Allylsilane-interrupted homo-Nazarov cyclization and synthesis of bicyclo[3.2.1]octan-8-ones

Article abstract of DOI:10.1016/j.tetlet.2014.02.018

The combination of homo-Nazarov cyclization of 2-(tert- butyldiphenylsilylmethyl)cyclopropyl vinyl ketone leading to oxyallyl cation and its subsequent [3+2] capture by allylsilane has been demonstrated as an useful strategy for the construction of functionalized bicyclo[3.2.1]octan-8-ones. The [3+2] capture proceeds exclusively in the exo mode to make the overall reaction diastereoselective. The less substituted end of the oxyallyl cation was found to react nearly two times faster than the more substituted end.

Full text of DOI:10.1016/j.tetlet.2014.02.018

Tetrahedron Letters 55 (2014) 2015–2018
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Allylsilane-interrupted homo-Nazarov cyclization and synthesis
of bicyclo[3.2.1]octan-8-ones
a
b
Veejendra K. Yadav a, , Vijaya K. Naganaboina , Vijaykumar Hulikal
⇑
a Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
b Bioorganics & Applied Materials Pvt. Ltd, B64/1, III Stage PIA, Peeneya, Bangalore 560058, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The combination of homo-Nazarov cyclization of 2-(tert-butyldiphenylsilylmethyl)cyclopropyl vinyl
ketone leading to oxyallyl cation and its subsequent [3+2] capture by allylsilane has been demonstrated
as an useful strategy for the construction of functionalized bicyclo[3.2.1]octan-8-ones. The [3+2] capture
proceeds exclusively in the exo mode to make the overall reaction diastereoselective. The less substituted
end of the oxyallyl cation was found to react nearly two times faster than the more substituted end.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 8 January 2014
Revised 4 February 2014
Accepted 9 February 2014
Available online 15 February 2014
Keywords:
Cyclopropyl vinyl ketone
Homo-Nazarov cyclization
Oxyallyl cation
Allylsilane
Bicyclo[3.2.1]octan-8-one
Designing efficient short routes for the construction of polycyclic
skeletons is a continuing challenge in organic synthesis. Among such
skeletons, the bicyclo[3.2.1]octane skeleton constitutes the basic
framework of many important biologically active natural products,
the tricyclic and tetracyclic sesquiterpenes and diterpenes in partic-
ular.1 During the course of construction of carbocycles and heterocy-
cles using silylmethyl-substituted cyclopropanes bearing a vicinal
electron-withdrawing group as inherent 1,3- and 1,4-dipoles,2 we
have investigated homo version of the Nazarov cyclization3,4 and re-
ported the construction of 4-tert-butyldiphenylsilylmethylcyclo-
hexanones fused to heterocyclic rings.5 The first homo-Nazarov
cyclization, reported by Murphy and Wattanasinin 1980 using an al-
kyl or phenyl substituent as the electron-releasing group, was
limited in synthetic scope.6 Since then, other researchers have inves-
tigated many versions of this reaction.7 This prompted us to
investigate the homo-Nazarov cyclization of 2-(tert-butyldiphe-
nylsilylmethyl)cyclopropyl vinyl ketones to explore the possibility
of capturing the in situ formed oxyallyl cation by dipolarophiles to
generate the bicyclo[3.2.1]octan-8-one skeleton.8 Since the tert-
butyldiphenylsilylmethyl group is known to be latent to the
synthetically more useful hydroxymethyl group,2d,5,9 its use as a
cyclopropane-activating motif enhances the synthetic potential of
any protocol enormously.
We report herein a hitherto unknown fully diastereoselective
protocol for a novel assembly of the bicyclo[3.2.1]octan-8-one
skeleton10 which involves the Lewis acid-mediated homo-Nazarov
cyclization of a 2-(tert-butyldiphenylsilylmethyl)cyclopropyl vinyl
ketone to generate an oxyallyl cation and its capture by allylsilane
in an overall [3+2] manner. It is to be noted that West and co-
workers.11 have previously reported the construction of bicy-
clo[2.2.1]heptan-7-one skeleton from
a non-diastereoselective
capture of an oxyallyl cation which was formed from the usual
Nazarov cyclization by allyl-TMS.
