Highly stereoselective and scalable anti-aldol reactions using N-(p-dodecylphenylsulfonyl)-2-pyrrolidinecarboxamide: Scope and origins of stereoselectivities

Article abstract of DOI:10.1021/jo1015008

A highly enantio- and diastereoselective anti-aldol process (up to >99% ee, >99:1 dr) catalyzed by a proline mimetic-N-(p-dodecylphenylsulfonyl)-2- pyrrolidinecarboxamide-has been developed. Catalyst loading as low as 2 mol % can be employed. Use of industry-friendly solvents for this transformation as well as neat reaction conditions have been demonstrated. The scope of this transformation on a range of aldehydes and ketones is explored. Density functional theory computations reveal that the origins of enhanced diastereoselectivity are due to the presence of nonclassical hydrogen bonds between the sulfonamide, the electrophile, and the catalyst enamine that favor the major anti-Re aldol TS in the Houk-List model.

Full text of DOI:10.1021/jo1015008

pubs.acs.org/joc
Highly Stereoselective and Scalable anti-Aldol Reactions Using
N-(p-Dodecylphenylsulfonyl)-2-pyrrolidinecarboxamide: Scope and
Origins of Stereoselectivities
Hua Yang, Subham Mahapatra,† Paul Ha-Yeon Cheong,*,† and Rich G. Carter*
Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon 97331.
These researchers conducted the computational studies. Paul Ha-Yeon Cheong is the corresponding author
for the computational portions of the article.
†
paul.cheong@oregonstate.edu; rich.carter@oregonstate.edu
Received July 30, 2010
A highly enantio- and diastereoselective anti-aldol process (up to >99% ee, >99:1 dr) catalyzed by a
proline mimetic;N-(p-dodecylphenylsulfonyl)-2-pyrrolidinecarboxamide;has been developed.
Catalyst loading as low as 2 mol % can be employed. Use of industry-friendly solvents for this
transformation as well as neat reaction conditions have been demonstrated. The scope of this
transformation on a range of aldehydes and ketones is explored. Density functional theory
computations reveal that the origins of enhanced diastereoselectivity are due to the presence of
nonclassical hydrogen bonds between the sulfonamide, the electrophile, and the catalyst enamine
that favor the major anti-Re aldol TS in the Houk-List model.
Introduction
protocols, which generate a net aldol adduct but do not
utilize traditional aldol starting materials.7-9 Zimmerman
and Traxler proposed over 50 years ago that controlling syn-
versus anti-aldol adducts may be explained through the use
of cyclic, chairlike transition states.10 Alternate acyclic tran-
sition states have also been proposed to address stereochem-
ical outcome of other aldol reactions.11 Metal-based enolates
have proven particularly useful in controlling both the
enantio- and diastereoselectivity in aldol processes through
The aldol reaction has been a central focus of the chemical
community since observed by Kane1 and later by Borodin,2
4
Kekule, and Wurtz. Subsequent decades led to numerous
ꢀ 3
advances that helped to address the stereoselectivity of the
reaction process.5 These accomplishments have paved the
way for modern polyketide and macrolide synthesis.6 In
addition to these important discoveries, a wealth of effort
has been directed to the development of alternate reaction
(7) (a) Jung, M. E.; Salehi-Rad, R. Angew. Chem., Int. Ed. 2009, 48, 8766–
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DOI: 10.1021/jo1015008
r
Published on Web 10/08/2010
J. Org. Chem. 2010, 75, 7279–7290 7279
2010 American Chemical Society

Yang et al.
JOCArticle
the use of chiral directing groups.11,12 Metal-catalyzed, en-
antioselective protocols have also been developed which
offer the ability to reduce the dependence on auxiliary-based
approaches for accomplishing these transformations.13
Considerable recent excitement has been generated by the
resurgence14 of organocatalysis as a practical and user-friendly
method for facilitating these types of transformations.15 Many
of the catalyst scaffolds are based on amino acid architecture
with 2ꢀ amines proving particularly useful. Consequently, a
large percentage of organocatalysts find their origins in proline.
These organocatalyzed reactions often are performed at ambi-
ent or near-ambient temperatures and do not require the care-
ful exclusion of moisture and oxygen. Organocatalyzed aldol
reactions also tend to provide access to the anti-aldol adduct as
the major product from the transformation. While alternate
approaches to accessing enantioenriched anti-aldol adducts do
exist,16,17 organocatalyzed protocols have often proven attrac-
tive based on stereoselectivity and practicality. The mechanistic
underpinnings of the proline-catalyzed aldol reaction transfor-
mation have been previously explored;18 however, the divergent
nature of the stereoselectivities based on catalyst modification
and substrate scope is not fully understood. In this article, we
FIGURE 1. N-(p-Dodecylphenylsulfonyl)-2-pyrrolidinecarboxamide.
provide a full account of our organocatalyzed process for facili-
tating highly enantio- and diastereoselective aldol reactions
using the practical proline mimetic N-(p-dodecylphenylsulfo-
nyl)-2-pyrrolidinecarboxamide (1)19 and a detailed analysis
of the enhanced stereoselectivities of this catalyst (Figure 1).
Results and Discussion
Proline and proline-derived organocatalysts have proven
useful in a range of transformations.20 Our interest in this
field arose during our synthetic work toward the alkaloid
lycopodine (Scheme 1).21,22 We required an organocatalyst
for an intramolecular Michael addition which possessed
both a 2ꢀ amine and an organic acid motif. These efforts
ultimately resulted in the development of a catalyst scaffold
based on a proline sulfonamide. A more detailed discussion
of this transformation has been published elsewhere.23
Solvent Effects. With our development of catalyst 1 for
enantioselective, intramolecular keto-sulfone Michael reac-
tions, we became intrigued by the possibility that this catalyst
scaffold would have more widespread applicability. Given the
considerable importance of the aldol reaction in modern syn-
thetic organic chemistry, the application of our sulfonamide
catalyst system in this setting seemed appropriate.24 While
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SCHEME 1. Development of an Organocatalyzed, Intramolecular Michael Addition and Application to Lycopodine
been reported based on proline, we were struck by the fact that
the vast majority of these catalyst systems employed polar,
aprotic solvent systems (e.g., DMSO, DMF). This solvent
choice is primarily based on the poor solubility of proline and
related derivatives in nonpolar, aprotic solvents. We reasoned
that if improved levels of stereoselectivity could be realized in
nonpolar media, it would allow for the more subtle differentia-
tion between competing transition states.
While use of these polar aprotic solvents (e.g., MeCN,
DMF, DMSO) are common in academic research labora-
tories, their polarity poses serious challenges in industrial
settings with aqueous phase miscibility, product isolation
issues,25 and solvent reclamation.26 DMSO is a particularly
challenging solvent, as solvent recycling on an industrial
scale is typically impossible.27 The development of new
catalysts and processes that use relatively inexpensive, non-
polar solvents is crucial, due to the specific industrial ad-
vantages associated with efficient aqueous phase splits and
ease of recycle at scale.28 Alternatively, the development of
synthetic processes performed in the absence of solvents (or
minimal solvent volumes) can effectively eliminate chemical
waste in industrial processes (in some cases, solvent is always
required to effectively dissipate exotherms and ensure even
heat transfer throughout the medium).29
We first explored the optimization of the catalyst on the
aldol reaction between cyclohexanone and p-nitrobenzal-
dehyde (Table 1). This reaction has become the standard
reaction explored in most organocatalyzed aldol publica-
tions and provided us with a good metric for analysis. We
were primarily interested in developing a protocol that pre-
formed well in nonpolar organic solvents. Given the high solu-
bility of the sulfonamide catalyst in dichloromethane (DCM),
we started our screening process using it as the baseline solvent
(entry 1). While the reaction performed reasonably well in that
solvent, we observed dramatic improvements in yield by switch-
ing the solvent to 1,2-dichloroethane (DCE) (entry 2). We23,30
and others31 have observed similar results by replacement of
DCM with DCE. Reduction of catalyst loading led to a reduc-
tion in chemical efficiency (entry 3). Use of ethanol as a potential
proton source had minimal impact on the transformation
(entries 4-6). Fortunately, the addition of a single equivalent
of water had a dramatic positive impact on the rate and selecti-
vity of the transformation (entry 7). Pihko and others have
observed the significant importance of water in the rate, yield,
and stereoselectivity of the reaction.32 Excess water did not
appear to have a significant additional impact on the transfor-
mation (entry 8). Use of DCE as solvent continued to be
preferred to DCM for achieving optimum levels of diastereos-
electivity (entries 7 and 9). The reaction could alternatively be
performed in water as solvent (entry 10) at reduced catalyst
loading or in 2-methyltetrahydrofuran (2-Me-THF, entry 11)
with excellent levels of selectivity. Reduction in the reaction
temperature to 4 ꢀC led to the outstanding levels of chemical
The proline dodecylphenylsulfonamide 1 has greatly im-
proved solubility properties in nonpolar solvents, as com-
pared to proline, as illustrated in Figure 2. Comparable
weights (300 mg) of proline and our sulfonamide 1 were
mixed with 1.0 mL of dichloromethane at ambient tempera-
ture. Proline is essentially insoluble in this solvent (calculated
solubility <5 mg/mL in CH2Cl2), as were proline tetrazole
and proline phenylsulfonamide. In contrast, dodecylphenyl-
sulfonamide 1 is completely soluble under these conditions.
