New (S)-proline derivatives as catalysts for the enantioselective aldol reaction

Article abstract of DOI:10.1007/s11172-009-0259-0

(2S)-N-{2-[(R)-Hydroxy(phenyl)methyl]phenyl}pyrrolidine-2-carboxamide, (2 S)-N-{2-[(S)-hydroxy(phenyl)methyl]phenyl}pyrrolidine-2-carboxamide, and (S)-N-{2-[hydroxy-(diphenyl)methyl]phenyl}pyrrolidine-2-carboxamide were obtained and tested as catalysts for an asymmetric aldol reaction of 4-nitrobenzaldehyde with acetone.

Full text of DOI:10.1007/s11172-009-0259-0

Russian Chemical Bulletin, International Edition, Vol. 58, No. 9, pp. 1903—1907, September, 2009
1903
New (S)ꢀproline derivatives as catalysts
for the enantioselective aldol reaction
V. I. Maleev, Z. T. Gugkaeva, M. A. Moskalenko, A. T. Tsaloev, and K. A. Lyssenko
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5085. Eꢀmail: vim@ineos.ac.ru
(2S)ꢀNꢀ{2ꢀ[(R)ꢀHydroxy(phenyl)methyl]phenyl}pyrrolidineꢀ2ꢀcarboxamide, (2S)ꢀNꢀ{2ꢀ
[(S)ꢀhydroxy(phenyl)methyl]phenyl}pyrrolidineꢀ2ꢀcarboxamide, and (S)ꢀNꢀ{2ꢀ[hydroxyꢀ
(diphenyl)methyl]phenyl}pyrrolidineꢀ2ꢀcarboxamide were obtained and tested as catalysts for
an asymmetric aldol reaction of 4ꢀnitrobenzaldehyde with acetone.
Key words: organocatalysis, asymmetric synthesis, aldol reaction, (S)ꢀproline.
An asymmetric aldol reaction is one of the most effiꢀ
cient tools for designing complex chiral polyols.1 List et
al.2 demonstrated that this reaction is effectively cataꢀ
lyzed by (S)ꢀproline. This discovery gave a strong impetus
to investigations into asymmetric organocatalysis that alꢀ
lowed the synthesis of many catalysts (mostly derived
from (S)ꢀproline) for the aldol reaction. The best cataꢀ
lysts are compounds combining an amino group* and a
fragment capable of hydrogen bonding to a carbonyl comꢀ
pound (electrophile). Examples include proline amides,
Nꢀsulfonyl carboxamides, 5ꢀ(2ꢀ(S)ꢀpyrrolidinyl)tetrazole,
and prolineꢀcontaining peptides (Fig. 1).3—12
Based on these data, we obtained new (S)ꢀproline
derivatives, namely, proline amides containing an alcoꢀ
hol OH group (Scheme 1). Compounds 7—9 may be reꢀ
garded as potential catalysts because they contain the
chiral pyrrolidine ring and the amide and hydroxy groups
capable of hydrogen bonding to substrate molecules for
stabilization of the enamine in activated intermediate
complex A. In addition, these systems allow variation of
the steric environment for optimization of their catalytic
properties.
It has been convincingly proved by many researchꢀ
ers13—16 that the presence of an acid component in a cataꢀ
lytic system and the spatial structure of the catalyst are
critical for its activity and stereoselective ability.
Proline derivatives 7—9 were obtained by reactions of
Bocꢀ(S)ꢀproline (2) with aromatic amino alcohols 4 and
6 in the presence of N,N´ꢀdicyclohexylcarbodiimide
(DCC). Diastereomers 7 and 8 were separated by column
chromatography.
Acid
function
To determine the absolute configurations of comꢀ
pounds 7 and 8, we converted them into the corresponding
Nꢀbenzyl derivatives 10 and 11 (Scheme 2). The absolute
configuration of compound 11 (isolated in the individual
state by column chromatography from a mixture with its
diastereomer) was proved by Xꢀray diffraction.
