Lewis base assisted Bronsted base catalysis: Direct regioselective asymmetric vinylogous alkylation of allylic sulfones

Article abstract of DOI:10.1002/chem.201100534

A Lewis base assisted Brnsted base catalysis (LBABB) strategy is applied for direct asymmetric vinylogous alkylation of allylic sulfones with Morita-Baylis-Hillman (MBH) carbonates, in which a strong Brnsted base, tert-butoxy anion, generated in situ from a tertiary amine catalyst and MBH carbonate, is crucial in activating unstabilized nucleophiles. The γ-regioselective alkylation products were obtained with good to excellent enantiomeric excess values when catalyzed by a modified cinchona alkaloid. Copyright

Full text of DOI:10.1002/chem.201100534

FULL PAPER
DOI: 10.1002/chem.201100534
Lewis Base Assisted Brønsted Base Catalysis: Direct Regioselective
Asymmetric Vinylogous Alkylation of Allylic Sulfones
Lin Jiang,[a] Qian Lei,[a] Xin Huang,[a] Hai-Lei Cui,[a] Xue Zhou,[a] and
Ying-Chun Chen*[a, b]
Abstract: A Lewis base assisted Brønsted base catalysis (LBABB) strategy is ap-
plied for direct asymmetric vinylogous alkylation of allylic sulfones with Morita–
Keywords: alkylation · allylic sul-
Baylis–Hillman (MBH) carbonates, in which a strong Brønsted base, tert-butoxy
fones · Brønsted bases · Lewis bas-
anion, generated in situ from a tertiary amine catalyst and MBH carbonate, is cru-
es · Morita–Baylis–Hillman carbo-
cial in activating unstabilized nucleophiles. The g-regioselective alkylation products
nates
were obtained with good to excellent enantiomeric excess values when catalyzed
by a modified cinchona alkaloid.
Introduction
an a-electron-withdrawing group, especially in the field of
metal-free organocatalysis.[5] On the other hand, the utiliza-
tion of sulfone species possessing a simple functional group,
such as allyl phenyl sulfone (pKa =22.5 in DMSO),[6] the so-
called “hard” or unstabilized nucleophile,[7] seems to be a
more challenging and formidable task because the removal
of the a proton usually needs a much stronger base (nBuLi
or NaH).[8] To the best of our knowledge, no successful ex-
ample for such useful synthons has been presented in an
asymmetric catalytic manner to date.
The applications of sulfone-containing materials in asym-
metric catalysis have recently triggered increasing interest,
not only because of their synthetic versatility,[1] but also for
their wide distribution in biologically active products,[2] such
as the commercial drugs illustrated in Scheme 1.[3] Apart
Inspired by our recent studies on the Lewis basic tertiary
amine catalyzed asymmetric allylic alkylation (AAA) reac-
tions with Morita–Baylis–Hillman (MBH) carbonates,[9,10]
we recognized that a rather strong Brønsted base, tert-
butoxy anion (pKa (tBuOH)=32.2 in DMSO),[6] would be
generated in situ in the catalytic cycle, which should be ca-
pable of deprotonating a series of otherwise inert nucleo-
philes. We envisaged that such a catalytic strategy, hence-
forth abbreviated as Lewis base assisted Brønsted base cat-
alysis (LBABB), would be applicable to the direct vinylo-
gous[11] alkylation of allylic sulfones, as outlined in
Scheme 2.
Scheme 1. Typical commercial drugs containing sulfone groups.
from activating a conjugated double bond for Michael addi-
tion,[4] sulfone functionality could stabilize a-carbanions,
which allows sulfone-containing alkanes to perform as well
as C-nucleophilic reagents. Although significant progress
has been made in the past few years, almost all reported
asymmetric reactions are confined to sulfones substituted by
Results and Discussion
[a] L. Jiang, Q. Lei, X. Huang, H.-L. Cui, X. Zhou, Prof. Dr. Y.-C. Chen
Key laboratory of Drug-Targeting
The initial reaction of commercially available allyl phenyl
sulfone (2a) and MBH carbonate 3a was conducted with
1,4-diazabicycloACTHNUTRGNEUNG[2.2.2]octane (DABCO; 1a) catalyst in 1,2-
and Drug Delivery System of the Education Ministry
Department of Medicinal Chemistry
dichloroethane (DCE) at 408C. The starting materials could
be consumed smoothly, but a mixture of g-selective product
4a (E/Z >99:1) and a-selective product 5a (a diastereo-
meric mixture) was obtained (Table 1, entry 1).[12] Fortunate-
ly, the g-alkylation product 4a was dominantly produced in
59% yield after 72 h when catalyzed by a modified cinchona
alkaloid 1b,[13] and more importantly, the enantioselectivity
was promising (Table 1, entry 2). To facilitate reactivity, allyl
West China School of Pharmacy, Sichuan University
Chengdu, 610041 (P.R. China)
Fax : (+86)28-85502609
E-mail: ycchenhuaxi@yahoo.com.cn
[b] Prof. Dr. Y.-C. Chen
State Key Laboratory of Biotherapy, West China Hospital
Sichuan University, Chengdu, 610041 (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100534.
