Convergent Evolution of Diastereomeric Mixtures of 5-Methoxy-pentylzirconocenes toward Trans-1,2-substituted Cyclopentanes

Article abstract of DOI:10.1021/acs.orglett.0c03998

The access to 1,2- and 1,1,2-substituted trans cyclopentanes via a sequential hydrozirconation/TMSOTf-mediated cyclization applied to 5-methoxypent-1-enes is presented. Involving a transient carbocation, the reaction was shown to be diastereo-convergent. Possibly performed in a nonracemic version, the reaction proved compatible with a range of functional groups affording a large panel of cyclopentanes.

Full text of DOI:10.1021/acs.orglett.0c03998

pubs.acs.org/OrgLett
Letter
Convergent Evolution of Diastereomeric Mixtures of 5‑Methoxy-
pentylzirconocenes toward Trans-1,2-substituted Cyclopentanes
́
Aurelien Coelho, Carine Machado-Rodrigues, Jean-Bernard Behr, and Jean-Luc Vasse*
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ABSTRACT: The access to 1,2- and 1,1,2-substituted trans cyclopentanes via a sequential hydrozirconation/TMSOTf-mediated
cyclization applied to 5-methoxypent-1-enes is presented. Involving a transient carbocation, the reaction was shown to be diastereo-
convergent. Possibly performed in a nonracemic version, the reaction proved compatible with a range of functional groups affording
a large panel of cyclopentanes.
ommon to a plethora of biologically active molecules,1
the cyclopentane skeleton constitutes a privileged
high tolerance toward a large range of functional groups.14
Although the resulting zirconocenes suffer from a lack of
reactivity, it may be compensated by generating a more
reactive nucleophilic species through transmetalation.15
C
scaffold for the design of chiral ligands for catalysis as well as
a locking element to build conformationnally constrained
mimics. In this context, strategies which allow the stereo-
controlled construction of carbocyclic structures including 1,2-
disubstituted cyclopentanes remain of considerable interest.
A number of synthetic tools enable efficient ring closure to
the five-membered carbocyclic motif, such as metathesis,2
transition-metal-mediated intramolecular transformations in-
cluding ene-reaction,3 alkyl-Heck,4 allylation,5 cascade hydro-6
and carbometalation7/cyclizations, and palladium-mediated [3
+ 2] cycloaddition.8 However, to achieve stereoselectivity,
most of these strategies require configurational control of all
the stereogenic centers prior to cyclization or via a subsequent
stereoselective transformations of the intracyclic CC double
bond.
Since the pioneering work of Hansawa and Taguchi related
to the formation of cyclopropylmethanols from vinylepoxides
under hydrozirconation conditions,16 a complementary
approach, based on the Lewis acid mediated activation of the
electrophilic site, was developed to promote the ring closure.
First, Hansawa and Taguchi showed that homoallylic epoxides
underwent a similar and stereospecific ring closure affording
cyclopentanols (Figure 1).17 In this case, an epoxide activation
is required due to the absence of a proximal Zr−O interaction.
Second, Szymoniak described the access to trans cyclopropanes
through a sequential hydrozirconation/BF3·OEt2-assisted
contraction.18 The stereoselectivity was reported to result
from a diastereoselective hydrozirconation, the consequence of
a directed approach of the zirconium hydride mediated by Zr−
O interaction, followed by a stereospecific ring-closure step.
The mechanism was studied in-depth using marked com-
pounds by Casey, and a W-shaped transition state evolving
with an SN2 like selectivity was proposed.19 Additionally an
access to cyclopropylboronic esters via a similar sequence
hydroboration/hydrozirconation applied to propargylic sily-
lethers was reported by Talbot.20 Recently, we described the
Additionally, Lewis acid mediated9 or organo-catalyzed ring
closure implying prochiral enolic systems have been reported
as direct stereoselective routes to polysubstituted cyclo-
pentanes.10 Cascade Michael addition/cyclization11 also
constitutes an attractive and stereoselective approach for
accessing polysubstituted cyclopentanes. Alternatively, radical
ring-closure processes involving prochiral systems12 might also
be used to reach five-membered carbocycles.
