Carboazidation of chiral allylsilanes: Experimental and theoretical investigations

Article abstract of DOI:10.1002/chem.200701401

The carboazidation of chiral allylsilanes has been investigated by varying the nature of the substituents at the silicon center and on the carbon framework. The influence of temperature and the nature of the sulfonyl azide, as well as the stereochemistry of the remote stereogenic center, on the 1,2-diastereocontrol of the process were considered. Good to excellent levels of diastereocontrol were generally observed, with the syn-β-azidosilane always being formed as the major isomer. An illustration of the value of this methodology has been provided with a short and efficient synthesis of an analogue of castanospermine. EPR spectroscopy was carried out on various β-silyl radicals providing useful information about their conformations in the ground state. Based on this experimental evidence and DFT calculations, reactant-like transition state models were finally proposed that rationalize the observed 1,2-stereoinduction.

Full text of DOI:10.1002/chem.200701401

DOI: 10.1002/chem.200701401
Carboazidation of Chiral Allylsilanes: Experimental and Theoretical
Investigations
Laurent Chabaud,[a] Yannick Landais,*[a] Philippe Renaud,*[b] FrØdØric Robert,[a]
FrØdØric Castet,[a] Marco Lucarini,[c] and Kurt Schenk[d]
Abstract: The carboazidation of chiral
allylsilanes has been investigated by
varying the nature of the substituents
at the silicon center and on the carbon
framework.The influence of tempera-
ture and the nature of the sulfonyl
azide, as well as the stereochemistry of
the remote stereogenic center, on the
1,2-diastereocontrol of the process
were considered.Good to excellent
levels of diastereocontrol were general-
ly observed, with the syn-b-azidosilane
always being formed as the major
isomer.An illustration of the value of
this methodology has been provided
with a short and efficient synthesis of
an analogue of castanospermine.EPR
spectroscopy was carried out on vari-
ous b-silyl radicals providing useful in-
formation about their conformations in
the ground state.Based on this experi-
mental evidence and DFT calculations,
reactant-like transition state models
were finally proposed that rationalize
the observed 1,2-stereoinduction.
Keywords: allylsilanes
· azides ·
carboazidation · density functional
calculations · radicals
Introduction
velopments.For instance, N radical species were shown to
3
add to electron-rich olefins by using a combination of hyper-
The azido group constitutes a versatile functional group[1]
that may be involved in many important synthetic transfor-
mations,[2] including 1,3-dipolar cycloadditions onto alkynes,
which have recently attracted a lot of interest through the
click-chemistry concept.[1b] Numerous methods have been
developed to incorporate an azido group into a carbon
framework.[3–8] Amongst these, radical processes hold a spe-
cial place and have recently led to useful and attractive de-
valent iodine reagents and TMSN3.[5] Activation of the C H
ꢀ
bond and formation of the corresponding alkyl azide by
using similar reagents[6] or IN3 was also reported.[7] More
challenging intermolecular transfer of azido groups has also
been described by using sulfonyl azides following a fragmen-
tation process.[8] Recently, we developed a useful carboazi-
ꢀ
dation process, allowing the formation of both a C C bond
ꢀ
and a C N bond in a one-pot operation starting from simple
olefins (Scheme 1).[9] The reaction works well with radical
species which have an electrophilic character that add effi-
ciently onto olefins in a first step.This intermolecular pro-
cess then generates a second radical species with nucleophil-
ic character, which reacts with the sulfonyl azide to provide
[a] Dr.L.Chabaud, Prof.Y.Landais, Dr.F.Robert, Dr.F.Castet
UniversitØ Bordeaux 1
Institut des Sciences MolØculaires, UMR-CNRS 5255
351, cours de la libØration, 33405 Talence (France)
Fax : (+33)540-002-289
E-mail: y.landais@ism.u-bordeaux1.fr
[b] Prof.P.Renaud
Universität Bern, Department für Chemie und Biochemie
Freiestrasse 3, 3000 Bern 9 (Switzerland)
Fax : (+41)31-631-43-59
E-mail: philippe.renaud@ioc.unibe.ch
[c] Prof.M.Lucarini
Dipartimento di Chimica Organica “A.Mangini”
Via S.Giacomo, 11- 40136 Bologna (Italy)
[d] Prof.K.Schenk
Institut de cristallographie, UniversitØ de Lausanne
BPS CH-1015, Lausanne-Dorigny (Switzerland)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Scheme 1.The carboazidation reaction of olefins. Application to chiral
allylsilanes.
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FULL PAPER
the carboazidation product.The process is general and was
found to be efficient for a number of radical precursors and
olefins.This method is a valuable tool in organic synthesis,
as illustrated by some recent applications in natural product
synthesis.[10]
Allylsilanes are known to react efficiently with radical
species and possess an enhanced reactivity, compared to
other olefins, toward radical intermediates with electrophilic
character.[11] Allylsilanes exhibit a unique reactivity towards
electrophiles, and while a wealth of data is available on the
1,2-stereocontrol arising from the reaction of chiral allylsi-
lanes with ionic electrophilic reagents,[12] little is known on
the stereocontrol occurring during radical reactions of chiral
allylsilanes.[13] Porter et al.first demonstrated that an allylic
silicon group could control the stereochemistry of a new ste-
reogenic center created during the radical addition (atom
transfer) of a-bromo and a-iodo amides onto a chiral allylsi-
lane.[14] Based on these premises, we started an investigation
on the 1,2-stereocontrol arising from free-radical mediated
carboazidation of chiral allylsilanes (Scheme 1).[15] We
report here a full account of our studies, including the scope
and limitations of the process, and an EPR study on the b-
silyl radical species issued from the addition of the carbon
radical onto chiral allylsilanes.Finally, ab initio calculations
were performed, providing support for the transition state
models proposed to rationalize the stereoinduction.
Scheme 2.Chiral allylsilanes 1a,b and anti-b-hydroxy allylsilanes 3a–h.
(entry 4) for which the resulting titanium alcoholate was
found to be difficult to hydrolyze.The nature of the silicon
moiety was also varied, with PhMe2Si, Ph3Si, and (iPr)3Si
groups introduced following this approach.[18]
syn-Diastereomers of chiral allylsilanes were prepared in
parallel, starting from allyltin intermediates (not shown).[19]
Under Lewis acid conditions, allylsilanes 4a,b were thus ob-
tained in moderate to good yield, but with high levels of
syn-stereocontrol (Scheme 3).
Results and Discussion
Scheme 3.Synthesis of syn-b-hydroxy allylsilanes 4a,b.
Synthesis of chiral allylsilane precursors: A series of simple
allylsilanes possessing one or two stereogenic centers (bear-
ing the silicon group) were prepared (Table 1).Racemic al-
lylsilanes 1a,b bearing a single stereogenic center were pre-
pared through the intermediacy of a silylcuprate-mediated
SN2’ displacement of the corresponding allyl chlorides
(Scheme 2).[16] Allylsilanes 3a–h, with two contiguous ste-
reogenic centers, were prepared following the Yamamoto–
Reetz allylation[17] procedure from allylsilanes 2a–c and the
corresponding aldehydes (Table 1).Good yields and high
levels of anti stereoselectivity were generally observed using
this strategy.This method is general and provided good re-
sults with most aldehydes that were tried, except glyoxal
Finally, g-hydroxyallylsilanes were prepared by reacting
the lithium carbanion of allylsilanes 2a,b with racemic phe-
nyloxirane (Scheme 4).[20] While good yields of the desired
and separable diastereomers 5b and 6b were obtained from
2b, only the anti isomer 5a could be isolated from the reac-
tion mixture starting from 2a.In this case, a large amount
of the g isomer was formed along with the desired a-anti 5a
and a-syn isomers.
Table 1.Synthesis of chiral b-hydroxy allylsilanes 3a–h (Scheme 2).
Product
R
R’
R’’
Yield
[%][a]
anti/
syn[b]
Scheme 4.Synthesis of g-allylsilanes 5a,b and 6b.
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3 f
3g
3h
Me
Me
Me
Me
Me
iPr
Ph
Ph
Ph
Ph
Ph
Ph
iPr
Ph
Ph
Ph
iPr
tBu
CO2Et
66
79
69
22
68
79
63
68
>98:<2
>98:<2
>98:<2
>98:<2
>98:<2
>98:<2
>98:<2
>98:<2
Carboazidation of achiral allylsilanes: Carboazidation was
first carried out on simple allylsilanes 2a,b, lacking a stereo-
genic center, in order to test the influence of the substitu-
ents on the silicon center on the course of the carboazida-
tion.Xanthate 7 was used as a radical precursor,[21] and
DTBHN (di-tert-butyl hyponitrite) as an initiator, and the
reaction was carried out as previously reported.[9] Under
CH=CHMe (E)
Ph
Ph
iPr
Ph
[a] Isolated yield.[b] Estimated from 1H NMR analysis of the crude reac-
tion mixture.
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2745

Y.Landais, P.Renaud et al.
Table 2.Carboazidation of chiral allylsilanes 3a–h, 4a, and 5a,b.
these conditions (1.5 h at 808C), 2a,b provided the carboazi-
dation products 8a,b in excellent yields,[22] indicating that
the nature of the silicon group has little effect on the rate of
the radical process on simple allylsilanes (Scheme 5).This
contrasts with observations by Hwu et al., who noticed sub-
stituent effects in the radical addition of ketones onto allyl-
silanes.[23] This reactivity is also very different from that of
allylsilanes toward electrophiles under ionic conditions, for
which 2b is far less reactive than 2a.[24]
Allyl-
silane
Product
R
R’
R’’
Yield
[%][a]
syn/
anti[b]
1
2
3
4
5
6
7
8
9
3a
3g
3 f
3b
3c
3e
4a
5a
5b
10a
10b
10c
10d
10e
10 f
11a
12a
12b
Me
Ph
Ph
Ph
iPr
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
iPr
71
30
34
56
45
–
47
72
30
63:37
90:10
>95:<5
>90:<10
>90:<10
–
iPr
Me
Me
Me
Me
Me
Ph
tBu
CH=CHMe
Ph
–
–
73:27
80:20
80:20
[a] Isolated yield.[b] Estimated from 1H NMR and 28Si NMR analysis of
the crude reaction mixture.
isolated pure diastereomers with tetrabutyl ammonium fluo-
ride (TBAF).Under these conditions, the syn and anti-b-azi-
dosilanes led to the corresponding Z- and E-olefins, respec-
tively, in high yield through an anti-stereospecific b-elimina-
tion.[25,26] The syn configuration of the major isomer was
later confirmed through X-ray structure determination of a
crystalline b-azidosilane (vide infra).