Chem. Eur. J. 2011, 17, 9489 – 9493
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9489

However, the other analogous cinchona alkaloids 1c and 1d
exhibited very low catalytic activity, and both g- and a-alky-
lation products 4c and 5c could be detected (Table 1, en-
tries 5 and 6).[14,15] The reaction proceeded efficiently when
catalyzed by 1e, while product 4c with the opposite configu-
ration was formed in much lower enantioselectivity (Table 1,
entry 7). Solvent screening showed that a slower reaction
rate was observed in toluene and m-xylene (Table 1, en-
tries 8 and 9), but better data were delivered in PhCF3 with
regard to reactivity, yield, and enantiocontrol (Table 1,
entry 10). When the reaction was conducted at room tem-
perature, the enantioselectivity was slightly improved, but
the reaction rate became sluggish (Table 1, entry 11). Finally,
it was found that good results could still be obtained with
catalyst 1b (5 mol%; Table 1, entry 12), even at lower cata-
lytic loading (2 mol%; Table 1, entry 13). In addition, the ee
value was also improved with catalyst 1e under optimal con-
ditions (Table 1, entry 14).
Scheme 2. A LBABB strategy for direct vinylogous alkylation of allylic
sulfones. Boc: tert-butyloxycarbonyl.
Table 1. Screening studies of vinylogous alkylation of allyl sulfones 2 and
MBH carbonate 3a.[a]
Consequently, we then examined the substrate scope and
limitations of the new vinylogous alkylation reaction. The
reaction was performed with catalyst 1b (5 mol%) in PhCF3
(1 mL) at 408C. The results are summarized in Table 2. For
the reaction of 2c, an array of MBH carbonates equipped
with diverse electron-deficient or -rich aryl or heteroaryl
groups have been explored, providing g-alkylation products
4c–4n in high to excellent enantioselectivities and moderate
Entry
1
1
2
Solvent
DCE
t [h]
Product
(yield [%][b]
)
ee [%][c]
G
Table 2. Substrate scope and limitations.[a]
4a (60)
5a (25[d]
4a (59)
4b (67)
4c (77)
1a
2a
4
–
)
2
3
4
5
6
7
8
9
1b
1b
1b
1c
1d
1e
1b
1b
1b
1b
1b
1b
1e
2a
2b
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
DCE
DCE
DCE
DCE
DCE
DCE
toluene
m-xylene
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
72
92
15
72
72
16
66
66
22
65
48
70
48
88
89
89
–
–
4c, 5c (<10)
4c, 5c (<10)
4c (77)
4c (63)
4c (73)
4c (73)
4c (68)
4c (80)
4c (70)
Entry
2
Ar
EWG
t [h] Product
(yield [%][b]
)
ee [%][c]
À44
G
86
88
92
95
92
90
À63
1
2
3
4
2c
2c
2c
2c
2c
2c
2c
2c
Ph
COOMe 48
COOMe 66
COOMe 71
COOMe 71
COOMe 32
COOMe 61
4c (80)
4d (69)
4e (83)
4 f (79)
4g (79)
4h (72)
4i (72)
4j (82)
92
98
90
97
89
89
96
95
10
11[e]
12[f]
13[f,g]
14[f]
p-ClC6H4
m-ClC6H4
o-BrC6H4
3,4-Cl2C6H3
p-NO2C6H4
5
6[d]
7
4c (68)
p-MeOC6H4 COOMe 75
m-MeC6H4
[a] Unless otherwise noted, reactions were performed with
(0.05 mmol), 3a (0.1 mmol), and 1 (10 mol%) in solvent (0.5 mL) at
408C. [DHQD]2AQN: hydroquinidine (anthraquinone-1,4-diyl) diether;
2
8
COOMe 88
9
2c
COOMe 66
4k (74)
93
[DHQD]2PHAL:
hydroquinidine
1,4-phthalazinediyl
diether;
[DHQD]2PYR: hydroquinidine-2,5-diphenyl-4,6-pyrimidinediyl diether;
[DHQ]2AQN: hydroquinine (anthraquinone-1,4-diyl) diether. [b] Yield
of isolated product. [c] Enantiomeric excess (ee) was determined by
chiral HPLC analysis; E/Z >99:1. [d] Diastereomeric ratio (d.r.)=63:37,
determined by 1H NMR spectroscopic analysis. [e] At RT. [f] At
0.1 mmol scale, with 5 mol% of 1b in 1 mL of PhCF3. [g] With 2 mol%
of 1b.
