One-pot synthesis of 1-aryl-3-methyl-1,3-dienes using methallyl(trimethyl) silane and aldehydes and their low temperature (Z)→(E) isomerization induced by sulfur dioxide

Article abstract of DOI:10.1016/j.tet.2004.11.070

2-Methylprop-2-ene-1-sulfonyl fluorides can be easily prepared via the ene reaction of methallylsilanes and SO₂. In the presence of a base, aldehydes and 2-methylprop-2-ene-1-sulfonyl fluorides give 1,3-(E) and (Z)-dienes. Their (Z)→(E) isomerization by classical means fails or leads to their polymerization. It is shown that SO₂ can isomerize 1-aryl-3-methyl-1,3-dienes at low temperature, without formation of sulfolenes (cheletropic addition/elimination). Preliminary mechanistic studies suggest that SO₂ adds to 1,3-dienes forming 1,4-diradical intermediates that are responsible for the (Z)→(E) isomerizations.

Full text of DOI:10.1016/j.tet.2004.11.070

Tetrahedron 61 (2005) 1523–1530
One-pot synthesis of 1-aryl-3-methyl-1,3-dienes using
methallyl(trimethyl)silane and aldehydes and their low
temperature (Z)/(E) isomerization induced by sulfur dioxide
*
Srinivas Reddy Dubbaka and Pierre Vogel
` ´ ´ ´
Laboratoire de glycochimie et de synthese asymetrique, Ecole polytechnique Federale de Lausanne (EPFL),
BCH CH-1015 Lausanne, Switzerland
Received 29 October 2004; revised 22 November 2004; accepted 25 November 2004
Available online 15 December 2004
Dedicated to Professor Manfred Schlosser on the occasion of his 70th birthday
Abstract—2-Methylprop-2-ene-1-sulfonyl fluorides can be easily prepared via the ene reaction of methallylsilanes and SO2. In the presence
of a base, aldehydes and 2-methylprop-2-ene-1-sulfonyl fluorides give 1,3-(E) and (Z)-dienes. Their (Z)/(E) isomerization by classical
means fails or leads to their polymerization. It is shown that SO2 can isomerize 1-aryl-3-methyl-1,3-dienes at low temperature, without
formation of sulfolenes (cheletropic addition/elimination). Preliminary mechanistic studies suggest that SO2 adds to 1,3-dienes forming
1,4-diradical intermediates that are responsible for the (Z)/(E) isomerizations.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
under thermal, oxidative, or reductive conditions. Kagabu et
al. have found that phenylmethanesulfonyl fluoride can be
condensed with carbonyl compounds13 or with activated
haloalkanes14 to give mixtures of alkenes. Recently,
sulfonyl chlorides have become important intermediates
for carbon–carbon single bond formation via desulfitative
Heck–Mizoroki,15 Stille and carbonylative Stille,16 Suzuki–
Miyaura17 and Sonogashira–Hagihara18 cross-couplings.
Nader et al.19 have shown, on their side, that methanesulfo-
nyl fluoride reacts with electron-deficient ketones to give
mixtures of alkenes. Hawkins et al.20 reported that alkenes
can be obtained by reaction of sulfonic esters with
benzophenone in the presence of t-BuLi. Here we present
the further development of the methodology proposed
initially by Kagabu.13 Our main contribution exploits the
efficient preparation of 2-methylprop-2-ene-1-sulfonyl flu-
oride via the ene reaction of methallylsilane and SO2.
Furthermore, when mixtures of (E) and (Z)-dienes are
formed, we have found that pure (E)-isomers can be
obtained by (Z)/(E)-isomerization induced by sulfur
dioxide at low temperature. This discovery is crucial as
other known means for the (Z)/(E)-alkene isomerization
failed or led to polymerization of the dienes. As we shall
show, our conditions for the SO2-induced isomerization of
(Z)-dienes into (E)-isomers does not involve the cheletropic
addition/elimination of SO2. Preliminary mechanistic
studies suggest the intermediacy of diradicals.
1,3-Dienes are important synthetic intermediates, with
particular application in the Diels–Alder reaction.1
A
selective cross-metathesis between a terminal alkyne and
a terminal alkene yielding 1,3-disubstituted butadienes has
been reported by Blechert and co-workers.2 1,4-Disubsti-
tuted butadienes can be prepared via the palladium(II)-
mediated homocoupling of organostannanes.3 1,1,4,4-Tetra-
substituted 1,3-butadienes have been obtained by the
homocoupling of alkenyl(2-pyridyl)silanes.4 There are
also numerous regio- and stereospecific methods presently
available to prepare di- or trisubstituted carbon–carbon
double bonds in 1,3-dienes, for example, applying Wittig,5
6
7
´
Julia–Kocienski, and other reactions.
Sulfur based olefinations employing sulfides,8 sulfoxides,9
sulfones,10 sulfinamides,11 sulfoximides12 have been
developed. They each require two or more synthetic steps,
one involving the addition of a sulfur stabilized carbanion to
carbonyl compounds, and one, or several further steps for
the activation and the elimination of the resulting alcohols
Keywords: Aldehydes; Diradical intermediates; Ene reaction; Methallyl-
silane; (Z)/(E)-Isomerization; Sulfonyl fluoride; Sulfur dioxide.
