Lithiated anions derived from (alkenyl)pentamethyl phosphoric triamides: Useful synthons for the stereoselective synthesis of 9-oxo- and 10-hydroxy-2(E)-decenoic acids, important components of queen substance and royal jelly of honeybee Apis mellifera

Article abstract of DOI:10.1016/j.jorganchem.2010.06.020

Lithiated anions derived from (alkenyl)pentamethyl phosphoric triamides as homoenolate equivalents are used in the reaction with halogenated acetal and ketal giving regioselectively the γ-alkylation adducts. Chemoselective acidic hydrolysis of the enephosphoramide moiety in the presence of acetal or ketal groups leads to expected carbonyl products, key intermediates in the synthesis of natural compounds. The synthetic potential of the presented strategy is illustrated by stereoselective synthesis of two pheromones namely, 9-oxo-2(E)-decenoic acid 1 from queen substance and 10-hydroxy-2(E)-decenoic acid 2 from royal jelly of honeybee Apis mellifera.

Full text of DOI:10.1016/j.jorganchem.2010.06.020

Journal of Organometallic Chemistry 695 (2010) 2354e2358
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Lithiated anions derived from (alkenyl)pentamethyl phosphoric triamides:
Useful synthons for the stereoselective synthesis of 9-oxo- and 10-hydroxy-2(E)-
decenoic acids, important components of queen substance and royal jelly of
honeybee Apis mellifera
*
Tomasz K. Olszewski , Catherine Bomont, Philippe Coutrot, Claude Grison
Centre D’Ecologie Fonctionnelle et Evolutive, Unité Mixte de Recherche 5175, Campus CNRS, 1919 Route de Mende, 34293 Montpellier cedex 5, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 June 2010
Received in revised form
Lithiated anions derived from (alkenyl)pentamethyl phosphoric triamides as homoenolate equivalents
are used in the reaction with halogenated acetal and ketal giving regioselectively the
g-alkylation
adducts. Chemoselective acidic hydrolysis of the enephosphoramide moiety in the presence of acetal or
ketal groups leads to expected carbonyl products, key intermediates in the synthesis of natural
compounds. The synthetic potential of the presented strategy is illustrated by stereoselective synthesis of
two pheromones namely, 9-oxo-2(E)-decenoic acid 1 from queen substance and 10-hydroxy-2(E)-
decenoic acid 2 from royal jelly of honeybee Apis mellifera.
21 June 2010
Accepted 22 June 2010
Available online 6 August 2010
Keywords:
Ó 2010 Elsevier B.V. All rights reserved.
Homoenolate anion
Enephosphoramide carbanion
Carbonyl protecting group
Chemical messengers
Natural compounds
1. Introduction
at the g position with electrophiles. Additionally, the conjugate
enephosphoramide group which is stable to bases liberates the
carbonyl group under mild acidic conditions [7] (Fig. 1).
Carbonecarbon bond formation represents one of the most
important events in organic chemistry. A large number of available
protocols take advantage of the activation of methyl/methylene
produced by the electron withdrawing effect of adjacent carbonyl
functionality that facilitates the reaction of an electrophile at the
Herein, we wish to demonstrate the flexibility offered with the
application of lithium anions derived from (alkenyl)pentamethyl
phosphoric triamides, by the stereoselective synthesis of two
natural products namely 9-oxo-2(E)-decenoic acid (9-ODA) 1 from
queen substance and 10-hydroxy-2(E)-decenoic acid (10-HDA) 2,
pheromones of honeybee A. mellifera (Fig. 2).
In addition to their role as interesting synthetic targets both
pheromones represent very promising biological activity. 9-ODA
was found to possess antibacterial, anti-inflammatory and antidotal
activity and also can act as an accelerator of graft wound or thermal
burn healing and as an immunomodulator [8]. In turn, 10-HDA has
demonstrated an antitumor [9], antibacterial [10], immunomodu-
latory [11] and antioxidant activities [12] and was recently found to
promote neurogenesis [13]. The majority of the reported syntheses
rely upon the a priori most obvious concept namely, condensation
of the corresponding carbonyl compound (7-oxo-octanal in the
case of pheromone 1 and 8-hydroxy-octanal for 2) with either
malonic acid (the Doebner reaction) or their use in selective olefi-
nation reaction (Fig. 2) [14]. The main problem related with this
most straightforward approach is the preparation of the carbonyl
a-carbon via enol/enolate intermediate. The reaction at the b-
carbon is also feasible and proceeds via intermediacy of homo-
enolates. In contrast to enolate anions however, most homoenolate
anions are difficult to handle, and not easy to prepare therefore,
numerous efforts have been expended to understand the factors
affecting the reactivity and regioselectivity of these reagents [1e6].
