New rearrangement of conjugated cyclic ene nitroso O-trimethylsilyl acetals: Convenient synthesis of dihydro-2H-pyran-3-one and dihydrofuran-3-one oximes

Article abstract of DOI:10.1055/s-0030-1260108

The nucleophile-induced rearrangement of cyclic N-alkoxy-N-(silyloxy) enamines was investigated. As a result, a new strategy for the synthesis of β-pyranone and β-furanone derivatives from nitro compounds is suggested. Georg Thieme Verlag Stuttgart New Yo

Full text of DOI:10.1055/s-0030-1260108

PAPER
2415
New Rearrangement of Conjugated Cyclic Ene Nitroso O-Trimethylsilyl
Acetals: Convenient Synthesis of Dihydro-2H-pyran-3-one and Dihydrofuran-
3-one Oximes
N
ew
R
earrangeme
n
nt of Cyclic
d
E
ne Nitroso
r
-Trime
e
thylsilyl
A
c
y
etals A. Tabolin,*a Yulia A. Khomutova,a Yulia V. Nelyubina,b Sema L. Ioffe,*a Vladimir A. Tartakovskya
a
N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prosp. 47, Moscow 119991, Russian Federation
Fax +7(499)1355328; E-mail: iof@ioc.ac.ru
A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str. 28, Moscow 119991,
Russian Federation
b
Received 4 April 2011; revised 25 May 2011
Cyclic ene nitroso acetals 1 are readily accessible as indi-
vidual diastereomers by silylation3 of the corresponding
nitronates 2 (n = 0, 1) (Scheme 2).4,5 Their reactivity is
very close to BSENAs.3 The relative configuration of ring
carbon atoms in derivatives 2 should not change in their
common reactions, which may allow the application of ni-
troso acetals 1 in stereocontrolled synthesis. At the same
time, the chemistry of derivatives 1 has remarkable fea-
tures compared to BSENA chemistry. This results from
enamine 1 possessing different oxy groups connected to
the nitrogen atom. Furthermore, cleavage of the endocy-
clic N–O bond does not lead to a simplification of the car-
bon skeleton of initial enamine 1, because the oxygen
atom remains tethered to the other part of the initial mol-
ecule. This makes the unique rearrangement 1 → 3, de-
picted in Scheme 2, possible. If a nitroso acetal 1 is treated
with a nucleophile Nu– with an affinity to silicon, elimina-
tion of the trimethylsilyl moiety leads to the anion A, that
may be in equilibrium with the open-chain anion B. The
latter possesses a highly reactive a-nitrosoalkene frag-
ment. In this case, ring closure leading to the oximino an-
ion C is preferred. Finally, desilylation of 1 with anion C
results in silylated oxime 3, and anion A, formed from 1,
propagates the chain process. (Another possible formation
of oxime 3 cannot be excluded, i.e. interaction of anion C
with NuSiMe3, leading to regeneration of the nucleophile
Nu–.)
Abstract: The nucleophile-induced rearrangement of cyclic N-
alkoxy-N-(silyloxy)enamines was investigated. As a result, a new
strategy for the synthesis of b-pyranone and b-furanone derivatives
from nitro compounds is suggested.
Key words: nitroso acetals, rearrangement, nitrogen heterocycles,
1,2-oxazines, aliphatic nitro compounds
A quarter of a century has passed since the discovery of
N,N-bis(silyloxy)enamines (BSENAs) (i.e., nitro com-
pound double silylation products).1 Due to their availabil-
ity and unique reactivity, BSENAs now play an important
role among other nitro compound derivatives. Common
chemical transformations of BSENAs are presented in
Scheme 1. As can be seen, BSENAs are able to react with
quite different types of reagents, like electrophiles, nu-
cleophiles and radicals. Also worthy of note is the peculiar
BSENA rearrangement, 1,3-N,C migration of the silyloxy
group, that occurs under the action of Lewis acids, leading
to the corresponding b-oximino alcohol derivatives.2
Nu–
Nu(R)
N
NOSiMe3
O
R
– NuSiMe3
– Me3SiO–
Nu–
Desilylation of derivatives 3 can be considered as a simple
approach to the synthesis of furanone (n = 0) or pyranone
(n = 1) oximes 4. Products of this kind possess biological
activity (cytotoxic,6a muscarinic6b,c and implantation pre-
vention in rats6d) and have been used in carbohydrate7 and
cyclic amino alcohol6d,8 syntheses. A general method for
the synthesis of oximes 4, however, is lacking and these
derivatives are usually obtained via oximation of the cor-
responding carbonyl compounds. In contrast, the strategy
pointed out in Scheme 2 would allow for the assembly of
oximes 4 from simple initial compounds. In this situation,
oximes 4 could themselves be considered as convenient
precursors in the design of target b-pyranones and b-fura-
nones.
E+
silylation
E
NO2
NO2
N(OSiMe3)2
BSENA
OSiMe3
NOSiMe3
Scheme 1 General chemistry of BSENAs (Nu– = nitronate anions,
azoles, amines, CN–, N3 , etc.; R = R¢SO2CH2, R¢O2CCH2;
–
E+ = carboxonium, iminium and episulfonium cations, diaryl carbo-
cations, bromine)
Recently, we tentatively mentioned two examples of the
rearrangement 1 → 3;9 however, the target oximes 4 were
obtained in rather low yields (17% and 27%) and those re-
SYNTHESIS 2011, No. 15, pp 2415–2422
Advanced online publication: 14.07.2011
DOI: 10.1055/s-0030-1260108; Art ID: T35811SS
© Georg Thieme Verlag Stuttgart · New York
x
x.
x
x
.
2
0
1
1

2416
A. A. Tabolin et al.
PAPER
NOSiMe3
NOH
desilylation
silylation
n
n
n
n
N
–
N
+
O
O
O
OSiMe3
O
O
2
1
3
4
Nu–
n = 0, 1
1
NuSiMe3
–
NO
n
n
n
N
N
–
–
O
O
O
O
O
A
B
C
Scheme 2 Rearrangement 1 → 3 as the key step in the general strategy for the synthesis of oximes 4 from nitronates 2
Table 1 Optimization of the Conditions for Enamine 1a Rearrangement
An
O
An
O
NOR
N
OSiMe3
1a
An = 4-MeOC6H4
3a R = SiMe3
4a R = H
Entry
1
Reaction conditionsa
Product
4a
Yield (%)
33d
TBAFb (0.1 equiv), CH2Cl2, –78 °C to r.t., then NH4F, AcOH, MeOHc
TBAF·3H2O (0.1 equiv), CH2Cl2, –78 °C to r.t., then NH4F, AcOH, MeOH
TBAFb (0.1 equiv), 3 Å MS, CH2Cl2, –78 °C to r.t.
