Stereoselective Lewis acid mediated [1,3] ring contraction of 2,5-dihydrooxepins as a route to polysubstituted cyclopentenes

Article abstract of DOI:10.1002/anie.200500088

(Chemical Equation Presented) A room-temperature diastereoselective [1,3] rearrangement results from treatment of 2,5-dihydrooxepins with EtAlCl ₂ (see scheme). A modular synthesis of dihydrooxepins allows substituents to be incorporated at any

Full text of DOI:10.1002/anie.200500088

Communications
Synthetic Methods
Stereoselective Lewis Acid Mediated [1,3] Ring
Contraction of 2,5-Dihydrooxepins as a Route to
Polysubstituted Cyclopentenes**
Christopher G. Nasveschuk and Tomislav Rovis*
The Diels–Alder reaction is a cornerstone of organic syn-
thesis, and its ability to enable the production of cyclohexenes
in a stereocontrolled manner is unparalleled. In contrast, no
method exists for the synthesis of cyclopentanes that matches
the scope and power of the Diels–Alder reaction in spite of
the prevalence of these ring systems in natural products.
Among numerous methods that have been used to target
these cores, vinylcyclopropane ring-expansion strategies have
been intensively investigated and have provided some
spectacular successes.[1,2] Nevertheless, most reports result in
mono- or disubstituted cyclopentanes and cyclopentenes,
while approaches to polysubstituted systems are rare.[3] We
were interested in addressing this deficiency and developing a
diastereoselective approach to tri-, tetra-, and pentasubsti-
tuted cyclopentanes from readily available precursors, and
herein we report our results.
We have previously reported the [1,3] rearrangement of
vinyl acetals which proceeds through a metalloenolate and
oxocarbenium ion pair.[4] To extend this concept to other
stabilized cations, we initiated a program to study the
[1,3] rearrangement of allylvinyl ethers that would form a
metalloenolate and an allylic cation ion pair under Lewis
acidic conditions. However, we were mindful that these
substrates could also undergo a Lewis acid (LA) accelerated
Claisen rearrangement, which if concerted, would form the
[3,3] rearrangement product exclusively.[5] A number of
workers have documented that the Lewis acid mediated
Claisen rearrangement proceeds stepwise[6] and occasionally
provides the [1,3] adduct with some selectivity.[7] To favor the
[1,3] over the [3,3] product, we envisioned that a cyclic
allylvinyl ether or 2,5-dihydrooxepin could provide access to
densely functionalized cyclopentenes under ionizing condi-
tions as the [3,3] rearrangement should be disfavored because
of ring strain in the cyclopropane product [Eq. (1)]. We report
the successful implementation of this strategy, in which a
unique Lewis acid promoted ring contraction of 2,5-dihy-
drooxepins to cyclopentenes was used.
[*] C. G. Nasveschuk, Prof. T. Rovis
Department of Chemistry
Colorado State University
Fort Collins, CO 80523 (USA)
Fax: (+1)970-491-1801
E-mail: rovis@lamar.colostate.edu
[**] Financial support was provided by the National Institute of General
Medical Sciences (GM65407). We also thank Merck Research
Laboratories, GlaxoSmithKline, Amgen, and Eli Lilly for unrestricted
support. We thank Professor Andre Charette (Montreal) for helpful
discussions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200500088
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could be converted into 9 in a one-pot
oxidation/retro-Claisen sequence and, after
some optimization, it was found that
1.5 equivalents of the Dess–Martin period-
inane in CH2Cl2 at 408C provided the desired
2,5-dihydrooxepin. The nature of the equilib-
rium meant that the unrearranged aldehyde 8
could be isolated and subsequently converted
Despite their apparent complexity, 2,5-dihydrooxepins
are readily prepared by a retro-Claisen reaction of the
corresponding cyclopropyl aldehyde 1, itself available by a
modular approach using established methods (see below).[8,9]
An equilibrium between 1 and 2 has been predicted computa-
tionally,[10] and may be shifted towards the 2,5-dihydrooxepin
with p-stabilizing substituents [Eq. (2); EDG = electron-
donating group, EWG = electron-withdrawing group].[9]
into 9 by heating overnight in toluene at 1108C.
With a convergent approach to the requisite 2,5-dihy-
drooxepins in hand, we began our studies on the stereo-
selective [1,3] ring contraction by conducting a brief screen of
Lewis acids. Cu(OTf)2, TiCl4, and SnCl4 yielded no product
under a variety of conditions (Table 1, entries 1–3). In the
presence of EtAlCl2 (entries 4–6), the starting material was
Table 1: Optimization of [1,3] ring contraction.
Entry
Lewis acid
Conditions
Yield [%] (cis/trans)
1
2
3
4
5
6
7
8
Cu(OTf)2
TiCl4
SnCl4
EtAlCl2
EtAlCl2
EtAlCl2
EtAlCl2
EtAlCl2
various
various
various
0.1m, À788C, 30 min
0.1m, 238C, 30 min
0.001m, À788C, 60 min
0.001m, 238C, 5 min
0.02m, 238C, 5 min[b]
NP[a]
NP[a]
NP[a]
We began our investigations by developing a highly
modular approach to the 2,5-dihydrooxepin skeleton. A
representative synthesis is illustrated in Scheme 1. A Sono-
gashira cross-coupling between propargyl alcohol and aryl
halides provided 4.[11] Selective formation of the (Z)-vinyl
iodide 5 was effected with red-Al/I2,[12] and a Negishi coupling
NP[a]
NP[a]
NP[a]
89 (90:10)
53 (93:7)
[a] Starting material consumed. [b] Slow addition of substrate to dilute
Lewis acid. NP=no product.
was then employed to insert an additional alkene.[13]
A
directed Simmons–Smith cyclopropanation afforded 7 in near
quantitative yield as a single regioisomer.[14] We felt that 7
