Enantioselective organocatalytic domino oxa-michael/aldol/ hemiacetalization: Synthesis of polysubstituted furofuranes containing four stereocenters

Full text of DOI:10.1002/anie.200901333

Angewandte
Chemie
DOI: 10.1002/anie.200901333
Synthetic Methods
Enantioselective Organocatalytic Domino Oxa-Michael/Aldol/
Hemiacetalization: Synthesis of Polysubstituted Furofuranes
Containing Four Stereocenters**
Efraꢀm Reyes, Garazi Talavera, Jose L. Vicario,* Dolores Badꢀa, and Luisa Carrillo
The discovery of new methodologies for the synthesis of
complex molecules in the shortest and most efficient way is a
key field of research. In this context, domino or cascade
reactions represent an advantage for the straightforward
construction of biologically relevant compounds because they
allow construction of complex molecules in an efficient way,
thereby minimizing the number of laboratory operations and
the generation of waste chemicals.[1] Additionally, when
stereochemistry is a fundamental parameter to be controlled,
domino processes arise as an effective approach for con-
structing the target molecule with good stereoselectivity.
Among the different methodologies described in the chemical
literature, organocatalytic enantioselective domino reactions
represent a useful and competitive tool for the generation of
molecular complexity from readily available and cheap
starting materials, as well as displays exceptional performance
with regard to stereochemical control.[2] More advantages of
this methodology are related to the fact that organocatalysts
are very often commercially available, environmentally
friendly, water compatible, air stable, and robust reagents.
Additional benefits are associated with the tolerance of the
catalysts and the reactive intermediates to the presence of
moisture or air in the reaction medium, which leads to an
advantage in operational simplicity when carrying out the
reaction.[3]
by conjugate additions have been reported, most of them
À
involving a C C bond formation in the cascade-initiating
Michael reaction step and also some examples can be found in
which a hetero-Michael reaction has been employed to start
the process. Importantly, it has to be pointed out that oxa-
Michael-initiated domino reactions have received little atten-
tion, just as the organocatalytic oxa-Michael reaction, which
still remains a rather unexplored transformation. This lack of
attention is mainly a result of the reversibility of the conjugate
addition process,[4] which very often makes the oxa-Michael
addition products configurationally unstable. An additional
difficulty associated with this reaction is related to the low
nucleophilicity of the alcohol functionality, which therefore
requires a prior deprotonation step to activate it as an
alkoxide ion. As a consequence of this the scope of the
alcohols suitable candidates to be used as oxygen nucleo-
philes in oxa-Michael reactions is restricted to compounds of
enhanced acidity.[5] In fact, literature examples are exclusively
limited to the use of functionalized phenols as nucleophiles
(in oxa-Michael-initiated cascade reactions or intramolecular
versions)[6] and also a couple of elegant procedures have been
reported by Jørgensen and co-workers[7] for the b-hydroxyl-
ation of a,b-unsaturated aldehydes and by List and co-
workers[8] for the b-hydroxylation of enones using oximes and
hydroperoxides, respectively, as O nucleophiles.
A particularly interesting situation is the use of chiral
amines as catalysts in domino processes which are initiated by
Michael-type reactions.[4] Chiral amines can activate a,b-
unsaturated aldehydes or ketones by the reversible formation
of an iminium ion which, after the conjugate addition step,
delivers in intermediate enamine ready to participate in a
subsequent reaction, therefore providing an opportunity for a
domino process to occur. Related to this topic, several
stereoselective amine-catalyzed cascade reactions initiated
In this context, and in connection with our ongoing efforts
to develop new organocatalytic reactions, we report herein a
novel amine-promoted asymmetric domino reaction between
dihydroxyacetone dimer and a,b-unsaturated aldehydes,
which leads to the enantioselective formation of
hexahydrofuro[3,4-c]furanes in a single step (Scheme 1).