2
4a
25d
3
3a
89e
4
TBAFb (0.1 equiv), 3 Å MS, CH2Cl2, –78 °C
3a
87e
5
TBAFf (0.1 equiv), CH2Cl2, –78 °C to r.t., then NH4F, AcOH, MeOH
TBAFf (0.1 equiv), CH2Cl2, r.t., then NH4F, AcOH, MeOH
TBAFb (0.1 equiv), 3 Å MS, CH2Cl2, –78 °C to r.t., then NH4F, AcOH, MeOH
4a
97d
6
4a
91e
7
4a
100e
8
TBAF·3H2O (0.1 equiv), 3 Å MS, CH2Cl2, MeOH, –78 °C to r.t., then NH4F, AcOH, MeOH unidentified mixture of products
9
NH4F (2 equiv), Et2O, MeOH, –78 to –30 °C, then NH4F, AcOH, MeOH
ZnF2 (0.6 equiv), HMPA, CH2Cl2, r.t., 3 d
unidentified mixture of products
10
11
12
13
14
15
16
17
4a
4a
4a
4a
4a
10e (77g)
21e (41g)
77e
LiF (0.6 equiv), NMP, r.t., 3 d
t-BuOK (0.1 equiv), CH2Cl2, THF, –78 °C to r.t., then NH4F, AcOH, MeOH
TMSONa (3 equiv), CH2Cl2, THF, ca. –60 °C to r.t., then AcOH
BuLi (0.87 equiv), THF, –78 °C, then AcOH, MeOH
NaOH (0.2 equiv), MeOH, r.t., then NH4F, AcOH, MeOH
Et3N (0.1 equiv), CH2Cl2, r.t.
95e
83e
unidentified mixture of products
3a
3a
41e
87e
DMAP (0.1 equiv), CH2Cl2, r.t.
a Reaction times: 0.5–1.5 h at lower temperature, 0.5–1 h at higher temperature, 1 h for desilylation, unless otherwise mentioned (for details,
see Supporting Information).
b Commercially available 1-M solution in THF.
c Here and in other entries: NH4F (1.5 equiv), AcOH (2 equiv), MeOH (2 mL/1 mmol of enamine).
d Isolated yield after column chromatography.
e Based on 1H NMR spectrum integration [ratio: signals (CH2O)/(CHArom)].
f Dried by heating under reduced pressure.10
g Approx. recovery of 1a.
Synthesis 2011, No. 15, 2415–2422 © Thieme Stuttgart · New York

PAPER
New Rearrangement of Conjugated Cyclic Ene Nitroso O-Trimethylsilyl Acetals
2417
sults could not be considered as constituting a useful syn- (Table 1, entries 10, 11) led to low yields, presumably due
thetic method for the synthesis of derivatives 4. Here, we to extremely low solubility even in polar aprotic solvents.
report a detailed investigation of the 1 → 3 rearrangement, For best results, commercially available tetrabutylammo-
including optimization of the conditions and elucidation nium fluoride trihydrate or tetrabutylammonium fluoride
of the scope of this reaction.
solution in tetrahydrofuran must be dried either by heating
under reduced pressure (Table 1, entries 5, 6)10 or with
molecular sieves (entries 3, 4, 7).11 Other nucleophiles
(e.g., BuLi, t-BuOK or DMAP) are also applicable for re-
alizing the rearrangement (Table 1, entries 12–14, 16, 17).
Optimization of the conditions was performed using mod-
el enamine 1a (Table 1). Desilylation (step 3a → 4a) was
not optimized separately, because nearly quantitative
yields of oxime 4a can be achieved on treatment of the re-
action mixture with ammonium fluoride–acetic acid– The action of tetrabutylammonium fluoride dried with 3 Å
methanol (cf. Table 1, entries 3, 4 and 5–7). As can be molecular sieves (MS) in dichloromethane (see Table 1,
seen, major attention was paid to fluoride, which is known entry 7) was chosen as the standard procedure for further
to have a high affinity to silicon. Indeed, the rearrange- investigations. These conditions were used for the rear-
ment with F– in aprotic solvents already proceeds at – rangement of other enamines 1b–n (Table 2). The rear-
78 °C (Table 1, entry 4). The application of protic sol- rangement was successfully applied to a wide variety of
vents did not lead to the desired result (Table 1, entries 8, ene nitroso acetals 1, regardless of ring substituents, tri-
9). The presence of moisture also resulted in a decreased alkylsilyl-group character or ring size; however, some
yield of oxime 4a (cf. Table 1, entries 1, 2 and 5–7). Inor- enamines 1 required an additional procedure optimiza-
ganic fluorides like zinc fluoride or lithium fluoride tion. Thus, for the bulky tert-butyldimethylsilyl derivative
Table 2 Synthesis of Oximes 4 via Enamine 1 Rearrangementa
R1
R1
O
TBAF, 3 Å MS
CH2Cl2, –78 °C to r.t.
R2
R3
R2
R3
NOH
n
n
then NH4F, AcOH, MeOH
N
O
OSiMe2R5
R4
R4
1a–n
4a–m
Entry
1
1
a
b
c
d
e
f
4
a
b
c
d
e
f
n
1
1
1
1
1
1
1
1
1
1
1
1
0
0
1
R1
R2
R3
R4
Me
Me
Me
Me
H
SiMe2R5
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TBS
Yield (%)
97
4-MeOC6H4
Ph
H
Me
Me
Me
Me
Pr
2
H
91
3
OBz
H
71
4
Me
H
80
5
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
H
H
95
6
H
Ph
H
76
7
g
g
h
i
g
g
h
i
(CH2)4
H
53
8
(CH2)4
H
61b
53
c
9
(CH2)2O
H
10
11
12
13
14
15
H
H
H
–
OMe
OEt
H
Me
H
83d
76d
90d
83
j
j
k
l
k
l
OEt
H
Ph
m
m
H
–
CO2Me
Me
Me
Me
60
n
a
4-MeOC6H4
H
77e
a General reaction conditions: enamine 1 (0.5 M), TBAF (0.1 equiv, 0.05 M), 3 Å MS, CH2Cl2, –78 °C for 1.6 h, r.t. for 1 h, then NH4F, AcOH,
MeOH.
b Reaction conditions: enamine 1g (0.1 M), TBAF (0.1 equiv, 0.01 M), 3 Å MS, CH2Cl2, –78 °C to r.t., then NH4F, AcOH, MeOH.
c dr ~4:1. The relative configuration of R1 in the major diastereomer was opposite to that shown above.
d For desilylation, NH4F, MeOH was used instead of NH4F, AcOH, MeOH.
e Reaction conditions: enamine 1n (0.33 M), TBAF (0.3 equiv, 0.05 M), 3 Å MS, CH2Cl2, –78 °C for 0.5 h, r.t. for 1 h, then TFA (3 equiv),
MeOH.