10
2c
2c
2c
2d Ph
2e Ph
2c
2c
2c
1-naphthyl
2-thienyl
2-furyl
COOMe 23
COOMe 72
COOMe 51
COOMe 64
COOMe 69
4l (69)
4m (77)
4n (67)
4o (70)
4p (82)
4q (53)
4r (55)
4s (61)
96
94
83
11[e]
12[e]
13
99[f,g]
93[f]
95
14
15
16
17
Ph
p-BrC6H4
p-MeC6H4
COMe
COMe
COMe
31
24
24
95
96
[a] Unless otherwise noted, reactions were performed with 2 (0.1 mmol),
3 (0.2 mmol), and 1b (5 mol%) in PhCF3 (1.0 mL) at 408C. EWG: elec-
tron-withdrawing group. [b] Yield of isolated product. [c] Determined by
chiral HPLC analysis; E/Z> 99:1. [d] At 08C. [e] At RT. [f] D.r.> 99:1,
by 1H NMR spectroscopic analysis. [g] The absolute configuration of 4o
was determined by X-ray analysis (see the Supporting Information).[16]
The other products were assigned by analogy.
sulfones with a more electron-withdrawing heteroaryl group
were designed. Although similar results were obtained for 2-
pyridyl-substituted sulfone 2b (Table 1, entry 3), it was
pleasing that the reaction could be greatly accelerated for
sulfone 2c with a benzothiazol-2-yl group (Table 1, entry 4).
9490
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Chem. Eur. J. 2011, 17, 9489 – 9493

Lewis Base Assisted Brønsted Base Catalysis
FULL PAPER
to good isolated yields (Table 2, entries 1–12). It is notewor-
thy that the g-substituted allylic sulfones 2d and 2e could be
efficiently used, giving the corresponding g-selective alkyla-
tion products 4o and 4p, respectively, with remarkable dia-
stereo- and enantioselectivity (Table 2, entries 13 and 14).
Additionally, higher reactivity was observed for MBH car-
bonates derived from methyl vinyl ketone, affording g-alky-
lation products 4q–4s with outstanding ee values, albeit in
modest yields due to some side reactions (Table 2, en-
tries 15–17). It should be noted that unwanted side reactions
often occurred when MBH carbonates with b-alkyl substitu-
tion were used.
plex, the reaction intermediate for the selective formation
of the g-regioselective substitution product (R,R)-4o with
catalyst 1b is assumed. As shown in Scheme 5, the MBH
carbonate, presumably first undergoes Michael-type addi-
tion at the nitrogen atom of the quinuclidine, and the result-
ing complex might be stabilized through p–p stacking be-
tween the quinoline moiety and phenyl ring.[18] Such a sand-
wich-like geometry would effectively block the Si face of
this complex, thus favoring attack from the Re face. In addi-
tion, the relatively bulky linkage of 1b seems to be incom-
patible with the sulfone moiety, therefore, the less-hindered
g-regioselective product is generated.[14,15]
To further illustrate the efficacy of this LBABB strategy,
we tested other alkyl-substituted sulfones. As shown in
Scheme 3, sulfone 2 f could be smoothly alkylated with
Scheme 3. Expansion of the “hard” alkyl sulfones.
Scheme 5. Proposed transition state for the formation of product (R,R)-
4o.