(27) Vinci, D.; Donaldson, M.; Hallett, J. P.; John, E. A.; Pollet, P.;
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Usano, A.; Pihko, P. M. Synlett 2004, 1891–1896. (b)Pihko, P. M.;Laurikainen,
K. M.; Usano, A.; Nyberg, A. I.; Kaavi, J. A. Tetrahedron 2006, 62, 317–328.
(c) Hayashi, Y.; Sumiya, T.; Takahashi, J.; Gotoh, H.; Urushima, T.; Shoji,
M. Angew. Chem., Int. Ed. 2006, 45, 958–961. (d) Aratake, S.; Itoh, T.;
Okano, T.; Nagae, N.; Sumiya, T.; Shoji, M.; Hayashi, Y. Chem.;Eur. J.
2007, 13, 10246–10256. (e) Li, X.-J.; Zhang, G.-W.; Wang, L.; Hu, M.-Q.;
Ma, J.-A. Synlett 2008, 1255–1259. Effects of water on organocatalyzed
Mannich reactions: (f) Itoh, T.; Yokoya, M.; Miyauchi, K.; Nagata, K.;
Ohsawa, A. Org. Lett. 2003, 5, 4301–4304. (g) Notz, W.; Watanabe, S.;
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Adv. Synth. Cat. 2004, 346, 1131–1140.
FIGURE 2. Solubility comparison of organocatalysts in CH2Cl2.
The left image illustrates that 300 mg of sulfonamide 1 is soluble in 1
mL of CH2Cl2, and the right image demonstrates that 300 mg of
proline is not readily soluble in 1 mL of CH2Cl2.
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J. Org. Chem. Vol. 75, No. 21, 2010 7281

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TABLE 1. Optimization of Reaction Conditionsa
entry
conditionsa
mol % (1)
temp
time (h)
yield (%)
ee %b
drc
1
2
3
4
5
6
7
8
9
10
11
12
13
14d
CH2Cl2
DCE
DCE
DCE/EtOH (99:1)
DCE/EtOH (99:1)
EtOH
DCE, H2O (1 equiv)
DCE, H2O (10 equiv)
CH2Cl2, H2O (1 equiv)
H2O
2-Me-THF, H2O (1 equiv)
DCE/EtOH (99:1)
DCE, H2O (1 equiv)
neat, H2O (1 equiv)
20
20
10
20
10
20
20
20
20
10
20
20
20
2
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
4 ꢀC
4 ꢀC
rt
36
46
36
36
36
36
14
16
16
36
16
36
30
36
51
92
55
96
60
82
96
96
95
95
85
97
95
96
97
95
95
97
95
80
97
98
97
95
94
97
99
96
15:1
12:1
11:1
14:1
18:1
6:1
36:1
31:1
18:1
20:1
30:1
53:1
>99:1e
>99:1
aAll reactions were performed at 2 M concentration of 6 in solution and with 5 equiv of 5. bDetermined by chiral HPLC analysis. cDetermined by 1H NMR
analysis. dPerformed using 2 equiv of 5. e1H NMR expansions are provided in the Supporting Information to demonstrate the diastereomeric selectivity for
product 7.
yield and stereoselectivities with 99% ee, >99:1 dr, and 95%
chemical yield (entry 12). It is important to note that 1 equiv of
water is not readily soluble under the DCE, 4 ꢀC reaction
conditions. Immiscible phases can be visually observed, indicat-
ing that far less than 1 equiv of water is actually present in the
organic phase under these reaction conditions. It is possible
that the organocatalyst is acting in part as a phase transfer
catalyst between the aqueous and organic phases. We have
also developed an alternative procedure requiring just
2 mol % of catalyst loading and reduced equivalents of
the cyclohexanone (entry 14). This protocol was performed
in the absence of solvent at room temperature with a single
of equivalent of water and again provides excellent levels of
chemical yield and stereoselectivity.
Catalyst Structure Effects on Stereoselectivities. Next, we
screened a range of known organocatalysts under our optimized
reaction conditions to provide a comparative analysis. We
recognize that each catalyst structure may have reaction para-
meters which are favorable for optimum performance, but a
standardized set of conditions were important to gauge how
much of the high diastereoselectivity is a function of the unique
structural characteristics of the new sulfonamide catalyst 1. We
chose to look at reaction conditions containing 1% ethanol or 1
equiv of water to provide two separate data sets for evaluation.
With each catalyst we screened, conditions B (which employed 1
equiv of water) provided higher levels of diastereoselectivity
than the ethanol conditions (conditions A). The increased chem-
ical yield with proline under conditions A is likely due to the
improved solubility of proline with 1% ethanol present in the
reaction mixture. As mentioned previously, immiscible phases
are readily formed under conditions B. From the data collected,
it appears that the long alkyl chain on the sulfonamide is acting
principally to improve solubility in the transformations, leading
to improved chemical yields. That said, proline sulfonamides as
a group provided superior levels of diastereoselectivity as
compared to both proline and proline tetrazole.
with proline-sulfonamide-catalyzed aldol reactions (Table 2).
The intermolecular aldol reaction between the proline sulfonam-
ide enamine of cyclohexanone and benzaldehyde was explored
computationally using DFT (B3LYP/6-31G*) geometries and
thermochemistries,33 augmented with SCS-MP2 energies extra-
polated to infinite basis (extrapolated from cc-pVTZ and cc-
pVQZ results).34 The use of the two-point extrapolation to in-
finite basis addresses the problem of basis set truncation, a major
source of error in computations. Solvation single points were
computed for DCM at the B3LYP/6-31þG** level, using the
PCM solvation model and UAKS radii.
The mechanism of the aldol reaction catalyzed by proline has
been reported previously by Houk and co-workers.18 A strong
hydrogen-bonding network between the carboxylic acid and the
aldehyde oxygen in the stereodetermining C-C bond-forming
transition state (TS) is key for stereocontrol and catalysis. In
addition, staggering of the substituents around the forming
C-C bond minimizes steric repulsions in the TS. There are four
possible diastereomeric enamine aldol transition states that meet
these two requirements: TS-anti-Re, TS-anti-Si, TS-syn-Re, and
TS-syn-Si. anti/syn refers to the arrangement of the enamine
with respect to the organic acid moiety while Re/Si denotes the
(33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci,
B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada,
M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.;
Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.;Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski,
J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin,
R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; and Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.:
Wallingford CT, 2004.
€
€
(34) (a) Weigend, F.; Kohn, A.; Hattig, C. J. Chem. Phys. 2002, 116, 3175.
