Unexpected Lewis acid mediated reactions of 1-arylbut-3-en-1-ols with trimethyl orthoformate - A new synthesis of homoallyl ethers and chlorides

Article abstract of DOI:10.1055/s-2008-1042928

1-Arylbut-3-en-1-ols react with trimethyl orthoformate in the presence of Lewis acids like InCl₃, BiCl₃, TiCl₄, or BF₃·OEt₂ providing homoallyl ethers or homoallyl chlorides in high yields. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1042928

LETTER
903
Unexpected Lewis Acid Mediated Reactions of 1-Arylbut-3-en-1-ols with Tri-
methyl Orthoformate – A New Synthesis of Homoallyl Ethers and Chlorides
L
ewis Acid Me
l
e
r
ansforma
n
tion of 1-Ary
a
lbut-3-en-1-ols Merişor, Jürgen Conrad, Chandi C. Malakar, Uwe Beifuss*
Bioorganische Chemie, Institut für Chemie, Universität Hohenheim, Garbenstraße 30, 70599 Stuttgart, Germany
Fax +49(711)45922951; E-mail: ubeifuss@uni-hohenheim.de
Received 12 October 2007
Lewis acid
+
Abstract: 1-Arylbut-3-en-1-ols react with trimethyl orthoformate
in the presence of Lewis acids like InCl3, BiCl3, TiCl4, or BF3·OEt2
providing homoallyl ethers or homoallyl chlorides in high yields.
CH(OR)3
HC(OR)2
B
A
Key words: alcohols, ethers, indium, halides, Lewis acids
R'
R'
R'
Nu
Nu
R'
B
+
OH
+
CHOR
Relatively little is known about reactions between ortho
esters and alcohols.1 Among the best known transforma-
tions of this kind is the acid-catalyzed transesterification
of simple ortho esters with an excess of a less volatile al-
cohol. The resulting products are higher ortho esters with
three identical OR groups. More difficult to achieve is the
selective access to mixed ortho esters as no general prep-
aration method is known. Albizati et al. could demonstrate
that the synthesis of mono-exchanged products can be
achieved by reacting an alcohol with a large excess of an
ortho ester in the presence of MgCl2.2 This method has
also been applied to the synthesis of a mixed ortho ester of
a homoallyl alcohol. Using the substrates in almost
equimolar amounts leads, of course, to mixtures of prod-
ucts derived from mono-, bis- and tris-exchange.
OCH(OR)2
O
O
OR
C
D
E
F
Scheme 1 Synthesis of tetrahydropyrans by reaction of homoallyl
alcohols and orthoformates
InCl3 (1 equiv)
CH2Cl2, r.t., 3 h
+
HC(OMe)3
OMe
OH
88 %
1a
2
3a
Scheme 2 Reaction of 1a and 2 with InCl3
we focused on the development of a general method for
the use of ortho esters as precursors for dioxenium ions in
intramolecular olefin cyclizations. For this purpose, we
chose the reactions between 1-arylbut-3-en-1-ols and
orthoformates. To start with, 1-phenylbut-3-en-1-ol (1a)
was prepared in 90% yield by reacting benzaldehyde with
allylmagnesium bromide in Et2O.9 Subsequently, one
equivalent 1a was reacted with one equivalent trimethyl
orthoformate (TMOF, 2) in the presence of one equivalent
of InCl3 at room temperature (Scheme 2). InCl3 is a re-
agent that has proved to be an efficient Lewis acid in a
number of Prins reactions.10 The transformation proceed-
ed cleanly and without any side reactions. Surprisingly,
instead of the expected tetrahydropyran or a mixed ortho
ester we isolated the homoallyl ether 3a with 88% yield
upon workup.11
When the reaction of the homoallyl alcohol with the
orthoformate is not performed with MgCl2, but in the
presence of other Lewis acids like SnCl4,3a,b SnBr4,3a,b
3b
ZnBr2,3a,b MgI2 or Mg(CO2CF3)2,3b the stereoselective
formation of tetrahydropyrans is observed. It can be as-
sumed that the reaction sequence starts with the Lewis
acid mediated transformation of the orthoformate A into
the electrophilic dioxenium cation B,4 which in turn is at-
tacked by the allyl alcohol C as a nucleophile to yield the
mixed ortho ester D. The latter is transformed into the di-
oxenium ion E – again in a Lewis acid mediated reaction.