Our exploration commenced with a 1:1.6 mixture of cis- and
trans-2-(tert-butyldiphenylsilylmethyl)cyclopropyl vinyl ketone
1a, as shown in Scheme 1. The reaction with the use of 10 equiv
of allyl-TMS in the presence of 1.2 equiv of SnCl4 in CH2Cl2 at
ꢀ78 °C for 3 h generated bicyclo[3.2.1]octan-8-one 4a in 82% yield
as a single isomer.12 Employing 5 equiv of allyl-TMS resulted in
very poor yield. Use of SnCl4 in lesser amounts, 0.2 equiv for in-
stance, also led to poor conversion even after 18 h at ꢀ78 °C. A
raise in temperature to ꢀ40 °C led to decomposition. Other Lewis
acids such as TiCl4, BF3ꢁOEt2, TMSOTf, and Sc(OTf)3 were found to
be ineffective, as poor yield of 4a was obtained in each case.
As illustrated, bicyclo[3.3.1]nonan-9-one 6a, which is likely to
arise from 1,2-migration of the SiMe3 group through a siliranium
ion13 was not observed. Cleavage of the activated rCAC bond of
the cyclopropane ring under the Lewis acid condition was believed
to generate the enolate-cum-siliranium ion 2a. This, in turn,
involved ring-closure to form the oxyallyl cation 3a and its capture
⇑
Corresponding author. Tel.: +91 512 259 7439; fax: +91 512 259 7436.
E-mail address: vijendra@iitk.ac.in (V.K. Yadav).
http://dx.doi.org/10.1016/j.tetlet.2014.02.018
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

2016
V. K. Yadav et al. / Tetrahedron Letters 55 (2014) 2015–2018
O
R3
H
R3
R3
H
2α
SnCl4
R
O-
O-
α
'
R2
R2
R1
TBDPS
TBDPS
DCM, -78 o
C
R1
3
R1
β
TBDPS
1
2
H R3
O
R3
R3
O
H
O
H
R2
R2
R2
TMS
TBDPS
R1
TBDPS
TBDPS
H R3
R1
TMS
R1
allyl-TMS
TMS
4
5
6
R3
O
O
H
R2
R2
TBDPS
TBDPS
R1
7
R1
8
Scheme 1. Homo-Nazarov cyclization and the possible products from exo-capture of the oxyallyl cation by allyl-TMS.
by allyl-TMS to generate 4a. Since allyl-TMS was believed to react
with an oxyallyl cation in stepwise manner as well,11 chloride ion-
initiated desilylation in 2-trimethylsilyl-substituted cation (not
shown) in Hosomi–Sakurai manner14 could be a distinct possibility
to generate the simply allylated species 7/8, as shown in Scheme 1.
Further, since the desilylation involves SN2 attack of chloride ion
on silicon, the reaction condition is likely to play an important con-
trol factor in as much as the extent of formation of 7/8 is con-
cerned. The collective size of the substituents on silicon is yet
another control factor as bulky substituents shield the silicon from
nucleophilic attack and thereby, reduce or even completely pre-
vent the desilylation pathway. TBDPS fits this requirement extre-
mely well15 and hence, we have used allyl-TBDPS to exclude this
desilylation possibility.
1), the b-methyl-substituted substrate 1d required 8 h (Table 1, en-
try 4).
The capture of oxyallyl cation by allyl-TBDPS was observed to
be considerably slow in comparison to the capture by allyl-TMS.
For instance, while the reaction of 1b with allyl-TMS took only
40 min to complete (Table 1, entry 2), the corresponding reaction
with allyl-TBDPS required 5 h (Table 1, entry 5). The large bulk of
TBDPS in allyl-TBDPS is a likely factor to exert greater resistance
to its approach to the oxyallyl cation in comparison to the TMS
in allyl-TMS.