Fig. 1. Structural features of the proline derivatives used for
aldol reaction catalysis.
In crystal structure 11, the unit cell consists of two
crystallographically independent molecules. They are
* The secondary amino group, which serves to form an enamine
from a methylene component.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1843—1847, September, 2009.
1066ꢀ5285/09/5809ꢀ1903 © 2009 Springer Science+Business Media, Inc.

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Russ.Chem.Bull., Int.Ed., Vol. 58, No. 9, September, 2009
Maleev et al.
Scheme 1
HO
i. (Boc)2O, NaOH, NaHCO3
ii. 1) DCC, CH2Cl ; 2) CF3CO2H; 3) NaHCO3
2
Scheme 2
conformers with different positions of the carbinol OH
group relative to the amide fragment. The difference in
the conformations of the independent molecules is partly
due to the Hꢀbond system. For instance, in one molecule
(A), the H(2N) atom of the amide group is linked by a
bifurcate intramolecular Hꢀbond to the N(1) atom and
the hydroxyl O(2) atom, while molecule B contains the
normal hydrogen bond H(2N)—N(1´) (Fig. 2). The biꢀ
C(23)
C(24)
C(22)
C(22´)
C(25´)
C(23´)
C(25)
C(21)
C(24´)
C(17´)
C(21´)
C(20)
C(20´)
C(19´)
C(16´)
C(18´)
H(2O)
C(19)
O(2)
C(15´)
C(11´)
C(13´) C(10´)
C(6´)
N(1)
C(12)
C(7)
C(14´)
H(2N)
C(5)
N(1´)
C(8)
C(4)
H(2N´)
C(5´)
C(6)
C(9)
C(10)
C(4´)
C(1)
N(2)
C(9´)
C(11)
C(13)
C(3)
C(7´)
C(8´)
N(2´)
C(1´)
O(1)
C(18)
C(14)
C(3´)
C(12´)
C(2)
H(2O´)
O(1´)
C(17)
C(16)
C(2´)
C(15)
B
A
Fig. 2. General view of two crystallographically independent molecules A and B in structure 11 with atomic thermal displacement
ellipsoids (p = 50%).

Proline derivatives in the aldol reaction
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 9, September, 2009 1905
Table 1. Effects of the additives and the solvent system on the yield
and enantiomeric purity of the aldol adduct obtained from acetone
and 4ꢀnitrobenzaldehyde in the presence of compounds 7—9
furcate bond in molecule A weakens the intramolecular
bond H(2N)—N(1) so that the distance N...N increases
to 2.711(2) Å (against 2.663(2) Å in molecule B).
The configuration of the chiral centers in compound
11 is (S,R) (see Fig. 2). Accordingly, diastereomer 10 was
assigned the (S,S)ꢀconfiguration.
Compounds 7—9 were tested as organocatalysts in a
model reaction of acetone with 4ꢀnitrobenzaldehyde
(Scheme 3).
Catalyst
Solvent
Yield (%)
eе (%)
7
NaClaq
H2O
CH2Cl2
*
73
0
20
0
85
96
74
55
0
9
—
27
—
38
31
41
4
—
20
—
29
21
37
0
DMSO
DMSO—H2O
Dioxane—H2O
DMF—H2O
Scheme 3
8
9
NaClaq
H2O
CH2Cl2
DMSO
*
25
0
DMSO—H2O
Dioxane—H2O
DMF—H2O
87
82
92
60
35
11
0
75
85
71
NaClaq
H2O
CH2Cl2
DMSO
*
15
17
—
13
9
DMSO—H2O
Dioxane—H2O
DMF—H2O
13
i. Catalyst (5 mol.%), solvent, 25 °C, 72 h.
* The saturated solution.
