The first general enantioselective catalytic Diels-Alder reaction with simple (α,β-unsaturated ketones

Article abstract of DOI:10.1021/ja017641u

The first general approach to enantioselective catalysis of the Diels-Alder reaction with simple ketone dienophiles has been accomplished. The use of iminium catalysis has enabled enantioselective access to a fundamental Diels-Alder reaction variant that

Full text of DOI:10.1021/ja017641u

Published on Web 02/23/2002
The First General Enantioselective Catalytic Diels-Alder Reaction with
Simple r,â-Unsaturated Ketones
Alan B. Northrup and David W. C. MacMillan*
DiVision of Chemistry and Chemical Engineering,California Institute of Technology, Pasadena, California 91125
Received November 29, 2001
For more than 70 years, the Diels-Alder reaction has remained
arguably the most powerful organic transformation in chemical
synthesis.1 In particular, asymmetric catalytic variants of this
venerable reaction2 have received unprecedented attention,3 presum-
ably due to their capacity to rapidly provide complex enantioen-
riched carbocycles from simple substrates. It is remarkable,
therefore, that simple acyclic ketone dienophiles have yet to be
successfully employed in enantioselective catalytic [4 + 2] cy-
cloadditions,4 a synthetic deficiency that is underscored by the
prevalence of enantiopure ketones throughout natural product
architecture. In our recent studies, we reported that the LUMO-
lowering activation of R,â-unsaturated aldehydes using chiral
imidazolidinones 1 is a valuable platform for the development of
a wide range of enantioselective carbon-carbon bond-forming
reactions. In this communication, we extend this organocatalytic
strategy5 to the activation of R,â-unsaturated ketones using a new
chiral amine catalyst. In this context, we document the first
enantioselective catalytic Diels-Alder reaction with simple ketone
dienophiles.
tion is replaced by the requirement for selective π-bond formation
(eq 2), a precedented task with respect to iminium construction.6,7
Our enantioselective catalytic ketone-Diels-Alder was first
evaluated with 4-hexen-3-one and cyclopentadiene and a series of
amine‚HClO4 salts 1-5 (Table 1). Surprisingly, this cycloaddition
Table 1. Effect of Amine Architecture on the Diels-Alder
Reaction between 4-Hexen-3-one and Cyclopentadiene
entry cat.
R
1
R (R )
time (h) % yield endo:exo %eea,b
2
3
1
2
3
4
5
1
2
3
4
5
Bn Me (Me)
Bn t-Bu (H)
Ph Ph (H)
Bn Ph (H)
48
48
22
42
22
20c
27c
88
83
89
7:1
9:1
21:1
23:1
25:1
0
0
47
82
90
Bn 5-Me-furyl (H)
a Product ratios determined by chiral GLC. b Absolute configuration
assigned by chemical correlation to a known compound. c Less than 30%
conversion of starting material after 48 h.
The success of chiral Lewis acid mediated Diels-Alder reactions
is founded upon the use of dienophiles such as aldehydes, esters,
quinones, and bidentate chelating carbonyls that achieve high levels
of lone pair discrimination in the metal association step, an
organizational event that is essential for enantiocontrol. In contrast,
Lewis acid coordination is traditionally a nonselective process with
ketone dienophiles since the participating lone pairs are positioned
in similar steric and electronic environments (eq 1). The capacity
for diastereomeric activation pathways in this case often leads to
poor levels of enantiocontrol and ultimately has prevented the use
of simple ketone dienophiles in asymmetric catalytic Diels-Alder
reactions.
