Design and synthesis of novel bis-annulated caged polycycles via ring-closing metathesis: Pushpakenediol

Article abstract of DOI:10.3762/bjoc.10.280

Intricate caged molecular frameworks are assembled by an atom economical process via a Diels-Alder (DA) reaction, a Claisen rearrangement, a ring-closing metathesis (RCM) and an alkenyl Grignard addition. The introduction of olefinic moieties in the penta

Full text of DOI:10.3762/bjoc.10.280

Design and synthesis of novel bis-annulated caged
polycycles via ring-closing metathesis: pushpakenediol
Sambasivarao Kotha* and Mirtunjay Kumar Dipak
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 2664–2670.
Department of Chemistry, Indian Institute of Technology-Bombay,
Powai, India, Fax: 022-2572 7152
doi:10.3762/bjoc.10.280
Received: 11 August 2014
Accepted: 31 October 2014
Published: 13 November 2014
Email:
Sambasivarao Kotha* - srk@chem.iitb.ac.in
* Corresponding author
Associate Editor: I. Marek
Keywords:
© 2014 Kotha and Dipak; licensee Beilstein-Institut.
License and terms: see end of document.
Diels–Alder cycloaddition; Grignard addition; pentacycloundecane
(PCUD); ring-closing metathesis
Abstract
Intricate caged molecular frameworks are assembled by an atom economical process via a Diels–Alder (DA) reaction, a Claisen
rearrangement, a ring-closing metathesis (RCM) and an alkenyl Grignard addition. The introduction of olefinic moieties in the
pentacycloundecane (PCUD) framework at appropriate positions followed by RCM led to the formation of novel heptacyclic cage
systems.
Introduction
Caged polycyclic compounds draw the attention of synthetic [25,26] and golcondane (8) [27] were assembled by employing
organic chemists due to their unusual reactivity patterns as well various novel synthetic routes.
as their strained nature [1-9]. Several pentacycloundecane
(PCUD) related molecules were found to be key structural Results and Discussion
elements in various drugs [10-12], high-energy materials [13- In connection with our interest to prepare annulated PCUD, we
15] and supramolecules [16,17].
proposed various dialkylated pentacyclic diones such as 1,9-
dialkylpentacyclo[5.4.0.02,6.03,10.05,9]undeca-8,11-dione 12 by
In addition, caged molecules possess unusual and often unique using [4 + 2] and [2 + 2] cycloaddition strategies, which involve
properties that are associated with their rigid carbocyclic frame- the DA reaction of 2,5-dialkyl-1,4-benzoquinone 10 and 1,3-
work [1,2]. They are useful synthons in the design and syn- cyclopentadiene (9) followed by the formation of the cyclobu-
thesis of natural as well as non-natural products [3,4]. Several tane ring through a [2 + 2] photocycloaddition reaction
intricate targets (Figure 1), such as snoutane (1) [18], basketane (Figure 2). Later on, one can introduce two allyl groups by
(2) [19], tetrahedrane (3) [20], triprismane [21], rocketene (4) using traditional carbonyl chemistry. The addition of alkyl
[22], cubane (5) [23], garudane (6) [24], dodecahedrane (7) groups can take place from the less hindered exo side of this
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Beilstein J. Org. Chem. 2014, 10, 2664–2670.
Figure 1: Selected theoretically interesting molecules.
caged system. If the R and R’ groups contain unsaturated possible rearranged diallylated products 16 and 17 [29,30] in
systems one can think of constructing additional rings on the equimolar ratio. When 2,5-diallyl-1,4-hydroquinone (17) was
pentacyclic framework by utilizing ring-closing metathesis subjected to MnO2 oxidation in acetone at room temperature the
(RCM). By varying the length of unsaturated component one corresponding 2,5-diallyl-1,4-benzoquinone (18) was obtained
can generate diverse PCUD ring systems.
in good yield (67%). Since the quinone 18 is prone to polymer-
ization (it gave a long streak on TLC when exposed to air at rt),
it was immediately subjected to the [4 + 2] cycloaddition reac-
tion with freshly prepared 1,3-cyclopentadiene (9) at 0–5 ºC to
deliver compound 19 in 71.5% yield (Scheme 1).