Synthesis 2011, No. 15, 2415–2422 © Thieme Stuttgart · New York

2418
A. A. Tabolin et al.
PAPER
1n, an increase in the tetrabutylammonium fluoride
amount and the use of trifluoroacetic acid for desilylation
were necessary. For bicyclic enamine 1g, some improve-
ment was achieved by decreasing the concentration of the
reagents (cf. Table 2, entries 7, 8). It is remarkable that
enamines 1h–k bearing an alkoxy substituent at C-6 (R3
or R4 = OAlk) can be successfully involved in the rear-
rangement. Evidently, the rate of intramolecular cycliza-
tion of anion B (see Scheme 2) is greater than the rate of
RO– elimination from C-6 (Scheme 3).
R1
R1
R2
R2
F–
N
N
–
O
AlkO
O
OSiMe3
AlkO
O
R3
R3
B
1h–k
–
– AlkO
R1
R1
O
–
NO
R2
R2
R3
3h–k
Figure 1 X-ray data of compound 4a (ORTEP drawing)
N
O
AlkO
O
R3
rivatives 4 were obtained as a mixture of E/Z-isomers (for
assignment and further details, see experimental section).
Scheme 3 Proposed mechanism for the rearrangement of 6-alkoxy-
substituted enamines 1h–k
The only limitation of the rearrangement is that it requires
the absence of an R6 substituent in the starting enamines
1, i.e. the C–C double bond should be terminal. If this was
not the case, other processes occurred (Scheme 4). Unfor-
tunately, all our attempts to overcome these problems
The structures of oximes 4 were confirmed by 1H and 13C
NMR spectroscopy and elemental analysis (or HRMS)
data. In addition, an X-ray crystallographic analysis of
oxime 4a was performed (Figure 1).12 In most cases, de-
An
R6
unidentified mixture
of products
N
R3
O
–
O
(for 1r–t)
F–
R4
B
R6
An
R6
An
O
OR
N
N
in situ
(for 1o–q)
R3
O
OSiMe3
R3
R4
R4
An
O
R6
1o–t
7o–q R = SiMe3
8o–q R = H
silylation
(Me3SiX, base)
desilylation
+
N
–
O
R3
R4
An
O
R6
An
R6
2o–t
retro-[4+2]
o R3, R4, R6 = Me
N
R3
OSiMe3
p R3 = Pr, R4 = H, R6 = Me
q R3 , R4 = Me, R6 = Ph
r R3, R4 = Me, R6 = CO2Me
s R3 = OMe, R4, R6 = Me
N
R4
OSiMe3
5o
6o
t R3 = OMe, R4 = Me, R6 = CH2CO2Me
Scheme 4 Side processes in the preparation and rearrangement of internal enamines 1o–t
Synthesis 2011, No. 15, 2415–2422 © Thieme Stuttgart · New York

PAPER
New Rearrangement of Conjugated Cyclic Ene Nitroso O-Trimethylsilyl Acetals
2419
failed. One problem related to internal nitroso acetals 1 is All reactions were performed in oven-dried (150 °C) glassware.
Melting points were determined on a Koffler melting-point appara-
connected with complications in their preparation. In par-
tus and are uncorrected. Chromatographic separations were per-
formed on silica gel (Acros, 40–60 mm, 60 Å) with analytical grade
solvents, driven by air pressure. Analytical thin-layer chromatogra-
ticular, the synthesis of enamine 1o is accompanied by
formation of enoxime 6o which is the product of [4+2] cy-
cloreversion of endocyclic enamine 5o.9 Moreover, bulky
phy was performed on Fluka silica gel plates with fluorescent indi-
Hünig’s base turned out to be too weak to afford enamine
1o with trimethylsilyl bromide. Switching from trimethyl-
silyl bromide to the more powerful trimethylsilyl trifluo-
romethanesulfonate led to some rearrangement to 7o in
the reaction mixture.9 Increasing the steric hindrance in
the initial nitronate was also unsuccessful. Thus, only ox-
azine 7p was observed on silylation of nitronate 2p. This
might be due to the rearrangement of 1p9 under the silyla-
tion conditions or during workup. Increasing the acidity of
the exocyclic hydrogen by using an electron-withdrawing
group as R6 favored exocyclic hydrogen abstraction, but
enamine 1q (R6 = Ph) rearranged to 7q, just as described
for 1p. The methoxycarbonyl-containing enamine 1r was
stable, but failed to undergo rearrangement 1 → 3 and led
to a complex mixture of products. Enamines 1s,t,9 al-
though stable, also led to a complex mixture of products
upon treatment with tetrabutylammonium fluoride. Pre-
sumably, this results from shielding of the nitrosoalkene
moiety in anion B due to R6 bulkiness.
cator (254 nm). Visualization was accomplished with UV light and/
or anisaldehyde and/or ninhydrin. 1D and 2D NMR spectra were re-
corded on a Bruker AV-300 NMR spectrometer (1H: 300.13 MHz,
13C: 75.47 MHz, 29Si: 59.63 MHz) for CDCl3 solutions at 298 K
(unless otherwise mentioned) with residual solvent peak as an inter-
nal standard.13 The INEPT pulse sequence was used for observation
of the 29Si signals. Multiplicities are indicated by s (singlet), d (dou-
blet), t (triplet), q (quartet), m (multiplet) and br (broad). Atom nu-
mering is shown in Figure 2. Coupling constants, J, are reported in
hertz. Determination of the C=N bond configuration (E or Z) was
made based on the chemical shifts in the 13C NMR spectra: the car-
bon atom in syn arrangement to the OR group (R = H, SiMe3) is
shifted to higher field compared to the signal from the same carbon
in the opposite isomer.14 Elemental analyses were performed by the
Analytical Centre of the N.D. Zelinsky Institute of Organic Chem-
istry. IR spectra were recorded as thin layers on a Bruker Vector 22
spectrometer. HRMS were recorded on a Bruker microTOF mass
spectrometer.