The regio- and stereochemistry of the products were deter-
mined from nOe measurements in some instances, and on account
of the observed differential site-selectivity (vide supra) in the reac-
tion of allylsilane with oxyallyl cation, in the other instances. For
example, the product formed from the addition of allylsilane at less
substituted end of the oxyallyl cation was more abundant than the
product formed from such an addition at the more substituted end.
The saturation of TMS in 4b resulted in 7% nOe enhancement of the
signal for the quaternary methyl group. Such a feature was absent
in 5b. Likewise, saturation of TMS in 4c resulted in nOe enhance-
ment in the aromatic region, a feature that was absent in 5c. These
nOe results suggest vicinal syn arrangement of CH2TMS with the
methyl in 4b and phenyl in 4c, as shown. The same groups are
too far from each other in 5b and 5c to allow the nOe. Thus, the al-
lyl-TMS added to the oxyallyl cation exclusively in the exo mode to
generate 4b and 4c.
The products 5b and 5c were also formed from the exo addition
of allyl-TMS to oxyallyl cation as the endo addition is untenable on
account of the severe steric interactions between CH2TMS and the
axial hydrogen on CH2TBDPS-bearing carbon in the oxyallyl cation
3 in the requisite TS. This argument finds strong support in the for-
mation of a single isomer of 4a. Had the endo addition of allyl-TMS
also taken place, we must have obtained an isomer of 4a as well. It
is significant to note that West and co-workers have observed both
endo and exo additions in the [3+2] capture of the oxyallyl cation
formed from the usual Nazarov cyclization by allyl-TMS.11 The ste-
ric interactions in the TS required for endo addition in the construc-
tion of the bicyclo[3.2.1]octane skeleton thus appear to be
significantly more stringent than in the construction of the bicy-
clo[2.2.1]heptane skeleton. The observed stereo-control at C3 of
the bicyclo[3.2.1]octane skeleton arises from the least hindered
six-membered cyclic transition state resembling 2a–d which is re-
quired for ring closure between the olefinic bond and the silirani-
um ion;13,16 the large TBDPSCH2 group assuming the exclusive
equatorial position, as shown in Scheme 1.
Notably, the product 4a was formed from separate reactions of
cis-1a and trans-1a. Quenching experiments revealed that cis-1a
was first isomerized to trans-1a before it reacted in the homo-Naz-
arov fashion. However, trans-1a reacted directly without any isom-
erization to cis-1a. Since both cis-1a and trans-1a furnished the
same product 4a, the cis/trans-mixtures of 1b (1:1.5), 1c (1:20),
and 1d (1:1.6) were used in other instances.
The capture of oxyallyl cations was formed from the a -
- and a0
substituted ketones 1b and 1c by allyl-TMS generated 2:1 regioiso-
meric mixtures of 4b and 5b (Table 1, entry 2) and 4c and 5c
(Table 1, entry 3), respectively. The nucleophilic reaction of allyl-
TMS at the less substituted end of oxyallyl cation was thus nearly
two times more prevalent than the reaction at the more
substituted end. This could be for reasons such as (a) the higher
electrophilicity of the less substituted end of oxyallyl cation in
comparison to the more substituted end on steric grounds and
(b) the larger equilibrium concentration of the more substituted
enolate amounting to the less substituted cation than the less
substituted enolate amounting to the more substituted cation on
account of relative thermodynamic stability. Here, it is to be noted
that West and co-workers.11 have previously observed exclusive
bond formation at the less substituted end of the oxyallyl species.
