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Construction of Quaternary Stereocenters New Perspectives through Enantioselective Michael Reactions.

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Highlights
Enantioselective Michael Addition
Construction of Quaternary Stereocenters: New
Perspectives through Enantioselective Michael Reactions
Jens Christoffers* and Angelika Baro
Keywords:
asymmetric catalysis · C C coupling ·
enantioselectivity · Michael addition · transition
metals
S
tereoselective C C bond-forming reactions are of particular importance for
the preparation of enantiopure natural
compounds and pharmaceuticals. The
wide variety of available chiral auxiliaries, reagents, and catalysts nowadays
enables the generation of tertiary stereocenters in most cases without any
difficulty. However, the approach to
complex compounds with quaternary
stereocenters is still a challenge for
synthetic organic chemists, and every
enantioselective procedure in which a
fully substituted carbon center is constructed is of value.[1]
The Michael reaction, the conjugated or 1,4-addition of enolates to acceptor-substituted olefins, is a fundamental
C C-coupling reaction, which is catalyzed not only by Brønsted bases,[2] but
also by a number of metal compounds.[3]
An important breakthrough in the field
of extensively investigated metal-catalyzed, enantioselective Michael reactions[4] was recently reported by Sodeoka and co-workers.[5] For the first time,
quaternary stereocenters could be generated with > 90 % ee at a relatively
high temperature of 20 8C in a Michael
reaction of b-dicarbonyl compounds 1
with enones 2 using palladium(ii) as the
catalytically active transition metal. The
PdII–diaquadiphosphane complexes 3 a
and 3 b are formed with chiral ligands
(R)-tol-binap and (R)-binap, respective-
[*] Prof. Dr. J. Christoffers, Dr. A. Baro
Institut f0r Organische Chemie
Universit3t Stuttgart
Pfaffenwaldring 55
70569 Stuttgart (Germany)
Fax: (+ 49) 711-685-4269
E-mail: jchr@po.uni-stuttgart.de
1688
ly. Scheme 1 shows four typical reaction
products 4 a–d as examples.
Both cyclic (1 a) and acyclic (1 b) bketoesters react with methyl vinyl ketone (2 a) at 20 8C in a Michael reaction promoted by 3 a to give the corresponding products 4 a and 4 b in moderate to good yields with > 90 % ee. Noteworthy is the use of the sterically
demanding tert-butyl and phenyl b-ketoesters. The conversion of b-diketone
1 c (X = CH2Ar) in the presence of
catalyst 3 b (10 mol %) at 10 8C is an
outstanding example of an asymmetric
Michael addition to acyclic triketones
such as 4 c with very high stereoselectivity, an achievement thus far not reported
in the literature. Furthermore, the reaction of 1 d with substituted enone 2 b
catalyzed by 3 b at 20 8C affords a
mixture of two diastereoisomers (d.r. =
8:1), and an enantiomeric excess of 99 %
for the major diastereomer of 4 d.
The high efficiency of the method
developed by Sodeoka and co-workers
is clearly demonstrated in view of the
work published so far: In 1975 Wynberg
and co-workers applied cinchona alkaloids in base-catalyzed, enantioselective
Michael reactions.[6] About 10 years later, the first of these reactions catalyzed
by a transition metal, cobalt, was published by Brunner and Hammer.[7] Both
methods result in quaternary stereocenters with up to 68 % ee, which was
improved in the following years by
Desimoni et al. to 75 % ee.[8] The introduction of rhodium catalysts in 1992 by
Ito and co-workers in Michael reactions
of a-methyl-substituted cyanoacetates
and vinyl ketones or acrolein represented an important breakthrough in this
field. The catalyst was generated in situ
from the chiral ligand 2,2’’-bis[1-(diphenylphosphanyl)ethyl]-1,1’’-biferrocene
[(R,R)-(S,S)-Phtrap]
(5)
and
Scheme 1. Enantioselective Pd catalysis according to Sodeoka and co-workers. 4 c, d: conversion
at 10 8C. The ee value of the major diastereoisomer of 4 d is given. tol-binap = 2,2’-bis(di-p-tolylphosphanyl)-1,1’-binaphthyl, binap = 2,2’-bis(diphenylphosphanyl)-1,1’-binaphthyl.