(b) Helgaker, T.; Klopper, W.; Koch, H.; Noga, J. J. Chem. Phys. 1997, 106,
9639–9646.
We have developed a model to explain the considerable im-
provements in diastereoselectivity that are uniquely observed
7282 J. Org. Chem. Vol. 75, No. 21, 2010

Yang et al.
JOCArticle
facial attack of the aldehyde electrophile. The most stable
transition structures are shown in Figure 3.
The major diastereomer arises from the TS-anti-Re and the
minor isomer from the TS-anti-Si. All theoretical methods
employed (B3LYP/6-31G*, B3LYP/6-31G*//SCS-MP2/¥,
and B3LYP/6-31G*//MP2/¥) agree in the substantial ener-
getic preference for TS-anti-Re. Our calculations indicate that
the energetic difference between these two TSs is >4.5 kcal/
mol (or >3.7 with solvation corrections). It should be noted
that the computed difference in this series is 3 kcal/mol more
selective than observed for proline.18
TABLE 2. Comparison of Organocatalystsa
The greatly enhanced diastereoselectivity of proline sulfon-
amide catalysts over proline is due to two nonclassical hydrogen
bonds: between the sulfonamide oxygens and (1) the hydrogens
˚
of the aldehyde electrophile (distance O-HCOPh = 2.9 A) and
(2) the cyclohexyl enamine (distance O-enamineH = 3.9 and
˚
2.6 A) that stabilize the TS-anti-Re C-C bond-forming TS.
In the disfavored TS-anti-Si, these favorable interactions are
replaced by steric repulsions from the intercalating phenyl ring
˚
(analogous distance O-enamineH = 4.9 and 4.0 A). This loss
of stabilizing electrostatic interactions and gain of repulsive
steric interactions in the TS-anti-Si cause the proline sulfonam-
ide catalysts to be ∼3 kcal/mol more selective than other simpler
proline-type catalysts.
entry
conditionsa
catalyst
yield (%)
ee %b
drc
6:1
13:1
2:1
An extensive conformational analysis was performed in
order to determine the most stable nonclassical hydrogen-
bonding pattern by the sulfonamide. Three such arrange-
ments for the TS-anti-Re are shown in Figure 4. The sulfon-
amide conformation found in TS-anti-Re, in which the
sulfonamide oxygens are in close proximity to the aldehyde
hydrogen and the cyclohexyl protons, were found to be much
more stable than TS-anti-Re0 or TS-anti-Re00, in which one of
these stabilizing nonclassical hydrogen-bonding interactions
is missing. This preference is somewhat diminished with
solvation corrections (DCE results shown in Figure 4), as
expected for any electrostatic interactions.
1
2
3
4
5
6
7
8
9
A
B
A
B
A
B
A
B
A
B
A
B
8
8
9
84
22
81
91
34
42
55
49
54
52
97
95
96
98
96
98
97
99
99
98
98
99
97
99
9
7:1
10
10
11
11
12
12
1
24:1
65:1
40:1
83:1
35:1
92:1
53:1
>99:1
10
11
12
1
aConditions A: DCE/EtOH (99:1), 4 ꢀC, 36 h. Conditions B: DCE,
H2O (1 equiv), 4 ꢀC, 30 h. All reactions were performed at 2 M
concentration of 6 in solution and with 5 equiv of 5. bDetermined by
chiral HPLC analysis. cDetermined by 1H NMR analysis.
Nonclassical hydrogen bonds, or electrostatic interactions,
are prevalent in literature. Corey was one of the pioneers to
propose formyl δþCH-Oδ- nonclassical hydrogen bonds as
FIGURE 3. Lowest energy transition structures for the aldol reaction between cyclohexanone enamine of proline-sulfonamide and
benzaldehyde leading to the formation of each of the diastereomeric products;35 anti/syn refers to the arrangement of the enamine with
respect to the organic acid moiety while Re/Si denotes the facial attack of the aldehyde electrophile. Distances are in angstroms, energies in
kcal/mol. Green lines designate possible stabilizing electrostatic interactions. Structures and thermodynamic corrections computed using
B3LYP/6-31G* in the gas phase. Numbers in parentheses include solvation corrections for DCE; ¥ designates infinite basis set extrapolation
from the Dunning cc-pVTZ and cc-pVQZ basis sets.
J. Org. Chem. Vol. 75, No. 21, 2010 7283

Yang et al.
JOCArticle
FIGURE 4. Three anti-Re transition structures for the aldol reaction between cyclohexanone enamine of proline sulfonamide and
benzaldehyde, showing three different conformations of the sulfonamide.35 Distances are in angstroms, energies in kcal/mol. Green lines
designate possible stabilizing electrostatic interactions. Structures and thermodynamic corrections computed using B3LYP/6-31G* in the gas
phase. Numbers in parentheses include solvation corrections for DCE; ¥ designates infinite basis set extrapolation from the Dunning cc-pVTZ
and cc-pVQZ basis sets.
TABLE 3. Comparison of Proline Sulfonamidesa
stabilizing interactions.36 Nonclassical hydrogen bonds have
been subsequently invoked by other groups to explain stabiliz-
ing relationships.37 With respect to organocatalysis, Houk and
co-workers pointed out that stabilizing electrostatic interactions
between the CH vicinal to the forming iminum and the forming
alkoxide (referred to as the δþNCH-Oδ- interactions) con-
tributes to the stereoselectivity of proline-catalyzed aldol reac-
tions.18a These nonclassical hydrogen-bonding electrostatic in-
teractions were also demonstrated as stabilizing interactions in
stereoselective additions of chiral alcohols to ketenes.38 The
magnitude of these stabilizing interactions has also been quan-
tified and has been demonstrated to be significant even in the
solution.39
We did consider the possibility that the enhanced diaster-
eoselectivity of the proline sulfonamide catalysts over proline
may be due to increased torsional strain around the forming
bond that stabilizes the TS-anti-Re over other transition states.
Interestingly, juxtaposing the proline sulfonamide transi-
tion structures with those published for proline,18b we find
no evidence that proline sulfonamide transition structures
exhibit drastically different torsional strain than proline transi-
tion structures. The torsional differences that do exist cannot
explain the >3 kcal/mol greater selectivity exhibited by proline
sulfonamides compared to proline.
entry
catalyst
time (h)
yield (%)
ee %b
drc
1
2
3
4
1
13
14
15
16
60
18
24
96
97
96
88
97
97
d
36:1
26:1
1.5:1
2.4:1
Variation of Sulfonamide Scaffold. We also explored addi-
tional modification of the sulfonamide scaffold (Table 3).
For this series, the aldol reaction was conducted at room
d
aAll reactions were performed at 2 M concentration of 6 in solution
and with 5 equiv of 5. bDetermined by chiral HPLC analysis. Deter-
c
(35) Legault, C. Y. CYLview BETA 1.0, Copyright 2006-2010.
(36) (a) Corey, E. J.; Rohde, J. J.; Fischer, A.; Azimioara, M. D. Tetra-
hedron Lett. 1997, 38, 33–36. (b) Corey, E, J.; Rohde, J. J. Tetrahedron Lett.
1997, 38, 37–40. (c) Corey, E. J.; Barnes-Seeman, D.; Lee, T. W. Tetrahedron
Lett. 1997, 38, 1699–1702. (d) Corey, E. J.; Barnes-Seeman, D.; Lee, T. W.
Tetrahedron Lett. 1997, 38, 4351–4354.
(37) For a selected subset of examples, see: (a) Gung, B. W.; Xue, W.;
Roush, W. R. J. Am. Chem. Soc. 2002, 124, 10692–10697. (b) Fujiyama, R.;
Goh, K.; Kiyooka, S. I. Tetrahedron Lett. 2005, 46, 1211–1215. (c) Paton,
R. S.; Goodman, J. M. Org. Lett. 2006, 7, 4299–4302. (d) Paton, R. S.;
Goodman, J. M. J. Org. Chem. 2008, 73, 1253–1263.