Importantly, a substrate bearing a
-/a0-substituent was found to
react faster than a substrate lacking the same. For instance, the sub-
strates 1b and 1c reacted with allyl-TMS completely within 40 min
(Table 1, entry 2) and 5 min (Table 1, entry 3), respectively, in com-
parison to the reaction of 1a which required 3 h for completion. The
following two conclusions could be drawn from these observations:
(i) an a0-substituent facilitates cyclopropane ring cleavage to form
the more substituted enolate and (ii) an electron-releasing
tuent provides an additional electronic push for the intramolecular
nucleophilic -capture of the siliranium ion. Accordingly, a b-substi-
tuent was expected to retard the rate due to reversal in the polarity
of the bond. Indeed, while the reaction of the unsubstituted sub-
strate 1a with allyl-TMS required 3 h for completion (Table 1, entry
a-substi-
p
The simply a-allylated product 7c was indeed formed in 10%
yield from the reaction of 1c at ꢀ78 °C. A similar reaction at
ꢀ30 °C also generated 7c in 20% yield and the combined yield of
the bicyclic products 4c and 5c was compromised to just 60% from
p

V. K. Yadav et al. / Tetrahedron Letters 55 (2014) 2015–2018
2017
Table 1
Homo-Nazarov reactions of 1a–d followed by capture of the in situ generated oxyallyl cation by allylsilane12
O
R3
R2
a
SnCl4
DCM, -78 oC
a
'
+
Allylsilane
Products
R1
TBDPS
b
1
1a: R1 = R2 = R3 = H; 1b: R1 = R3 = H, R2 = Me; 1c: R1 = R2 = H, R3 = Ph;
1d: R1 = Me, R2 = R3 = H;1e: R1 = H, R2 = R3 = Ph
Entry
1
Ketone
Allylsilane
Allyl-TMS
Time
3 h
Products
Yield (%), ratio
O
TBDPS
1a
82
4a
4b
TMS
O
O
81
TBDPS
TBDPS
2
3
1b
1c
Allyl-TMS
Allyl-TMS
40 min
5 min
4b:5b = 2:1
TMS
O
5b
5c
TMS
Ph
Ph
O
76
TBDPS
TBDPS
TBDPS
TBDPS
4c:5c = 2:1
TMS
TMS
O
4c
4d
9b
O
72
4
5
6
1d
1b
1d
Allyl-TMS
8 h
4d:5d = 1.2:1
TMS
O
TMS
5d
O
78
TBDPS
TBDPS
TBDPS
TBDPS
Allyl-TBDPS
Allyl-TBDPS
5 h
9b:10b = 2:1
TBDPS
O
TBDPS
10b
O
76
12 h
9d:10d = 1.2:1
TBDPS
TBDPS
9d
10d
the previous 76%. The identity of 7c was discerned from the
appearance of two quaternary carbons in the aliphatic region.
The alternate isomer 8c has only one quaternary carbon. The pro-
pensity of chloride ion-induced elimination of the TMS group in
Hosomi–Sakurai manner was, thus, indeed raised at higher tem-
peratures (vide supra). The use of allyl-TBDPS prevented this path-
way and allowed formation of only the bicyclic products.
The TBDPSCH2 substituent, necessary to provide the required
activation for smooth cleavage of rCAC bond of cyclopropane ring,
may be conveniently replaced by an aryl group. For instance, trans-
2-phenylcyclopropyl 2-propenyl ketone 11 reacted rapidly with al-
lyl-TMS to generate an inseparable 2:1 mixture of 12 and 13 in a
combined 70% yield, as shown in Eq. 1.
tert-BuOOH in DMF at 100 °C for 100 h, 4a was transformed into
the corresponding alcohol 14 in 70% yield as shown in Eq. 2.17
The elevated temperature was necessary as the reaction was found
to be slow at lower temperatures.
In conclusion, we have presented first examples of [3+2] cap-
ture of oxyallyl cations formed from homo-Nazarov cyclizations
of 2-(tert-butyldiphenylsilylmethyl)cyclopropyl vinyl ketones by
allylsilanes in the construction of bicyclo[3.2.1]octan-8-ones in
high yields. The allylsilane was discovered to add to the oxyallyl
cation in the exo mode exclusively. Coupled with the oxidative
cleavage of rCATBDPS bond, the present protocol opens up new
avenues for construction of the challenging polycyclic skeletons.