The formation of cycloadduct 19 was confirmed on the basis of
1H and 13C NMR spectral data. In 1H NMR data the integra-
tion of the olefinic region (ring olefinic proton) corresponds to
the three protons and, as the cycloadduct is unsymmetrical, all
the seventeen carbons appeared in the 13C NMR spectrum.
The stereochemistry of adduct 19 was expected to be endo as
the reaction was performed at low temperature [28], and gener-
ally, the kinetically controlled product was produced under
these reaction conditions. Our assumption was found to be
correct, when we found that the DA adduct undergoes a smooth
[2 + 2] photocyclization [31] upon exposure to UV light to
Figure 2: Retrosynthetic approach toward bis-annulated PCUD.
To realize the strategy depicted in Figure 2, we chose 2,5- generate caged dione 20 in good yield (80%). The structure of
diallyl-1,4-benzoquinone (18) as a viable option. We deliber- the photoadduct was assigned as 1,9-diallyl PCUD 20 on the
ately choose two unsaturated R groups at non-vicinal positions, basis of the spectral data (Scheme 2).
because we have shown in an earlier report that the vicinal allyl
groups in PCUD can be converted to a six-membered ring by Having prepared the pentacyclic diallyldione 20, we ventured
RCM [28]. Since the two allyl groups in the PCUD framework into the synthesis of PCUD based novel heptacyclic systems.
are far apart, they did not undergo a RCM reaction. Later, by The dione 20 was subjected to an allyl Grignard addition reac-
introducing additional unsaturated fragments in the PCUD tion, which resulted in the formation of tetra-allyldiol 21. The
system one can generate multiple rings in the PCUD system by structure of diol 21 was established on the basis of high field
using the RCM protocol. To this end, we began with the Claisen 1H NMR (400 MHz) spectral data and further supported by
rearrangement of bis(allyloxy)benzene 15 to deliver the two 13C NMR spectral data (Scheme 3).
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Beilstein J. Org. Chem. 2014, 10, 2664–2670.
Scheme 1: The synthesis of diallylated tricyclic diene 19.
Scheme 2: The synthesis of diallylated pentacyclic dione 20.
Figure 3: (a) Optimized structure of 22 (b) Ancient flying machine
“Pushpak Viman”.
The Grignard addition at a trigonal carbon may result in the for-
mation of C–C bond in two possible ways (en face and zu face),
but due to steric reasons only the exo–exo allyl addition prod- the structure was visualized with the Mercury software
uct was isolated in the present case. The assumption of exo–exo (Figure 3).
stereochemistry has been further revived when the tetra-allyl-
diol 21 underwent a smooth RCM [32-37] upon its exposure to We turned our attention toward the next target, i.e., the symmet-
Grubbs’ second generation catalyst to deliver bis-annulated rical heptacyclic diol 27, by adopting the ring-rearrangement
heptacyclic diol 22 in good yield (72%) (Scheme 3).
metathesis (RRM) protocol [42]. To this end, hexacyclic diones
23 [28] and 25 [28] were treated with an excess amount of allyl-
The molecular model and the optimized structure of hepta- magnesium bromide (6 equiv) at room temperature to give the
cyclic diol 22 resembles the ancient flying machine “Puspak desired diallylated adducts 24 and 26 as sole products in 82%
Viman” designed by the famous ancient aeronautical engineer and 85% yield, respectively (Scheme 4). The structures of the
saint Bhardwaj [38-40]. Therefore, we coined the name ‘push- adducts 24 and 26 were established based on high field
pakenediol’ [41] for the heptacyclic diol. The optimization of 1H NMR (400 MHz) spectral data and further supported by
the structure was carried out by means of Chem Draw 3D and 13C NMR spectral data.
Scheme 3: The synthesis of heptacyclic diol 22.
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Beilstein J. Org. Chem. 2014, 10, 2664–2670.
Experimental
General
Reactions involving organometallic species were carried out
under nitrogen by using oven-dried glassware and syringes.
THF and Et2O were distilled from sodium/benzophenone
under nitrogen immediately prior to use. Dichloromethane
was distilled over P2O5. TLC was performed by using
(10 × 5 cm) glass plates coated with Acme’s silica gel GF254
(containing 13% calcium sulfate as a binder). Flash-column
chromatography was performed by using Aceme silica gel
(100–200 mesh). Solvents were concentrated at reduced pres-
sure on a Büchi R-114 rotary evaporator. 1H NMR (400 MHz)
and 13C NMR (75.1 MHz) spectra were recorded at rt on a
Bruker AX 400 with TMS (δ = 0.0 ppm, 1H NMR spectra) and
CDCl3 (δ = 77.0 ppm, 13C NMR spectra) as internal standards.