MBH carbonate 3b by using catalyst 1b, giving compound 7
as a diastereomeric mixture. Subsequent desulfonylation af-
forded the allylic benzylation product 8 with high enantio-
purity. Nevertheless, the simple sulfone 2g exhibited no re-
activity under the same conditions and remains to be ex-
plored further.
Interestingly, the thermodynamic preference for allylic
sulfone 9 over its conjugated counterpart 4c has been ob-
served in the presence of 1,8-diazabicycloACTHNUTRGNE[NUG 5.4.0]undec-7-ene
(DBU). Tandem desulfonylation of 9 with SmI2 could pro-
vide the valuable allylic alkenylation[17] derivative 10, which
is not easily accessible by common synthetic protocols
(Scheme 4).
Conclusion
We have demonstrated that the LBABB strategy is helpful
for the direct C C bond-forming reaction of “hard” nucleo-
À
philes, as exemplified in the first asymmetric vinylogous al-
kylation of allylic sulfones with MBH carbonates. The g-re-
gioselective products were obtained in high to excellent
enantioselectivities and good yields with catalyst 1b,[14] from
which a formal allylic alkenylation procedure has been fur-
ther developed. We believe that this catalytic strategy will
be applicable to more direct asymmetric reactions of di-
versely structured “hard” nucleophiles,[7] and the results will
be reported in due course.
Experimental Section
General procedure for the g-regioselective alkylation reaction: Sulfone
2c (23.9 mg, 0.1 mmol), MBH carbonate 3a (58.4 mg, 0.2 mmol), 1b
(4.3 mg, 5 mol%) in dry PhCF3 (1.0 mL) were stirred at 408C, and the re-
action was monitored by TLC. After 48 h, the mixture was directly puri-
fied by flash column chromatography on silica gel (petroleum ether/
EtOAc) to give the g-regioselective alkylation product 4c.
Compound 4c: 80% yield; [a]D20 =À52.7 (c=1.20 in CHCl3); 92% ee, de-
Scheme 4. A formal allylic alkenylation sequence.
termined by HPLC analysis (Daicel chiralpak AD, nhexane/iPrOH=80/
20, 1.0 mLminÀ1
,
l=254 nm,
tmajor =14.889 min,
tminor =16.537 min);
1H NMR (400 MHz, CDCl3): d=8.20 (d, J=8.4 Hz, 1H), 8.00 (d, J=
8.0 Hz, 1H), 7.66–7.57 (m, 2H), 7.16–7.03 (m, 6H), 6.52 (d, J=15.6 Hz,
1H), 6.33 (s, 1H), 5.62 (s, 1H), 4.06 (t, J=8.0 Hz, 1H), 3.66 (s, 3H),
2.95–2.90 (m, 1H), 2.82–2.76 ppm (m, 1H); 13C NMR (100 MHz, CDCl3):
Although the conformational ability of dimeric cinchona
alkaloids in solution makes it difficult to accurately analyze
the transition-state structure of the substrate–catalyst com-
Chem. Eur. J. 2011, 17, 9489 – 9493
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9491

Y.-C. Chen et al.
d=166.6, 166.5, 152.7, 149.4, 141.7, 140.1, 136.8, 129.6, 128.5, 127.8, 127.7,
127.5, 126.9, 125.3, 125.2, 122.2, 51.9, 44.9, 36.3 ppm; HRMS (ESI): m/z
calcd for C21H20NO4S2 [M+H]+: 414.0834; found: 414.0839.
(400 MHz, CDCl3): d=7.31–7.27 (m, 2H), 7.22–7.17 (m, 3H), 6.32 (s,
1H), 5.70 (ddd, J=16.4 Hz, 7.6 Hz, 1.6 Hz, 1H), 5.58 (s, 1H), 5.43–5.36
(m, 1H), 4.58 (d, J=7.6 Hz, 1H), 3.68 (s, 3H), 1.69 ppm (d, J=6.4 Hz,
3H); 13C NMR (100 MHz, CDCl3): d=167.1, 143.1, 141.6, 131.6, 128.2,
128.1, 127.2, 126.3, 125.6, 51.7, 49.3, 17.8 ppm; HRMS (ESI): m/z calcd
for C14H16O2Na [M+Na]+: 239.1048; found: 239.1053.