In contrast, strategies relying on the connection of two Csp3
through a nucleophilic substitution remain rare. Hard reaction
conditions,13 including strong bases, are often required to
promote the cyclization, limiting the method to substrates
bearing inert substituents. To prevent the use of strong bases
for generating the C−M bond, hydrometalation constitutes an
efficient and regioselective variation, particularly in the case of
terminal alkenes. Among them, the hydrozirconation exhibits a
Received: December 2, 2020
Published: January 13, 2021
https://dx.doi.org/10.1021/acs.orglett.0c03998
© 2021 American Chemical Society
Org. Lett. 2021, 23, 772−776
772

Organic Letters
pubs.acs.org/OrgLett
Letter
conditions. While structural flexibility is tolerated in the β-
position of the methoxy group (R2), the choice of the proximal
substituent (R1) might be guided by its propensity to activate
the departure of the methoxy group under Lewis acid
conditions. Indeed, alkyl (Table 2, entry 5) and a deactivating
a
Table 2. Synthesis of Cyclopentanes 2
b
c
entry
R1
R2
1, dr
2 yield (%), dr
1
2
3
4
5
6
7
8
9
10
PMP
PMP
1c, 88:12
1d, 94:6
1e, 90:10
1f, >95:5
1g, 75:25
1h, 100:0
1h, 0:100
1i, 93:7
2c (44), >95:5
2d (53), >95:5
NR
2f (52), >95:5
NR
2h (64), >95:5
2h (50), >95:5
2i (90), >95:5
2j (39), 83:17
2k (62), 85:15
Figure 1. Context of the study.
4-iBu-C6H4
4-Cl-C6H4
Ph
CH2Ph
Ph
4-MeO-C6H4
4-Cl-C6H4
CH2Ph
synthesis of disubstituted alkenes from a diastereoisomeric
mixture of homoallylic ethers following a similar sequence. The
exclusive E selectivity was assumed to result from a cationic
intermediate.21 Therefore, if a transient carbocation could be
generated, a convergent access to cyclopentanes may be
envisioned from a mixture of diastereomers, thus avoiding a
fastidious diastereoselective synthesis of the precursor.
In this letter, we disclose the diastereoselective preparation
of trans di- and trisubstituted cyclopentanes from 5-methoxy
alkenes through a sequential hydrozirconation/Lewis acid
mediated deoxygenative cyclization.
Ph
CH2OTBDPS
CH2OTBDPS
Ph
CH2OTBDPS
CH2OTBDPS
Ph
d
2-thiophenyl
2-thiophenyl
Ph−CHCH
1j, 56:44
1k, 52:48
a
Reaction conditions: 3 (1 mmol), Cp2Zr(H)Cl (1.3 mmol), CH2Cl2
(3 mL), rt, 20−30 min, then TMSOTf (1 mmol), −50 °C for 30 min,
then 30 min rt. Isolated yield Determined from H NMR of the
b
c
1
d
crude reaction, TMSOTf was added at 0 °C.
Preliminary studies were devoted to identifying optimal
conditions to promote the cyclization from model substrates
susceptible to favor the delivery of a carbocation (Table 1).
phenyl group (entry 3) are not suitable for promoting the
cyclization. In contrast, aryl (entries 1, 3−4, and 6−7), 2-
thiophenyl (entries 8 and 9), and a styryl group (entry 10) are
sufficiently activating to ensure the formation of the
corresponding cyclopentanes. Except for 2j and 2k, good
trans-selectivity was observed with most substrates, even
starting from mixtures of diastereomers.