1
d
mined by H NMR analysis. Not determined due to poor diastereos-
electivity.
temperature. The baseline experiment with the sulfonamide
1 provides high chemical yield, diastereoselectivity, and
enantioselectivity (entry 1). Recently, we reported the devel-
opment of a second generation (compound 13) of our cata-
lyst system in which a dodecyl ester is placed on the aromatic
ring.40 This catalyst proved more effective than catalyst 1 in
(38) Cannizzaro, C. E.; Houk, K. N. J. Am. Chem. Soc. 2004, 126, 10992–
11008.
(39) Cannizzaro, C. E.; Houk, K. N. J. Am. Chem. Soc. 2002, 124, 7163–
7169.
(40) Yang, H.; Carter, R. G. Org. Lett. 2010, 12, 3108–3111.
7284 J. Org. Chem. Vol. 75, No. 21, 2010

Yang et al.
JOCArticle
FIGURE 5. Effect of carbonyl motif on catalyst performance.
the formation of cyclohexanones containing a stereogenic
all-carbon quaternary center. Interestingly, this catalyst 13
proved significantly less reactive than its parent 1 in the aldol
reaction (entry 2). While the enantioselectivity and dias-
tereoselectivity were comparable to catalyst 1, a nearly 4-fold
decrease in reactivity was observed. This difference is likely due
to the presumed increased acidity of the sulfonamide N-H in
the modified catalyst 13. In addition to this electronic impact,
catalyst 13 is also less soluble than catalyst 1. This solubility
difference may be explained by the difference in the dode-
cyl chain;linear, homogeneous C12H25 chain in catalyst 13
and a mixture of branched and linear isomers (e.g., 1-dodecyl,
2-doceyl, etc.) in catalyst 1. As a complement to this study,
we also explored the removal of the carbonyl of the amide carbon
(catalyst 14). This previously unknown catalyst 14 led to dra-
matically lower levels of diastereoselectivity (entry 3). Removal
of the amide carbonyl should decrease the sulfonamide N-H
acidity. Finally, we also probed the placement of an additional
Lewis base on the sulfonamide scaffold through pyridyl sulfon-
amide 15. While catalyst 15 had not been previously prepared, a
closely related desmethyl version was utilized by Nakamura and
co-workers in an aldol reaction between acetaldehyde and 4,6-
dibromoisatin with modest success (77% yield, 80% ee).41 In our
system, pyridyl catalyst 15 gave greatly reduced diastereoselec-
tivity in the aldol reaction (entry 4). These experiments point to
the delicate balance between the nature of the sulfonamide
moiety and reaction rate and diastereoselectivity.
moiety in the anti-Re TS is ideally suited to benefit energetically
from this conformational freedom. This is in contrast to the Re-
face approach, in which the sterically bulky phenyl group is
universally poorly accommodated by the any conformation of
the sulfonamide (Figure 3). Thus, sulfonamides in catalysts 1
and 13 are conformationally more flexible and can change
conformations to better electrostatically suit the anti-Re TS,
leading to high selectivity. The sulfonamide in catalyst 14 is more
conformationally rigid (the double bond between NdS makes
the sulfonamide more rigid; top, Figure 5) and poorly stabilizes
the anti-Re TS, leading to the observed poor selectivity.
The lower diastereoselectivities observed with catalysts 14 and
15 can be rationalized through analysis of their structural fea-
tures. In catalyst 14, the sulfonamide is the only functional group
stabilizing the nitrogen anion. We hypothesize that the remark-
able degradation in diastereoselectivity of catalyst 14 is due
to the sulfonamide conformation being locked to achieve this
conjugation. This in effect results in the loss of electrostatic
distinction between the anti-Re and anti-Si TS and leads to loss
of selectivity. The change from sp2 to sp3 hybridization at the
starred carbon in catalyst 14 likely introduces flexibility in the
sulfonamide group that may decrease the differentiation be-
tween the anti-Re and anti-Si TS, leading to lower selectivities.
Wang and co-workers have shown that further increase in the
electron-withdrawing capacity of the sulfonamide scaffold by
incorporation of a trifluoromethyl moiety can counterbalance
this effect.24n In catalyst 15, we believe the additional internal
base may participate in the key proton transfer event of the
critical C-C bond-forming transition state, as previously sug-
gested by Nakamura.41 Such participation will impart confor-
mational rigidity to the sulfonamide moiety, leading to poor
selectivity.
We hypothesize that the sulfonamide conformation is key for
stereocontrol. The electronics of the sulfonamide moiety is
critical in determining this conformation of the sulfonamide
(Figure 5). In catalysts 1 and 13, the attenuation of the devel-
oping negative charge on the nitrogen via conjugation to the
carbonyl allows the sulfonamide conformational freedom to
electrostatically complement the incoming electrophile in the
critical C-C bond-forming TS (bottom, Figure 5). The expo-
sure of the aldehyde hydrogen to the catalyst organic base
Alternate explanations for the differences in selectivities
between the various sulfonamide catalysts are possible. One
alternative is based on the pKa differences between the
various catalysts. The pKa values of sulfonamides are gen-
erally viewed as similar to those of carboxylic acids24c and
can conceivably be modulated through either the addition of
electron-withdrawing or electron-donating moiety on the
(41) (a) Hara, N.; Nakamura, S.; Shibata, N.; Toru, T. Chem.;Eur. J.
2009, 15, 6790–6793. (b) Nakamura, S.; Hara, N.; Nakashima, H.; Kubo, K.;
Shibata, N.; Toru, T. Chem.;Eur. J. 2008, 14, 8079–8081.
J. Org. Chem. Vol. 75, No. 21, 2010 7285

Yang et al.
JOCArticle
TABLE 4. Scope of Aldehyde Moiety at 4ꢀCa
TABLE 5. Scope of Aldehyde Moiety at Room Temperaturea
yield %
time (h) (product) ee %b
yield %
time (h) (product) ee %b
drc
entry
R
drc
entry
R
1
2
3
4
5
6
pentafluorophenyl-
2,4-dinitrophenyl-
2,6-dichlorophenyl-
4-pyridyl-
2-naphthyl-
phenylethyl-
13
40
48
12
48
72
97 (17)
99 (17)
99 (17)
92 (17)
63 (17k)
32 (17l)
>99
96
99
99
>99
59
>99:1
57:1
>99:1
50:1
>99:1
29:1
a
b
c
d
e
f
g
h
i
pentafluorophenyl-
2,4-dinitrophenyl-
2,6-dichlorophenyl-
4-chlorophenyl-
2-nitrophenyl-
2-chlorophenyl-
4-methoxyphenyl-
phenyl-
36
72
72
48
72
72
72
60
24
72
72
91 (17a)
51 (17b)
94 (17c)
68 (17d)
91 (17e)
79 (17f)
16 (17g)
60 (17h)
98 (17i)
19 (14j)
51 (14k)
>99
97
99
99
99
98
98
99
99
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
80:1
>99:1
29:1
>99:1
>99:1
aAll reactions were performed at 2 M concentration of 16 in solution
and with 5 equiv of 5. bDetermined by chiral HPLC analysis. Deter-
c
mined by 1H NMR analysis.
4-pyridyl-
1-naphthyl-
2-naphthyl-
j
k
99
>99
aAll reactions were performed at 2 M concentration of 16 in solution
SCHEME 2. Scope of Aldehyde Moiety at 2-Methyltetrahy-
c
and with 5 equiv of 5. bDetermined by chiral HPLC analysis. Deter-
drofuran as Solventa
mined by 1H NMR analysis.
sulfonamide aromatic ring40 or modification of the carbonyl
moiety. Wu and co-workers have previously postulated that
the repulsive interaction between the proton being trans-
ferred and the electrophile substituent destabilize the anti-Si
TS and could contribute to the enhanced selectivity of pro-
line-amide-catalyzed aldol reactions.42 The modulation of
the sulfonamide pKa could effect this interaction and may be
a factor in the resulting differential selectivities.
