Organocatalysis with azahelicenes: The first use of helically chiral pyridine-based catalysts in the asymmetric acyl transfer reaction

Article abstract of DOI:10.1135/cccc2009067

The successful utilisation of a helicene-based organocatalyst in acylative kinetic resolution of racemic secondary alcohol was described. Employing rac-1-phenylethanol, optically pure (-)-(M)-2-aza[6]helicene (5-20 mole %), isobutyric anhydride and N-ethyldiisopropylamine in chloroform at 22-40 °C, an asymmetric acyl transfer reaction took place overwhelming the uncatalysed background process. Moderate reactivity as well as selectivity factor (s = 9, 10) were observed. An effective control of the enantiodiscriminating step in acylative kinetic resolution by the helically chiral organocatalyst was proven for the first time.

Full text of DOI:10.1135/cccc2009067

Organocatalysis with Azahelicenes
1151
ORGANOCATALYSIS WITH AZAHELICENES: THE FIRST USE OF
HELICALLY CHIRAL PYRIDINE-BASED CATALYSTS IN THE
ASYMMETRIC ACYL TRANSFER REACTION
a3,b,c,
Michal ŠÁMALa1,b, Jiří MÍŠEKa,b, Irena G. STARÁa2,b,* and Ivo STARÝ
*
a
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic,
v.v.i., Flemingovo nám. 2, 166 10 Prague 6, Czech Republic; e-mail: samal@uochb.cas.cz,
1
2
3
stara@uochb.cas.cz, stary@uochb.cas.cz
b
c
Center for Biomolecules and Complex Molecular Systems, Institute of Organic Chemistry and
Biochemistry, Academy of Sciences of the Czech Republic, v.v.i., Flemingovo nám. 2,
166 10 Prague 6, Czech Republic
Department of Organic Chemistry, Charles University, Albertov 2030, 128 40 Prague 2,
Czech Republic
Received May 15, 2009
Accepted Jun e 17, 2009
Publish ed on lin e July 22, 2009
Dedicated to Dr Alfred Bader on the occasion of his 85th birthday.
Th e successful utilisation of a h elicen e-based organ ocatalyst in acylative kin etic resolution of
racem ic secon dary alcoh ol was described. Em ployin g rac-1-ph en yleth an ol, optically pure
(–)-(M)-2-aza[6]h elicen e (5–20 m ole %), isobutyric an h ydride an d N-eth yldiisopropylam in e
in ch loroform at 22–40 °C, an asym m etric acyl tran sfer reaction took place overwh elm in g
th e un catalysed backgroun d process. Moderate reactivity as well as selectivity factor (s = 9,
10) were observed. An effective con trol of th e en an tiodiscrim in atin g step in acylative kin etic
resolution by th e h elically ch iral organ ocatalyst was proven for th e first tim e.
Keyw ord s: Azah elicen es; Organ ocatalysis; Asym m etric catalysis; Acyl tran sfer; Kin etic resolu-
tion .
Azah elicen es represen t un ique h elically ch iral polyarom atic h eterocycles1
but th ey were for decades practically un explored. As far as pyridoh elicen es,
a subcategory of azah elicen es, are con cern ed, th ere are on ly scattered but
sign ifican t studies focused on th eir self-assem bly2,3, basicities4,5 an d
ch iroptical properties con trolled by m etal coordin ation 6. Th ese com poun ds
can n ow be syn th esised n ot on ly by usin g th e classical ph otodeh ydro-
cyclisation m eth od7--9 but also em ployin g in tram olecular [2 + 2 + 2] cyclo-
isom erisation of triyn es10,11 or th e Stille–Kelly reaction of dih alogen ated
cis-stilben e-type precursors12. Becom in g m ore easily accessible in a racem ic
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1151–1159
© 2009 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc2009067

1152
Šámal, Míšek, Stará, Starý:
or n on racem ic form , azah elicen es attract an in creasin g atten tion an d a sys-
tem atic study of th eir properties an d a search for th eir prom isin g applica-
tion s h as recen tly been com m en ced. In particular, rem arkably h igh proton
affin ities of pyridoh elicen e derivatives were m easured by m ass spectrom et-
ric tech n iques13, ch iral discrim in ation in silver(I)-boun d dim ers probed by
electrospray m ass spectrom etry dem on strated a pron oun ced preferen ce for
th e form ation of h om och iral dim ers14 sim ilarly to th e solid state10 an d
acid–base dissociation con stan ts of azah elicen es were determ in ed by capil-
lary zon e electroph oresis15.
