Domino asymmetric conjugate addition-conjugate addition

Article abstract of DOI:10.1021/ol5005752

Enantioenriched Al-, Mg-, and Zn-enolates undergo electrophilic trapping by nitroolefins and vinylsulfones to afford 1,4-diketones and 2- (bis(phenylsulfonyl)ethyl)ketones in good yield and excellent diastereoselectivity. A one-pot preparation of indenes and enantiopure syntheses of tetrahydrobenzofurans, tetrahydrobenzopyrroles, and azulenes are disclosed. A site-selective two-step sequence of three conjugate additions is also demonstrated.

Full text of DOI:10.1021/ol5005752

Letter
pubs.acs.org/OrgLett
Domino Asymmetric Conjugate Addition−Conjugate Addition
Nicolas Germain, Doriane Schlaefli, Mathieu Chellat, Stephane Rosset, and Alexandre Alexakis*
́
Department of Organic Chemistry, University of Geneva, Quai Ernest Ansermet, 30, 1211 Geneva 4, Switzerland
S
* Supporting Information
ABSTRACT: Enantioenriched Al-, Mg-, and Zn-enolates undergo electrophilic trapping by nitroolefins and vinylsulfones to
afford 1,4-diketones and 2-(bis(phenylsulfonyl)ethyl)ketones in good yield and excellent diastereoselectivity. A one-pot
preparation of indenes and enantiopure syntheses of tetrahydrobenzofurans, tetrahydrobenzopyrroles, and azulenes are disclosed.
A site-selective two-step sequence of three conjugate additions is also demonstrated.
Scheme 1. Functionalization of Enolates Using MVK (Methyl
Vinyl Ketone) and Nitrostyrene
ccess to enantiopure small molecules is a continuous
Achallenge for organic chemists. Asymmetric conjugate
addition (ACA) offers a selective tool for the preparation of
molecules bearing one or multiple stereogenic centers.1 Many
transition metals have been reported to catalyze this reaction, but
copper salts are now accepted to be the most versatile of them.2
Copper not only is among the cheapest transition metals used in
asymmetric catalysis but also is easily transmetalated from
organoaluminium, -magnesium, and -zinc. As a result, method-
ologies involving these organometallic reagents complement
each other, and their association with various optimized catalytic
complexes brings an accurate tool to the organic chemistry
community.3 Noteworthy is the synthetic potential of metal-
enolates resulting from the conjugate addition of organocuprates
to enones: in the presence of an additional electrophile, a second
stereocenter can be created.4,5
Due to the aggregation state of metal-enolates, and the Lewis
acidity of the metal, Al-, Mg-, and Zn-enolates have a different
reactivity toward a given electrophile. While Mg-enolates are the
most reactive, Al-counterparts are often more problematic except
when coordination with the entering electrophile is possible like
aldehydes or imines.6 Another way to increase the reactivity of
Al-enolates is to form in situ an Alanate by addition of 1 equiv of
MeLi.7 Zn-enolates can be successfully reacted in situ to form
aldol, Mannich, Blaise, or Dieckmann adducts, but in these
examples, at least two diastereomers are isolated.4g,8,9
Furthermore, despite numerous examples of alkylation of
metal-enolates, the electrophilic trapping by Michael acceptors
could broaden the scope of ACA. The synthetic challenge
associated with this cascade reaction10 combined with the
constant need for straightforward syntheses of functionalized
small molecules prompted us to consider this topic.
