P*,P*-Bidentate diastereoisomeric bisdiamidophosphites based on N-benzyltartarimide and their applications in asymmetric catalytic processes

Article abstract of DOI:10.1016/j.tetasy.2009.10.022

Two novel P^*-stereogenic bisdiamidophosphites derived from (3R,4R)-N-benzyltartarimide as a chiral 1,2-diol have been prepared from readily available starting materials. Palladium and rhodium catalytic systems containing these new P^*

Full text of DOI:10.1016/j.tetasy.2009.10.022

Tetrahedron: Asymmetry 20 (2009) 2490–2496
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
P*,P*-Bidentate diastereoisomeric bisdiamidophosphites based on
N-benzyltartarimide and their applications in asymmetric catalytic processes
a
b,
Konstantin N. Gavrilov a, , Sergey V. Zheglov , Eduard B. Benetsky , Anton S. Safronov b,
,
*
Eugenie A. Rastorguev a, Nikolay N. Groshkin a, Vadim A. Davankov b, , Benjamin Schäffner c,à
,
Armin Börner c,à
a Department of Chemistry, Ryazan State University, 46 Svoboda Street, 390000 Ryazan, Russian Federation
b Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Street, 119991 Moscow, Russian Federation
c Leibniz-Institut für Katalyse an der Universität Rostock e.V., A.-Einstein-Str. 29a, 18059 Rostock, Germany
a r t i c l e i n f o
a b s t r a c t
Two novel P*-stereogenic bisdiamidophosphites derived from (3R,4R)-N-benzyltartarimide as a chiral 1,2-
diol have been prepared from readily available starting materials. Palladium and rhodium catalytic sys-
tems containing these new P*,P*-bidentate ligands afforded 96%, 83% and 65% ee in asymmetric allylic
substitution, hydrogenation and addition processes, respectively. These diastereomeric diamidophosph-
ites were found to be complementary stereoselectors.
Article history:
Received 10 August 2009
Accepted 21 October 2009
Available online 24 November 2009
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
of very promising P,P-bidentate phosphite-type ligands with stere-
ogenic phosphorus atoms in the literature.3h,5
Transition-metal asymmetric catalysis has found wide-spread
application and is the subject of intensive research both in industry
and academia.1 Phosphite-type chiral compounds have become
one of the most successful, versatile, and commonly used classes
of organophosphorus ligands for asymmetric catalysis due to their
ready accessibility, modular nature and applicability in a wide
range of metal-catalysed transformations. The most important
advantages of phosphite-type ligands include their pronounced
The sterically rigid fragment is also a desirable element of struc-
ture of the successful stereoinductors. For example, ligands LA and
LB with a pyrrolidine-2,5-dione backbone afford high enantioselec-
tivities for the Rh-catalysed hydrogenation of enamides and
Pd-catalysed allylic alkylation (Fig. 1).6 Motivated by our contin-
uing efforts in the design and synthesis of novel P*-chiral ligands
for use in asymmetric catalysis,7 we have prepared P*,P*-bidentate
bisdiamidophosphites (SC,RP)-3 and (RC,SP)-3 as diastereomeric
stereoselectors. It should be noted that (SC,RP)-3 and (RC,SP)-3 have
a rigid pyrrolidine-2,5-dione fragment and diazaphospholidine
cycles. Notably, diazaphospholidines have several important
p-acidity, oxidation stability, as well as their synthetic availability
and low cost.2,3 In contrast to monodentate phosphites, libraries of
chiral bidentate phosphite-type ligands are scarce, despite the
importance of bidentate phosphorus ligands in asymmetric transi-
tion-metal-catalysed reactions.1 This is attributed to the intrinsi-
cally more complicated synthesis of bidentate ligands compared
to that of monodentate ligands. This is especially true for bidentate
ligands with the chirogenic phosphorus donor atoms. In their com-
plexes, the asymmetric phosphorus atoms bind directly to the me-
tal atom. This factor eliminates potentially inefficient secondary
transfer of chirality from the ligand backbone and, thus, provides
a more efficient chiral environment at the site where the enantio-
selection originates.4 Nevertheless, there are only a few examples
advantages. In general, they are good
p
-acceptor ligands, thanks
to the availability of low-lying pꢀPN orbitals, and good
r
-donor
ligands. Incorporation of the phosphorus atom in a cyclic structure,
in particular a five-membered ring, is a key feature, as it increases
stability towards air and moisture.8
2. Results and discussion
2.1. Synthesis of the ligands
(3R,4R)-N-Benzyltartarimide 2 was chosen as a suitable build-
ing block for the synthesis of novel P*,P*-bidentate ligands
(Scheme 1). This diol is readily prepared by the direct condensa-
tion of tartaric acid with benzylamine.9 Then, bisdiamidophosph-
ites (SC,RP)-3 and (RC,SP)-3, based on rigid pyrrolidine-2,5-dione
backbone, were synthesised stereospecifically in two steps from
* Corresponding author. Tel./fax: +7 4912 28 0580.
E-mail addresses: k.gavrilov@rsu.edu.ru (K.N. Gavrilov), eduardben@mail.ru (E.B.
Benetsky), armin.boerner@catalysis.de (A. Börner).
Fax: +7 4951 35 6471.
à
Fax: +49 381 12815000.
0957-4166/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2009.10.022

K. N. Gavrilov et al. / Tetrahedron: Asymmetry 20 (2009) 2490–2496
2491
O
O
P
O
R
P
Ph
O
O
N
N
P
O
R
O
O
P
O
O
LA
LB
Figure 1. Bidentate organophosphorus ligands with a pyrrolidine-2,5-dione backbone.
4/
5/
6/
N
P
N
7/
8/
O
O
O
N
P
N
O
N
(SC,RP)-3
OH
yield 74%
O
2 Et3N, THF
- 2 HEt3NCl
N
Cl
P
N
+
OH
2
or
N
O
4/
5/
6/
2
N
(SC,RP)-1
or
(RC,SP)-1
P
N
7/
8/
O
O
O
N
P
N
N
O
(RC,SP)-3
yield 70%
Scheme 1. Synthesis of the diastereomeric bisdiamidophosphites (SC,RP)-3 and (RC,SP)-3.