Substrate Scope. We next set out to screen a range of
aromatic aldehydes to probe the scope of this transformation
(Table 4). In the vast majority of cases, the transformation
performed remarkably well. High levels of enantioselectivity,
diastereoselectivity, and chemical yield were observed in most
cases. Aldol reactions on even sterically congested alde-
hydes 16b and 16c proved reasonably effective. Single ortho
substitution did not appear to adversely impact reactivity
(entries b,c and e,f). Current limitations to the protocol
appear to include electron-rich aldehydes, which showed
sluggish reactivity (entry g).
We also wanted to screen the scope of these reactions per-
formed at room temperature (Table 5). While it was expected
that increased reaction temperature would increase reactivity
while possibly reducing stereoselectivity, the added practicality
of avoiding externally cooling a reaction was attractive. For-
tunately, the overall stereoselectivities in most cases at room
temperature were still high. In fact, the dinitrophenyl case gave
significantly higher yields at room temperature than at 4 ꢀC
(entry 2). One substrate class that was ineffective at lower
temperatures was aliphatic aldehydes. Fortunately, it appears
that improved reactivity provided by conducting the experi-
ment at room temperature did facilitate a modestly effective
transformation with 59% ee and 32% chemical yield (entry 6).
These types of aldehydes are particularly challenging as self-
aldol condensation is often a major competing transforma-
tion.43 Currently, 4-hydroxyproline derivatives have proven
most effective at facilitating related aldol reactions with alipha-
tic aldehydes.32c,d
aAll reactions were performed at 2 M concentration of 16 in solution
and with 5 equiv of 5. Enantiomeric excess was determined by chiral
HPLC analysis. Diastereomeric ratios (dr) were determined by 1H NMR
analysis.
While DCE proved to be an excellent solvent for facilitat-
ing highly stereoselective aldol reactions, it has been desig-
nated a class 1 genotoxin.44,45 Consequently, we sought to
explore alternative solvent choices, which might be viewed as
more industrial friendly (Scheme 2). 2-Methyltetrahydrofur-
an (2-Me-THF) is rapidly becoming an industrially friendly
alternative to chlorinated solvents.46 The cost of production
of this solvent has come down significantly in recent years.
Unlike THF, 2-Me-THF provides good phase splits with
water, which can facilitate easier solvent recycling. We were
pleased to see that 2-Me-THF can be substituted for DCE
with only minimal disruption in the levels of enantioselec-
tivity. The experiments were conducted at room temperature
and provided high levels of chemical yield, enantioselectivity,
and diastereoselectivity. These examples coupled with the
entry detailed in Table 1 indicate that there is significant
promise in utilizing the catalyst 1 in nonchlorinated solvents
such as 2-Me-THF.
We also explored the utility of neat reaction conditions using
the low catalyst loading conditions (2 mol % of 1, neat, rt) on a
range of aldehyde substituents (Table 6). Please note that these
€
(44) Muller, L.; Mauthe, R. J.; Riley, C. M.; Andino, M. M.; De Antonis,
D.; Beels, C.; DeGeorge, J.; De Knaep, A. G. M.; Ellison, D.; Fagerland,
J. A.; Frank, R.; Fritschel, B.; Galloway, S.; Harpur, E.; Humfrey, C. D. N.;
Jacks, A. S.; Jagota, N.; Mackinnon, J.; Mohan, G.; Ness, D. K.; O’Donovan,
M. R.; Smith, M. D.; Vudathala, G.; Yotti, L. Regul. Toxicol. Pharmacol. 2006,
44, 198–211.
(42) Tang, Z.; Jiang, F.; Yu, L.-T.; Cui, X.; Gong, L.-Z.; Mi, A. Q.; Jiang,
Y.-Z.; Wu, Y.-D. J. Am. Chem. Soc. 2003, 125, 5262–5263.
(43) List, B.; Pojarliev, P.; Castello, C. Org. Lett. 2001, 3, 573–575.
(45) (a) Robinson, D. I. Org. Process Res. Dev. 2010, 14, 946–959. (b)
McGovern, T.; Jacobson-Kram, D. Trends Anal. Chem. 2006, 25, 790–795.
(46) Aycock, D. F. Org. Process Res. Dev. 2007, 11, 157–159.
7286 J. Org. Chem. Vol. 75, No. 21, 2010

Yang et al.
JOCArticle
SCHEME 3. Large-Scale Example of Aldol Reactiona
aEnantiomeric excess was determined by chiral HPLC analysis. Diastereomeric ratios (dr) were determined by 1H NMR analysis.
TABLE 6. Scope of Aldehyde Moiety under Low Catalyst Loadinga
SCHEME 4. Use of Substituted Cyclohexanones in Aldol Re-
actionsa
yield %
(product) ee %b
entry
R
time (h)
drc
1
2
3
4
5
6
7
2,4-dinitrophenyl-
4-chlorophenyl-
2-chlorophenyl-
4-hydroxyphenyl-
phenyl-
48
48
72
72
48
36
72
50 (17b)
49 (17d)
67 (17f)
22 (17m)
75 (17h)
91 (17i)
36 (17j)
93
92
99
90
99
91
88
>99:1
>99:1
>99:1
26:1
55:1
7:1
aAll reactions were performed at 2 M concentration of 6 in solution and
5 equiv of ketone 18. Enantiomeric excess was determined by chiral HPLC
analysis. Diastereomeric ratios (dr) were determined by 1H NMR analysis.
4-pyridyl-
1-naphthyl-
96:1
aAll reactions were performed with 2 equiv of 5. bDetermined by
cyclohexanones. In each case, high levels of stereoselectivity and
chemical yield were observed. In each experiment, a total of
three stereogenic centers are established.
chiral HPLC analysis. cDetermined by 1H NMR analysis.
2 mol % catalyst experiments were performed with a reduced
loading of 5 (2 equiv) and only a single equivalent of both the
aldehyde and water. These transformations generally performed
well, with enantioselectivities greater than 90% ee in each case.
The diastereoselectivity in neat reactions was somewhat reduced
as compared to the experiments conductedinanorganicsolvent.
We attribute that result to the impact of the solvent’s polarity on
the diastereomeric transition states as compared to the neat
component reactions. It should be noted that, in most cases,
both 5 and the aldehyde 16 were not readily miscible with the 1
equiv of water. We hypothesize that the polar pyrrolidine
portion and the aliphatic side arm of the sulfonamide catalyst
may function as a phase transfer agent for this transformation.
Given the procedural ease for accomplishing these aldol
reactions, we thought it would be valuable to demonstrate
the utility of this protocol for conducting a mole scale
reaction (Scheme 3). The experiment was conducted in a
single 500 mL round-bottom capped flask using 235 g of
cyclohexanone and 181 g of 4-nitrobenzaldehyde with just
2 mol % (10.1 g) of the sulfonamide catalyst 1 and a single
equivalent of water (21.6 mL). Shown in Scheme 3 is the
photograph of the experiment. The solid stirring around in
the reaction is the product 7, which was filtered and washed
with hexanes to isolate. Over 60% of the catalyst 1 could be
recovered via simple purification of the mother liquor
through a plug of silica gel followed by a single recrystalliza-
tion in methanol.
The use of acetone as a potential nucleophile in the aldol
process was also screened (Table 7). This reaction appeared to
work most effectively using our optimized conditions [DCE,
H2O (1equiv), 4 ꢀC] to provide 66% yield of the desired product
in a reasonable (84% ee) enantioselectivity. Performing the
transformation at room temperature led to reduced chemical
yields and enantioselectivity. Interestingly, the same reaction
performed in the absence of added water gave comparable levels
of enantioselectivity to the optimized conditions. We are unsure
as to the rationale of this result, but this outcome may be in part
due to the hydroscopic nature of acetone, which was not
rigorously dried prior to use.
We explored the impact of utilizing cyclopentanone in-
stead of cyclohexanone in the aldol process (Table 8).