Th e robust h elical skeleton of azah elicen es alon g with th e presen ce of a
basic, n ucleoph ilic an d coordin atin g h eteroatom calls for th e application s
to asym m etric catalysis. Th e prelim in ary study in gas ph ase revealed th at
th e in teraction between proton ated azah elicen e an d 2-alkan ols, both in opti-
cally pure form s, leads to prom isin g en an tiodiscrim in ation 13. Most sign ifi-
can tly, optically pure azah elicen e N-oxides were successfully used in
desym m etrisation of meso epoxides ach ievin g up to 94% ee 12. To th e best
of our kn owledge, th is is th e on ly exam ple publish ed in th e literature so
far, wh ich describes th e utilisation of azah elicen e derivatives in en an tio-
selective catalysis an d, in particular, organ ocatalysis.
Herein , we report our prelim in ary results m an ifestin g th e first use of
n on racem ic azah elicen es as organ ocatalysts in th e asym m etric acyl tran sfer
reaction leadin g to kin etic resolution of racem ic secon dary alcoh ol.
RESULTS AND DISCUSSION
4-(Dim eth ylam in o)pyridin e (DMAP) is a powerful catalyst in a variety of
acyl tran sfer reaction s16--18. Owin g to th e presen ce of th e electron -don atin g
dim eth ylam in o group, sign ifican tly in creased n ucleoph ilicity of th e pyri-
din e n itrogen atom leads, on reaction with carboxylic acid an h ydride, to
th e favourable form ation of th e acylpyridin ium –carboxylate ion pair in ter-
m ediate, wh ich serves as an effective acyl tran sfer-reagen t in acylation reac-
tion s16. It goes with out sayin g th at ch iral con gen ers of DMAP could be of
outstan din g utility in asym m etric acylation s as, for in stan ce, kin etic resolu-
tion of racem ic secon dary alcoh ols. In deed, startin g from th e m id-n in eties,
m yriads of ch iral an alogues of DMAP h ave em erged alon g with n on -DMAP
ch iral catalyst such as fused dih ydroim idazoles, im idazole-con tain in g
peptides, N-h eterocyclic carben es an d tertiary ph osph in es19. Am on g th em ,
Fu’s ch iral ferrocen yl DMAP catalysts20,21 an d Birm an ’s 3,4-dih ydro-
2H-pyrim ido- or 2,3-dih ydroim idazo[2,1-b][1,3]ben zoth iazole catalysts22,23
(Ch art 1) h ave gain ed a privileged position due to th eir efficien cy in asym -
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1151–1159

Organocatalysis with Azahelicenes
1153
m etric acylative kin etic resolution of secon dary alcoh ols. Moreover, th e
24
22
Fu’s catalyst 1
is com m ercially available an d th e Birm an ’s catalyst 2
can be prepared in on ly two steps from catalogue ch em icals. Classifyin g
acyl tran sfer catalysts accordin g to th e type of ch irality, elem en ts of cen-
tral22, axial25 an d plan ar ch irality h ave been explored exem plified by 1–3
(Ch art 1). To th e best of our kn owledge, n o h elically ch iral catalyst h as so
far been used in th is regard.
