Facile conversion of CF3-containing propargylic alcohol derivatives via the corresponding allenes

Article abstract of DOI:10.1016/j.tet.2012.05.131

Empirical information on the acidity of the propargylic proton from our previous work allowed us to develop novel synthetic transformations of readily available terminally trifluoromethylated propargylic alcohols 1 into the corresponding allenyl tosylates 3a, 1-tosyloxy- or 1-acyloxy-4,4,4- trifluorobutan-2-ones 4, and 2-(2,2,2-trifluoroethyl)prop-2-en-1-ones 5, which was enabled by such common bases as NaOH and tertiary amines for affecting ready abstraction of this proton.

Full text of DOI:10.1016/j.tet.2012.05.131

Tetrahedron 68 (2012) 6665e6673
Contents lists available at SciVerse ScienceDirect
Tetrahedron
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Facile conversion of CF3-containing propargylic alcohol derivatives via the
corresponding allenes
*
Takashi Yamazaki , Yohsuke Watanabe, Nao Yoshida, Tomoko Kawasaki-Takasuka
Division of Applied Chemistry, Institute of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakamachi, Koganei 184-8588, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 April 2012
Received in revised form 29 May 2012
Accepted 30 May 2012
Available online 8 June 2012
Empirical information on the acidity of the propargylic proton from our previous work allowed us
to develop novel synthetic transformations of readily available terminally trifluoromethylated
propargylic alcohols 1 into the corresponding allenyl tosylates 3a, 1-tosyloxy- or 1-acyloxy-4,4,4-
trifluorobutan-2-ones 4, and 2-(2,2,2-trifluoroethyl)prop-2-en-1-ones 5, which was enabled by such
common bases as NaOH and tertiary amines for affecting ready abstraction of this proton.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Propargylic alcohols
Trifluoromethyl group
Allenes
a,b-Unsaturated ketones
Stereoselectivity
1. Introduction
the isomeric allenes as the important synthetic intermediates.
On the basis of our previous reports2,13 on the redox type isomer-
Organofluorine compounds have attracted considerable interest
in the field of medicines and functional materials due to their
unique effects, which are not usually attained by any other groups
nor elements.1 Our research group has recently paid attention for
clarification of synthetic utility of propargylic alcohols 1 with
a trifluoromethyl (CF3) group at the terminal position of the triple
bond.2 This type of compounds was first prepared from 3,3,3-
trifluoropropyne,3 while its gaseous nature and high cost might
prevent further expansion of this method in the organic chemistry
field. We originally devised convenient construction of 1 from the
commercially available oily materials such as 2-bromo-2a and
1-chloro-3,3,3-trifluoropropene.4 Due to more facile availability
and handling, these propargylic alcohols 1 have been employed
thus far in such a variety of procedures as [2þ2þ2] cycloadditions
with other alkynes,5 1,3-dipolar cycloadditions with azides,6 allene
formation,7,8 Pd-catalyzed cyclocarbonation for butenolide syn-
thesis,9 and elaboration to biologically interesting sugars2a,10 and
amino acids.11 All of these transformations basically employed the
carbonecarbon triple bond or the corresponding double bond after
partial reduction as the key functional groups. During our study for
further development of the novel utilization method of 1, we
wondered the lability of its propargylic proton: if this proton is
acidic enough, its abstraction would open totally different routes to
a variety of materials previously unable to be accessed by way of
ization of 1 into the corresponding a,b-unsaturated ketones, it is
quite reasonable to consider that deprotonation was actually and
easily occurred at this position, and utilization and modification of
this information allowed us to find out an intriguing conversion
of propargylic tosylates 2a into the corresponding allenyl tosylates
3a.12 Moreover, this 2a or the related propargylic esters
were smoothly transformed into the 1-substituted 1-tosyloxy- or
1-acyloxy-4,4,4-trifluorobutan-2-ones 4 via the intramolecular
attack of carbonyl oxygen in hydroxy protective groups to the
central carbon atom of the intermediary allenes 3. Alkanesulfonyl
moieties like ethanesulfonyl (R0¼CH3, R00¼H) were proved to be
a special case, which led to the formation of 2-(2,2,2-trifluoroethyl)
prop-2-en-1-ones (E)-5 by the intramolecular attack of carbanion
a
to the sulfur atom in these instances to the same position of 3.
These three processes are quite unique and intriguing because of
hitherto unknown types of reactions initiated from abstraction of
the propargylic proton.13 In this article, we would like to report full
detail of these reactions of 1 as common substrates (Scheme 1).
2. Results and discussion
2.1. Base-mediated transformation of propargylic tosylates
with a CF3 group to the corresponding allenyl tosylates
Toluenesulfonates 2a employed in this study as key substrates
were synthesized from alcohols 12a by the condensation with
* Corresponding author. E-mail address: tyamazak@cc.tuat.ac.jp (T. Yamazaki).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.05.131

6666
T. Yamazaki et al. / Tetrahedron 68 (2012) 6665e6673
Table 1
Transformation of propargylic tosylates to the corresponding allenes
Entry R1
Basea
Solvent
Time (h) Isolated yield (%)
3a
6
1
2
3
4
5
6
7
8
PhCH2CH2e (a)
DBU
DBU
NaOMe MeOH
NaOEt EtOH
AcONa MeOH
MeCN
MeOH
4.5
3
3
3
3
3
2
3
23b
0
PhCH2CH2e (a)
PhCH2CH2e (a)
PhCH2CH2e (a)
PhCH2CH2e (a)
PhCH2CH2e (a)
PhCH2CH2e (a)
PhCH2CH2e (a)
PhCH2CH2e (a)
PhCH2CH2e (a)
Phe (b)
46b 19b,c (6aa)
56b 42b,c (6aa)
1b 89b (6ba)
0
0
K2CO3
K2CO3
K2CO3
NaOH
MeOH
EtOH
15 69b,c (6aa)
Scheme 1. Various transformation of protected propargylic alcohols 2.