The sequence is completed by the intramolecular cationic
olefin cyclization of E to give the tetrahydropyran F
(Scheme 1).3,5
Homoallyl ethers are important and versatile building
blocks amenable to further transformations so that several
synthetic methods can be found in the literature,12 most of
them based on the allylation of acetals.
Such intramolecular Prins reactions have been developed
into an efficient method for the synthesis of tetrahydropy-
rans.6 In particular, allylsilanes and related compounds
have been employed as intramolecular traps for oxocarbe-
nium ions which are generated in various ways.7
Several reagents have been used to effect this transforma-
tion. These include TiCl4,13a AlCl3,13b BF3·OEt2,13b
Me3SiOTf,13c Me3SiI,13d TrClO4,13e Ph2BOTf,13e montmo-
During the course of our studies towards the construction
of pyrans and pyrones by means of cationic cyclizations8
rillonite,13f
PbBr2/Al/AlBr3,13g
TMSN(SO2F)2,13h
TiCp2(CF3SO3)2,13i BiBr3,13j TMSNTf2,13k Sc(OTf)3,13l
Bi(OTf)3,13m and TMSOTf in ionic liquids.13n In addition
to the very popular allyltrimethylsilane,13a–f allyl bro-
mide,13g and allylborates14 have served as allylating re-
SYNLETT 2008, No. 6, pp 0903–0907
x
x.
x
x.
2
0
0
8
Advanced online publication: 11.03.2008
DOI: 10.1055/s-2008-1042928; Art ID: G33707ST
© Georg Thieme Verlag Stuttgart · New York

904
E. Merişor et al.
LETTER
InCl3 (1 equiv)
CH2Cl2, r.t.
+
HC(OMe)3
+
+
Cl
OMe
OH
O
R
R
R
R
R
1
2
3
4
5
Scheme 3 InCl3-mediated reaction of homoallyl alcohols 1 with TMOF (2)
Table 1 Results of the InCl3-Mediated Reactions between 1 and 2
Entry 1
R
Time 3 Yield 4 Yield 5 Yield
(h)
(%)
(%)
(%)
1
2
1a
H
3
3a 88
–
–
–
–
–
–
–
–
–
–
–
1b
1c
1d
1e
1f
2-Me
2
3b 86
3c 84
3d 92
3e 90
3f 72
3g 63
3h 61
–
–
–
3
3-Me
2
–
–
–
4
4-Me
2
–
–
–
5
2,5-dimethyl
4-F
2
–
–
–
6
2
4f
9
–
7
1g
1h
1i
4-Cl
3
4g 14
4h 14
–
Figure 1 Part of 1H NMR spectrum of 4f in CDCl3, displaying the
signals for selected protons of the two diastereomers D1 and D2
8
4-Br
3
–
9
4-CF3
pentafluoro
4-CN
4-O2N
3
3i
3j
–
–
–
–
–
–
–
–
–
–
5i 82
5j 83
5k 88
5l 67
Lewis acid (1 equiv)
CH2Cl2, r.t.
+
3a or 5a
1a
2
10
11
12
1j
3
Scheme 4 Reactions of 1a and 2 with different Lewis acids
1k
1l
3
3k –
3l
3
–
and flash chromatography (3f–h, 4f–h, 5i–l), respectively,
the products 3–5 were obtained analytically pure. The
structures of all products were established unambiguous-
ly.
agents. As the reaction shown in Scheme 2 promised a
new approach to homoallyl ethers, which benefits from
very cheap and easily available substrates as well as the
use of a particular mild Lewis acid under mild reaction
conditions, we decided to investigate scope and limitation
of the new reaction.