CHART 1
To un derstan d th e role of h elical ch irality in asym m etric acyl tran sfer re-
action s, we decided to explore 1-aza[6]h elicen e 4 an d 2-aza[6]h elicen e 5
(Ch art 2) as organ ocatalysts in kin etic resolution of secon dary alcoh ols. We
described earlier such azah elicen es are accessible with in a reason able n um -
ber of syn th etic steps to receive th em in optically pure form s such as (M)
an d (P) en an tiom ers10. We were aware th at DMAP an d its derivatives, com -
pared to paren t pyridin e, are approxim ately 104 tim es m ore active wh en
used as acylation catalysts18. Regardless of th e absen ce of th e activatin g
dialkylam in o group in th e para position to n itrogen in th e an n ulated pyri-
din e rin g, we speculated th at azah elicen es 4 an d 5 m igh t still be reactive
en ough to prom ote th e acylation reaction possibly with en an tio-
discrim in ation .
CHART 2
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1154
Šámal, Míšek, Stará, Starý:
First, we exam in ed th e acyl tran sfer capability at (+)-(P)-1-aza[6]h elicen e
4 (Eq. (1)). Racem ic 1-ph en yleth an ol 6 was treated with isobutyric an h y-
dride in th e presen ce of a catalytic am oun t of (+)-(P)-4 an d N-eth yldiiso-
propylam in e as a base in ch loroform at room tem perature. However, to our
discouragem en t, even after a prolon ged reaction period (20 h ) we detected
on ly a n egligible am oun t of th e acylated product 7 (<5%), wh ose form ation
could be ascribed to an un catalysed backgroun d reaction . We reason ed th at
th e catalytic in efficien cy of (+)-(P)-4 could be ascribed to a lim ited accessi-
bility of th e n itrogen atom in th e m ost sterically con gested position 1
rath er th an to its low n ucleoph ilicity. Th us, th e catalytic activity of
(+)-(P)-4 was n ot furth er studied.
(1)
(i-PrCO)2O (0.75 equiv.), (+)-(P)-4 (10 mole %), N-ethyldiisopropylamine (0.75 equiv.), chloro-
form, r.t., 20 h, 6:7 = >95:<5 (GC)
Accordin gly, we turn ed our atten tion to isom eric (–)-(M)-2-aza[6]h elicen e
5, wh ich exh ibited differen t beh aviour. We foun d th at, un der an alogous
con dition s, (–)-(M)-5 served as an organ ocatalyst prom otin g th e acyl tran s-
fer reaction (Eq. (2)). Wh en ach ievin g ca. 50% con version , we could detect
en couragin g en an tiom eric excess both of alcoh ol 6 an d ester 7 by m ean s of
HPLC on a Ch iralcel OD-H colum n (ee of ester determ in ed after
sapon ification ).
(2)
(i-PrCO)2O (1.0 equiv.), (–)-(M)-5 (5 mole %), N-ethyldiisopropylamine (0.75 equiv.), chloroform,
r.t., 110 h, 6:7 = 47:53 (GC)
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1151–1159

Organocatalysis with Azahelicenes
1155
For th e first tim e, we observed an asym m etric acyl tran sfer process
catalysed by a h elicen e m olecule. Hen ce, we decided to explore th e effects
of th e organ ocatalyst loadin g, tem perature, solven ts, bases an d an h ydrides
on en an tiom eric excess of (+)-(R)-6 as well as selectivity of th e reaction .
Obviously, th e lon ger reaction period, th e h igh er ee of (+)-(R)-6 was ob-
served (Table I, en tries 1–4). Th e m oderate selectivity factor s ran ged be-
tween 5 an d 9. However, at th e sam e tim e, we observed m in or un catalysed
backgroun d acylation , wh ich n egatively in fluen ced th e values of selectivity
factor. Un fortun ately, we were un able to extract th e backgroun d reaction
n um erically as th e selectivity factor was h igh ly sen sitive to con version an d,
in th is case, we foun d it too diffusive due to subtractin g two values affected
by an experim en tal error. A h igh er catalyst loadin g (Table I, en tries 5 an d 6)
led to a faster reaction but selectivity factor was practically un ch an ged.
Sim ilarly, kin etic resolution perform ed at sligh tly elevated tem perature
(Table I, en try 5) resulted in an alogous selectivity factor (s = 10).