Scheme 5.Carboazidation of achiral allylsilanes 2a,b.Study on the effects
of substituents at silicon.
Carboazidation of OH-free chiral b- and g-hydroxyallylsi-
lanes: With these results in hand, we then started our inves-
tigations on the carboazidation of chiral allylsilanes 1a,b,
3a–c, 3e–g, 4a, and 5a,b (Scheme 6).Several trends emerge
The influence on the 1,2-stereocontrol and of the nature
of the substituents on the carbon backbone and the silicon
center were studied, as well as the effect of the additive ste-
reogenic centers located in the b- and g-positions relative to
the silicon group.Modest-to-high levels of syn-stereoselec-
tivity were thus observed, with diastereocontrol increasing
with the size of the R’’ substituents (entries 1, 4, and 5).Sim-
ilar observations were made by Porter et al.during their
studies on the radical azidation of b-silyl Barton esters.[26]
The diastereocontrol also increased with the size of the
silicon group, as shown by the good to excellent diastereo-
mer ratio observed with (iPr)3Si and Ph3Si groups, albeit
with moderate yield (entries 2,3).It should also be noted
that the Ph3Si group always led to lower yields than other
silicon groups, probably due to the decomposition of the
azide product under our radical conditions.This contrasts
with the results observed with the achiral allylsilane 2b
(Scheme 5). b-Fragmentation of the b-silyl radical inter-
mediate was ruled out as no olefinic proton could be detect-
1
ed in the H NMR spectrum of the crude reaction mixture.
Aryl migration and homolytic aromatic substitution may
also be envisioned, but are not supported by the isolation of
the corresponding byproducts.
The stereocontrol appears to rely mainly on the nature of
the substituents on the a-stereocenter bearing the silicon
group (i.e., 1a,b).Hence, the introduction of a second ste-
reogenic center (as in 3a–g and 4a) did not greatly influence
the level of stereocontrol (entries 1–7).The same conclusion
may be drawn if the additional stereogenic center is more
remote, that is, in the g-position as in 5a,b (entries 8,9).A
slightly better diastereocontrol was observed with the syn-b-
hydoxysilane 4a than anti-3a (entries 1 and 7).However,
this result should be treated with caution as the yield in this
case is moderate and decomposition of the minor isomer
can not be completely ruled out.Overall, we observed that
b-benzylic alcohols (3a,f,g, 4a) were quite sensitive to the
Scheme 6.Carboazidation of OH-free chiral allylsilanes.
from the preliminary studies summarized in Table 2.Car-
boazidation of chiral allylsilanes led to moderate to good
yields of the desired b-azidosilanes 9a,b, 10a–f, 11a, and
12a,b with syn selectivities.The moderate yields were gener-
ally ascribed to the tedious purification of the products,
from excess sulfonyl azide.The diastereomer ratio was esti-
1
mated from H and 28Si NMR studies after removal of tin
residues through a short pad of silica, and the relative con-
figuration of the major isomer was obtained by treating the
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Carboazidation of Chiral Allylsilanes
FULL PAPER
reaction conditions, although again no b-elimination prod-
ucts could be detected from the crude reaction mixture.In
line with this observation, only decomposition was observed
upon carboazidation of diene 3e with an allylic alcohol
moiety (entry 6).
The influence of various parameters on the stereocontrol
(nature of the substituents on the allylsilane framework and
at silicon, additional stereogenic centers) was studied as
above.Optimization of the carboazidation conditions, vary-
ing the temperature and the nature of the azide reagent,
was also considered.
Carboazidation of OH-protected chiral b- and g-hydroxyal-
lylsilanes: Although free OH groups are usually inert under
radical conditions, the sometimes modest yields observed
above, along with the reported failure[27] during carboazida-
tion of olefins bearing a free hydroxyl group, led us to con-
sider the carboazidation process onto OH-protected b-hy-
droxy allylsilanes (Scheme 7).As summarized in Table 3,
Decreasing the temperature from 80 to 608C led to a sig-
nificant improvement of the yield of product, but with simi-
lar stereocontrol, indicating that b-azidosilanes are sensitive
to thermal degradation (entries 1 and 3).The reaction was
also carried out under irradiation at a lower temperature
(entries 4,5).A slightly higher level of stereocontrol was ob-
tained at the expense of the yields, with a large amount of
xanthate 7 left unreacted even after 8 h of irradiation.As
above for 3e, the diene 13e led mainly to decomposition
(entry 8), and allylsilane 13 f provided the desired b-azidosi-
lane 15 f (entry 9) with good selectivity but poor yield. syn-
b-Hydroxy allylsilanes 14a,b afforded all-syn azides 16a,b in
good yields and selectivities (entries 10,11), which compare
well with those observed with anti-diastereomers, showing
again the weak effect of the stereochemistry of the b-stereo-
center on the stereochemical course of the carboazidation.
As above, increasing the size of the R group improved the
diastereocontrol (entries 3 and 6).The excellent diastereo-
control obtained reproducibly with benzyl acetate 13d is
misleading as a careful examination of 1H and 28Si NMR
spectra of the crude reaction mixture led to the conclusion
that a partial degradation of the minor diastereomer was
likely to be the cause of the overestimation of the diastereo-
meric ratio.Pleasingly, b-silylazide 15d crystallized, allowing
the determination of its relative configuration through X-
ray crystallography, thus confirming the syn configuration
determined through chemical correlation[25] (Figure 1).
Optimization of the reaction conditions also led us to in-
vestigate the nature of the sulfonyl azide.Increasing the size
of the aryl (or alkyl) sulfonyl group led to no improvement
of the diastereocontrol (entry 13).The absence of a steric
effect of the R’ sulfonyl group may be explained in the light
of recent work by Masterson et al.[28] who demonstrated that
the linear azide reacts with the carbon radical center,
Scheme 7.Carboazidation of OH-protected chiral allylsilanes.
Table 3.Carboazidation of OH-protected chiral allylsilanes (Scheme 7).
Allyl- Product
silane
R
PG
T
[8C]
Yield syn/
[%][a] anti[b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
13a
13b
13a
13a
13a
13c
13d
13e
13 f
14a
14b
13a
13a
13a
15a
15b
15a
15a
15a
15c
15d
15e
15 f
16a
16b
15a
15a
15a
iPr
iPr
iPr
iPr
iPr
tBu
Ph
Ac
SiEt3 80
80
67[c]
61[c]
74[c]
78:22
50:50
77:23
84:16
82:18
83:17
>95:<5
–
88:12
83:17
82:18
77:23
77:23
76:24
Ac
Ac
Ac
Ac
Ac
60
10–15[d] 44[e]
ꢀ30[d]
48[e]
72[e]
65[e]
–
60
60
60
60
60
80
60
60
60
CH=CHMe Ac
CO2Et
Ph
iPr
iPr
Ac
Ac
Ac
Ac
Ac
Ac
34[e]
78[e]
67[e]
74[e]
50[f]
64[g]
iPr
iPr
[a] Isolated yield.[b] Estimated from 1H NMR analysis of the crude reac-
tion mixture.[c] PhSO 2N3 was used.[d] hn.[e] 3-PyrSO 2N3 was used.
[f] 2,4,6-PhSO2N3 (Trisyl) was used.[g] EtSO N3 was used.
2
yields with acetate-protected allylsilanes were generally
higher and more reproducible, indicating that in our case
the OH group probably interferes with the radical process
for reasons that are currently unknown.The diastereocon-
trol was in turn slightly lower (see Table 2, entry 4, and
Table 3, entry 1).The acetate led to the best results in terms
of both yield and diastereocontrol.In comparison, a triethyl-
silyl (TES) group, as in 13b, led to the b-azidosilane 15b
(Entry 2) in reasonable yield, but with no stereocontrol.
This may be attributed to the small size differentiation, on
the allylic stereogenic center, between the large PhMe2Si
group and the medium-sized substituent iPrCHOSiEt3.
Figure 1.X-ray structure determination of b-azidosilane 15d.
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Y.Landais, P.Renaud et al.
through its terminal nitrogen center, so that the R’ group is
rather remote from the reacting center.In line with this, 3-
pyridyl[9c] and ethylsulfonyl azides led to similar diastereo-
control, but with better yields than the trisyl azide[3] (en-
tries 12 and 14).3-Pyridylsulfonyl azide is, however, recom-
mended when purification of the b-azidosilane is tedious, as
the excess of azide may simply be removed through a mild
acidic workup.
The nature of the substituents at silicon was again varied,
with Ph3Si leading invariably to lower yields (i.e., 19,
Scheme 8), and only decomposition products with b-silylace-
Scheme 9.Varying the nature of the xanthate fragment.
Scheme 10.Carboazidation of 2,2-disubstituted allylsilanes and allylic
ethers.
Scheme 8.Influence of the nature of the silicon group on the carboazida-
tion.
pated, based on some precedent, that we could take advan-
tate 17a.In this case, we observed the formation of olefinic
products probably resulting from the elimination of Ph3SiN3
or Ph3SiOAc fragments.These byproducts could unfortu-
nately not be isolated in pure form from the complex reac-
tion mixture.Finally, triisopropylsilylallylsilane 17b led to
the corresponding b-azidosilane as a single stereoisomer,
thus demonstrating that bulky silicon groups promote an op-
timum level of diastereocontrol.However, for the sake of
simplicity, the PhMe2Si group was used in most of our stud-
ies, since it provides a reasonable level of diastereocontrol,
is stable, easily available from commercial sources, and can
be oxidized into the corresponding OH group[29] under mild
conditions (vide infra).
G
Carboazidation with various xanthates and olefins: Interest-
ingly, other xanthates may be added to our allylsilanes, al-
lowing attractive further functionalization of the resulting b-
azidosilanes.This is illustrated with the ketone 21a and the
Weinreb amide 21b, both adding to allylsilane 13a to afford
the desired b-azido silanes 22a,b in good yields and with
reasonable selectivities (Scheme 9).
In contrast to the above results, we experienced some lim-
itations with the carboazidation of chiral 2,2-disubstituted
allylsilanes and allylic ethers (Scheme 10).Carboazidation
of allylsilane 23 thus afforded, as expected, azide 24 in good
yield, but without any selectivity.In parallel, it was antici-
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Carboazidation of Chiral Allylsilanes
FULL PAPER
Scheme 12.Allylsilane carboazidation pathway.
cavity, at room temperature, by photolytic cleavage of
(Bu3Sn)2 in the presence of xanthate 7 and allylsilanes
(13a,d, 14a, and 17b) in benzene.All the spectra were
straightforwardly interpreted on the basis of the coupling of
the unpaired electron with the a- and three unequivalent b-
protons; the spectroscopic parameters are reported in
Table 4.