General procedure for synthesis of allylic benzylation product 8: Sulfone
2 f (28.9 mg, 0.1 mmol), MBH carbonate 3b (65.2 mg, 0.2 mmol), and 1b
(4.3 mg, 5 mol%) in dry PhCF3 (1.0 mL) were stirred at 408C for 72 h.
Then, the mixture was directly purified by flash column chromatography
on silica gel (petroleum ether/EtOAc) to give 7. The desulfonylation pro-
cess: Mg powder (24.0 mg, 0.10 mmol) and a solution of AcOH/AcONa
(8m, 0.5 mL) were added to a stirred solution of 7 (33.0 mg, 0.07 mmol)
in DMF (1.0 mL) under argon at room temperature. The resulting mix-
ture was stirred at the same temperature for 4 h. The mixture was diluted
with Et2O (5 mL) and the organic layer was isolated. Then, the aqueous
phase was extracted with Et2O (3ꢁ5 mL) and the combined organic
phase was washed with water and brine, dried over Na2SO4, and concen-
trated under reduced pressure. The residue was purified by flash column
chromatography on silica gel (petroleum ether/AcOEt=200:1) to afford
the pure desulfonylation product 8.
Compound 7: 75% yield; d.r.=60:40; 1H NMR (400 MHz, CDCl3)
(major diastereomer): d=8.20 (d, J=8.4 Hz, 1H), 7.82 (d, J=7.2 Hz,
1H), 7.63–7.57 (m, 1H), 7.54–7.49 (m, 2H), 7.33–7.27 (m, 2H), 7.20–7.18
(m, 2H), 7.05–6.98 (m, 3H), 6.83–6.81 (m, 1H), 6.33 (s, 1H), 6.16 (s, 1H),
6.08 (d, J=12.0 Hz, 1H), 5.00 (d, J=12.0 Hz, 1H), 3.71 ppm (s, 3H);
13C NMR (100 MHz, CDCl3) (major diastereomer): d=165.7, 165.6,
152.2, 139.2, 137.0, 136.0, 132.6, 131.0, 130.5, 129.4, 128.8, 128.5, 128.3,
128.1, 127.8, 127.6, 127.4, 125.4, 122.0, 71.2, 52.0, 48.9 ppm; HRMS (ESI):
m/z calcd for C25H20ClNO4S2Na [M+Na]+: 520.0420; found: 520.0422.
General procedure for the one-pot synthesis of a-regioselective conjugat-
ed sulfones: The reaction was carried out with sulfone 2c (23.9 mg,
0.1 mmol), MBH carbonate 3a (58.4 mg, 0.2 mmol), and 1d (8.8 mg,
10 mol%) in dry PhCF3 (1.0 mL) at 508C for 52 h. Then, DBU (4.5 mL,
0.03 mmol) was added and the reaction was stirred at room temperature
for 0.5 h. After completion, the mixture was directly purified by flash
column chromatography on silica gel (petroleum ether/EtOAc) to give
6a.
Compound 6a: 43% yield; [a]D20 =À59.0 (c=0.70 in CHCl3); E/Z> 99:1;
82% ee, determined by HPLC analysis (Daicel chiralpak AD, nhexane/
iPrOH=90/10, 1.0 mLminÀ1
,
l=254 nm,
tmajor =17.462 min, tminor =
19.257 min); 1H NMR (400 MHz, CDCl3): d=8.15 (d, J=8.0 Hz, 1H),
7.94–7.92 (m, 1H), 7.62–7.53 (m, 3H), 7.13–7.05 (m, 5H), 6.34 (s, 1H),
5.61 (s, 1H), 5.49 (s, 1H), 3.58 (s, 3H), 1.71 ppm (d, J=7.2 Hz, 3H);
13C NMR (100 MHz, CDCl3): d=166.6, 166.5, 152.7, 145.8, 140.9, 138.7,
138.0, 137.3, 128.4, 128.3, 127.7, 127.4, 126.9, 125.3, 122.1, 52.2, 44.9,
15.3 ppm; ESI-HRMS: m/z calcd for C21H19NO4S2Na [M+Na]+:
436.0653; found: 436.0653.