TABLE I
Acylative kin etic resolution of racem ic alcoh ol 6 em ployin g (–)-(M)-5 as organ ocatalysta
(–)-(M)-5 Tem p.
Tim e
h
Con version
%
ee (%) of
ee (%) of
Selectivity
En try
b
m ole %
°C
(+)-(R)-6c
(–)-(S)-7d
factore
s
1
2
3
4
5
6
5
5
30
30
30
30
40
22
37
62
20.3 (3.4)f
42.2 (8.6)f
46.9 (10.5)f
69.2 (27.6)f
72.0 (19.0)f
65.0
19.1
50.4
53.1
84.3
99.0
92.0
n d
9
9
n d
5
84
n d
7
5
146
112
68
n d
5
10
20
n d
10
9
50.0
a
Isobutyric an h ydride (0.75 equiv.), N-eth yldiisopropylam in e (0.75 equiv.), ch loroform , un -
b
c
less stated oth erwise. Determ in ed by GC-MS. Determ in ed by HPLC on a Ch iralcel OD-H
colum n . Determ in ed by HPLC on a Ch iralcel OD-H colum n after sapon ification of th e es-
ter (n d: n ot determ in ed). Not corrected to th e un catalysed backgroun d reaction . Con ver-
sion of un catalysed reaction (th e values in paren th eses are related to th e blan k experim en ts
with out (–)-(M)-5 un der oth erwise iden tical con dition s). Isobutyric an h ydride (1.25 equiv.)
d
e
f
g
h
added in two portion s: 0.75 equiv. at th e begin n in g an d 0.5 equiv. after 84 h . Isobutyric
an h ydride (1.0 equiv.). Isobutyric an h ydride (2.3 equiv.), N-eth yldiisopropylam in e (2.3
i
equiv.).
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1151–1159

1156
Šámal, Míšek, Stará, Starý:
We reason ed con version as well as selectivity factor m igh t h ave been sol-
ven t-depen den t an d, th erefore, we perform ed acylative kin etic resolution of
racem ic alcoh ol 6 un der (–)-(M)-5 catalysis in a series of solven ts (Table II).
Accordin gly, we m on itored th e h igh est reactivity in solven ts with a low di-
electric con stan t (ε < 5) such as toluen e an d ch loroform (Table II, en tries 1
an d 2). Sim ilarly, th e h igh est selectivity factor we observed in ch loroform
(s = 9; Table II, en try 2).
TABLE II
Solven t-depen den t acylative kin etic resolution of racem ic alcoh ol 6 em ployin g (–)-(M)-5 as
organ ocatalysta
Dielectric
con stan tb
Con version
%
ee (%) of
Selectivity
En try
Solven t
c
(+)-(R)-6d
factore
s
1
2
3
4
5
6
7
8
toluen e
2.4
4.8
37.3
25.9
13.2
16.1
5.4
36.2
26.1
10.8
8.4
6
9
7
3
4
4
3
2
ch loroform
2-m eth yl-2-butan ol
eth yl acetate
THF
5.8
6.1
7.5
3.2
dich lorom eth an e
aceton e
8.9
16.4
4.5
10.2
2.1
21.0
36.6
aceton itrile
6.5
2.6
a
Isobutyric an h ydride (1.0 equiv.), N-eth yldiisopropylam in e (0.75 equiv.), (–)-(M)-5 (5 m ole %),
r.t., 48 h . For ε, see ref.31 Determ in ed by GC-MS. Determ in ed by HPLC on a Ch iralcel
OD-H colum n . Not corrected to th e un catalysed backgroun d reaction .
b
c
d
e
Sin ce carryin g out acylative kin etic resolution of racem ic alcoh ol 6
catalysed by (–)-(M)-5 in ch loroform resulted in th e h igh est selectivity fac-
tor an d good reactivity, we exam in ed th e role of th e base an d an h ydride
n ature un der such con dition s (Table III). Th e experim en ts sh owed selectiv-
ity factor was practically in varian t to th e base added but th e presen ce of
N-eth yldiisopropylam in e led to th e h igh est con version (Table III, en tries
1–3). A direct com parison of various an h ydrides revealed th at th e use of
acetan h ydride led to th e h igh est con version wh ile th e addition of
isobutyric an h ydride resulted in th e h igh est selectivity factor in th is series
(Table III, en tries 4–7).