Inspection of Table 4 shows that the value of a(Ha) is es-
sentially unaffected by changing the nature of SiR3 and R’’
substituents in I (Scheme 12).Its value depends on the spin
density at the a-carbon, which is expected to be close to 1
(for comparison, in the isopropyl radical, a(Ha) is
22.22 Gauss). In contrast, the value of a(Hb) shows a signifi-
cant dependence on the nature of substrate and a good cor-
relation is observed between this value and the carboazida-
Scheme 11.Synthesis of an analogue of castanospermine.
ing tosylate 30.Pd/C reduction of the azido group of 30
then led to the desired amine, which cyclized spontaneously
to provide a lactam that was reduced with LiAlH4 into the
bicyclic intermediate 31.Deprotection of the acetonide and
treatment of the resulting chlorhydrate with basic Dowex
led to 1-deoxy-6,8-diepicastanospermine 32,[32a] a potent in-
hibitor of a-l-fucosidase,[32b] in 45% overall yield and in ten
steps from allylsilane 27.
[35]
tion ratio.According to the Heller–McConnel equation,
the value of a(Hb) depends on the time-average value of
2
ꢀ
cos q, in which q is the dihedral angle between a b-C H
bond and the axis of the p orbital on Ca.The low value of
a(Hb) indicates that hcos2qi is close to zero.On this basis,
two different conformations I-A and I-B could be drawn,
Carboazidation of chiral allylsilanes—EPR and quantum
chemical studies—transition-state models: The course of the
carboazidation process under radical conditions is believed
to follow the pathway depicted in Scheme 12,[8a,b] with the
with eclipsing interactions between the sCꢀ bond and the
Si
radical SOMO (Figure 2).These conformations are similar
to those obtained through other spectroscopic methods[36]
and in good agreement with our recent calculations on radi-
cal cyclizations.[34]
To further support the putative conformations of the b-
silyl radicals and explain the stereochemistry of the process,
we carried out a quantum chemical study on the radical azi-
dation step.The reaction of 1a with phenylsulfonyl azide to
give 9a was chosen as a model system (see Scheme 6).To
reduce the number of conformational possibilities for the
ꢀ
C N bond formation as the stereochemistry-determining
step.It was reasoned that during the reaction of radical I,
which has nucleophilic character, with electrophilic sulfonyl
azide, the transition state (II) should resemble the starting
b-silyl radical I in the ground state.[33] Spectroscopic mea-
surement and recent ab initio calculations have shown that
the minimum energy conformations of acyclic b-silyl radicals
such as I exhibit a sCꢀ bond eclipsing the radical SOMO,
Si
the maximum energy having a
sCꢀ bond orthogonal to the
Si
radical orbital.[34]
Table 4.EPR spectral parameters [in Gauss] for radical I (Scheme 12) deriving from allylsilanes 13a, 13d,
14a, 17b.
EPR experiments were car-
ried out to provide additional
information relative to the con-
formation of radical species I in
the ground state.Radical I was
generated inside the EPR
SiR3
Allylsilane
R’’
a(Ha)
a(Hb)
a
(H’b)
a
G
syn/anti
1
2
3
4
SiMe2Ph
SiMe2Ph
SiMe2Ph
13a
13d
14a
17b
iPr
Ph
Ph
Ph
20.64
20.70
20.71
20.82
8.66
7.65
8.00
5.75
25.82
23.05
23.34
24.63
25.96
24.75
24.61
25.58
78:22
>95:5
83:17
Si
G
>95:5
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2749

Y.Landais, P.Renaud et al.
Figure 2.Conformations of the b-silyl radical obtained through EPR
measurements.
ethyl group and thus lower the computational costs, we re-
placed the CO2Et group by a simpler CO2Me group, assum-
ing that the alkyl chain on the terminal ester has no direct
influence on the course of the carboazidation process.
B3LYP/6–31G(d) calculations performed using the reaction
temperature of 808C (see Computational Methods) led to
two stable radical conformers that confirm the putative
schemes I-A and I-B in Figure 2.The radical I-B is 2.66 kcal
molꢀ1 more stable than I-A, due to the weaker gauche inter-
actions in the former, while the interconversion barrier from
I-A to I-B amounts to 0.55 kcalmolꢀ1.Moreover, as suspect-
ed from EPR experiments, both radicals present an almost
Figure 3.Transition-state models for the carboazidation of chiral allylsi-
lanes.
coplanar arrangement between the sCꢀ bond and the radi-
Si
cal SOMO, I-B presenting an angle of 868 between Hb and
the silicon group with no pyramidalization of the carbon
radical center. I-A presents larger deformations with an
angle of 788 between Hb and SiMe2Ph and a pyramidaliza-
tion of 58, which probably account for its higher energetic
level.Concerning the azide addition, we considered only an
attack at the terminal nitrogen center of the azide.[28]
PhSO2N3 may then approach radicals I syn or anti relative
to the SiMe2Ph group.The syn approach led to much higher
energetic transition states (TS) with major rearrangement of
the radicals in a conformation in which the silicon group is
orthogonal to the radical orbital.Thus, only transition states
resulting from an anti approach relative to silicon were con-
sidered, the azide moiety preferring to avoid strong steric in-
teractions with the silicon group.Two diastereomeric transi-
tion states II-A and II-B, resembling conformations I-A and
I-B, thus emerged, which led to syn and anti b-azidosilanes
respectively (Figure 3).
A total of eight different transition states (TS) were then
considered (Figure 4), four TS named IIA-i leading to the
syn product and four TS named IIB-i leading to the anti
product.The structures IIA-i (IIB-i) with i=1 and i=2 (or
with i=3 and i=4) differ from the others by a rotation of
about 1208 of the sulfonyl group.The activation barriers and
the relative Gibbs free energies (DGi) of the eight TS, as cal-
culated at the B3LYP and ROMP2//B3LYP levels, are re-
ported in Table 5.The lower TS arises from the less stable
radical I-A, and the activation barriers (between 15–
19 kcalmolꢀ1), being one order of magnitude higher than
the interconversion barrier between I-A and I-B, mean that
the reaction should follow a Curtin–Hammett regime, in
which product ratios are only controlled by the DGi values.
The relative populations were thus estimated by performing
Maxwell–Boltzmann statistics using the relative free ener-
Figure 4.Representation of the eight transition structures leading to 9a.
gies of the eight possible TS.[33b] Populations of products
connected to IIA-i and IIB-i were eventually summed to
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Chem. Eur. J. 2008, 14, 2744 – 2756

Carboazidation of Chiral Allylsilanes
FULL PAPER
Table 5.Relative Gibbs free energies [ DG, kcalmolꢀ1] of the transition
structures IIA-i and IIB-i and relative product populations [Rel.pop.
%], calculated at 353 K.
Conclusion
Carboazidation of chiral allylsilanes provides the corre-
sponding b-azidosilanes with 1,2-stereocontrol ranging from
7:3 to >95:5.This process is particularly powerful as it pro-
vides straightforward access to acyclic systems with several
contiguous stereogenic centers, the stereochemistry of which
can be controlled efficiently.EPR experiments and ab initio
calculations have also been performed, which led to transi-
tion state models allowing the prediction of the stereochem-
ical course of the carboazidation reaction.These investiga-
tions offer valuable information and better insight into the
factors at the origin of the 1,2-stereocontrol in these sys-
tems.They also give information on the behavior of acyclic
radical systems such as I (Scheme 12) that should be rele-
vant for other transformations as well as carboazidations.[40]
B3LYP/6–31G(d)
ROMP2/6–31G(d)//
B3LYP/6–31G(d)
DG
Rel.pop.
DG
Rel.pop.
TS syn
IIA-1
IIA-2
IIA-3
IIA-4
0.00
0.83
2.41
2.46
60
18
2
0.00
0.73
1.16
2.60
52
18
10
1
2
TS anti
IIB-1
IIB-2
IIB-3
IIB-4
1.75
2.07
2.42
1.46
5
3
2
8
2.19
2.44
1.87
1.05
2
1
4
12
syn/anti ratio
82:18
81:19
obtain the final populations of the syn and anti b-azidosi-
lanes, respectively (last line of Table 5).The relative ener-
gies indicate that the major product is formed through the
IIA-i structure, in which the sulfonyl azide approaches on
the Hb side, thus avoiding steric interactions with the methyl
group (as in IIB-i).It is worth noting that, as observed ex-
perimentally, increasing the size of this medium-sized group
(iPr, tBu) led to better diastereomeric excess.
Experimental Section
Computational methods: Geometry optimizations were performed in
vacuum using the density functional theory (DFT) with the three-param-
eter hybrid functional B3LYP and the 6–31G(d) basis set.Thermal cor-
rections were calculated from the unscaled B3LYP/6–31G(d) harmonic
vibrational frequencies using experimental temperature and pressure
conditions.Every transition structure was characterized by a single imagi-
nary frequency in the diagonalized mass-weighted Hessian matrix associ-
ꢀ
The pyramidalization of the carbon radical center
amounts to 258 and the length of the incipient bond is equal
to 2.30 Š, consistent with the picture of an early transition
ated with the normal mode corresponding to the formation of the C N
bond.The largest eigenvalue of the total spin operator was equal to
0.7596, which suggests no error due to spin contamination. The electronic
energies were further refined by using the restricted open shell second-
order Møller-Plesset (ROMP2) level of theory with the same basis set.
This procedure is referred to as ROMP2/6–31G(d)//B3LYP/6–31G(d)
and abbreviated as ROMP2//B3LYP.All calculations were performed
using Gaussian03.[41] The geometries of all the TS are provided as Sup-
porting Information.
state.Besides, the dihedral angle between the sCꢀ bond and
Si
ꢀ
the incipient C N bond is ꢀ157.88.All the other transition
states present similar features, confirming the stabilizing
effect of an eclipsed conformation between the sCꢀ bond
Si
and the SOMO of the radical.Although the relative order-
ing of the eight transition states depends on the level of
theory used to determine the electronic wave function, the
B3LYP and ROMP2//B3LYP levels predict very similar syn/
anti selectivities that reproduce fairly well the experimental
ratio (73:27, see Scheme 6).