12
OMe
11
12
OMe
11
10
10
To demonstrate the synthetic utility of oximes 4, we have
performed the deoximation of 4a to ketone 9 and the re-
duction of 4a to amine 10. The latter was characterized as
its hydrochloride, 10·HCl (Scheme 5).
9
9
8
4
8
4
8
4
7
7
3
NOH
NOR
3
5
5
7
3
5
14
13
2
6
6
6
2
13
N
O
O
O
O
OSiMe3
11
14
9
An
An
An
10
O
NOH
NH2
3a R = SiMe3
4a R = H
H2, Raney Ni
4j
1d
TiCl3
82%
82%
dr = 1:1
O
O
O
12
11
OMe
10
OMe
11
9
4a
10
HCl
10
9
10⋅HCl (92%)
9
8
6
8
7
4
Scheme 5 Deoximation and reduction of oxime 4a
7
+
NOH
4
3
NH3
3
4
5
O
3
5
8
–
5
6
Cl
2
1
O
6
7
2
9
In conclusion, the rearrangement of cyclic N,N-
bis(oxy)enamines 1, prepared by the silylation of cyclic
nitronates 2, was investigated. As a result, a new strategy
for the assembly of dihydro-2H-pyran-3-ones and dihy-
drofuran-3-ones from nitroethane and other simple mole-
cules has been suggested (Scheme 6).
13
12
2
O
O
14
13
10
9
4l
10⋅HCl
Figure 2 Atom-numbering in selected compounds
The following compounds were prepared according to literature
procedures: nitronates 2a,d,15 2e,16 and enamines 1a,l,m,17 1b,g,i–
k,3 1c,f,s,18 1t.9 Previously unknown nitronates 2h,p–r were pre-
pared by procedures similar to those known3 (for details, see Sup-
porting Information). For oxazines 8p,q, also see Supporting
Information.
EtNO2,
EtNO2,
O
(for n = 1)
(for n = 0)
,
O
( )n
H
4,6,6-Trimethyl-3-methylene-2-(trimethylsilyloxy)-1,2-oxazi-
nane (1d); Typical Procedure
O
Et3N (0.24 g, 0.33 mL, 2.4 mmol, 1.2 equiv) and TMSBr (0.34 g,
0.29 mL, 2.2 mmol, 1.1 equiv) were successively added to a stirred
soln of nitronate 2d (0.32 g, 2 mmol) in CH2Cl2 (4 mL) at –78 °C
under argon atmosphere. The mixture was maintained at –78 °C for
1 d, then the cooling bath was removed. The mixture was diluted
with hexane (10 mL) and transferred to a mixture of hexane (40 mL)
and H2O (20 mL). The organic layer was washed with a soln of
Scheme 6 Assembly of pyranones and furanones from nitroethane
and other readily available precursors
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2420
A. A. Tabolin et al.
PAPER
NaHSO4 (60 mg/mmol of 2) in H2O (20 mL), and brine (2 × 20
mL), then treated with activated charcoal, filtered and dried
(Na2SO4). The solvent was evaporated under reduced pressure to
give target enamine 1d as a colorless oil, that was sufficiently pure
(NMR analysis) and used without additional purification; yield:
0.29 g (63%).
ture of Et2O (40 mL) and H2O (30 mL). The aqueous layer was
extracted with Et2O (20 mL). The combined organic layer was
washed with brine (2 × 30 mL) and concentrated under reduced
pressure. The residue was subjected to column chromatography
(hexane–EtOAc, 10:1 to 2:1) to give target oximes 4.
Oximes 4i–k; General Procedure (GP-2)
1H NMR: d = 0.19 (s, 9 H, 11-CH3), 1.11 (d, J = 6.6 Hz, 3 H, 8-
CH3), 1.20 (s, 3 H, 9-CH3), 1.32 (dd, J = 14.0, 12.5 Hz, 1 H, Hax5),
1.43 (s, 3 H, 10-CH3), 1.60 (dd, J = 12.5, 5.1 Hz, 1 H, Heq5), 2.53–
2.68 (m, 1 H, H4), 4.40 (s, 1 H, Ha7), 4.99 (s, 1 H, Hb7).
13C NMR: d = –0.8 (C11), 17.7, 24.1 (br), 28.9 and 30.1 (C4, C8, C9
and C10), 45.7 (br, C5), 76.8 (C6), 93.3 (C7), 159.7 (C3).
Molecular sieves were dried at ca. 200 °C under reduced pressure
(0.7 mbar) for 10 min prior to use. 1 M TBAF in THF (0.1 equiv)
(or TBAF·3H2O) was added to CH2Cl2 (2 mL/mmol of enamine 1)
under argon atmosphere and stirred with 3 Å MS (1 g/1 mmol of
TBAF) for 20 min. This suspension was cooled to –78 °C and a 0.5
M soln of enamine 1 in CH2Cl2 (0.8–2.2 mmol, 1 equiv) was added
during 5 min. The resulting mixture was stirred at –78 °C for 1 h,
then the cooling bath was removed, and the mixture was stirred for
an additional 1 h and concentrated. The residue was dissolved in
MeOH (4 mL/mmol of enamine 1) and NH4F (ca. 10 mg) was add-
ed. The resulting mixture was maintained for 1 d, filtered through a
short pad of Celite® and the Celite® was washed with EtOAc (ca. 25
mL). The resulting solution was poured into a mixture of Et2O (40
mL) and H2O (30 mL). The aqueous layer was extracted with Et2O
(20 mL). The combined organic layer was washed with brine
(2 × 30 mL) and concentrated under reduced pressure. The residue
was subjected to column chromatography (hexane–EtOAc, 3:1) to
give target oximes 4.
HRMS: m/z [M + H]+ calcd for C11H24NO2Si: 230.1571; found:
230.1568.
Previously unknown enamines 1e,h,r were prepared by silylation of
nitronates 2e,h,r with TMSBr, similar to the procedure for 1d,
while enamine 1n was prepared by silylation of nitronate 2a with
TBSOTf, similar to a known procedure3 (for details, see Supporting
Information).
Optimization of the Reaction Conditions 1a → 3a or 4a
See Supporting Information.