* Corresponding author. Tel.: C41 21 693 9371; fax: C41 21 693 9375;
e-mail: pierre.vogel@epfl.ch
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.11.070

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S. R. Dubbaka, P. Vogel / Tetrahedron 61 (2005) 1523–1530
For practical reasons, a method to isomerize the (Z)/(E)
mixtures into pure (E)-diene was sought. Irradiation of the
dienes (Pyrex vessel, high pressure Hg lamp),24 failed to
isomerize our dienes. (Z)-Stilbenes have been isomerized
into (E)-stilbenes through radical addition and elimination
reactions (e.g. heating with ArSSAr or I2).25
Sulfur dioxide is capable to isomerize 1,3-dienes below
room temperature. We have found that pure 1-[(1Z)-3-
methylbuta-1,3-dienyl]-40-nitrobenzene was converted in
the presence of sulfur dioxide (0.2 to 1 equiv) in CH2Cl2
(0.13 M) at K20 8C. After 15 h, a 7:1 mixture of (Z)-8 and
(E)-8. After additional 15 h at K20 8C, (E)-8/(Z)-8 ratio
reached 98:2. Elimination of the sulfur dioxide must be done
by evaporation at K78 8C under vacuum. Otherwise, at
higher temperature, the dienes add to SO2 giving the
corresponding sulfolenes or/and polymeric products. For
instance, a 1:1.3 mixture of (Z)-8e (1-[3-methylbuta-1,3-
dienyl]-40-(trifluoromethyl) benzene) was left at K20 8C in
the presence of an excess sulfur dioxide (up to 5 equiv) in
CH2Cl2 (0.13 M) gave, after 16 h, a 3:1 mixture of the (E)-
8e and 9e (Scheme 3). We could not detect any changes
after heating this mixture at 40 8C under reduced pressure.
When the reaction was performed at 55 8C and under
250 mbar, the diene was polymerized and the sulfolene 9e
remained unchanged after 15 h.
Scheme 1. Efficient preparation of 2-methylprop-2-ene-1-sulfonyl deriva-
tives by ene reaction of methallyl(trimethyl)silane and SO2. Reaction
conditions: (i) 5 mol% TBSOTf, SO2 in CH3CN then 1, K40 8C, 12 h;
(ii) NCS or Cl2 at K20 8C, 2 h.
2. Results and discussion
Methallyl(trimethyl)silane (1) undergoes the ene reaction
with sulfur dioxide at K40 8C in acetonitrile in the presence
of 5 mol% of TBSOTf and yields the corresponding silyl
sulfinate 2.21 The latter is then reacted with N-chloro-
succinimide or chlorine to afford the corresponding sulfonyl
chloride 322 (Scheme 1), which, on its turns, reacts in situ
with KF in the presence of dibenzo(18-crown-6) giving 2-
methylprop-2-ene-1-sulfonyl fluoride (4). In the presence of
a base 4 reacts with all kinds of aromatic aldehydes 7
yielding the corresponding alkenes 8 as mixtures of (Z)/(E)-
isomers (Scheme 2). These mixtures can be separated by
column chromatography on silica gel. Sulfonic ester 5 or
thioester 6 (Scheme 1) can also react with carbonyl
compounds in the presence of base to give mixtures of
alkenes (Scheme 2) (Table 1). When the sulfonyl chloride 3
was purified by distillation,23 overall yield increased
significantly.
In the case of a 1:6 mixture of (Z)-8f and (E)-8f (1-methyl-
40-[3-methylbuta-1,3-dienyl]benzene) fast formation of
sulfolene 9f was observed at K20 8C already. This sulfolene
(9f) remained stable at room temperature for 24 h. However,
on heating to 80 8C only polymeric material was obtained.
Diene (Z)-8f can be isomerized into (E)-8f at K60 8C in the
presence of SO2 (0.5–1 equiv). All the other dienes tested
were isomerized with SO2 (0.2 to 1 equiv) at K40 to
Scheme 2. Olefinations of aromatic aldehydes.
Table 1. Olefination of aldehydes 7 with 4, 5, 6 to give dienes (Z)-8 and (E)-8
Entry
Aldehyde 7
Diene 8 yield (%)a
Product ratio (Z)-8/(E)-8b
1
2
3
4
5
6
7
8
Benzaldehyde (7a)
64(28)c(8)d
0.74
1.4
1.4
1.15
0.95
0.66
0.33
!1:99
—
4-Nitrobenzaldehyde (7b)
3-Nitrobenzaldehyde (7c)
4-Chlorobenzaldehyde (7d)
4-(Trifluoromethyl)benzaldehyde (7e)
p-Tolualdehyde (7f)
1-Naphthaldehyde (7g)
p-Anisaldehyde (7h)
Undecanal
70(30)c(12)d
73(24)c(9)d
63
61
60(20)c(9)d
54
40
0
9
10
11
Cl3CCHO
CH3CHO
0
0
—
—
a Yields with respect to the sulfonyl fluoride 4.
b (Z) and (E) ratio determined for the product mixture purified by flash chromatograpy on silica gel.
c Yields for reactions of sufonic ester 5.
d Yields for reactions of sulfonic thioester 6.

S. R. Dubbaka, P. Vogel / Tetrahedron 61 (2005) 1523–1530
1525
K60 8C. The results show that their rate of isomerization
strongly depends on the nature of the diene. Approximate
rate of their SO2-induced isomerization at K20 8C
(in CH2Cl2) follow the order 8b (p-NO2)w8c (m-
NO2)O8e (p-CF3)O8d (p-Cl)O8f (p-Me). This reactivity
order is not consistent with the formation of zwitterionic
intermediate of type 114110, but is consistent with the
formation of diradicals of type 104100 (Scheme 4).
Alternatively, sultines of type 12 (Scheme 5) could be
formed as intermediates via reversible hetero-Diels–Alder
additions26 (Scheme 5) and be responsible for the (Z)-
diene/(E)-diene isomerization. As we have found that
TEMPO inhibits these SO2-induced isomerizations, we
propose that the mechanism involves the formation of
diradicals of type 10 and 100. We verified that TEMPO
alone does not isomerize (Z)-diene/(E)-diene at K20 8C
or at room temperature.
Scheme 3. Sulfur dioxide induced (Z)-1,3-diene/(E)-1,3-diene
isomerization.
Scheme 4. Diradical mechanism confirmed by inhibition by TEMPO.
Scheme 5. Reversible hetero-Diels–Alder additions of SO2.
Treatment of the 2-methylprop-2-ene-1-sulfonyl fluoride (4)
with aromatic ketones 13, in the presence of K2CO3 gave
exclusively the corresponding trisubstituted (Z)-alkenes
(Scheme 6), (Table 2), but with poor yields.