(S)-2-(anilinomethyl)pyrrolidine or (R)-2-(anilinomethyl)pyrroli-
dine. In turn, these enantiomeric diamines are easily accessible
in two steps from (S)- or (R)-glutamic acid, respectively.10 These
compounds react with PCl3 in benzene to yield phosphorylating
reagents (SC,RP)-1 and (RC,SP)-1.7a Subsequent treatment of suit-
able chlorodiamidophosphites (SC,RP)-1 or (RC,SP)-1 with diol 2
in THF affords the desired compounds. The reaction requires the
presence of Et3N as an HCl scavenger. Flash chromatography of
the crude products affords pure ligands (SC,RP)-3 and (RC,SP)-3 as
yellow powders in 74% and 70% yields, respectively. It is notewor-
thy that owing to the ease of each step, as well as the easy-to-
handle nature of all the related intermediates, (SC,RP)-3 and
During the phosphorylation process, the exclusive formation of
stereospecific bisdiamidophosphites (SC,RP)-3 and (RC,SP)-3 takes
place. The 31P NMR spectra of their solutions in CDCl3 exhibited
narrow singlets at dP 130.8 and 128.6, respectively. According to
the literature data, both natural (SC,RP)-3 and unnatural (RC,SP)-3
diastereomers [based on natural (S)- and unnatural (R)-glutamic
acid, respectively] have the pseudoequatorial orientation of the
exocyclic substituent at the phosphorus atom and anti-orientations
between this substituent and the –(CH2)3– part of the pyrrolidine
fragment of phosphabicyclic skeleton and, as consequence, cis-ori-
entations of the phosphorus atom lone pair and C(80).7a,10b,11 This
2
was concluded from the large J(C(80),P) value (35.0 Hz) in their
(RC,SP)-3 can be prepared on
a
multigram scale. These bis-
13C NMR spectra (see Section 4).7a,10b,12 In the case of the ligand
(SC,RP)-3 based on natural (S)-glutamic acid, this corresponds to
the (R)-configuration at the P*-stereocentres. Correspondingly,
diamidophosphites were found to be stable enough to allow
manipulation in open air and can be stored under a dry atmo-
sphere for several months at room temperature without any
degradation. Besides, they are rather stable in solutions in
common organic solvents.
*
the unnatural ligand (RC,SP)-3 with a (R)-C (50) stereocentre in
the phosphabicyclic skeleton is characterised by an (S)-configura-
tion of the asymmetric phosphorus atoms (Fig. 2).7c,10b,11e

2492
K. N. Gavrilov et al. / Tetrahedron: Asymmetry 20 (2009) 2490–2496
(83–90%) and enantioselectivity (up to 92%, Table 1, entries 1–4).
(S)
(R)
Natural diastereomer (SC,RP)-3 provides much higher asymmetric
induction than unnatural (RC,SP)-3. In the case of allylic alkylation
with (SC,RP)-3, reactions in CH2Cl2 gave higher conversion and
enantioselectivities than those in THF. The molar ratio L/Pd = 1 is
optimal. In contrast to allylic sulfonylation, unnatural ligand
(RC,SP)-3 is slightly more stereoselective (Table 1, entries 5 and 9)
demonstrating excellent enantioselectivity (up to 96%). Surpris-
ingly, palladium complexes with (RC,SP)-3 did not show any activ-
ity in THF solutions (Table 1, entries 11 and 12).
..
P
..
P
8/
8/
N
N
(
N
N
(R)
(S)
X
X = exocyclic substituent
unnatural (RC,SP)-diastereomer
X
natural (SC,RP)-diastereomer
Figure 2. Stereochemistry of the phosphabicyclic part in ligands (SC,RP)-3 and
(RC,SP)-3.
Further studies focused on the asymmetric allylic amination of
(E)-1,3-diphenylallyl acetate 4 (Table 2). Firstly, the new ligands
were tested in the Pd-catalysed allylic amination of 4 with dipro-
pylamine as an N-nucleophile, under standard conditions.
2.2. Pd-catalysed asymmetric allylic substitution
The enantioselective Pd-catalysed allylic substitution has
emerged as a powerful synthetic tool. In addition to a high level
of asymmetric induction, the advantages of this method include
its tolerance of a wide range of functional groups and great flexibil-
ity in the type of bonds that can be formed.13 It should be noted
that many ligands have been designed for the benchmark allylic
substitution with (E)-1,3-diphenylallyl acetate and led to excellent
enantioselectivity. Nevertheless, the availability of new diverse
chiral ligands and a necessity to estimate their efficiency are strong
motivators for further investigations.1,2,13 With novel P*,P*-biden-
tate ligands in hand, we set out to study their application in the
Pd-catalysed asymmetric allylic sulfonylation and alkylation of
(E)-1,3-diphenylallyl acetate 4 with sodium para-toluene sulfinate
and dimethyl malonate (Table 1). Both reactions are common
benchmark tests for novel groups of stereoselectors, and powerful
tools in the total synthesis of natural products.13b
Table 2
Pd-catalysed allylic amination of (E)-1,3-diphenylallyl acetate 4a
Nu
*
OAc
Ph
NuX, cat
Solvents
Ph
Ph
Ph
4
5c,d
Nu = N(Pr)2, X = H, 5c
Nu = N(CH2)4, X = H, 5d
Entry
Ligand
L/Pd
Solvent
Conv. (%)
ee (%)
Allylic amination with dipropylamineb
1
2
3
4
5
6
7
8
(SC,RP)-3
(SC,RP)-3
(SC,RP)-3
(SC,RP)-3
(RC,SP)-3
(RC,SP)-3
(RC,SP)-3
(RC,SP)-3
1/1
2/1
1/1
2/1
1/1
2/1
1/1
2/1
CH2Cl2
CH2Cl2
THF
70
50
18
20
51 (+)
50 (+)
20 (+)
53 (+)
73 (ꢁ)
60 (ꢁ)
70 (ꢁ)
45 (ꢁ)
THF
Table 1
73 (66)c
100
75
Pd-catalysed allylic sulfonylation and alkylation of (E)-1,3-diphenylallyl acetate 4a
CH2Cl2
CH2Cl2
THF
OAc
Ph
Nu
*
THF
98
NuX, cat
Solvents
Ph
Ph
Allylic amination with pyrrolidined
Ph
9
10
11
12
13
14
15
16
(SC,RP)-3
(SC,RP)-3
(SC,RP)-3
(SC,RP)-3
(RC,SP)-3
(RC,SP)-3
(RC,SP)-3
(RC,SP)-3
1/1
2/1
1/1
2/1
1/1
2/1
1/1
2/1
CH2Cl2
CH2Cl2
THF
100
100
42
26
95 (85)
98
97
99
46 (R)
60 (R)
30 (R)
10 (R)
73 (S)
71 (S)
57 (S)
67 (S)
4
5a,b
Nu = SO2pTol, X = Na, 5a
Nu = CH(CO2Me)2 X = H, 5b
THF
e
CH2Cl2
CH2Cl2
THF
Entry
Ligand
L/Pd
Solvent
Conv.b (%)
ee (%)
THF
Allylic sulfonylation with sodium para-toluene sulfinatec
a
1
2
3
4
(SC,RP)-3
(SC,RP)-3
(RC,SP)-3
(RC,SP)-3
1/1
2/1
1/1
2/1
THF
THF
THF
THF
83
90
86
89
58 (R)
92 (R)
55 (S)
46 (S)
All reactions were carried out with 2 mol % of [Pd(allyl)Cl]2 at room tempera-
ture for 48 h.