Interestingly, the level of diastereoselectivty (anti/syn) was
greatly eroded as compared to the cyclohexanone examples.
This diastereoselectivity difference between cyclopentanone
and cyclohexanone was first observed by Gong47 and Zhao48
independently; however, we are unaware of a mechanistic
explanation for the difference outside of the increased
reactivity of cyclopentanone-derived enamines.49 We screened
both the low catalyst loading conditions [2 mol % 1, H2O
(1 equiv), neat] and the optimized 20 mol % catalyst loading
(47) Tang, Z.; Yang, Z.-H.; Chen, X.-H.; Cun, L.-F.; Mi, A.-Q.; Jiang,
Y.-Z.; Gong, L.-Z. J. Am. Chem. Soc. 2005, 127, 9285–9289.
(48) Samantha, S.; Liu, J.; Dodda, R.; Zhao, C.-G. Org. Lett. 2005, 7,
5321–5323.
(49) Shechter, H.; Collis, M. J.; Dessy, R.; Okuzumi, Y.; Chen, A. J. Am.
Chem. Soc. 1962, 84, 2905–2910.
We investigated the impact of substitution of the cyclohex-
anone ring in the aldol process (Scheme 4). This reaction worked
similarly well with either the methyl or tert-butyl-substituted
J. Org. Chem. Vol. 75, No. 21, 2010 7287

Yang et al.
JOCArticle
TABLE 7. Use of Acetone in Organocatalyzed Reactiona
entry
aldehyde
mol % of 1
conditions
time (h)
yield (%)
ee %b
1
2
6
6
6
6
16h
20
20
20
2
DCE, H2O (1 equiv), 4 ꢀC
DCE/EtOH (99:1), 4 ꢀC
DCE, rt
neat, H2O (1 equiv), rt
DCE/EtOH (99:1), 4 ꢀC
62
72
30
48
72
66
64
80
52
42
84
82
80
71
87
3
4c
5
20
aAll reactions were performed at 2 M concentration of aldehyde in solution and with 5 equiv of 20 unless otherwise noted. bDetermined by chiral
HPLC analysis. cThis reaction was performed with 2 equiv of 20.
TABLE 8. Use of Cyclopentanone in Aldol Reactiona
entry
aldehyde
mol % (1)
conditions
yield (%)
ee %b
drc
1d
3
4
5
6
6
6
6
16f
2
2
20
20
20
neat, H2O (1 equiv), 15 h, rt
neat, 24 h, rt
DCE, H2O (1 equiv), 16 h, 4 ꢀC
DCE/EtOH (99:1), 36 h, 4 ꢀC
DCE, H2O (1 equiv), 48 h, 4 ꢀC
87
63
94
87
51
77 (syn), 91 (anti)
73 (syn), 74 (anti)
87 (syn), 95 (anti)
85 (syn), 92 (anti)
81 (syn), 93 (anti)
1.05:1
3.2:1
1.35:1
2:1
6
1.5:1
aAll reactions were performed at 2 M concentration of aldehyde in solution and with 5 equiv of 22 unless otherwise noted. bDetermined by chiral
HPLC analysis. cDetermined by 1H NMR analysis. dThis reaction was performed with 2 equiv of 22.
TABLE 9. Use of 4-Pyranone in Aldol Reactionsa
conditions [DCE, H2O (1 equiv), 4 ꢀC]. Under both sets of con-
ditions, similar results were obtained.
We explored the potential use of 4-pyranone in the aldol
reaction process with a series of aromatic aldehydes (Table 9).
4-Pyranones are present in a wide range of natural product
scaffolds.50 Xiao and co-workers have focused specifically on
exploiting the utility of organocatalysis for the heteroatom-
containing cyclic ketone substrates.51 In each case, our opti-
mized reaction conditions [DCE, H2O (1 equiv), 4 ꢀC] provided
superior results to alternative procedures performed at room
time yield ee
temperature or using alternative additives. Aldehyde 16d was
particularly effective as excellent levels of enantioselectivity and
diastereoselectivity were achieved (entry 4).
b
entry aldehyde
conditions
(h) (%)
%
drc
4:1
5:1
13:1
1
2
3
4
5
6
6
6
16d
16d
16h
13h
DCE/EtOH (99:1), rt
16
16
92
52
60
27
47
88
96
94
DCE, H2O (1 equiv), 4 ꢀC 52
DCE, H2O (1 equiv), rt 16
DCE, H2O (1 equiv), 4 ꢀC 72
DCE, H2O (1 equiv), rt 16
DCE, H2O (1 equiv), 4 ꢀC 72
The potential use of 4-thiopyranone in the aldol reac-
tion process was also explored (Scheme 5). Thiopyranones
have been exploited as polyproprioate surrogates through
97 >99:1
90
94
41:1
41:1
aAll reactions were performed at 2 M concentration of aldehyde in
solution and with 5 equiv of 24. bDetermined by chiral HPLC analysis.
cDetermined by 1H NMR analysis.
(50) (a) Turner, W. B. J. Chem. Soc., Perkin Trans. 1 1978, 1621. (b)
Goehrt, A.; Grabley, S.; Thiericke, R.; Zeeck, A. Liebigs Ann. 1996, 627–634.
(c) Pittayakhajonwut, P.; Suvannakad, R.; Thienhirun, S.; Prabpai, S.;
Kongsaeree, P.; Tanticharoen, M. Tetrahedron Lett. 2005, 46, 1341–1344.
(d) Lopez, S. N.; Sierra, M. G.; Gattuso, S. J.; Furlan, R. L.; Zacchino, S. A.
Phytochem. 2006, 67, 2152–2158. (e) Krick, A.; Kehraus, S.; Gerhaeuser, C.;
Klimo, K.; Nieger, M.; Maier, A.; Fiebig, H.-H.; Atodiresei, I.; Raabe, G.;
Fleischhauer, J.; Koenig, G. M. J. Nat. Prod. 2007, 70, 353–360. (f) Ahmed,
S. A.; Ross, S. A.; Slade, D.; Radwan, M. M.; Khan, I. A.; ElSohly, M. A.
Tetrahedron Lett. 2008, 49, 6050–6053. (g) Tantray, M. A.; Shawl, A. S.; Ali,
N.; Khuroo, M. A. Chem. Nat. Prod. 2008, 44, 424–426.
Raney-Ni removal of the carbon-sulfur bonds after coupling.52
The aldol reaction proved sufficiently effective that they could
be performed at room temperature with high levels of stereo-
selectivity. We attribute the difference between pyranone and
thiopyranone to the modulated reactivity imparted by the
presence of the sulfur moiety.
(51) Chen, J.-R.; Li, X.-Y.; Xing, X.-N.; Xiao, W.-J. J. Org. Chem. 2006,
71, 8198–8202.
(52) (a) Jheengut, V.; Ward, D. E. J. Org. Chem. 2007, 72, 7805–7808. (b)
Beye, G. E.; Goodman, J. M.; Ward, D. E. Org. Lett. 2009, 11, 1373–1376.
Glycolate aldol reactions have proven particularly significant
for construction of 1,2-anti- and 1,2-syn-diol functionality.53
7288 J. Org. Chem. Vol. 75, No. 21, 2010

Yang et al.
JOCArticle
SCHEME 5. Use of 4-Thiopyranone in Aldol Reactionsa
with generally high levels of stereoselectivity. The practicality of
this protocol has been demonstrated on a 1 mol scale reaction
conducted in a single 500 mL round-bottom flask. Detailed
computational analysis established that nonclassical hydrogen
bonds to the sulfonamide lead to improved stereoselectivity
in this system. We anticipate that this catalyst system will
find application in natural product synthesis and industrial
processes.