First we studied the influence of the solvent on the
yield and enantiomer excess of the reaction product. Seven
types of solvents were used; the best results were obtained
in dioxane—water and DMF—water with catalyst 7
(Table 1). In all the cases, the reaction was carried out at
room temperature for 72 h. It turned out that catalyst 9 is
inferior to catalysts 7 and 8 in all parameters (see Table 1).
Apparently, two phenyl groups in compound 9 present
steric hindrances to the approach and activation of an
aldehyde molecule. Interestingly, the reaction in the presꢀ
ence of catalyst 9 occurred in water with ee 15%.
The reaction conditions were further optimized for
the solvent DMF—water and catalyst 7. Earlier,17—20 it
has been demonstrated that addition of acids to catalysts
substantially influences both the yield and enantiomeric
purity of the product, presumably because of stabilization
of an intermediate enamine by the acid.
Table 2. Effect of the acid additives on the yield and enantioꢀ
meric purity of the aldol adduct obtained from acetone and 4ꢀnitroꢀ
benzaldehyde in DMF—water in the presence of compound 7*
Acid
Yield of the
product (%)
ee (%)
(configuration)
—
74
99
60
27
77
61
41 (R)
56 (R)
54 (R)
62 (R)
60 (R)
58 (R)
Benzoic
Salicylic
Oxalic
(R,R)ꢀTartaric
Citric
* Reaction conditions: 4ꢀnitrobenzaldehyde (0.09 mmol),
acetone (0.5 mmol), compound 7 as a catalyst (5 mol.%), the
additive (5 mol.%), DMF—water (10 : 1, 0.3 mL), 25 °C, 72 h.
electronꢀwithdrawing properties into the phenyl ring) will
enhance their catalytic activity and stereoselective ability.
Experimental data obtained in the presence of benꢀ
zoic, salicylic, oxalic, tartaric, and citric acids are given in
Table 2. The best results (yield 99%, ee 56%) were achieved
with benzoic acid. The use of oxalic acid provided the
highest ee (62%); however, the yield of the product was
only 27%.
To sum up, we obtained new organocatalysts that cataꢀ
lyze (when used in 5 mol.%) an aldol reaction of 4ꢀnitroꢀ
benzaldehyde with acetone. Although the maximum ee of
the reaction product was 62%, one can expect that further
modification of the catalyst structure (e.g., by introducꢀ
tion of bulkier substituents or substituents with different
Experimental
1H NMR spectra were recorded on Bruker Avance 300 and
Avance 400 spectrometers (300.13 and 400.13 MHz, respecꢀ
tively); the chemical shifts are referenced to the signal for the
residual protons of CDCl3. Optical rotation was measured on a
PerkinꢀElmer 341 polarimeter (cell thickness 5 cm, temperature
control, 25 °C). Column chromatography was carried out on
Kieselgel 60 (Merck). Elemental analysis was performed at the

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Russ.Chem.Bull., Int.Ed., Vol. 58, No. 9, September, 2009
Maleev et al.
elemental analysis laboratory of the Institute of Organoelement
Compounds of the Russian Academy of Sciences. All solvents
were purified according to standard procedures.
1H NMR (CDCl3), δ: 10.42 (s, 1 H, NH); 8.44 (d, 1 H, Harom
,
J = 9 Hz); 7.35 (m, 6 H, Harom); 7.15 (m, 2 H, Harom); 5.41
(s, 1 H, CH); 3.82 (q, 1 H, CH, J = 10.2 Hz); 2.98 (m, 1 H);
2.75 (m, 1 H); 2.10 (m, 1 H); 1.81 (m, 1 H); 1.64 (m, 1 H); 1.47
(m, 1 H).