b
The conversion of the substrate 4 and enantiomeric excess of 5c were deter-
mined by HPLC (Daicel Chiralcel OD-H, C6H14/i-PrOH/HN(Et)2 = 1000/1/1, 0.4 ml/
min, 254 nm, t(+) = 8.2 min, t(ꢁ) = 9.1 min).
Allylic alkylation with dimethyl malonate (BSA, KOAc)d
c
Isolated yield of 5c is given in brackets.
The conversion of the substrate 4 and enantiomeric excess of 5d were deter-
5
6
7
8
9
10
11
12
(SC,RP)-3
(SC,RP)-3
(SC,RP)-3
(SC,RP)-3
(RC,SP)-3
(RC,SP)-3
(RC,SP)-3
(RC,SP)-3
1/1
2/1
1/1
2/1
1/1
2/1
1/1
2/1
CH2Cl2
CH2Cl2
THF
100
63
90
89 (S)
70 (S)
85 (S)
40 (S)
96 (R)
84 (R)
—
d
mined by HPLC (Daicel Chiralcel OD-H, OD-H, C6H14/i-PrOH/HN(Et)2 = 200/1/0,1
0.9 ml/min, 254 nm).
THF
34
e
CH2Cl2
CH2Cl2
THF
98 (92)e
Isolated yield of 5d is given in brackets.
97
0
0
THF
—
Both enantiomers of the product 5c can be obtained with mod-
erate to good enantioselectivity. The influence of the stereochem-
istry of the diastereoisomeric ligands, solvent, and L/Pd molar
ratio on the catalytic activity and enantioselectivity was investi-
gated. Listed factors strongly affect activity and enantioselectivity
of the allylic amination. As a rule, CH2Cl2 is the solvent of choice
and a molar ratio L/Pd = 1 is optimal. The best result (up to 73%
ee) was obtained with unnatural ligand (RC,SP)-3 (Table 2, entry
5). Pyrrolidine was also employed as an N-nucleophile in the allylic
amination of 4. Analogously to the reaction with dipropylamine,
CH2Cl2 is the solvent of choice, and unnatural diastereomer
(RC,SP)-3 is a good stereoinductor providing up to 73% ee (Table 2,
entry 13).
a
All reactions were carried out with 2 mol % of [Pd(allyl)Cl]2 at room tempera-
ture for 48 h.
b
Isolated yield of 5a in allylic sulfonylation.
c
Enantiomeric excess of 5a was determined by HPLC (Daicel Chiralcel OD, C6H14
/
i-PrOH = 4/1, 0,5 ml/min, 254 nm).
d
The conversion of substrate 4 and enantiomeric excess of 5b were determined
by HPLC (Daicel Chiralcel OD-H, C6H14/i-PrOH = 99/1, 0.6 ml/min, 254 nm).
e
Isolated yield of 5b is given in brackets.
The test reactions were evaluated using catalysts generated
in situ by interaction of [Pd(allyl)Cl]2 and an appropriate bis-
diamidophosphite. In particular, these new catalytic systems led
to the expected sulfonylated product 5a with very good conversion

K. N. Gavrilov et al. / Tetrahedron: Asymmetry 20 (2009) 2490–2496
2493
Table 4
To investigate further the potential of the new readily available
Rh-catalysed hydrogenation of a
-dehydrocarboxylic acid estersa
ligands, they were tested in the Pd-catalysed allylic alkylation of a
cyclic substrate, namely cyclohex-2-enyl ethyl carbonate 6, with
dimethyl malonate (Table 3). The enantioselectivity for the cyclic
substrate 6 is usually more difficult to control than for the
hindered substrate 4. For high ees to be achieved it is crucial that
ligands create a small chiral pocket around the metal centre,
mainly because of the presence of less sterically demanding syn
substituents.14
CO2R3
CO2R3
R2
R2
H2, [Rh(cod)2]BF4 / L
*
R1
R1
Solvents
8a-c
9a-c
It is clear that (RC,SP)-3 demonstrates a higher catalytic activity
and enantioselectivity than (SC,RP)-3 (Table 3, entries 1–4 and 5–8).
As in the aforementioned catalytic reactions, the process is rather
sensitive to the solvent used; best ee-values were observed in
CH2Cl2. In general, unnatural bisdiamidophosphite (RC,SP)-3
showed enantioselectivity of up to 65% in the asymmetric synthe-
sis of 7, which is appreciable for the sterically undemanding
substrate 6 (see Ref. 14 and references cited therein).
R1 = H, R2 = CH2CO2Me, R3 = Me, 8a and 9a
R1 = Ph, R2 = NHAc, R3 = Me, 8b and 9b
R1 = H, R2 = NHAc, R3 = Me, 8c and 9c
Entry Substrate Ligand
Solvent Time (min) Conv.b,c,d (%) ee (%)
1
2
3
4
5
6
7
8
8a
8a
8a
8b
8b
8b
8c
8c
(SC,RP)-3 CH2Cl2
(RC,SP)-3 PCe
(RC,SP)-3 CH2Cl2
(SC,RP)-3 CH2Cl2
(RC,SP)-3 PC
1440
1200
1200
1440
960
100
16
94
100
85
16 (S)
59 (R)
83 (R)
26 (R)
73 (S)
83 (S)
30 (R)
66 (S)
(RC,SP)-3 CH2Cl2
(SC,RP)-3 CH2Cl2
(RC,SP)-3 CH2Cl2
1200
1440
1440
93
100
100
Table 3
Pd-catalysed allylic alkylation of cyclohex-2-enyl ethyl carbonate 6 with dimethyl
malonatea
a
All reactions were carried out with 1 mol % of [Rh(cod)2]BF4 at 25 °C and 1 bar
O
H2.
b
The conversion of substrate 8a and enantiomeric excess of 9a were determined
by GC (Lipodex E, 25 m ꢂ 0.25 mm, 80 °C, 1 ml/min) or HPLC (Daicel Chiralcel OD-H,
C6H14/i-PrOH = 98/2, 0.8 ml/min, 220 nm, t(R) = 9.1 min, t(S) = 16.1 min).