Quite remarkably, the result of the transformations is cru-
cially dependent on the nature of the substituent R of the
aryl ring of 1. In the case of +I,+M-substituents, the ho-
moallyl ethers 317 were isolated exclusively with yields
ranging from 84% to 92% (Table 1, entries 2–5). With
halogen atoms as the substituents R, the homoallyl ethers
3f–h (61–72%) were accompanied by bishomoallyl ethers
4f–h18 (9–14%) as side products (Table 1, entries 6–8). If
the aryl groups of the homoallyl alcohols 1 carry several
halogen atoms or a –I,–M-substituent, the homoallyl chlo-
rides 5i–l19 were exclusively formed in yields of between
67% and 88% (Table 1, entries 9–12).
For these studies a number of aryl-substituted homoallyl
alcohols 1 were prepared. Apart from compounds having
aryl groups with +I,+M-substituents 1b-e compounds
with –I,+M-substituents (1f–h,j) and –I,–M-substituents
(1i,k,l) were also synthesized. Most of the homoallyl alco-
hols were obtained according to the simple method that
had already been employed for the preparation of 1a in
high yield. Reaction of the correspondingly substituted al-
dehydes with allylmagnesium bromide in Et2O delivered
the homoallyl alcohols 1b–j with yields ranging from
70% to 99%. Only the alcohols 1k,l with cyano or nitro
groups as substituents on the aromatic ring were prepared
according to the method of Baba.15
The structures of all products 3–5 were established unam-
biguously. In the case of the bishomoallyl ethers 4 sepa-
rate signals can be observed for two diastereomers
(Figure 1). For example, the 1H NMR spectrum of 4f ex-
hibits two signals for the olefinic proton 3-H at d = 5.67
ppm and d = 5.77 ppm in a 1:1 ratio, indicating a 1:1 mix-
ture of two diastereomers D1 and D2.20 Complete proton
assignment for both diastereomers has been achieved us-
ing 1D NMR (1H, 13C) and 2D NMR (gCOSY, decoupling
experiments, gHSQC, gHMBC, gHSQCAD, gHM-
BCAD).
Then each of the alcohols 1b–l was reacted with TMOF
(2) in the presence of InCl3 in CH2Cl2 at room temperature
(Scheme 3, Table 1).16 Progress of the reactions was mon-
itored by TLC. The reactions were stopped by adding wa-
ter according to the reaction times specified in Table 1.
After extractive workup followed by purification of the
crude products by means of Kugelrohr distillation (3b–e)
Synlett 2008, No. 6, 903–907 © Thieme Stuttgart · New York

LETTER
Lewis Acid Mediated Transformation of 1-Arylbut-3-en-1-ols
905
Table 2 Results of the Reactions of 1a and 2 with Different Lewis
Acids
+
+
1
+
HC(OR)2
OCH(OR)2
OCHOR
Entry
Lewis acid
Temp (°C) Time Yield of 3a Yield of 5a
A
(h)
(%)
88
80
86
–
(%)
R
R
B
C
1
2
3
4
InCl3
r.t.
3
–
BiCl3
r.t.
2
–
+
+
ROH
+
BF3·OEt2
TiCl4
–78 °C
–78 °C
2
–
SN1
– HCO2R
OCHOR
2
92
R
R
R
C
3
D
BF3·OEt2 (1 equiv)
CH2Cl2, –78 °C to r.t.