Having demonstrated the utility of iminium activation to provide
LUMO-lowering catalysis outside the mechanistic confines of lone
pair coordination,6 we hypothesized that amine catalysts might also
enable simple ketone dienophiles to function as useful substrates
for enantioselective Diels-Alder reactions. In this case, the capacity
to perform substrate activation through specific lone pair coordina-
strategy was unsuccessful with amine salts that have previously
been identified4 as valuable catalysts for aldehyde activation (entries
1 and 2, 20-27% yield, 0% ee). In contrast, the cis-2,5-diphenyl-
substituted amine 3 provided useful reaction rates in conjunction
with moderate enantiocontrol (entry 3, 88% yield, 21:1 endo:exo,
47% ee).8 Efforts to increase enantioselectivity through iminium
geometry control were successful with the introduction of a benzyl
group at the C(2) catalyst site9 (entry 4, 83% yield, 82% ee).
Moreover, a survey of aryl and heteroaromatic architecture at the
C(5) position has established that the 2-(5-methylfuryl)-derived
imidazolidinone10 5 affords superior levels of enantiofacial dis-
crimination while maintaining reaction efficiency (entry 5, 89%
yield, 25:1 endo:exo, 90% ee).
Experiments that probe the scope of the dienophile component
are summarized in Table 2. Variation in the alkyl ketone group
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10.1021/ja017641u CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Organocatalyzed Diels-Alder Cycloadditions between
Cyclopentadiene and Representative Acyclic Enones
highlight both the functional tolerance and the preparative utility
of this new organocatalytic strategy, the cycloaddition of dieny-
lamine 6 to ethyl vinyl ketone was performed on a 25 mmol scale
to afford the corresponding Diels-Alder adduct in 98% ee and
91% yield (Table 3, entry 2).11 Notably, recovery of amine catalyst
5 was accomplished in 91% yield after chromatography.
The sense of asymmetric induction observed in all cases is
consistent with selective engagement of the diene substrate with
the si-face of the cis-iminium isomer 8 (MM3-8). Indeed, compu-
tational models provide supporting evidence that the trans-iminium
isomer MM3-7 will be energetically disfavored on the basis of
nonbonding interactions between the benzyl and CH2-b (green)
substituents.12 Moreover, the calculated cis-iminium isomer MM3-8
is selectively exposed to cycloaddition at the si-face, while both
enantiofacial sites of the trans-isomer are shielded by structural
impediment when R2 ) Et (MM3-7, green circle ) Me), n-Bu or
isoamyl. In the singular case of methyl ketone (R2 ) Me, green
circle ) H), it is apparent that the trans-iminium MM3-7 will
also be exposed at the olefin re-face, thereby allowing enantiodi-
vergent pathways to compete in the [4 + 2] event. Significantly,
this computational prediction is in complete accord with the
disparity in enantiocontrol observed with methyl and ethyl ketone
dienophiles (Table 2, R2 ) Me, 61% ee; R2 ) Et, 90% ee).
entry
R
R
% yield
endo:exo
%eea,b
1
2
1
2
3
4
5
6
7
Me
Me
Me
Me
Me
n-Pr
i-Pr
Me
Et
n-Bu
i-Am
i-Pr
Et
85
89
83
86
24
84
78
14:1
25:1
22:1
20:1
8:1
61
90
92c
92
0
15:1
6:1
92
90
Et
a Product ratios determined by chiral GLC. b Absolute configuration
determined by chemical correlation to a known compound or by analogy.
c Reaction performed without solvent.
Table 3. Organocatalyzed Diels-Alder Cycloadditions between
Ethyl Vinyl Ketone and Representative Dienes
a Product ratios determined by chiral GLC or HPLC. b Absolute con-
figuration determined by chemical correlation to a known compound or by
analogy. c Reaction conducted at -40 °C. d Regiomeric ratio. e Reaction
conducted at -20 °C. f Reaction performed without solvent.
To test the validity of these models and the accompanying
stereochemical analysis we conducted several experiments with
cyclic dienophiles (Table 4). The principal issue in these studies is
(R2) reveals that ethyl, n-butyl, and isoamyl substituents (entries
2-4) provide superior enantiocontrol (90-92% ee), while surpris-
ingly the methyl ketone is subject to moderate levels of induction
(entry 1, 61% ee). Furthermore, the isopropyl-substituted ketone
leads to racemic cycloadducts in poor yield (entry 5, 0% ee, 24%
yield), presumably as a result of steric inhibition of iminium
formation and the attendant potential for Brønsted acid catalysis.