MeO2C
CO2Me
O
O
c
CH2(CO2Me)2, cat
Solvents
The conversion of the substrate 8b and enantiomeric excess of 9b were deter-
*
mined by GC (Lipodex E, 25 m ꢂ 0.25 mm, 145 °C, 1 ml/min).
d
The conversion of substrate 8c and enantiomeric excess of 9c were determined
6
7
by GC (XE-valin(tert-butylamide) 4 ꢂ 0.25 mm, 85 °C, 1 ml/min).
e
Propylene carbonate.
Entry
Ligand
L/Pd
Solvent
Conv.b (%)
ee (%)
1
2
3
4
5
6
7
8
(SC,RP)-3
(SC,RP)-3
(SC,RP)-3
(SC,RP)-3
(RC,SP)-3
(RC,SP)-3
(RC,SP)-3
(RC,SP)-3
1/1
2/1
1/1
2/1
1/1
2/1
1/1
2/1
CH2Cl2
CH2Cl2
THF
100
47
63
30
99
20 (R)
30 (R)
19 (R)
26 (R)
39 (S)
65 (S)
27 (S)
35 (S)
entry 8) was observed in the Rh-catalysed hydrogenation of sub-
strate 8c. Notably, in propylene carbonate, products 9a and 9b
were formed with lower enantioselectivity than in CH2Cl2. It is
important to note that rhodium catalysts, which are formed
in situ by the interaction of [Rh(cod)2]BF4 with one equivalent of
the appropriate bisdiamidophosphite in CH2Cl2, are cationic metal
chelates [Rh(cod)(L)]BF4. For (SC,RP)-3 and (RC,SP)-3, only doublets
of the desired complexes were observed in the 31P NMR spectra
of the reaction solutions (dP 121.1, 1J(P,Rh) = 214.3 Hz; dP 121.8,
1J(P,Rh) = 210.4 Hz, respectively). MALDI TOF/TOF mass spectral
examination of the resulting solutions revealed the mononuclear
nature of the complexes (m/z (I, %)): 841 (19) [MꢁBF4]+, 733
(100) [MꢁBF4ꢁcod]+.
THF
CH2Cl2
CH2Cl2
THF
84 (77)c
65
43
THF
a
All reactions were carried out with 2 mol % of [Pd(allyl)Cl]2 at room tempera-
ture for 48 h (BSA, KOAc).
The conversion of the substrate 6 and enantiomeric excess of 7 were deter-
mined by HPLC (Daicel Chiralcel AD, C6H14/i-PrOH = 200/1, 1 ml/min, 219 nm,
t(R) = 11.3 min, t(S) = 12.1 min).
b
c
Isolated yield of 7 is given in brackets.
Finally, novel P*,P*-bidentate ligands were applied as stereose-
lectors in the Rh-catalysed addition of phenylboronic acid to
2-methoxybenzaldehyde 10 (Table 5). The reaction was carried
out in 1,4-dioxane/H2O (1:1) in the presence of [Rh(C2H4)2Cl]2,
L/Rh = 1.
2.3. Rh-catalysed asymmetric hydrogenation and addition of
phenylboronic acid
Catalytic systems containing bisdiamidophosphites (SC,RP)-3
and (RC,SP)-3 were used in the Rh-catalysed asymmetric hydroge-
Unnatural bisdiamidophosphite (RC,SP)-3 exhibited quantitative
conversion, but asymmetric induction was very low. In contrast,
natural diastereomer (SC,RP)-3 provided a satisfactory conversion
and enantioselectivity (65%, Table 5, entry 1). The result achieved
with (SC,RP)-3 is rather remarkable, since the well-known bisphos-
phine (R,R)-i-Pr-DuPHOS and (S)-BINOL-based bisphosphoramidite
gave only up to 50% ee.15
nation of certain
a-dehydrocarboxylic acid esters. It is necessary
to note that the asymmetric catalytic hydrogenation of unsatu-
rated bonds, which employs dihydrogen and small amounts of
transition-metal complexes intrinsically modified by chiral
ligands, is now recognised as being the most promising strategy
for the synthesis of large amounts of enantiomerically pure prod-
ucts.4 In this regard, the results of the Rh-catalysed hydrogenation
of benchmark substrates 8a–c (Table 4) are presented. One can
thus directly compare the efficacy of different ligand systems.
Hydrogenation experiments were performed in propylene carbon-
ate or CH2Cl2 at room temperature (with [Rh(cod)2]BF4 as a
catalyst precursor, L/Rh = 1).
3. Conclusion
In conclusion, new diastereomeric P*,P*-bidentate bisdiamidoph-
osphites with a 3-phenyl-1,3-diaza-2-phosphabicyclo[3.3.0]octane
skeleton and pyrrolidine-2,5-dione backbone were synthesised
and evaluated in the Pd-catalysed asymmetric allylic substitution
and Rh-catalysed asymmetric hydrogenation and addition
reactions. It was shown that the novel stereoinductors are comple-
In all cases, unnatural bisdiamidophosphite (RC,SP)-3 was found
to be a superior sereoselector. In the transformation of 8a,b to 9a,b,
ligand (RC,SP)-3 showed good enantioselectivity (83% ee, Table 4,
entries 3 and 6). Moderate enantioselectivity (66% ee, Table 4,

2494
K. N. Gavrilov et al. / Tetrahedron: Asymmetry 20 (2009) 2490–2496
Table 5
substrate 6 with dimethyl malonate were performed according to
Rh-catalysed addition of phenylboronic acid to 2-methoxybenzaldehydea
the appropriate procedures.7a,18–21 The Rh-catalysed hydrogena-
tion of
a-dehydrocarboxylic acid esters 8a–c was performed as
O
published.22,23 The Rh-catalysed addition of phenylboronic acid
to 2-methoxybenzaldehyde 10 was performed according to the
known procedure.24 Conversion of substrate 10 and ee of product
11 were determined using HPLC (Daicel Chiralcel OD-H column)
according to the literature.15b,24 Starting substrates 4, 6 and 8b
were synthesised as published.16,25,26 Dimethyl malonate, BSA
(N,O–bis(trimethylsilyl) acetamide), sodium para-toluene sulfi-
nate, dimethyl itaconate, methyl 2-acetamidoacrylate, 2-methoxy-
benzaldehyde, phenylboronic acid and [Rh(C2H4)2Cl]2 were
purchased from Aldrich and Acros Organics and used without
further purification.
