Nucleophilic fluoroalkylation of α,β-unsaturated carbonyl compounds with α-fluorinated sulfones: Investigation of the reversibility of 1,2-additions and the formation of 1,4-adducts

Article abstract of DOI:10.1002/hlca.201200445

A detailed investigation of the reactions of PhSO₂CF ₂H and PhSO₂CH₂F with (E)-chalcone (=(E)-1,3-diphenylprop-2-en-1-one) at low temperatures revealed that these two reactions were kinetically controlled, and the ratios of 1,2- vs. 1,4-adducts, which did not change much over time at these temperatures, reflect the relative rates of the two reaction pathways. The controlled experiments of converting the PhSO₂CF₂- and PhSO₂CHF-substituted 1,2-adducts to 1,4-adducts showed that these isomerizations are not favored due to the low stability and hard-soft nature of PhSO₂CF₂^- and PhSO₂CHF^- anions. Moreover, taking advantage of the remarkable stability and softness of (PhSO₂)₂CF ^- anion, an efficient thermodynamically controlled isomerization of (PhSO₂)₂CF-substituted 1,2-adduct to 1,4-adduct was achieved for the first time. Copyright

Full text of DOI:10.1002/hlca.201200445

Helvetica Chimica Acta – Vol. 95 (2012)
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Nucleophilic Fluoroalkylation of a,b-Unsaturated Carbonyl Compounds with
a-Fluorinated Sulfones: Investigation of the Reversibility of 1,2-Additions and
the Formation of 1,4-Adducts
by Xiao Shen, Chuanfa Ni, and Jinbo Hu*
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, 345 Ling-Ling Road, Shanghai 200032, P. R. China
(phone: þ 862154925174; e-mail: jinbohu@sioc.ac.cn)
Dedicated to Professor Dieter Seebach on the occasion of his 75th birthday
A detailed investigation of the reactions of PhSO2CF2H and PhSO2CH2F with (E)-chalcone (¼(E)-
1,3-diphenylprop-2-en-1-one) at low temperatures revealed that these two reactions were kinetically
controlled, and the ratios of 1,2- vs. 1,4-adducts, which did not change much over time at these
temperatures, reflect the relative rates of the two reaction pathways. The controlled experiments of
converting the PhSO2CF2- and PhSO2CHF-substituted 1,2-adducts to 1,4-adducts showed that these
isomerizations are not favored due to the low stability and hard-soft nature of PhSO2CFꢀ2 and
PhSO2CHFꢀ anions. Moreover, taking advantage of the remarkable stability and softness of
(PhSO2)2CFꢀ anion, an efficient thermodynamically controlled isomerization of (PhSO2)2CF-substituted
1,2-adduct to 1,4-adduct was achieved for the first time.
Introduction. – Nucleophilic fluoroalkylation, typically involving the transfer of an
a-fluoro carbanion to an electrophile, represents one of the major synthetic methods to
synthesize organofluorine compounds [1 – 7]. a,b-Unsaturated carbonyls (such as a,b-
enones) are ambident electrophiles, which have been extensively used in organic
synthesis [8][9]. However, the addition chemistry to a,b-unsaturated CO compounds
can be of practical synthetic utility only if one of the two regioisomers is generated
selectively [10]. Therefore, regioselective incorporation of a fluoroalkyl into the C(1)
or C(3) position of a,b-unsaturated carbonyls has attracted much attention these years
[11 – 18].
Due to the high electronegativity of the F-atom, many a-fluoro carbanions such as
F3Cꢀ are considered as hard nucleophiles and thus usually undergo 1,2-addition
reactions with a,b-unsaturated carbonyl compounds [11][16]. On the contrary, the high
regioselective nucleophilic introduction of a fluoroalkyl group in C(3) position of a,b-
unsaturated carbonyl compounds (1,4-addition) is a challenging task, which is usually
attributed to the intrinsic unmatched hard/soft nature between the fluorinated
nucleophiles and the C(3) position of a,b-unsaturated carbonyl compounds [16].
Reported methods for 1,4-addition of a fluoroalkyl group in a,b-enones and a,b-enals
include: 1) in situ protection of the CO group with a sterically hindered Lewis acid such
as aluminum tris(2,6-diphenylphenoxide) (Scheme 1,a) [12]; 2) activation of the b-
position with an electron-withdrawing group (EWG) or an aryl group (Scheme 1, b
ꢀ 2012 Verlag Helvetica Chimica Acta AG, Zꢁrich