4-(4-Methoxyphenyl)-6,6-dimethyldihydro-2H-pyran-3(4H)-
one O-(Trimethylsilyl)oxime (3a)
For data for oximes 4b–i,k,m, see Supporting Information.
A 0.5-M soln of enamine 1a in CH2Cl2 (1 mL, 0.5 mmol) was added
to a soln of DMAP (6 mg, 0.05 mmol, 0.1 equiv) in CH2Cl2 (1 mL).
The resulting mixture was stirred for 2 h, then poured into a mixture
of Et2O (30 mL) and NaHSO4 [0.12 g in H2O (20 mL)]. The organic
layer was washed with brine (2 × 30 mL) and concentrated under
reduced pressure to give silyl derivative 3a; yield: 0.15 g (94%);
E/Z = 4.5:1.
4-(4-Methoxyphenyl)-6,6-dimethyldihydro-2H-pyran-3(4H)-
one Oxime (4a)
From Enamine 1a
Oxime 4a was obtained as a white solid from enamine 1a (1 mmol)
according to GP-1; yield: 242 mg (97%); E/Z = 1.5:1.
From Enamine 1n
Molecular sieves were dried at ca. 200 °C under reduced pressure
(0.7 mbar) for 10 min prior to use. TBAF·3H2O (28 mg, 0.09 mmol,
0.3 equiv) was added to CH2Cl2 (1.8 mL) under argon atmosphere
and stirred with 3 Å MS (90 mg) for 20 min. This suspension was
cooled to –78 °C and a 0.33 M soln of enamine 1n in CH2Cl2 (1 mL,
0.3 mmol, 1 equiv) was added during 5 min. The resulting mixture
was stirred at –78 °C for 0.5 h, then the cooling bath was removed,
the mixture was stirred for an additional 1 h, and a mixture of TFA
(103 mg, 67 mL, 0.9 mmol, 3 equiv) and MeOH (1 mL) was added.
The resulting mixture was stirred for 1.3 h, then poured into a mix-
ture of Et2O (40 mL) and H2O (30 mL). The aqueous layer was ex-
tracted with Et2O (20 mL). The combined organic layer was washed
with brine (2 × 30 mL) and concentrated under reduced pressure.
The crude product was purified by column chromatography (hex-
ane–EtOAc, 5:1 then 3:1) to give oxime 4a as a white solid; yield:
57 mg (77%); E/Z = 1:1.5.
E-Isomer
1H NMR: d = 0.07 (s, 9 H, 7-CH3), 1.10 (s, 3 H, 13-CH3), 1.25 (s, 3
H, 14-CH3), 1.98 (dd, J = 14.5, 6.7 Hz, 1 H, Ha5), 2.14 (dd, J = 14.5,
7.5 Hz, 1 H, Hb5), 3.80 (s, 3 H, 12-CH3), 4.25 (d, J = 13.7 Hz, 1 H,
Ha2), 4.37 (dd, J = 7.5, 6.7 Hz, 1 H, H4), 4.46 (d, J = 13.7 Hz, 1 H,
Hb2), 6.85 (d, J = 8.7 Hz, 2 H, H10), 7.09 (d, J = 8.7 Hz, 2 H, H9).
13C NMR: d = –1.0 (C7), 27.5 (C13), 28.5 (C14), 36.5 (C4), 41.4
(C5), 55.2 (C12), 62.3 (C2), 72.5 (C6), 113.8 (C10), 128.2 (C9),
133.3 (C8), 157.9 (C11), 161.3 (C3).
Z-Isomer
1H NMR: d = 0.08 (s, 9 H, 7-CH3), 1.26 (s, 3 H, 13-CH3), 1.36 (s, 3
H, 14-CH3), 1.86–2.08 (m, 2 H, 5-CH2), 3.73–3.82 (m, 1 H, H4),
3.81 (s, 3 H, 12-CH3), 4.46 (d, J = 16.9 Hz, 1 H, Ha2), 4.73 (d,
J = 16.9 Hz, 1 H, Hb2), 6.85 (d, J = 8.7 Hz, 2 H, H10), 7.15 (d,
J = 8.7 Hz, 2 H, H9).
Mp 149–155 °C; Rf = 0.62 and 0.53 (hexane–EtOAc, 1:1) (anisal-
dehyde).
13C NMR: d = –0.9 (C7), 25.9 (C13), 28.3 (C14), 40.9 (C4), 43.4
(C5), 55.2 (C12), 58.7 (C2), 72.2 (C6), 113.4 (C10), 129.5 (C9),
132.6 (C8), 157.9 (C11), 163.4 (C3).
IR: 3244, 2970, 2933, 1614, 1518, 1458, 1369, 1250, 1178, 1033,
944 cm–1.
Oximes 4a–h,l,m; General Procedure (GP-1)
E-Isomer
Molecular sieves were dried at ca. 200 °C under reduced pressure
(0.7 mbar) for 10 min prior to use. 1 M TBAF in THF (0.1 equiv)
(or TBAF·3H2O) was added to CH2Cl2 (1.9–2.2 mL/1 mmol of
enamine 1) under argon atmosphere and stirred with 3 Å MS (1 g/1
mmol of TBAF) for 20 min. This suspension was cooled to –78 °C
and a 0.46–0.53 M soln of enamine 1 in CH2Cl2 (0.64–1.27 mmol,
1 equiv) was added over 5 min. The resulting mixture was stirred at
–78 °C for 1.6 h, then the cooling bath was removed, the mixture
was stirred for an additional 1 h, and a mixture of NH4F (1.5 equiv),
AcOH (2 equiv) and MeOH (2 mL/1 mmol of enamine 1) was add-
ed. The resulting mixture was stirred for 1 h, then poured into a mix-
1H NMR: d = 1.08 (s, 3 H, 13-CH3), 1.25 (s, 3 H, 14-CH3), 2.00 (dd,
J = 14.7, 6.6 Hz, 1 H, Ha5), 2.11 (dd, J = 14.7, 7.3 Hz, 1 H, Hb5),
3.80 (s, 3 H, 12-CH3), 4.19 (d, J = 13.6 Hz, 1 H, Ha2), 4.34 (dd,
J = 7.3, 6.6 Hz, 1 H, H4), 4.43 (d, J = 13.6 Hz, 1 H, Hb2), 6.87 (d,
J = 8.4 Hz, 2 H, H10), 7.13 (d, J = 8.4 Hz, 2 H, H9), 8.12 (br s, 1 H,
H7).