O
OH
O
PhB(OH)2, [Rh(C2H4)2Cl]2/ 2L
*
1,4-dioxane/H2O
10
11
Entry
Ligand
Conv.b (%)
ee (%)
1
2
(SC,RP)-3
(RC,SP)-3
72 (63)c
100
65 (S)
3 (R)
a
All reactions were carried out with 1 mol % of [Rh(C2H4)2Cl]2 at room temper-
ature for 48 h (KFꢄ2H2O).
b
The conversion of the substrate 10 and enantiomeric excess of 11 were deter-
4.2. General procedure for the synthesis of ligands (SC,RP)-3 and
(RC,SP)-3
mined by HPLC (Daicel Chiralcel OD-H, C6H14/i-PrOH = 9/1, 1 ml/min, 230 nm,
t(S) = 14 min, t(R) = 14.3 min).
c
Isolated yield of 11 is given in brackets.
A solution of compound 2 (1.11 g, 5 mmol) in THF (15 ml)
was added dropwise at 20 °C over 20 min to a vigorously stirred
solution of (SC,RP)-1 or (RC,SP)-1 (2.41 g, 10 mmol) and Et3N (1.45
ml, 10.4 mmol) in THF (30 ml). The mixture was then heated to
the boiling point, stirred for 1.5 h, cooled to 20 °C and filtered.
The filtrate was concentrated in vacuo (40 Torr). The residue
was purified by flash chromatography on silica gel (hexane/
AcOEt 1:2).
mentary to each other in enantioselective catalysis. Inversion of the
*
absolute configuration at the C (50) stereocentre and, as a result, at
the P*(20) stereocentre in the phosphabicyclic frameworks of the
novel ligands allows us to control the degree and direction of asym-
metric induction. Thus, in each of the tested catalytic reactions,
diastereoisomeric ligands afforded products with opposite absolute
configurations. Furthermore, in the Pd-catalysed allylic sulfonyla-
tion of (E)-1,3-diphenylallyl acetateand inthe Rh-catalysed addition
of phenylboronic acid to 2-methoxybenzaldehyde, natural diaste-
reomer (SC,RP)-3 is a more efficient stereoselector; however, in the
Pd-catalysed allylic alkylation of (E)-1,3-diphenylallyl acetate and
cyclohex-2-enyl ethyl carbonate, allylic amination of (E)-1,3-
diphenylallyl acetate and in the Rh-catalysed hydrogenation of
4.2.1. 3,4-Bis[(20R,50S)-30-phenyl-10,30-diaza-20-phosphabicy-
clo[3.3.0]octyloxy]-(3R,4R)-1- benzylpyrrolidine-2,5-dione
(SC,RP)-3
(Hexane/AcOEt 1:2 Rf = 0.80) (2.33 g, yield 74% as yellow pow-
der).
½
a 2D0
ꢃ
¼ ꢁ288:9 (c 1.0, CHCl3). IR (CHCl3):
m(C@O) =
1708 cmꢁ1
.
1H NMR (CDCl3) d: 1.83 (dq, 2H, J = 10.7 Hz), 1.9 (m,
a-dehydrocarboxylic acid esters, unnatural diastereomer (RC,SP)-3
4H), 2.1 (dq, 2H, J = 10.9 Hz), 2.4 (ddd, 2H, J = 8.3, 6.9, 5.7 Hz),
3.1 (m, 4H), 3.6 (m, 4H), 4.68 (m, 4H), 6.65 (d, 4H, J = 7.7 Hz),
7.09 (t, 4H, J = 7.6 Hz), 7.2 (m, 3H), 7.36 (d, 2H, J = 7.7, 7.3 Hz),
7.74 (d, 2H, J = 8.3 Hz). 13C NMR (CDCl3) d: 26.4 [d, 3J = 4.4 Hz,
is superior. Further modifications of the new ligands and their appli-
cations to other asymmetric reactions are currently in progress in
our laboratories.
2
C(70)], 31.9 [s, C(60)], 42.4 (s, CH2N), 48.1 [d, J = 35.0 Hz, C(80)],
2
2
53.8 [d, J = 7.3 Hz, C(40)], 62.9 [d, J = 8.0 Hz, C(50)], 74.7 (s,
4. Experimental
CHO), 115.5 (d, 3J = 13.1 Hz, CHAr), 119.6 (s, CHAr), 128.0 (s, CHAr
,
4.1. General methods
Bn), 128.7 (s, CHAr, Bn), 128.9 (s, CHAr, Bn), 129.2 (s, CHAr), 135.3
(s, CAr, Bn), 145.2 (d, 2J = 16.0 Hz, CAr), 172.4 (s, C@O). MS (MALDI
TOF/TOF), m/z (I, %): 669 (61) [M+K]+, 653 (100) [M+Na]+, 631
(11) [M+H]+, 408 (76) [MꢁC11H15N2OP]+. Anal. Calcd for
C33H37N5O4P2: C, 62.95; H, 5.92; N, 11.12. Found: C, 63.20; H,
5.79; N, 11.22.
IR spectra were recorded on Specord M-80 spectrophotometer
in CHCl3 (NaCl cuvette). 31P, 13C and 1H NMR spectra were re-
corded with a Bruker AMX 400 instrument (162.0 MHz for 31P,
100.6 MHz for 13C and 400.13 MHz for 1H). Complete assignment
of all the resonances in 13C NMR spectra was achieved by the use
of DEPT techniques. Chemical shifts (ppm) are given relative to
4.2.2. 3,4-Bis[(20S,50R)-30-phenyl-10,30-diaza-20-phosphabicyclo-
[3.3.0]octyloxy]-(3R,4R)-1- benzylpyrrolidine-2,5-dione (RC,SP)-
3
Me4Si (1H and 13C) and 85% H3PO4 31P NMR). Mass spectra were
(
recorded with a Bruker Daltonics Ultraflex spectrometer (MALDI
TOF/TOF). Optical rotations were measured on a Perkin–Elmer
341 polarimeter. Elemental analyses were performed at the Labo-
ratory of Microanalysis (Institute of Organoelement Compounds,
Moscow).