Interestingly, this transition-state model is reminiscent of
the Felkin–Anh model[37] proposed for nucleophilic addi-
tions to chiral carbonyl compounds.Similar to Felkin–Anh
model, the more favorable transition state II-A, that is, the
approach of the reagent anti with respect to the largest sub-
stituent (SiR3) on the side of the smallest group (H), occurs
through the highest energy conformation of the starting rad-
ical (i.e., I-A), under a Curtin–Hammett regime.[37c] The par-
tial positive charge developing at the carbon radical center
at the transition state is also stabilized further by the elec-
EPR spectroscopy: EPR spectra were obtained using a Bruker ESP300
spectrometer equipped with an NMR gaussmeter for field calibration
and a Hewlett Packard 5350B microwave frequency counter for the de-
termination of the g factors.Photolysis was carried out by focusing the
unfiltered light from a 500 W high pressure mercury lamp on the EPR
cavity.The instrument settings were as follows: microwave power
5.0 mW, modulation amplitude 0.05 mT, modulation frequency 100 kHz,
scan time 180 s.
General remarks: 1H NMR and 13C NMR spectra were recorded on a
Brüker Avance 300 (1H: 300 MHz, 13C: 75.5 MHz) or a Brüker AC-250
FT (1H: 250 MHz, 13C: 62.9 MHz), with CDCl3 as internal reference.The
chemical shifts (d) and coupling constants (J) are expressed in ppm and
hertz, respectively.IR spectra were recorded on a Perkin–Elmer Paragon
1000 FT-IR spectrophotometer as neat films on NaCl windows or as KBr
pellets.HRMS were recorded on a Varian MAT 311 apparatus (for EI)
or a Micromass ZABSpec TOF apparatus (for ESI).Melting points were
determined by using a Büchi Totolli apparatus and are uncorrected.
Merck silica gel (0.043–0.063 mm) was used for flash chromatography.
All reactions were carried out under a nitrogen atmosphere unless speci-
fied otherwise.CH 2Cl2 was distilled over CaH2.Benzene and THF were
distilled from sodium/benzophenone prior to use.All reagent-grade
chemicals were obtained from commercial suppliers and were used as re-
ceived, unless otherwise stated.Spectroscopic data for the starting allylsi-
lanes is available in the Supporting Information.
tron-rich sCꢀ bond through hyperconjugation (b-silicon
Si
effect).[38] A related model had been proposed earlier by
Giese and Curran[39] to rationalize the 1,2-stereoinduction
occurring in reactions of acyclic radicals.At the transition
state, the largest groups are orthogonal to each other with a
pronounced pyramidalization (ꢁ258), a key feature to ex-
plain the preference for the approach of sulfonyl azide fol-
lowing TS-IIA instead of that illustrated by TS-IIB.
Preparation of b-hydroxyallylsilanes (general procedure A): 1. 6m solu-
tion of n-butyllithium in hexane (1.1 equiv) was added to a solution of al-
lylsilane (1 equiv) in dry THF, followed by TMEDA (1.5 equiv) at 08C.
The resulting pale yellow solution was stirred at 08C for 2 h, turning red-
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2751

Y.Landais, P.Renaud et al.
dish brown, then cooled to ꢀ788C.Titanium isopropoxide (1 equiv) was
added at ꢀ788C and the mixture was stirred for 1 h at this temperature.
Finally, a solution of aldehyde (1 equiv) in dry THF was added at ꢀ788C,
and the reaction mixture was then stirred for 75 min.The reaction was
quenched with an aqueous solution of NH4Cl at ꢀ788C.The organic
layer was decanted and the aqueous layer extracted with diethyl ether.
The combined organic layers were then washed with H2O, dried over
MgSO4, and the solvents were removed under reduced pressure.The resi-
due was purified by chromatography.
MS (ESI): m/z (%) : 342 (100) [M+Na]+, 277 (74) [MꢀN3]+, 135 (80)
[PhMe2Si];.HRMS [ M+Na]+ C16H25N3O2SiNa calcd: 342.1613; found:
342.1613.
4-Azido-5-(dimethylphenylsilanyl)-5-phenylpentanoic acid ethyl ester
(9b): Prepared according to general procedure C from ethyl ester 7
(70.2 mg, 0.34 mmol), 1b (161 mg, 0.64 mmol), benzenesulfonylazide
(199 mg, 1.09 mmol), (Bu3Sn)2 (0.25 mL, 0.49 mmol), and DTBHN
(10 mg, 0.06 mmol) in dry benzene (1 mL). The crude was purified by
chromatography (EP/EtOAc : 98/2) to afford an inseparable mixture of
the two diastereomers (syn/anti 80:20) as a colorless oil (74.4 mg, correct-
ed yield 55%).IR (neat): nmax =3069–2958, 2099, 1733, 1250, 1112,
Acetylation of b- and g-hydroxysilanes (general procedure B): Acetic an-
hydride (2 equiv), NEt3 (2 equiv), and a catalytic amount of 4-DMAP
(0.1 equiv) were added to a stirred solution of alcohol (1 equiv) in dry
CH2Cl2 (6.5 mLmmolꢀ1).The resulting mixture was then stirred at room
temperature (RT) under nitrogen for 18 h and was treated with saturated
aqueous NaHCO3 solution.The organic layer was decanted and the
aqueous layer extracted with diethyl ether.The combined extracts were
washed with brine and dried over MgSO4, and the solvents were concen-
trated in vacuo.The crude product was purified by chromatography
through silica.
1
702 cmꢀ1; H NMR (250 MHz, CDCl3), major + minor: d=7.50–7.00 (m,
20H), 4.08 (q, J=7.0 Hz, 2H), 4.06 (q, J=7.0 Hz, 2H), 3.77 (ddd, J=8.5,
7.3, 7.0 Hz, 1H), 3.61 (m, 1H), 2.47 (d, J=7.3 Hz, 1H), 2.42 (d, J=
10.7 Hz, 1H), 2.30 (t, J=7.6 Hz, 2H), 2.29 (t, J=7.0 Hz, 2H), 1.93–1.62
(m, 4H), 1.21 (t, J=7.0 Hz, 3H), 1.20 ppm (t, J=7.0 Hz, 3H); 13C NMR
(63 MHz, CDCl3), major + minor: d=172.6, 139.2, 137.2, 133.9, 129.3,
129.1, 128.3, 127.9, 126.7, 64.0, 60.5, 42.5, 42.0, 30.9, 30.6, 29.5, 29.1, 14.1,
13.7, ꢀ2.4, ꢀ2.6, ꢀ3.9, ꢀ3.9, ꢀ4.6 ppm; MS (ESI): m/z (%): 404 (8)
[M+Na]+, 339 (100) [MꢀN3]+; HRMS [M+Na]+ C21H27N3O2SiNa calcd:
404.1770; found: 404.1748.
Radical carboazidation (general procedure C): DTBHN (3 mol% or
18 mol%) was added every 90 min to a solution of ethyl 2-iodoacetate or
xanthate (1 or 2 equiv), olefin (2 or 1 equiv), arylsulfonylazide (3 equiv)
and (Bu3Sn)2 (1.5 equiv) in dry benzene (2 mLmmolꢀ1) at reflux (80 or
608C) under N2.The reaction was monitored by TLC.Upon completion,
the solvent was removed under reduced pressure and the crude product
was filtered through silica gel (pentane, then pentane/EtOAc).
4-Azido-6-hydroxy-6-phenyl-5-triisopropylsilanylhexanoic acid ethyl ester
(10c): Prepared according to general procedure C from ethyl ester 7
(221.5 mg, 1.06 mmol), 3 f (609 mg, 2 mmol), benzenesulfonylazide
(549 mg, 3 mmol), (Bu3Sn)2 (0.76 mL, 1.5 mmol), and DTBHN (30 mg,
0.18 mmol) in dry benzene (2 mL). The crude was purified by chromatog-
raphy (pentane/EtOAc : 95/5) to afford a yellow oil (syn/anti >95:5,
192.2 mg, yield 34%). 1H NMR (300 MHz, CDCl3): d=7.39–7.32 (m,
5H), 5.21 (m, 1H), 4.02 (q, J=7.1 Hz, 2H), 3.69 (m, 1H3), 2.39 (d, J=
4.5 Hz, 1H), 2.21–2.11 (m, 2H), 1.95–1.81 (m, 1H), 1.79 (m, 1H), 1.70–
1.51 (m, 1H), 1.18 ppm (m, 24H); 13C NMR (75 MHz, CDCl3): d=172.5,
145.1, 128.5, 128.4, 125.2, 72.4, 62.0, 60.4, 37.8, 31.8, 31.3, 19.4, 19.3, 14.1,
11.7 ppm; MS (ESI): m/z (%): 456 (16) [+Na]+, 391 (80) [MꢀN3]+, 347
(100); HRMS [M+Na]+ C23H39N3O3SiNa calcd: 456.265841; found:
456.265502.
4-Azido-5-(dimethylphenylsilanyl)pentanoic acid ethyl ester (8a): Pre-
pared according to general procedure C from ethyl ester 7 (86.8 mg,
0.42 mmol), dimethylphenylallylsilane (136.2 mg, 0.77 mmol), benzenesul-
fonylazide (220.3 mg, 1.20 mmol), (Bu3Sn)2 (0.29 mL, 0.57 mmol), and
DTBHN (12 mg, 0.07 mmol) in dry benzene (0.8 mL). The crude product
was purified by chromatography (pentane/EtOAc 95:5) to afford
yellow oil (103.9 mg, 81%). IR (neat): nmax =3069–2958, 2102, 1728, 1252,
1110 cmꢀ1 1H NMR (250 MHz, CDCl3): d=7.56–7.30 (m, 5H), 4.11 (q,
a
;
J=7.3 Hz, 2H), 3.38 (m, 1H), 2.37 (t, J=7.0 Hz, 2H), 1.93–1.63 (m, 2H),
1.24 (t, J=7.0 Hz, 3H), 1.21 (dd, J=14.3, 7.3 Hz, 1H), 1.11 (dd, J=14.6,
7.0 Hz, 1H), 0.39 (s, 3H), 0.37 ppm (s, 3H); 13C NMR (63 MHz, CDCl3):
d=172.8, 137.9, 133.4, 129.2, 127.9, 60.5, 59.5, 32.2, 30.8, 21.8, 14.1 ppm;
MS (LSIMS): m/z (%): 328 (12) [M+Na]+, 263 (100) [MꢀN3]+; HRMS
[M+Na]+ C15H23N3O2SiNa calcd: 328.145911; found: 328.145725.