In contrast, variation in the steric contribution of the olefin
substituent (R1) can be accomplished without loss in enantiocontrol
or reaction efficiency (entries 2, 6, and 7, R1 ) Me, n-Pr, i-Pr,
g78% yield, 6-25 to 1 endo:exo, 90-92% ee).
This organocatalytic Diels-Alder reaction is also quite general
with respect to diene structure, allowing enantioselective access to
a broad range of alkyl-, alkoxy-, amino-, and aryl-substituted
cyclohexenyl ketones (Table 3, entries 1-5, 79-92% yield, >100
to 1 endo:exo, 85-98% ee). Of particular note is the fact that all
entries in Table 3 produce a single regio- and diastereoisomer as
determined by GLC (>200:1) or HPLC (>100:1) analysis. To
Table 4. Organocatalyzed Diels-Alder Cycloadditions between
Cyclopentadiene and Representative Cyclic Enones
entry
n
time (h)
% yield
endo:exo
%eea,b
1
2
3
4
5
0
1
2
3
10
12
17
28
72
72
81
81
85
83
88c
15:1
12:1
18:1
6:1
48
63
90
91
93
5:1
a Product ratios determined by chiral GLC. b Absolute configuration
determined by chemical correlation to a known compound or by analogy.
c Reaction conducted with (E)-cyclopentadecene-2-one to provide the
corresponding 1,2-trans-tricyclo[15.2.1.0]eicos-18-en-3-one.
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C O M M U N I C A T I O N S
116, 2812. (c) White, J. D.; Choi, Y. Org. Lett. 2000, 2, 2373. (d) Engler,
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that small rings (ring size ) 5 or 6) were expected to exhibit
selectivities in accord with methyl ketone dienophiles because the
inherent conformational restrictions should allow participation of
both cis- and trans-iminium isomers in the Diels-Alder event.
However, as the dienophile ring size is expanded, we would expect
enhanced torsional freedom about the NdC-alkyl bond, a trend that
should increasingly shield the trans-iminium re-face (see MM3-
7) and accordingly improve asymmetric induction. In agreement
with this hypothesis, cyclopentenone and cyclohexenone provide
modest enantiocontrol (12-15:1 endo:exo, 48-63% ee, 81% yield),
while cycloheptenone (n ) 2), cyclooctenone (n ) 3), and (E)-
cyclopentadecene-2-one (n ) 10) are highly enantioselective (entries
3-5, 5-18:1 endo:exo, 90-93% ee, 83-88% yield).
Last, with respect to the operational and environmental advan-
tages of organocatalysis, it is important to note that all of the
cycloadditions reported herein were conducted in aqueous (Tables
1, 2, and 4) or ethanolic (Table 3) media. While the capacity of
protic solvents to accelerate [4 + 2] cycloadditions has long been
established,13 such reaction media are rarely amenable to chiral
metal salt catalysis.14 In this and in preceding reports,6 we have
demonstrated that iminium-catalysis and the accompanying imi-
dazolidinone catalysts are robust to aqueous conditions. We hope
this organocatalytic strategy will continue to provide a new platform
for the development of enantioselective processes that utilize
environmentally benign solvents.
(5) For notable examples of organocatalytic reactions, see: Aldol reaction:
(a) Hajos, Z. G.; Parrish, D. R. J. Org. Chem. 1974, 39, 1615. (b) Eder,
U.; Sauer, G.; Wiechert, R. Angew. Chem., Int. Ed. Engl. 1971, 10, 496.
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(e) List, B.; Pojarliev, P.; Castello, C. Org. Lett. 2001, 3, 573. (f) List, B.