Experimental Section
Sulfonamide 14: To a solution of 1 (0.422 g, 1.00 mmol) in
THF (10 mL) was added lithium aluminum hydride (0.190 g,
5.0 mmol) slowly at 0 ꢀC. After stirring at 0 ꢀC for 2 h, the
solution was quenched with H2O (10 mL) and extracted with
CH2Cl2 (3 ꢀ 25 mL). The dried (Na2SO4) extract was concen-
trated in vacuo and purified by chromatography over silica gel,
eluting with 10% MeOH/dichloromethane, to give 14 (0.346 g,
0.848 mmol, 85%) as a colorless oil: [R]23D = þ21.4 (c = 1.9,
CHCl3); IR (neat) 3276, 2954, 2916, 2856, 1451, 1408, 1331,
1146, 1097, 825, 645 cm-1; 1H NMR 400 MHz, CDCl3) δ 7.77
(d, J = 7.6 Hz, 2H), 7.27-7.33 (m, 2H), 3.87 (br s, 2H),
3.31-3.33 (m, 1H), 2.57-3.04 (m, 4H), 0.75-1.86 (m, 28H);
13C NMR (100 MHz, CDCl3) δ 137.3, 128.3, 127.7, 127.0, 57.2,
47.2, 46.3, 40.0, 38.9, 38.2, 36.7, 36.4, 31.9, 29.7, 29.3, 28.9, 27.5,
27.2, 25.8, 22.7, 22.0, 20.6, 14.1, 12.1; HRMS (EIþ) calcd for
C23H40N2O2S (Mþ) 408.2811, found 408.2787.
aAll reactions were performed at 2 M concentration of aldehyde in
solution and with 5 equiv of 26. Enantiomeric excess was determined by
chiral HPLC analysis. Diastereomeric ratios were determined by 1H
NMR analysis.
SCHEME 6. Use of Glycolates in Aldol Reactionsa
aAll reactions were performed at 2 M concentration of 6 in solution
and with 5 equiv of 24. Enantiomeric excess was determined by chiral
HPLC analysis. Diastereomeric ratios (dr) were determined by 1H NMR
analysis.
We were pleased to see that our organocatalyzed process could
again provide reasonable levels of stereoselectivity in this type of
transformation (Scheme 6). While the chemical yield and en-
antioselectivity appear for this transformation to be insensitive to
reaction temperature, the diastereoselectivity was improved
when the reaction was carried out at lower temperatures.
(Z)-L-Sulfonamide 32: To a solution of (Z)-L-proline 30 (1.25
g, 5.0 mmol) in CH2Cl2 (50 mL) were added sulfonamide 31
(1.04 g, 6.0 mmol), EDCI (0.96 g, 5.0 mmol), and DMAP (0.122
g, 1.0 mmol). The reaction mixture was stirred at room tem-
perature for 48 h before being partitioned between EtOAc (100
mL) and water (50 mL). The organic layer was washed with half-
saturated brine (2 ꢀ 50 mL). The dried (Na2SO4) extract was
concentrated in vacuo and purified by chromatography over
silica gel, eluting with 20-70% EtOAc/CH2Cl2, to give 32 (1.59
Conclusion
A practical asymmetric aldol process has been developed
based on the proline aryl sulfonamide scaffold. These reac-
tions have been shown to be effective on a wide range of
substrates in generally high levels of enantioselectivity. The
presence of a single equivalent of water was shown to be
beneficial to the reaction yield, enantioselectivity, and dia-
stereoselectivity. Particular utility has been showcased in
nonpolar organic solvents such as DCE and 2-Me-THF.
Nonpolar solvents increase the practicality of the chemistry
for applications in industrial settings as the solvents are more
readily recyclable. Alternatively, the sulfonamide-catalyzed
aldol reaction was also shown to be effective in aqueous
media with generally good levels of stereoselectivity. We
hypothesized that the proline sulfonamide organocatalyst
may be acting as a phase transfer catalyst between the
aqueous and nonpolar media. Catalyst loading as low as
2 mol % was demonstrated using neat reaction conditions
g, 3.95 mmol, 79%) as a white solid: mp 136-137 ꢀC; [R]23
=
D
-129.7 (c = 3.1, CHCl3); IR (neat) 3069, 2953, 2871, 1704, 1684,
1455, 1420, 1357, 1190, 1132, 1108, 739, 700, 665 cm-1 1H
;
NMR (400 MHz, DMSO-d6, two rotamers) δ 8.37 (s, 1H), 8.30
(s, 1H), 7.59-7.76 (m, 2H), 7.21-7.35 (m, 5H), 4.78-5.03 (m,
2H), 4.02-4.06 (m, 1H), 2.33 (s, 3H), 2.27 (s, 3H), 1.71-2.10 (m,
3H); 13C NMR (100 MHz, MeOD, two rotamers) δ 181.3, 155.9,
155.6, 154.9, 148.9, 148.7, 139.5, 139.3, 137.9, 137.8, 136.6,
128.2, 128.0, 127.7, 127.3, 127.2, 126.2, 127.7, 66.7, 66.2, 62.5,
62.1, 47.1, 46.7, 31.1, 30.3, 23.8, 23.1, 17.2; HRMS (EIþ) calcd
for C19H21N3O5S (Mþ) 403.1202, found 403.1219.
Sulfonamide 15: To a solution of (Z)-L-sulfamide 32 (0.12 g,
0.298 mmol) in MeOH (3 mL) was added Pd/C (12 mg, 10%).
The mixture was stirred at rt under an atmosphere of hydrogen.
After 13 h, the reaction was filtered through Celite and a silica
gel pad, and the filtrate was concentrated in vacuo to give a
white solid. The crude product was purified by chromatography
over silica gel, eluting with 10% MeOH/CH2Cl2, to give the
product 15 (71.2 mg, 0.265 mmol, 89%) as a white solid: mp
192-194 ꢀC; [R]23D = -44.5 (c = 0.2, EtOH); IR (neat) 3432,
3070, 3031, 2992, 1610, 1564, 1455, 1381, 1315, 1264, 1248, 1151,
1089, 789, 696, 649, 560 cm-1; 1H NMR (400 MHz, MeOD) δ
8.41 (s, 1H), 7.97 (d, J = 8.0 Hz, 1H), 7.80 (d, J = 7.2 Hz, 1H),
(53) (a) Crimmins, M. T.; She, J. J. Am. Chem. Soc. 2004, 126, 12790–
12791. (b) Vaughn, J. F.; Hitchcock, S. R. Tetrahedron: Asymmetry 2005, 15,
3449–3455. (c) Davies, S. G.; Nicholson, R. L.; Smith, A. D. Org. Biomol.
Chem. 2005, 3, 348–359. (d) Andrus, M. B.; Liu, J.; Ye, Z.; Cannon, J. F. Org.
Lett. 2005, 7, 3861–3864. (e) Denmark, S. E.; Chung, W.-J. J. Org. Chem.
2008, 73, 4582–4595. (f) Fanjul, S.; Hulme, A. N. J. Org. Chem. 2008, 73,
9788–9791. (g) Gawas, D.; Kazmaier, U. J. Org. Chem. 2009, 74, 1788–1790.
J. Org. Chem. Vol. 75, No. 21, 2010 7289

Yang et al.
JOCArticle
4.02-4.06 (m, 1H), 3.21-3.39 (m, 2H), 2.45 (s, 3H), 1.93-2.42
(m, 4H); 13C NMR (100 MHz, MeOD) δ 173.3, 157.3, 148.9,
137.8, 136.5, 121.9, 62.3, 45.8, 29.2, 23.4, 16.9; HRMS (ESþ)
calcd for C11H16N3O3S (M þ 1) 270.0895, found 270.0912.
General Procedure for Aldol Reaction. Procedure A (DCE, 20
mol % of Catalyst, 4 ꢀC): To a solution of aldehyde (0.5 mmol)
and cyclohexanone (0.245 g, 0.26 mL, 2.5 mmol, 5 equiv) in
DCE (0.24 mL) were added sulfonamide 123 (42.2 mg, 0.1 mmol)
and water (0.5 mmol, 9 mg, 1 equiv) at 4 ꢀC. After stirring at the
same temperature, the reaction was loaded directly onto silica
gel and was purified by chromatography, eluting with 10-30%
EtOAc/hexanes, to give the corresponding aldol product.