13C NMR (DEPT): d = 27.4 (C13), 28.5 (C14), 36.0 (C4), 41.4 (C5),
55.3 (C12), 62.1 (C2), 72.7 (C6), 114.1 (C10), 128.0 (C9), 132.9
(C8), 158.5 (C11), 161.0 (C3).
Synthesis 2011, No. 15, 2415–2422 © Thieme Stuttgart · New York

PAPER
New Rearrangement of Conjugated Cyclic Ene Nitroso O-Trimethylsilyl Acetals
Z-Isomer
2421
Z-Isomer
1H NMR: d = 1.30 (s, 3 H, 13-CH3), 1.36 (s, 3 H, 14-CH3), 1.89–
2.04 (m, 2 H, 5-CH2), 3.73 (dd, J = 12.5, 5.1 Hz, 1 H, H4), 3.79 (s,
3 H, 12-CH3), 4.41 (d, J = 16.9 Hz, 1 H, Ha2), 4.65 (d, J = 16.9 Hz,
1 H, Hb2), 6.87 (d, J = 8.1 Hz, 2 H, H10), 7.13 (d, J = 8.1 Hz, 2 H,
H9), 7.91 (br s, 1 H, H7).
1H NMR: d = 2.68–2.76 (m, 1 H, Ha4), 3.05 (dd, J = 16.1, 5.9 Hz, 1
H, Hb4), 4.55 (dd, J = 16.1, 1.5 Hz, 1 H, Ha2), 4.83 (d, J = 16.1 Hz,
1 H, Hb2), 5.06–5.11 (m, 1 H, H5), 7.30–7.41 (m, 5 H, H8, H9 and
H10), 8.89 (br s, 1 H, H6).
13C NMR (DEPT): d = 37.7 (C4), 66.6 (C2), 80.4 (C5), 125.9, 128.1
and 128.6 (C8, C9 and C10), 140.0 (C7), 163.4 (C3).
13C NMR (DEPT): d = 25.8 (C13), 28.1 (C14), 40.6 (C4), 42.6 (C5),
55.3 (C12), 58.3 (C2), 72.3 (C6), 114.0 (C10), 129.3 (C9), 131.5
(C8), 158.5 (C11), 161.0 (C3).
Anal. Calcd for C10H11NO2: C, 67.78; H, 6.26; N, 7.90. Found (for
E/Z-mixture): C, 67.68; H, 6.42; N, 7.82.
Analytically pure sample was obtained after drying over P2O5 under
reduced pressure at 78 °C in a drying pistol.
4-(4-Methoxyphenyl)-6,6-dimethyldihydro-2H-pyran-3(4H)-
one (9)
Anal. Calcd for C14H19NO3: C, 67.45; H, 7.68; N, 5.62. Found (for
Compound 9 was prepared using a modification of a literature
procedure19 used for another compound. To a soln of oxime 4a (111
mg, 0.45 mmol), NH4OAc (0.5 g) and AcOH (0.1 mL) in dioxane
(1.2 mL) and H2O (0.1 mL) was added a 10% soln of TiCl3 in 20–
30% HCl (1.45 mL, 1.13 mmol, 2.4 equiv) during 4 min. The mix-
ture was stirred for 1 h, diluted with Et2O (ca. 10 mL), stirred for 2
h, then poured into a mixture of Et2O (30 mL) and brine (20 mL).
The aqueous layer was washed with Et2O (20 mL). The combined
organic layer was washed with sat. aq NaHCO3 soln (20 mL), H2O
(20 mL) and brine (2 × 30 mL), then dried (Na2SO4) and concentrat-
ed under reduced pressure to give pyranone 9 as a slightly yellow
oil; yield: 86 mg (82%).
E/Z-mixture): C, 67.23; H, 7.56; N, 5.60.
rel-(4R,6R)-6-Ethoxy-4-(4-methoxyphenyl)dihydro-2H-pyran-
3(4H)-one Oxime (4j)
Oxime 4j was obtained as a white solid from enamine 1j (0.8 mmol)
according to GP-2; yield: 161 mg (76%); Z/E = 1.7:1.
Mp 129–132 °C (MeOH); Rf = 0.55 (hexane–EtOAc, 1:1) (anisal-
dehyde).
Z-Isomer
1H NMR: d = 1.27 (t, J = 7.0 Hz, 3 H, 14-CH3), 2.01–2.10 (m, 1 H,
Ha5), 2.27–2.36 (m, 1 H, Hb5), 3.55 (dq, J = 9.9, 7.0 Hz, 1 H, Ha13),
3.79–3.86 (m, 1 H, Hb13), 3.80 (s, 3 H, 12-CH3), 3.95 (dd, J = 10.5,
5.5 Hz, 1 H, H4), 4.22 (d, J = 15.4 Hz, 1 H, Ha2), 4.87 (d, J = 15.4
Hz, 1 H, Hb2), 4.97 (br s, 1 H, H6), 6.85 (d, J = 8.5 Hz, 2 H, H10),
7.15 (d, J = 8.5 Hz, 2 H, H9), 8.30 (br s, 1 H, H7).
Rf = 0.30 (hexane–EtOAc, 3:1) (anisaldehyde).
IR: 2972, 2933, 1732, 1613, 1516, 1464, 1369, 1302, 1248, 1180,
1161, 1124, 1093, 1034, 831 cm–1.
1H NMR: d = 1.37 (s, 3 H, 12-CH3), 1.48 (s, 3 H, 13-CH3), 2.14–
2.21 (m, 2 H, 5-CH2), 3.81 (s, 3 H, 11-CH3), 3.87 (dd, J = 11.5, 7.6
Hz, 1 H, H4), 4.09–4.22 (m, 2 H, 2-CH2), 6.91 (d, J = 8.8 Hz, 2 H,
H9), 7.07 (d, J = 8.8 Hz, 2 H, H8).
13C NMR: d = 15.1 (C14), 36.7 (C5), 39.3 (C4), 55.2 (C12), 55.7
and 63.2 (C2 and C13), 96.2 (C6), 114.0 (C10), 129.2 (C9), 131.6
(C8), 158.0 (C11), 158.5 (C3).
Characteristic NOE contacts: 13-CH2–H6, Ha2–H4, H6–5-CH2,
H4–Ha5. Contact 13-CH2–2-CH2 was not observed.
