Achieving molecular complexity by organocatalytic one-pot strategies-a fast entry for synthesis of sphingoids, amino sugars, and polyhydroxylated α-amino acids

Full text of DOI:10.1002/anie.200901446

Communications
DOI: 10.1002/anie.200901446
Asymmetric Catalysis
Achieving Molecular Complexity by Organocatalytic One-Pot
Strategies—A Fast Entry for Synthesis of Sphingoids, Amino Sugars,
and Polyhydroxylated a-Amino Acids**
Hao Jiang, Petteri Elsner, Kim L. Jensen, Aurelia Falcicchio, Vanesa Marcos, and
Karl Anker Jørgensen*
A major driving force for the intriguing developments in the
field of total synthesis over the past century is the proficiency
with which biological systems transform simple starting
materials into complex molecular frameworks. Although
necessary issues such as selectivity and synthetic efficiency
to construct intricate biological structures can be addressed
nowadays to a high degree, new aspects such as diversity and
operational efficiency are becoming more important, because
of the demand for making complex molecular architectures
by effective and simple methodologies.[1] In this respect,
catalytic cascade reactions involving two or more selective
transformations in one pot are emerging as an attractive tool
to overcome the operational limitations associated with
traditional “Stop-and-Go” synthesis.[2]
Scheme 1. Synthesis of 4,5-disubstituted isoxazoline-N-oxides by using
organocatalysis. TMS=trimethylsilyl.
Organocatalysis has been shown to be a powerful tool for
forming multiple stereocenters in a one-pot protocol by
employing either a single catalyst[3a–k] or a combination of
catalysts.[3i–l] We became interested in the 4,5-disubstituted
isoxazoline-N-oxide motif, since it has the potential to serve
as an important building block for diversity orientated total
synthesis. Several approaches to isoxazoline-N-oxides are
present in the literature either in a racemic fashion,[4] starting
from enantiomerically pure compounds,[5] or by employing
stoichiometric amounts of a chiral reagent.[6] We envisioned
that 4,5-disubstituted isoxazoline-N-oxides having up to three
stereocenters could be obtained through a highly stereose-
lective one-pot procedure using simple and commercially
available starting materials in combination with one or two
organocatalysts (Scheme 1).
Herein, we report a new enantio- and diastereoselective
one-pot protocol to access 4,5-disubstituted isoxazoline-N-
oxides, as well as demonstrate the use of this protocol for the
de novo synthesis of b,g-dihydroxylated and b,g,d-trihydroxy-
lated a-amino acid derivatives, phytosphingosines, and amino
sugars.
Recently, our group reported an efficient and highly
enantioselective procedure for the formation of optically
active a-bromo aldehydes.[7a,b] Encouraged by the size and
leaving group ability of the bromine, we evaluated the
possibility of an in situ entrapment, thereby, generating a
new class of chiral 1,2-dielectrophiles to participate in multi-
ple-bond-forming cascade sequences. To our delight, the
chirality stored within this a-carbonyl sp3-carbon center,
formed by the direct a-bromination of aldehydes 1 by the
electrophilic bromination reagent 2 catalyzed by the TMS-
protected diaryl-prolinol 3, is fully exploited by a base-
promoted face-selective Henry addition of nitroacetates and
subsequent stereospecific O-alkylation, furnishing the enan-
tio- and diastereoselective synthesis of 4,5-disubstituted
isoxazoline-N-oxides 4 in one pot (Table 1). The generality
of this one-pot, three-step sequence was explored and the
results are outlined in Table 1. It appears that b-branched
aldehydes 1a–c provided the 4,5-disubstituted isoxazoline-N-
oxides 4a–c as single diastereomers in high yield (68–85%)
and excellent enantioselectivity (94–96% ee; Table 1,
entries 1–3). Nonconjugated unsaturated systems 1d and
linear unbranched substrates 1e–f were also well-tolerated,
giving the isoxazoline-N-oxide products 4d–f in yields of 50–
90%, d.r. values ranging from 73:27 to greater than 20:1, and
ee values of 92–94% (Table 1, entries 4–8).
[*] H. Jiang, Dr. P. Elsner, K. L. Jensen, A. Falcicchio, V. Marcos,
Prof. Dr. K. A. Jørgensen
Center for Catalysis, Department of Chemistry
Aarhus University, 8000 Aarhus C (Denmark)
Fax: (+45)8619-6199
E-mail: kaj@chem.au.dk
[**] This work was made possible by a grant from The Danish National
Research Foundation, OChemSchool, and the Carlsberg Founda-
tion.
To expand the product diversity, the described three-step,
one-pot protocol was extended to the formation of the
corresponding Schiff base of the a-bromoaldehyde, which
subsequent to an aza-Henry/alkylation cascade provided the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901446.