O
O
O
SnCl4 / DCM
-78 oC, 30 min
70%
Ph
Ph
+
+
SiMe3
TMS
Ph
ð1Þ
TMS
11
12
13
O
O
Acknowledgment
KH, t-BuOOH
HO
TBDPS
ð2Þ
DMF,100 o
100 h, 70%
C
We gratefully acknowledge the financial support from the
Council of Scientific & Industrial Research and the Department of
Science & Technology, Government of India. This work is dedicated
to the fond memory of Professor Michael Benn of the University of
Calgary, Canada.
TMS
TMS
4a
14
The rCATBDPS bond was cleaved oxidatively to the correspond-
ing alcohol in one instance and this demonstrates the synthetic
potential of the present protocol. On reaction with KH and

2018
V. K. Yadav et al. / Tetrahedron Letters 55 (2014) 2015–2018
Schmitt, O.; Wanzl, G. Chem. Commun. 1999, 1737; For an exhaustive review on
Supplementary data
the subject, see: (f) Jones, G. R.; Landais, Y. Tetrahedron 1996, 52, 7599.
10. For recent alternative preparations of bicyclo[3.2.1]octan-8-ones, see: (a)
Barluenga, J.; Ballesteros, A.; Santamaria, J.; de la Rua, R. B.; Rubio, E.; Tomás,
M. J. Am. Chem. Soc. 2000, 122, 12874; (b) McDougal, N. T.; Schaus, S. E. Angew.
Chem., Int. Ed. 2006, 45, 3117.
11. Giese, S.; Kastrup, L.; Stiens, D.; West, F. G. Angew. Chem., Int. Ed. 2000, 39, 1970.
12. General procedure for the SnCl4-induced homo-Nazarov reaction followed by
capture of the resultant oxyallyl cation by allyl-SiR3: The solution of a mixture of
cis- and trans-2-(tert-butyldiphenylsilylmethyl) cyclopropyl vinyl ketones
(0.66 mmol) and allyl-SiR3 (6.6 mmol) in CH2Cl2 (60 mL) was cooled to
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.02.
018.
References and notes
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ꢀ78 °C and mixed with SnCl4 (94
lL, 0.79 mmol). The resultant was stirred
for 3 h at ꢀ78 °C and quenched with saturated NaHCO3 (20 mL). The content
was stirred vigorously for 10 min. The layers were separated and the aqueous
layer was extracted with CH2Cl2 (2 ꢂ 10 mL). The combined organic solution
was dried, filtered, and concentrated. The crude material was purified by radial
chromatography over silica gel using mixtures of EtOAc and hexanes as the
eluant to obtain the product. 4a: Colorless liquid, 82% yield. 1H NMR
(400 MHz):
d 7.67–7.60 (4H, m), 7.39–7.34 (6H, m), 2.93–2.86 (1H, q,
J = 8.5 Hz), 2.54–2.45 (1H, m), 2.14–2.04 (1H, m), 2.01–1.85 (2H, m), 1.58–
1.51 (1H, m), 1.42–1.28 (3H, m), 1.16 (1H, d, J = 12.2 Hz), 1.05 (9H, s), 1.00–0.92
(2H, m), 0.82–0.72 (2H, m), 0.02 (9H, s). 13C NMR (100 MHz): d 209.4, 136.2,
136.0, 135.8, 134.9, 134.6, 129.0, 127.5, 127.4, 57.3, 36.2, 27.8, 27.0, 26.3, 25.6,
21.4, 20.5, 18.1, 16.5, 7.9, -1.0. IR (thin film): mmax 1690 cmꢀ1. Calcd m/z for
C
29H43OSi2 [M+H]+ = 463.2852; found m/z = 463.2855.
13. For 1,2-migration of TBDPS leading to ring-enlargement, see: Narhe, B. D.;
Sriramurthy, V.; Yadav, V. K. Org. Biomol. Chem. 2012, 10, 4390.