J. Am. Chem. Soc. 2000, 122, 9336. Phase-transfer catalysis: (g)
O’Donnell, M. J.; Bennett, W. D.; Wu, S. D. J. Am. Chem. Soc. 1989,
111, 2353. (h) Corey, E. J.; Bo, Y. X.; Busch-Petersen, J. J. Am. Chem.
Soc. 1998, 120, 13000. (i) Corey, E. J.; Xu, F.; Noe, M. C. J. Am. Chem.
Soc. 1997, 119, 12414. Epoxidation: (j) Yang, D.; Yip, Y. C.; Tang, M.
W.; Wong, M. K.; Zheng, J. H.; Cheung, K. K. J. Am. Chem. Soc. 1996,
118, 491. (k) Yang, D.; Wong, M. K.; Yip, Y. C.; Wang, X. C.; Tang, M.
W.; Zheng, J. H.; Cheung, K. K. J. Am. Chem. Soc. 1998, 120, 5943. (l)
Tu, Y.; Wang, Z. X.; Shi, Y. J. Am. Chem. Soc. 1996, 118, 9806. (m)
Tian, H. Q.; She, X. G.; Shu, L. H.; Yu, H. W.; Shi, Y. J. Am. Chem.
Soc. 2000, 122, 11551. (n) Denmark, S. E.; Wu, Z. C. Bayliss-Hillman
reaction: (o) Iwabuchi, Y.; Nakatani, M.; Yokoyama, N.; Hatakeyama,
S. J. Am. Chem. Soc. 1999, 120, 10219. (p) Corey, E. J.; Zhang, F. Y.
Org. Lett. 1999, 1, 1287. Asymmetric Strecker synthesis: (q) Vachal, P.;
Jacobsen, E. N. Org. Lett. 2000, 2, 867. (r) Sigman, M. S.; Vachal, P.;
Jacobsen, E. N. Angew. Chem., Int. Ed. 2000, 39, 1279. (s) Corey, E. J.;
Grogan, M. J. Org. Lett. 1999, 1, 157. Acyl transfer: (t) Jarvo, E. R.;
Copeland, G. T.; Papaioannou, N.; Bonitatebus, P. J.; Miller, S. J. J. Am.
Chem. Soc. 1999, 121, 11638. (u) For an excellent review on enantiose-
lective organocatalysis see: Dalko, P. I.; Moisan, L. Angew. Chem., Int.
Ed. 2001, 40, 3726.
In summary, the scope of LUMO-lowering organocatalysis has
been extended to accomplish the first general enantioselective
ketone Diels-Alder reaction. A full disclosure of these studies will
be reported in due course.
(6) Diels-Alder: (a) Ahrendt, K. A.; Borths, C. J.; MacMillan, D. W. C. J.
Am. Chem. Soc. 2000, 122, 4243. Nitrone cycloaddition: (b) Jen, W. S.;
Wiener, J. J. M.; MacMillan, D. W. C. J. Am. Chem. Soc. 2000, 122,
9874. Friedel-Crafts: (c) Paras, N. A.; MacMillan, D. W. C. J. Am. Chem.
Soc. 2001, 123, 4370. (d) Austin, J. F.; MacMillan, D. W. C. J. Am. Chem.
Soc. 2002, 124, 1172.
(7) The Beilstein database reports 7924 examples of tetrasubstituted iminium
ions. Prior to this investigation, only examples that involve intramolecular
formation of tetrasubstituted iminium ions have been accomplished
selectively.
(8) Only catalysts that bear an aryl substituent at C(5) of the imidazolidinone
ring exhibit enantioselectivity in this reaction. Investigations are currently
underway to determine the origins of this phenomenon.
Acknowledgment. Financial support was provided by kind gifts
from AstraZeneca, Boehringer-Ingelheim, Dupont, GlaxoSmith-
Kline, Johnson and Johnson, Lilly, Materia, Merck Research
Laboratories, Pfizer, Pharmacia, and Roche Biosciences. We also
thank Great Lakes for their generous donation of (S)-phenylalanine.