In this context, we have previously reported on the synthetic
utility of lithium anions derived from (alkenyl)pentamethyl
phosphoric triamides as an effective homoenolate synthetic
equivalents for the preparation of aldehydes and ketones [7]. The
value of this synthetic strategy is based on the remarkable ability of
lithiated allylphosphoramide ambident anions to react exclusively
*
Corresponding author. Tel./fax: þ33 (0) 467 61 32 44.
E-mail address: tomasz.olszewski@pwr.wroc.pl (T.K. Olszewski).
0
022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.06.020

T.K. Olszewski et al. / Journal of Organometallic Chemistry 695 (2010) 2354e2358
2355
4
γ
R1 R4
O
R1 R4
base
O
R1
R
O
E
E
R
(
Me N) P
(Me N) P
(Me
2
N)
2
P
3
2
2
R3
2
2
α
R3
N
N
N
β
Me R2
Me R2
Me R2
H O
3
R1
R4
R1 R4
E
R
3
R3
O
O
R2
R2
Fig. 1. Concept of (alkenyl)pentamethyl phosphoric triamides as homoenolate synthetic equivalents.
precursors which is not a trivial issue and usually requires multi-
step protocols. Conceptually, 7-oxo-octanal and 8-hydroxy-octanal
could be prepared by Michael reaction involving the conjugate
addition of functional organometallics to acrolein. Although this
reaction between 6-(2-methyl-1,3-dioxolan-2-yl)hexanal 4 (for the
synthesis of 1) or 7-(1,3-dioxolan-2-yl)heptanal 5 (in the case of 2)
with lithiated anion 3 used earlier in our group [23] (Scheme 1).
Compounds 4 and 5 can be viewed as equivalents of 7-oxo- and
8-hydroxy-octanals respectively (Fig. 2). Both molecules 4 and 5
were easily prepared, in the key step of the synthesis, from ene-
phosphoramides 6a, b issued from the reaction of halogenated
derivatives possessing a masked carbonyl function 7a, b with
carbanions derived from allylphosphoramide 8.
strategy is limited by the instability of acrolein, the acrolein dia-
0
lkylacetal approach raised the problem of competitive SN
2
allylic
substitution [15]. An interesting solution is the copper catalyzed
reaction to protected acrolein [16]. Most recent and effective
syntheses of 7-oxo-octanal involve: a) preparation and reduction of
7
-oxooctanoic acid to corresponding diol and oxidation with PCC
[17]; b) preparation of 7,7-dimethoxy-heptanal followed by its
2.1. Synthesis of queen substance, 9-oxo-2(E)-decenoic acid 1
methylenation, oxidation and deprotection [18] or c) the use of
heptane-1,7-diol as substrate followed by its selective protection
with THP group, and subsequent sequence oxidation, methylena-
tion, oxidation and finally deprotection and again oxidation [19]. In
turn, the most convenient protocols leading to 8-hydroxy-octanal
require: a) the use of 8-nonenoic acid as substrate, reduction of the
carboxylic acid followed by hydroxylation of the terminal C]C and
oxidative cleavage of the intermediate 1,2-diol [20] or b) selective
bromination of octane-1,8-diol, protection of the free hydroxyl with
THP group and oxidation at the halogenated carbon atom followed
by deprotection [21]. Simple and efficient preparation of 7-oxo-
octanal and 8-hydroxy-octanal, their synthetic equivalents and
analogues would not only facilitate the access to the pheromones 1
and 2 but also would be of great interest for the synthesis of cyclic
compounds via intermolecular aldol reactions that use unmodified
dialdehydes, ketoaldehydes and diketones [17,18,22].
The starting material for the preparation of 1, the 2-(3-iodo-
propyl)-2-methyl-1,3-dioxolane 7a was prepared in a two step
reaction sequence from commercially available 5-chloro-2-penta-
none 9 (Scheme 2, Supplementary material).