Scheme 4: The synthesis of diallylated hexacyclic diols.
It was anticipated that the two allyl groups and the cyclohexene IR spectra were recorded on a Nicolet Impact-400 FTIR spec-
moiety present in 24 would undergo a ring-rearrangement trometer. HRMS were determined on a Micromass Q-ToF spec-
metathesis (RRM), [42] which involves the ring-opening and trometer.
ring-closing metathesis sequence (ROM–RCM) in a single step
to generate the novel heptacyclic system 27. However, even Materials
under harsh reaction conditions, we did not observe the forma- Grubbs’ 1st and 2nd generation and Grubbs–Hoveyda catalysts
tion of the required RRM product (Scheme 5).
were purchased from Aldrich, Milwaukee (USA). Compounds
18, 19, 20, and 21 were prepared by procedures similar to those
described in [28] for analogous compounds.
Preparation of 2,5-diallyl-1,4-benzoquinone (18)
To a solution of 2,5-diallyl-1,4-hydroquinone (17, 1.8 g,
9.45 mmol) in acetone (50 mL) was added MnO2 in excess
(8 equiv) at rt. After completion of the reaction (TLC moni-
toring, 15 h), the reaction mixture was filtered off by using a
pad of celite. The filtrate was evaporated to dryness and the
residue was purified by distillation under reduced pressure at
106–107 °C (1 mmHg) to give 18 as a dark yellow oil (1.2 g,
67%). Bp: 106–107 °C at 1.0 mmHg (Lit. Bp: 105 °C at
1.0 mmHg) [16].
Scheme 5: The attempted synthesis of heptacyclic diol via ring-
rearrangement metathesis.
The RRM reaction was carried out under various reaction
conditions with different metathesis catalysts, for example,
with Grubbs’ 1st and 2nd generation catalysts at rt as well as
under reflux conditions. However, even the presence of
ethylene during the metathesis sequence did not deliver the
expected product, and the starting material remained unaltered
(Scheme 5).
Preparation of 2,5-diallyltricyclo[6.2.1.02,7]undeca-
4,9-diene-3,6-dione (19)
To a cooled solution (at 0 °C) of 2,5-diallyl-1,4-benzoquinone
(18, 1.17 g, 6.21 mmol) in methanol (25 mL) was added freshly
prepared 1,3-cyclopentadiene (0.48 mL, 6.24 mmol) in a drop-
Conclusion
We demonstrated that the Grignard reaction in combination wise manner. After completion of the reaction (TLC moni-
with the RCM reaction provides a useful strategy for the syn- toring, 11 h), the solvent was evaporated under reduced pres-
thesis of novel and intricate molecular frameworks such as 22, sure, and the residue was purified by silica-gel column chroma-
which is suitable for studying stereoelectronic effects [43]. The tography (3% ethyl acetate/petroleum ether) to give 19 (1.13 g,
strategy shown here is an atom economical process. The syn- 71.5%) as a thick yellow liquid. IR (Neat) νmax: 1337, 1666,
thetic sequence opens up a new route to complex caged 2991 cm−1; 1H NMR (400 MHz, CDCl3) δ 1.4 (d, J = 8.8 Hz,
systems. At the same time non-participation of 24 in the RRM 1H), 1.6 (d, J = 9.2 Hz, 1H), 2.16 (dd, J1 = 8.8 Hz, J2 = 8.2 Hz,
reaction reveals that systems which contain a sterically hindered 1H), 2.82 (dd, J1 = J2 = 6.7 Hz, 1H), 2.90–3.10 (m, 4H), 3.3 (s,
cyclohexene ring are not suitable candidates for the tandem 1H), 4.42–5.12 (m, 4H), 5.92–5.94 (m, 1H), 5.63–5.73 (m, 1H),
metathesis sequence.
5.93 (dd, J1 = 2.0 Hz, J2 = 3.0 Hz, 1H), 6.05 (dd, J1 = 2.0 Hz,
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Beilstein J. Org. Chem. 2014, 10, 2664–2670.