D.W.C.M is grateful for support from Research Corporation under
the Cottrell Scholarship program. A.B.N. is grateful for a NSF
predoctoral fellowship.
(9) The C(2)-CH2Ar motif in catalyst 1 is required for selective iminium
formation; Ahrendt, K. A., unpublished results.
(10) Preparation of (2S,5S)-5-benzyl-3-methyl-2-(5-methyl-furan-2-yl)-imida-
zolidin-4-one (catalyst 5): To Sm(OTf)3 (1.20 g, 2.0 mmol) were added
powdered 4 Å molecular sieves (4.0 g), followed by (S)-phenylalanine
methyl amide (8.91 g, 50 mmol) in THF (80 mL). To the resulting mixture
was added 5-methylfurfural (3.98 mL, 40 mmol). After stirring for 29 h
at 23 °C, the mixture was filtered through a plug of silica gel, concentrated,
and purified by silica gel chromatography to afford the title compound as
a clear, colorless oil in 46% yield (4.93 g, 18.2 mmol). The faster eluting
(2R,5S)-5-benzyl-3-methyl-2-(5-methyl-furan-2-yl)-imidazolidin-4-one iso-
mer was isolated in 38% yield.
(11) Representative procedure: To a flask containing catalyst 5 (20 mol %) in
H2O (3-7 M) at 0 °C was added the R,â-unsaturated ketone (100 mol
%) followed by aqueous perchloric acid (20 mol %). After stirring for 5
min, freshly distilled cyclopentadiene (150 mol %) was added dropwise.
The resulting mixture was stirred at 0 °C until consumption of the
unsaturated ketone as determined by TLC analysis. The reaction mixture
was then diluted with the appropriate eluent and then purified by silica
gel chromatography.
Note Added in Proof. Shortly prior to publication it came to
the authors attention that an enantioselective boron-catalyzed ketone
Diels-Alder reaction has been accomplished. This work will be
submitted for communication in the near future. Joel M. Hawkins,
Pfizer Global Research, personal communication.
Supporting Information Available: Experimental procedures,
structural proofs, and spectral data for all new compounds are provided
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org.
(12) A Monte Carlo simulation using the MM3 force-field; Macromodel V6.5.
(13) (a) Breslow, R.; Rideout, D. J. Am. Chem. Soc. 1980, 102, 7816. (b)
Grieco, P. A.; Garner, P.; He, Z. Tetrahedron Lett. 1983, 24, 1897. (c)
Gajewski, J. J. J. Org. Chem. 1992, 57, 5500. (d) Otto, S.; Blokzijl, W.;
Engberts, J. B. F. N. J. Org. Chem. 1994, 59, 5372. (e) Engberts, J. B. F.
N. Pure Appl. Chem. 1995, 67, 823. (f) Otto, S.; Engberts, J. B. F. N.
Pure Appl. Chem. 2000, 72, 1365.
(14) For examples of highly enantioselective (>90% ee) metal-catalyzed
transformations that employ H2O as the bulk reaction medium see: (a)
Green Chemistry; Anastas, P. T., Williamson, T. C., Eds.; ACS Sympo-
sium Series 626; American Chemical Society: Washington: DC, 1996
and references therein. (b) Li, C.-J.; Chan, T.-H. Organic Reactions in
Aqueous Media; Wiley: New York, 1997. (c) Grieco, P. A., Ed. Organic
Synthesis in Water; Kluwer Academic Publishers: Dordrecht, The
Netherlands, 1997. (d) Uozumi, Y.; Shibatomi, K. J. Am. Chem. Soc. 2001,
123, 2919. (e) Tokunaga, M.; Larrow, J. F.; Kakiuchi, F.; Jacobsen, E.
N. Science 1997, 277, 936.
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(3) A survey of the CAS database (Scifinder) reveals >500 manuscripts or
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(4) Only quinone and chelating ketone dienophiles have been successfully
utilized in asymmetric Diels-Alder reactions. Quinones: (a) Mikami, K.;
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