+
1
2
4
3
+
+
OR
F
6
R
TiCl3OR + H+ + Cl–
Scheme 5 Reactions of 1 and 2 with BF3·OEt2
TiCl4 + ROH
Table 3 Results of the BF3·OEt2-Mediated Reactions between 1 and
2
Cl–
5
+
+
OCHOR
SN2
Entry 1
R
Time 3 Yield 4 Yield 6 Yield
R
(h)
(%)
(%)
(%)
C
1
2
3
4
5
6
7
8
9
1a
H
2
3a 86
–
–
–
–
–
–
–
–
–
–
–
Scheme 6 A plausible reaction mechanism for the formation of dif-
ferent products
1b 2-Me
1c 3-Me
1d 4-Me
1e 2,5-dimethyl
1f 4-F
1
3b 70
3c 71
3d 70
3e 71
3f 77
3g 72
3h 68
–
–
–
1-ols 1a–l was reacted with TMOF (2) in the presence of
BF3·OEt2 in CH2Cl2 (Scheme 5). The results summarized
in Table 3 clearly demonstrate that the reactions with
BF3·OEt2 largely resemble the InCl3-mediated transfor-
mations. For example, the transformations of 1a–e also
led to the exclusive formation of homoallyl ethers 3a–e
(Table 3, entries 1–5), though with somewhat lower
yields. The reactions of 1f–h are also similar to the corre-
sponding InCl3-mediated transformations. The homoallyl
ethers 3f–h were the main products accompanied by small
amounts of the bishomoallyl ethers 4f–h (Table 3, entries
6–8). The BF3·OEt2-mediated reactions with 1j–l were
unsatisfactory as in all but one case, a multitude of prod-
ucts of unknown structure was formed. Only with the pen-
tafluorophenyl-substituted butenol 1j as the substrate
could the homoallyl fluoride 6j be isolated in analytically
pure form (Table 3, entry 9).
1
–
–
–
1
–
–
–
1
–
–
–
2
4f
3
–
1g 4-Cl
2
4g 10
4h 12
–
1h 4-Br
2
–
1j pentafluoro
6
–
–
–
–
6j 21
In the face of the exceptional dependence of product dis-
tribution on the nature of the aryl substituents the influ-
ence of the Lewis acid on the reactions was briefly studied
(Scheme 4, Table 2). It was found that transformations
with BiCl3 could also be performed at room temperature.
The reaction of equimolar amounts of 1a, TMOF (2) and
BiCl3 in CH2Cl2 delivered the homoallyl ether 3a in high
yield (80%) as expected (Table 2, entry 2). With strong
Lewis acids like BF3·OEt2 and TiCl4, however, the reac-
tions could be performed at low temperatures. Employing
BF3·OEt2 as the Lewis acid the homoallyl ether 3a could
again be isolated exclusively and in high yield (86%,
Table 2, entry 3), whereas the reaction with TiCl4 provid-
ed an unexpected result, namely the exclusive formation
of the homoallyl chloride 5a in 92% yield (Table 2, entry
4). These results definitely prove that product distribution
can also be controlled by choosing the appropriate Lewis
acid.
Mechanistic considerations accounting for the formation
of the products 3–5 rely on the carbenium ion C as the
central intermediate (Scheme 6). If the aryl ring of C car-
ries +I,+M- or –I,+M-substituents, an SN1 process is fa-
vored and the formation of the carbenium ion D, which is
both an allyl cation and a benzyl cation, occurs. If InCl3,
BiCl3, or BF3·OEt2 are used as Lewis acids, the carbenium
ion D is subsequently trapped with the alcohol ROH as the
nucleophile to yield the homoallyl ether 3. If the homoal-
lyl alcohol 1 itself acts as the nucleophile, the formation
of a bishomoallyl ether 4 takes place. If TiCl4 is used, the
alcohol ROH undergoes an irreversible reaction with the
Lewis acid, and thus, the carbenium ion D reacts with the
remaining chloride ion to afford the allyl halide 5. If the
aryl ring of C carries several halogen atoms or a –I,–M-
Compared to InCl3 and BiCl3, BF3·OEt2 is a very cheap
Lewis acid. This is why the entire range of 1-arylbut-3-en-
Synlett 2008, No. 6, 903–907 © Thieme Stuttgart · New York

906
E. Merişor et al.
LETTER
K.; Robinson, N. P.; Whelan, J.; Bosnich, B. Tetrahedron
Lett. 1993, 34, 4309. (j) Komatsu, N.; Uda, M.; Suzuki, H.;
Takahashi, T.; Domae, T.; Wada, M. Tetrahedron Lett.
1997, 38, 7215. (k) Ishii, A.; Kotera, O.; Saeki, T.; Mikami,
K. Synlett 1997, 1145. (l) Yadav, J. S.; Reddy, B. V. S.;
Srihari, P. Synlett 2001, 673. (m) Wieland, L. C.; Zerth, H.
