New strategies for organic catalysis: The first enantioselective organocatalytic 1,3-dipolar cycloaddition [20]

Full text of DOI:10.1021/ja005517p

9874
J. Am. Chem. Soc. 2000, 122, 9874-9875
Table 1. Effect of Catalyst Structure on the Dipolar Cycloaddition
between Crotonaldehyde and Nitrone 3
New Strategies for Organic Catalysis: The First
Enantioselective Organocatalytic 1,3-Dipolar
Cycloaddition
Wendy S. Jen, John J. M. Wiener, and
David W. C. MacMillan*,†
Department of Chemistry, UniVersity of California
Berkeley, California 94720
DiVision of Chemistry and Chemical Engineering
California Institute of Technology
Pasadena, California 91125
ReceiVed August 8, 2000
Our laboratory has been engaged in the design of broadly useful
new strategies for enantioselective catalysis that utilize organic
chemicals as reaction catalysts. In our recent study, we reported
that the LUMO-lowering activation of R,â-unsaturated aldehydes
using the reversible formation of iminium ions with chiral
imidazolidinones 1 (eq 1) is a valuable platform for the develop-
ment of enantioselective organocatalytic Diels-Alder reactions
(eq 2).1 In this work, we reveal that this catalytic strategy is also
amenable to [3 + 2] cycloadditions between nitrones and R,â-
unsaturated aldehydes to provide isoxazolidines (eq 3), useful
synthons for the construction of biologically important amino
acids, â-lactams, amino carbohydrates, and alkaloids.2 To our
knowledge, this is the first example of an organocatalytic 1,3-
dipolar cycloaddition.3 Moreover, this study further documents
that chiral amines can be employed as asymmetric catalysts for
a range of transformations that traditionally utilize metal salts.
a Product ratios determined by HPLC using a Chiralcel OD-H column
after reduction of the formyl group with NaBH4. b Absolute and relative
configurations assigned by chemical correlation or by analogy (Sup-
porting Information).
R,â-unsaturated aldehydes are poor substrates for metal-catalyzed
nitrone cycloadditions, presumably due to the preferential coor-
dination of Lewis acids to nitrone oxides in the presence of
monodentate carbonyls (eq 4).4 In contrast, we expected amine
catalysts to be inert to nitrone association, thereby enabling R,â-
unsaturated aldehydes to undergo iminium activation (eq 5) and
subsequent [3 + 2] cycloaddition.
Our catalytic [3 + 2] addition strategy was first evaluated using
N-benzylidenebenzylamine N-oxide (3) with (E)-crotonaldehyde
and a series of chiral imidazolidinone‚HCl salts 1.5 As revealed
in Table 1, this reaction was successful with a variety of amine
catalysts (entries 1-7, 45-77% yield, 20-93% ee) in CH3NO2-
H2O at +4 °C. A survey of catalyst architecture reveals that
incorporation of benzylic substituents at the C(3) position on the
imidazolidinone framework provides the highest levels of enan-
tiofacial discrimination (1a, R ) CH2Ph, 93% ee; 1e, R ) CH2-
2-naphthyl, 86% ee; 1f, R ) CH2C6H4OMe-4, 89% ee). In accord
with our Diels-Alder studies,1 the phenylalanine-derived catalyst
1a was found to be most general with respect to reaction substrates
(vide infra).
Given the operational and economical advantages associated
with organocatalysis, we recently sought to further demonstrate
the value of our iminium-activation strategy through the develop-
ment of reaction variants that are currently unavailable using
Lewis acid catalysts. In this context, it has been established that
Variation in the Brønsted acid component of the benzyl
imidazolidinone catalyst was next examined (Table 2). A number
of imidazolidinone acid salts were found to catalyze the formation
† Current address: Division of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, California 91125.
(1) Ahrendt, K. A.; Borths, C. J.; MacMillan, D. W. C. J. Am. Chem. Soc.
2000, 122, 4243.
(4) Chiral Lewis acid mediated [3 + 2] cycloadditions involving bidentate
dipolarophiles have been reported: (a) Gothelf, K. V.; Jørgensen, K. A. J.
Org. Chem. 1994, 59, 5687. (b) Gothelf, K. D.; Marti, R. E.; Hintermann, T.
HelV. Chim. Acta 1996, 79, 1710. (c) Hori, K.; Kodama, H.; Ohta, T.;
Furukawa, I. Tetrahedron Lett. 1996, 37, 5947. (d) Ukaji, Y.; Taniguchi, K.;
Sada, K.; Inomata, K. Chem. Lett. 1997, 547. (e) Jensen, K. B.; Gothelf, K.
V.; Hazell, R. G.; Jørgensen, J. Org. Chem. 1997, 62, 2471. (f) Kobayashi,
S.; Kawamura, M. J. Am. Chem. Soc. 1998, 120, 5840. (g) Kanemasa, S.;
Oderaotoshi, Y.; Tanaka, J.; Wada, E. J. Am. Chem. Soc. 1998, 120, 12355.
(5) Derived in two steps from the respective amino acid (see Supporting
Information).
(2) For a recent review on the utilty of amino-oxy synthons, see:
Fredrickson, M. Tetrahedron 1997, 53, 403.