4-Azido-5-(dimethylphenylsilanyl)-6-hydroxy-7,7-dimethyloctanoic acid
ethyl ester (10e): Prepared according to general procedure C from ethyl
ester 7 (208 mg, 1 mmol), allylsilane 3c (524 mg, 2 mmol), benzenesulfo-
nylazide (549 mg, 3 mmol), (Bu3Sn)2 (0.76 mL, 1.5 mmol), and DTBHN
(30 mg, 0.18 mmol) in dry benzene (2 mL). The crude was purified by
chromatography (pentane/EtOAc 95:5) to afford a colorless oil as an in-
separable mixture of azidosilane and PhSO2N3 (syn/anti >90:10,
360.6 mg, estimated yield 45%). IR (neat): nmax =3520, 3069–2871, 2097,
4-Azido-5-triphenylsilanylpentanoic acid ethyl ester (8b): Prepared ac-
cording to general procedure C from ethyl ester 7 (86.7 mg, 0.42 mmol),
triphenylallylsilane
(233.4 mg,
0.78 mmol),
benzenesulfonylazide
1732, 1252, 1112 cmꢀ1 1H NMR (250 MHz, CDCl3): d=7.61–7.52 (m,
;
(230.3 mg, 1.26 mmol), (Bu3Sn)2 (0.29 mL, 0.57 mmol), and DTBHN
(12 mg, 0.07 mmol) in dry benzene (0.8 mL). The crude was purified by
chromatography (pentane/EtOAc : 95/5) to afford a yellow oil (133.5 mg,
2H), 7.39–7.31 (m, 3H), 4.17 (q, J=7.3 Hz, 2H), 4.03 (m, 1H), 3.49 (d,
J=4 Hz, 1H), 2.52–2.27 (m, 2H), 1.86–1.63 (m, 2H), 1.56 (d, J=4 Hz,
1H), 1.38 (m, 1H), 1.29 (t, J=7.3 Hz, 3H), 0.73 (s, 9H), 0.47 (s, 3H),
0.41 ppm (s, 3H); 13C NMR (75 MHz, CDCl3): d=172.9, 138.5, 134.3,
129.0, 127.8, 75.9, 62.4, 60.4, 37.1, 35.3, 32.2, 32.1, 26.0, 14.2, ꢀ1.8,
ꢀ3.3 ppm; MS (LSIMS): m/z (%): 414 (48) [M+Na]+, 364 (69), 349 (100)
[M+N3]+; HRMS [M+Na]+ C20H33N3O3SiNa calcd: 414.218890; found:
414.218667.
74%).IR (neat): nmax =3071–2910, 2101, 1728, 1110 cmꢀ1 1H NMR
;
(250 MHz, CDCl3): d=7.58–7.30 (m, 15H), 4.07 (q, J=7.3 Hz, 2H), 3.51
(m, 1H), 2.33 (m, 2H), 1.90–1.75 (m, 3H), 1.68 (dd, J=14.9, 6.6 Hz, 1H),
1.20 ppm (t, J=7.3 Hz, 3H); 13C NMR (63 MHz, CDCl3): d=172.7,
135.6, 133.9, 129.7, 128.0, 60.5, 59.4, 32.6, 31.0, 19.6, 14.1 ppm; MS
(LSIMS): m/z (%): 452 (12) [M+Na]+, 387 (31) [MꢀN3]+, 259 (100)
[Ph3Si]; HRMS [M+Na]+ C25H27N3O2SiNa calcd: 452.175932; found:
452.177025.
Ethyl
4-azido-6-hydroxy-5-(dimethylphenylsilyl)-6-phenylhexanoate
(11a): Prepared according to general procedure C from ethyl ester 7
(110 mg, 0.53 mmol), allylsilane 4a (303.7 mg, 1.06 mmol), benzenesulfo-
nylazide (288 mg, 1.59 mmol), (Bu3Sn)2 (0.40 mL, 0.79 mmol), and
DTBHN (15 mg, 0.09 mmol) in dry benzene (1 mL). The crude was puri-
fied by chromatography (pentane/EtOAc 9:1) to afford a colorless oil
(syn/anti 73:27, 102.1 mg, 47%). IR (neat): nmax =3500, 3069–2854, 2098,
1732, 1251, 1111 cmꢀ1. : 1H NMR (250 MHz, CDCl3), major diastereo-
mer: d=7.61–7.19 (m, 10H), 4.91 (dd, J=8.2, 3.7 Hz, 1H), 4.04 (q, J=
7.0 Hz, 2H), 3.36 (m, 1H), 2.29–2.07 (m, 3H), 1.93–1.66 (m, 2H), 1.19 (t,
J=7 Hz, 3H), 0.50 (s, 3H), 0.42 ppm (s, 3H); 13C NMR (75 MHz,
CDCl3): d=172.6, 144.6, 139.1, 134.3, 128.9, 128.6, 127.7, 126.3, 74.9, 64.4,
60.4, 42.1, 31.7, 29.3, 14.1, ꢀ0.7, ꢀ1.0 ppm.
4-Azido-5-(dimethyl-phenyl-silanyl)-hexanoic acid ethyl ester (9a): Pre-
pared according to general procedure C from ethyl ester 7 (91.5 mg,
0.44 mmol), 1a (334 mg, 0.88 mmol), benzenesulfonylazide (241 mg,
1.32 mmol), (Bu3Sn)2 (0.33 mL, 0.65 mmol), and DTBHN (13 mg,
0.06 mmol) in dry benzene (1.5 mL). The crude was purified by chroma-
tography (pentane/EtOAc : 98/2) to afford the two diastereomers (syn/
anti 73:27) as a colorless oil (90.3 mg, 64%). IR (neat): nmax =3069–2874,
2097, 1734, 1251, 1112, 702 cmꢀ1 1H NMR (250 MHz, CDCl3), major +
;
minor: d=7.60–7.30 (m, 10H), 4.11 (q, J=7.0 Hz, 2H), 4.10 (q, J=
7.0 Hz, 2H), 3.45 (m, 1H), 3.35 (m, 1H), 2.41–2.26 (m, 4H), 1.80–1.60
(m, 4H), 1.43–1.35 (m, 2H), 1.25 (t, J=7.0 Hz, 3H), 1.23 (t, J=7.0 Hz,
3H), 1.02 (d, J=7.3 Hz, 6H), 0.38 (s, 6H), 0.34 ppm (s, 6H); 13C NMR
(63 MHz, CDCl3), major + minor: d=172.9, 137.4, 133.8, 129.1, 127.8,
65.4, 60.5, 31.2, 28.6, 27.4, 25.5, 25.2, 14.1, 10.4, 9.6, ꢀ3.8, ꢀ3.9, ꢀ4.4 ppm;
6-Acetoxy-4-azido-5-(dimethylphenylsilanyl)-7-methyloctanoic acid ethyl
ester (15a): Prepared according to general procedure C from ethyl ester
7 (50 mg, 0.24 mmol), allylsilane 13a (139.4 mg, 0.48 mmol), 3-pyridylsul-
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Chem. Eur. J. 2008, 14, 2744 – 2756

Carboazidation of Chiral Allylsilanes
FULL PAPER
fonylazide (132 mg, 0.72 mmol), (Bu3Sn)2 (0.18 mL, 0.36 mmol), and
DTBHN (9 mg, 0.05 mmol) added by 3% portion every 90 min. The
crude was purified by chromatography (hexane/EtOAc 95:5) to afford a
yellow oil (syn/anti 77:23, 75 mg, 74%).IR (neat): nmax =3070–2965, 2106,
7.2 Hz, 1H), 4.02 (q, J=7.1 Hz, 1H), 3.72 (m, 1H), 2.20–2.00 (m, 2H),
2.03 (s, 3H), 1.72–1.55 (m, 2H), 1.45–1.31 (m, 1H), 1.19 (t, J=7.2 Hz,
3H), 0.47 (s, 3H), 0.39 ppm (s, 3H); 13C NMR (75 MHz, CDCl3): d=
172.4, 169.8, 140.5, 137.5, 134.0, 129.2, 128.4, 127.9, 127.5, 125.4, 74.1,
62.2, 60.4, 41.8, 31.6, 31.4, 21.4, 14.1, ꢀ2.0, ꢀ2.6 ppm; MS (ESI): m/z
(%): 476 (100) [M+Na]+, 433 (29), 299 (18), 217 (48); HRMS [M+Na]+
C24H31N3O4SiNa calcd: 476.1981; found: 476.1961.
1732, 1427, 1372, 1235, 1112, 837, 817 cmꢀ1 1H NMR (300 MHz, CDCl3),
;
major diastereomer: d=7.58–7.51 (m, 2H), 7.38–7.32 (m, 3H), 4.93 (dd,
J=8.1, 2.1 Hz, 1H), 4.14 (q, J=7.1 Hz, 2H), 3.89 (m, 1H), 2.38 (m, 2H),
2.00 (s, 3H), 1.84 (m, 1H), 1.68 (m, 2H), 1.45 (dd, J=2.0, 1.4 Hz, 1H),
1.26 (t, J=7.2 Hz, 3H), 0.77 (d, J=7.0 Hz, 3H), 0.74 (d, J=7 Hz, 3H),
0.41 (s, 3H), 0.40 ppm (s, 3H); minor diastereomer: d=7.58–7.48 (m,
2H), 7.40–7.32 (m, 3H), 5.03 (dd, J=6.7, 5.2 Hz, 1H), 4.12 (q, J=7.1 Hz,
2H), 3.75 (m, 1H), 2.36 (m, 2H), 2.04 (s, 3H), 1.94 (m, 1H), 1.69 (m,
2H), 1.43 (m, 1H), 1.25 (t, J=7.2 Hz, 3H), 0.81 (d, J=6.8 Hz, 3H), 0.71
(d, J=6.6 Hz, 3H), 0.43 (s, 3H), 0.42 ppm (s, 3H); 13C NMR (75 MHz,
CDCl3), major diastereomer: d=172.8, 170.7, 137.7, 134.2, 129.1, 127.7,
76.8, 63.0, 60.5, 36.5, 32.5, 32.0, 31.7, 21.6, 19.4, 18.5, 14.2, ꢀ2.1,
ꢀ3.1 ppm; minor diastereomer : d?172.7, 170.5, 137.8, 133.7, 129.2, 127.9,
77.0, 62.8, 60.4, 33.7, 32.2, 31.7, 30.3, 21.1, 19.9, 17.5, ꢀ2.3, ꢀ2.4 ppm; MS
(ESI): m/z: 442 (50) [M+Na]+; HRMS [M+Na]+ C21H33N3O4SiNa calcd:
442.2138; found: 442.2137.