M.; Mohan, R. S. Tetrahedron Lett. 2002, 43, 4597.
(n) Zerth, H. M.; Leonard, N. M.; Mohan, R. S. Org. Lett.
2003, 5, 55.
substituent, it is supposed that an SN2 reaction takes place.
In this case, C reacts with the chloride ion as the best nu-
cleophile available under the reaction conditions and af-
fords the allyl halide 5.
In summary, this is the first report on the Lewis acid me-
diated transformation of 1-arylbut-3-en-1-ols and trimeth-
yl orthoformates into their corresponding homoallyl
ethers and homoallyl halides.
(14) Hunter, R.; Michael, J. P.; Tomlinson, G. D. Tetrahedron
1994, 50, 871.
(15) Onishi, Y.; Ito, T.; Yasuda, M.; Baba, A. Eur. J. Org. Chem.
2002, 1578.
(16) General Procedure for InCl3-Mediated Reactions of
Acknowledgment
We thank Dr. R. Frank, Ms. I. Klaiber, Dr. H. Leutbecher, and Ms.
S. Mika for recording of UV, MS and NMR spectra.
Homoallyl Alcohols 1 with TMOF (2)
InCl3 (3 mmol) was added to a stirred solution of homoallyl
alcohol 1 (3 mmol) and TMOF (2, 3 mmol) in CH2Cl2 (25
mL). The reaction mixture was stirred at r.t. under Ar until 1
had been completely consumed (TLC). Then H2O (25 mL)
was added and the resulting mixture was extracted with
CH2Cl2 (75 mL). The extracts were washed with brine (25
mL), dried over MgSO4, filtered and concentrated in vacuo.
The products were purified by Kugelrohr distillation or flash
chromatography on silica gel.
References and Notes
(1) Pindur, U. The Chemistry of Acid Derivatives, In The
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1988, 29, 2023.
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1987, 16, 75.
(5) For examples, see: (a) Keck, G. E.; Covel, J. A.; Schiff, T.;
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I. E.; Adams, H. Tetrahedron Lett. 1991, 32, 4783.
(c) Martin, V. A.; Perron, F.; Albizati, K. F. Tetrahedron
Lett. 1990, 31, 301.
(17) Selected data for 3e: Rf = 0.20 (PE–CH2Cl2, 7:3). UV/Vis
(MeCN): lmax (log e) = 269 (3.17), 277 (3.18) nm. IR
(ATR): 3090, 2927, 2819, 1641 (C=C), 1500, 1448, 1357,
1189, 1156, 1108 (C–O–C), 1091, 974, 912, 808, 729 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.33 (s, 3 H, 2¢-CH3), 2.38
(s, 3 H, 5¢-CH3), 2.44 (br ddd, 1 H, 2Jgem = 14.5 Hz, 3J = ca.
7.4 Hz, 3J = ca. 7.4 Hz, 2-H), 2.53 (br ddd, 1 H, 2Jgem = 14.5
Hz, 3J = ca. 7.0 Hz, 3J = ca. 7.0 Hz, 2-H), 3.27 (s, 3 H, 1-
OCH3), 4.48 (br dd, 3J = 7.9 Hz, 3J = 8.0 Hz, 1 H, 1-H), 5.11
(ddt, 1 H, 3Jcis = 9.8 Hz, 2J = 1.3 Hz, 4-H), 5.17 (dq, 1 H,
3Jtrans = 17.2 Hz, 2J = 1.3 Hz, 4-H), 5.87 (dddd, 1 H,
3Jcis = 10.1, 3Jtrans = 17.1 Hz, 3J = ca. 7.0 Hz, 3J = ca. 7.0 Hz,
3-H), 7.04 (br d, 3J = 7.5 Hz, 1 H, 3¢-H), 7.08 (br d, 3J = 7.8
Hz, 1 H, 4¢-H), 7.24 (s, 1 H, 6-H). 13C NMR (75 MHz,
CDCl3): d = 18.7 (2¢-CH3), 21.1 (5¢-CH3), 41.6 (C-2), 56.6
(1-OCH3), 80.0 (C-1), 116.6 (C-4), 126.4 (C-6¢), 127.8 (C-
4¢), 130.2 (C-3¢), 132.2 (C-1¢), 135.2 (C-3), 135.6 (C-5¢),
139.5 (C-1¢). MS (EI, 70 eV): m/z (%) = 149 (100) [M+ –
41], 143 (12), 133 (45), 119 (59), 105 (65), 103 (15), 91 (31),
77 (32), 65 (8), 51 (10). HRMS (EI): m/z calcd for C13H18O:
190.1357; found: 190.1316 [M+]. Anal. Calcd for C13H18O:
C, 82.06; H, 9.53. Found: C, 82.05; H, 9.58.