J2 = 3.0 Hz, 1H), 6.3 (s, 1H) ppm; 13C NMR (100.6 MHz, was evaporated and the resulting residue was purified by silica
CDCl3) δ 32.8, 44.9, 46.4, 49.3, 53.1, 54.4, 57.4, 118.2, 132.8, gel column chromatography (25% ethyl acetate/petroleum
133.1, 134.9, 137.4, 139.55, 139.58, 152.3, 198.8, 202.0 ppm; ether) to give 22 (15 mg, 72%) as a white crystalline solid. Mp:
HRMS (Q-ToF ES+) m/z: calcd for C17H18O2K, 293.0944; 206–207 °C; IR (KBr) νmax: 3691, 3054, 2987, 2305, 1422,
found, 293.1086 [M + K]+.
1266 cm−1; 1H NMR (400 MHz, CDCl3) δ 1.38 (1/2 AB q, J1 =
J2 = 11.0 Hz, 2H), 1.79 (q, J = 1.8 Hz, 1H), 1.99–2.56 (m,
13H), 5.54–5.72 (m, 4H) ppm; 13C NMR (100.6 MHz, CDCl3)
δ 31.0, 31.4, 32.4, 36.2, 37.1, 37.7, 44.2, 44.5, 45.1, 47.3, 49.6,
Preparation of 1,9-diallylpenta-
cyclo[5.4.0.02,6.03,10.05,9]undeca-8,11-dione (20)
Tricyclic dione 19 (125 mg, 0.49 mmol) was dissolved in dry 51.4, 58.6, 74.7, 75.0, 123.1, 124.1, 124.4, 126.3 ppm; HRMS
ethyl acetate (500 mL) and irradiated in a Pyrex immersion well (Q-ToF ES+) m/z: calcd for C19H22O2Na, 305.1517; found,
by a 125 W lamp (homemade) for 1.5 h under nitrogen at rt. 305.1523 [M + Na]+.
After completion of the reaction (TLC monitoring), the solvent
was evaporated under reduced pressure and the residue was Preparation of hexacyclic diallyldiol 24
purified by silica gel column chromatography (5% ethyl acetate/ To a freshly prepared solution of allylmagnesium bromide
petroleum ether) to give 20 (80 mg, 80%) as a thick pale yellow (6 equiv) in ether was added the ethereal solution of hexacyclic
liquid. IR (KBr) νmax: 3073, 2974, 1752, 1636, 924 cm−1; dione 23 (200 mg, 0.88 mmol) in a dropwise manner over a
1H NMR (400 MHz, CDCl3) δ 1.82 (d, J = 11.4 Hz, 1H), 2.12 period of 10–15 min under nitrogen at rt. After completion of
(d, J = 11.4 Hz, 1H) 2.15–2.4 (m, 6H), 2.55 (dd, J1 = J2 = 1.0 the reaction (TLC monitoring, 8 h), the reaction mixture was
Hz, 1H), 2.7 (dd, J1 = J2 = 1.0 Hz, 1H), 2.8–3.1 (m, 2H), quenched with saturated aqueous NH4Cl solution at 0 °C. Then,
5.0–5.2 (m, 4H), 5.6–5.9 (m, 2H) ppm; 13C NMR (100.6 MHz, the aqueous layer was extracted by ethyl acetate (3 × 25 mL).
CDCl3) δ 33.7, 35.3, 35.6, 39.7, 42.9, 44.0, 47.3, 47.8, 51.5, The combined organic layer was washed with brine and dried
60.2, 61.6, 118.5, 118.7, 132.9, 133.7, 212.5, 213.6 ppm; over anhydrous Na2SO4. After removal of the solvent under
HRMS (Q-ToF ES+) m/z: calcd for C17H19O2Na, 277.1204; reduced pressure, the resulting residue was purified by silica-gel
found, 277.1210 [M + Na]+.