13C NMR: d = 26.6 (C12), 27.3 (C13), 41.7 (C5), 49.3 (C4), 55.2
(C11), 68.5 (C2), 72.9 (C6), 114.1 (C9), 129.5 (C8), 129.9 (C7),
158.8 (C10), 210.5 (C3).
E-Isomer
HRMS: m/z [M + H]+ calcd for C14H19O3: 235.1329; found:
235.1329.
1H NMR: d = 1.21 (t, J = 7.0 Hz, 3 H, 14-CH3), 2.01–2.10 (m, 1 H,
Ha5), 2.27–2.36 (m, 1 H, Hb5), 3.47 (dq, J = 9.5, 7.0 Hz, 1 H, Ha13),
3.79–3.86 (m, 1 H, Hb13), 3.80 (s, 3 H, 12-CH3), 4.22 (d, J = 14.0
Hz, 1 H, Ha2), 4.42 (d, J = 14.0 Hz, 1 H, Hb2), 4.48 (dd, J = 5.5, 3.3
Hz, 1 H, H4), 4.77 (dd, J = 7.7, 4.1 Hz, 1 H, H6), 6.87 (d, J = 8.8
Hz, 2 H, H10), 7.19 (d, J = 8.8 Hz, 2 H, H9), 8.30 (br s, 1 H, H7).
4-(4-Methoxyphenyl)-6,6-dimethyltetrahydro-2H-pyran-3-
amine (10) and 4-(4-Methoxyphenyl)-6,6-dimethyltetrahydro-
2H-pyran-3-aminium Chloride (10·HCl)
A 50% slurry of Raney Ni in H2O (2 mL) was washed with MeOH
(4 × 3 mL) prior to use. A soln of oxime 4a (107 mg, 0.43 mmol) in
MeOH (7 mL) was hydrogenated at 20 bar and 70 °C in a steel au-
toclave for 2 h. The resulting mixture was filtered through Celite®
and the Celite® was washed with MeOH (30 mL). The filtrate was
concentrated under reduced pressure. The residue was purified by
column chromatography (EtOAc, then CHCl3–MeOH, 20:1) to give
amine 10 as a colorless oil; yield: 83 mg (82%); dr = 1:1; Rf = 0.25
(CHCl3–MeOH, 10:1) (ninhydrin).
13C NMR: d = 15.2 (C14), 34.3 (C5), 35.3 (C4), 55.2 (C12), 61.7
and 63.6 (C2 and C13), 97.2 (C6), 114.0 (C10), 128.5 (C9), 131.6
(C8), 157.2 (C3), 158.0 (C11).
Characteristic NOE contacts: 13-CH2–H6.
Anal. Calcd for C14H19NO4: C, 63.38; H, 7.22; N, 5.28. Found (for
E/Z-mixture): C, 63.20; H, 7.37; N, 5.09.
5-Phenyldihydrofuran-3(2H)-one Oxime (4l)
Oxime 4l was obtained as a colorless oil, that solidifies upon stor-
age, from enamine 1l (0.75 mmol) according to GP-1; yield: 111 mg
(83%); E/Z = 5:1.
Amine 10 was dissolved in toluene (1 mL) and 4 M HCl in dioxane
(0.1 mL) was added. Slow formation of a white precipitate was ob-
served. The mixture was maintained for 3 h, then concentrated un-
der reduced pressure to give hydrochloride 10·HCl as a brownish
solid; yield: 96 mg (82%, based on initial oxime). Recrystallization
(acetone) afforded a white powder; yield: 47 mg (49%, based on
amine); trans/cis = 2.5:1; mp 255–258 °C (dec). Concentration of
the mother liquor afforded a brownish oil; yield: 41 mg (43%, based
on amine); trans/cis = 1:2.5.
Mp 106–113 °C; Rf = 0.62 (UV) and 0.48 (UV, anisaldehyde) (hex-
ane–EtOAc, 1:1).
E-Isomer
1H NMR: d = 2.69 (ddd, J = 18.4, 8.8, 1.5 Hz, 1 H, Ha4), 3.26 (dd,
J = 18.4, 6.6 Hz, 1 H, Hb4), 4.42 (d, J = 13.9 Hz, 1 H, Ha2), 4.64 (dd,
J = 13.9, 1.5 Hz, 1 H, Hb2), 5.10 (dd, J = 8.8, 6.6 Hz, 1 H, H5),
7.30–7.41 (m, 5 H, H8, H9 and H10), 8.89 (br s, 1 H, H6).
trans-Isomer
1H NMR (COSY, NOESY, DMSO-d6): d = 1.17 (s, 3 H, 13-CH3),
1.27 (s, 3 H, 14-CH3), 1.55–1.69 (m, 2 H, 5-CH2), 3.13–3.28 (m, 2
H, H3 and H4), 3.65–3.73 (m, 1 H, Ha2), 3.73 (s, 3 H, 12-CH3),
13C NMR (DEPT): d = 35.4 (C4), 67.9 (C2), 80.6 (C5), 125.9, 128.1
and 128.6 (C8, C9 and C10), 140.4 (C7), 162.2 (C3).
Synthesis 2011, No. 15, 2415–2422 © Thieme Stuttgart · New York

2422
A. A. Tabolin et al.
PAPER
3.89–3.94 (m, 1 H, Hb2), 6.91 (d, J = 8.5 Hz, 2 H, H10), 7.27 (d,
J = 8.5 Hz, 2 H, H9), 8.06 (br s, 3 H, H7).
(4) For reviews on cyclic nitronate chemistry, see:
(a) Denmark, S. E.; Thorarensen, A. Chem. Rev. 1996, 96,
137. (b) Ioffe, S. L. In Nitrile Oxides, Nitrones and
Nitronates in Organic Synthesis; Feuer, H., Ed.; John Wiley
& Sons: Hoboken, 2008, 444, 531.
1H NMR (COSY, NOESY, D2O): d = 1.23 (s, 3 H, 13-CH3), 1.31 (s,
3 H, 14-CH3), 1.77–1.80 (m, 2 H, 5-CH2), 3.03 (dt, J = 11.7, 8.1 Hz,
1 H, H4), 3.40 (ddd, J = 11.7, 10.3, 5.3 Hz, 1 H, H3), 3.68–3.77 (m,
1 H, Hax2), 3.77 (s, 3 H, 12-CH3), 3.94 (dd, J = 11.7, 5.3 Hz, 1 H,
Heq2), 6.96 (d, J = 7.9 Hz, 2 H, H10), 7.27 (d, J = 7.9 Hz, 2 H, H9).