Diethyl 2-acetoxy-4-azido-3-(dimethylphenylsilyl)heptanedioate (15 f):
Prepared according to general procedure C from ethyl ester 7 (73 mg,
0.35 mmol), allylsilane 13 f (213 mg, 0.66 mmol), 3-pyridylsulfonylazide
(197 mg, 1.07 mmol), (Bu3Sn)2 (0.25 mL, 0.5 mmol), and DTBHN (10 mg,
0.06 mmol) added by 3% portion every 90 min. The crude was purified
by chromatography (pentane/EtOAc 9:1) to afford a yellow oil (syn/anti
88:12, 54 mg, 34%).Major diastereomer : IR (neat) nmax =3071–2950,
2109, 1740, 1731, 1373, 1113, 817 cmꢀ1 1H NMR (300 MHz, CDCl3): d=
;
7.56–7.49 (m, 2H), 7.40–7.34 (m, 3H), 5.11 (d, J=1.9 Hz, 1H), 4.14 (m,
4H), 3.72 (m, 1H), 2.44–2.24 (m, 2H), 2.03 (s, 3H), 1.86 (m, 1H), 1.78–
1.66 (m, 2H), 1.25 (m, 6H), 0.47 ppm (s, 6H); 13C NMR (75 MHz,
CDCl3): d=172.6, 170.1, 170.0, 136.6, 133.9, 129.5, 128.0, 71.4, 62.9, 61.7,
60.5, 36.7, 31.5, 31.1, 20.9, 14.2, 14.0, ꢀ2.3, ꢀ2.7 ppm; MS (LSIMS): m/z
(%): 472 (30) [M+Na]+, 407 (65) [MꢀN3]+, 372 (26), 235 (22); HRMS
[M+Na]+ C21H31N3O6SiNa calcd: 472.187984; found: 472.187837.
4-Azido-5-(dimethylphenylsilanyl)-7-methyl-6-triethylsilanyloxyoctanoic
acid ethyl ester (15b): Prepared according to general procedure C from
ethoxythiocarbonylsulfanyl-acetic acid ethyl ester 7 (59 mg, 0.283 mmol),
allylsilane 13b (205 mg, 0.566 mmol), benzenesulfonylazide (161 mg,
0.849 mmol), (Bu3Sn)2 (0.21 mL, 0.424 mmol) and DTBHN (9 mg,
0.05 mmol) added by 3% portion every 90 min. The crude was purified
by chromatography (hexane/EtOAc : 98/2) to afford a yellow oil (50/50,
Ethyl
6-acetoxy-4-azido-5-(dimethylphenylsilyl)-6-phenylhexanoate
(16a): Prepared according to general procedure C from ethyl ester 7
(58 mg, 0.278 mmol), allylsilane 14a (151 mg, 0.466 mmol), 3-pyridylsulfo-
nylazide (156 mg, 0.85 mmol), (Bu3Sn)2 (0.18 mL, 0.36 mmol), and
DTBHN (16 mg, 0.09 mmol) added by 3% portion every 90 min. The
crude was purified by chromatography (hexane/EtOAc 95:5) to afford a
colorless oil (syn/anti 83:17, 98 mg, 78%).IR (neat): nmax =3069–2950,
75 mg, 61%).IR (neat)
nmax =2857, 2099, 1738, 1250, 1111 cmꢀ1
;
1H NMR (400 MHz, CDCl3) two diastereomers: d=7.54–7.49 (m, 4H),
7.39–7.30 (m, 6H), 4.17–4.04 (m, 4H), 3.98 (m, 1H), 3.87 (m, 1H), 3.85–
3.79 (m, 2H), 2.48–2.23 (m, 2H), 2.21–2.00 (m, 2H), 1.92-.159 (m, 4H),
1.61-.154 (m, 2H), 1.45–1.31 (m, 2H), 1.29–1.23 (m, 6H), 1.01–0.95 (m,
18H), 0.88 (d, J=6.8 Hz, 3H), 0.80 (d, J=6.8 Hz, 3H), 0.68–0.59 (m,
18H), 0.46 (s, 3H), 0.44 (s, 3H), 0.43–0.41 ppm (m, 6H); 13C NMR
(75 MHz, CDCl3) two diastereomers: d=172.8, 172.8, 139.2, 139.1, 134.1,
133.7, 128.9, 128.8, 127.8, 127.6, 76.3, 75.1, 63.5, 63.0, 60.4, 60.3, 35.8, 35.7,
35.2, 34.8, 32.1, 32.1, 32.0, 30.5, 19.5, 17.7, 17.6, 17.6, 14.2, 7.1, 5.6, 5.6,
ꢀ0.9, ꢀ1.5, ꢀ1.6, ꢀ2.7 ppm; MS (ESI): m/z (%): 514 (100) [M+Na]+,
471 (40); HRMS [M+Na]+ C25H45N3O3Si2Na calcd: 514.2897; found:
514.2892.
2099, 1737, 1229, 1111, 1022 cmꢀ1 1H NMR (250 MHz, CDCl3), major +
;
minor: d=7.60–7.50 (m, 2H), 7.40–7.22 (m, 5H), 6.09 (d, J=7.0 Hz, 1H),
5.93 (d, J=8.5 Hz, 1H), 4.04 (q, J=7.0 Hz, 2H), 3.38 (m, 1H), 2.30–2.06
(m, 2H), 1.96 (s, 3H), 1.95 (m, 2H), 1.89 (s, 3H), 1.76–1.64 (m, 2H), 1.20
(t, J=7.3 Hz, 3H), 1.19 (t, J=7.3 Hz, 3H), 0.46 (s, 3H), 0.41 (s, 3H),
0.34 ppm (s, 3H); 13C NMR (63 MHz, CDCl3) major diastereomer: d=
172.3, 169.4, 140.5, 138.4, 133.8, 129.0, 128.5, 127.9, 127.8, 126.6, 76.2,
63.4, 60.4, 39.6, 31.5, 29.2, 21.1, 14.1, ꢀ1.2, ꢀ1.3 ppm; minor diastereo-
mer: d=172.2, 169.5, 141.1, 138.4, 133.6, 126.4, 74.8, 62.3, 60.4, 38.4, 31.2,
29.3, 21.1, 13.7, ꢀ1.3, ꢀ2.3 ppm; MS (LSIMS): m/z (%): 476 (100)
[M+Na]+, 329 (50), 307 (36), 288 (67); HRMS [M+Na]+
C24H31N3O4SiNa calcd: 476.198155; found: 476.198755.
Ethyl
6-acetoxy-4-azido-7,7-dimethyl-5-(dimethylphenylsilyl)octanoate
(15c): Prepared according to general procedure C from ethyl ester 7
(144 mg, 0.69 mmol), allylsilane 13c (425 mg, 1.4 mmol), 3-pyridylsulfo-
6-Acetoxy-4-azido-5-(dimethylphenylsilanyl)-7-methyloctanoic acid ethyl
ester (16b): Prepared according to general procedure C from ethyl ester
7 (55 mg, 0.265 mmol), allylsilane 14b (154 mg, 0.53 mmol), benzenesul-
fonylazide (145 mg, 0.795 mmol), (Bu3Sn)2 (0.20 mL, 0.4 mmol), and
DTBHN (5 mg, 0.05 mmol) added by 3% portion every 90 min. The
crude was purified by chromatography (hexane/EtOAc : 95/5) to afford a
yellow oil (syn/anti 82:18, 74 mg, 67%).IR (neat) nmax =2970–2874, 2100,
A
DTBHN (22 mg, 0.13 mmol) added by 3% portion every 90 min. The
crude was purified by chromatography (pentane/EtOAc 95:5) to afford a
yellow oil (syn/anti 83:17, 108 mg, corrected yield 72%).IR (neat): nmax
=
3071–2872, 2106, 1738, 1370, 1235, 1113, 1051, 837 cmꢀ1 1H NMR
;
1732, 1427, 1372, 1236, 1112, 838, 817 cmꢀ1 1H NMR (300 MHz, CDCl3),
;
(300 MHz, CDCl3), major + minor: d=7.62–7.53 (m, 2H), 7.39–7.31 (m,
3H), 5.01 (m, 1H), 4.14 (q, J=7.2 Hz, 1H), 4.13 (q, J=7.0 Hz, 1H), 4.07
(m, 1H), 2.51–2.28 (m, 2H), 2.13 (s, 3H), 2.10 (s, 3H) 1.83 (m, 1H), 1.67
(m, 1H), 1.38 (m, 1H), 1.26 (t, J=7.2 Hz, 3H), 1.25 (t, J=7.2 Hz, 3H),
0.69 (s, 9H) 0.67 (s, 9H),0.48 (s, 3H) 0.43 (s, 3H), 0.41 (s, 3H) 0.40 ppm
(s, 3H); 13C NMR (75 MHz, CDCl3), major + minor: d=172.8, 172.7,
170.5, 170.3, 138.0, 137.9, 134.4, 134.2, 129.3, 129.1, 127.8, 127.7, 76.7,
76.5, 63.5, 62.7, 60.5, 60.4, 37.6, 37.2, 34.9, 32.7, 32.4, 32.0, 31.7, 26.2, 26.1,
21.9, 21.0, 14.2, ꢀ2.0, ꢀ2.4, ꢀ2.5, ꢀ3.7 ppm; MS (LSIMS): m/z (%): 456
(100) [M+Na]+, 391 [MꢀN3]+, 349, 305, 288; HRMS [M+Na]+
C22H35N3O4SiNa calcd: 456.229455; found: 456.230196.
major + minor: d=7.61–7.50 (m, 2H), 7.40–7.32 (m, 3H), 5.07 (dd, J=
9.4, 1.5 Hz, 1H), 5.00 (dd, J=9.0, 2.3 Hz, 1H), 4.11 (q, J=7.2 Hz, 2H),
4.06 (q, J=7.1 Hz, 2H), 3.52 (m, 1H), 2.33 (m, 2H), 2.07 (s, 3H), 2.04 (s,
3H), 1.92–1.58 (m, 7H), 1.48 (dd, J=5.4, 1.6 Hz, 1H), 1.23 (t, J=7.2 Hz,
3H), 1.21 (t, J=7.3 Hz, 3H), 0.91 (d, J=6.6 Hz, 3H), 0.89 (d, J=6.4 Hz,
3H), 0.80 (m, 3H), 0.78 (d, J=6.6 Hz, 3H), 0.52 (s, 6H), 0.51 (s, 3H),
0.50 ppm (s, 3H). 13C NMR (75 MHz, CDCl3), major diastereomer: d=
172.7, 170.2, 138.4, 133.8, 129.1, 127.8, 78.8, 63.9, 60.4, 36.7, 32.9, 31.3,
27.8, 21.2, 19.8, 18.2, 14.1, ꢀ0.7, ꢀ1.9 ppm; minor diastereomer: d=
172.3, 170.4, 138.8, 133.7, 129.1, 127.9, 78.4, 63.1, 60.5, 35.2, 32.6, 31.0,
30.2, 21.2, 19.9, 18.5, 14.1, ꢀ0.3, ꢀ2.7 ppm; MS (ESI): m/z (%): 442 (75)
[M+Na]+; HRMS [M+Na]+ C21H33N3O4SiNa calcd: 442.2138; found:
442.2126.
