Proline-catalyzed Mannich reaction of aldehydes with N-Boc-imines

Article abstract of DOI:10.1002/anie.200603188

(Chemical Equation Presented) A curious cat.: The proline-catalyzed Mannich reaction between aldehydes and N-Boc-imines (Boc = tert-butoxycarbonyl) gives crystalline β-amino aldehydes with exceptionally high diastereoselectivities and enantioselectivities

Full text of DOI:10.1002/anie.200603188

Angewandte
Chemie
DOI: 10.1002/anie.200603188
Organocatalysis
Proline-Catalyzed Mannich Reaction of Aldehydes with N-Boc-
Imines**
Jung Woon Yang, Michael Stadler, and Benjamin List*
The catalytic asymmetric Mannich reaction is arguably the
most useful approach to synthesize chiral b-amino carbonyl
compounds.[1] Six years ago, we discovered a proline-cata-
lyzed version of this powerful reaction.[2] Originally, ketones,
aldehydes, and an aniline as the amine component were used
in a catalytic asymmetric three-component reaction. Since
then, the reaction has found utility in the synthesis of chiral
nonracemic nitrogenous compounds, such as amino acids and
amino alcohols. Recently, several new catalysts and substrate
classes have been developed to expand the scope of the
reaction and to modify its remarkably high syn diastereo- and
enantioselectivity.[3] Despite its frequent use, both in an
academic as well as an industrial context, the main limitation
of the proline-catalyzed Mannich reaction has been the
requirement to use anilines as the amine component.
Although optically enriched p-anisidylamines are of potential
utility in asymmetric synthesis, facile and efficient removal of
the N-protecting group to yield the unfunctionalized amine is
required. Generally, the removal of the most commonly used
p-methoxyphenyl (PMP) group from nitrogen requires rather
drastic oxidative conditions involving harmful reagents, such
as ceric ammonium nitrate, which are not compatible with all
substrates. We have now identified reaction conditions that
allow for the use of simple preformed aromatic N-Boc-imines
(Boc = tert-butoxycarbonyl) in proline-catalyzed Mannich
reactions. Remarkably, the reaction provides chiral b-amino
aldehydes and ketones as stable, crystalline compounds in
generally high diastereo- and enantioselectivities without the
requirement for chromatographic purification.
and enantioselectivity (Table 1, entry 2). In this case, product
3b did not precipitate but its isolation proved similarly
simple: After an aqueous workup, the crude product was
triturated with cool hexanes to afford the pure, crystalline
product.
The remarkably high enantioselectivity is at least not
entirely based on an enantioenrichment during the precip-
Table 1: Proline-catalyzed asymmetric Mannich reaction of aldehydes
with N-Boc-imines.[a]
Entry
1
Product
Yield [%]
84
d.r.
e.r.[a]
>99:1
>99:1[b]
2
3
91
88
>99:1
>99:1
>99:1
>99:1
4
80
59
82
74
>99:1
99:1
>99:1
98.5:1.5
>99:1[c]
99:1
After a short screening of reaction conditions, an optimal
procedure was found. If the benzaldehyde-derived N-Boc-
imine 2a (R3 = Ph) was treated with hexanal in the presence
of (S)-proline (20 mol%) in acetonitrile at 08C for 8 h, the
desired product 3a precipitated during the reaction and could
be isolated by filtration. The product had an e.r. value greater
than 99:1, a d.r. value greater than 99:1, and the yield of the
isolated product was 84% (Table 1, entry 1). Similarly, the
reaction of 2a with propionaldehyde resulted in the formation
of crystalline product 3b with the same diastereoselectivity
5
6
>99:1
97:3
7[d]
[*] Dr. J. W. Yang, M. Stadler, Prof. Dr. B. List
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany)
Fax: (+49)208-306-2999
8
<5
n.d.[f]
n.d.[f]
E-mail: list@mpi-muelheim.mpg.de
9[e]
73
>99:1
[**] We thank the DFG (Priority program “Organocatalysis” SPP1179)
for partially funding this work. Generous support by the Max Planck
Society and by Novartis (Young Investigator Award to B.L.) is
gratefully acknowledged. We also thank BASF, Degussa, Merck,
Saltigo, and Wacker for general support and for donating chemicals.
Boc=tert-butoxycarbonyl.
[a] Yields, diastereoselectivities, and enantioselectivities of precipitated
products. [b] Crude e.r. 99:1. [c] Crude e.r. 96:4. [d] Product isolated by
chromatography. [e] Reaction run at room temperature in acetone.
[f] Not determined.
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Communications
itation. When product 3a was isolated through an aqueous/
organic extraction, its crude e.r. value was determined to be