column chromatography (4% ethyl acetate/petroleum ether) to
give 24 (232 mg, 82%) as a white crystalline solid. Mp:
177–178 °C; IR (KBr) νmax: 3339, 3054, 2985, 1422, 1265
Preparation of pentacyclic tetraallyldiol 21
To a freshly prepared solution of allylmagnesium bromide in cm−1; 1H NMR (400 MHz, CDCl3) δ 1.38 (1/2 ABq, J1 = J2 =
ether was added an ethereal solution of pentacyclic dione 20 10.3 Hz, 2H), 1.9–2.0 (m, 6H), 2.13–2.25 (m, 6H), 2.32 (d, J =
(300 mg, 1.18 mmol) in a dropwise manner over a period of 1.6 Hz, 2H), 5.10–5.17 (m, 4H), 5.91–6.02 (m, 4H) ppm;
10–15 min at rt under nitrogen. After completion of the reac- 13C NMR (100.6 MHz, CDCl3) δ 26.2, 34.7, 42.1, 43.4, 43.7,
tion (TLC monitoring, 10 h), the reaction was quenched with a 48.8, 51.4, 78.3, 117.9, 128.2, 134.0 ppm; HRMS (Q-ToF ES+)
saturated aqueous NH4Cl solution at 0 °C. Then, the aqueous m/z: calcd for C20H27O2, 311.2011; found, 311.2012 [M + H]+.
layer was extracted by ethyl acetate (3 × 25 mL). The combined
organic layer was washed with brine, and dried over anhydrous Preparation of hexacyclic diallyldiol 26
Na2SO4. After the removal of the solvent under reduced pres- To a freshly prepared solution of allylmagnesium bromide
sure, the residue was purified by silica gel column chromatog- (6 equiv) in ether was added the ethereal solution of hexacyclic
raphy (4% ethyl acetate/petroleum ether) to give 21 (280 mg, dione 25 (250 mg, 1.12 mmol) in a dropwise manner over a
70%) as a white solid. Mp: 158–160 °C; IR (KBr) νmax: 3309, period of 10–15 min under nitrogen at rt. After completion of
3055, 2977, 1265, 743, 705 cm−1; 1H NMR (400 MHz, CDCl3) the reaction (TLC monitoring, 8 h), the reaction mixture was
δ 1.3 (1/2 ABq, J1 = 10.9 Hz, J2 = 11.0 Hz, 2H), 1.96–2.19 (m, quenched with saturated aqueous NH4Cl solution at 0 °C. Then,
6H), 2.23–2.46 (m, 8H) 5.08–5.10 (m, 8H), 5.9–6.1 (m, 4H) the aqueous layer was extracted with ethyl acetate (3 × 25 mL).
ppm; 13C NMR (100.6 MHz, CDCl3) δ 33.1, 35.2, 36.6, 38.1, The combined organic layer was washed with brine and
41.1, 42.2, 43.1, 44.2, 45.2, 49.0, 49.5, 55.5, 55.7, 78.9, 80.5, collected over anhydrous Na2SO4. After removal of the solvent
116.71, 116.72, 117.4, 118.3, 134.1, 134.7, 136.1, 136.3 ppm; under reduced pressure, the resulting residue was purified by
HRMS (Q-ToF ES+) m/z: calcd for C23H30O2Na, 361.2144; silica-gel column chromatography (4% ethyl acetate/petroleum
found, 361.2146 [M + Na]+.
ether) to give 26 (292 mg, 85%) as a white crystalline solid.
Mp: 191–192 °C; IR (KBr) νmax: 3339, 3055, 2961, 1439, 1265
cm−1; 1H NMR (400 MHz, CDCl3) δ 0.90 (d, J = 10.8 Hz, 1H),
Preparation of heptacyclic diol 22
To a solution of 21 (25 mg, 0.074 mmol) was added Grubbs’ 1.40 (d, J = 10.8 Hz, 1H), 2.02–2.07 (m, 2H), 2.26 (s, 2H),
2nd generation catalyst (4 mg, 6 mol %) under argon at rt. After 2.34–2.39 (m, 4H), 2.73 (s, 2H), 4.84 (brs, 2H), 5.10–5.15 (m,
completion of the reaction (TLC monitoring, 8 h), the solvent 4H), 5.56 (dd, J1 = 10.7 Hz, J2 = 2.7 Hz, 2H), 5.89–6.01 (m,
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Beilstein J. Org. Chem. 2014, 10, 2664–2670.
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Supporting Information File 1
Copies of 1H, 13C NMR and HRMS spectra for all new
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Acknowledgements
We would like to thank the DST (New Delhi) for financial
support. MKD thanks CSIR (New Delhi) for awarding a
research fellowship. SK thanks DST for the award of a J. C.
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