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1151–1159

Organocatalysis with Azahelicenes
1157
TABLE III
Base- an d an h ydride-depen den t acylative kin etic resolution of racem ic alcoh ol 6 em ployin g
(–)-(M)-5 as organ ocatalysta
Con version
%
ee (%) of
Selectivity
En try
Base
An h ydride
b
(+)-(R)-6c
factord
s
1
2
3
4
5
6
7
Et3N
(i-PrCO)2O
(i-PrCO)2O
(i-PrCO)2O
(i-PrCO)2Oe
(EtCO)2Oe
(Ph CO)2Oe
(CH3CO)2Oe
16.6
14.5
21.5
25.9
32.9
0.6
14.4
12.2
19.7
26.1
19.6
n d
7
i-Pr2NH
i-Pr2NEt
i-Pr2NEt
i-Pr2NEt
i-Pr2NEt
i-Pr2NEt
7
7
9
3
n d
3
61.6
48.8
a
b
An h ydride (0.75 equiv.), base (0.75 equiv.), (–)-(M)-5 (5 m ole %), r.t., 48 h . Determ in ed
c
d
by GC-MS. Determ in ed by HPLC on a Ch iralcel OD-H colum n (n d: n ot determ in ed). Not
corrected to th e un catalysed backgroun d reaction (n d: n ot determ in ed). An h ydride (1.0
e
equiv.).
METHODS
Con version C (%) was calculated from en an tiom eric excesses of un reacted
alcoh ol (eeA, %) an d form ed ester (eeE, %; estim ated after its sapon ification )
accordin g to th e equation 26,27 C = 100 × eeA/(eeA + eeE) or, in m ost cases,
from th e alcoh ol/ester relative ratio obtain ed by GC-MS an alyses (corrected
to th e alcoh ol/ester calibration lin e). Acylative kin etic resolution of racem ic
secon dary alcoh ol was treated as sim ple first-order kin etics for th e substrate
in th e absen ce of n on lin ear effects in volvin g th e catalyst28. Selectivity fac-
tor s was calculated from con version (C, %) an d en an tiom eric excess of
un reacted alcoh ol (eeA, %) accordin g to th e equation s = ln [(1 – 0.01C) × (1 –
0.01eeA)] / ln [(1 – 0.01C) × (1 + 0.01eeA)] or en an tiom eric excess of form ed
ester (eeE, %) accordin g to th e equation s = ln [(1 – 0.01C) × (1 + 0.01eeA)] /
ln [1 – 0.01C × (1 + 0.01eeA)] 28,29
.
CONCLUSION
We described, for th e first tim e, th e successful utilisation of a h elicen e-
based organocatalyst in acylative kinetic resolution of racemic alcohol. Employ-
in g rac-6, (–)-(M)-5 (5–20 m ole %), isobutyric an h ydride an d N-eth yldiiso-
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1151–1159

1158
Šámal, Míšek, Stará, Starý:
propylam in e in ch loroform at 22–40 °C, we could m on itor an asym m etric
catalytic acyl tran sfer reaction overwh elm in g th e un catalysed backgroun d
process. Furth erm ore, we recorded m oderate selectivity factor (s = 9, 10). Al-
th ough oth er organ ocatalysts kn own to date exh ibited h igh er catalytic ac-
tivities as well as selectivity factors, we succeeded in applyin g a bare
pyridoh elicen e m olecule with out th e activatin g dim eth ylam in o group. We
proved th e h elically ch iral organ ocatalyst could effectively con trol th e
en an tiodiscrim in atin g acyl tran sfer step ach ievin g up to 99% ee of (+)-(R)-6
at 72% con version . Furth er studies on th e application of oth er n on racem ic
azah elicen es to organ ocatalysis are in progress.