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Table 1: Scope of the enantioselective a-bromination/Henry reaction/
include the use of chiral epoxy aldehydes, generated by
cyclization sequence.[a]
aminocatalysis, which would result in the creation of one
additional stereocenter. This approach utilizes the organo-
catalytic epoxidation of a,b-unsaturated aldehydes 6 by
hydrogen peroxide and TMS-protected diarylprolinol 3 as
the catalyst.[7c] Preliminary results indicated that the epoxy
motif gave poor or no selectivity in the subsequent Henry
reaction, in which weak bases such as imidazole were applied.
A thorough screening of diverse chiral and nonchiral bases
and hydrogen-bonding catalysts favored the use of solid
CsOH as the base under phase-transfer conditions, with the
Lygo-type chiral ammonium salt 7 as the catalyst[7d] (Table 3).
Aromatic, aliphatic, and functionalized a,b-unsaturated alde-
hydes 6a–c all participated in the desired reaction sequence,
assembling highly enantioenriched to enantiopure products in
moderate to good yields and diastereoselectivities (Table 3,
entries 1–3).
Entry[a]
1 (R)
Yield [%][b]
d.r.[c]
ee [%][d]
1
2
3
4
1a (iPr)
1b (cPent)
1c (cHex)
1d (cis-2-pentenyl)
1e (nBu)
1 f (C15H31)
1 f (C15H31)
1 f (C15H31)
4a: 77
4b: 86
4c: 68
4d: 85
4e: 69
4 f: 90
>20:1
>20:1
>20:1
>20:1
77:23
73:27
79:21
79:21
95
96
96
94
93 (93)
94 (94)
94 (94)
92 (92)
5[e]
6
7[e,f]
8[e,g]
4g: 50
ent-4g: 66
[a] Reactions performed on 0.2 mmol scale (see the Supporting
Information). [b] Yield of the products isolated as mixture of
Table 3: Enantioselective
epoxidation/Henry
reaction/cyclization
sequence.[a]
a
diastereoisomers. [c] Determined by 1H NMR spectroscopy. [d] Deter-
mined by HPLC methods on a chiral stationary phase (see the
Supporting Information). The value in parentheses is the ee value for
the minor diastereoisomer. [e] tert-Butyl nitroacetate used as the
nucleophile. [f] Reaction performed on a 2 mmol scale. [g] The enantio-
mer of catalyst 3 was used.
4-amino isoxazoline-N-oxides 5 in one operation (Table 2).
As exemplified by the cascade reaction of aldehydes 1a, b,
and d, good yields and diastereoselectivities of 5a–c were
achieved during this four-step sequence. However, the Schiff
base is prone to rapid enamine formation, leading to a slight
decrease in the observed optical purity of the products
(Table 2, entries 1–3).
Having been successful in our approach to the develop-
ment of the one-pot cascade procedure for the formation of
the optically active 4,5-disubstituted isoxazoline-N-oxides 4
and the 4-amino isoxazoline-N-oxides 5, we decided to
expand this organocatalytic chiral leaving group strategy to
Entry[a]
6 (R)
Yield [%][b]
d.r.[c]
ee [%][d]
1
2
3
6a (iPr)
6b (Ph)
6c (CH2OBn)
8a: 71
8b: 65
8c:67
73:27
78:22
73:27
99 (99)
99 (99)
94 (94)
[a] Reactions performed on a 0.2 mmol scale (see the Supporting
Information). [b] Yield of the products isolated as mixture of
a
diastereoisomers. [c] Determined by 1H NMR spectroscopy. The relative
and absolute configurations of the products were determined by NOE
experiments or by analogy to known compounds. [d] Determined by
HPLC methods on a chiral stationary phase (see the Supporting
Information). The value in the parentheses is the ee value for the minor
diastereoisomer.
Table 2: Extension to an enantioselective a-bromination/imination/aza-
Henry/cyclization sequence.[a]
The diversity in the transformations of the obtained
isoxazoline-N-oxide products 4, 5, and 8 is demonstrated by
their rapid conversion into important and biorelevant natural
and non-natural products. Starting from ent-4g, the b,g-
dihydroxylated a-amino acid ester 11 can be formed in three
high yielding steps, while maintaining the enantiomeric excess
obtained in the one-pot cascade reaction, including O-
protection with TBS (9a; see the Supporting Information),
deoxygenation to 10a, and reductive ring-opening (11) as
presented in Scheme 2.
Entry[a]
1 (R)
Yield [%][b]
d.r.[c]
ee [%][d]
1
2
3
1a (iPr)
1c (cHex)
1d (cis-2-pentenyl)
5a: 57
5b: 47
5c: 47
>20:1
>20:1
>20:1
86
81
82
Sphingoid bases, such as sphingosine, phytosphingosine,
and sphinganine, which are amide-linked with a fatty acid
chain, form the indispensable structural motif of sphingolipids
found in all eukaryotic cell membranes. Moreover, sphingo-
lipids and their metabolites have been recognized to actively
[a] Reactions performed on a 0.2 mmol scale (see the Supporting
Information). [b] Yield of the products isolated as
a mixture of
diastereoisomers. [c] Determined by 1H NMR spectroscopy. [d] Deter-
mined by HPLC methods on a chiral stationary phase (see the
Supporting Information).