a
Table 1. Lewis Acid Screening
Nucleophilic substitution of activated alcohols22 promoted
by a Lewis or Brønsted acid is established to follow an SN1
pattern.23 Eventually, the nucleophilic attack of the transient
carbocation may be stereoinduced by a chiral structural
element. Thus, a high level of diastereocontrol could be
reached in intramolecular reactions.24 Possibly, the diaster-
eoselectivity may decline from a thermodynamic equilibration
when the cyclization step is reversible.24d In contrast,
cyclization involving the SN2 process from activated alcohols
is rare and requires a transient chiral intimate ion pair,25 or a
simultaneous activation of both the leaving group and the
nucleophile.26
b
c
entry
1, R
LA
2a, yield (%)
dr
d
1
2
3
4
5
6
1a, CH3
1a, CH3
1a, CH3
1a, CH3
1b, TMS
1b, TMS
BF3·OEt2
AlCl3
TiCl4
TMSOTf
BF3·OEt2
AlCl3
50
ND
77
80
88
>98:2
>98:2
>98:2
77:23
ND
d
40
20
a
Reaction conditions: 3 (1 mmol), Cp2Zr(H)Cl (1.2 mmol), CH2Cl2
(3 mL), rt, 20−30 min, then Lewis acid (1 mmol), −50 °C for 30
min, then 30 min rt. Isolated yield. Calculated from the crude
b
c
d
1
reaction by H NMR spectrometry. Conversion
In the present case, the formation of trans cyclopentanes 227
would plausibly originate from a carbocation, generated via
TMSOTf-mediated activation, which irreversibly evolves in a
selective manner (Figure 2). This was supported by applying
identical reaction conditions to each diastereomeric form of
1h, which both led to trans 2h (Table 2, entries 6 and 7).28
Structural diversity remains a major objective in synthetic
methodology development. In this context, it would be
appealing to extend the present strategy to more versatile
substrates. From that perspective, the introduction of a CC
triple bond as the additional substituent was envisioned. This
would (i) meet the electronic criteria for generating a cationic
intermediate, which should even be facilitated; (ii) possibly
secure a diastereoselective ring-closure step since the disparity
in size between the two lateral chains would be retained; and
(iii) offer multiple possibilities of further functionalization.
However, the access to the corresponding cyclopentane relies
Thus, 1a,b reacted with Schwartz’s reagent (1.2 equiv) in
CH2Cl2. While the hydrozirconation proceeded well with 1a,
providing a limpid yellow solution within 30−40 min, a turbid
solution was observed with 1b. Subsequently, a Lewis acid was
added to promote the cyclization. As anticipated from the
latter experimental observations, lower conversions were
obtained in the case of 1b. Regarding Lewis acids, the use of
BF3 allowed the cyclization to occur, however in low yield
along with degradation. In contrast, good conversions were
obtained using TiCl4 and AlCl3. Finally, the best conditions
were obtained using TMSOTf with 1a as the starting material.
Interestingly, the cyclopentane was obtained with almost
complete diastereoselectivity.
To estimate the scope of the reaction, a series of diversely
substituted substrates was prepared next (for details, see
Supporting Information) and tested in the optimized
773
https://dx.doi.org/10.1021/acs.orglett.0c03998
Org. Lett. 2021, 23, 772−776

Organic Letters
pubs.acs.org/OrgLett
Letter
Scheme 1. Synthetic Applications
Figure 2. Rationale of evolution of the diastereomeric zirconocenes.
Finally, the possibility to synthesize cyclopentanes in a
nonracemic version was evaluated. Since the reaction was
proven to involve a transient carbocation, the strategy for
preparing the cyclopentane precursor was devoted toward
solely controlling the configuration of the irremovable
stereogenic center at the homoallylic position. We opted for
a diastereoselective acylation of Evans-type substrates31
followed by a reduction, which did not need to be
diastereoselective, to afford the desired diol.32 Thus, 633 was
deprotonated using LDA, and benzoyl chloride or p-
methoxybenzoyl chloride was added to give respectively
ceto-carbamates 7l and 7m respectively, isolated as single
on the chemoselective hydrozirconation of the CC double
bond, which remains possible owing to the higher substitution
degree of the CC triple bond.
A series of substrates 3 were prepared and submitted to the
same cyclization conditions. Pleasantly, cyclopentanes 429
incorporating diverse combinations of substituents (Table 3,
entries 1−4 and 7−12) were obtained in good yield, except in
the particular case of 3f (R1 = Bn, entry 6), where a
competitive evolution toward the enyne seemed to prevail.30
Interestingly, a high level of diastereocontrol was observed in
most cases (dr 93:7 to >95:5), except for 4d, presumably due
to low discrimination between the n-butyl and the alkynyl
chain. Noteworthy, a large panel of alkynyl fragment (R3 = Ph,
SiMe3 or hydroxyl-protected chains) is compatible with a
selective hydrozirconation. In comparison with the previous
series, the incorporation of an alkynyl chain seems to
compensate the presence of an alkyl chain (entry 4) or a
deactivating aryl fragment (entry 3), enlarging the structural
diversity achievable. However, it remains necessary to
incorporate a conjugated alkynyl fragment to generate the
carbocation (entry 4 vs 5), and the reaction time must be
increased to 1 h.