92 8b (6ba)
i-PrOH
6
89
99
df
90
80
99
78
97
0
0
0
0
0
0
0
0
0
9
THF/H2Oe 40
10
11
12
13
14
15
16
NaOHd THF/H2Oe
NaOHd THF/H2Oe
5
1
p-toluenesulfonyl chloride (TsCl) mediated by Et3N and a catalytic
amount of 4-(dimethylamino)pyridine. However, 2ab14 was the
exception, which was obtained from 1b (R¼Ph) and TsCl in the
presence of Ag2O and KI, and the resultant 2ab should be used
without purification due to its relative instability (Scheme 2).15 For
the base-mediated conversion to the corresponding allenyl tosy-
lates 3a, various bases and solvents were screened using 2aa
(R¼PhCH2CH2e) as the representative substrate (Table 1). As
expected, this isomerization was found to proceed by the action of
DBU in MeOH, which was more effective than CH3CN, but the for-
C9H19e (c)
NaOHd THF/H2Oe 24
NaOHd THF/H2Oe 24
NaOHd THF/H2Oe 48
BnO(CH2)2e (d)
c-Hex (e)
Ph(MOMO)CHe (f) NaOHd THF/H2Oe 48
t-Bue (g)
NaOHd THF/H2Oe 48
a
Base of 1.1 equiv was employed unless otherwise noted.
b
c
d
e
f
Yields were determined by 19F NMR using PhCF3 as an internal standard.
Compound 6aa: R2¼Me, 6ba: R2¼Et.
Base of 3 equiv was used.
A mixture of a solvent and H2O in a ratio of 2:1 was used.
A complex mixture was obtained.
mer solvent furnished a small amount of b-methoxyketone 6aa as
a byproduct (entries 2 and 3). Higher nucleophilicity of a base
seemed to prefer the formation of this byproduct, giving a larger
amount of 6aa by use of NaOMe (entry 3), and this tendency be-
came more intense under the NaOEt/EtOH system to produce
89% yield of 6ba (entry 4). Formation of 6aa would be interpreted as
the initial attack to the electronically positive sulfur atom in 3aa by
these stronger alkoxide bases, which would further participate for
In the case of 2ab, the fact that only a complex mixture was formed
would be elucidated by the presence of the ‘too activated’ hydrogen
atom at the propargylic position by an additional phenyl group
(entry 11). Other compounds were found to nicely follow this
process to produce 3a in high to excellent yields (entries 12 and 13)
in spite of requirement of longer reaction times for 2a with bulkier
R1 groups (entries 14e16).
the conjugate addition to the resultant a,b-unsaturated ketone. This
mechanism was supported by the independent reaction of this
ketone with NaOEt in EtOH at room temperature for 3 h, actually
affording 6ba in excellent 92% yield. Change of a base to weaker
K2CO3 succeeded in suppression of such removal of a tosyl group
with retaining this isomerization ability, and the yield of 3aa
increased to 92% along with 6ba forming in only 8% (entry 7). Steric
bulkiness of the solvent had a crucial role in this process, and
For obtaining experimental proof on the electronic character of
a CF3 group, the non-fluorinated counterpart 7aa was synthesized
for comparison and subjected to the similar reaction conditions
(Scheme 3). Although the propargylic tosylate 2aa reacted
smoothly to almost quantitatively afford the corresponding allenyl
tosylate 3aa as shown in Table 1, exchange of a CF3 group to a n-Bu
moiety totally suppressed this pathway and 7aa was recovered in
77% yield even after prolonged reaction time. It is apparent that
a CF3 group plays an important role in stabilization of the depro-
tonated intermediate while an electron-releasing n-Bu moiety
should work in an opposite manner. This discrepancy directly led to
estimation of the lower acidity of the propargylic proton in 7aa,
which would be the major reason for the failure of this reaction.
During the course of optimization of this isomerization, we
have also noticed that aqueous DMF affected in a different manner
methanol preferentially afforded the byproduct, b-methoxyketone
6aa, in 42% yield along with the desired 3aa in 56% yield (entry 3),
while bulkier i-PrOH almost did not promote the reaction (entry 8).
Although it required longer time by using 1.1 equiv of NaOH (entry
9), we have eventually found out NaOH (3 equiv) as a base and
a THF/H2O mixed solvent in a ratio of 2:1 as the best conditions, and
after 5 h at room temperature, the desired product 3aa was isolated
almost in a quantitative fashion16 (entry 10).
This base-mediated isomerization was carried out for a variety
of substrates 2a under the optimized conditions as described above.
Scheme 2. Preparation of CF3-containing propargylic tosylates.
Scheme 3. Comparison of reactivity.

T. Yamazaki et al. / Tetrahedron 68 (2012) 6665e6673
6667
to form 1-substituted 4,4,4-trifluoro-1-tosyloxybutan-2-ones 4a as
the major products. Thus, although the case of entry 9 in Table 1
furnished the allenyl tosylate 3aa selectively, simple exchange
the solvent from THF to DMF completely altered the reaction
course, and 4aa was obtained in 44% yield without formation of
3aa (entry 1 in Table 2). This intriguing observation allowed us to
dimethylamine and sodium formate, and participation of either
(or both) of them for the formation of 4a from 3a. Actually, hy-
drolytic decomposition of DMF was reported to proceed much
faster than the case of DMA and DMPU.17 On the basis of this fact,
the isolated allenyl tosylate 3aa was independently treated with
some selected bases whose results were summarized in Table 3.
Table 3
Table 2
Effect of a base for the conversion of 3aa into 4aa
Conversion of 2a into the corresponding 1-substituted 4,4,4-trifluoro-1-
tosyloxybutan-2-ones 4a
Entry
Base
Isolated yield (%)
Recovery (%)
4aa
10aa
3aa
1
2
3
4
5
HCO2Na
Me2NH
Me3N
BuNH2
Pyrrolidine
0
48
0
62
44
0
15
0
7
30
99
28
99
20
22
Entry
R
Solvent 1a Time (h) Solvent 2a Isolated yield (%)
3a
4a
10a
1b
2b
3b
4b
5
PhCH2CH2e (a)
PhCH2CH2e (a)
PhCH2CH2e (a)
PhCH2CH2e (a)
PhCH2CH2e (a)
PhCH2CH2e (a)
PhCH2CH2e (a)
PhCH2CH2e (a)
PhCH2CH2e (a)
n-C9H19e (c)
DMF
EtOH
i-PrOH
DMSO
MeCN
THF
THF
THF
THF
THF
1
1
1
1
4
5
5
5
5
24
24
24
24
48
48
d
0
44 dc
d
71d
83d
47d
8d
0
0
0
0
0
0
d
d
Although sodium formate, one of the DMF hydrolysis products, did
not work at all (entry 1), another component dimethylamine in fact
converted 3aa to 4aa in 48% yield (entry 2). Primary and secondary
amines like butylamine and pyrrolidine properly promoted this
reaction (entries 4 and 5, respectively), while this was not the case
for the representative tertiary amine, trimethylamine, only the
starting 3aa being quantitatively recovered (entry 3).