Xꢀray diffraction analysis of compound 11 was carried out
on a Bruker SMART 1000 CCD automatic diffractometer
(graphite monochromator, λ(MoꢀKα) = 0.71073 Å, ω scan mode,
2θmax = 58°). At 120 K, colorless orthorhombic crystals (C25H26N2O2,
M = 386.48), space group P212121, a = 11.3842(18) Å, b =
(S)ꢀNꢀ{2ꢀ[Hydroxy(diphenyl)methyl]phenyl}pyrrolidineꢀ
2ꢀcarboxamide (9). A solution of DCC (0.47 g, 2.3 mmol) in
CH2Cl2 (3 mL) was added in one portion to a stirred solution of
NꢀBocꢀ(S)ꢀproline (0.5 g, 2.3 mmol) and (2ꢀaminophenyl)ꢀ
diphenylmethanol (6) (0.64 g, 2.3 mmol) in CH2Cl2 (10 mL).
The reaction mixture was stirred for 24 h and urea derivatives
were filtered off. The filtrate was washed with a saturated solution
of NaHCO3, 1 M HCl, and water, dried with anhydrous Na2SO4,
and concentrated on a rotary evaporator. The residue was reꢀ
crystallized from benzene to give tertꢀbutyl (S)ꢀ2ꢀ({2ꢀ[hydroxyꢀ
(diphenyl)methyl]phenyl}carbamoyl)ꢀ2,3,4,5ꢀtetrahydroꢀ1Hꢀ
pyrroleꢀ1ꢀcarboxylate, m.p. 195 °C, [α]D25 +56.7 (c 1, CHCl3).
3
12.831(2) Å, c = 28.420(4) Å, V = 4151.1(11) Å , Z = 8 (Z´ = 2),
dcalc = 1.237 g cm–3, F(000) = 1648, μ = 0.79 cm–1. The number
of measured reflections was 46 376 (Rint = 0.0332), out of which
11 059 independent reflections were used in subsequent calculaꢀ
tions and refinement.
The structure was solved by the direct methods and refined
by the fullꢀmatrix leastꢀsquares method on F2hkl in the anisotroꢀ
pic approximation for all nonꢀhydrogen atoms. Hydrogen atoms
were located from difference electronꢀdensity maps and refined
using a riding model. Final residuals are R1 = 0.0512 (on F2 for
8510 observed reflections with I > 2σ(I)) and wR2 = 0.1260;
GOOF = 1.004, the number of parameters refined is 530. All
calculations were performed with the SHELXTL PLUS proꢀ
gram package.
1H NMR (CDCl3), δ: 9.88 (s, 1 H, NH); 8.44 (d, 1 H, Harom
,
J = 6 Hz); 7.31 (m, 10 H, Harom); 7.11 (d, 1 H, Harom, J = 5 Hz);
6.90 (t, 1 H, Harom, J = 7 Hz); 6.46 (d, 1 H, Harom, J = 6 Hz);
4.13 (m, 1 H, CH); 3.30 (m, 1 H, CH2); 3.14 (m, 1 H, CH2);
2.35 (m, 1 H, CH2); 1.91 (m, 1 H, CH2); 1.47 (s, 9 H, CH3);
1.33 (m, 1 H, CH2); 1.25 (m, 1 H, CH2). A solution of this Bocꢀ
precursor (0.5 g) in CH2Cl2 (3 mL) was treated with a 30%
solution of trifluoroacetic acid in CH2Cl2 (5 mL). The mixture
was kept for 0.5 h, concentrated to dryness on a rotary evaporaꢀ
tor, and treated with a saturated solution of NaHCO3. The
product was extracted with hot ethyl acetate (3×50 mL). The
combined organic extracts were dried with anhydrous Na2SO4,
concentrated on a rotary evaporator, and recrystallized from
NꢀtertꢀButyloxycarbonylꢀ(S)ꢀproline (2) was prepared acꢀ
cording to a standard procedure.21 Yield 90%, m.p. 132—134 °C
25
(cf. Ref. 22: m.p. 134—136 °C), [α]D +57 (c 1, CHCl3).
