzene (55%); annulated product. Heteroaryl-substituted and bulky
however, a complex mixture of unidentified products adamantyl-substituted alkylidene malononitriles were
was formed in acetonitrile and THF (entry 2). Screen- good substrates for this reaction, giving products 3n
t
ing of other bases, for example LiO Bu, NaH, NaOMe and 3o in 57% and 70% yields, respectively. Impor-
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[a]
Table 2. Scope of the vinylogous annulation cascade.
Pleasingly, annulation reactions with substrates bearing
combination of a methyl and electronically distinct
benzyl and allyl substituents at the quaternary center
proceeded smoothly under the optimized conditions,
rendering products 3aa–3ag in very high yields (65–
8
4%). Bioactive estrone derived alkylidene malononi-
trile also underwent facile annulation to offer 3ah and
ai in 78% and 76% yields, respectively. Here, a
3
diastereomeric ratio of 2:1 was observed for both
cases. This late-stage functionalization of pharmaceuti-
cally important natural product is noteworthy.
Also, the reaction of 3-cyano-4-methylcoumarin 4
with cyclopentene-1,3-dione 2a under the standard
conditions was fruitful to forge annulated product 5 in
52% yield, demonstrating further scope of this vinyl-
ogous annulation cascade (Scheme 3).
Scheme 3. Vinylogous annulation with coumarin derivative.
Following our synthetic planning, we then set out
for the chemo- and regioselective reduction of annu-
lated product 3 to access 3-hydroxy indanone Scaffold.
To our delight, treatment the annulated product 3a
with Zn powder in AcOH at 100°C for a short period
of time (20 min) effected selective reduction of one
keto-functionality to offer densely functionalized 3-
hydroxy indanone 6a as a single diastereomer in 86%
[10]
isolated yield (Table 3).
This reduction conditions
turned out quite general for a series of annulated
product 3 bearing diverse substitutions and typically
single diastereomer was obtained for the substrates
examined (Table 3). Substrates having propargyl (6b),
heterocyclic thienyl (6c), adamantyl (6d) and ferro-
cenyl (6e) groups gave desired product in very high
yields. The single-crystal X-ray diffraction analysis of
product 6c confirmed its structure and relative
[
a]
Reaction conditions: 1a (0.22 mmol), 2a (0.36 mmol),
t
KO Bu (1 equiv.), N , DCE (4 mL), rt, 1.5 h. Yields of
2
isolated products were given.
Reaction time was 1 h.
[9]
[b]
configuration. Fused polycyclic annulated products
along with pharmaceutically relevant estrone derivative
underwent smooth reduction to give desired products
6
f–h in excellent yields (>90%). Alteration of
tantly, ferrocenyl functionalized product 3p was also substitutions at the quaternary center did not hamper
prepared in 74% yield. Reaction efficacy of cyclic the reduction process and selectivity; single diastereo-
alkylidene malononitriles was also investigated. Six- meric products with allyl (6i), 2-methylbenzyl (6j),
membered and five-membered alkylidene malononi- and of 1-naphthalenemethyl (6k) groups were obtained
triles prepared from α-tetralone (3q), 4-chromanone in very high yields (86–90%). We believe the steric
(
3r), 4-thiochromanone (3s), 1-indanone (3t) and factor embedded in the substrates is responsible for the
substituted 1,3-cyclopentanedione (3u) readily ren- high diastereoselectivity observed in these cases.
dered expected products in good to high yields (51– The synthetic utility was further highlighted
8%). through the preparation of tetracyclic
Next, we explored the reaction scope with respect indenoquinoline derivative. Accordingly, annulated
to 2,2-disubstituted cyclopentene-1,3-diones (Table 2). product 3ac having well-placed nitro-group was
7
[11]
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Table 3. Synthesis of 3-hydroxy indanones via stereoselective PO), refuting the involvement of a radical species in
[a]
reduction.
this process (Scheme 5a). The gas chromatography
[
a]
b]
Reaction conditions: 3 (0.2 mmol), Zn (16 equiv.), AcOH
(
2 mL), 100°C, 20 min. Yields of isolated products were
given.
[
Reaction was performed with annulated product 3ah having
2
:1 dr.
Scheme 5. Control experiments.
exposed to Zn/AcOH reduction conditions; complete
reduction of both keto and nitro groups followed by analysis of the benchmark reaction also disproved the
intramolecular cyclization proceeded cleanly, furnish- liberation of hydrogen gas during the course of the
ing compound 7 in 73% yield (Scheme 4). It is worth reaction (Scheme 5b). Standard reaction of 1a and 2a
in the presence of Michael acceptor vinyl sulfone 8
(1 equiv.) gave 9 in 6% yield (Scheme 5c). All these
findings are in line with the propose hydride transfer
mechanism.