It is noteworthy, that the choice of the protecting group for the
carbonyl function present in 9 was crucial for the success of the
synthesis. After several attempts, the acetal group proved to be
the best option. This protecting group was found, in the further
steps of the synthesis, to be stable under mild acidic conditions
(pH 3.2), which were necessary to hydrolyse the enephosphor-
amide 6a in order to obtain the desired aldehyde 4, while it liber-
ates the carbonyl function under strong acidic conditions (1 N aq.
HCl, room temp., 4 h) producing the desired compound 1
(Scheme 3).
With compound 7a in hand a series of experiments was per-
formed to establish the best conditions for the reaction with
lithiated enephosphoramide anion 11 (Supplementary material,
Scheme 3).
2
. Results and discussion
Our approach to the synthesis of pheromones 1 and 2 was based
on a simple retrosynthetic pathway involving the WittigeHorner
The highest yields were obtained by using two equivalents of
lithiated anion 11 with respect to the iodo derivative 7a, and in the
OMe
O
O
MeO
Ref.16
Ref.17
O
O
H
O
O
COOH
O
OH
Ref.18
OMe
O
7
-oxo-octanal
Doebner reaction
or
1
MeO
HO
H
selective olefination
Ref.19
(
Ref.14)
OTHP
H
COOH
Ref20
O
2
CO H
OH
OH
Ref.21
THPO
2
8-hydroxy-octanal
Br
Fig. 2. Examples of synthetic strategies leading to pheromones 1 and 2.

2356
T.K. Olszewski et al. / Journal of Organometallic Chemistry 695 (2010) 2354e2358
O
COOH
HO
COOH
1
2
O
O
O
O
O
CHO
O
(
EtO) P-CHCOO
(
EtO) P-CHCOO
2
2
CHO
H
3
3
4
5
O
Me
N P(NMe2)
O
6
a n=3, R=CH3
2
6b n=4, R=H
R
n
O
O
O
P(NMe2)2
7
a n=3, R=CH3
O
I
n
N
7b n=4, R=H
R
Me
8
Scheme 1. Retrosynthetic analysis that includes application of lithium anions derived from (alkenyl)pentamethyl phosphoric triamides.
five phosphorylated compounds: HMPA (
expected ethylenic stereoisomers 6a (
¼ 24.28 for E and 25.45 ppm
for Z) and both Z and E transposed allyl(pentamethyl)phosphor-
amides derived from -hydrolysis of carbanion 11 (respectively
2.97 and 22.48 ppm) (Fig. 3). The assignment of 6a Z and E was
realized by analogy with P NMR chemical shifts of other ene-
phosphoramides described in our previous communications [7c].
The E stereoisomer was the more stable one, whereas the formation
of Z stereoisomer can be explained as the result of an internal
chelation of lithium with the free nitrogen orbital, in the carba-
nionic precursor 11.
d
¼ 25.68 ppm), both
O
O
O
i
ii
d
Cl
I
O
I
9
g
10
7a
2
Scheme 2. Preparation of compound 7a. Reagents and conditions: i) NaI, acetone,
reflux, 88%; ii) ethylene glycol, cat. TsOH, toluene, reflux 1 h, 95%.
31
presence of one equivalent of HMPA (as a chelation agent for
lithium) with respect to anion 11. The lithiated anion 11 was
prepared from corresponding enephosphoramide 8 by deprotona-
ꢁ
Treatment of 6a with a strong acid led to the hydrolysis of both
dioxalane and enephosphoramide functions, followed by direct
cyclization of the dihydroxyaldehyde [7c]. Therefore here, an
accurate study on the hydrolysis of 6a was performed. It was
tion with n-BuLi at ꢀ50 C in THF. The addition of iodo acetal 7a to
ꢁ
ꢁ
C
a cooled (ꢀ50 C) solution, followed by gentle warming to 20
produced, after neutral hydrolysis, the expected conjugate ene-
3
1
phosphoramide 6a with 89% yield (calculated on the basis of
NMR of the crude mixture) as a mixture of two stereoisomers E and
Z in a ratio 85:15. It should be noted here, that the reaction was
clean and the ambident anion reacted exclusively at
The H NMR spectrum of the crude reaction mixture was
complex and difficult to assign however, P NMR data clearly show
P
established that treatment of 6a with 1 N H
a reaction time of 4 h, during which time the pH was readjusted
every hour to its initial value by adding 1 N aqueous H SO , created
2 4
SO until pH 3.2 and
2
4
g
position.