Mg-enolate, the desired product could not be isolated; an
inseparable mixture of products was obtained instead, probably
resulting from a polymerization process through 4, while the Zn-
enolate led to a mixture of products from which the desired 1,5-
diketone could be isolated in 30% yield. These results can be
rationalized by looking at the reaction pathway: after the first
conjugate addition, a parent metal-enolate 1 is trapped by MVK,
generating another enolate 2. Yet, 1 and 2 compete and MVK can
be indifferently trapped by the two enolates to give 3 or 4. To
circumvent this selectivity issue, a different Michael acceptor is
needed with charge-stabilizing functionality. Unlike the enolate
We initiated our work by optimizing the Cu-catalyzed
conjugate addition of organometallic reagents to cyclohexenone
followed by addition of MVK (Scheme 1a).11,12 In the case of
Received: February 22, 2014
Published: March 24, 2014
© 2014 American Chemical Society
2006
dx.doi.org/10.1021/ol5005752 | Org. Lett. 2014, 16, 2006−2009

Organic Letters
Letter
derived from the conjugate addition to enones, 1,4-additions to
nitroolefin and vinyl sulfone should result in less reactive species,
lowering potential over-reactions.
In all the cases, and with Mg, Al, and Zn, the straightforward
ACA-trapping proceeded smoothly via in situ Nef reaction23,24

no nitro-ketones were detectedand 1,4-diketones were
isolated in good to excellent yields and dr. Experimentally, 2-
nitropropene and -butene showed comparable reactivity.
Indeed, starting from the same Mg-enolate, 7bb and 7bc
needed both 2 h at 0 °C to reach full conversion (Table 1, entries
To test this concept, a first reaction was run with commercially
available nitro-styrene. The desired ketone 5 was obtained in
42% yield, but no diastereocontrol at C-7 was observed (Scheme
1b)13,14 Hence, Michael acceptors should not be β-substituted
and must be compatible with Al-, Mg-, and Zn-enolates. Ideal
electrophiles for the domino ACA-Conjugate Addition would
also lead to easily derivatizable adducts in high enantiomeric
excess (ee) and diastereoselective ratio (dr).
Table 1. Synthesis of Chiral 1,4-Diketones via Domino
Conjugate Additions
Herein, we propose 1-alkyl-1-nitroolefin and bis-
(phenylsulfonyl) ethylene as valuable substitutes to our early
attempts (Scheme 2). This choice was also inspired by the work
of Mayr et al. who recently stated that nitroolefins and vinyl
disulfones have comparable electrophilicity.15
Scheme 2. Reactivity of Enolates towards Michael Acceptors
a
b
c
entry
1
product
R
yield (%)
dr
ee (%)
7a
EtMgBr
Me3Al
64
84
63
66
82
68
71
81
72
66:34
82:18
86:14
90:10
88:12
57:43
66:34
97:3
87
d
2
7ba
7bb
7bc
7bd
7ca
7cb
7da
7db
95
3
4
5
6
7
EtMgBr
EtMgBr
Et2Zn
97
97
>99.5
90
EtMgBr
EtMgBr
Me3Al
90
d
8
93
9
Me2Zn
92:8
>99.5
a
Since the present study was motivated by the need for
enantioenriched complex molecules, our methodology was
designed to allow for the preparation of relevant bicyclic
scaffolds. The core structures of the three sesquiterpenes
(Valerenic acid,16 Ambrosin,17 Halipanacine18) and meroterpe-
noid Liphagal19 are potential targets of this work (Figure 1).
1H NMR spectroscopy and GC showed full conversion for all
b
c
examples. Trans/cis ratio. Measured before trapping and compared
to reported data. No erosion has been noted. Reaction scaled up to 5
mmol. Equimolar addition of methyllithium was used prior to the
addition of nitroolefin.
d
3 and 4). In contrast, Al-enolates react sluggishly. To enhance the
reactivity, 1 equiv of MeLi was added to form the corresponding
more nucleophilic Alanate. The reaction rate was then
comparable to Mg-enolate (entry 2 vs 3). To our delight, Zn-
enolate resulting from the ACA of diethylzinc to 6b gave
diketone 7bd in only 1.5 h at 0 °C in 82% yield and 88:12 dr
(entry 5). We can conclude that, under optimized conditions, our
three enolates have comparable reactivity profiles toward 1-
alkylnitroolefins. The concept was further extended to seven-
membered rings with even higher dr’s up to 97:3 (entries 8 and
9). The isolation of 7a in 64% yield demonstrated that five-
membered rings were also tolerated (entry 1). Gratifyingly, the
reaction was compatible with sterically demanding vicinal
quaternary centers: diketones 7ca and 7cb were obtained in up
to 71% isolated yield. As expected, better diastereocontrol was
obtained for ketones bearing contiguous tertiary centers (entry 4
vs 7). Our methodology was eventually amenable to 5 mmol in
81% yield (entry 8).
