Highly enantioselective diethylzinc addition to aldehydes catalyzed by D-glucosamine derivatives

Article abstract of DOI:10.1016/S0957-4166(02)00053-8

Synthesis of α-hydroxy sulfonamides derived from D-glucosamine and their application as ligands in the titanium tetraisopropoxide-promoted enantioselective addition of diethylzinc to benzaldehyde and selected aromatic and aliphatic aldehydes is presented. The reaction is highly enantioselective and enantiomeric excesses of up to 97% for benzaldehyde and 88% for n-hexanal were obtained.

Full text of DOI:10.1016/S0957-4166(02)00053-8

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 13 (2002) 77–82
Highly enantioselective diethylzinc addition to aldehydes
catalyzed by -glucosamine derivatives
D
Tomasz Bauer,* Joanna Tarasiuk and Konrad Pas´niczek
Department of Chemistry, Warsaw University, Pasteura 1, PL-02-093 Warsaw, Poland
Received 10 January 2002; accepted 4 February 2002
Abstract—Synthesis of a-hydroxy sulfonamides derived from
D-glucosamine and their application as ligands in the titanium
tetraisopropoxide-promoted enantioselective addition of diethylzinc to benzaldehyde and selected aromatic and aliphatic alde-
hydes is presented. The reaction is highly enantioselective and enantiomeric excesses of up to 97% for benzaldehyde and 88% for
n-hexanal were obtained. © 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
Herein, we would like to present the application of
N-tosylated derivatives of -glucosamine as chiral lig-
ands in the addition of diethylzinc to aldehydes. We
have chosen -glucosamine as an amino alcohol
because it is relatively cheap and allows for stereochem-
ical manipulations at stereogenic centers thus allowing
the fine-tuning of its asymmetric induction capabilities.
D
The enantioselective addition of organometallic
reagents to aldehydes is a valuable method for the
synthesis of optically active secondary alcohols.
Although several efficient catalysts have been developed
so far, full optimization of the asymmetric addition of
diethylzinc to aldehydes still remains a challenge.
D
Various types of chiral amino alcohols were developed
as chiral ligands for such additions. Since 1986, when
DAIB was introduced by Noyori,1 the number of very
useful and efficient dialkylamino alcohols has grown
dramatically,2 and now includes not only dialkylamino
alcohols but also amino thiols,3 oxazolines4–6 and even
diols, such as TADDOLs7,8 and BINOLs.9–11 Further
progress in enantioselective additions to aldehydes was
achieved, when Ohno and co-workers reported
diethylzinc additions in the presence of titanium tetra-
isopropoxide and chiral disulfonamide ligands.12 Since
then, extensive synthetic studies were conducted and
a-hydroxy sulfonamides and disulfonamides were
proven as very efficient ligands for additions of
dialkylzinc reagents.13–17 Among various amino alco-
hols used as ligands, derivatives of simple and easily
available carbohydrates are only rarely reported,18,19
and, in our opinion, remain underestimated. We are
unaware of any carbohydrate-derived ligand possessing
an a-hydroxy sulfonamide functionality.
2. Results and discussion
2.1. Synthesis of chiral ligands
Starting from the commercially available
amine 1, methyl 2-deoxy-2-p-toluenesulfonamido-4,6-
O-benzylidene-a- -glucopyranoside 5 was synthesized
using known procedures. Thus, -glucosamine 1 was
D-glucos-
D
D
treated with methyl chloroformate in the presence of
base, yielding the corresponding carbamate.20 The
product of this reaction was dissolved in 4% methanolic
hydrogen chloride and heated under reflux to give
almost exclusively 2 in 61% yield from
D-glucosamine.
The free hydroxyl groups at C(4) and C(6) (carbohy-
drate nomenclature) were protected as their benzylidene
acetal by treatment with C6H5CH(OMe)2 in the pres-
ence of a catalytic amount of p-toluenesulfonic acid.21
After crystallization, compound 3 was obtained in 83%
yield. Finally, the carbamate protecting group had to
be converted to a p-toluenesulfonamide. The methyl
carbamate appeared to be even more stable than
expected and under standard conditions (reflux in 1.2
M aqueous NaOH in 1,4-dioxane) we were able
* Corresponding author. Tel.: (+48-22)-822-02-11, ext. 273; fax: (+48-
22)-822-59-96; e-mail: tbauer@chem.uw.edu.pl
0957-4166/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(02)00053-8