DMF
DMF
DMF
DMPU
DMA
DMF
DMF
DMF
DMF
DMF
DMF
57d dc
76 17
54d dc
0d dc
3d dc
58 14
61 12
26 16
6
0
0
7e
8
88d
85d
0
9
10
11
12f
13
14
BnO(CH2)2e (d)
c-Hexe (e)
Ph(MOMO)CHe (f) THF
THF
THF
Trace
13
0
86
75
41d
0
0
0
0
0
A possible mechanism for the present formal hydration is shown
in Scheme 4. After facile base-mediated isomerization of prop-
argylic tosylates 2a to allenyl tosylates 3a, attack of dimethylamine
at the electropositive sulfonyl sulfur atom would facilitate the
intramolecular cyclization by polarized sulfonate oxygen to the
allene central carbon atom of 3a to furnish Int-A. Elimination of this
amine allowed the transformation to the cyclic sulfonates Int-B,
which should be in an equilibrating relationship with Int-C and Int-
D, the former of which was expected to be predominant due to the
energetically favorable Na/F intramolecular chelation.18 Final
protonation of this intermediate Int-C would reasonably explain
the base-mediated formation of 1-substituted 4,4,4-trifluoro-1-
tosyloxybutan-2-ones 4a from propargylic tosylates 2a. Absence
of the regioisomeric 4,4,4-trifluoro-2-tosyloxybutan-1-ones would
be the good support of the energetic superiority of Int-C over Int-D.
Consideration of the electron-withdrawing property of a CF3 group
close to an ester moiety1,19 led us to speculation of the higher
acidity of methylene protons between CF3 and a carbonyl moiety in
4a, and these protons would be readily eliminated under stronger
basic conditions. Then, the following defluorination furnished the
t-Bue (g)
THF
THF
15f,g t-Bue (g)
a
The ratio of solvent 1/H2O/solvent 2 was 7:4:1 unless otherwise noted.
The second step was omitted.
b
c
d
e
f
Not determined.
Determined by 19F NMR by using PhCF3 as the internal standard.
LiOH was used instead of NaOH.
The solvent system of THF/H2O/DMF¼2:1:1 was employed.
g
The second step was heated to 90 ꢀC.
further investigate a variety of factors related to this formal
hydration of 2a. Other water-soluble solvents like EtOH, i-PrOH,
and DMSO resulted in the unequivocal construction of 3aa, and
4aa was not produced at all (entries 2e4). Importance of DMF was
recognized at this point, and addition of DMF was found to be
more effective after completion of isomerization to the allene,
which is apparent from the comparison between the result of
entry 1 and the ones of entries 5 and 6. It is interesting to note that
N,N0-dimethyl-propyleneurea (DMPU) and N,N-dimethylacetamide
(DMA) with a similar structure to DMF did not exhibit such
product preference, and 2aa was selectively converted into the
allenyl tosylate 3aa with the negligible yield of 4aa (entries 8 and
9). At this stage, we have determined the conditions in entry 6 as
the best, and scope and limitation of substituents R in 2a was
checked. In the case of entries 10, 11, and 13, ketones 4a were
isolated in good yields along with a small amount of the totally
difluorinated 10a. Substrate 2ae with a cyclohexyl moiety was
found to be reluctant to the present conversion, requiring an in-
creased amount of DMF for promotion of the reaction (under the
standard conditions, reaction did not proceed at all) to afford 4ae
only in 26% yield. The sterically most demanding 2ag among
substrates tested furnished 3ag in excellent yields in spite of such
forcing conditions as a larger amount of DMF and heating to 90 ꢀC
(entry 15).
reactive Int-E with the highly electrophilic
a,b
b-carbon not only by the
-unsaturated carbonyl framework but by this sp2-hybridized
carbon with two fluorine atoms giving rise to p-
p repulsive in-
teraction.1 Successive sequence of addition of hydroxide and the
following elimination of fluoride ions20 affected total defluorina-
tion to furnish sodium salt of
termediate eventually followed facile decarboxylation to produce
the byproduct 10a.
b-ketoacid Int-F, and this in-
2.2. Base-mediated conversion of CF3-containing propargylic
carboxylates
Having succeeded in the base-mediated transformation of
We have paid our attention to the possibility of DMF hydrolysis
under the present aqueous basic conditions to furnish
propargylic tosylates 2a to the corresponding allenyl tosylates 3a
and
a-tosyloxy ketones 4a, we next turned our attention to the

6668
T. Yamazaki et al. / Tetrahedron 68 (2012) 6665e6673
Scheme 4. Possible reaction mechanism.
similar type of process employing hydrolytically more labile
propargylic esters. Investigation of the optimal reaction conditions
for the model substrate 2ia was undertaken with an appropriate
amine base in an aprotic THF solvent because our preliminary ex-
periment has already clarified that the simple acetate functionality
was readily cleaved under isomerization conditions shown in Table
1 (3 equiv of NaOH in THF/H2O¼2:1, rt, 3 h furnished the hydro-
lyzed alcohol 1a in 50% yield along with 48% recovery of the ace-
tate). Tertiary amines and K2CO3 did not afford 4ia at all with high
to quantitative recovery of the starting material 2ia (Table 4, entries
1e5). On the other hand, DBU and DBN with higher basicity spe-
cifically promoted the reaction nicely to afford the desired ketone
4ia in excellent and moderate yields (entries 6 and 7, respectively).
Solvent effect was briefly studied with DBU, and although Et2O
worked well, CH2Cl2 and EtOH were found to be irrelevant (entries
8e10). Consideration of the mechanism on the basis of the corre-
sponding tosylates 2a as depicted in Scheme 4 anticipated us that
a catalytic amount of DBU should suffice, while reduction of this
base to 20 mol % led to recovery of 2ia in excellent yield without
formation of the desired 4ia (entry 11). Increase of the reaction
temperature from 0 ꢀC to rt resulted in decrease of the yield of 4ia
to some extent.