This transformation consists of an initial oxa-Michael reac-
tion, a subsequent intramolecular aldol reaction, and lastly a
hemiacetalization step, and it proceeds with the generation of
four new stereocenters. Remarkably, the intramolecular aldol
reaction step involves the participation of a ketone as internal
electrophile, therefore generating a quaternary stereocenter.
This reaction is in contrast with the other reported organo-
[*] Prof. E. Reyes, G. Talavera, Prof. J. L. Vicario, Prof. Dr. D. Badꢀa,
Prof. L. Carrillo
Departamento de Quꢀmica Orgꢁnica II
Facultad de Ciencia y Tecnologꢀa
Universidad del Paꢀs Vasco/Euskal Herriko Unibertsitatea
P.O. Box 644, 48080 Bilbao (Spain)
E-mail: joseluis.vicario@ehu.es
Homepage: http://www.ehu.es/GSA
[**] This work was supported by the University of the Basque Country
(GIU07/06), the Spanish MICINN (CTQ2008-00136/BQU), the
Diputaciꢂn Foral de Bizkaia (DIPE08/03), and the Basque Govern-
ment (a fellowship to G.T.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901333.
Scheme 1. One-step synthesis of hexahydrofuro[3,4-c]furanes by an
oxa-Michael/aldol/hemiacetalization domino process.
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catalytic cascade oxa-Michael/aldol processes in which an
aldehydes having different substituents. As shown in
Table 2, the reaction protocol had to be slightly modified to
obtain similar results to those obtained in the screening
experiments, with respect to the yield and stereoselectivity, by
aldehyde moiety is chosen as a more reactive internal
electrophile.[6] Notably, and to the best of our knowledge,
this is the first example of a highly enantioselective direct
b-alkoxylation of a,b-unsaturated aldehydes catalyzed by a
chiral amine,[9] showing that even an aliphatic alcohol having
a low pKa value, such as dihydroxyacetone dimer, is able to
participate as an oxygen nucleophile in a conjugate addition
reaction under iminium activation.
Table 2: Scope of the reaction.[a]
Our studies began with the identification of the best
catalyst and reaction conditions for this transformation using
(E)-2-hexenal as a model substrate (Table 1). We started
Entry 1 (R)
Yield of 4 [%][b] d.r.[c]
ee [%][d]
1
2
3
4
5
6
7
8
1a (nPr)
1b (Me)
1c (Et)
1d (nBu)
1e (nC5H11)
1 f (nC6H13)
1g (Z-EtCH CHCH2CH2)
1h (nC8H17)
1i (Ph)
1j (o-MeOC6H4)
1k ((3-MeO)(4-AcO)C6H3) 67
1d (nBu)
1h (nC8H17)
1i (Ph)
1d (nBu)
1h (nC8H17)
1i (Ph)
96
89
86
89
92
78
83
76
76
71
>10:1 99
Table 1: Screening of the optimal reaction conditions for the reaction.[a]
7:1 92
7:1 95
>10:1 97
>10:1 96
>10:1 95
>10:1 94
>10:1 98
>10:1 98
>10:1 90
>10:1 94
>10:1 98
>10:1 97
>10:1 98
>10:1 97
>10:1 98
>10:1 98
=
9
Entry Catalyst Additive
Conv. [%][b] d.r.[c]
ee [%][d]
10
11
12[e]
13[e]
14[e]
15[f]
16[f]
17[f]
1
2
3
4
5
6
7
3a
3a
3a
3a
3a
3a
3a
3b
3b
3b
–
<10
50
–
–
PhCO2H (10 mol%)
PhCO2H (20 mol%)
PhCO2H (100 mol%)
DABCO (10 mol%)
Et3N (10 mol%)
NaOAc (10 mol%)
PhCO2H (10 mol%)
PhCO2H (100 mol%)
PhCO2H (200 mol%)
>10:1 n.d.
90
74
77
98
96
80
99 (93)[e] >10:1 99
99 (96)[e] >10:1 97
<10
<10
20
<10
50
–
–
–
–
n.d.