78
T. Bauer et al. / Tetrahedron: Asymmetry 13 (2002) 77–82
2.2. Enantioselective addition of diethylzinc to
benzaldehyde
to obtain the free amine 4 only in a very low yield. We
tested various reaction conditions and found that the
best results can be achieved using 4 M ethanolic KOH
in the presence of 2-methoxyethanol.22,23 Compound 4
was obtained in 45% yield and selective N-tosylation
gave methyl 2-deoxy-2-p-toluenesulfonamido-4,6-O-
With ligands 5 and 9 in hand, we studied the influence
of the temperature and amount of the catalyst on the
enantioselective ethylation of aromatic and aliphatic
aldehydes. We started with the most popular model
benzaldehyde. The first reaction of diethylzinc (3
equiv.) with benzaldehyde 10 was performed at −20°C
in the presence of 0.2 equiv. of ligand 5 and titanium-
(IV) isopropoxide (7 equiv.) in methylene chloride.
(R)-1-Phenyl-1-propanol 11 was obtained in 89% iso-
lated yield and 92% e.e. Repeating the reaction at room
temperature afforded the product in 92% isolated yield
with 94.7% e.e. Ligand 5 is equally or even more
efficient when used in smaller amounts. The best
results, as far as chemical yield and enantiomeric excess
concerns were obtained when using 0.1 equiv. of 5,
which afforded 11 in 99% yield and with e.e. of 97.0%.
Reactions conducted in the presence of 0.05 and 0.025
equiv. of ligand 5 are still highly enantioselective,
although in the latter case the yield of the reaction
drops significantly. These results place our ligand
among the most active a-hydroxy- and disulfonamide
catalysts,13–17 but below the excellent Verdi disulfon-
amide catalyst, which can be effectively used at levels of
benzylidene-a-
D
-glucopyranoside
5
in 75% yield
(Scheme 1).
The second ligand, the known compound 9 was synthe-
sized in order to evaluate the influence of the configura-
tion at the stereogenic center adjacent to the metal
binding amino alcohol. Ligand 9 has b-configuration at
the anomeric center, which is opposite to that of com-
pound 5.
D-Glucosamine was peracetylated using acetic
anhydride and pyridine and then treated with benzyl
,
alcohol in the presence of tin tetrachloride and 4 A
molecular sieves to give b-benzyl glucopyranoside 6 in
25% yield.24 The acetate protecting groups were selec-
tively removed using sodium methoxide in methanol
and the crude product of the reaction was subjected to
benzylidenation as previously described, yielding
derivative 7 (65%).21 Hydrolysis of acetamide 7, fol-
lowed by selective N-tosylation gave ligand 9 in 76%
yield (Scheme 1).25
HO
HO
HO
HO
O
i
ii
O
O
O
Ph
OH
OMe
OMe
O
61%
83%
NH2.HCl
NHCO2Me
NHCO2Me
HO
HO
HO
1
2
3
O
O
O
O
iv
iii
O
O
Ph
Ph
OMe
NH2
OMe
NHTs
45%
75%
HO
HO
4
5
HO
HO
AcO
AcO
O
O
O
O
O
Ph
v, vi
25%
vii,ii
65%
OH
OBn
NHAc
OBn
NHAc
NH2.HCl
HO
AcO
HO
1
6
7
O
O
O
iv
O
O
viii
45%
Ph
Ph
OBn
NH2
OBn
NHTs
O
76%
HO
HO
9
8
Scheme 1. Synthesis of methyl 2-deoxy-2-p-toluenesulfonamido-4,6-O-benzylidene-a-
D-glucopyranoside 5 and benzyl 2-deoxy-2-
p-toluenesulfonamido-4,6-O-benzylidene-b- -glucopyranoside 9. Reaction conditions: (i) Ref. 20; (ii) PhCH(OMe)2, p-TsOHcat
D
,
,
DMF; (iii) 4N KOH/EtOH, MeOCH2CH2OH; (iv) p-TsCl, Na2CO3, dioxan–H2O; (v) Ac2O/Py; (vi) BnOH, SnCl4, 4 A MS; (vii)
2N MeONa/MeOH; (viii) 4N KOH, EtOH.