Table 4
Investigation of the reaction conditions for the conversion of 2ia into 4ia
At the next stage, the scope of propargylic esters was searched
basically under the condition of entry 6 in Table 4. As shown in
Table 5, not only esters 2ia, 2ja, and 2ka (entries 1e3) but also the
carbonate 2ma and phosphate 2na (entries 5 and 7) participated
nicely for the present transformation to afford the corresponding
ketones 4 in moderate to excellent yields, while the carbamate 2la,
tosylate 2aa, and ether 2oa (entries 4, 6, and 8) didn’t work at all
and, instead, decomposition of the substrates was observed. For the
examination of the effect of substituents R at the propargylic po-
sition, 2jb and 2jh unfortunately failed to react properly, which
would be as a consequence of the existence of an activated prop-
argylic proton by the participation of an additional carbonecarbon
double bond in R (entries 9 and 14). Steric crowding around the
propargylic position of 2jg might avoid the approach of a base to
this site, allowing recovery of this substrate in 87% yield. This is in
sharp contrast to the case of base-mediated isomerization with
smaller NaOH in size where successful abstraction of the proton
was realized (entry 14 in Table 2). On the other hand, the formal
hydration of benzoates with saturated primary and secondary alkyl
substituents as R was performed in moderate to high yields and the
former appeared to proceed even at lower temperature (entries
10e12, and 15). Entries 16 and 17 demonstrated the examples of
non-fluorinated substrates 7ja and 8ja with CO2Et and n-Bu moi-
eties instead of a CF3 group, respectively. These substrates did not
Entry
Base
Solvent
Yielda (%)
Recoverya (%)
1
2
3
4
5
6
7
8
Et3N
THF
THF
THF
THF
THF
THF
THF
CH2Cl2
EtOH
Et2O
THF
0
0
0
0
0
87
44
0
0
60
0
>99
84
71
89
>99
<1
<1
9
60
0
83
0
Pyridine
DMAP
DABCO
K2CO3
DBU
DBN
DBU
DBU
DBU
9
10
11b
12c
DBU
DBU
THF
52
a
Yields were determined by 19F NMR.
The amount of DBU was reduced to 20 mol %.
Reaction was carried out at room temperature.
b
c

T. Yamazaki et al. / Tetrahedron 68 (2012) 6665e6673
6669
Table 5
the former 7ja seemed to experience hydrolysis at the terminal
ethyl ester function. Thus, a characteristic property of a CF3 group as
a non-hydrolyzable strong electron-withdrawing moiety proved to
effectively promote this process.
DBU-mediated transformation of 2 into 4
2.3. Novel transformation of CF3-containing propargylic
alkanesulfonates to
manner
a,b-unsaturated ketones in an (E)-specific
Entry
1
R
P
Substrate Temp Product Yielda
(ꢀC)
(%)
During examination of the formal hydration of various
CF3-containing propargylic sulfonates for clarification of the scope
and limitation of this reaction, we unexpectedly discovered that
PhCH2CH2e
2-Furyle
C(O)e
PhC(O)e
4-O2Ne
C6H4e
2ia
0
4ia
87
2
3
PhCH2CH2e
PhCH2CH2e
2ja
2ka
rt
rt
4ja
4ka
85
51
the corresponding mesylate 2ca afforded a,b-unsaturated ketones
(E)-5ba in a stereospecific fashion (entry 9 in Table 6). Consider-
ation of its utility allowed us to further search this interesting
conversion using ethanesulfonate 2ba as the model substrate
(Table 6). THF and 1,4-dioxane were found to be acceptable sol-
vents in the presence of NaOH at room temperature for 3 h (en-
tries 1e4), and NaOH was the hydroxide bases of choice (entries 1
and 5e7). Further attempt for increase of the yield of the product
(E)-5ba21 to the acceptable level was noticed by simply raising the
reaction temperature to 50 ꢀC (entry 8). At the next stage, we have
examined the effect of the sulfonate substituent (R2R3CHe). In the
case of the methanesulfonate 2ca (entry 9), although the reaction
smoothly proceeded even at rt, sensitivity of the product 5ca to-
ward silica gel drastically reduced the isolated yield to 22% in spite
of 63% before purification (determined by 19F NMR). The bulkier
sulfonate 2da behaved in a different manner and the allenyl sul-
fonate 3ca was obtained in 62% yield possibly as the result of
steric congestion around the reacting carbanion, rendering its
access to the allene central carbon atom difficult (entry 10). In the
case of sulfonates 2ea and 2fa, only complex mixtures were
C(O)e
NH2C(O)e
MeOC(O)e 2ma
Tse 2aa
(EtO)2P(O)e 2na
4
5
6
7
8
9
10
11
12
13
14
15
16c
17d
PhCH2CH2e
PhCH2CH2e
PhCH2CH2e
PhCH2CH2e
PhCH2CH2e
Phe
n-C9H19e
c-C6H11e
PhCH(MOMO)e
t-Bue
2la
0
4la
0
68
0
53
rt
rt
0
rt
0
4ma
4aa
4na
4oa
4jb
4jd
4je
4jf
4jg
4jh
4jc
11ja
12ja
MOMe
2oa
2jb
2jc
2je
2jf
2jg
2jh
2ji
0
PhC(O)e
PhC(O)e
PhC(O)e
PhC(O)e
PhC(O)e
Trace
47
0
rt
rt
rt
rt
0
77
85b
Trace (87)
0
65
0 (9)
0 (88)
(E)-C3H7CH]CHe PhC(O)e
n-C6H13e
PhCH2CH2e
PhCH2CH2e
PhC(O)e
PhC(O)e
PhC(O)e
7ja
8ja
rt
rt
a
Isolated yield and in the parenthesis was shown the recovery of the starting
materials.
b
c
Diastereomer mixture of 83:17 was obtained.
A CF3 group was substituted for a CO2Et moiety.