(Hexane/AcOEt 1:2 Rf = 0.90) (2.2 g, yield 70% as yellow pow-
der).
½
a 2D0
ꢃ
¼ þ252:4 (c 1.0, CHCl3). IR (CHCl3):
m(C@O) =
1704 cmꢁ1
.
1H NMR (CDCl3) d: 1.86 (dq, 2H, J = 10.3 Hz), 2.1 (m,
4H), 2.2 (dq, 2H, J = 11.1 Hz), 2.4 (ddd, 2H, J = 8.1, 7.0, 5.9 Hz),
3.0 (m, 4H), 3.7 (m, 4H), 4.72 (m, 4H), 6.66 (d, 4H, J = 7.5 Hz),
7.19 (t, 4H, J = 7.6 Hz), 7.3 (m, 3H), 7.41 (d, 2H, J = 7.7, 7.2 Hz),
7.8 (d, 2H, J = 8.2 Hz). 13C NMR (CDCl3) d: 26.6 [d, 3J = 4.4 Hz,
All reactions were carried out under a dry argon atmosphere
in flame-dried glassware and in freshly dried and distilled
solvents; triethylamine, pyrrolidine and dipropylamine were dis-
tilled over KOH and then over a small amount of LiAlH4 before
use. Phosphorylating reagents—(2R,5S)-2-chloro-3-phenyl-1,3-dia-
za-2-phosphabicyclo[3.3.0]octane (SC,RP)-1 and (2S,5R)-2-chloro-3-
phenyl-1,3-diaza-2-phosphabicyclo[3.3.0]octane (RC,SP)-1 were
2
C(70)], 31.5 [s, C(60)], 42.4 (s, CH2N), 48.0 [d, J = 35.0 Hz, C(80)],
2
2
53.7 [d, J = 6.6 Hz, C(40)], 62.8 [d, J = 8.0 Hz, C(50)], 73.7 (s,
CHO), 115.3 (d, 3J = 12.4 Hz, CHAr), 119.5 (s, CHAr), 128.0 (s, CHAr
,
Bn), 128.7 (s, CHAr, Bn), 128.8 (s, CHAr, Bn), 129.2 (s, CHAr), 135.4
(s, CAr, Bn), 145.3 (d, 2J = 17.5 Hz, CAr), 172.9 (s, C@O). MS (MALDI
TOF/TOF), m/z (I, %): 669 (100) [M+K]+, 653 (89) [M+Na]+, 631
(17) [M+H]+, 408 (55) [MꢁC11H15N2OP]+. Anal. Calcd for
C33H37N5O4P2: C, 62.95; H, 5.92; N, 11.12. Found: C, 63.15; H,
6.0; N, 11.0.
16
prepared analogously to the known procedure.7a Pd(allyl)Cl]2 and
17
[Rh(cod)2]BF4 were prepared as described earlier. Pd-catalysed
allylic substitution: sulfonylation of substrate 4 with sodium
para-toluene sulfinate, alkylation with dimethyl malonate, amina-
tion with dipropylamine and pyrrolidine; and alkylation of

K. N. Gavrilov et al. / Tetrahedron: Asymmetry 20 (2009) 2490–2496
2495
4.3. General procedure for the synthesis of the rhodium com-
plexes with ligands (SC,RP)-3 and (RC,SP)-3 for the 31P NMR and
MALDI TOF/TOF experiments
diphenylallyl)pyrrolidine 5d. In order to evaluate the ee and
conversion, the obtained residue was dissolved in an appropriate
eluent mixture (8 ml) and a sample was taken for HPLC analysis.
Cationic rhodium complexes with diamidophosphites (SC,RP)-3
and (RC,SP)-3 were synthesised for the instrumental investigations
4.4.4. General procedure for the Rh-catalysed hydrogenation of
a-dehydrocarboxylic acid esters 8a–c
as follows:
a
solution of the appropriate ligand (0.031 g,
A solution of [Rh(cod)2]BF4 (0.002 g, 0.005 mmol) and the
0.05 mmol) in CH2Cl2 (1 ml) was added dropwise to a stirred solu-
tion of [Rh(cod)2]BF4 (0.02 g, 0.05 mmol) in the same solvent
(1 ml). A sample of the resulting solution (1 ml) was then trans-
ferred to an NMR tube and spectroscopic experiments were carried
out. In the case of MALDI TOF/TOF, a sample of the resulting solu-
tion (0.1 ml) was diluted with CH2Cl2 (10 ml) before spectroscopic
experiments.
appropriate ligand (0.003 g, 0.005 mmol) in 8 ml of the appropriate
solvent was stirred for 40 min. Then, the resulting solution and the
prochiral olefin (0.5 mmol) were transferred into the hydrogena-
tion device (a standard device for hydrogenation under 1 bar
hydrogen pressure) under a hydrogen atmosphere. The hydrogena-
tion was followed by measurement of the gas-consumption under
isothermic (25 °C) and isobaric (1 bar) conditions with an automat-
ically registering gas measuring device. When no further gas
consumption occurred the reaction was finished. The conversions
were determined by GC/HPLC simultaneously with determination
of the ee values. In some cases, conversions were also determined
by 1H NMR.
4.4. General experimental procedures for asymmetric catalytic
reactions
4.4.1. General procedure for the Pd-catalysed allylic sulfony-
lation of (E)-1,3-diphenylallyl acetate 4 with sodium para-
toluene sulfinate
4.4.5. General procedure for Rh-catalysed addition of phenyl-
boronic acid to 2-methoxybenzaldehyde 10
A solution of [Pd(allyl)Cl]2 (0.0037 g, 0.01 mmol) and the appro-
priate ligand (0.013 g or 0.026 g, 0.02 mmol or 0.04 mmol) in THF
(2 ml) was stirred for 40 min. (E)-1,3-Diphenylallyl acetate
(0.1 ml, 0.5 mmol) was added and the solution stirred for 15 min,
then sodium para-toluene sulfinate (0.178 g, 1 mmol) was added
and the reaction mixture stirred for a further 48 h, quenched with
brine (5 ml) and extracted with THF (3 ꢂ 3 ml). The organic layer
was washed with brine (2 ꢂ 3 ml) and dried over MgSO4. The sol-
vent was evaporated at reduced pressure (40 Torr). Crystallisation
of the residue from EtOH, followed by desiccation in vacuo
(10 Torr, 12 h), gave (E)-1,3-diphenyl-3-tosylprop-1-ene 5a as
white crystals. The enantiomeric excess of 5a was determined by
HPLC.