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200201614
Angew. Chem. Int. Ed. 2003, 42, 1688 – 1690
Angewandte
Chemie
[RhH(CO)(PPh3)3].[9] The diphosphane
ligand with both planar and central
chiral elements forms a trans-chelate
complex with rhodium. Under the same
conditions, cyanoacetic acid amides such
as 1 e can be alkylated in the a position
with vinyl ketones or acrolein
(Scheme 2),[9d] using a catalyst formed
In the past years, Shibasaki's group
has
attempted
to
develop a La–binol catalyst (binol
= 2,2’-dihydroxy-1,1’-binaphthyl) that is
air-stable, storable, and reusable. These
efforts resulted in the so-called linkedbinol system 6 b (Scheme 3), which
combines all the desired attributes.[14, 15]
For the synthesis of tertiary stereocenters, this
type of catalyst is almost
optimal with regard to
stereoselectivity,
handling, and reuse. However, as exemplified in
Scheme 3 for the preparation of 4 f, these ex- Scheme 3. La-catalyzed enantioselective Mipectations cannot be ex- chael reaction following the method of Shibatended to quaternary saki and co-workers.
stereocenters (75 % ee
at 30 8C). The replace- vinyl ketones as Michael acceptors)
ment of lanthanum by stoichiometric amounts of a Lewis acid
Scheme 2. Rhodium-catalyzed asymmetric Michael reaction aczinc as the central metal, or high-pressure conditions are necescording to Ito and co-workers. Cp = cyclopentadienyl,
Phtrap = 2,2’-bis[1-(diphenylphosphanyl)ethyl]-1,1’-biferrocene,
however, improves the sary. Following the method of d'Angelo
acac = acetylacetonato.
selectivities for this and co-workers,[18] the reaction of alinked-binol ligand in alkoxyimine 7 a and acrylonitrile derivin situ from the phosphane-free [Rh- the conversion of a-hydroxyketone do- ative 2 c, which may be regarded as a
(acac)(CO)2]. The resulting Weinreb nors such as 1 g. For example, tertiary synthetic equivalent of acetaldehyde
amide 4 e, which is obtained in good alcohols 4 g were obtained at 20 8C (polarity reversal), affords the product
yield and selectivity, can be utilized as a with up to 96 % ee (Scheme 4).[16] in good yield with excellent diastereoprecursor for further carbonyl deriva- Hence, from this point of view, the and enantioselectivity (Scheme 5).
tives. A very similar reaction was re- above-mentioned latest Pd-catalyzed recently published by Motoyama et al.:[10] action developed by Sodeoka and coIn the presence of the catalyst, prepared workers appears to be a milestone in
in situ from [{RhCl(coe)2}2] and [(Phe- generating quaternary stereocenters by
box)SnMe3], the reaction of a-cyano- Michael reactions.
Apart from catalytic procedures,
propionates with acrolein proceeds with
up to 86 % ee (coe = cyclooctene, phe- auxiliary-mediated asymmetric Michael
reactions are of great practical relebox = 2,6-bis(oxazolinyl)).
The decisive breakthrough in the vance, as a wide range of substrates are
field of metal-catalyzed, asymmetric tolerated under relatively mild and neuMichael reactions, however, came in tral reaction conditions. In 1985 Pfau
1994 with the class of widely applicable et al. reported the use of 1-phenylethylheterobimetallic catalysts developed by amine, which is now established as an
Shibasaki and co-workers.[11] These cat- easily accessible and universal auxilialysts currently define the state-of-the- ary.[17] The C C coupling proceeds as an
Scheme 5. Auxiliary-assisted Michael reaction of
art for the formation of tertiary stereo- enamine-Michael reaction via a cyclic 7 a according to d'Angelo and co-workers and of
centers at 0 8C in high yields and with transition state related to that of an aza- piperidone derivative 7 b according to Christoffers
excellent selectivities (up to 99 % ee).[12] ene reaction. In some cases, elevated and co-workers. Boc = tert-butoxycarbonyl,
The generation of quaternary stereo- temperatures and (especially for simple Bn = benzyl.
centers, however, requires low temperatures (Scheme 3).[13] Furthermore, relatively high amounts of the catalyst 6 a
(lanthanum–sodium
binaphtholate).
The rare-earth metal lanthanum in 6 a
is octahedrally coordinated to three
chiral binaphtholate ligands, which are
bridged in pairs by alkali-metal ions Scheme 4. Zn-catalyzed enantioselective Michael reaction with the linked-binol ligand according
(Scheme 3).
to Shibasaki and co-workers. binol = 2,2’-dihydroxy-1,1’-binaphthyl.
Angew. Chem. Int. Ed. 2003, 42, 1688 – 1690
www.angewandte.org
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1689
Highlights
a-Amino acid amides were introduced recently as efficient auxiliaries for
copper-catalyzed Michael reactions. In
the presence of a catalytic amount of
Cu(OAc)2·H2O, enamines such as 7 b
react with simple vinyl ketones 2 a at
ambient temperature in acetone as the
solvent, yielding the product with selectivities of up to 99 % ee after acidic
hydrolysis.[19] The auxiliaries, readily
available from natural a-amino acids,
can be recovered almost quantitatively
after workup. As proved in the synthesis
of bicyclic compound 8 starting from
piperidone derivative 7 b (Scheme 5),
the copper-catalyzed Michael reaction
is compatible with substrates bearing
donor functions, for example, a carbamate moiety, and is therefore superior to
the lanthanide-catalyzed procedure of
Shibasaki and co-workers.[20]
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