1
optimum conditions for the chemoselective cleavage of the
carbonenitrogen bond leading to the desired aldehyde 4 with an
31
O
O
O
i
iI
Me
P(NMe2)2
P(NMe2)2
P(NMe2)3
Cl
N
H
iii
O
7a
Li
O
I
O
O
iv
γ
α
O
P(NMe2)2
P(NMe2)2
Me
N
O
N
N
β
P(NMe2)2
Me
8
Me
11
6a
O
v
O
O
vi
O
vii
O
O
COOH
COOH
CHO
1
12
4
ꢁ
ꢁ
Scheme 3. Synthesis of 1. Reagents and conditions: i) P(O)Cl
3
, 0.5 equiv., 2 h, 160 C, 100%; ii) CH
3
NH
2
excess, Et
2
O, 48 h, 20 C, 100%; iii) NaH/THF, allyl bromide, room temp., 80%; iv)
P(O)CH CO H, 91%; vii) 1 N aq. HCl, room temp., 4 h, 95% (47% overall
ꢁ
ꢁ
n-BuLi, ꢀ50 C, HMPA then 7a, 89%; v) 1 N aq. H
2
SO
4
to pH 3.2, room temp., 4 h, 90%; vi) n-BuLi, ꢀ60 C, (EtO)
2
2
2
yield from commercially available 9).

T.K. Olszewski et al. / Journal of Organometallic Chemistry 695 (2010) 2354e2358
2357
O
O
O
i
O
I
Cl
H
H
13
7b
Scheme 4. Preparation of compound 7b. Reagents and conditions: i) NaI, acetone,
reflux, 88%.
completely stereoselective and resulted in the exclusive formation
of only the E stereoisomer.
In the last step of the synthesis, the protecting group of 12 was
easily removed in acidic conditions using 1 N HCl producing the
desired 9-oxo-2(E)-decenoic acid (9-ODA) 1 with a 95% yield and
with an excellent 47% overall yield from commercially available 9.
The formation of only
E stereoisomer was unambiguously
confirmed by comparison of the NMR data of the obtained sample
with those previously reported in the literature (Supplementary
material). The efficient preparation of 1 prompted us to extend
this methodology to the synthesis of second pheromone namely
10-hydroxy-2(E)-decenoic acid 2 from royal jelly.
2.2. Synthesis of 10-hydroxy-2(E)-decenoic acid 2
In the synthesis of compound 2 the already protected and
commercially available starting material 13 was used rendering
the desired 2-(4-iodobutyl)-1,3-dioxolane 7b with 88% yield
(Scheme 4, Supplementary material).
Next, the iodo-precursor 7b was reacted with lithiated ene-
phosphoramide anion 11 using analogous conditions to that
described earlier for the synthesis of pheromone 1. The expected
conjugate enephosphoramide 6b was obtained with 74% yield
Fig. 3. 31P NMR spectra (CDCl
) of enephosphoramide 6a (crude product), HMPA,
3
transposed all(pentamethyl)phosphoramide.
31
calculated on the basis of P NMR as a mixture of two stereoiso-
31
mers E and Z (Scheme 5). On the P NMR spectra the E stereo-
isomer presented an upfield signal at 24.11 ppm, and the Z
stereoisomer presented a downfield signal at 25.32 ppm.
excellent yield (90%) (Scheme 3). It is worth mentioning, that the
presented protocol leading to aldehyde 4 is, to the best of our
knowledge, the most efficient described so far (67% overall yield; 4
steps from commercially available 9). Aldehyde 4 is a useful
building block in the synthesis of natural compounds e.g. ketoal-
kenes and ketoalkenynes from Echinacea pallida [24].
Dioxolane acetal derivatives are not as robust as dioxolane ketals
hence they are hydrolyzed more readily than their ketal counter-
parts [25]. The treatment of 6b with 1 N H SO until pH 3.2 and
2
4
a reaction time of 4 h, during which time the pH was readjusted
every hour to its initial value byadding 1 N aqueous H SO , led to the
2
4
The aldehyde was further used in the WittigeHorner reaction
with dianion 3, prepared from diethylphosphonoacetic acid. The
desired (E)-8-(2-methyl-1,3-dioxolan-2-yl)oct-2-enoic acid 12 was
obtained with 91% yield. It is noteworthy that, the reaction was
chemoselective hydrolysis of the enephosphoramide moiety and
formation of the expected aldehyde 5 with a 90% yield (Scheme 5).
