Enantioselective organocatalytic amine conjugate addition

Article abstract of DOI:10.1021/ja063267s

The first enantioselective organocatalytic amine conjugate addition has been successfully developed. The application of LUMO-lowering iminium catalysis has enabled the highly chemo- and enantioselective 1,4-addition of a rationally designed N-silyloxycarb

Full text of DOI:10.1021/ja063267s

Published on Web 06/30/2006
Enantioselective Organocatalytic Amine Conjugate Addition
Young K. Chen, Masanori Yoshida, and David W. C. MacMillan*
DiVision of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
Received May 10, 2006; E-mail: dmacmill@caltech.edu
Table 1. Effect of Catalyst and Solvent on the Conjugate
Amination
The prevalence of amines in pharmaceutical agents and natural
products places C-N stereogenicity among the most important
synthon targets for enantioselective reaction development.1 While
the 1,4-addition of stoichiometric chiral amines to electron-deficient
olefins has long been established as a principal strategy for C-N
bond construction,2 it is surprising that corresponding catalytic
variants have only recently been realized using Lewis acids with
R,â-unsaturated ketones,3a,b imides,3b,c and amides.3d Moreover, the
enantioselective conjugate amination of R,â-unsaturated aldehydes
has remained elusive, a notable deficiency given the broad utility
of â-amino aldehydes. Over the last six years, our laboratory has
demonstrated that the reversible formation of iminium ions from
chiral amine catalysts and R,â-unsaturated aldehydes is a useful
platform for conjugate addition reactions involving carbon4a-c and
hydrido4d nucleophiles. In this communication, we further advance
this activation concept to describe the first enantioselective orga-
nocatalytic amine conjugate addition, a highly chemo- and stereo-
selective transform that is founded upon a rationally designed
N-centered nucleophile. This operationally simple protocol allows
rapid and predictable access to enantioenriched â-amino aldehydes
and â-amino acids using an inexpensive amine catalyst.
entry
catalyst
solvent
temp (
°C)
% conversiona
% eeb
1
2
3
4
5
6
7
8
L-proline
1•TFA
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
EtOAc
THF
0
-20
-20
-20
-20
-20
-20
-20
0
25
29
32
70
12
45
95
3
81
69
87
92
92
92
2•TFA
2•HCl
2•pTSA
2•pTSA
2•pTSA
2•pTSA
CHCl3
a Conversion determined by 1H NMR analysis. b Enantiomeric excess
determined by chiral HPLC analysis on the corresponding amino alcohol.
stereodefining heteroatom addition step must be accompanied by
irreVersible loss of the nucleophile’s proton. More specifically, this
deprotonation step removes the possibility of an equilibrium-
controlled process involving a reversible N-addition step (a
thermodynamic pathway that would demand the formation of
racemic products). In consideration of these requirements, we
selected N-silyloxycarbamates as the nucleophilic component A on
the basis that the N-O functionality would enhance nucleophilicity
at the nitrogen center via the R-effect,5,6 while the carbamate
functionality would render the amino aldehyde product effectively
nonbasic (silyloxycarbamate N-H, estimated pKa ∼ 9.0).7 With
respect to the catalyst component B, we focused upon the use of
imidazolidinone amines given their established capacity4 to par-
ticipate in asymmetric iminium activation with enals and enones
while selectively avoiding heteroconjugate addition.
To our great delight, exposure of crotonaldehyde to benzyl tert-
butyldimethylsilyloxycarbamate in the presence of imidazolidinone
catalyst 2‚TFA did indeed provide the desired â-amino aldehyde
product with good levels of enantiocontrol (Table 1, entry 3, 81%
ee). Further evaluation of a variety of catalyst salts (entries 3-5)
revealed that imidazolidinone 2‚pTSA exhibited superior conversion
and selectivity. Last, a survey of reaction media (entries 5-8)
demonstrated that CHCl3 was the optimal solvent (entry 8, 92%
ee). The superior levels of induction and reaction efficiency
exhibited by catalyst 2‚pTSA in CHCl3 to provide the amino
aldehyde 3 in 92% ee and 92% yield prompted us to select these
conditions for further reaction exploration (Table 2, entry 1).
The scope of the R,â-unsaturated aldehyde component in this
enantioselective heteroconjugate addition has been examined. As
Design Plan. From the outset, we recognized that a number of
chemoselectivity issues must be addressed if an enantioselective
conjugate amination were to be realized using an iminium catalysis
platform. First, an amine A must be identified that will selectively
function as a 1,4-addition nucleophile (TS-1), yet will not participate
in iminium activation (TS-2, a racemic pathway). Moreover, a
second amine B must be found that will perform as an iminium
catalyst (TS-1) while avoiding a nucleophilic role (a pathway that
would lead to catalyst consumption). Third, to ensure that the
reaction can proceed under asymmetric (kinetic) control, the