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Helvetica Chimica Acta – Vol. 95 (2012)
Scheme 1. Reprentative Examples for the Regioselective Nucleophilic Introduction of a Fluoroalkyl
Group (Rf) in the b-Position of Enones
and c) [13 – 15]; and 3) modification of the fluoroalkyl moiety with EWGs that can
soften the corresponding a-fluorinated carbanion (Scheme 1,d and e) [16][17].
However, all these methods focus on the direct 1,4-addition reactions. It is known
that 1,4-adducts are usually thermodynamically more stable than the 1,2-adducts, and
thus the 1,2-adducts could be transformed to 1,4-adducts especially when the 1,2-
addition reaction is reversible [9][19]. The isomerization of 1,2- to 1,4-adducts in aldol
reaction of enones to ketones has been reported previously [19]. In this article, we
discuss the reversibility of the 1,2-addition in nucleophilic fluoroalkylation of a,b-
enones and a,b-enals with a-fluorinated sulfones and the isomerization of 1,2- to 1,4-
adducts at different temperatures (Scheme 2).
Scheme 2. Proposed Conversion of 1,2- to 1,4-Adducts
Results and Discussion. – Sulfur stabilization plays an important role in selective
nucleophilic di- and monofluoromethylation, and by this strategy, CF2H and CH2F
have been introduced in various electrophiles containing CꢀX (X ¼ leaving groups),
C¼O, C¼N, C¼C, and CꢁC bonds [3 – 7]. Among various S-based fluoroalkylation
reagents, a-fluorinated sulfones represent the ideal reagents due to the versatile
transformation of the sulfone functionality [3][4]. In recent years, with the aid of
sulfonyl substituent, fluoroalkylation of a,b-enones and a,b-enals with bis(benzene-

Helvetica Chimica Acta – Vol. 95 (2012)
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sulfonyl)fluoromethane ((PhSO2)2CHF; 1) in 1,4-addition manner has been achieved
by us and others [16][17][20 – 22]. We also examined the nucleophilic fluoroalkylation
of chalcones with difluoromethyl phenyl sulfone (PhSO2CF2H; 2) and fluoromethyl
phenyl sulfone (PhSO2CH2F; 3), and, in all cases, the full control of C(3)
regioselectivity was difficult to achieve [16]. However, by comparing the fluoroalky-
lation of a series of chalcones, it was found that the reaction with PhSO2CF2H (2)
favored 1,2-adducts, while the reaction with PhSO2CH2F preferred the 1,4-adducts. This
indicates that the number of F-substituents affects the hard/soft nature of the
carbanions [3][4][16]. According to the product ratios 1,4-/1,2-adduct, the order of
softness of a-fluoro carbanions was repored approximately as: (PhSO2)2CFꢀ >
PhSO2CHFꢀ > PhSO2CFꢀ2 [16].
For a better understanding of the reaction, we carried out a further study of the
nucleophilic addition of PhSO2CF2H (2) and PhSO2CH2F (3) to chalcone (¼(E)-1,3-
diphenylprop-2-en-1-one; 4) at different temperatures. Lithium hexamethyldisilazide
(LiHMDS) was chosen as the base with THF as the solvent, and no additive was used.
The results for the reaction between PhSO2CF2H (2) and chalcone 4 are compiled in
Table 1. When the reaction was conducted at ꢀ 788 and quenched after 30 min, 1,2- and
1,4-adducts were obtained in 90% total yield with a 81:19 ratio (Entry 1). Prolonged
reaction time (8 h) did not influence the product ratio and overall yield (Entry 2). It is
interesting to observe that the product ratio was also not significantly affected by
increasing the temperature (ꢀ 50 to 08), albeit the total yield decreased (Entries 3, 5,
and 7). Moreover, at a given temperature (ꢀ 50, ꢀ 25, or 08), the total yield did not
change much over time (Entries 4, 6, and 8).
Table 1. Reaction of PhSO2CHF2 (2) with (E)-Chalcone (4)
Entry
T [8]
Time [h]
Ratio 5/6a)
Total yield [%] 5 þ 6a)
1
2
3
4
5
6
7
8
9
ꢀ 78
ꢀ 78
ꢀ 50
ꢀ 50
ꢀ 25
ꢀ 25
0
0.5
8
0.5
8
0.5
7
0.5
7
81 : 19
82 : 18
81 : 19
81 : 19
83 : 17
85 : 15
83 : 17
80 : 20
67: 33
n.d.b)
90 (73 þ 17)
90 (74 þ 16)
72 (58 þ 14)
74 (60 þ 14)
46 (38 þ 8)
44 (37 þ 7)
24 (20 þ 4)
20 (16 þ 4)
6 (4 þ 2)
0
r.t.
r.t.
0.5
7
10
trace
a) The ratios and yields were determined by 19F-NMR with PhCF3 as an internal standard. b) n.d. ¼ Not
determined.
The results for the reaction between PhSO2CH2F (3) and chalcone 4 are collected in
Table 2. When the reaction was conducted at ꢀ 788 and quenched after 0.1 h, 1,2- and