A solution of [Rh(C2H4)2Cl]2 (0.004 g, 0.0104 mmol) and the
appropriate ligand (0.013 g, 0.0208 mmol) in 1,4-dioxane (1.5 ml)
was stirred for 40 min. Then, PhB(OH)2 (0.25 g, 2.1 mmol),
KF ꢂ 2H2O (0.2 g, 2.1 mmol), water (1.5 ml) and 2-methoxybenzal-
dehyde (0.14 g, 1.04 mmol) were added sequentially and the reac-
tion mixture was stirred for further 48 h at room temperature. The
resulting mixture was extracted with CH2Cl2 (3 ꢂ 3 ml) and the or-
ganic layer was dried over MgSO4. The solvent was evaporated at
reduced pressure (40 Torr). The resulting (2-methoxyphenyl)(phe-
nyl)methanol 11 was purified by column chromatography on silica
gel (hexane/AcOEt 9:1) and subjected to enantiomeric excess anal-
ysis by chiral HPLC.
4.4.2. General procedure for Pd-catalysed allylic alkylation of
(E)-1,3-diphenylallyl acetate 4 or cyclohex-2-enyl ethyl carbon-
ate 6 with dimethyl malonate
Acknowledgements
We acknowledge the financial support from the INTAS Open
Call 2005-2006 Grant (INTAS Ref. No. 05-1000008-8064) and RFBR
Grant No. 08-03-00416-a.
A solution of [Pd(allyl)Cl]2 (0.0037 g, 0.01 mmol) and the appro-
priate ligand (0.013 g or 0.026 g, 0.02 mmol or 0.04 mmol) in the
appropriate solvent (2 ml) was stirred for 40 min. Then, (E)-1,3-
diphenylallyl acetate or cyclohex-2-enyl ethyl carbonate
(0.5 mmol) was added and the solution stirred for 15 min. Di-
methyl malonate (0.10 ml, 0.87 mmol), BSA (0.22 ml, 0.87 mmol)
and potassium acetate (0.002 g) were added. The reaction mixture
was stirred for 48 h, diluted with CH2Cl2 or THF (2 ml) and filtered
through Celite. The filtrate was evaporated at reduced pressure
(40 Torr) and dried in vacuo (10 Torr, 12 h) to afford a residue con-
taining dimethyl 2-((E)-1,3-diphenylallyl)malonate 5b or dimethyl
2-(cyclohex-2-enyl)malonate 7. In order to evaluate the ee and
conversion, the obtained residue was dissolved in appropriate elu-
ent mixture (8 ml) and a sample was taken for HPLC analysis.
References
1. (a) Brown, J. M.. In Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.;
Comprehensive Asymmetric Catalysis; Springer: Berlin, 1999; Vol. I, pp 121–
182; (b) Ohkuma, T.; Kitamura, M.; Noyori, R. In Catalytic Asymmetric Synthesis;
Ojima, I., Ed.; Wiley-VCH: New York, 2000; pp 1–110; (c) Blaser, H.-U.;
Schmidt, E. Asymmetric Catalysis on Industrial Scale; Wiley-VCH: Weinheim,
2004; (d) Goudriaan, P. E.; van Leeuwen, P. W. N. M.; Birkholz, M.-N.; Reek, J. N.
H. Eur. J. Inorg. Chem. 2008, 2939–2958.
2. Claver, C.; Pamies, O.; Dieguez, M. In Phosphorus Ligands in Asymmetric
Catalysis; Börner, A., Ed.; Vo1. II; Wiley-VCH: Weinheim, 2008; pp 507–528.
3. (a) Ansel, J.; Wills, M. Chem. Soc. Rev. 2002, 31, 259–268; (b) Alexakis, A.;
Benhaim, C. Eur. J. Org. Chem. 2002, 19, 3221–3236; (c) Molt, O.; Schrader, T.
Synthesis 2002, 2633–2670; (d) Reetz, M. T.; Mehler, G.; Meiswinkel, A.; Sell, T.
Tetrahedron Lett. 2002, 43, 7941–7943; (e) Dieguez, M.; Pamies, O.; Claver, C.
Tetrahedron: Asymmetry 2004, 15, 2113–2122; (f) Gavrilov, K. N.; Bondarev, O.
G.; Polosukhin, A. I. Russ. Chem. Rev. 2004, 73, 671–699; (g) Jerphagnon, T.;
Renaud, J.-L.; Bruneau, C. Tetrahedron: Asymmetry 2004, 15, 2101–2111; (h)
Reetz, M. T.; Mehler, G.; Bondarev, O. Chem. Commun. 2006, 2292–2294; (i) de
Vries, J. G.; Lefort, L. Chem. Eur. J. 2006, 12, 4722–4734; (j) Jagt, R. B. C.; Toullec,
P. Y.; Geerdink, D.; de Vries, J. G.; Feringa, B. L.; Minnaard, A. J. Angew. Chem., Int.
Ed. 2006, 45, 2789–2791.
4. Crepy, K. V. L.; Imamoto, T. Adv. Synth. Catal. 2003, 345, 79–101.
5. (a) Pastor, S. D.; Rodebaugh, R. K.; Odorisio, P. A.; Pugin, B.; Rihs, G.; Togni, A.
Helv. Chim. Acta 1991, 74, 1175–1193; (b) Dros, A. C.; Meetsma, A.; Kellogg, R.
M. Tetrahedron 1999, 55, 3071–3090.
6. (a) Enthaler, S.; Erre, G.; Junge, K.; Addis, D.; Kadyrov, R.; Beller, M. Chem. Asian
J. 2008, 3, 1104–1110; (b) Sanhes, D.; Gual, A.; Castillon, S.; Claver, C.; Gomez,
M.; Teuma, E. Tetrahedron: Asymmetry 2009, 20, 1009–1014.
7. (a) Tsarev, V. N.; Lyubimov, S. E.; Shiryaev, A. A.; Zheglov, S. V.; Bondarev, O. G.;
Davankov, V. A.; Kabro, A. A.; Moiseev, S. K.; Kalinin, V. N.; Gavrilov, K. N. Eur. J.