Ethyl
6-acetoxy-4-azido-5-(dimethylphenylsilyl)-6-phenylhexanoate
(15d): Prepared according to general procedure C from ethyl ester 7
(52 mg, 0.25 mmol), allylsilane 13d (162 mg, 0.50 mmol), benzenesulfo-
A
(4S,5R,6S)-Ethyl
6-acetoxy-4-azido-5-(triisopropylsilyl)-6-phenylhexa-
DTBHN (7 mg, 0.04 mmol) added by 3% portion every 90 min. The
crude was purified by chromatography (hexane/EtOAc 95:5) to afford a
yellow oil (syn/anti >95:5, 74 mg, 65%).IR (neat): nmax =3440, 2105,
noate ethyl ester (18b): Prepared according to general procedure C from
ethyl ester 7 (92 mg, 0.44 mmol), allylsilane 17c (77 mg, 0.22 mmol), 3-
pyridylsulfonylazide (124 mg, 0.67 mmol), (Bu3Sn)2 (0.17 mL, 0.32 mmol),
and DTBHN (14 mg, 0.08 mmol) added by 3% portion every 2 h. The
crude was purified by chromatography (hexane/EtOAc 95:5) to afford a
1
1737, 1373, 1229, 1112, 1033 cmꢀ1; H NMR (300 MHz, CDCl3): d=)7.42–
7.35 (m, 5H), 7.31–7.18 (m, 5H), 6.05 (d, J=3.4 Hz, 1H), 4.03 (q, J=
Chem. Eur. J. 2008, 14, 2744 – 2756
ꢀ 2008 Wiley-VCH Verlag GmbH & Co.KGaA, Weinheim
www.chemeurj.org
2753

Y.Landais, P.Renaud et al.
yellow oil (syn/anti >95:<5, 65 mg, 62%).IR (neat): nmax =3063–2868,
2104, 1732, 1226 cmꢀ1 1H NMR (300 MHz, CDCl3): d=7.38–7.14 (m,
48.0, 35.9, 34.8, 29.4, 24.1, 23.2, 14.1, ꢀ1.0, ꢀ1.1, ꢀ2.0, ꢀ2.2 ppm; MS
(LSIMS): m/z (%): 418 [M+Na]+ (35), 353 (100), 318 (13), 288 (43);
HRMS [M+Na]+ C22H29N3O2SiNa calcd: 418.192505; found: 418.192676.
;
5H), 6.14 (d, J=1.9 Hz, 1H), 3.81 (qd, J=7.1, 2.1 Hz, 2H), 3.54 (ddd, J=
10.5, 4.1, 2.3 Hz, 1H), 2.03 (s, 1H), 1.99–1.91 (m, 2H), 1.64–1.56 (m, 2H),
1.11–0.87 ppm (m, 21H); 13C NMR (75 MHz, CDCl3): d=172.4, 169.9,
141.3, 128.6, 127.3, 124.9, 74.9, 62.2, 60.4, 37.7, 31.8, 31.7, 21.7, 19.3, 14.1,
12.2 ppm; MS (ESI): m/z: 498 (100) [M+Na]+; HRMS [M+Na]
C25H41N3O4SiNa calcd: 498.2764; found: 498.2744.
Ethyl 4-azido-5-(benzyloxy)hexanoate (26a): Prepared according to gen-
eral procedure C from ethyliodoacetate (222 mg, 1.04 mmol), olefin 25a
(324 mg, 2 mmol), benzenesulfonylazide (550 mg, 3 mmol), (Bu3Sn)2
(0.75 mL, 1.5 mmol) and DTBHN (5 mg, 0.03 mmol) in dry benzene
(2 mL).The crude was purified by chromatography (cyclohexane then cy-
clohexane/tBuOMe 95:5) to afford a yellow oil as an inseparable mixture
of PhSO2N3 and both diastereomers (54:46, 281 mg, estimated yield
Ethyl 7-acetoxy-4-azido-7-phenyl-5-(triphenylsilyl)heptanoate (20): Pre-
pared according to general procedure C from ethyl ester 7 (102 mg,
0.48 mmol), allylsilane 19 (380 mg, 0.82 mmol), 3-pyridylsulfonylazide
(265 mg, 1.44 mmol), (Bu3Sn)2 (0.36 mL, 0.72 mmol), and DTBHN
(16 mg, 0.09 mmol) added by 3% portion every 90 min. The crude was
purified by chromatography (pentane/EtOAc 9:1) to afford a colorless
43%).IR (neat):
nmax =3065–2871, 2127, 1732, 1449, 1372, 1170,
1087 cmꢀ1 1H NMR (300 MHz, CDCl3), major + minor: d=7.41–7.25
;
(m, 10H), 4.59 (m, 4H), 4.30 (m, 1H), 4.22 (m, 1H), 4.14 (q, J=7.2 Hz,
2H), 4.14 (q, J=7.2 Hz, 2H), 3.45 (m, 1H), 3.20 (m, 1H), 2.69–2.52 (m,
2H), 2.51–2.34 (m, 2H), 2.12 (m, 2H), 2.04 (m, 2H), 1.34 (d, J=6.0 Hz,
3H), 1.31–1.22 ppm (m, 9H); 13C NMR (75 MHz, CDCl3): d=172.6,
172.5, 138.2, 138.0, 128.3, 127.7, 126.6, 77.5, 77.2, 70.8, 70.7, 60.5, 60.4,
42.0, 40.3, 34.4, 34.3, 30.8, 30.4, 18.5, 17.9, 14.2 ppm.
oil (syn/anti 87:13, 89 mg, 31%).Major diastereomer : IR (neat): nmax
=
3070–2933, 2100, 1732, 1428, 1372, 1112 cmꢀ1 1H NMR (250 MHz,
;
CDCl3): d=7.53–7.28 (m, 20H), 5.78 (m, 1H), 4.06 (q, J=6.9 Hz, 2H),
3.95 (m, 1H), 2.44–2.26 (m, 2H), 2.25–2.16 (m, 2H), 2.04 (s, 3H), 2.02–
1.91 (m, 1H), 1.79–1.46 (m, 2H), 1.21 ppm (t, J=7.2 Hz, 3H); 13C NMR
(63 MHz, CDCl3): d=172.6, 170.1, 139.4, 136.0, 133.1, 129.7, 128.5, 128.0,
127.4, 126.3, 75.9, 64.6, 60.4, 33.9, 31.8, 28.4, 26.8, 21.3, 14.1 ppm; MS
(LSIMS): m/z: 614 (13) [M+Na]+, 504 (11), 259 (100) [Ph3Si]+; HRMS
[M+Na]+ C35H37N3O4SiNa calcd: 614.245105; found: 614.244118.
4-Azido-5-(tert-butyldimethylsilanyloxy)hexanoic acid ethyl ester (26b):
Prepared according to general procedure
C from ethyliodoacetate
(212 mg, 0.99 mmol), olefin 25b (373 mg, 2 mmol), benzenesulfonylazide
(550 mg, 3 mmol), (Bu3Sn)2 (0.75 mL, 1.5 mmol) and DTBHN (5 mg,
0.03 mmol) added by 3% portion every 90 min. The crude was purified
by chromatography (cyclohexane then cyclohexane/EtOAc 98:2) to
afford a yellow oil (57:43, 214 mg, 68%).IR (neat), major diastereomer:
nmax =2959–2858, 2103, 1736, 1463, 1257, 1179, 837 cmꢀ1; minor diastereo-
5-Azido-2-methyl-4-(dimethylphenylsilyl)-8-oxononan-3-yl acetate (22a):
Prepared according to general procedure C from xanthate 21a (123 mg,
0.69 mmol), allylsilane 13a (105 mg, 0.362 mmol), 3-pyridylsulfonylazide
(190 mg, 1.03 mmol), (Bu3Sn)2 (0.26 mL, 0.52 mmol), and DTBHN
(12 mg, 0.06 mmol) added by 3% portion every 90 min. The crude was
purified by chromatography (hexane/EtOAc 95:5) to afford a colorless
oil (syn/anti 72:28, 101 mg, 72%).IR (neat): nmax =2962, 2101, 1732,
mer: nmax =2930–2858, 2102, 1736, 1472, 1257, 837 cmꢀ1
;
1H NMR
(300 MHz, CDCl3), major diastereomer: d=4.15 (q, J=7.2 Hz, 2H), 3.89
(m, 1H), 3.32 (dt, J=10.6, 3.5 Hz, 1H), 2.43 (m, 2H), 1.81 (m, 1H), 1.58
(m, 1H), 1.27 (t, J=7.2 Hz, 3H), 1.17 (d, J=6.2 Hz, 3H), 0.90 (s, 9H),
0.08 ppm (s, 6H); minor diastereomer : d=4.15 (q, J=7.1 Hz, 2H), 3.83
(m, 1H), 3.00 (dt, J=9.9, 4.8 Hz, 1H), 2.46 (m, 2H), 1.85 (m, 1H), 1.27
(t, J=7.2 Hz, 3H), 1.21 (d, J=6.2 Hz, 3H), 0.91 (s, 9H), 0.1 (s, 3H),
0.09 ppm (s, 3H); 13C NMR (75 MHz, CDCl3), major diastereomer: d=
172.9, 71.3, 67.6, 60.5, 31.2, 25.7, 25.4, 18.7, 17.9, 14.2, ꢀ4.5, ꢀ4.9 ppm;
minor diastereomer: d=172.9, 71.4, 66.9, 60.5, 31.1, 25.8, 25.7, 20.7, 17.9,
14.2, ꢀ4.5, ꢀ5.0 ppm; MS (LSIMS): major diastereomer m/z (%): 316
[M+H]+ (17), 258 (47), 185 (16), 159 (100), 115 (34); minor diastereomer
m/z (%): 316 [M+H]+ (21), 258 (97), 185 (37), 159 (100), 115 (44);
HRMS [M+H]+ C14H30N3O3Si calcd: 316.205646; found: 316.205260.