The best yields of alkenylation are observed with sulfonyl
fluoride 4 and for electron-poor aromatic aldehydes
Scheme 6. Olefination of aromatic ketones.
Table 2. The reaction of 2-methylprop-2-ene-1-sulfonyl fluoride (4) with aromatic ketones (13) in the presence of K2CO3
Entry
R
Y
Yielda
Isomer
1
2
3
4
CF3 (13a)
CF2Cl (13b)
CH3 (13c)
CH3 (13d)
H
H
NO2
H
48
40
15
Traces
14a
14b
14c
14d
a Yields of products determined after flash chromatography on silica gel.

1526
S. R. Dubbaka, P. Vogel / Tetrahedron 61 (2005) 1523–1530
Scheme 7. Possible explanation for the variation of (Z)- versus (E)-stereoselectivity of the olefinations as a function of the electron-demand of the aromatic
aldehydes and ketones.
(Table 1) and ketones (Table 2). The reaction fails with
aliphatic aldehydes and with electron-rich methyl ketones
(Table 2, entry 4). This can be interpreted in terms of the
relative ease by which 4, 5 or 6 are deprotonated by K2CO3
(4 is more acidic than 5 and 6) and the intrinsic energy
barrier of the subsequent addition of the conjugate base of
the sulfonyl compound to the aldehydes or ketones. The
latter depends on electrophilicity of the carbonyl compound
and on steric hindrance. Electrophilicy increases with
electron-withdrawing substituents and with the polariza-
bility of the carbonyl compound (aromatic vs. aliphatic).
possible at low temperature and without concurrent
formation of sulfolenes and polymeric materials. The latter
reactions involve probably the formation of 1,4-diradical
intermediates. The reaction of 2-methylprop-2-ene-1-sulfo-
nyl fluoride with ketones is less successful, except for
electron-poor derivatives.
4. Experimental
4.1. General
The (Z)- versus (E)-diene ratio ((Z)-8/(E)-8) varies from
!1:99 for p-anisaldehyde to 1.4 for 4- and 3-nitrobenz-
aldehyde (Table 1). This can be interpreted in terms of
electrostatic interactions between the SO2F group of the
attacking carbanionic species and the aromatic groups of the
aldehydes. As the SO2F group bears partial negative charges
on its oxygen and centers, the latter recognizes the electron-
deficient aromatic ring (4-NO2C6H4) better than electron-
rich aromatic ring (4-MeOC6H4). Thus, transition structure
15, leading to adducts 16, that generate the (Z)-dienes (Z)-8
All solvents were distilled prior to use: THF and Et2O from
Na and benzophenone; CH2Cl2, and CH3CN from P2O5,
CaH2, respectively. Solvent after reactions and extractions
were evaporated in a rotatory evaporator under reduced
pressure. Liquid/solid flash chromatography (FC): columns
of silica gel (0.040–0.63 mm, Merck No.9385 silica gel 60,
240–400 mesh). Thin layer chromatography (TLC) for
reaction monitoring: Merck silica gel 60F254 plates;
detection by UV light. Pancaldi reagent ((NH4)6MoO4,
Ce(SO4)2, H2SO4, H2O), or KMnO4. Reagents were from
Fluka or Aldrich and used without purification. Mp:
uncorrected; Tottoli (Bu¨chi SMP-20) apparatus. IR Spectra:
Perkin–Elmer-1420 or Beckman -IR4230 spectrometer; n in
13
by syn-elimination of SO3 is prepared over the less
sterically crowded transition structure 16 with electron-poor
aromatic aldehydes. In the case of electron-rich aromatic
aldehydes (7f, 7g, 7h), transition structures of type 17
are preferred. This leads to adducts 18 that undergo
syn-elimination13 with formation of the corresponding
(E)-dienes ((E)-8) (Scheme 7).
cmK1 1H NMR Spectra: Bruker-DPX-400, or Bruker-
.
ARX-400 spectrometer; d(H) in ppm rel. to internal Me4Si
(Z0.00 ppm) or to the solvent’s residual 1H-signal (CHCl3,
d(H) 7.27; C6HD5, d(H) 7.16; CHD2COCD3, d(H) 1.95;
CD2HCN, d(H) 2.50; CHD2SOCD31, d(H) 2.50, CH2OD,
d(H) 3.31) as internal reference, all H-signal assignments
were confirmed by double irradiation experiments or by 2D
COSY-DQF or COSY-45 spectra. 13C NMR Spectra: same
instruments as above (101.61 MHz); d(C) in ppm rel. to
internal Me4Si (Z0.00 ppm) or to solvents 13C-signal
(CDCl3, C6D6, d(C) 128.4; (CD3)2CO d(C) 29.8; CD3CN,
d(C) 1.3; (CD3)2SO, d(C) 39.5, CD3OD, d(C) 49.2) as
internal reference; coupling constants J in Hz (G0.5 Hz).
3. Conclusions
The Kagabu’s olefination has been applied to prepare
1-aryl-3-methyl-1,3-dienes using aromatic aldehydes and
2-methylprop-2-ene-1-sulfonyl fluoride. When mixtures of
(Z)-diene and (E)-dienes are formed, efficient (Z)-diene/
(E)-diene isomerization induced by sulfur dioxide is

S. R. Dubbaka, P. Vogel / Tetrahedron 61 (2005) 1523–1530
1527
Ms: Nermag R-10–10C, chemical ionization (NH3) mode
m/z amu [% relative base peak (100%)].
C(2)). 13C NMR (100.6 MHz, CDCl3): 132.3, 122.5, 121.9,
64.6, 59.5, 22.3. CI-MS (NH3): 236 (33, [MCNH4]C), 218
(1, [MCH]C), 109 (100), 72 (28). HRMS (MALDI-TOF):
(C6H9F3O3SNaC), calcd 241.0122; found: 241.0125.