Org. Chem. 2004, 2214–2222; (b) Gavrilov, K. N.; Tsarev, V. N.; Shiryaev, A. A.;
4.4.3. General procedure for the Pd-catalysed allylic amination
of (E)-1,3-diphenylallyl acetate 4 with dipropylamine or pyrrol-
idine
A solution of [Pd(allyl)Cl]2 (0.0037 g, 0.01 mmol) and the appro-
priate ligand (0.013 g or 0.026 g, 0.02 mmol or 0.04 mmol) in the
appropriate solvent (2 ml) was stirred for 40 min. Then (E)-1,3-
diphenylallyl acetate (0.1 ml, 0.5 mmol) was added and the solu-
tion stirred for 15 min, then freshly distilled dipropylamine or pyr-
rolidine (1.5 mmol) was added and the reaction mixture was
stirred for further 48 h. The resulting solution was filtered through
Celite. The filtrate was evaporated at reduced pressure (40 Torr)
and dried in vacuo (10 Torr, 12 h) affording a residue containing
(E)-1,3-diphenyl-N,N-dipropylprop-2-en-1-amine 5c or 1-((E)-1,3-

2496
K. N. Gavrilov et al. / Tetrahedron: Asymmetry 20 (2009) 2490–2496
Bondarev, O. G.; Lyubimov, S. E.; Benetsky, E. B.; Korlyukov, A. A.; Antipin, M.
13. (a) Trost, B. M.; Crawley, M. L. Chem. Rev. 2003, 103, 2921–2943; (b) Graening,
T.; Schmalz, H.-G. Angew. Chem., Int. Ed. 2003, 42, 2580–2584; (c) Lu, Z.; Ma, S.
Angew. Chem., Int. Ed. 2008, 47, 258–297.
14. Mata, Y.; Dieguez, M.;Pamies, O.;Claver,C.Adv. Synth.Catal.2005, 347, 1943–1947.
15. (a) Suzuki, K.; Kondo, K.; Aoyama, T. Synthesis 2006, 1360–1364; (b) Jagt, R. B.
C.; Toullec, P. Y.; de Vries, J. G.; Feringa, B. L.; Minnaard, A. J. Org. Biomol. Chem.
2006, 4, 773–775.
16. Auburn, P. R.; McKenzie, P. B.; Bosnich, B. J. Am. Chem. Soc. 1985, 107, 2033–2046.
17. RajanBabu, T. V.; Ayers, T. A.; Halliday, G. A.; You, K. K.; Calabrese, J. C. J. Org.
Chem. 1997, 62, 6012–6028.
Y.; Davankov, V. A.; Gais, H.-J. Eur. J. Inorg. Chem. 2004, 629–634; (c) Tsarev, V.
N.; Lyubimov, S. E.; Bondarev, O. G.; Korlyukov, A. A.; Antipin, M. Y.; Petrovskii,
P. V.; Davankov, V. A.; Shiryaev, A. A.; Benetsky, E. B.; Vologzhanin, P. A.;
Gavrilov, K. N. Eur. J. Org. Chem. 2005, 2097–2105; (d) Gavrilov, K. N.;
Lyubimov, S. E.; Bondarev, O. G.; Maksimova, M. G.; Zheglov, S. V.; Petrovskii, P.
V.; Davankov, V. A.; Reetz, M. T. Adv. Synth. Catal. 2007, 349, 609–616; (e)
Gavrilov, K. N.; Zheglov, S. V.; Vologzhanin, P. A.; Maksimova, M. G.; Safronov,
A. S.; Lyubimov, S. E.; Davankov, V. A.; Schäffner, B.; Börner, A. Tetrahedron Lett.
2008, 49, 3120–3123.
8. Buono, G.; Toselli, N.; Martin, D.. In Börner, A., Ed.; Phosphorus Ligands
in Asymmetric Catalysis; Wiley-VCH: Weinheim, 2008; Vol. II, pp 529–
546.
9. Gonsalves, A. M. d’. A. R.; Serra, M. E. S.; Murtinho, D.; Silva, V. F.; Beja, A. M.;
Paixao, J. A.; Silva, M. R.; da Veiga, L. A. J. Mol. Catal. A: Chem. 2003, 195, 1–9.
10. (a) Iriuchijima, S. Synthesis 1978, 684–685; (b) Brunel, J. M.; Constantieux, T.;
Buono, G. J. Org. Chem. 1999, 64, 8940–8942.
11. (a) Brunel, J. M.; Chiodi, O.; Faure, B.; Fotiadu, F.; Buono, G. J. Organomet. Chem.
1997, 529, 285–294; (b) Reymond, S.; Brunel, J. M.; Buono, G. Tetrahedron:
Asymmetry 2000, 11, 1273–1278; (c) Ngono, C. J.; Constantieux, T.; Buono, G.
Eur. J. Org. Chem. 2006, 1499–1507; (d) Bondarev, O. G.; Goddard, R.
Tetrahedron Lett. 2006, 47, 9013–9015; (e) Kimura, M.; Uozumi, Y. J. Org.
Chem. 2007, 72, 707–714; (f) Mariz, R.; Briceno, A.; Dorta, R.; Dorta, R.
Organometallics 2008, 27, 6605–6613.
18. Lyubimov, S. E.; Davankov, V. A.; Gavrilov, K. N. Tetrahedron Lett. 2006, 47,
2721–2723.
19. Gavrilov, K. N.; Lyubimov, S. E.; Zheglov, S. V.; Benetsky, E. B.; Davankov, V. A. J.
Mol. Catal. A: Chem. 2005, 231, 255–260.
20. Frolander, A.; Lutsenko, S.; Privalov, T.; Moberg, C. J. Org. Chem. 2005, 70, 9882–
9891.
21. Pamies, O.; Dieguez, M. Chem. Eur. J. 2008, 14, 944–960.
22. Korostylev, A.; Monsees, A.; Fischera, C.; Börner, A. Tetrahedron: Asymmetry
2004, 15, 1001–1005.
23. Vallianatou, K. A.; Kostas, I. D.; Holz, J.; Börner, A. Tetrahedron Lett. 2006, 47,
7947–7950.
24. Duan, H.-F.; Xie, J.-H.; Shi, W.-J.; Zhang, Q.; Zhou, Q.-L. Org. Lett. 2006, 8, 1479–
1481.
25. Genet, J.-P.; Juge, S.; Achi, S.; Mallart, S.; Buiz, J.; Montes, J. R.; Levif, G.
Tetrahedron 1988, 44, 5263–5275.
26. Gladiali, S.; Pinna, L. Tetrahedron: Asymmetry 1991, 2, 623–632.
12. Arzoumanian, H.; Buono, G.; Choukrad, M.; Petrignani, J.-F. Organometallics
1988, 7, 59–62.