EXPERIMENTAL
1H NMR spectra were m easured at 499.8 or 500.13 MHz, 13C NMR spectra at 125.7 MHz, in
CDCl3 with TMS as an in tern al stan dard. Com poun ds (+)-(P)-4 an d (–)-(M)-5 were prepared
earlier in th e auth ors′ laboratory10. Com m ercially available reagen t grade m aterials were
used as received. Toluen e, dich lorom eth an e, eth yl acetate, aceton e, aceton itrile,
N-eth yldiisopropylam in e, diisopropylam in e an d trieth ylam in e were distilled from CaH2 un der
n itrogen (aceton e was quickly distilled to m in im ise its decom position ). Ch loroform stabi-
lised with eth an ol (1 vol.%) was wash ed with water (3 ×), dried over an h ydrous CaCl2 over-
n igh t, distilled from fresh an h ydrous CaCl2 an d redistilled from CaH2 un der argon (dry
ch loroform was stored over 3Å m olecular sieves in refrigerator). THF was fresh ly distilled
from sodium /ben zoph en on e un der n itrogen . 2-Meth yl-2-butan ol (Reagen tPlus®) was used as
purch ased. TLC was perform ed on Silica gel 60 F254-coated alum in ium sh eets (Merck) an d
spots were detected with a solution of Ce(SO4)2·4H2O (1%) an d H3P(Mo3O10
ric acid (10%).
) (2%) in sulfu-
4
En an tiom eric excess of alcoh ol 6 was determ in ed by ch iral HPLC on a Ch iralcel OD-H
colum n (250 × 4.6 m m , n-h eptan e–isopropan ol 95:5, 0.6 m l/m in , 18 °C) usin g UV (254 n m )
an d polarim etric detectors; reten tion tim es: 16 m in for (+)-(R)-6, 19 m in for (–)-(S)-6. Th e (R)
absolute con figuration of (+)-en an tiom er an d (S) con figuration of (–)-en an tiom er were as-
sign ed accordin g to literature data30. Con version (th e alcoh ol 6 to ester 7 ratio) was deter-
m in ed by GC-MS usin g an Agilen t 19091s-443 colum n (HP-5MS, 5% ph en ylm eth ylsiloxan e;
n om in al len gth , diam eter an d film th ickn ess: 30 m × 250 µm × 0.25 µm ).
General procedure: A Sch len k flask was ch arged with (–)-(M)-2-aza[6]h elicen e 5 (4.1 m g,
0.013 m m ol, 5 m ole %) an d flush ed with argon . Dry ch loroform (1 m l), racem ic 1-ph en yl-
eth an ol 6 (30 µl, 0.25 m m ol) an d N-eth yldiisopropylam in e (32 µl, 0.19 m m ol, 0.75 equiv.)
were added and the solution was treated with isobutyric anhydride (31 µl, 0.19 mmol, 0.75 equiv.).
Th e reaction m ixture was stirred un der argon at 30 °C for a period specified in Tables I–III.
Sam ples were con tin uously taken up an d an alysed by GC-MS an d ch iral HPLC.
This research was supported by the Czech Science Foundation under Grant No. 203/07/1664 and
203/09/1766, by the Grant Agency of the Academy of Sciences of the Czech Republic under Grant
No. IAA400550916, by the Ministry of Education, Youth and Sports of the Czech Republic under
Project No. MSM0021620857 and LC512 (the Center for Biomolecules and Complex Molecular
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1151–1159

Organocatalysis with Azahelicenes
1159
Systems) and by the Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the
Czech Republic (this work is part of the Research Project Z4 055 0506).
REFERENCES
1. Sato K., Arai S. in: Cyclophane Chemistry for the 21st Century (H. Takemura, Ed.), pp. 173–197.
Research Signpost, Trivandrum 2002.