A CF3 group was substituted for a n-Bu moiety.
d
undergo the present hydration at all: this result would be inter-
preted as the lower acidity of the propargylic proton by loosing an
effective activation unit like a CF3 group in the latter 8ja case, and
formed presumably due to their higher reactivity (Cl
a to the
electron-attracting SO3 group and the acidic hydrogen on the
sulfur bound carbon atom; entries 11 and 12, respectively). Finally,
Table 6
Stereoselective formation of (E)-a,b-unsaturated ketones (E)-5
Entry
R1
R2
R3
Suba
Temp (ꢀC)
Time (h)
Solvent
Base
Prob
Isolated yieldc (%)
1
2
3
4
5
6
7
8
PhCH2CH2e
PhCH2CH2e
PhCH2CH2e
PhCH2CH2e
PhCH2CH2e
PhCH2CH2e
PhCH2CH2e
PhCH2CH2e
PhCH2CH2e
PhCH2CH2e
PhCH2CH2e
PhCH2CH2e
n-C9H19e
CH3e
CH3e
CH3e
CH3e
CH3e
CH3e
CH3e
CH3e
He
CH3e
CF3e
Cle
CH3e
CH3e
CH3e
CH3e
CH3e
He
He
He
He
He
He
He
He
He
CH3e
He
He
He
He
He
He
He
2ba
2ba
2ba
2ba
2ba
2ba
2ba
2ba
2ca
2da
2ea
2fa
rt
rt
rt
rt
rt
rt
rt
50
rt
50
50
rt
rt
rt
rt
rt
rt
3
3
3
3
3
3
3
3
1
3
3
3
24
3
24
3
3
THF
NaOH
NaOH
NaOH
NaOH
LiOH
5ba
5ba
5ba
5ba
5ba
5ba
5ba
5ba
5ca
5da
5ea
5fa
(45)
(25)
(44)
(4)
(37)
(30)
(31)
75
22 (63)
(6) [62]d
Trace
Trace
56
CH3CN
1,4-Dioxane
DMSO
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
KOH
Me4NOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
9
10
11
12
13
14
15
16
17
2bc
2bd
2be
2bf
5bc
5bd
5be
5bf
d
BnOCH2CH2e
c-C6H11e
PhCH(OMOM)e
PhCH2CH2e
40
63
60
7bae
0f
a
Substrates.
Products.
b
c
d
e
f
Yields in parentheses were determined by 19F NMR by using PhCF3 as an internal standard.
Isolated yield of 6,6,6-trifluoro-1-phenylhexa-3,4-dien-3-yl 1-methylethanesulfonate 3da.
A CF3 group in 2ba was substituted for a n-Bu moiety.
Compound 7ba was recovered in 97% yield.

6670
T. Yamazaki et al. / Tetrahedron 68 (2012) 6665e6673
some other ethanesulfonates 2b with a variety of primary as well
as secondary R1 moieties were employed for clarification of the
applicability of this intriguing novel transformation, and they
worked in a similar fashion to furnish the desired products 5b in
good yields with unequivocal E stereospecificity (entries 13e16).
To gain mechanistic insight on the present process, we have
performed two independent reactions. One was to employ the non-
fluorinated substrate 7ba (entry 17 in Table 6), and like the other
instances, this substrate 7ba was totally intact under the above
standard reaction conditions and was recovered almost in quanti-
tative yield. The other was to subject the separately prepared
allenyl ethanesulfonate 3ba to the same alkaline conditions
(Scheme 5). Treatment of ethanesulfonate 2ba with a weaker base
like K2CO3 in EtOH at rt affected only the isomerization reaction
and 16ha, and time-dependent transformation of 14ha into the
other two compounds was noticed.
A possible mechanism for the above conversion of propargyl
sulfonates 2 would be explained as shown in Scheme 7. With re-
ferring to the results in Scheme 5, 2 would experience the isom-
erization to the corresponding allenyl sulfonates 3 initiated by
deprotonation of the acidic propargylic proton. Further action of
NaOH to abstract the
a proton to the sulfur atom furnished the
anionic species, which would construct a new carbonecarbon bond
comparison22 of the model compounds 14bg and 17bg with
R1¼R2¼CH3 clarified the ca. 9 kcal/mol energetic preference of the
former whose results led to semi-quantitative estimation that 14
would be the major intermediate at the cyclization.
The ready isomerization to the energetically more stable
a,b-unsaturated carbonyl system by NaOH-mediated depro-
tonationeprotonation sequence in an aqueous media
was reasonably expected by the presence of the electron-
withdrawing R2 group in 14. Moreover, such substituents R2 also
played an important role in increase of the acidity of the proton
next to the CF3 group in 15, rendering the following dehydro-
fluorination and hydrolysis of the resultant terminally difluorinated
vinyl moiety easier (see, similar decomposition of a CF3 group in 4a
to 10a in Scheme 4). This transformation should eventually produce
13 by losing all the fluorine atoms, and the stronger electron-
withdrawing ester group allowed 13ha to affect further de-
carboxylation to 16ha. The possible route to the final product (E)-5
would be by way of attack of hydroxide on the sulfur atom of 14 to
open the sultone ring, and the produced ketone was successively
converted to the enolate Int-I whose stereochemistry would be
controlled by the favorably stabilizing intramolecular chelation
(one possible structure was shown in Scheme 7). At the last stage,
the sulfur-containing leaving group should occupy the position
orthogonal to the CeC double bond, and because the steric re-
pulsion between enolate oxygen and R2 seemed to be the con-
trolling factor, the less crowded Int-I1 would be preferable to afford
(E)-5 with high selectivity.
Scheme 5. Independent transformation of 2ba into (E)-5ba via 3ba.
suppressing the successive intramolecular attack to afford 3ba in
45% yield which, after chromatographic purification, was in-
troduced to a mixed solvent (THF/H2O¼2:1) containing 3 equiv of
NaOH at rt for 3 h to in fact promote conversion into the desired
product (E)-5ba in 48% isolated yield.
Sulfonates 2ga and 2ha with vinyl and ethoxycarbonyl groups as
R2, respectively, were found to behave in a different fashion and
they offered a series of sultones 13 to 16 (Scheme 6). For example,
the totally defluorinated carboxylic acid 13ga was obtained in
30% yield as the major isolated product possibly by way of 15ga. In
the case of 2ha, the reaction also proceeded in a quick manner
within 0.1 h to furnish the isomeric mixture of sultones 14ha, 15ha,
As shown in Scheme 5, this process was considered to be initi-
ated from the formation of the allenyl sulfonates 3, while we also
obtained an interesting proof for this reaction proceeding via a di-
rect attack of the carbanion to the triple bond carbon
b to the CF3
group. Thus, formation of the sulfonate 2hj without propargylic
hydrogen did not yield the desired material but the sultone 15hj in
32% yield along with 55% recovery of the starting material 1j
(Scheme 8). This fact would be elucidated in a similar manner to the
case of intramolecular lactone formation with the allene central
carbon atom in an intramolecular manner to give the in-
termediates, 14 and/or 17. Computational of ethyl propargyl malo-
nates reported by Arcadi et al., which occurred in the presence of
NaH in DMSO at 80 ꢀC.23 The ready room temperature reaction and
applicability of a weaker base in our instance should be the re-
flection of the electrophilic activation of the carbonecarbon triple
bond by the CF3 group, which effectively lowered the LUMO energy
level of the possible initial product 2hj. At present, this ‘direct’
mechanism cannot be ruled out as the possible pathway.