4.1.1. 2-Methylprop-2-ene-1-sulfonyl chloride (3). (t-Bu)-
Me2SiOSO2CF3 (0.23 g; 0.85 mmol, 0.05 equiv) in anh.
CH3CN (6 mL) was degassed by freeze–thaw cycles on the
vacuum line. Sulfur dioxide (15 mL, 0.34 mol, 20 equiv),
dried through a column packed with phosphorus pentoxide
and aluminum oxide, was transferred on the vacuum line to
the CH3CN solution frozen at K196 8C. The mixture was
allowed to melt and to warm to K40 8C. After 30 min at this
temperature the methallylsilane (2.2 g, 17 mmol, 1 equiv) in
CH3CN (6 mL) was added slowly. The mixture was stirred
at K40 8C for 6 h. After cooling to K78 8C, the excess of
SO2 and the solvent were evaporated under reduced
pressure (10K3 Torr) to dryness (ca. 1 h). Halogenating
agent (NCS 2.7 g, 20.4 mmol, 1.2 equiv, dissolving in
CH3CN) was added to the reaction mixture at K20 8C. After
2 h at this temperature, methallylsulfonyl chloride formed,
which was distilled at 66–68 8C under 8 mbar affording
(2.2 g; 82%) colourless oil. UV (MeCN): 234 (1100), 202
(690). IR (film): 3093, 2986, 2923, 1449, 1369, 1244, 1170,
4.1.4. S-(20,20,20-trifluoroethyl)-2-methylprop-2-ene-1-
sulfothioate (6). Same procedure as for the preparation of
3. The crude product 3 in CH3CN solution was mixed with
the trifluorothioethanol (2.6 g, 22.1 mmol, 1.3 equiv) and
triethylamine (2.8 mL, 20 mmol, 1.2 equiv) dissolved in
CH3CN at K20 8C. The reaction mixture was allowed to
reach 20 8C within 2 h. The mixture was poured into ice-
water (40 mL) and extracted with CH2Cl2 (25 mL, 3 times).
The combined organic extracts were washed with brine
(20 mL), dried (Na2SO4) and the solvent eliminated under
reduced pressure and under reflux. The residue was purified
by FC (8.5:1.5 light petroleum ether/EtOAc, RfZ0.37)
affording (2 g, 50%), brownish oil. UV (CH3CN): 205
(1650). IR (film): 2925, 1450, 1405, 1375, 1305, 1265,
1
1130, 1240, 1130, 1075, 915, 840. H NMR (400 MHz,
CDCl3): 5.20 (s, 1H, H–C(3)), 5.13 (s, 1H, H–C(3)), 3.86 (s,
3
2H, H–C(1)), 3.75 (q, 1H, JZ9.0 Hz, H–C(10)), 3.63 (q,
1H, 3JZ9.0 Hz, H–C(10)), 1.93 (s, 3H, Me–C(2)). 13C NMR
(100.6 MHz, CDCl3): 135.1, 125.2, 120.3, 65.8, 33.6, 23.3.
CI-MS (NH3): 252 (33, [MCNH4]C), 234 (15, [MCH]C),
201 (100), 169 (96), 118 (86), 84 (83), 72 (70). HRMS
(MALDI-TOF): (C6H9F3O2S2 NaC), calcd 256.9894;
found: 256.9817.
1
1146, 923, 885, 674. H NMR (400 MHz, CDCl3): 5.41
(br.s, 1H, H–C(3)), 5.32 (br.s, 1H, H–C(3)), 4.34 (s, 2H,
H–C(1)), 2.01 (s, 3H, Me–C(2)).13C NMR (100.6 MHz,
CDCl3): 132.0, 124.2, 73.2, 22.2. CI-MS (NH3): 172 (4,
[MCNH4]C), 154 (1, [M]C.), 90 (10), 72 (100). Anal.
Calcd for C4H7ClO2S (154.61): C 31.07; H 4.56; found: C
31.01; H 4.55.
4.1.5. Olefination procedure 1. The crude sulfonyl fluoride
4 (2.3 g, 17 mmol, 1 equiv) was mixed with various
aromatic aldehydes or aromatic ketones (22.1 mmol,
1.3 equiv) in the presence of a K3CO3 (9.4 g, 68 mmol,
4 equiv). After stirring at 20 8C for 2–4 h, the mixture was
poured into ice-water (50 mL) and extracted with CH2Cl2
(20 mL, 3 times). The combined organic extracts were dried
(Na2SO4) and the solvent evaporated under reduced
pressure and under reflux with cooling at K20 8C. The
residue was purified by FC, affording mixtures of the
corresponding (Z)/(E)-1-aryl-2-methyl-1,3-dienes (8, 14).
All 4 steps starting from 1 were carried out in a single pot.
If intermediate 2-methylprop-2-ene-1-sulfonyl chloride (3)
is purified by distillation, overall yield increases slightly.
4.1.2. 2-Methylprop-2-ene-1-sulfonyl fluoride (4). The
crude methallylsulfonyl chloride formed (3), was treated
with KF (2 g, 34 mmol, 2 equiv) in the presence of
dibenzo18-crown-6 (0.31 g, 0.85 mmol, 0.05 equiv) giving
a brownish solution after staying at 20 8C overnight. Cold
ether was added and the solution filtered through Celite.
The solvent was evaporated under reflux with cooling at
K20 8C. Yield (1.5 g, 80%). IR (film): 3065, 2930, 2885,
1635, 1595, 1505, 1455, 1405, 1325, 1250, 1200, 1130,
1065, 945, 885, 800, 740, 610. 1H NMR (400 MHz, CDCl3):
5.32 (br.s, 1H, H–C(3)), 5.25 (br.s,1H, H–C(3)), 4.04 (d, 2H,
3JC–FZ3.2 Hz, H–C(1)), 1.98 (s, 3H, Me–C(2)). 13C NMR
(100.6 MHz, CDCl3): 131.0, 122.9, 69.3, 20.7. CI-MS
(NH3): m/zZ156 (47, [MCNH4]C), 138 (17, [MCH]C),
119 (6), 72 (100).