1H NMR (CDCl3), δ: 8.90 (s, 1 H); 4.35 (m, 1 H); 3.56 (m,
2 H); 2.40—1.85 (m, 4 H); 1.43 (s, 9 H). Found (%): C, 55.78;
H, 7.98; N, 6.33 C10H17NO4. Calculated (%): C, 55.80;
H, 7.96; N, 6.51.
(2ꢀAminophenyl)phenylmethanol (4) was prepared according
to a known procedure.23 Yield 82%, m.p. 115 °C (cf. Ref. 23:
m.p. 110—112 °C).
toluene, m.p. 212 °C; [α]D –9.1 (c 1, CHCl3). 1H NMR
25
(CDCl3), δ: 10.53 (d, 1 H, NH, J = 8 Hz); 9.73 (m, 13 H,
(2ꢀAminophenyl)diphenylmethanol (6) was prepared accordꢀ
ing to a known procedure.24 Yield 85%, m.p. 120 °C (cf. Ref. 24:
m.p. 120—121 °C).
Harom); 9.39 (t, 1 H, Harom, J = 7.7 Hz); 9.01 (d, 1 H, Harom,
J = 7.7 Hz); 5.89 (q, 1 H, CH, J = 15 Hz); 5.29 (m, 1 H, CH2);
5.18 (m, 1 H, CH2); 4.31 (m, 1 H, CH2); 3.92 (m, 1 H, CH2);
3.77 (m, 1 H, CH2); 3.65 (m, 1 H, CH2).
Synthesis of compounds 7 and 8. A solution of dicyclohexylꢀ
carbodiimide (DCC) (0.5 g, 2.5 mmol) in CH2Cl2 (3 mL) was
added in one portion to a stirred solution of NꢀBocꢀ(S)ꢀproline
(0.5 g, 2.5 mmol) and (2ꢀaminophenyl)phenylmethanol (4)
(0.49 g, 2.5 mmol) in CH2Cl2 (10 mL). The reaction mixture
was stirred for 24 h and urea derivatives were filtered off. The
filtrate was washed with a saturated solution of NaHCO3, 1 M
HCl, and water, dried with anhydrous Na2SO4, and concenꢀ
trated on a rotary evaporator. The residue was dissolved in
CH2Cl2 (10 mL) and treated with a 30% solution of trifluoroꢀ
acetic acid in CH2Cl2 (30 mL). The mixture was left for 0.5 h
and concentrated to dryness on a rotary evaporator. The residue
was treated with a saturated solution of NaHCO3 and the prodꢀ
uct was extracted with hot ethyl acetate (3×50 mL). The
combined organic extracts were dried with anhydrous Na2SO4,
concentrated on a rotary evaporator, and chromatographed on
silica gel with CHCl3—methanol—aqueous NH3 (90 : 10 : 1) as
an eluent. The total yield of compounds 7 and 8 was 60%.
(2S)ꢀNꢀ{2ꢀ[(S)ꢀHydroxy(phenyl)methyl]phenyl}pyrrolidineꢀ
2ꢀcarboxamide (7), Rf = 0.4, [α]D25 +10 (c 1, CHCl3). 1H NMR
(CDCl3), δ: 10.32 (s, 1 H, NH); 8.35 (d, 1 H, Harom, J = 8.4 Hz);
7.35 (m, 6 H, Harom); 7.13 (m, 2 H, Harom); 5.40 (s, 1 H, CH);
3.77 (t, 1 H, CH, J = 9 Hz); 3.05 (m, 2 H, CH2); 2.17 (m, 1 H);
2.03 (m, 1 H, CH2); 1.76 (m, 2 H, CH2).
(S)ꢀNꢀ{2ꢀ[(S)ꢀHydroxy(phenyl)methyl]phenyl}ꢀ1ꢀbenzylꢀ
pyrrolidineꢀ2ꢀcarboxamide (10). A mixture of compound 7
(0.3 g, 1 mmol), benzyl bromide (0.17 g, 1 mmol), and Na2CO3
(0.5 g) was refluxed with stirring in acetonitrile (10 mL) under
argon for 4 h. Then the reaction mixture was concentrated to
dryness on a rotary evaporator and diluted with water (10 mL).