3. Conclusion
In summary, we have demonstrated the successful NaOH-
mediated transformation of propargylic tosylates with a CF3
group 2 into the corresponding allenyl tosylates 3. The same sub-
strates 2 with other hydroxy protective groups realized formation
of 4,4,4-trifluorobutan-2-ones 4 with a protected hydroxy moiety
at the 1-position, or
a,b-enones (E)-5 with a 2,2,2-trifluoroethyl
moiety at the -position, both by way of the allenyl derivatives 3.
a
Characteristics of these novel reactions is the facile deprotonation
of the activated propargylic proton by such commercially available
Scheme 6. Sultone formation from 2ga and 2ha.

T. Yamazaki et al. / Tetrahedron 68 (2012) 6665e6673
6671
Scheme 7. Possible reaction mechanism.
Scheme 8. Direct formation of sultone 15hj.
and low cost common bases as NaOH and DBU, and further appli-
cation of these protocols is underway in this laboratory.
downfield from internal tetramethylsilane (Me4Si:
d
0.00, for 1H
and 13C) or hexafluorobenzene (C6F6:
d
ꢁ163.00 for 19F). Data were
tabulated in the following order: multiplicity (s, singlet; d, dou-
blet; t, triplet; q, quartet; quint, quintet; sex, sextet; m, multiplet;
br, broad peak), number of protons, coupling constants in hertz.
Infrared (IR) spectra were obtained on a JASCO A-302 spectrom-
eter and reported in wave numbers (cmꢁ1). Elemental analyses
were performed by PerkineElmer SeriesII CHNS/O analyzer.
Electrospray ionization mass spectrometry by Thermofisher
Exactive was obtained using the negative and positive ionization
modes.
4. Experimental section
4.1. General
All reactions were carried out under an atmosphere of argon in
dried glassware with magnetic stirring. Analytical thin-layer
chromatography (TLC) was routinely used for monitoring re-
actions by generally using a mixture of hexane and AcOEt (v/v).
Spherical neutral silica gel (63e210 mm) was employed for column
chromatography. Anhydrous Et2O, THF, and CH2Cl2 were pur-
chased and were used without further purification. 1H (300 MHz),
13C (75 Hz), and 19F (283 Hz) NMR spectra were recorded on
a JEOL AL 300 spectrometer in CDCl3 unless otherwise noted and
chemical shifts were recorded in parts per million (ppm),
4.2. Typical procedure for the preparation of propargyl
sulfonates
To a solution of 1a (2.28 g, 10.0 mmol), p-toluenesulfonyl chlo-
ride (2.09 g, 11.0 mmol), and 4-(N,N-dimethyl)pyridine (0.024 g,

6672
T. Yamazaki et al. / Tetrahedron 68 (2012) 6665e6673
0.20 mmol) in CH2Cl2 (20 mL) was added at 0 ꢀC triethylamine
(1.67 mL, 12.0 mmol) and the resultant mixture was stirred at room
temperature for 2 h. The reaction mixture was quenched with
saturated NH4Cl aq (30 mL) and extracted with CH2Cl2 (3ꢂ70 mL).
Concentration by rotary evaporator after dried over Na2SO4 fur-
nished a crude material, which was purified by silica gel column
chromatography (hexane/EtOAc¼15:1) to give 3.59 g (9.39 mmol)
of pure propargyl tosylate 2aa in 94% yield.
In the case of preparation of 2ab, after 1b (1.00 g, 5.0 mmol),
p-toluenesulfonyl chloride (1.14 g, 6.0 mmol), Ag2O (1.39 g,
6.0 mmol), and KI (1.00 g, 6.0 mmol) in CH2Cl2 (15 mL) was refluxed
for 4 h, the resultant mixture was filtered off and concentration
furnished crude 2ab, which was used without further purification.
1271,1376,1418,1437,1454,1496,1598,1737, 2928, 3030 cmꢁ1;HRMS
(FAB): m/z calcd for C19H18F3O3S [MþH]þ 383.0929; found 383.0950.
4.5. Typical procedure for the preparation of 4-
tosyloxyketones
To a solution of 2aa (0.33 g, 0.40 mmol) in THF (2.1 mL) was
added an aqueous 1 M NaOH aq solution (1.2 mL) at rt. The mixture
was stirred at that temperature for 4 h before addition of DMF
(0.3 mL). After the resulting solution was stirred for additional 1 h,
the reaction mixture was quenched with 1 M HCl aq (3 mL) and
extracted with hexane/EtOAc (1:3, 3ꢂ15 mL). Concentration by
rotary evaporator after dried over Na2SO4 furnished a crude mix-
ture, which was purified by silica gel chromatography (hexane/
EtOAc¼7:1) to afford 4aa in 73% yield (0.121 g, 0.30 mmol).
4.2.1. Compound 2aa.24 Yield: 94%, Rf¼0.46 (hexane/EtOAc, 4:1). 1H
NMR (CDCl3, 300 MHz):
2.71e2.84 (m, 2H), 2.71e2.84 (m, 2H), 7.13e7.36 (m, 7H), 7.80 (d,
J¼8.4 Hz, 2H).
d
¼2.08e2.28 (m, 2H), 2.45 (s, 3H),
4.5.1. Compound 4aa. Yield: 75%, colorless oil, Rf¼0.43 (hexane/
EtOAc, 4:1). 1H NMR (CDCl3, 300 MHz):
d
¼1.91e2.10 (m, 2H),
2.39e2.49 (m, 1H), 2.47 (s, 3H), 2.58 (ddd, J¼14.7, 9.3, 6.0 Hz, 1H),
3.40 (dq, J¼18.0, 9.9 Hz, 1H), 3.50 (dq, J¼18.0, 9.9 Hz, 1H), 4.65 (dd,
J¼7.8, 5.1 Hz, 1H), 6.96e6.98 (m, 2H), 7.17e7.25 (m, 3H), 7.38
(d, J¼8.1 Hz, 2H), 7.79 (d, J¼8.4 Hz, 2H); 13C NMR (CDCl3, 75 MHz):
4.3. Typical procedure for the preparation of propargyl esters
To a solution of 1a (2.28 g, 10.0 mmol), 2-furyloyl chloride
(1.08 g, 11.0 mmol), and 4-(N,N-dimethylamino)pyridine (0.024 g,
0.20 mmol) in CH2Cl2 (20 mL) was added at 0 ꢀC triethylamine
(1.67 mL, 12.0 mmol) and the resultant mixture was stirred at room
temperature for 2 h. The reaction mixture was quenched with
saturated NH4Cl aq (30 mL) and extracted with CH2Cl2 (3ꢂ70 mL).