Notably, reaction of 1a with 2a was sluggish under
oxygen atmosphere, and the intermediate C was
isolated in 28% yield along with product 3a
Scheme 4. Synthesis of indenoquinoline derivative.
(Scheme 5d). When isolated intermediate C was
exposed to molecular oxygen, formation of product 3a
was not significant even after prolonging the reaction
noting that, unlike other cases (Table 3), this cycliza- time (Scheme 5e, left). Improvement was also insignif-
tion cascade involved a reductive deoxygenation of icant when the reaction under oxygen atmosphere was
t
aromatic ketone, which was confirmed through X-ray performed in the presence of KO Bu; 3a was obtained
[9]
analysis.
in only 12% yield and TLC showed the presence of
To gain insights of the reaction mechanism, various unreacted intermediate C (Scheme 5e, right). Further,
t
control experiments were conducted. Significant treatment of intermediate C with KO Bu under nitro-
amount of product 3a was formed in the presence of gen atmosphere did not produce product 3a. However,
radical scavengers like butylated hydroxyltoluene when the same reaction was executed in the presence
(BHT) or 2,2,6,6-tetramethylpiperidine-1-oxyl (TEM- of hydride acceptor 2a, product 3a was isolated in
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0% yield (Scheme 5e, below). These findings clearly Experimental Section
validate a base promoted hydride transfer mechanism
under nitrogen atmosphere for the aromatization step
and distinct our methodology from others where O2
was claimed to be crucial for the aromatization step.
Notably, this base promoted hydride transfer under
General procedure of vinylogous annulation cascade reac-
tion: The alkylidene malononitriles 1 (0.22 mmol, 1 equiv.),
cyclopentene-1,3-diones 2 (0.36 mmol, 1.6 equiv.), and anhy-
drous potassium tert-butoxide (1 equiv.) were taken in a 16×
1
00 mm oven dried reaction tube equipped with a magnetic stir.
nitrogen atmosphere is also in line with the prior work The reaction tube was capped with a rubber septum, evacuated
[8c]
by Nikolai et al..
and backfilled with nitrogen gas. Then, dry DCE (4 mL) was
Based on the above control experiments, a plausible added via syringe. The mixture was allowed to stir at room
reaction mechanism is proposed in the Scheme 6. temperature for 1–1.5 h. After completion (TLC monitored), the
crude reaction mixture was loaded directly onto silica gel
column and purified with a gradient eluent of hexane and ethyl
acetate to provide pure 1,3-indandiones 3.
General procedure for the synthesis of 3-hydroxy indanones:
The compound 3 (0.1 mmol, 1 equiv.) and zinc (16 equiv.) were
taken in a 50 mL round bottom flask equipped with a magnetic
stir. Acetic acid (2 mL) was added and the flask was capped
with a rubber septum. The mixture was allowed to stir at 100°C
for 20 minutes. Then the crude reaction mixture was quenched
carefully with saturated NaHCO3 solution (aq) and was
extracted three times with EtOAc (10 mL×3). The combined
organic layers was washed with brine and dried over anhydrous
Na SO . The volatiles were carefully evaporated and the crude
2
4
reaction mixture was loaded directly onto silica gel column and
purified with a gradient eluent of hexane and ethyl acetate to
provide pure 3-hydroxy indanones 6.
Scheme 6. Plausible reaction mechanism.
Deprotonation of alkylidene malononitrile 1 with KO
t
Bu produces enolate A. It reacts with cyclopentene-
Acknowledgements
1
,3-dione 2 and gives B after proton transfer from the
We gratefully acknowledge DST (EMR/2014/000225) for
financial support. V.B. and S.M. thank IIT Madras for HTRA.
M.B. thanks IIT Madras for the support through the Institute
Research Development Award (IRDA). We also thank the
Department of Chemistry, IIT-Madras for instrumental facili-
ties.
reaction medium. Intermediate B isomerises to more
stable intermediate C. Further deprotonation generates
the enolate D, which triggers the hydride transfer to
acceptors (1, 2 or derivatives of 2) to deliver the
product 3.
In conclusion, we have developed a succinct syn-
thesis of pharmaceutically important 3-hydroxy inda-
t
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UPDATES
Vinylogous Annulation Cascade Toward Stereoselective
Synthesis of Highly Functionalized Indanone Derivatives
Adv. Synth. Catal. 2019, 361, 1–7
V. Bhajammanavar, S. Mallik, M. Baidya*