4.1.6. Olefination procedure 2. 20,20,20-Trifluoroethyl-2-
methylprop-2-ene-1-sulfonate (5, 0.5 g, 2.3 mmol, 1 equiv)
was dissolved in 10 mL of THF under argon atmosphere.
After cooling to K78 8C, the aldehydes or aromatic ketones
(2.99 mmol, 1.3 equiv) were added, followed by t-BuLi
(1.5 M in pentane) (1.8 mL, 2.76 mmol, 1.2 equiv). The
mixture allowed to reach 20 8C within 2 h. The mixture was
poured into brine (30 mL) and was extracted with ether
(20 mL, 3 times). The combined organic extracts were dried
(Na2SO4) and the solvent was evaporated under reduced
pressure and under reflux with cooling to K20 8C. The
residue was purified by FC, affording (Z)/(E)-1-aryl-2-
methyl-1,3-dienes 8, 14.
4.1.3. 20,2020-Trifluoroethyl-2-methylprop-2-ene-1-sulfo-
nate (5). Same procedure as for the preparation of 3.
The crude product 3 in CH3CN solution was mixed
with trifluoroethanol (2.2 g, 22.1 mmol, 1.3 equiv) and
triethylamine (2.8 mL, 20 mmol, 1.2 equiv) dissolved in
CH3CN at K20 8C. The reaction mixture was allowed to
reach 20 8C within 2 h. The mixture was poured into ice-
water (40 mL) and extracted with CH2Cl2 (25 mL, 3 times).
The combined organic extracts were washed with brine
(20 mL), dried (Na2SO4) and the solvent eliminated under
reduced pressure and under reflux. The residue was purified
by FC (9:1 light petroleum ether/EtOAc, RfZ0.3) giving
(2.6 g, 70%), colourless oil. UV (CH3CN): 202 (1450). IR
(film): 3093, 2984, 2923, 1651, 1452, 1414, 1370, 1284,
4.1.7. Olefination procedure 3. S-(20,20,20-Trifluoroethyl)
2-methylprop-2-ene-1-sulfothioate 6 (0.54 g, 2.3 mmol,
1 equiv) was dissolved in 10 mL of THF under Ar
atmosphere. After cooling to K78 8C, the aldehyde or
aromatic ketone (2.99 mmol, 1.3 equiv) was added followed
1
1177, 1044, 962. H NMR (400 MHz, CDCl3): 5.28 (br.s,
3
1H, H–C(3)), 5.19 (br.s, 1H, H–C(3)), 4.52 (q, 2H, JZ
8.0 Hz, H–C(10)), 3.92 (s, 2H, H–C(1)), 1.98 (s, 3H, Me–

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S. R. Dubbaka, P. Vogel / Tetrahedron 61 (2005) 1523–1530
by t-BuLi (1.5 M in pentane) (1.8 mL, 2.76 mmol,
1.2 equiv). The mixture was allowed to reach 20 8C within
2 h. The mixture was poured into brine (30 mL) and
extracted with ether (20 mL, 3 times). The combined
organic extracts were dried (Na2SO4) and the solvent
evaporated under reduced pressure and under reflux with
cooling to K20 8C. The residue was purified by FC,
affording mixtures of the corresponding (Z)/(E)-1-aryl-2-
methyl-1,3-dienes (8, 14).
(9792), 240 (11,960), 212 (9697). IR (KBr): 2920, 1610,
1592, 1509, 1449, 1334, 1109, 966, 951, 909, 873, 831, 751,
1
3
684, 524. H NMR (400 MHz, CDCl3): 8.19 (d, 2H, JZ
3
9.0 Hz, H–C(30)), 7.57 (d, 2H, JZ9.0 Hz, H–C(20)), 7.02
3
3
(d, 1H, JZ16.0 Hz, H–C(2)), 6.57 (d, 1H, JZ16.0 Hz,
H–C(1)), 5.26 (br.s, 1H, H–C(4)), 5.24 (br.s, 1H, H–C(4)),
2.00 (s, 3H, Me–C(3)).13C NMR (100.6 MHz, CDCl3):
146.6, 144.8, 141.1, 136.2, 129.7, 127.2, 123.4, 118.3, 22.3.
CI-MS (NH3): 207 (50, [MCNH4]C), 190 (31, [MCH]C),
189 (74, [MC%]), 172 (32), 142 (84), 128 (100), 115 (15).
Anal. Calcd for C11H11O2N (189.21): C 69.83; H 5.86;
found: C 69.98; H 6.00.
4.1.8. [(1Z)-3-Methylbuta-1,3-dienyl]benzene ((Z)-8a)
and [(1E)-3-methylbuta-1,3-dienyl]benzene ((E)-8a).
Applying the olefination procedure 1, yield 1.6 g (64%),
of a 0.74:1 mixture of (Z)-8a and (E)-8a.
4.1.10. 1-Chloro-40-[(1Z)-3-methylbuta-1,3-dienyl]ben-
zene ((Z)-8d) and 1-chloro-40-[(1E)-3-methylbuta-1,3-
dienyl]benzene ((E)-8d). Applying the olefination pro-
cedure 1, yield: 2.1 g (70%), 1.15:1 mixture of (Z)-8d and
(E)-8d.
Data of (Z)-8a. Same spectral data as those reported for this
compound.27 Purification by FC (heptane, RfZ0.5). Light
yellowish oil. IR (film): 3026, 2925, 2855, 1606, 1494,
1
1454, 1378, 960, 886, 753. H NMR (400 MHz, CDCl3):
7.29–7.27 (m, 5H, H–Ph), 6.46 (d, 1H, 3JZ12.5 Hz,
Data of (Z)-8d. Purification by FC (hexane, RfZ0.63). Light
yellowish oil. UV (CH3CN): 273 (5990), 269 (6000), 216
(5550), 208 (5290), 202 (5420), 194 (3800). IR (film): 2970,
3
H–C(2)), 6.20 (d, 1H, JZ12.5 Hz, H–C(1)), 5.03 (br.s,
1H, H–C(4)), 4.99 (br.s,1H, H–C(4)), 1.74 (s, 3H, Me–
C(3)). 13C NMR (100.6 MHz, CDCl3): 142.1, 137.9, 132.7,
129.4, 128.9, 127.8, 126.8, 116.9, 22.2. CI-MS (NH3): 145
(100, [MCH]C), 129 (8), 128 (6).