1236 cmꢀ1 1H NMR (300 MHz, CDCl3), major isomer: d=7.54 (m, 2H),
;
7.33 (m, 3H), 4.93 (dd, J=8.3, 1.9 Hz, 1H), 3.85 (m, 1H), 2.61–2.40 (m,
2H), 2.12 (s, 3H), 2.00 (s, 3H), 1.85 (m, 1H), 1.75–1.49 (m, 2H), 1.47 (m,
1H), 0.78 (d, J=6.8 Hz, 3H), 0.76 (d, J=6.8 Hz, 3H), 0.40 (s, 3H),
0.39 ppm (s, 3H); 13C NMR (75 MHz, CDCl3): d=207.7, 170.7, 137.7,
134.3, 129.0, 127.8, 76.8, 63.1, 40.8, 36.6, 32.0, 31.3, 29.9, 21.7, 19.4, 18.5,
ꢀ2.2, ꢀ3.1 ppm; MS (ESI): m/z (%): 412 (100) [M+Na]+.HRMS
[M+Na]+ C20H31N3O3SiNa calcd: 412.2032; found: 412.2017.
7-(N-Methoxy-N-methylcarbamoyl)-5-azido-2-methyl-4-(dimethylphenyl-
silyl)heptan-3-yl acetate (22b): Prepared according to general procedure
C
from xanthate 21b (97 mg, 0.43 mmol), allylsilane 13a (157 mg,
Tosylate (30): p-Tolylsulfonyl chloride (270 mg, 1.4 mmol) was added to a
solution of diol 29 (71.5 mg, 0.236 mmol) in pyridine (3.5 mL). The mix-
ture was stirred at RT for 5 h then quenched with brine.The aqueous
layer was extracted with EtOAc and the combined extracts washed with
Cu2SO4.The solvent was evaporated under vacuum and the crude puri-
fied through silica gel (pentane/EtOAc 8:2) to afford a colorless oil
(101.5 mg, 94%). IR (neat): nmax =3500, 2953, 2106, 1738, 1361, 1177,
0.89 mmol), 3-pyridylsulfonylazide (247 mg, 1.29 mmol), (Bu3Sn)2
(0.32 mL, 0.64 mmol), and DTBHN (13 mg, 0.07 mmol) added by 3%
portion every 90 min.The crude was purified by chromatography (pen-
tane/EtOAc 8:2) to afford a yellow oil (syn/anti 77:23, 209 mg, estimated
yield 75%).Major diastereomer: IR (neat): nmax =2964, 2104, 1731, 1664,
1373, 1235, 1111 cmꢀ1 1H NMR (300 MHz, CDCl3): d=7.59–7.52 (m,
;
2H), 7.38–7.31 (m, 3H), 4.92 (dd, J=8.3, 2.3 Hz, 1H), 3.95 (m, 1H), 3.69
(s, 3H), 3.18 (s, 3H), 2.59–2.44 (m, 2H), 1.99 (s, 3H), 1.92–1.63 (m, 3H),
1.49 (m, 1H), 0.78 (d, J=6.8 Hz, 3H), 0.74 (d, J=6.8 Hz, 3H), 0.41 (s,
3H), 0.40 ppm (s, 3H); 13C NMR (75 MHz, CDCl3): d=170.8, 137.8,
134.3, 129.0, 127.7, 76.6, 63.3, 61.2, 36.7, 32.2, 31.9, 29.1, 21.7, 19.4, 18.5,
ꢀ2.0, ꢀ3.1 ppm; MS (LSIMS): m/z (%): 457 [M+Na]+ (100); HRMS
[M+Na]+ C21H34N4O4SiNa calcd: 457.2247; found: 457.2249.
1069 cmꢀ1 1H NMR (300 MHz, CDCl3): d=7.81 (d, J=8.2 Hz, 2H), 7.36
;
(d, J=7.9 Hz, 2H), 4.34 (dd, J=10.2, 2.0 Hz, 1H), 4.20 (m, 1H), 4.17
(dd, J=6.1, 4.6 Hz, 1H), 4.07 (dd, J=10.0, 6.4 Hz, 1H), 3.97 (dd, J=9.5,
6.1 Hz, 1H), 3.69 (s, 3H), 3.56 (m, 1H), 2.89 (d, J=5.5 Hz, 1H), 2.50 (m,
2H), 2.45 (s, 3H), 2.16–1.85 (m, 2H), 1.40 (s, 3H), 1.29 ppm (s, 3H);
13C NMR (75mhz, CDCl3): d=173.4, 145.1, 132.5, 129.9, 128.0, 109.1,
79.6, 75.7, 72.4, 67.8, 58.4, 51.8, 30.5, 26.9, 24.7, 21.6 ppm; MS (LSIMS):
m/z (%): 480 [M+Na]+ (100), 430 (35); HRMS [M+Na]+
C19H27N3O8SNa calcd: 480.141657; found: 480.141685.
Ethyl 4-azido-4-methyl-5-(dimethylphenylsilyl)-5-phenylpentanoate (24):
Prepared according to general procedure C from ethyl ester 7 (62 mg,
0.3 mmol), olefin 23 (160 mg, 0.6 mmol), 3-pyridylsulfonylazide (175 mg,
0.9 mmol), (Bu3Sn)2 (0.22 mL, 0.45 mmol), and DTBHN (9 mg,
0.05 mmol) added by 3% portion every 90 min. The crude was purified
by chromatography (pentane/EtOAc 98:2) to afford a yellow oil as an in-
separable of xanthate and both diastereomers (50:50, 90.2 mg, estimated
(3aR,4R,9aR,9bS)-Octahydro-2,2-dimethyl
N
N
4-ol (31): The tosylate 30 (104 mg, 0.227 mmol) was dissolved in MeOH
(2.3 mL) and 10% Pd/C (2 mg) was added. The suspension was stirred at
RT under H2 (1 atm) for 2 h.The solution was filtered through Celite
(MeOH, 10 mL) then the residue was concentrated under vacuum.The
crude product was dissolved in MeOH (3 mL), Et3N (0.16 mL,
1.14 mmol) was added, and the solution was heated under reflux over-
night.The solvent was evaporated and the resulting crude material dis-
solved in THF (2.8 mL). A 1m solution of LiAlH4 in Et2O (0.64 mL) was
added dropwise at 08C.The solution was stirred under reflux for 3 h.
Water (0.03 mL), NaOH 10% (0.03 mL), and water (0.05 mL) were suc-
cessively added.The residue was extracted with EtOAc.The organic
yield 71%).IR (neat): nmax =3070–2950, 2099, 1732, 1252, 1112 cmꢀ1
;
1H NMR (250 MHz, CDCl3), major + minor: d=7.47 (m, 4H), 7.34 (m,
6H), 7.28–7.05 (m, 10H), 4.05 (m, 4H), 2.49 (s, 1H), 2.47 (s, 1H), 2.32–
2.15 (m, 4H), 1.96–1.67 (m, 4H), 1.22 (s, 3H), 1.21 (s, 3H), 1.18 (m, 6H),
0.47 (s, 3H), 0.43 (s, 3H), 0.20 (s, 3H), 0.15 ppm (s, 3H); 13C NMR
(63 MHz, CDCl3): d=172.9, 172.8, 139.5, 139.3, 138.8, 138.7, 134.1, 134.0,
129.0, 128.9, 128.1, 128.0, 127.7, 127.6, 125.9, 66.7, 66.4, 60.5, 60.4, 48.3,
2754
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Chem. Eur. J. 2008, 14, 2744 – 2756

Carboazidation of Chiral Allylsilanes
FULL PAPER
layer was dried over Na2SO4 and the solvent evaporated under vacuum.
The crude was purified through silica gel (CH2Cl2/MeOH 9:1) to afford a
Chem. Soc. 1969, 91, 2273–2279; d) R.A.Abramovitch, W.D.Hol-
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white solid (37 mg, 77%).IR (neat): nmax =3376, 2924, 1379, 1207 cmꢀ1
;
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1H NMR (300 MHz, CDCl3): d=4.31 (dd, J=6.1, 2.7 Hz, 1H), 4.12 (dd,
J=6.4, 4.6 Hz, 1H), 3.78 (m, 1H), 3.00 (dd, J=11.0, 6.1 Hz, 1H), 2.91
(m, 1H), 2.55 (m, 1H), 2.47–2.36 (m, 2H), 2.07–1.65 (m, 5H), 1.58 (s,
3H), 1.37 ppm (s, 3H); 13C NMR (75MHz, CDCl3): d=109.4, 74.7, 74.5,
66.3, 60.4, 54.1, 53.5, 26.1, 25.9, 25.3, 22.4 ppm; MS (LSIMS): m/z (%):
214 (100) [M+H]+; HRMS [M+H]+ C11H20NO3 calcd: 214.144319;
found: 214.143925.
(6R,7R,8S,8aR)-Octahydroindolizine-6,7,8-triol (32):[32a] The acetonide
31 (37 mg, 0.173 mmol) was dissolved in MeOH/HCl (v/v 2:1) (0.9 mL).
The solution was stirred under reflux for 1 h.The solvent was evaporated
and the chlorhydrate was purified through DOWEX 1X10 (OHꢀ form)
and eluted with water to afford 32 as a colorless oil (29 mg, 97%).
1H NMR (300 MHz, CD3OD): d=3.83 (m, 2H), 3.45 (m, 1H) 3.17 (dd,
J=12.1, 2.6 Hz, 1H), 3.02 (m, 1H), 2.17 (m, 2H), 2.06 (m, 1H), 1.98–
1.65 ppm (m, 4H); 13C NMR (75 Mhz, CD3OD): d=72.3, 71.7, 71.2, 67.9,
57.7, 54.6, 25.5, 22.4 ppm.
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Acknowledgement
We gratefully acknowledge MNERT for a grant (L.C.), CNRS, the
RØgion Aquitaine and COST D28 for financial support.PR thanks the
Swiss National Science Foundation (Project 200020-112250) and the State
Secretariat for Education and Research (Project C03.0047) for funding.
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Received: September 5, 2007
Published online: January 22, 2008
2756
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Chem. Eur. J. 2008, 14, 2744 – 2756