(5) (a) For the synthesis of five-membered cyclic nitronates
from nitroethane, see, e.g.: Kunetsky, R. A.; Dilman, A. D.;
Struchkova, M. I.; Belyakov, P. A.; Tartakovsky, V. A.;
Ioffe, S. L. Synthesis 2006, 2265. (b) For the synthesis of
six-membered cyclic nitronates from nitroethane, see ref.3
(6) (a) Elomri, A.; Skaltsounis, A.-E.; Michel, S.; Tillequin, F.;
Koch, M.; Rolland, Y.; Pierre, A.; Atassi, G. Chem. Pharm.
Bull. 1996, 44, 2165. (b) Wu, E. S. C.; Griffith, R. C.; Loch,
J. T. III; Kover, A.; Murray, R. J.; Mullen, G. B.; Blosser, J.
C.; Machulskis, A. C.; McCreedy, S. A. J. Med. Chem. 1995,
38, 1558. (c) Tsukamoto, S.; Fujii, M.; Yasunaga, T.;
Matsuda, K.; Wanibuchi, F.; Hidaka, K.; Furuya, T.;
Tamura, T. Chem. Pharm. Bull. 1995, 43, 842. (d) Arora, P.
K.; Kole, P. L.; Ray, S. Indian J. Chem., Sect. B: Org. Chem.
Incl. Med. Chem. 1985, 24, 845.
Characteristic NOE contacts: H3–5-CH2, H4–Hax2. Contact H3–H4
was not observed.
13C NMR (HSQC, DMSO-d6): d = 21.8 (C13), 30.8 (C14), 40.2
(C4), 44.0 (C5), 51.7 (C3), 55.5 (C12), 61.8 (C2), 72.4 (C6), 114.6
(C10), 129.5 (C9), 132.7 (C8), 158.9 (C11).
cis-Isomer
1H NMR (COSY, NOESY, DMSO-d6): d = 1.23 (s, 3 H, 13-CH3),
1.27 (s, 3 H, 14-CH3), 1.45 (dd, J = 13.9, 2.9 Hz, 1 H, Ha5), 2.27 (t,
J = 13.9 Hz, 1 H, Hb5), 3.26–3.40 (m, 2 H, H3 and H4), 3.73 (s, 3
H, 12-CH3), 3.89 (s, 2 H, 2-CH2), 6.91 (d, J = 8.8 Hz, 2 H, H10),
7.23 (d, J = 8.8 Hz, 2 H, H9), 7.87 (br s, 3 H, H7).
(7) (a) Tsuda, Y.; Okuno, Y.; Iwaki, M.; Kanemitsu, K. Chem.
Pharm. Bull. 1989, 37, 2673. (b) Brehm, M.; Goeckel, V.
H.; Jarglis, P.; Lichtenthaler, F. W. Tetrahedron: Asymmetry
2008, 19, 358. (c) Jarglis, P.; Goeckel, V. H.; Lichtenthaler,
F. W. Tetrahedron: Asymmetry 2009, 20, 952.
(8) (a) Georgiadis, M. P.; Haroutounian, S. A.; Couladouros, E.
A.; Apostolopoulos, C. D. J. Heterocycl. Chem. 1991, 28,
697. (b) Georgiadis, M. P.; Haroutounian, S. A.
J. Heterocycl. Chem. 1992, 29, 391.
1H NMR (COSY, NOESY, D2O): d = 1.23 (s, 3 H, 13-CH3), 1.31 (s,
3 H, 14-CH3), 1.72–1.77 (m, 1 H, Ha5), 2.06 (t, J = 13.9 Hz, 1 H,
Hb5), 3.52–3.59 (m, 2 H, H3 and H4), 3.77 (s, 3 H, 12-CH3), 3.77–
3.83 (m, 1 H, Ha2), 4.12 (d, J = 13.2 Hz, 1 H, Hb2), 6.96 (d, J = 7.9
Hz, 2 H, H10), 7.25 (d, J = 7.9 Hz, 2 H, H9).
13C NMR (HSQC, DMSO-d6): d = 21.8 (C13), 31.1 (C14), 33.9
(C5), 36.5 (C4), 50.9 (C3), 55.5 (C12), 62.5 (C2), 72.5 (C6), 114.6
(C10), 129.2 (C9), 132.1 (C8), 158.8 (C11).
(9) Tabolin, A. A.; Lesiv, A. V.; Khomutova, Yu. A.;
Nelyubina, Yu. V.; Ioffe, S. L. Tetrahedron 2009, 65, 4578.
(10) Cox, D. P.; Terpinski, J.; Lawrynowicz, W. J. Org. Chem.
1984, 49, 3216.
Anal. Calcd for C14H22ClNO2: C, 61.87; H, 8.16; N, 5.15; Cl, 13.04.
Found (for trans/cis-mixture): C, 61.10; H, 8.14; N, 5.19; Cl, 13.14.
(11) (a) Gerlach, H.; Kuenzler, P. Helv. Chim. Acta 1978, 61,
2503. (b) Marti, R. E.; Heizner, J.; Seebach, D. Liebigs Ann.
1995, 1193.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.
(12) Crystallographic data for the structure 4a have been
deposited with the Cambridge Crystallographic Data Centre
as supplementary publication number CCDC 805539.
Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK; Fax: +44(1223)336033; E-mail:
Acknowledgment
This work was supported by the Russian Foundation for Basic Re-
search (grant 11-03-00737) and the Federal Agency of Science and
Innovation (grants #64800.2010.3 and #02.740.11.0258).
deposit@ccdc.cam.ac.uk.
(13) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J. Org. Chem.
1997, 62, 7512.
(14) Danilenko, V. M.; Tishkov, A. A.; Ioffe, S. L.; Lyapkalo, I.
M.; Strelenko, Y. A.; Tartakovsky, V. A. Synthesis 2002,
635.
(15) Sukhorukov, A. Yu.; Klenov, M. S.; Ivashkin, P. E.; Lesiv,
A. V.; Khomutova, Yu. A.; Ioffe, S. L. Synthesis 2007, 97.
(16) Sukhorukov, A. Y.; Lesiv, A. V.; Eliseev, O. L.;
Khomutova, Yu. A.; Ioffe, S. L.; Borissova, A. O. Eur. J.
Org. Chem. 2008, 4025.
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