1
1591, 1516, 1489, 1454, 1218, 1091. H NMR (400 MHz,
3
CDCl3): 7.28 (s, 4H, H–C(20), H–C(30)), 6.37 (d, 1H, JZ
12.2 Hz, H–C(2)), 6.19 (d, 1H, 3JZ12.2 Hz, H–C(1)), 5.02
(br.s, 2H, H–C(4)), 1.74 (s, 3H, Me–C(3)). 13C NMR
(100.6 MHz, CDCl3): 142.0, 136.7, 133.8, 132.9, 130.6,
128.5, 128.4, 117.6, 22.5. CI-MS (NH3): 179 (10, [MC
H]C), 178 (21, [MC%]), 163 (26), 143 (100), 128 (77), 115
(8). HR-CIMS m/z (MCH)C calcd 178.0549, obsd
178.0546.
Data of (E)-8a. Same spectral data as those reported for this
compound.27 Purification by FC (heptane RfZ0.38). Light
yellowish oil. UV (CH3CN): 274 (6596), (4760), 206
(4770), 199 (5846). IR (film): 3026, 2924, 2853, 1606, 1493,
1454, 1376, 960, 886, 753, 694. 1H NMR (400 MHz,
3
4
CDCl3): 7.45 (dd, 2H, JZ8.0 Hz, JZ1.2 Hz, H–C(20)),
3
4
7.45 (td, 2H, JZ8.0 Hz, JZ1.2 Hz, H–C(30)), 7.24 (tt,
Data of (E)-8d. Purification by FC (hexane, RfZ0.55).
Colourless powder, mp 47.5–48.5 8C. UV (CH3CN): 286
(11,550), 279 (10,050), 219 (7900), 208 (6550), 194 (8970).
IR (KBr): 2941, 1626, 1605, 1490, 1455, 1434, 1404, 1376,
1285, 1180, 1089, 1039, 1011, 969, 941, 898, 867. 1H NMR
(400 MHz, CDCl3): 7.37 (d, 2H, 3JZ8.3 Hz, H–C(20)), 7.31
3
4
3
1H, JZ8.0 Hz, JZ1.2 Hz, H–C(40)), 6.89 (d, 1H, JZ
16.0 Hz, H–C(2)), 6.55 (d, 1H, 3JZ16.0 Hz, H–C(1)), 5.13
(br.s, 1H, H–C(4)), 5.09 (br.s, 1H, H–C(4)), 1.99 (s, 3H,
Me–C(3)). 13C NMR (100.6 MHz, CDCl3): 142.0, 137.4,
131.6, 128.7, 128.6, 127.4, 126.4, 117.4, 18.6. CI-MS
(NH3): 145 (100, [MCH]C), 129 (36), 128 (21), 115 (3).
3
3
(d, 2H, JZ8.3 Hz, H–C(20)), 6.86 (d, 1H, JZ16.0 Hz,
H–C(2)), 6.49 (d, 1H, 3JZ16.0 Hz, H–C(1)), 5.16 (br.s, 1H,
H–C(4)), 5.13 (br.s, 1H, H–C(4)), 1.99 (s, 3H, Me–C(3)).
13C NMR (100.6 MHz, CDCl3): 142.2, 136.3, 133.4, 132.7,
129.2, 128, 127.8, 118.3, 18.9. CI-MS (NH3): 179 (100,
[MCH]C), 178 (25, [MC%]), 163 (5), 143 (22), 128 (22),
115 (3). HRMS (MALDI-TOF): (C11H11ClNaC), calcd
201.0447; found: 201.0782.
4.1.9. 1-[(1Z)-3-Methylbuta-1,3-dienyl]-40-nitrobenzene
((Z)-8b) and 1-[(1E)-3-methylbuta-1,3-dienyl]-40-nitro-
benzene ((E)-8b). Applying the olefination procedure 1,
yield: 2.3 g (70%), 1.4:1 mixture of (Z)-8b and (E)-8b.
Data of (Z)-8b. Purification by FC (9:1 pentane/CH2Cl2,
RfZ0.24). Yellow oil. UV (CH3CN): 339 (7340), 334
(7770), 322 (8220), 308 (8340), 295 (7670), 289 (6760), 238
(7030), 225 (7040), 213 (5870), 205 (5300). IR (film): 3081,
4.1.11. 1-[(1Z)-3-Methylbuta-1,3-dienyl]-40-(trifluoro-
methyl)benzene ((Z)-8e) and 1-[(1E)-3-methylbuta-1,3-
dienyl]-40-(trifluoromethyl)benzene ((E)-8e). Applying
the olefination procedure 1, yield 2.2 g (60%), 0.95:1
mixture of (Z)-8e and (E)-8e.
1
2971, 1597, 1516, 1449, 1343, 1108, 900, 856. H NMR
3
(400 MHz, CDCl3): 8.145 (d, 2H, JZ8.6 Hz, H–C(30)),
3
7.49 (d, 2H, JZ8.6 Hz, H–C(20)), 6.43 (d, 1H, 3JZ
12.2 Hz, H–C(2)), 6.32 (d, 1H, 3JZ12.2 Hz, H–C(1)), 5.04
(s, 1H, H–C(4)), 5.01 (s, 1H, H–C(4)), 1.75 (s, 3H, Me–
C(3)).13C NMR (100.6 MHz, CDCl3): 146.6, 144.9, 141.0,
136.2, 129.6, 127.2, 123.4, 118.3, 22.3. CI-MS (NH3): 207
(43, [MCNH4]C), 190 (7, [MCH]C), 189 (15, [MC%]), 172
(56), 142 (90), 128 (100), 115 (17). HRCIMS m/z (MCH)C
calcd 189.0789, obsd 189.0792.