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Chem. 2006, 2045.
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(b) Leutbecher, H.; Schmidt, D.; Conrad, J.; Beifuss, U.
Synlett 2007, 2545.
(18) Selected data for 4f: Rf = 0.62 (PE–CH2Cl2, 7:3). UV/Vis
(MeCN): lmax (log e) = 271 (3.01), 265 (3.05) nm. IR
(ATR): 3077, 2980, 2905, 1641 (C=C), 1604, 1508, 1431,
1343, 1295, 1220, 1156, 1073 (C–O–C), 1013, 991, 915,
831, 785, 722 cm–1. MS (EI, 70 eV): m/z (%) = 273 (4)
[M+ – 41], 150 (20), 149 (100), 109 (35), 95 (3), 53 (2).
HRMS (EI): m/z calcd for the fragment [C10H10F+]:
149.0761; found: 149.0750.
(9) Ropp, G. A.; Coyner, E. C. J. Am. Chem. Soc. 1950, 72,
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triethyl orthoformate (TEOF).
(12) Yamamoto, Y.; Asao, N. Chem. Rev. 1993, 93, 2207.
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(19) Selected data for 5j: Rf = 0.50 (PE). UV/Vis (MeCN): lmax
(log e) = 266 (1.97) nm. IR (ATR): 3080, 2980, 1654 (C=C),
1523, 1501, 1449, 1424, 1359, 1316, 1148, 1130, 1042, 991,
955, 928, 802, 761, 741, 684 cm–1. 1H NMR (500 MHz,
CDCl3): d = 2.96 (br ddd, 1 H, 2J = 14.3 Hz, 3J = ca. 7.8 Hz,
3J = ca. 7.8 Hz, 2-H), 3.05 (br ddd, 1 H, 2J = 14.3 Hz, 3J = ca.
7.1 Hz, 3J = ca. 7.1 Hz, 2-H), 5.15 (ddt, 1 H, 3J = 10.1 Hz,
2J = 1.4 Hz, 4J = 1.2 Hz, 4-H), 5.19 (dq, 1 H, 2J = 1.4 Hz,
3Jtrans = 17.1 Hz, 4-H), 5.22 (br dd, 1 H, 3J = ca. 8.0 Hz, 3J =
ca. 8.0 Hz, 1-H), 5.73 (dddd, 1 H, 3Jtrans = 17.2 Hz,
(e) Mukaiyama, T.; Nagaoka, H.; Murakami, M.; Ohshima,
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(h) Trehan, A.; Vij, A.; Walia, M.; Kaur, G.; Verma, R. D.;
Trehan, S. Tetrahedron Lett. 1993, 34, 7335. (i) Hollis, T.
3Jcis = 10.1 Hz, 3J = ca. 7.0 Hz, 3J = ca. 7.0 Hz, 3-H). 13
C
NMR (125 MHz, CDCl3): d = 41.1 (C-2), 49.5 (C-1), 114.5
(C-1¢), 119.5 (C-4), 132.5 (C-3) 138.7 (d, C-4¢), 141.2 (d, C-
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Lewis Acid Mediated Transformation of 1-Arylbut-3-en-1-ols
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3¢and C-5¢), 144.75 (d, C-2¢and C-6¢). MS (EI, 70 eV): m/z
134.7 (C-3), 137.70 (d, JC–F = 2.9 Hz, C-1¢), 162.5 (d,
1JC–F = 245.5 Hz, C-4¢).