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Table 2. Reaction of PhSO2CH2F (3) with (E)-Chalcone (4)
Entry
T [8]
Time [h]
Ratio of 7/8a)
Total yield [%] 7 þ 8a)
1
2
3
4
5
6
7
8
9
ꢀ 78
ꢀ 78
ꢀ 78
ꢀ 50
ꢀ 50
ꢀ 25
ꢀ 25
0
0.1
0.5
7
0.5
7
0.5
7
0.5
7
43 : 57
42 : 58
43 : 57
52 : 48
54 : 46
64 : 36
64 : 36
71 : 29
70 : 30
71 : 29
15 : 85
96 (41 þ 55)
96 (40 þ 56)
96 (41 þ 55)
97 (50 þ 47)
97 (52 þ 45)
96 (61 þ 35)
97 (62 þ 35)
94 (67 þ 27)
90 (63 þ 27)
94 (67 þ 27)
39 (6 þ 33)
0
r.t.
r.t.
10
11
0.5
7
a) The ratios and yields were determined by 19F-NMR with PhCF3 as an internal standard.
1,4-adducts were obtained in 96% total yield with a 43 :57 ratio (Entry 1). Similar to the
reaction with PhSO2CF2H (2), prolonged reaction time (0.5 or 7 h) did not significantly
influence the product ratio and overall yield (Entries 2 and 3). At the temperatures
tested (ꢀ 50, ꢀ 25, and 08), the total yield and product ratio did not change over time
(Entries 5, 7, and 9). Different from the reaction with PhSO2CF2H, the product ratio of
this reaction was temperature-dependent, and the yield of 1,2-adduct increased
gradually with the elevation of temperature (Entries 2, 4, 6, and 8). When the reaction
was performed at r.t., due to the relative higher stability of PhSO2CHFLi compared to
that of PhSO2CF2Li, PhSO2CHFLi could react with 1 smoothly to give the 1,2- and 1,4-
adducts in 94% total yield with a 71:29 ratio (Entry 10). However, at this temperature,
the 1,2-adduct gradually decomposed over time, and, therefore, only a small amount of
1,2-adduct isomerized into the 1,4-adduct (Entry 11).
The data compiled in Tables 1 and 2 indicate that the nucleophilic fluoroalkylation
of chalcone 4 with PhSO2CF2Li and PhSO2CHFLi is kinetically controlled, and both
the lithium alcoholate (1,2-adduct) and the lithium enolate (1,4-adduct) are stable at
temperatures below 08. Therefore, the product ratios can reflect the relative rates of the
1,2- and 1,4-addition reactions. These data also suggest that the 1,4-addition reaction
between PhSO2CH2F and 4 is kinetically more favored than that between PhSO2CF2H
and 4.
To get more insights into the reversibility of the 1,2-addition reaction and the
kinetic preference of the formation of the 1,4-adducts, we examined the isomerization
of the 1,2-adducts 5, 7, and 9 under basic conditions (Table 3). When alcohol 5 was
treated with 1.2 equiv. of LiHMDS at ꢀ 788, and the reaction mixture was stirred at the
same temperature for 12 h, no 1,4-adduct 6 was detected by 19F-NMR spectroscopy
after quenching the reaction with saturated aqueous solution of NH4Cl (Entry 1).
Similar results were obtained for alcohols 7 and 9 (Entries 2 and 3). It is worthy of