T. Bauer et al. / Tetrahedron: Asymmetry 13 (2002) 77–82
79
only 0.01 mole equivalents.15 The product, (R)-11
appears not to be completely stable under the reaction
conditions, and prolonged reaction times result in lower
yields. We also observed slightly decreased enantiose-
lectivity, which suggest that the (R)-enantiomer decom-
poses faster than the (S)-isomer. Finally, we have
investigated influence of the configuration at the
anomeric center. In the presence of 0.1 equiv. of ligand
9 (R)-1-phenyl-1-propanol was obtained in almost
quantitative yield, but enantiomeric excess was very
low—only 10%. Results of the addition of diethylzinc
to benzaldehyde are presented in Table 1.
than the configuration at C(1). Although the exact
mechanism of the titanium-promoted addition of
diethylzinc to aldehydes is so far unknown, recent
structural and mechanistic investigations have shown
that the active catalytic intermediate could be an
ethyltitanium species derived from the transfer of an
ethyl group from zinc to titanium. The catalytic species
in our case can also be formed in the way proposed by
Paquette et al., where, in the final step of the formation
of the reacting species, coordination of aldehyde is
concomitant with the migration of the ethyl substituent
to an ‘apical’ position.15 Thus, a plausible explanation
2.3. Enantioselective addition of diethylzinc to some
aromatic and aliphatic aldehydes
Table 2. Enantioselective addition of diethylzinc to
selected aldehydes catalyzed by ligand 5
Taking into account the results presented in Table 1, we
carried out diethylzinc additions to some other repre-
sentative aromatic and aliphatic aldehydes in the pres-
ence of ligand 5. All reactions were performed at
ambient temperature in the presence of 0.1 equiv. of
chiral ligand using the standard procedure as described
for benzaldehyde. As shown in Table 2 reaction of
aromatic aldehydes proceeded with high enantioselec-
tivity, (albeit slightly lower than in the case of benzalde-
hyde). Ethylation of cinnamaldehyde 12c gave alcohol
13c with good enantioselectivity (86%). The addition to
cycloaliphatic aldehyde 12d proceeded with much lower
induction. Very good enantioselectivity (88% e.e.) was
observed for a simple aliphatic aldehyde n-hexanal 12c.
OH
5, ZnEt2 (3 equiv)
RCHO
TiCl4 (1.4 equiv), CH2Cl2
R
R
12
13
a R=3-ClPh
b R=4-MeOPh
c R=PhCH=CH
d R=c-C6H11
e R=CH3(CH2)5
Entry Aldehyde
Time Yielda E.e.b,c
(h)
(%)
(%)
1
2
3
4
5
3-Chlorobenzaldehyde 12a
4-Methoxybenzaldehyde 12b
trans-Cinnamaldehyde 12c
Cyclohexanecarboxaldehyded 12d
Hexanald 12e
2.5
7
3
5
1.5
85
95
94
73
80
90 (R)
89 (R)
86 (R)
39 (R)
88 (R)
2.4. Mechanistic considerations
The contrasting induction abilities of ligands 5 and 9
call for an explanation. These two ligands differ from
each other only by the configuration and substitution at
the anomeric center, so this center has to be responsible
for such dramatic changes in enantioselectivity. We
believe that the type of substitution is less important
a Isolated yield.
b Determined by HPLC analysis using a Daicel Chiracel OD column.
c Configuration of the major enantiomer in parentheses (based on the
known elution order).
d HPLC analysis of the benzoate.
Table 1. Enantioselective addition of diethylzinc to benzaldehyde catalyzed by ligands 5 and 9
OH
CHO
R
5 or 9, ZnEt2 (3 equiv)
TiCl4 (1.4 equiv), CH2Cl2
10
11
Entry
Ligand
Amount of ligand (equiv.)
Temp. (°C)
Time (h)
Yielda (%)
E.e.b,c (%)
1
2
3
4
5
6
7
8
9
5
5
5
5
5
5
5
5
5
9
0.2
0.2
0.1
0.1
−20
20
−20
20
20
−20
20
20
20
20
24
3
24
3
20
24
5
48
5
89
92
81
99
97
75
99
90
65
98
92 (R)
95 (R)
94 (R)
97 (R)
95 (R)
89 (R)
95 (R)
93 (R)
95 (R)
10 (R)
0.1
0.05
0.05
0.05
0.025
0.1
10
5
a Isolated yield.
b Determined by HPLC analysis using a Daicel Chiracel OD column.
c Configuration of the major enantiomer in parentheses.