–
n.d.
–
>10:1 98
>10:1 98
8[f]
9[f]
10[f]
[a] Reaction conditions: 2 (0.2 mmol), 1 (0.3 mmol), 3a (20 mol%), and
PhCO2H (40 mol%) in CHCl3 (2.0 mL). [b] Yield of isolated 4. [c] Deter-
mined by NMR analysis of the crude reaction mixture. [d] Determined by
HPLC analysis (see the Supporting Information). [e] Reaction condi-
tions: 2 (1.0 mmol), 1 (1.5 mmol), 3a (20 mol%) PhCO2H (40 mol%) in
CHCl3 (10 mL). [f] Reaction conditions: 2 (1.0 mmol), 1 (10 mmol), 3a
(20 mol%), PhCO2H (40 mol%) in CHCl3 (50 mL).
99
[a] Reaction conditions: 1a (0.2 mmol), 2 (0.2 mmol), and catalyst 3
(10 mol%) in CHCl3 (5.0 mL) with stirring at RT for 16 h. [b] Conversion
1
determined from H NMR analysis of crude aliquots. [c] Determined by
1H NMR spectroscopy of the crude reaction mixture. [d] Determined by
HPLC analysis after conversion into the diacetylated product (see the
Supporting Information). [e] Yield of the isolated product given within
the parentheses. [f] The reaction was stirred for 5 days. n.d.=not
determined, TMS=trimethylsilyl, DABCO=1,4-diazabicyclo[2.2.2]-
octane.
increasing the amount of the catalyst to 20 mol%. Under
these conditions, a wide variety of differently substituted
hexahydrofuro[3,4-c]furanes 4a–k were obtained with excel-
lent yields and remarkably, as single diastereoisomers in
almost all cases (Table 2, entries 1–11).[10] Additionally, the
reaction proceeded with excellent enantioselectivity for all
the substrates tested, furnishing the final heterocycles 4a–k as
highly enantioenriched compounds. The reaction could be
carried out on larger scale (Table 2, entries 12–14), resulting
in similar yields and stereoselectivities. Slightly higher yields
of the final products 4 were obtained under more dilute
reaction conditions and in the presence of a large excess of the
enal reagent, while also maintaining a high diastereo- and
enantioselectivity (Table 2, entries 15–17).
A plausible mechanistic proposal for this transformation
is given in Scheme 2. The reaction could start with the
conjugate addition of 2[11] to the enal under iminium
activation and then the intermediate enamine would undergo
intramolecular aldol reaction delivering the final adducts 4
after releasing the catalyst by hydrolysis and undergoing a
final internal hemiacetal-formation step. We believe that the
high stereochemical control obtained in the overall process
using O-trimethylsilyldiphenylprolinol (3a) as the catalyst,
and after some experiments we concluded that chloroform
was the best solvent for the reaction. We also found that a
Brønsted acid co-catalyst was also required for the reaction to
proceed to completion (Table 1, entry 1 versus entries 2–4).
The amount of an acid additive used had an important
influence on the reaction: after reacting for 16 hours the
reaction proceeded with complete conversion when 20 mol%
of PhCO2H or more was used (Table 1, entries 2–4). The
possibility of using a base as an additive was also evaluated,
but with negative results (Table 1, entries 5–7). We also
evaluated the modified catalyst 3b, but the reactions pro-
ceeded more slowly compared to those reactions using the
catalyst 3a, and they required the addition of two equivalents
of benzoic acid, as an additive to undergo full conversion, as
well as much longer reaction times (Table 1, entries 8–10).
Having established the best protocol for the reaction, we
decided to extend this methodology to a,b-unsaturated
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Scheme 3. Survey of transformations carried out on the adduct 4a.
PCC=pyridinium chlorochromate.
Remarkably, the allylation reaction leading to 6a proceeded
in a fully diastereoselective fashion.