General Procedure for Aldol Reaction. Procedure B (DCE,
20 mol % of Catalyst, Room Temperature): To a solution of
aldehyde (0.5 mmol) and cyclohexanone (0.245 g, 0.26 mL, 2.5
mmol, 5 equiv) in DCE (0.24 mL) were added sulfonamide 123
(42.2 mg, 0.1 mmol) and water (0.5 mmol, 9 mg, 1 equiv) at room
temperature. After stirring at the same temperature, the reaction
was loaded directly onto silica gel and was purified by chroma-
tography, eluting with 10-30% EtOAc/hexanes, to give the
corresponding aldol product.
General Procedure for Aldol Reaction. Procedure C (Neat,
2 mol % of Catalyst, Room Temperature): To a solution
of aldehyde (0.5 mmol) and cyclohexanone (0.098 g, 0.1 mL,
1.0 mmol, 2 equiv) were added sulfonamide 123 (4.2 mg, 0.01
mmol) and water (0.5 mmol, 9 mg, 1 equiv) at room tempera-
ture. After stirring at the same temperature, the reaction was
loaded directly onto silica gel and was purified by chromato-
graphy, eluting with 10-30% EtOAc/hexanes, to give the
corresponding aldol product.
General Procedure for Aldol Reaction. Procedure D (DCE/
EtOH (99:1), 20 mol % of Catalyst, Room Temperature): To a
solution of aldehyde (0.5 mmol) and cyclohexanone (0.245 g,
0.26 mL, 2.5 mmol, 5 equiv) in DCE/EtOH (99:1, 0.24 mL) was
added sulfonamide 123 (42.2 mg, 0.1 mmol) at room tempera-
ture. After stirring at the same temperature, the reaction was
loaded directly onto silica gel and was purified by chromato-
graphy, eluting with 10-30% EtOAc/hexanes, to give the corres-
ponding aldol product.
General Procedure for Aldol Reaction. Procedure E (2-Methyl-
tetrahydrofuran, 20 mol % of Catalyst, Room Temperature): To a
solution of aldehyde (0.5 mmol) and cyclohexanone (0.245 g,
0.26 mL, 2.5 mmol, 5 equiv) in 2-methyltetrahydrofuran (0.24
mL) were added sulfonamide 1 (42.2 mg, 0.1 mmol) and water
(0.5 mmol, 9 mg, 1 equiv) at room temperature. After stirring at
the same temperature, the reaction was loaded directly onto
silica gel and was purified by chromatography, eluting with
10-30% EtOAc/hexanes, to give the corresponding aldol pro-
duct.
EtOAc/hexanes, to give the known aldol product 754 (118 mg,
0.474 mmol, 95%, 99% ee, >99:1 dr). Procedure B:Toasolutionof
p-nitrobenzaldehyde (75.5 mg, 0.5 mmol) and cyclohexanone
(0.245 g, 0.26 mL, 2.5 mmol, 5 equiv) in DCE (0.24 mL) were
added sulfonamide 123 (42.2 mg, 0.1 mmol) and water (0.5 mmol, 9
mg, 1 equiv) at room temperature. After 14 h, the reaction was
loaded directly onto silica gel and was purified by chromatography,
eluting with 10-30% EtOAc/hexanes, to give the aldol product 7
(119 mg, 0.478 mmol, 96%, 97% ee, 36:1 dr). Procedure C: To a
solution of p-nitrobenzaldehyde (75.5 mg, 0.5 mmol) and cyclohex-
anone (98.0 mg, 0.1 mL, 1.0 mmol, 2 equiv) were added sulfonam-
ide 123 (4.2 mg, 0.01 mmol) and water (0.5 mmol, 9 mg, 1 equiv) at
room temperature. After 36 h, the reaction was loaded directly onto
silica gel and was purified by chromatography, eluting with
10-30% EtOAc/hexanes, to give the aldol product 7 (119 mg,
0.478 mmol, 96%, 96% ee, >99:1 dr). Procedure D: To a solution of
p-nitrobenzaldehyde (75.5 mg, 0.5 mmol) and cyclohexanone (0.245
g, 0.26 mL, 2.5 mmol, 5 equiv) in DCE/EtOH (99:1, 0.24 mL) was
added sulfonamide 123 (42.2 mg, 0.1 mmol) at room temperature.
After 36 h, the reaction was loaded directly onto silica gel and was
purified by chromatography, eluting with 10-30% EtOAc/hex-
anes, to give the aldol product 7(119 mg, 0.478 mmol, 96%, 97% ee,
14:1 dr). Procedure E: To a solution of p-nitrobenzaldehyde (75.5
mg, 0.5 mmol) and cyclohexanone (0.245 g, 0.26 mL, 2.5 mmol, 5
equiv) in 2-Me-THF (0.24 mL) were added sulfonamide 123 (42.2
mg, 0.1 mmol) and water (0.5 mmol, 9 mg, 1 equiv) at room
temperature. After stirring at the same temperature for 16 h, the
reaction was loaded directly onto silica gel and was purified by
chromatography, eluting with 10-30% EtOAc/hexanes, to give the
known aldol product 7 (106 mg, 0.425 mmol, 85%, 94% ee, 30:1
dr): [R]23D = þ8.0 (c = 1.3, CHCl3); 1H NMR (400 MHz, CDCl3)
δ 8.23 (d, J = 8.7 Hz, 2H), 7.53 (d, J = 8.7 Hz, 2H), 4.92 (dd, J =
8.4, 3.2 Hz, 1H), 4.10 (d, J= 3.2 Hz, 1H), 2.38-2.62(m, 3H), 2.11-
2.16 (m, 1H), 1.38-1.87 (m, 5H); 13C NMR (100 MHz, CDCl3) δ
214.8, 148.4, 147.6, 127.9, 123.6, 74.0, 57.2, 42.7, 30.8, 27.7, 24.7;
HPLC (Daicel Chiralpak AD) hexanes/i-PrOH, 90:10, 1.0 mL
min-1, 254 nm: tR (major) = 41.1 min; tR (minor) = 32.7 min.
Acknowledgment. Financial support was provided by the
National Institutes of Health (NIH) (GM63723) and Oregon
State University. National Science Foundation (CHE-
0722319) and the Murdock Charitable Trust (2005265) are
acknowledged for their support of the NMR facility. The
authors would like to thank Professor Max Deinzer and Dr.
ꢀ
Jeff Morre (OSU) for mass spectra data and Synthetech, Inc.
for the generous gift of Cbz-proline. Finally, the authors are
grateful to Dr. Roger Hanselmann (Rib-X Pharmaceuticals)
for helpful discussions. We acknowledge Christopher M.
Sullivan and Matthew Peterson of the Center for Genome
Research and Biocomputing (CGRB), Oregon State Uni-
versity, for computing cluster management.
2-[Hydroxy-(4-nitrophenyl)methyl]cyclohexan-1-one (7): Pro-
cedure A: To a solution of p-nitrobenzaldehyde (75.5 mg, 0.5 mmol)
and cyclohexanone (0.245 g, 0.26 mL, 2.5 mmol, 5 equiv) in DCE
(0.24 mL) were added sulfonamide 123 (42.2 mg, 0.1 mmol) and
water (0.5 mmol, 9 mg, 1 equiv) at 4 ꢀC. After stirring at the same
temperature for 30 h, the reaction was loaded directly onto silica gel
and was purified by chromatography, eluting with 10-30%
Supporting Information Available: Complete experimental
1
procedures are provided, including H and 13C spectra, of all
new compounds. Cartesian coordinates and energies for struc-
tures present in Figures 3 and 4 are also provided. This material
is available free of charge via the Internet at http://pubs.acs.org.
(54) Wang, C.; Jiang, Y.; Zhang, X.; Huang, Y.; Li, B.; Zhang, G.
Tetrahedron Lett. 2007, 48, 4281–4285.
7290 J. Org. Chem. Vol. 75, No. 21, 2010