80
T. Bauer et al. / Tetrahedron: Asymmetry 13 (2002) 77–82
of the stereochemical outcome of the reaction can be
based on model transition states I–VI depicted at
Scheme 2. In ligand 5 the axial methoxy substituent
‘pushes’ the tosyl group toward titanium. The isopro-
poxy group takes position in the least crowded environ-
ment and benzaldehyde coordinates in-between.
Additionally, p-stacking effect of phenyl rings from
benzaldehyde and tosyl group can further rigidify the
transition state I. The low enantioselectivity observed
for the cyclohexanecarboxaldehyde 12d is in good
agreement with this assumption. The coordination of
titanium to one of the sulfonamide oxygens, as demon-
strated by Walsh et al.17 cannot also be excluded, as
shown in model transition state II. Ligand 9 at the
anomeric center has an equatorial benzyloxy sub-
stituent, which is located in a space remote from the
reacting center. It allows for at least four possible
transition states III–VI and seems to be the main cause
of the low levels of asymmetric induction observed for
ligand 9.
3. Conclusions
We have presented synthesis of methyl 2-deoxy-2-p-
toluenesulfonamido-4,6-O-benzylidene-a-D-glucopyran-
O
O
O
O
Ph
Ph
O
O
O
O
Et
O
N
S
Et
O
N
S
O
O
Ti
Ti
O
O
O
O
O
O
H
H
re-attack
re-attack
II
I
O
O
O
O
O
O
Ph
Ph
O
O
OBn
O
OBn
O
Et
N
Et
N
S
S
Ti
O
Ti
O
O
O
O
O
H
H
re-attack
si-attack
III
IV
O
O
O
O
N
Ph
Ph
H
O
O
OBn
O
OBn
O
O
O
Et
O
N
Et
O
S
S
Ti
Ti
O
O
O
O
H
si-attack
re-attack
V
VI
Scheme 2. Stereochemical models for an ethylation of benzaldehyde.