34
stereoisomers. Subsequent global reduction using LiBH4
followed by silylation of the primary alcohol and methylation
of the secondary completed the enantio-enriched synthesis of
1l and 1m (Scheme 2). For the preparation of 3m, a similar
approach was employed. The deprotonation/benzylation
sequence35 applied to 636 provided 8 as a single diastereomer.
Subsequent conversion into the Weinreb amide 9, followed by
successive additions of 4-methoxyphenyllithium and [6-(tert-
butyldiphenylsilyloxy) hex-1-ynyl]lithium afforded the tertiary
alcohol which was ultimately methylated to afford 3m (Scheme
2). Substrates 1l,m and 3m were submitted to the previously
described cyclization conditions to give enantioenriched
cyclopentanes with nearly complete conservation of the
enantiomeric integrity. Products 2l,37 2m, obtained in their
free alcohol form, and 4m were obtained with ee values greater
than 90% (Scheme 2).
The synthetic interest of the method was illustrated by first
accessing cyclopentane 4′h bearing both an alkyl and an aryl
substituent at the same position through Pd-catalyzed
hydrogenation. Second, tricyclic compound 5 was prepared
via intramolecular alkyne−azide cycloaddition (Scheme 1).
a
Table 3. Synthesis of Cyclopentanes 4
b
c
entry
3
R1
R2
R3
4 yield (%), dr
1
2
3b
3c
3d
3e
3f
3g
3h
3i
1-Naphtyl
4-MeO-C6H4
4-F-C6H4
n-Bu
CH2Ph
(CH2)2OBn
(CH2)3OBn
(CH2)3OBn
(CH2)3OBn
Ph
Ph
Ph
Ph
Ph
4a (51), 93:7
4b (70), 92:8
4c (66), 96:4
4d (74), 79:21
−
(CH2)2OBn
4-Me-C6H4
Ph
(CH2)2OBn
Ph
SiMe3
(CH2)2OBn
SiMe3
d
3
d
4
d
5
6
7
8
9
10
11
12
CH2Ph
ND
4-MeO-C6H4
4-MeO-C6H4
2-thiophenyl
4-MeO-C6H4
2-thiophenyl
4-MeO-C6H4
4g (84), 94:6
4h (76), >95:5
4i (90), >95:5
4j (58), 90:10
4k (65), >95:5
3j
3k
3l
Ph
Ph
Ph
2-thiophenyl
CH2OBn
CH2OBn
e
3m
CH2OTBDMS
4l (80), >95:5
a
Reaction conditions: 3 (1 mmol), Cp2Zr(H)Cl (1.3 mmol), CH2Cl2 (3 mL), rt, 20−30 min, then TMSOTf (1 mmol), −50 °C for 30 min, then
b
c
d
e
1
30 min rt. Isolated yield. Determined from H NMR of the crude reaction mixture. 1 h at −50 °C, 1 h, then, 30 min rt. Free alcohol was
obtained.
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pubs.acs.org/OrgLett
Letter
a
Notes
Scheme 2. Synthesis of Enantio-enriched Cyclopentanes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
The CNRS and the Universite de Reims-Champagne Ardenne
for financial support, and Alexis Vallee (ICMR-UMR 7213) for
technical assistance, are gratefully acknowledged.
́
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
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Experimental procedures, characterization, and copies of
1H and 13C NMR spectra of new compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Jean-Luc Vasse − Institut de Chimie Moléculaire de Reims-
CNRS (UMR 7312) and Université de Reims Champagne
Ardenne, 51687 Reims, France; orcid.org/0000-0003-
0802-1095; Email: jean-luc.vasse@univ-reims.fr
̌
́
́
Authors
́
Aurelien Coelho − Institut de Chimie Moléculaire de Reims-
CNRS (UMR 7312) and Université de Reims Champagne
Ardenne, 51687 Reims, France
Carine Machado-Rodrigues − Institut de Chimie Moléculaire
de Reims-CNRS (UMR 7312) and Université de Reims
Champagne Ardenne, 51687 Reims, France
Jean-Bernard Behr − Institut de Chimie Moléculaire de Reims-
CNRS (UMR 7312) and Université de Reims Champagne
Ardenne, 51687 Reims, France
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03998
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