In conclusion, we have developed a very efficient domino
process which leads to the synthesis of hexahydrofuro[3,4-
c]furanes in excellent yields and diastereo- and enantioselec-
tivities starting from readily available starting materials.
Remarkably, this sequence involves the consecutive forma-
Scheme 2. A plausible reaction pathway for the reaction.
À
À
À
relies on the irreversible intramolecular C C bond-formation
tion of two C O and one C C bonds and the fully
stereocontrolled generation of four stereocenters, one of
them being a quaternary center. Notably, this is also the first
example of a highly enantioselective b-alkoxylation of a,b-
unsaturated aldehydes catalyzed by a secondary amine. The
fact that a high pKa oxygen nucleophile is employed as a
Michael donor to initiate the conjugate addition process, and
the subsequent intramolecular aldol reaction takes place with
a less electrophilic ketone moiety are unique features
associated with this transformation. Moreover, the possibility
of the selective manipulation of the different functionalities
present within the obtained adducts allows the preparation of
a wide range of different compounds which demonstrates the
potential of this methodology for the enantioselective syn-
thesis of useful chiral building blocks.
step, taking into account the known reversibility of oxa-
Michael addition reactions.[7a,9] In this context, the efficiency
of the catalyst 3a to control the two stereocenters formed in
the intramolecular aldol reactions is well documented.[2a,6] In
contrast, with regard to the stereocontrol at the stereogenic
center formed in the oxa-Michael step, two possibilities might
explain the high selectivity observed: 1) a catalyst-controlled
oxa-Michael reaction and a subsequent fast intramolecular
aldol reaction which avoids the retro-oxa-Michael process, or
2) a dynamic kinetic resolution process, in which the chiral
catalyst accelerates the aldol reaction for one diastereoisomer
over the other, the later epimerizing because of the reversi-
bility of the oxa-Michael reaction. We have been unable to
detect the formation of or isolate the intermediate oxa-
Michael product by using NMR analyses of aliquots of the
reaction mixture or by carrying out the reaction under
stoichiometric conditions; this lack of identification is sug-
gestive of the first possible explanation discussed above.
However, this experiment is not definitive proof for com-
pletely ruling out the dynamic kinetic resolution pathway.
Finally, the last hydrolysis/hemiacetalization reaction should
take place under thermodynamic control, furnishing the most
stable diastereoisomer at the anomeric carbon center. Never-
theless, control experiments using a modified substrate
indicate that the hemiacetal formation is also important to
attaining full conversion.[12] We interpret this latter finding as
efficient product scavenging from the catalytic cycle by the
formation of a more stable bicyclic compound such as 4.
Regarding the role played by the additive, it is proposed that
PhCO2H participates in the reaction not only by assisting in
the formation of the iminium ion but also by activating the
ketone moiety in the intramolecular aldol addition step
through protonation.
Received: March 10, 2009
Revised: May 18, 2009
Published online: June 25, 2009
Keywords: asymmetric catalysis · domino reactions ·
heterocycles · organocatalysis · oxa-Michael reaction
.
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We also decided to survey the reactivity of the obtained
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building blocks in organic synthesis (Scheme 3). All these
transformations proceeded without epimerization at any of
the stereogenic centers present in the starting materials.
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Angew. Chem. Int. Ed. 2009, 48, 5701 –5704
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Communications
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[10] The absolute configuration of the compounds 4 was assigned by
analogy from the X-ray analysis of the corresponding O-acetyl
p-chlorobenzoyl derivative of 4b (see the Supporting Informa-
tion). CCDC 723339 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[11] We also think that it is the dihydroxyacetone dimer 2 itself which
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having the ability to retrodimerize 2, such as MeOH or THF,
proceeded with very low conversions.
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[12] The reaction of crotonaldehyde with hydroxyacetophenone
under the optimized reaction conditions did not furnish any
product; only unmodified starting materials were recovered.
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