Figure 1. Potential target of this work.
Nitroolefins have been used in two-step processes involving
silyl enol ether in Sakurai-type reactions mediated by tin
tetrachloride or aluminum trichloride.20 Yet, the reactivity of
nitroolefins with Al-, Mg-, and Zn-enolates remains underex-
plored. Li-enolates have been reacted with nitroolefins, but two
steps are needed and regioselectivity issues cannot be
avoided.5,21
The reactivity of the various enantioenriched metal enolates
was then explored.22 We assumed that the catalytic Cu/chiral
ligand complexes did not influence the outcome of the trapping
step.
With our optimized conditions in hand, we set different
experimental procedures in order to demonstrate the synthetic
versatility of 1,4-diketones (Scheme 3). After transformations of
our enantioenriched adducts, not only can the precursors of
molecules depicted in Figure 1 be targeted but also other natural
compounds featuring common molecular scaffolds can be
prepared. The regioselective aldol reaction is one possible
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Organic Letters
Letter
Scheme 3. One-Pot and Two-Pot Syntheses of
Enantioenriched Indenones and Azulene Derivatives
Table 2. Domino Michael Reactions with Vinyl gem-
Disulfones
a
b
c
entry
product
R′
yield(%)
dr
ee (%)
1
2
14a
EtMgBr
Me2Zn
Me3Al
Et2Zn
85
67
82
75
71
85
53:17
83:17
95:5
86
14ba
14ba
14bb
14bb
14c
93
d
3
96
4
5
6
85:15
81:19
53:47
>99.5
95
reaction.4o For condition A, no purification is needed and the
construction of functionalized indenes 8 and 9 proceeded
smoothly, while for condition B, the intermediate 1,4-diketones
was isolated prior to the cyclization step to yield indene 10 and
azulene-type 11, a useful guaiane skeleton.25
Et3Al
EtMgBr
90
a
b
1H NMR and GC showed full conversion for all examples. Trans/cis
c
ratio. Measured on 1,4-adduct before trapping and compared to
reported data. Reaction scaled up to 5 mmol. Equimolar addition of
methyllithium was used prior to the addition of disulfone.
d
In addition, 1,4-diketones are also pyrrole and furan precursors
(Scheme 4).26 Microwaves-assisted Paal−Knorr synthesis using
As stated above, sulfones derivatives are tunable synthetic
entities.33 The homolytic cleavage of disulfone has been
introduced many years ago, and methodologies using lithium
naphthalenide or samarium iodide are well-established proce-
dures for the synthesis of various compounds.34
Scheme 4. Syntheses of Enantiopure Tetrabenzopyrrole and
Tetrabenzofuran Derivatives
Mechanistically, the trapping step leads to an anion stabilized
by the gem-disulfone. A second trapping would be theoretically
possible, but only moderate yields were obtained. 14a−c feature
three potential acidic positions, but the selective deprotonation
of the most acidic of those was possible using sodium hydride.35
Treatment of 14ab under basic conditions followed by the
addition of vinyl triphenylphosphonium bromide yielded a
transient phosphonium ylide, which readily reacted with the
ketone to afford the Wittig adduct 15 in 76% yield (Scheme 5).36
This transformation was the result of three conjugate additions in
a row. By following the same procedure, allyl iodide was trapped
in a modest 33% yield.37
benzyl amine in acetic acid yielded one enantiomer of N-
benzylpyrrole derivative 12 in 79% yield.27 Analogously, acid-
catalyzed Paal−Knorr furan synthesis gave tetrahydrobenzofuran
13 in 76% yield.28 The preparations of enantiorenriched 4,5,6,7-
tetrahydropyrrole and -furan have been scarcely disclosed.29
As a significant class of small synthetic fragments, sulfones
have found a broad scope of applications.30 Also called chemical
chameleons, they are versatile derivatizable functions.31 As a
consequence, they have been extensively used in organic
synthesis and notably in organocatalyzed reactions.32
Scheme 5. Derivatization of Ketone 14ba by Conjugate
Addition−Wittig-like Condensation Sequence To Form
Indene 15
To the best of our knowledge, metal-enolates have never been
trapped by vinyl disulfones. We then decided to test the same
chiral enolates derived from ketones 6a−d (Table 2).