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Table 3. Conversion of 1,2-Adducts to 1,4-Adducts
Entry
Rf
T [8]
Time [h]
Conversion [%]a)
Yield [%]a)b)
1
2
3
4
5
6
PhSO2CF2
PhSO2CHF
CF3
PhSO2CF2
PhSO2CHF
CF3
ꢀ 78
ꢀ 78
ꢀ 78
12
10
10
8
10
11
5: < 1
7: < 1
9: < 1
5: > 99
7: > 99
9: < 1
6: 0
8: 0
10: 0
6: 0 (2: 60)
8: 9 (3: 73)
10: 0
r.t.
r.t.
r.t.
a) The conversion and yields were determined by 19F-NMR using PhCF3 as an internal standard.
b) Yields of RfꢀH were given in parentheses.
noting that the diastereomer ratio (4.3 :1) of 7 was maintained during the reaction.
These results suggest that the 1,2- adducts 5, 7, and 9 are relatively stable at ꢀ 788, and
the retro-1,2-addition can hardly take place under the above conditions.
Subsequently, we carried out the reactions under conditions such that the retro-1,2-
addition is favored (Table 3, Entries 4 – 6). After the addition of 1.2 equiv. of LiHMDS
to the THF solution of 5 at ꢀ 788, the solution was allowed to warm to room
temperature and stirred for 8 h. Although the 1,2-adduct 5 was consumed completely,
no 1,4-adduct 6 was detected (only 2 was formed in 60% yield; Entry 4). When 1,2-
adduct 7 was used as a substrate, 1,4-adduct 8 was obtained in only 9 % yield (Entry 5),
which is much lower than the kinetically controlled formation of 8 (27% yield, see
Table 2, Entry 10). These results demonstrate that, in our cases, although the 1,4-adduct
is thermodynamically more stable than the 1,2-adduct, the retro-1,2-addition reaction
results in the protonation and partial decomposition of the fluoroalkyl anions rather
than their 1,4-addition to chalcones under such conditions (Scheme 3). As a result, it is
difficult to isomerize the fluoroalkylated 1,2-adduct 5 and 7 to the 1,4-adducts 6 and 8,
respectively, under thermodynamically controlled conditions. Different from 5 and 7,
Scheme 3. Possible Competive Pathways That Influence the Formation of 1,4-Adduct under Thermody-
namically Controlled Conditions

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Helvetica Chimica Acta – Vol. 95 (2012)
compound 9 is stable even at room temperature, and its retro-1,2-addition was not
observed (Entry 6).
Based on aforementioned results and discussion, a successful isomerization of 1,2-
to 1,4-adduct should fulfill two prerequisites: one is the reversibility of the 1,2-addition,
the other is the stability of the fluoroalkyl anion. To the best of our knowledge, there
has been no report on the effective conversion of 1,2-adduct, which was generated by
the reaction of an a,b-unsaturated carbonyl compound with an a-fluoro carbanion, to
1,4-adduct. In 2011, we reported a successful addition reaction of (PhSO2)2CHF (1)
with an aldehyde at ꢀ 948 promoted by LiꢀO interaction, which was previously
assumed to be unattainable [23][24]. The reaction was proved to be general, and only
the kinetically controlled 1,2-adduct was observed in the case of (E)-cinnamaldehyde
[23]. The excellent 1,2-addition selectivity probably results from the higher reactivity
and the less steric hindrance of the a-C-atom. In 2009, Rios and co-workers, Cordova
and co-workers, and Wang and co-workers independently reported the catalytic
enantioselective conjugate addition of 1 to a,b-unsaturated aldehydes [20 – 22]. Based
on the above achievements of us and others, we envisioned that the 1,2-adduct of
(PhSO2)2CHF (1) with an a,b-unsaturated aldehyde might be converted to 1,4-adduct
under the activation of an organocatalyst.
The 1,2-adduct 12 was synthesized from 1 and aldehyde 11 in 90% yield according to
the procedure described in [23] (Scheme 4,a). It was found that, with 13 as a catalyst,
PhCOOH as an additive, and toluene as a solvent, 12 could be successfully isomerized
to the 1,4-adduct in a moderate yield (50%) and excellent enantiomeric excess (95%
ee) after the reduction with NaBH4 (Scheme 4,b).
Scheme 4. Synthesis of Alcohol 12 and Its Isomerization to 14
Conclusions. – In summary, an investigation of the reactions of PhSO2CF2H (2) and
PhSO2CH2F (3) with (E)-chalcone (4) at low temperatures revealed that these two
reactions were kinetically controlled, and the ratios of 1,2- vs. 1,4-adducts at these
temperatures did not change much over time. The controlled experiments of converting
the isolated PhSO2CF2- and PhSO2CHF-substituted 1,2-adducts to 1,4-adducts showed
that the relatively low thermal stability and/or hard/soft nature of a-fluoro carbanions,
such as PhSO2CFꢀ2 and PhSO2CHFꢀ, hampered the formation of the thermodynami-
cally more favorable 1,4-adducts via reversible 1,2-additions under thermodynamically