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Product 8c, obtained by the epoxidation/Henry alkylation
cascade, contains three contiguous stereocenters and may
serve as a fast entry into polyfunctionalized synthetic targets.
The concept is demonstrated in Scheme 4. Starting from 8c,
the protection of the hydroxy groups with TBS (15; see the
Supporting Information) and then deoxygenation leads to
compound 16, which is reductively cleaved to provide the
b,g,d-trihydroxylated a-amino acid derivative 17.
Scheme 2. Transformation of an isoxazoline-N-oxide into a b,g-dihy-
droxylated a-amino acid derivative. TBS=tert-butyldimethylsillyl,
Boc=tert-butoxycarbonyl.
participate in various signal transduction pathways as secon-
dary messengers, thereby, playing an essential role in vital
functions; for example, stress response regulation, cell cycle
arrest, cell differentiation, and apoptosis.[8] Because of their
biochemical significance and relevance, naturally occurring or
chemically modified sphingoids and their analogues are
considered attractive targets in total synthesis.[9] Initial
predictions suggested that the reduction of the tert-butyl
ester moiety of 10a and subsequent global deprotection
would provide the phytosphingosine product in a concise
manner. However, several attempts failed; the use of the tBu
ester in the 4,5-disubstituted isoxazoline-N-oxide had to be
avoided to suppress undesired side reactions. Only low yields
were obtained using LiAlH4 as the reductant, and subsequent
O-silyl deprotection provided the known N-Boc l-Ribo-
phytosphingosine.[9a] Instead, by employing the 4,5-disubsti-
tuted isoxazoline-N-oxide ent-4 f as the initial chiral building
block, the ethyl ester 10b was successfully reduced, after
deoxygenation and prior to ring-opening to provide the
isoxazoline 12 (Scheme 3). Treatment of 12 with nickel
borohydride and then removal of the silyl protecting group
furnished l-Ribo-phytosphingosine (14) in 93% yield and
82:18 d.r. Interestingly, the hydroxymethyl group plays a
crucial role in the sense of diastereoselectivity, since O-
silylation (with a TBS group) resulted in only a 60:40 mixture
of the diastereomers.
Scheme 4. Transformation of an isoxazoline-N-oxide into the b,g,d-
trihydroxylated a-amino acid derivative 17. Bn=benzyl, Tf=trifluoro-
sulfonyl.
The product 17 is also a member of the 2-amino 2-
deoxyaldonic acid class of compounds, which are highly
relevant in carbohydrate chemistry as equivalents of amino
sugars and other sugar derivatives—motifs frequently
observed in antibiotics and macromolecules of living tis-
sues.[10] Moreover, these 2-amino hexonates have also been
adopted as intermediates in several total syntheses of natural
products or fungicides.[11] Our approach to this class of
compound has the advantage over traditional enantiomeri-
cally pure compound synthesis because of the potential for
the simple introduction of orthogonal protecting groups,
which makes easier the efforts for additional synthetic
manipulations.
The mechanistic proposals for our one-pot reactions are
outlined in Scheme 5. Saturated aldehydes 1 enter the
catalytic cycle by condensation with the organocatalyst 3,
thereby forming a reactive enamine species, which reacts with
the electrophilic bromination reagent 2 to introduce the a-
bromine in an enantioselective manner. Upon hydrolysis and
liberation of the catalyst 3, the optically active a-bromo
intermediate 18 is formed (Scheme 5, left). Alternatively, by
employing a,b-unsaturated aldehydes 6 as substrates, an
activated iminium ion is obtained as result of the condensa-
tion with 3 (Scheme 5, right). Next, the conjugate addition of
hydrogen peroxide, subsequent epoxide formation, and then
catalyst hydrolysis provides the enantiomerically enriched
trans-epoxyaldehydes 19. Having completed the initial orga-
nocatalyzed functionalization cycles, aldehydes 18 and 19 are
subjected to a base-induced intermolecular Henry reaction
with nitro acetates as nucleophiles. Subsequent deprotonation
of the carbon atom a to the nitro group, and intramolecular
SN2-type O-alkylation utilizing either the bromine or epoxide
as chiral leaving groups, furnished the highly functionalized
Scheme 3. Synthesis of l-Ribo-phytosphingosine (14). TBAF=n-tetra-
butylammonium fluoride.
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Scheme 5. Mechanistic proposal for the one-pot reactions.
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In conclusion, we have demonstrated that by utilizing an
organo-mediated chiral leaving group strategy, molecular
complexity can be rapidly and efficiently achieved from
simple and commercially available starting materials, with
minimal manual operations. The 4,5-disubstituted isoxazo-
line-N-oxide products, obtained in high yields and excellent
enantioselectivities, serve as versatile building blocks for
natural product synthesis, as exemplified by the de novo
synthesis of Ribo-phytosphingosine, amino sugar derivatives,
and polyfunctionalized a-amino acid derivates.
Received: March 16, 2009
Published online: May 13, 2009
Keywords: asymmetric catalysis · divergent synthesis ·
.
molecular complexity · synthetic methods · organocatalysis
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