consumed with the formation of uncharacterized oligomeric
products. We hypothesized that this could happen in one of
two ways: 1) vinyl ether 9 could polymerize before ionization
À
of the C O bond or 2) the zwitterionic intermediate gener-
À
ated from the ionization of C O is stable enough so that
intramolecular ring closure is slower than the bimolecular
reaction. Thus, cyclopentene 10 was isolated in 89% yield
with 90:10 (cis/trans) selectivity (entry 7) when 9 was
subjected to dilute Lewis acid at ambient temperature. We
further note that slow addition of dilute 2,5-dihydrooxepin to
the Lewis acid generally provides an incremental increase in
selectivity (entry 8).
We then evaluated the scope of the [1,3] ring contraction
of the 2,5-dihydrooxepins. Electron-donating and electron-
withdrawing groups in the para position of the aromatic ring
are tolerated and give products in comparable yield and
selectivity (Table 2, entries 1–3). Additional substitution on
the dihydrooxepin unit is well-tolerated, with substrate 15
furnishing tetrasubstituted cyclopentene 16 in good selectiv-
ity. An increase in the steric bulk of the substituent from a
methyl group (9) to a phenethyl group (17) or protected
alcohol (19) results in a slight decrease in the yield and
selectivity, but still provides synthetically useful amounts of
product. Lastly, the use of aldehyde 21 as a substrate indicates
Scheme 1. Synthetic approach to the 2,5-dihydrooxepins.
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Table 2: Reaction scope.
Table 3: Aldehyde epimerization.
Entry substrate
Product
Yield [%] cis/trans
Entry
1
Substrate (R1, R2)
Product
Yield [%]
68
cis/trans
12 (Me, p-Tol)
12:88
1
89
90:10
À
2
18 ( CH2CH2Ph, Ph)
81
3:97
2
3
4
85
75
58
93:7
jected to the optimized reaction conditions, 26 was isolated in
70% yield and 95% ee [Eq. (3)]. The pre-existing stereo-
87:13
88:12
5
6
73
52
85:15
85:15
center controls the diastereoselective course of the reaction,
and the observed selectivity can be rationalized by our
proposed model (Figure 1). There is an interplay of minimi-
7
59
89:11
Figure 1. Proposed stereochemical model for the diastereoselective
rearrangement of 25. a) Minimization of the A1,2 and A1,3 strains and
b) the favored model brought about by this process.
p-Tol=para-toluene, TBDPS=tert-butyldiphenylsilyl.
that formation of the dihydrooxepin is not necessary to
achieve reaction. Furthermore, this substrate lacks the aryl
stabilization evident in the other substrates, thus suggesting
that aliphatic stabilization is sufficient in some cases. These
disubstituted cyclopentene carboxaldehydes are readily epi-
merized[14] to form the trans diastereomer upon treatment
with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU; Table 3,
entries 1 and 2).
zation between the A1,2 strain, between the phenyl group on
the allyl cation and the adjacent methyl, and the A1,3 strain,
between the alkoxide on the enolate and the methyl group,
that presumably leads to the observed levels of diastereose-
lectivity.
As noted above, we may also use the aldehydes as
precursors for the rearrangement [Table 2, entry 7; Eq. (4)].
A considerable advantage of this method is the ability to
introduce substitution at every position of the dihydrooxepin
ring. With this approach in mind, we sought to apply the
protocol of Charette et al.[15] as a means of introducing further
substitutents onto the cyclopropane and affording a tetrasub-
stituted cyclopentene on [1,3] rearrangement. 2,5-Dihydroox-
epin 25 was synthesized in an enantioenriched form using the
Charette–Simmons–Smith protocol.[15] When 25 was sub-
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J. Am. Chem. Soc. 1991, 113, 5488 – 5489; c) A. Gꢁnsauer, D.
We suggest that this arises from the initial Lewis acid
catalyzed retro-Claisen rearrangement that gives the 2,5-
dihydrooxepin prior to [1,3] bond migration. Another possi-
bility is that the Lewis acid accelerates the Claisen/retro-
Claisen equilibrium so that a Curtin–Hammett situation is
formed where the cyclopentene product is siphoned off from
either the 2,5-dihydrooxepin or the cyclopropyl aldehyde.
Although the exact mechanism at this stage remains unclear,
this observation makes the overall procedure operationally
simpler.
The presence of the olefin in the cyclopentene allows for
further diastereoselective functionalization. We investigated
one such approach and found that diastereoselective dihy-
droxylation produces a pentasubstituted cyclopentane in
modest yield but excellent selectivity [Eq. (5), NMO = 4-
methylmorphoꢀine N-oxide].[16]
Fielenbach, C. Stock, Adv. Synth. Catal. 2002, 344, 845 – 848;
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[10] D. Sperling, H.-U. Reißig, J. Fabian, Liebigs Ann. 1997, 2443 –
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In summary, we have developed a novel room-temper-
ature Lewis acid mediated diastereoselective [1,3] ring con-
traction of 2,5-dihydrooxepins. Our modular approach to
these seven-membered heterocycles allows for the installa-
tion of a variety of groups at every position. The reaction
provides access to cis and trans cyclopentene carboxalde-
hydes with good selectivities, and can lead to tetrasubstituted
cyclopentenes in high enantiomeric excess and diastereose-
lectivity.
Received: January 11, 2005
Published online: April 21, 2005
Keywords: cyclopentenes · diastereoselectivity · Lewis acids ·
.
rearrangement · ring contraction
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