Zn-enolates were converted to ketones 14ba and 14bb in 3 h
at −30 °C in up to 75% yield (entries 2 and 4). The reaction was
fast and produced the desired product in 83:17 and 85:15 dr
respectively. The trapping has been also successfully applied to
Al-enolates, affording adducts 14ba in 82%, 95:5 dr, 96% ee and
14bb in 71%, 81:19 dr, 95% ee (entries 3 and 5). However, 15 h
at −30 °C and activation by methyllithium were required to
achieve full conversion. Without methyllithium, the reaction
proceeded at a much lower rate and the concomitant formation
of undesired products was observed. Here also, the same results
were obtained when scaling up to 5 mmol for the synthesis of
14ba. Analogously to the preceding table, the excellent behavior
of Mg-enolates allowed us to validate the method with five-
membered rings and in the presence of bulky quaternary centers.
Hence, functionalized ketones were obtained in 85% yield in 2 h
at 0 °C (entries 1 and 5). Also, no erosion of the
enantioselectivity was noticed. From a practical point of view,
adducts 14 are easily isolated as white solids and can be further
purified by recrystallization in ethyl acetate.
The sequence of Michael additions involving ACA and enolate
trapping with nitroolefin and vinyl sulfone has been demon-
strated to be a reliable transformation for enantioenriched Al-,
Mg-, and Zn-enolates. The domino ACA−enolate trapping by
Michael acceptors afforded 1,4-diketones and γ-sulfonylated
ketones in high yield and diastereoselectivity. These Michael
adducts were then derivatized toward notable bicyclic structures.
This work emphasizes the versatility of metal-enolates and shows
that investigations are still needed in this field. Extension of the
methodology and its application toward the rapid synthesis of
natural products are currently underway in our laboratory and
will be reported in due time.
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Organic Letters
Letter
(11) In addition to our recent observation with Mg-enolate (ref 12),
the use of Zn-enolate lead to the same issue. See: Knopff, O., research
report No.1, University of Geneva, 2004.
ASSOCIATED CONTENT
* Supporting Information
■
S
(12) Germain, N.; Guen
2014, 16, 118.
́ ́
ee, L.; Mauduit, M.; Alexakis, A. Org. Lett.
Experimental procedures and complete characterizations (NMR
spectra, GC traces, and HRMS) are provided. This material is
available free of charge via the Internet at http://pubs.acs.org.
(13) The same trend has been observed in earlier work with Zn-
enolates. See: research report, Graham Trevitt N°1, University of
Geneva, 2001.
(14) A good 9:1 dr was obtained with Zn-enolate on acyclic system.
See: Guo, S.; Xie, Y.; Hu, X.; Huang, H. Angew. Chem., Int. Ed. 2010, 49,
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: alexandre.alexakis@unige.ch.
Notes
The authors declare no competing financial interest.
(18) Ye, B.; Nakamura, H.; Murai, A. Tetrahedron 1996, 52, 6361.
(19) Marion, F.; Williams, D. E.; Patrick, B. O.; Hollander, I.; Mallon,
R.; Kim, S. C.; Roll, D. M.; Feldberg, L.; Van Soest, R.; Andersen, R. J.
Org. Lett. 2006, 8, 321.
ACKNOWLEDGMENTS
■
This work was supported by the Swiss National Research
Foundation (Grant No. 200020-126663).
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