Helvetica Chimica Acta – Vol. 95 (2012)
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controlled conditions. Eventually, taking advantage of the remarkable stability and
softness of (PhSO2)2CFꢀ anion, an efficient and thermodynamically controlled iso-
merization of (PhSO2)2CF-substituted 1,2-adduct to 1,4-adduct was achieved for the
first time.
Support of our work by the National Natural Science Foundation of China (20825209 and 20832008),
the National Basic Research Program of China (2012CB215500 and 2012CB821600), the Chinese
Academy of Sciences, and the Syngenta PhD Studentship (to X. S.) is gratefully acknowledged.
Experimental Part
General. Unless otherwise mentioned, solvents and reagents were purchased from commercial
sources and used as received. Toluene, THF, and CH2Cl2 were distilled over Na. 1H-, 13C- and 19F-NMR
spectra: Bruker AM300, DPX-400, or Avance-500; 1H-NMR chemical shifts were determined relative to
internal TMS (d(H) 0.0 ppm) or to the signal of a residual protonated solvent (CDCl3 d(H) 7.26 ppm);
13C-NMR chemical shifts were determined relative to internal TMS (d(C) 0.0 ppm), and 19F-NMR
chemical shifts were determined relative to CFCl3 (d(F) 0.0 ppm) or PhCF3 (d(F) ꢀ 63.7 ppm). MS:
Ionspec 4.7 T mass spectrometer. HR-ESI-MS: FTMS-7 mass spectrometer.
Reactions of Difluoromethyl Phenyl Sulfone (2) and Fluoromethyl Phenyl Sulfone (3) with (E)-
Chalcone (¼(2E)-1,3-Diphenylprop-2-en-1-one; 4). General Procedure. Under N2, to a stirred mixture of
2 or 3 (0.5 mmol), and 4 (0.5 mmol) in dry THF (2.5 ml) at temps. as indicated in Tables 1 and 2, was
added lithium hexamethyldisilazide (LiHMDS; 1.0m in THF, 0.6 ml, 0.6 mmol). After the corresponding
reaction time, the reaction was quenched by an adding excess amount of sat. aq. soln. of NH4Cl (1 ml).
The org. layer was subjected to 19F-NMR analysis with PhCF3 as an internal standard, and the yield of the
products were calculated from the 19F-NMR integrals.
(3E)-1,1-Difluoro-2,4-diphenyl-1-(phenylsulfonyl)but-3-en-2-ol (5). 19F-NMR (282 MHz, THF):
ꢀ 104.3 (d, J ¼ 239, 1 F); ꢀ 106.3 (d, J ¼ 239, 1 F).
4,4-Difluoro-1,3-diphenyl-4-(phenylsulfonyl)butan-1-one (6). 19F-NMR (282 MHz, THF): ꢀ 99.3
(dd, J ¼ 233, 11.4, 1 F); ꢀ 106.0 (dd, J ¼ 233, 22.7, 1 F).
(3E)-1-Fluoro-2,4-diphenyl-1-(phenylsulfonyl)but-3-en-2-ol (7). 19F-NMR (282 MHz, THF):
ꢀ 179.9 (d, J ¼ 44, 1 F, isomer 1); ꢀ 181.9 (d, J ¼ 44, 1 F, isomer 2).
4-Fluoro-1,3-diphenyl-4-(phenylsulfonyl)butan-1-one (8). 19F-NMR (282 MHz, THF): ꢀ 180.3 (dd,
J ¼ 48, 18, 1 F, minor isomer); ꢀ 185.6 (dd, J ¼ 48, 28, 1 F, major isomer).
1,2-Addition of Bis(benzenesulfonyl)fluoromethane (¼1,1’-[(Fluoromethanediyl)disulfonyl]diben-
zene; 1) to (E)-3-(4-Bromophenyl)acrylaldehyde (¼(2E)-3-(4-Bromophenyl)prop-2-enal; 11) for the
Syntheses of Alcohol 12. Under N2, to a stirred mixture of 1 (157 mg, 0.5 mmol) and 11 (158 mg,
0.75 mmol) in dry CH2Cl2 (5 ml) at ꢀ 788 was added LiHMDS (1.0m in THF, 0.6 ml, 0.6 mmol). After
0.5 h, the reaction was quenched by adding excess amount of TFA (1 ml) at ꢀ 948, followed by extraction
of the mixture with CH2Cl2 and H2O. The org. layer was dried (MgSO4), and the solvent was removed
under reduced pressure. The residue was subjected to CC (SiO2 (acidified with 1% TFA in petroleum
ether before use)) to give product 12 (236 mg, 90%).
Data of (3E)-4-(4-Bromophenyl)-1-fluoro-1,1-bis(phenylsulfonyl)but-3-en-2-ol (12). IR (film):
3514, 3063, 1584, 1487, 1448, 1349, 1168, 1136, 1077, 998, 978, 849, 800, 708. 1H-NMR (300 MHz,
CDCl3): 8.01 (d, J ¼ 7.6, 2 H); 7.89 (d, J ¼ 7.7, 2 H); 7.74 (d, J ¼ 7.4, 1 H); 7.69 – 7.54 (m, 3 H); 7.49 – 7.40 (m,
4 H); 7.09 (d, J ¼ 8.3, 2 H); 6.53 (d, J ¼ 15.9, 1 H); 6.06 (dd, J ¼ 15.9, 5.1, 1 H); 5.01 (s, 1 H); 3.61 (s, 1 H).
19F-NMR (282 MHz, CDCl3): ꢀ 137.67 (s). 13C-NMR (100 MHz, CDCl3): 136.2; 135.6; 135.5; 135.0;
134.7; 132.7; 131.7; 131.2; 129.1; 129.0; 128.4; 122.8; 122.2; 112.1 (d, J ¼ 273.5); 72.2 (d, J ¼ 21.3). MALDI-
MS: 547 ([M þ Na]þ). HR-ESI-MS: 546.9664 ([M þ Na]þ, C22H18BrFNaO5Sþ2 ; calc. 546.9661).
Isomerization of 1,2-Addition Product 12 to 1,4-Addition Product 14. Under N2, 12 (131 mg,
0.25 mmol) and (2S)-2-{diphenyl[(trimethylsilyl)oxy]methyl}pyrrolidine (13; 16 mg, 0.05 mmol) were
dissolved in dry toluene (0.8 ml), and then PhCOOH (6 mg, 0.05 mmol) was added to the mixture. The
soln. was stirred at 08 for 90 h. The mixture was then directly purified by CC (SiO2) to give a mixture of 1
and 14. The mixture was dissolved in MeOH (2 ml), and then the soln. was cooled to 08. NaBH4 (72 mg,