Next, the aldehyde 5 was exposed to anion 3 in the WittigeHorner
olefination producing the desired (E)-9-formylnon-2-enoic acid 15
O
7b
Li
O
O
O
I
i
γ
α
H
O
Me
P(NMe2)2
P(NMe2)2
O
N
N
N P(NMe2)2
O
β
Me
8
Me
11
H
6b
ii
O
iv
O
iii
O
O
O
H
COOH
CHO
COOH
H
H
15
14
5
v
HO
COOH
2
ꢁ
ꢁ
Scheme 5. Synthesis of compound 2. Reagents and conditions: i) n-BuLi, ꢀ50 C, HMPA then 7b, 74%; ii) 1 N aq. H
SO
2 4
to pH 3.2, room temp., 4 h, 90%; iii) n-BuLi, ꢀ60 C, (EtO)
2
P(O)
2 2 4
CH CO H, 85%; iv) 1 N aq. HCl, room temp., 4 h, 92%; v) MeOH, NaBH , room temp., 2 h, 83% (49% overall yield from commercially available 13).

2358
T.K. Olszewski et al. / Journal of Organometallic Chemistry 695 (2010) 2354e2358
with 92% yield and total E stereoselectivity. In the last step of the
synthesis, the alcohol function in the acid 15 was reduced to an
[7] (a) P. Coutrot, C. Grison, C. Bômont, Phosphorus, Sulfur, and Silicon 77 (1993)
95;
1
(b) P. Coutrot, C. Grison, C. Bômont, Tetrahedron Lett. 35 (1994) 8381e8384;
c) P. Coutrot, C. Grison, C. Bômont, J. Organomet. Chem. 586 (1999) 208e217;
aldehyde by means of NaBH
4
generating with 83% yield the desired
(
1
0-hydroxy-2(E)-decenoic acid 2 with conserved E geometry of the
(d) C. Grison, A. Thomas, F. Coutrot, P. Coutrot, Tetrahedron 59 (2003)
101e2123;
e) C. Grison, A. Thomas, F. Coutrot, P. Coutrot, J. Organomet. Chem. 689 (2003)
530e1539.
2
(
1
double bond. The overall yield of the entire reaction sequence,
starting from the commercially available 13 was 49%.
In conclusion, a novel and versatile approach to the prepara-
tion of two known pheromones from honeybees A. mellifera
namely, 9-oxo-2(E)-decenoic acid 1 from queen substance (47%,
overall yield starting form commercially available substrate) and
[8] G.Yu. Ishmuratov, A.F. Ismagilova, A.A. Sharipov, O.N. Gerasyuta,
R.Ya. Kharisov, N.M. Ishmuratova, G.A. Tolstikov, Pharm. Chem. J. 37 (2003)
309e313.
[9] G.F. Townsend, W.H. Brown, E.E. Felauer, B. Hazlett, Can. J. Biochem. Physiol.
39 (1961) 1765e1770.
[
[
10] M.S. Blum, A.F. Novak, S. Taber, Science 130 (1959) 452e453.
11] D. Vucevic, E. Melliou, S. Vasilijic, S. Gasic, P. Ivanovski, I. Chinou, M. Colic, Int.
Immunopharmacol 7 (2007) 1211e1220.
10-hydroxy-2(E)-decenoic acid 2 from royal jelly (49%, overall
yield starting form commercially available material) and total E
stereoselectivity of the double bond present in both final struc-
tures was presented. The key step of the synthesis was the
reaction between homoenolate anion equivalent derived from
enephosphoramide and halogenated derivatives possessing
a masked carbonyl function. The synthetic value of this strategy
is based on the nature of the conjugate enephosphoramide group
which is stable to bases but liberates the aldehydic group under
mild acidic conditions. The results illustrate the benefit and
interest of the enephosphoramide strategy for the construction of
natural products. The potential in total synthesis of other natural
compounds is currently being investigated in our laboratory and
the results will be reported in due course.
[12] T. Nagai, M. Sakaia, R. Inouec, H. Inouec, N. Suzukia, Food Chem. 75 (2001)
37e240.