Data of (Z)-8e. Purification by FC (hexane, RfZ0.75).
Colourless oil. IR (KBr): 2972, 1616, 1406, 1325, 1165,
1126, 1108, 1067, 1017, 899, 875, 841. 1H NMR (400 MHz,
3
CDCl3): 7.55 (d, 2H, JZ8.3 Hz, H–C(30)), 7.44 (d, 2H,
3
3JZ8.3 Hz, H–C(20)), 6.44 (d, 1H, JZ12.5 Hz, H–C(2)),
6.27 (d, 1H, 3JZ12.5 Hz, H–C(1)), 5.02 (br.s, 2H, H–C(4)),
1.74 (s, 3H, Me–C(3)). 13C NMR (100.6 MHz, CDCl3):
141.8, 141.5, 134.8, 129.2, 129.1, 127.9, 125.0, 124.3,
117.8, 22.3. CI-MS (NH3): 230 (6, [MCNH4]C), 213 (100,
[MCH]C), 212 (15, [MC%]), 177 (8), 143 (9), 128 (10), 115
Data of (E)-8b. Purification by FC (9:1 pentane/CH2Cl2,
RfZ0.17). Yellow solid, mp 70.5–80.5 8C. UV (CH3CN):
360 (11,150), 333 (14,197), 329 (14,621), 321 (14,569), 260

S. R. Dubbaka, P. Vogel / Tetrahedron 61 (2005) 1523–1530
1529
(5), 99 (19), 76 (32). HRMS (MALDI-TOF):
(C12H11F3NaC), calcd 235.0711; found: 235.0719.
142.4, 135.0, 133.7, 132.8, 131.5, 128.3, 127.7, 126.7,
126.0, 125.8, 125.6, 125.2, 123.6, 118.7, 18.8. CI-MS
(NH3): 195 (10, [MCH]C), 194 (30, [MC%]), 179 (100),
152 (22), 139 (6), 128 (10), 115 (7), 89 (6). HR-CIMS m/z
(MCH)C calcd 194.1092, obsd 194.1095.
Data of (E)-8e. Purification by FC (hexane, RfZ0.64).
Colourless powder, mp 62–63 8C. IR (KBr): 2948, 1611,
1455, 1441, 1414, 1327, 1261, 1166, 1131, 1108, 1067,
1015, 969, 903, 873, 826. 1H NMR (400 MHz, CDCl3): 7.58
Data of (E)-8g. Purification by FC (hexane, RfZ0.32). Light
yellow oil. IR (film): 3058, 2970, 1601, 1510, 1434, 1394,
1345, 1170, 1016, 961, 887, 794, 773. 1H NMR (400 MHz,
3
(d, 2H, JZ8.3 Hz, H–C(30)), 7.52 (d, 2H, 3JZ8.3 Hz,
3
H–C(20)), 6.95 (d, 1H, JZ16.1 Hz, H–C(2)), 6.55 (d, 1H,
3JZ16.1 Hz, H–C(1)), 5.20 (br.s, 1H, H–C(4)), 5.17 (br.s,
1H, H–C(4)), 2.00 (s, 3H, Me–C(3)). 13C NMR
(100.6 MHz, CDCl3): 141.6, 140.9, 134.1, 127.2, 126.5,
125.5, 124.0, 119.1, 18.6. CI-MS (NH3): 230 (36, [MC
NH4]C), 213 (100, [MCH]C), 212 (82, [MC%]), 177 (13),
143 (15), 128 (26), 115 (4). HR-CIMS m/z(MCH)C calcd
212.0812, obsd 212.0821.
CDCl3): 8.18 (d, 1H, JZ8.32 Hz, H–C(80)), 7.88 (d, 1H,
3
3
3JZ8.0 Hz, H–C(50)), 7.79 (d, 1H, JZ8.0 Hz, H–C(80)),
3
7.68 (d, 1H, JZ7.0 Hz, H–C(20)), 7.51 (m, 3H, H–C(a
nap), 7.33 (d, 1H, 3JZ15.6 Hz, H–C(2)), 6.95 (d, 1H, 3JZ
15.6 Hz, H–C(1)), 5.20 (br.s, 1H, H–C(4)), 5.16 (br.s, 1H,
H–C(4)), 2.12 (s, 3H, Me–C(3)). 13C NMR (100.6 MHz,
CDCl3): 142.8, 135.4, 135.1, 134.1, 131.7, 129.0, 128.2,
126.4, 126.1, 126.0, 125.9, 124.1, 123.9, 118.1, 19.2. CI-MS
(NH3): 195 (56, [MCH]C), 194 (60, [MC%]), 179 (100),
152 (10), 139 (15), 128 (23), 115 (10), 89 (3). HRMS
(MALDI-TOF): (C15H14NaC), calcd 217.0993; found:
217.0997.
4.1.12. 1-Methyl-40-[(1Z)-3-methylbuta-1,3-dienyl]ben-
zene ((Z)-8f) and 1-methyl-40-[(1E)-3-methylbuta-1,
3-dienyl]benzene ((E)-8f). Applying the olefination pro-
cedure 1, yield 1.6 g (60%), 0.66:1 mixture of (Z)-8f and
(E)-8f.
4.1.14. 1-Methoxy-40-[(1E)-3-methylbuta-1,3-dienyl]-
benzene ((E)-8h). Applying the olefination procedure 1,
yield 1.2 g, (40%), !1:99 mixture of (Z)-8h and (E)-8h.
Data of (Z)-8f. Purification by FC (hexane, RfZ0.62).
Colourless oil. UV (CH3CN): 261 (9150), 215 (9790), 203
(11,020). IR (film): 2921, 1596, 1512, 1454, 1113, 893, 820.