2. Murguly E., McDonald R., Branda N. R.: Org. Lett. 2000, 2, 3169.
3. Tanaka K., Kitahara Y., Suzuki H., Osuga H.: Tetrahedron Lett. 1996, 37, 5925.
4. Staab H. A., Diehm M., Krieger C.: Tetrahedron Lett. 1994, 35, 8357.
5. Staab H. A., Zirnstein M. A., Krieger C.: Angew. Chem., Int. Ed. Engl. 1989, 28, 86.
6. Graule S., Rudolph M., Vanthuyne N., Autschbach J., Roussel C., Crassous J., Réau R.:
J. Am. Chem. Soc. 2009, 131, 3183.
7. Abbate S., Bazzini C., Caronna T., Fontana F., Gambarotti C., Gangemi F., Longhi G.,
Mele A., Sora I. N., Panzeri W.: Tetrahedron 2006, 62, 139.
8. Bazzini C., Brovelli S., Caronna T., Gambarotti C., Giannone M., Macchi P., Meinardi F.,
Mele A., Panzeri W., Recupero F., Sironi A., Tubino R.: Eur. J. Org. Chem. 2005, 1247.
9. Martin R. H., Deblecker M.: Tetrahedron Lett. 1969, 10, 3597.
10. Míšek J., Teplý F., Stará I. G., Tichý M., Šaman D., Císařová I., Vojtíšek P., Starý I.:
Angew. Chem. Int. Ed. 2008, 47, 3188.
11. Andronova A., Szydlo F., Teplý F., Tobrmanová M., Volot A., Stará I. G., Starý I., Rulíšek L.,
Šaman D., Cvačka J., Fiedler P., Vojtíšek P.: Collect. Czech. Chem. Commun. 2009, 74, 189.
12. Takenaka N., Sarangthem R. S., Captain B.: Angew. Chem. Int. Ed. 2008, 47, 9708.
13. Roithová J., Schröder D., Míšek J., Stará I. G., Starý I.: J. Mass Spectrom. 2007, 42, 1233.
14. Míšek J., Tichý M., Stará I. G., Starý I., Schröder D.: Collect. Czech. Chem. Commun. 2009,
74, 323.
15. Ehala S., Míšek J., Stará I. G., Starý I., Kašička V.: J. Sep. Sci. 2008, 31, 2686.
16. Xu S., Held I., Kempf B., Mayr H., Steglich W., Zipse H.: Chem. Eur. J. 2005, 11, 4751.
17. Scriven E. F. V.: Chem. Soc. Rev. 1983, 12, 129.
18. Höfle G., Steglich W., Vorbrüggen H.: Angew. Chem., Int. Ed. Engl. 1978, 17, 569.
19. Spivey A. C., McDaid P. in: Enantioselective Organocatalysis (P. I. Dalko, Ed.), pp. 287–329.
Wiley–VCH, Weinheim 2007.
20. Ruble J. C., Latham H. A., Fu G. C.: J. Am. Chem. Soc. 1997, 119, 1492.
21. Fu G. C.: Acc. Chem. Res. 2000, 33, 412.
22. Birman V. B., Li X.: Org. Lett. 2006, 8, 1351.
23. Birman V. B., Li X.: Org. Lett. 2008, 10, 1115.
24. Ruble J. C., Latham H. A., Fu G. C.: J. Am. Chem. Soc. 1998, 120, 11532.
25. Spivey A. C., Fekner T., Spey S. E.: J. Org. Chem. 2000, 65, 3154.
26. Spivey A. C., Arseniyadis S., Fekner T., Maddaford A., Leese D. P.: Tetrahedron 2006, 62, 295.
27. Yamada S., Misono T., Iwai Y., Masumizu A., Akiyama Y.: J. Org. Chem. 2006, 71, 6872.
28. Vedejs E., Jure M.: Angew. Chem. Int. Ed. 2005, 44, 3974.
29. Kagan H. B., Fiaud J. C.: Top. Stereochem. 1988, 18, 249.
30. Polavarapu P. L., Michalska D. F., Neergaard J. R., Smith H. E.: J. Am. Chem. Soc. 1984,
106, 3378.
31. Handbook of Chemistry and Physics, 77th ed. (D. R. Lide, Ed.). CRC Press, Boca Raton 1996.
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1151–1159