(3) For reviews of [3 + 2] cycloadditions, see: (a) Tufariello, J. J. In 1,3-
Dipolar Cycloaddition Chemistry; Padwa, A., Ed.; John Wiley & Sons:
Chichester, 1984; Vol. 2, p 83. (b) Torssell, K. B. G. Nitrile Oxides, Nitrones
and Nitronates in Organic Synthesis; VCH: Weinheim, 1988. (c) Gothelf,
K. V.; Jørgensen, K. A. Chem. ReV. 1998, 98, 863.
10.1021/ja005517p CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/26/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9875
Table 2. Effect of the Brønsted Acid Cocatalyst on the Dipolar
Cycloaddition between Crotonaldehyde and Nitrone 3
Table 3. Organocatalyzed Dipolar Cycloadditions between
Representative Nitrones and Dipolarophiles
a Product ratios determined by HPLC using a Chiralcel OD-H column
after reduction of the formyl group with NaBH4. b Reactions performed
at -20 °C.
of isoxazolidine (4S)-4 in good yield and in greater than 86% ee
(entries 1-6). An enantioselectivity/temperature profile documents
that optimal stereochemical control and reaction efficiency is
achieved at -10 °C with catalysts 1a, 5-7 (entries 1-5), while
the HClO4 salt 8 is most effective at -20 °C (entry 6). Preliminary
investigations suggest that the observed variation in enantiose-
lectivity as a function of cocatalyst can be attributed to the extent
of iminium activation in preference to achiral Brønsted acid
promotion. As such, it is important to note that the HCl- and
HClO4-derived catalysts successfully partition the [3 + 2]
cycloaddition toward the iminium pathway with >94% selectivity
(>94% ee). The superior levels of asymmetric induction and
diastereocontrol exhibited by the HClO4 salt 8 to afford isoxazo-
lidine (S)-4 in 94% ee, 94:6 dr, and 98% yield (20 mol % catalyst,
-20 °C) prompted us to select this catalyst for further exploration.
The scope of the organocatalytic 1,3-dipolar cycloaddition
between R,â-unsaturated aldehydes and various nitrones has been
investigated (Table 3).6 The reaction appears quite general with
respect to the nitrone structure (entries 1-8, 66-98% yield, 92:8
to 98:2 endo:exo, 91-99% ee). Variation in the N-alkyl group
(R1 ) Me, Bn, allyl, entries 1-3) is possible without loss in
enantioselectivity (endo 94-99% ee). As revealed with 4-chlo-
rophenyl- and 4-methoxyphenyl-substituted nitrones (entries 4-6),
the reaction is tolerant to a range of aromatic substituents on the
dipole (76-93% yield, 92:8 to 98:2 endo:exo, 91-95% ee).
Moreover, excellent levels of diastero- and enantioselectivity can
be achieved with alkyl-substituted nitrones (entry 9, 99:1 endo:
exo, 99% ee). To demonstrate the preparative utility, the addition
of nitrone 3 to crotonaldehyde was performed on a 25 mmol scale
with catalyst 8 to provide (4S)-4 (94% ee, 98% yield).
a Product ratios determined by HPLC using a Chiralcel OD-H column
after reduction of the formyl group with NaBH4. b Absolute and relative
configurations assigned by chemical correlation or by analogy (Sup-
porting Information). c Reactions conducted with catalyst 5.
consistent with (E)-iminium isomer MM3-97 in direct analogy
with our Diels-Alder studies.1 By inspection, it is evident that
enforced formation of the (E)-iminium isomer and the position
of the benzyl group on the catalyst framework effectively promote
cycloaddition from the si-face of the dipolarophile. Furthermore,
cycloaddition through an endo topography effectively alleviates
nonbonding interactions between the nitrone phenyl group and
the neopentyl methyl substituent on the catalyst framework.
Structural variation in the dipolarophile can also be realized
(Table 3, entries 10-15). Significantly, both the HClO4- and
TfOH-derived catalysts are compatible with acrolein (entries 10
and 11, 90-92% ee), with amine 5 providing optimal reaction
efficiency and stereocontrol (80% yield, 86:14 endo:exo). Finally,
it is noteworthy that all of the reactions described in this study
were performed under an aerobic atmosphere, using wet solvents
and a readily available bench-stable catalyst, further outlining the
operational advantages of organocatalysis.
In conclusion, we have further established LUMO-lowering
organocatalysis as a broadly useful concept for asymmetric
catalysis in the context of [3 + 2] dipolar cycloadditions. A full
account of this work will be forthcoming.
Enantioselective formation of the endo-(4S)-isoxazolidine ad-
duct was observed in all cases involving catalyst 8. Significantly,
this sense of asymmetric induction and diastereoselectivity are
Acknowledgment. W.S.J. and J.J.M.W. are grateful for predoctoral
NSF fellowships. This research has been supported by kind gifts from
Merck, Roche-Biosciences, and Warner-Lambert.
(6) In a representative procedure, crotonaldehyde (1.2 mmol) was added
to a solution of nitrone (0.3 mmol), catalyst 8 (0.04 mmol), and H2O (1.8
mmol) in CH3NO2 (3 mL) at -20 °C. The resulting solution was maintained
at this temperature until the nitrone was judged to be consumed by TLC
analysis (35-160 h). The resulting solution was then passed through a plug
of silica gel with EtOAc and concentrated. The resulting residue was then
purified by silica gel chromatography.
Supporting Information Available: Experimental procedures and
spectral data for all compounds are provided (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
JA005517P
(7) A Monte Carlo simulation, MM3 force-field; Macromodel V6.5.