Compound 8: 71% yield; [a]2D0 =À127.1 (c=0.35 in CHCl3); 87% ee, de-
termined by HPLC analysis (Daicel chiralcel OD, nhexane/iPrOH=98/2,
1.0 mLminÀ1, l=254 nm, tmajor =7.815 min, tminor =8.641 min); 1H NMR
(400 MHz, CDCl3): d=7.22–7.13 (m, 5H), 7.05 (d, J=8.4 Hz, 2H), 7.01
(d, J=7.2 Hz, 2H), 6.35 (s, 1H), 5.74 (s, 1H), 4.18–4.15 (m, 1H), 3.66 (s,
3H), 3.20 (dd, J=13.6, 6.4 Hz, 1H), 2.98 ppm (dd, J=13.6, 9.6 Hz, 1H);
13C NMR (100 MHz, CDCl3): d=167.0, 142.8, 140.3, 139.3, 132.2, 129.5,
129.0, 128.3, 128.2, 126.1, 124.8, 51.9, 47.5, 40.7 ppm; HRMS (ESI): m/z
calcd for C18H17ClO2Na [M+Na]+: 323.0815; found: 323.0811.
Acknowledgements
We are grateful for financial support from the NSFC (20972101 and
21021001) and the National Basic Research Program of China (973 Pro-
gram) (2010CB833300). We appreciate helpful comments by Diana Chen
of My Editor-in-Chief.
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General procedure for synthesis of alkenylation product 10: DBU
(31 mL, 0.21 mmol) was added to a solution of 4c (288.0 mg, 0.70 mmol,
92% ee) in CH2Cl2 (10.0 mL). The mixture was stirred at room tempera-
ture for 0.5 h and washed with 1m HCl, water and brine. The organic
layer was dried over Na2SO4 and concentrated under reduced pressure.
The residue was purified by flash column chromatography on silica gel
(petroleum ether/AcOEt=10:1) to give intermediate 9. For the desulfo-
nylation process: a solution of SmI2 (3 mL, 0.30 mmol; 0.1m in THF) was
slowly added by syringe to a stirred solution of 9 (41.4 mg, 0.10 mmol)
dissolved in dry THF (1.0 mL) under argon at room temperature. The re-
sulting mixture was stirred at the same temperature for 0.5 h. The mix-
ture was treated with 1m HCl (5 mL) and diluted with EtOAc (10 mL).
The aqueous phase was separated and extracted with EtOAc (3ꢁ10 mL).
The combined organic phase was washed with a saturated aqueous solu-
tion of sodium thiosulfate, water, and brine; dried over Na2SO4; and con-
centrated under reduced pressure. The residue was purified by flash
column chromatography on silica gel (petroleum ether/AcOEt=100:1)
to afford 10.
Compound 9: 95% yield; [a]D20 =À33.4 (c=2.15 in CHCl3); 93% ee, de-
termined by HPLC analysis (Daicel chiralcel OD, nhexane/iPrOH=70/
30, 1.0 mLminÀ1
,
l=254 nm,
t
minor =13.754 min,
tmajor =19.309 min);
1H NMR (400 MHz, CDCl3): d=8.21–8.19 (m, 1H), 8.00–7.98 (m, 1H),
7.67–7.59 (m, 2H), 7.11–7.08 (m, 3H), 6.94–6.92 (m, 2H), 6.18 (s, 1H),
6.03 (dd, J=15.6 Hz, 7.2 Hz, 1H), 5.44–5.40 (m, 1H), 5.32 (s, 1H), 4.59
(d, J=6.8 Hz, 1H), 4.25–4.22 (m, 2H), 3.63 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d=166.5, 165.0, 152.7, 143.1, 141.4, 139.5, 136.8,
128.4, 128.0, 127.6, 126.8, 126.6, 125.4, 122.3, 116.9, 58.1, 52.0, 49.2 ppm;
HRMS (ESI): m/z calcd for C21H19NO4S2Na [M+Na]+: 436.0653; found:
436.0659.
Compound 10: 68% yield; [a]D20 =À57.8 (c=0.60 in CHCl3); 92% ee, de-
termined by HPLC analysis (Daicel chiralcel OD, nhexane/iPrOH=98/2,
1.0 mLminÀ1, l=254 nm, tminor =4.320 min, tmajor =6.360 min]; 1H NMR
9492
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 9489 – 9493

Lewis Base Assisted Brønsted Base Catalysis
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Supporting Information.
Received: February 17, 2011
Published online: June 30, 2011
Chem. Eur. J. 2011, 17, 9489 – 9493
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9493