99:1. The reaction conditions turned out to be of use for
several substrate combinations (Table 1, entries 3–7). In
general, the derivatives of 3 either directly precipitated
from the reaction mixtures in the given stereoselectivities
and yields or were isolated through trituration with hexanes.
Only furan derivative 3g was isolated through chromatog-
raphy. Although several different aromatic imines could be
used, aliphatic imines proved less reactive and did not provide
the desired product under the reaction conditions (Table 1,
entry 8). However, ketones undergo the reaction with similar
enantioselectivities. Treating benzaldehyde-derived N-Boc-
imine 2a with (S)-proline (20 mol%) in acetone gave
Mannich product 3i in good yields and close to perfect
enantioselectivity.[4]
reaction. In our new procedure, aldehydes react with pre-
formed N-Boc-imines in the presence of proline to give the
corresponding b-amino aldehydes in excellent diastereose-
lectivities and enantioselectivites. Our reaction is useful for
the synthesis of a,b-branched-b-amino acids, which are of
great potential value in the synthesis of peptide derivatives
and related biologically active compounds.[6] In addition to
aldehydes, ketones can also be used. The reaction is highly
practical in that it does not require chromatographic purifi-
cation and in all cases provides crystalline products in almost
perfect enantioselectivities. The product either directly pre-
cipitates from the reaction mixture or is isolated after aqueous
workup and trituration with hexanes. A current limitation is
the incompatibility of aliphatic imines with our process and
the requirement to preform the N-Boc-imines. Although the
A
typical experimental procedure is illustrated in
Figure 1. Mixing the 2-naphthaldehyde-derived N-Boc-
imine (2 f; R3 = 2-naphthyl) with isovaleraldehyde in the
synthesis of the required imines is well established,[1g,u,7]
a
direct three-component reaction, which potentially would be
compatible with the use of aliphatic aldehydes similar to our
original protocol, would further improve the proline-cata-
lyzed Mannich reaction. Nonetheless, our new process is
expected to find application, particularly in the synthesis of b-
amino acids.
Experimental Section
General procedure for the (S)-Proline-catalyzed asymmetric Man-
nich reaction between N-Boc-protected imines and aldehyde donors:
The N-Boc-protected imine (0.5 mmol) was dissolved in anhydrous
acetonitrile (5 mL) and the corresponding aldehyde (2 equiv) was
added. The mixture was cooled to 08C and (S)-proline (0.1 mmol) was
added. After 8–12 h at 08C, the reaction was worked up either by
filtering off the precipitate and washing it with hexanes (À788C) or by
pouring the reaction mixture into distilled water and extracting with
diethyl ether (three times). In the latter case, the organic layers were
then combined, dried over MgSO4, filtered, concentrated, and
purified by trituration with cool hexanes (À788C) to afford the
corresponding pure syn-Mannich products. The enantiomeric ratios
of all products were determined by chiral-phase HPLC analysis.
Figure 1. The reaction of isovaleraldehyde with 2-naphthyl N-Boc-imine
in the presence of (S)-proline (20 mol%; Table 1, entry 6) in CH3CN.
a) Homogenous reaction mixture after mixing all components.
b) Reaction mixture after completion of the reaction (10 h).
presence of (S)-proline (20 mol%) in acetonitrile at 08C
resulted in an initially homogenous reaction mixture (Fig-
ure 1a). After complete consumption of the starting material
(10 h), a large amount of the desired product 3 f had
precipitated and could easily be collected by filtration
(Figure 1b).
Received: August 5, 2006
Published online: December 13, 2006
The N-Boc-imine-derived Mannich products 3a–g can
readily be converted into the corresponding a,b-branched-b-
amino acids (b2,3-amino acids). For example, oxidation of
product 3b to the carboxylic acid followed by acid-mediated
deprotection provided amino acid salt 4 without loss of
stereochemical integrity [Eq. (1); TFA = trifluoroacetic acid].
Measuring NMR spectra and optical rotation of the corre-
sponding HCl salt allowed us to confirm the expected
absolute and relative configuration of the product.[5]
In summary, we have developed a remarkably efficient
and enantioselective variant of the proline-catalyzed Mannich
Keywords: amino aldehydes · asymmetric synthesis · imines ·
Mannich reaction · organocatalysis
.
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