T. Bauer et al. / Tetrahedron: Asymmetry 13 (2002) 77–82
81
oside 5 and its application as chiral ligand in titanium
promoted enantioselective diethylzinc addition to aro-
matic and aliphatic aldehydes. To the best of our
knowledge, we have reported the first example of
diethylzinc addition to aldehydes catalyzed by a carbo-
hydrate derivative possessing an a-hydroxy sulfonamide
functionality. We have shown that addition to both
aromatic and aliphatic, but not cycloaliphatic alde-
hydes proceeds with high yield and enantioselectivity
even with 0.1 equiv. of our ligand. Investigations aimed
at defining the utility of this new ligand for other
substrates and reactions are in progress.
1N HCl and the volatiles were removed using rotatory
evaporator. Water (50 mL) was added to the residue
and the solution was extracted with chloroform (5×20
mL). The combined organic extracts were washed with
water, brine and dried over anhydrous MgSO4. Crude
product was purified by flash chromatography to afford
4 (yield 1.8 g, 45%) which was subjected to the next
1
step. Mp 174–177°C. H NMR (200 MHz): 7.60–7.25
(m, 5H, Ph), 5.50 (s, 1H, CHPh), 4.70 (d, J1,2=3.6, 1H,
H-1), 4.20 (m, 1H, H-6e), 3.80–3.60 (m, 3H, H-6a, H-2,
H-4), 3.50–3.40 (m, 2H, H-3, H-5), 3.40 (s, 3H, OCH3),
2.80 (d, 1H, OH). 13C NMR (50 MHz): 137.2, 129.2,
128.3, 126.2, 101.9 (PhCH), 101.1 (C1), 82.1, 71.7, 69.1,
62.5 (C6), 56.6, 55.4 (anomeric OCH3). [h]D +105.2 (c
0.73, CHCl3).
4. Experimental
4.1. General
4.2.3. Methyl 4,6-O-benzylidene-2-deoxy-2-p-toluenesul-
fonamido-a-D-glucopyranoside 5. To a solution of 4 (3.0
Melting points were determined using a Kofler hot
stage apparatus and are uncorrected. Optical rotations
were recorded using a Perkin–Elmer PE-241 polarime-
g, 8.84 mmol) and Na2CO3 (1.28 g, 12.0 mmol) in 1:1
water–dioxan (60 mL) p-TsCl (2.3 g, 12.0 mmol) was
added, and the mixture was stirred for 3.5 h at 5°C.
Solvents were then evaporated in vacuo and the solid
residue was extracted with chloroform, the organic
phases were washed with water, brine and dried over
anhydrous MgSO4. After filtration and evaporation
crude 5 was purified by flash chromatography (EtOAc–
hexane 3:7 v/v) to give pure 5 (3.48 g, 8.8 mmol, 75%).
1
ter with a thermally jacketed 10-cm cell. H and 13C
NMR spectra were recorded in CDCl3 using a Varian
200 Unity Plus and Varian 500 Unity Plus spectrome-
ters. All chemical shifts are quoted in parts per million
relative to tetramethylsilane (l, 0.00 ppm), and cou-
pling constants (J) are measured in hertz. Reactions
were carried using the Schlenk technique under argon
when necessary. Flash column chromatography was
completed using silica gel (Kieselgel-60, Merck, 230–
400 mesh).
1
Mp 187–189°C. H NMR (200 MHz): 7.85–7.40 (dd,
4H, PhCH3), 7.40–7.20 (m, 5H, Ph), 5.50 (s, 1H,
CHPh), 5.10 (d, 1H, NH), 4.40 (d, J1,2=4, 1H, H-1),
4.25 (m, 1H, H-6e), 3.85–3.60 (m, 3H, H-6a, H-2, H-4),
3.55–3.25 (m, 2H, H-3, H-5), 3.20 (s, 3H, OCH3), 2.60
(d, 1H, OH), 2.40 (s, 3H, CH3). 13C NMR (50 MHz):
143.9, 137.5, 136.9, 129.8, 129.2, 128.2, 127.1, 126.2,
101.9 (PhCH), 98.7 (C1), 81.2, 69.4, 68.7, 62.1 (C6),
58.2, 55.5 (C2), 21.5 (CH3). [h]D +34.4 (c 0.77, CHCl3).
Elemental analysis found: C, 57.92; H, 5.79; N, 3.22; S,
7.36. C21H25NO7S requires C, 57.99; H, 6.13; N, 2.77;
S, 7.43.
4.2. Preparation of methyl 2-deoxy-2-p-toluenesulfon-
amido-4,6-O-benzylidene-a-D-glucopyranoside 5
4.2.1. Methyl 4,6-O-benzylidene-2-deoxy-2-metoxycar-
bonylamido-a- -glucopyranoside 3. To the methyl 2-
deoxy-2-metoxycarbonylamido-a- -glucopyranoside
D
D
2
(5.1 g, 20 mmol), dissolved in anhydrous DMF (100
mL) was added benzaldehyde dimethyl acetal (6 mL, 60
mmol) and catalytic amount of p-toluenesulfonic acid.
Ethyl acetate (200 mL) was added and the solution was
washed with water (3×100 mL), dried over anhydrous
MgSO4, filtered and evaporated. The residue was crys-
tallized from MeOH–Et2O to give 3 (5.6 g, 83%) Mp
4.3. Typical procedure for diethylzinc addition reactions
To a solution of ligand 5 (43.5 mg, 0.1 mmol) in
methylene chloride (5 mL) was added titanium tetra-
isopropoxide (0.42 mL, 1.4 mmol). The mixture was
stirred for 1 h at room temperature, cooled to –78°C,
and diethylzinc (1.1 M toluene solution, 0.27 mL, 3
mmol) was added. Stirring was continued at this tem-
perature, and freshly distilled benzaldehyde (0.1 mL, 1
mmol) was added. The mixture was allowed to warm to
room temperature and stirred for the time indicated in
Table 1. The reaction was quenched with 1N HCl (10
mL), and insolubles were filtered off. The organic layer
was separated, and the aqueous layer was extracted
with ethyl acetate (3×5 mL). The combined organic
extracts were washed with brine, dried over MgSO4anh
and purified by flash chromatography (hexane–ethyl
acetate 5:1 v/v) to give (R)-1-phenylpropanol. Yield 135
mg; 99%. This product was subjected to HPLC analysis
using a Chiracel OD column (3% 2-propanol in
hexane).
1
214–216°C. H NMR (200 MHz): 7.50–7.25 (m, 5H,
Ph), 5.60 (s, 1H, CHPh), 5.20 (d, 1H, NH), 4.75 (d,
J1,2=2.6, 1H, H-1), 4.30 (m, 1H, H-6e), 4.95–4.85 (m,
4H, H-6a, H-2, H-3, H-4), 4.85 (s, 3H, COOCH3),
4.60–4.45 (m, 1H, H-5), 3.40 (s, 3H, OCH3), 3.00 (d,
1H, OH). 13C NMR (50 MHz): 161.8 (C=O) 136.9,
129.2, 129.2, 128.2, 126.29, 126.23, 101.9 (PhCH), 99.1
(C1), 81.87, 81.82, 76.2 (CNH), 68.9 (CHOH), 62.3
(C6), 55.4 (anomeric OCH3), 52.5 (ester OCH3). [h]D
+41.5 (c 1.13, CHCl3).
4.2.2. Methyl 4,6-O-benzylidene-2-deoxy-2-amino-a-
glucopyranoside 4. Compound 3 (5.05 g, 15 mmol)
was added to KOH solution in EtOH–
D-
4
M
MeOCH2CH2OH 10:1 v/v and the mixture was heated
under reflux for 14 h. The reaction mixture was cooled
to room temperature and acidified to pH 8 using cold

82
T. Bauer et al. / Tetrahedron: Asymmetry 13 (2002) 77–82
11. Kitajima, H.; Aoki, Y.; Katsuki, T. Bull. Chem. Soc. Jpn.
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