Concentration by rotary evaporator after dried over Na2SO4 fur-
nished a crude material, which was purified by silica gel column
chromatography (hexane/EtOAc¼12:1) to give 3.19 g (9.90 mmol)
of pure propargyl tosylate 2ia in 99% yield.
d
¼21.7, 30.4, 32.7, 41.7 (q, J¼29.1 Hz), 82.8 (q, J¼1.8 Hz), 123.4
(q, J¼276.7 Hz), 126.5, 128.1, 128.3, 128.6, 130.2, 132.1, 139.2, 146.0,
197.2 (q, J¼2.5 Hz); 19F NMR (CDCl3, 283 MHz):
d¼ꢁ63.66
(t, J¼6.9 Hz); IR (neat): 667, 700, 750, 816, 834, 930, 982, 1017, 1051,
1109, 1177, 1192, 1275, 1374, 1410, 1455, 1496, 1598, 1742, 2931,
3030 cmꢁ1; HRMS (FAB): m/z calcd for C19H20F3O4S [MþH]þ
401.1034; found 401.1028.
4.6. Typical procedure for the preparation of
acyloxyketones
a-
4.3.1. Compound 2ia. Yield: 99%, colorless oil, Rf¼0.66 (hexane/
EtOAc, 4:1). 1H NMR (CDCl3, 300 MHz):
d
¼2.20e2.39 (m, 2H), 2.84
To a solution of 2ia (0.193 g, 0.60 mmol) in THF (5 mL) were
added H2O (11.9 L, 0.60 mmol) and DBU (98.0 L, 0.66 mmol) at
(t, J¼7.5, 2H), 5.59e5.67 (m, 1H), 6.52 (dd, J¼3.6, 1.8 Hz, 1H),
m
m
7.20e7.30 (m, 6H), 7.63 (dd, J¼1.8, 0.9 Hz, 1H); 13C NMR (CDCl3,
0 ꢀC. After the resulting solution was stirred for 3 h at that tem-
perature, the reaction mixture was quenched with a 1 M HCl aq
solution (3 mL) and extracted with EtOAc (3ꢂ15 mL). Concentration
by rotary evaporator after dried over Na2SO4 furnished a crude
mixture, which was purified by silica gel chromatography (hexane/
EtOAc, 12:1) to afford 4ia (0.178 g, 0.522 mmol, 87%).
75 MHz):
d
¼30.9, 35.2, 62.2 (q, J¼1.3 Hz), 72.8 (q, J¼53.1 Hz), 83.9
(q, J¼6.6 Hz), 112.0, 113.8 (q, J¼257.4 Hz), 119.2, 126.4, 128.6, 139.6,
143.4, 147.1, 156.9; 19F NMR (CDCl3, 283 MHz):
¼ꢁ51.97 (s); IR
(neat): 700, 760, 885, 978, 1075, 1232, 1248, 1271, 1348, 1396, 1514,
d
1733, 2272, 3029, 3065 cmꢁ1
; HRMS (FAB): m/z calcd for
C17H14F3O3 [MþH]þ 323.0895; found 323.0867. Anal. Calcd for
C17H13O3F3: C, 63.36; H, 4.07; Found: C, 63.48; H, 4.46.
4.6.1. Compound 4ia. Yield: 87%, pale yellow oil, Rf¼0.46 (hexane/
EtOAc, 4:1). 1H NMR (CDCl3, 300 MHz):
2H), 2.71e2.85 (m, 2H), 3.28 (dq, J¼17.1, 9.9 Hz,1H), 3.40 (dq, J¼17.1,
10.2 Hz, 1H), 5.14 (t, J¼6.6 Hz, 1H), 6.58 (dd, J¼3.6, 1.8 Hz, 1H),
7.16e7.32 (m, 6H), 7.66e7.67 (m, 1H); 13C NMR (CDCl3, 75 MHz):
d
¼2.23 (dt, J¼7.8, 7.5 Hz,
4.4. Typical procedure for the preparation of allenyl tosylates
(3a)
d
¼30.9, 31.4, 41.9 (q, J¼28.7 Hz), 77.6 (q, J¼1.8 Hz),112.1,119.5,123.4
To a solution of 2aa (0.153 g, 0.40 mmol) in THF (2.4 mL) was
added an aqueous 1 M NaOH solution (1.2 mL) at rt and the mixture
was stirred at that temperature for 5 h. The reaction mixture was
quenched with 1 M HCl aq (3 mL) and extracted with EtOAc
(3ꢂ15 mL). Concentration by rotary evaporator after dried over
Na2SO4 furnished 3aa in an almost pure conditions. If necessary,
the residue was purified by short path column chromatography
(hexane/EtOAc¼8:1) to give pure allenyl tosylate 3aa in quantita-
tive yield (0.153 g, 0.40 mmol, quantitative yield).
(q, J¼276.3 Hz), 126.3, 128.3, 128.5, 139.7, 143.1, 147.3, 157.6, 197.1
(q, J¼1.9 Hz); 19F NMR (CDCl3, 283 MHz):
¼ꢁ63.52 (t, J¼10.3 Hz);
d
IR (neat): 644, 700, 1014, 1115, 1179, 1292, 1397, 1472, 1737, 2359,
2863, 2936, 3028, 3064, 3143 cmꢁ1; HRMS (FAB): m/z calcd for
C17H16F3O4 [MþH]þ 341.1001; found 341.1018.