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J. AM. CHEM. SOC. 2006, 128, 9328-9329
10.1021/ja063267s CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Scope of Enantioselective Organocatalytic Conjugate
Amination
was followed by in situ Wittig homologation to afford the
unsaturated ester 5 in a single operation (71% yield, 92% ee).
Subsequent exposure of amino enoate 5 to fluoride ion then enabled
both silyl group removal and intramolecular oxy-Michael addition
to afford isoxazolidine 6 with excellent diastereocontrol (99% yield,
10:1 syn/anti). Reduction of the N-O bond was accomplished with
SmI2 to afford the 1,3-amino alcohol 7 in excellent yield (99%
yield, 70% yield over three steps).11
Scheme 1. Enantioselective Synthesis of 1,3-Amino Alcohols
Acknowledgment. Financial support was provided by the
NIHGMS (R01 GM66142-01) and kind gifts from Amgen and
Merck. Y.K.C. thanks the National Cancer Institute for a postdoc-
toral fellowship (1 F32 CA108376-01).
Supporting Information Available: Experimental procedures,
structural proofs, and spectral data for all new compounds are provided
(9 pages, print/PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
References
(1) For recent reviews, see: (a) Xu, L.-W.; Xia, C.-G. Eur. J. Org. Chem.
2005, 633. (b) Hultzsch, K. C. AdV. Synth. Catal. 2005, 347, 367.
(2) For a review, see: Liu, M.; Sibi, M. P. Tetrahedron 2002, 58, 7991.
(3) Some recent examples of enantioselective conjugate amination: (a) Jin,
X. L.; Sugihara, H.; Daika, K.; Takeishi, H.; Jin, Y. Z.; Furuno, H.;
Inanaga, J. Tetrahedron 2002, 58, 8321. (b) Myers, J. K.; Jacobsen, E.
N. J. Am. Chem. Soc. 1999, 121, 8959. (c) Guerin, D. J.; Miller, S. J. J.
Am Chem. Soc. 2002, 124, 2134. (d) Yamagiwa, N.; Qin, H.; Matunaga,
S.; Shibasaki, M. J. Am. Chem. Soc. 2005, 127, 13419 and references
therein. (e) Sibi, M. P.; Prabagaran, N.; Ghorpades, S. G.; Jasperse, C. P.
J. Am. Chem. Soc. 2003, 125, 11796. (f) Sibi, M. P.; Shay, J. J.; Liu, M.;
Jasperse, C. P. J. Am. Chem. Soc. 1998, 120, 6615. (g) Zhuang, W.; Hazell,
R. G.; Jørgensen, K. A. Chem. Commun. 2001, 1240.
(4) (a) Paras, N. A.; MacMillan, D. W. C. J. Am. Chem. Soc. 2001, 123,
4243. (b) Austin, J. F.; MacMillan, D. W. C. J. Am. Chem. Soc. 2002,
124, 1172. (c) Brown, S. P.; Goodwin, N. C.; MacMillan, D. W. C. J.
Am. Chem. Soc. 2003, 125, 1192. (d) Ouellet, S. G.; Tuttle, J. B.;
MacMillan, D. W. C. J. Am. Chem. Soc. 2005, 127, 32.
(5) This FMO description is one of several explanations for the R-effect: (a)
Fleming, I. Frontier Orbitals and Organic Reactions. Wiley-Science: New
York, 1977; p 77. (b) Heaton, M. M. J. Am. Chem. Soc. 1978, 100, 2004.
(6) An alternative explanation involving transition-state charge stablization
by the adjacent heteroatom has also been described: (a) DePuy, C. H.;
Della, E. W.; Filley, J.; Grabowski, J. J.; Bierbaum, V. M. J. Am. Chem.
Soc. 1983, 105, 2481. (b) Buncel, E.; Um, I.-H. Chem. Commun. 1986,
595.
a Performed with 3 equiv of enal. b Enantioselectivity determined by
HPLC or SFC analysis. c Stereochemistry assigned by chemical correlation
or by analogy. d Performed with catalyst 2‚TFA.
highlighted in Table 2, enal substituents, including alkyl, alkenyl,
aryl, ether, amine, and ester groups, are readily tolerated (Table 2,
entries 2-9, 87-97% ee).8 Moreover, variation of the carbamate
protecting group from Cbz to Boc to Fmoc can be realized without
loss in enantiocontrol (entries 2-4, 95, 92, and 89% ee). Variation
of the silyloxy protecting group is also possible as illustrated by
the TBDPS derivative (entry 9, 97% ee).9 In accord with our design
plan, it is important to note that â-amino aldehyde products arising
from 1,4-catalyst incorporation were not detected in this study.
A demonstration of the utility of this organocatalytic amine
addition and the accompanying products is presented in the one-
pot (two-step) conversion of simple aldehydes to enantioenriched
â-amino acids. As revealed in eq 1, exposure of 2-hexenal to our
asymmetric amination conditions followed by in situ Pinnick
oxidation provided the corresponding â-amino acid 4 with excellent
enantioselectivity (92% ee). Notably, N-O bond removal can be
accomplished under mildly reducing conditions (Zn/AcOH).10
Alternatively, the amino-oxy moiety can be strategically exploited
to generate 1,3-amino alcohols with valuable levels of absolute and
relative stereocontrol (Scheme 1). In this case, hexenal amination
(7) Calculated using Advanced Chemistry Development (ACD/Labs) Software
V8.14 for Solaris (1994-2006 ACD/LABS).
(8) The catalysts shown are not successful with R-alkyl-substituted enals.
(9) The use of TBDPS-substituted systems does not generally provide an
increase in selectivities in comparison to the TBS-substituted analogues.
(10) Aschwanden, P.; Kvaerno, L.; Geisser, R. W.; Kleinbeck, F.; Carreira, E.
M. Org. Lett. 2005, 7, 5741.
(11) Keck, G. E.; Wagner, T. T.; McHardy, S. F. Tetrahedron 1999, 55, 11755.
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