(%) = 256 (33) [M+], 215 (100) [M+ – 41], 207 (57), 194
(35), 181 (45), 151 (12), 143 (11), 123 (7), 105 (3), 99 (6),
75 (5), 51 (5). HRMS (EI): m/z calcd for C10H6ClF5:
256.0078; found: 256.0076 [M+].
Compound 4f (D2): 1H NMR (500 MHz, CDCl3): d = 2.47
(dddq, 1 H, 2J = 14.1 Hz, 3J = ca. 7.1 Hz, 3J = ca. 7.1 Hz,
4J = 1.1 Hz, 2-H), 2.62 (dddt, 1 H, 2J = 14.1 Hz, 3J = ca. 7.1
Hz, 3J = ca. 7.1 Hz, 4J = 1.2 Hz, 2-H), 4.39 (dd, 1 H, 3J = ca.
6.4 Hz, 3J = ca. 6.4 Hz, 1-H), 5.04 (br d, 1 H, 3Jcis = 10.0 Hz,
4-H), 5.05 (dddd, 1 H, 3Jtrans = 17.5 Hz, 2J = ca. 1.6 Hz, 4J =
ca. 1.6 Hz, 2J = ca. 1.6 Hz, 4-H), 5.77 (dddd, 2 H,
(20) Compound 4f (D1): 1H NMR (500 MHz, CDCl3): d = 2.34
(dddq, 1 H, 2J = 13.9 Hz, 3J = ca. 6.5 Hz, 3J = ca. 6.5 Hz,
4J = 1.1 Hz, 2-H), 2.55 (br dddt, 1 H, 2J = 14.0 Hz, 3J = ca.
7.2 Hz, 3J = ca. 7.2 Hz, 4J = 1.3 Hz, 2-H), 4.08 (dd, 1 H,
3J = 7.3 Hz, 3J = 6.6 Hz, 1-H), 4.96 (dddd, 1 H, 3Jtrans = 17.2
Hz, 2J = ca. 1.6 Hz, 4J = ca. 1.6 Hz, 4J = ca. 1.6 Hz, 4-H), 4.98
(br d, 1 H, 3Jcis = 10.4 Hz, 4-H), 5.67 (dddd, 2 H,
3Jtrans = 16.9 Hz, 3Jcis = 10.5 Hz, 3J = ca. 7.2 Hz, 3J = ca. 7.2
Hz, 3-H), 6.94 (br dd, 2 H, 3J = ca. 8.8 Hz, 3JH–F = ca. 8.8 Hz,
3¢-H and 5¢-H), 7.14 (br dd, 2 H, 3J = 8.6 Hz, 4JH–F = 5.5 Hz,
2¢-H and 6¢-H). 13C NMR (125 MHz, CDCl3): d = 41.96 (C-
2), 79.4 (C-1), 115.06 (d, 2JC–F = 21.3 Hz, C-3¢and C-5¢),
117.54 (C-4), 128.51 (d, 3JC–F = 7.5 Hz, C-2¢ and C-6¢),
3Jtrans = 17.1 Hz, 3Jcis = 10.2 Hz, 3J = ca. 6.9 Hz, 3J = ca. 6.9
Hz, 3-H), 7.06 (br dd, 2 H, 3J = 8.7 Hz, 3JH–F = 8.0 Hz, 3¢-H
and 5¢-H), 7.23 (br dd, 2 H, 3J = 8.7 Hz, 4JH–F = 5.5 Hz, 2¢-H
and 6¢-H). 13C NMR (125 MHz, CDCl3): d = 43.05 (C-2),
77.85 (C-1), 115.44 (d, 2JC–F = 21.4 Hz, C-3¢ and C-5¢),
117.28 (C-4), 128.88 (d, 3JC–F = 8.5 Hz, C-2¢ and C-6¢),
4
134.5 (C-3), 137.19 (d, JC–F = 3.7 Hz, C-1¢), 162.20 (d,
1JC–F = 244.7 Hz, C-4¢).
Synlett 2008, No. 6, 903–907 © Thieme Stuttgart · New York