4.7. Typical procedure for transformation of propargylic
ethanesulfonates to the corresponding
ketones 5
a,b-unsaturated
4.4.1. Compound 3aa. Yield: quant., colorless oil, Rf¼0.70 (hexane/
EtOAc, 4:1). 1H NMR (CDCl3, 300 MHz):
d
¼2.45 (s, 3H), 2.56e2.63
To a solution of 2ba (0.128 g, 0.40 mmol) in THF (2.4 mL) was
added a 1 M NaOH aq solution (1.2 mL) at rt. After the resulting
solution was stirred for 3 h, the reaction mixture was quenched
with a 1 M HCl aq solution (3 mL) and extracted with EtOAc
(3ꢂ15 mL). Concentration by rotary evaporator after dried over
Na2SO4 furnished a crude mixture that was purified by silica gel
chromatography (hexane/EtOAc,12:1) to afford 5ba (0.0768 g, 75%).
(m, 2H), 2.70e2.76 (m, 2H), 5.66 (qt, J¼5.7, 3.0 Hz, 1H), 7.09e7.35
(m, 7H), 7.75e7.79 (m, 2H); 13C NMR (CDCl3, 75 MHz):
d¼21.3, 31.6,
33.2, 96.2 (q, J¼39.1 Hz), 120.5 (q, J¼271.6 Hz), 126.2, 128.1, 128.2,
128.3, 129.8, 130.3, 132.1, 139.3, 145.7, 199.3 (q, J¼5.6 Hz); 19F NMR
(CDCl3, 283 MHz):
745, 819, 846, 865, 940, 1033, 1071, 1087, 1128, 1145, 1181, 1195, 1214,
d
¼ꢁ63.38 (t, J¼6.9 Hz); IR (neat): 650, 680, 700,

T. Yamazaki et al. / Tetrahedron 68 (2012) 6665e6673
6673
13. We have already reported facile transformation of propargylic alcohols to the
corresponding -unsaturated ketones by Et3N in THF, where the abstraction
4.7.1. Compound 5ba. Yield: 75%, colorless oil, Rf¼0.49 (hexane/
EtOAc, 4:1). 1H NMR (CDCl3, 300 MHz):
¼1.94 (d, J¼7.2 Hz, 3H),
2.91e3.04 (m, 4H), 3.26 (q, J¼10.8 Hz, 2H), 7.03 (q, J¼7.2 Hz, 1H),
7.17e7.32 (m, 5H); 13C NMR (CDCl3, 75 MHz):
a,b
d
of the propargylic proton was considered to be the initiation of this reaction.
See reference 2d.
14. Alphabets in compound numbers describe the type of protective group by the
first, and the second for the substituent at the propargylic position.
d¼15.4 (q, J¼1.2 Hz),
29.0 (q, J¼30.4 Hz), 30.4, 38.8, 125.9 (q, J¼277.2 Hz), 126.1, 128.3,
ꢀ
15. Bouzide, A.; LeBerre, N.; Sauve, G. Tetrahedron Lett. 2001, 42, 8781e8783.
128.4, 132.6 (q, J¼2.6 Hz), 141.1, 143.6, 198.3; 19F NMR (CDCl3,
16. In the case of non-fluorinated compounds, harder conditions were usually
applied. For example: (a) Anderson, K. R.; Atkinson, S. L. G.; Fujiwara, T.; Giles,
M. E.; Matsumoto, T.; Merifield, E.; Singleton, J. T.; Saito, T.; Sotoguchi, T.; Tor-
nos, J. A.; Way, E. L. Org. Process Res. Dev. 2010, 14, 58e71 (t-BuOK in THF, 55 ꢀC,
1 h); (b) Lechel, T.; Gerhard, M.; Trawny, D.; Brusilowskij, B.; Schefzig, L.;
Zimmer, R.; Rabe, J. P.; Lentz, D.; Schalley, C. A.; Reissig, H.-U. Chem.dEur. J.
2011, 17, 7480e7491 (t-BuOK in toluene, 70 ꢀC, 1 h); (c) Deagostino, A.; Prandi,
C.; Toppino, A.; Venturello, P. Tetrahedron 2008, 64, 10344e10349 (n-BuLi in
THF, ꢁ78 ꢀC, 2 h). See also the following example occurring by weak bases: (d)
Sonye, J. P.; Koide, K. J. Org. Chem. 2006, 71, 6254e6257 (DABCO); (e) Sonye, J. P.;
Koide, K. J. Org. Chem. 2007, 72, 1846e1848 (NaHCO3).
283 MHz):
d
¼ꢁ65.83 (t, J¼9.0 Hz); IR (neat): 700, 751, 850, 1075,
1092,1111,1137,1254,1298,1337,1358,1454,1648,1676 cmꢁ1;HRMS
(FAB): m/z calcd for C14H15F3O [M]þ 256.1075; found 256.1059.
Supplementary data
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/0.1016/j.tet.2012.05.131.
€
17. (a) Buncel, E.; Symons, E. A. J. Chem. Soc. D 1970, 164e165; (b) Kankaanpera, A.;
Scharlin, P.; Kuusisto, I.; Kallio, R.; Bernoulli, E. J. Chem. Soc., Perkin Trans. 2 1999,
169e174.
References and notes
18. Yamazaki, T.; Kawashita, S.; Kitazume, T.; Kubota, T. Chem.dEur. J. 2009, 15,
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20. King, J. F.; Gill, M. S. J. Org. Chem. 1996, 61, 7250e7255.
21. (E)-stereochemistry was determined by NOESY spectra of this compound on
the basis of the observation of the clear trifluoroethyl-based cross peak with
the terminal methyl protons, not with the vinylic proton
1. (a) Hiyama, T. Organofluorine Compounds, Chemistry and Applications; Springer:
Berlin, 2000; (b) Uneyama, K. Organofluorine Chemistry; Blackwell: Oxford,
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2. (a) Yamazaki, T.; Mizutani, K.; Kitazume, T. J. Org. Chem. 1995, 30, 3043e3053;
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35, 5945e5948; (e) Obinata, R.; Kawasaki-Takasuka, T.; Yamazaki, T. Org. Lett.
2010, 12, 4316e4319.
ꢀ
ꢀ
22. Calculations were carried out with Gaussian 03W at the B3LYP/6-31þG
*
level of
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Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.;
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mani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota,
K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi,
R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador,
P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
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