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tert-BuOOH (583
lL, 70% solution, 6.06 mmol) in DMF (5 mL) was added
dropwise to 0 °C cooled suspension of KH (208 mg, 5.19 mmol, 30%
a
dispersion in mineral oil, washed by hexanes) in DMF (5 mL). The solution
was kept at 25 °C for 10 min and mixed dropwise with a solution of 4a
(200 mg, 0.43 mmol) in DMF (5 mL). After stirring for 100 h at 100 °C, solid
Na2S2O3 (400 mg) and water (10 mL) were added and the resultant was stirred
vigorously for 30 min. This was extracted with Et2O (5 ꢂ 10 mL). The combined
organic solution was washed by water (3 ꢂ 10 mL) and brine (1 ꢂ 10 mL),
dried and concentrated. The crude material was purified by column
chromatography over silica gel using increasing proportions of EtOAc in
hexanes as the eluant to obtain 14, 72 mg, 70% yield. Colorless liquid. 1H NMR
(400 MHz): d 3.67–3.60 (1H, m), 3.50–3.45 (1H, m), 2.96 (1H, dq, J = 8.3,
3.7 Hz), 2.63–2.51 (1H, m), 2.10–1.90 (2H, m), 1.87–1.78 (1H, m), 1.74–1.61
(1H, m), 1.60–1.51 (1H, m), 1.33–1.21 (2H, m), 0.95 (1H, dd, J = 14.2, 4.6 Hz),
0.90–0.85 (1H, m), 0.81–0.74 (1H, m), ꢀ0.03 (9H, s). 13C NMR (100 MHz): d
210.4, 64.8, 56.2, 36.1, 27.2, 26.6, 25.5, 24.5, 21.1, 14.6, ꢀ1.1. IR (CHCl3): mmax
8. For intramolecularly interrupted Nazarov cyclizations, see: (a) Bender, J. A.;
Blize, A. E.; Browder, C. C.; Giese, S.; West, F. G. J. Org. Chem. 1998, 63, 2430; (b)
Bender, J. A.; Arif, A. M.; West, F. G. J. Am. Chem. Soc. 1999, 121, 7443; (c) Nair,
V.; Bindu, S.; Sreekumar, V.; Chiaroni, A. Org. Lett. 2002, 4, 2821; (d) Giese, S.;
Mazzola, R. D., Jr.; Amann, C. M.; Arif, A. M.; West, F. G. Angew. Chem., Int. Ed.
2005, 44, 6545; For intermolecularly interrupted Nazarov cyclizations, see: (e)
Wang, Y.; Schill, B. D.; Arif, A. M.; West, F. G. Org. Lett. 2003, 5, 2747; (f) Dhoro,
F.; Tius, M. A. J. Am. Chem. Soc. 2005, 127, 12472; (g) Janka, M.; He, W.;
Haedicke, I. E.; Fronczek, F. R.; Frontier, A. J.; Eisenberg, R. J. Am. Chem. Soc.
2006, 128, 5312; (h) Basak, A. K.; Tius, M. A. Org. Lett. 2008, 10, 4073.
9. For oxidative
rCASi bond cleavage under basic conditions, see: (a) Smitrovich, J.
H.; Woerpel, K. A. J. Org. Chem. 1996, 61, 6044; (b) Liu, D.; Kozmin, S. A. Org. Lett.
2002, 4, 3005; For oxidative rCASi bond cleavage under acidic conditions, see:
(c) Fleming, I.; Henning, R.; Plaut, H. Chem. Commun. 1984, 29; (d) Fleming, I.;
Sanderson, P. E. J. Tetrahedron Lett. 1987, 28, 4229; (e) Knölker, H.-J.; Baum, G.;
3410, 1690 cmꢀ1
.
Calcd m/z for C13H25O2Si [M+H]+ = 241.1624; found
m/z = 241.1620.