The product was extracted with ethyl acetate (3×50 mL). The
organic extract was dried over Na2SO4 and concentrated on a
rotary evaporator. The residue was chromatographed on silica
gel with ethyl acetate as an eluent (Rf = 0.40). The yield of
compound 10 was 80%, colorless oil, [α]D25 +15.3 (c 1, CHCl3).
Found (%): C, 77.69; H, 6.65; N, 7.17. C22H26N2O2. Calcuꢀ
lated (%): C, 77.69; H, 6.78; N, 7.25. 1H NMR (CDCl3), δ:
10.02 (s, 1 H, NH); 8.05 (d, 1 H, Harom, J = 7.9 Hz); 7.36 (m,
12 H, Harom); 7.19 (t, 1 H, Harom); 5.92 (s, 1 H, CH); 4.18 (s,
1 H, OH); 3.82 (d, 1 H, CH2, J = 12 Hz); 3.52 (d, 1 H, CH2,
J = 12 Hz); 3.26—3.22 (m, 1 H, CH2); 3.08—3.03 (m, 1 H,
CH2); 2.40—2.16 (m, 1 H, CH2); 2.13—2.04 (m, 1 H, CH2);
1.68—1.48 (m, 3 H, CH2).
(S)ꢀNꢀ{2ꢀ[(R)ꢀHydroxy(phenyl)methyl]phenyl}ꢀ1ꢀbenzylꢀ
pyrrolidineꢀ2ꢀcarboxamide (11) was obtained in 79% yield as
described above for the synthesis of compound 10 (Rf = 0.45,
ethyl acetate as an eluent) and recrystallized from toluene,
m.p. 120 °C, [α]D25 –29.9° (c 1, CHCl3). Found (%): C, 77.65;
H, 6.73; N, 7.19. C22H26N2O2. Calculated (%): C, 77.69;
(2S)ꢀNꢀ{2ꢀ[(R)ꢀHydroxy(phenyl)methyl]phenyl}pyrrolidineꢀ
25
2ꢀcarboxamide (8), Rf = 0.3, [α]D –19.9 (c 1, CHCl3).

Proline derivatives in the aldol reaction
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 9, September, 2009 1907
H, 6.78; N, 7.25. 1H NMR (CDCl3), δ: 9.96 (s, 1 H, NH); 8.02
(d, 1 H, Harom, J = 7.9 Hz); 7.36 (m, 12 H, Harom); 7.17 (t, 1 H,
Harom); 5.90 (s, 1 H, CH); 3.95 (d, 1 H, CH2, J = 13 Hz); 3.54
(d, 1 H, CH2); 3.48 (s, 1 H, OH); 3.26—3.22 (m, 1 H,
CH2); 3.08—3.03 (m, 1 H, CH2); 2.40—2.16 (m, 1 H, CH2);
2.13—2.04 (m, 1 H, CH2); 1.68—1.48 (m, 3 H, CH2).
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(0.09 mmol) and acetone (0.5 mmol) were added at room temꢀ
perature to a solution of the catalyst (5 mol.%) in the system
solvent—H2O (10 : 1) (0.3 mL). The reaction mixture was stirred
for 72 h and concentrated on a rotary evaporator. The solid
residue was analyzed. When necessary, the product was addiꢀ
tionally purified by column chromatography on silica gel
with light petroleum—AcOEt (5 : 1) as an eluent. 4ꢀHydroxyꢀ4ꢀ
(4ꢀnitrophenyl)butanꢀ2ꢀone. 1H NMR (CDCl3), δ: 2.28 (s,
3 H, CH3CO); 2.92—2.90 (m, 2 H, CH2CO); 3.65 (d, 1 H, OH,
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,
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