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Helvetica Chimica Acta – Vol. 95 (2012)
1.9 mmol) was added in three portions, and the resulting mixture was stirred at 08 for 60 min. After the
addition of H2O (2 ml), the mixture was extracted with Et2O (15 ml ꢂ 3). The combined org. layers were
washed with brine, dried (Na2SO4), and concentrated. The residue was purified by CC (SiO2; petroleum
ether/AcOEt 5 :1) to give 14 (63 mg, 50 % yield).
Data of 3-(4-Bromophenyl)-4-fluoro-4,4-bis(phenylsulfonyl)butan-1-ol (14) [21]. ee 95% (SINO-
AD; 4.6 mm ꢂ 250 mm), hexanes/IPA 50 :50, 1.0 ml/min, l 214 nm, tR (major) 7.27 min, tR (minor)
11.33 min. 1H-NMR (300 MHz, CDCl3): 7.86 (d, J ¼ 7.5, 2 H); 7.79 (d, J ¼ 7.7, 2 H); 7.73 (d, J ¼ 7.5, 1 H);
7.65 (t, J ¼ 7.5, 1 H); 7.55 (t, J ¼ 7.8, 2 H); 7.46 (t, J ¼ 7.8, 2 H); 7.32 (d, J ¼ 8.5, 2 H); 6.94 (d, J ¼ 8.3, 2 H);
4.18 (d, J ¼ 11.5, 1 H); 3.80 (s, 1 H); 3.29 – 3.21 (m, 1 H); 3.11 – 2.97 (m, 1 H); 2.75 (t, J ¼ 13.0, 1 H). The
1H-NMR data are consistent with those given in [21].
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Received August 10, 2012