13] N. Hattori, H. Nomoto, H. Fukumitsu, S. Mishama, S. Furukawa, Biomed. Res.
8 (2007) 261e266.
2
[
2
[14] (a) For a review see: G.Yu. Ishmuratov, R.Ya. Kharisov, N.M. Botsman,
N.M. Ismuratova, G.A. Tolstikov Chem. Nat. Compounds 38 (2002) 1e23 [more
recent examples include];
(b) R.Ya. Kharisov, O.V. Botsman, L.P. Botsman, N.M. Ishmuratova,
G.Yu. Ishmuratov, G.A. Tolstikov, Chem. Nat. Compounds 38 (2002) 145e148;
(c) G.Yu. Ishmuratov, M.P. Yakovleva, L.P. Botsman, N.M. Ishmuratova,
R.R. Muslukhov, G.V. Khambalova, G.A. Tolstikov, Chem. Nat. Comp. 39 (2003)
28e30;
(d) J. Villieras, M. Rambaud, M. Graff, Tetrahedron Lett. 26 (1985) 53e56;
(e) G.Yu. Ishmuratov, M.P. Yakovleva, K.A. Tambovtsev, Yu.V. Legostaeva,
L.V. Kravchenko, N.M. Ishmuratova, G.A. Tolstikov, Chem. Nat. Compounds 44
(
2008) 74e76.
15] J.F. Normant, A. Commerçon, M. Bourgain, J. Villieras, Tetrahedron Lett. 16
1975) 3833e3836.
[
(
Appendix. Supplementary material
[
[
16] J. Villieras, M. Rambaud, M. Graff, Synth. Comm. 15 (1985) 569e580.
17] C. Ghobril, C. Sabot, Ch. Mioskowski, R. Baati, Eur. J. Org. Chem. (2008)
4104e4108.
18] F. Douelle, A.S. Capes, M.F. Greaney, Org. Lett. 9 (2007) 1931e1934.
19] J.S. Dickschat, E. Hemke, S. Schultz, Chem. Biodiversity 2 (2005) 318e352.
20] K. Sisido, M. Kawanisi, K. Kondo, T. Morimoto, A. Saito, N. Hukue, J. Org. Chem.
27 (1962) 4073e4076.
Detailed experimental procedures, spectroscopic characteriza-
tion of all compounds and copies of the NMR spectra for compounds
[
[
[
8
, 6a, 6b, 4, 5, 7a,12,14,15,1, 2. This information can be found, in the
online version, at doi:10.1016/j.jorganchem.2010.06.020.
[
[
21] R. Chiron, J. Chem. Ecol. 8 (1982) 709e713.
22] S. Mukherjee, J. Woon Yang, S. Hoffmann, B. List, Chem. Rev. 107 (2007)
References
5471e5569.
[23] (a) P. Coutrot, C. Grison, S. Genève, C. Didierjean, A. Aubry, A. Vicherat,
[
[
1] A. Nickon, J.L. Lambert, J. Am. Chem. Soc. 84 (1962) 4604e4605.
2] V. Nair, S. Vellalath, B.P. Babu, Chem. Soc. Rev. 37 (2008) 2691e2698 [and the
references cited therein].
M. Marraud, Lett. Pept. Sci. 4 (1997) 415e422;
(b) C. Grison, P. Coutrot, S. Genève, C. Didierjean, M. Marraud, J. Org. Chem. 70
(2005) 10753e10764;
[3] D. Hoppe, Synthesis 1 (2009) 43e55 [and the references cited therein].
[4] D. Hoppe, Angew. Chem. Int. Ed. Engl. 23 (1984) 932e948.
[5] D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 107 (2007) 5606e5655.
[6] B.E. Maki, E.V. Patterson, Ch. J. Cramer, K.A. Scheidt, Org. Lett. 11 (2009)
(c) C. Grison, S. Genève, E. Halbin, P. Coutrot, Tetrahedron 57 (2001)
4903e4923.
[24] M. Egger, P. Pellett, K. Nickl, S. Geiger, S. Graetz, R. Seifert, J. Heilmann,
B. Konig, Chem. Eur. J. 14 (2008) 10978e10984.
3942e3945.
[25] P.J. Kociensky, Protecting Groups. Georg Thieme Verlag Stuttgart, New York, 1994.