1H NMR (400 MHz, CDCl3): 7.24 (d, 2H, 3JZ8.0 Hz,
Data of (E)-8h. Purification by FC (9:1 pentane/CH2Cl2,
RfZ0.36). Colourless powder, mp 69.5–70.5 8C. UV
(CH3CN): 296 (28,050), 291 (28,100), 288 (28,900), 281
(28,050), 275 (26,000), 222 (17,300), 206 (11,900), 198
(13,700). IR (KBr): 2976, 2937, 1645, 1458, 1383, 1351,
3
H–C(20)), 7.1 (d, 2H, JZ8.0 Hz, H–C(30)), 6.41 (d, 1H,
3
3JZ12.3 Hz, H–C(2)), 6.15 (d, 1H, JZ12.3 Hz, H–C(1)),
5.02 (br.s, 1H, H–C(4)), 4.98 (br.s, 1H, H–C(4)), 2.35 (s,
3H, Me–C(10)), 1.75 (s, 3H, Me–C(3)). 13C NMR
(100.6 MHz, CDCl3): 142.3, 136.6, 135.1, 132.3, 129,
128.9, 128.6, 116.7, 22.3, 22.5. CI-MS (NH3): 159 (13,
[MCH]C), 158 (20, [MC%]), 143 (100), 128 (52), 115 (10).
HR-CIMS m/z (MCH)C calcd 158.1088, obsd 158.1095.
1
1330, 1202, 1143, 1127, 1019, 937, 790, 711. H NMR
(400 MHz, CDCl3): 7.38 (d, 2H, 3JZ8.6 Hz, H–C(30)), 6.87
3
3
(d, 2H, JZ8.6 Hz, H–C(20)), 6.77 (d, 1H, JZ16.0 Hz,
3
H–C(2)), 6.5 (d, 1H, JZ16.0 Hz, H–C(1)), 5.07 (br.s, 1H,
H–C(4)), 5.03 (br.s, 1H, H–C(4)), 3.83 (s, 3H, OMe), 1.97
(s, 3H, Me–C(3)). 13C NMR (100.6 MHz, CDCl3): 159.1,
142.2, 130.2, 129.7, 128.2, 127.6, 116.3, 114.1, 55.3, 18.7.
CI-MS (NH3): 175 (10, [MCH]C), 174 (46, [MC%]), 159
(74), 144 (22), 128 (34), 115 (19), 84 (100). HR-CIMS m/z
(MCH)C calcd 174.1044, obsd 174.1054.
Data of (E)-8f. Purification by FC (hexane, RfZ0.35).
Colourless powder, mp 36.5–37.5 8C. UV (CH3CN): 302
(7070), 296 (7030), 288 (7050), 277 (6850), 257 (6800), 227
(6200), 209 (5270). IR (KBr): 2921, 2358, 1601, 1513,
1435, 1375, 1114, 967, 885, 865, 810, 535. 1H NMR
(400 MHz, CDCl3): 7.34 (d, 2H, 3JZ8.0 Hz, H–C(20)), 7.14
3
3
(d, 2H, JZ8.3 Hz, H–C(30)), 6.85 (d, 1H, JZ16.0 Hz,
H–C(2)), 6.53 (d, 1H, 3JZ16.0 Hz, H–C(1)), 5.10 (br.s, 1H,
H–C(4)), 5.05 (br.s, 1H, H–C(4)), 2.35 (s, 3H, Me–C(10)),
1.98 (s, 3H, Me–C(3)). 13C NMR (100.6 MHz, CDCl3):
142.6, 137.6, 134.9, 131.1, 129.7, 129.0, 126.8, 117.2,
21.62, 19.01. CI-MS (NH3): 159 (71, [MCH]C), 158 (43,
[MC%]), 143 (100), 128 (57), 115 (15). HRMS (MALDI-
TOF): (C11H14NaC), calcd 181.0993; found: 181.0968.
4.1.15. [(1Z)-3-Methyl-1-(trifluoromethyl)buta-1,3-di-
enyl]benzene ((Z)-14a). Applying the olefination procedure
1, yield 1.7 g, 48%. Purification by FC (pentane, RfZ0.85).
Light yellow oil. UV (CH3CN): 280 (1900), 233 (2750). IR
(film): 3063, 2926, 2856, 1607, 1532, 1455, 1346, 1298,
1277, 1167, 1117, 1026, 930, 855, 708. 1H NMR (400 MHz,
CDCl3): 7.44–7.26 (m, 5H, H–C(Ph)), 6.85 (br.s, 1H,
H–C(2)), 5.16 (br.s, 2H, H–C(4)), 1.91 (s, 3H, Me–C(3)).
13C NMR (100.6 MHz, CDCl3): 144.9 (s, C(3)), 140.1 (s,
C(10)), 130.6, 128.9, 128.4, 128.0, 127.4, 124.2, 122, 22.7.
CI-MS (NH3): 230 (5, [MCNH4]C), 213 (3, [MCH]C),
212 (5, [MC%]), 194 (15), 177 (25), 150 (82), 136 (100), 104
(14), 93 (23). HRMS (MALDI-TOF): (C12H11F3NaC),
calcd 235.0711; found: 235.0789.
4.1.13. 1-[(1Z)-3-Methylbuta-1,3-dienyl]naphthalene
((Z)-8g) and 1-[(1E)-3-methylbuta-1,3-dienyl]naphtha-
lene ((E)-8f). Applying the olefination procedure 1, yield
1.8 g, (54%), 0.33:1 mixture of (Z)-8g and (E)-8f.
Data of (Z)-8g. Purification by FC (hexane, RfZ0.39).
Colourless oil. IR (film): 3057, 2970, 1600, 1506, 1434,
1394, 1344, 1168, 1024, 962, 890, 793, 774. 1H NMR
(400 MHz, CDCl3): 8.00–7.5 (m, 7H, H–C(a nap)), 6.86
(d, 1H, JZ12.5 Hz, H–C(2)), 6.53 (d, 1H, JZ12.5 Hz,
H–C(1)), 5.05 (br.s, 1H, H–C(4)), 4.96 (br.s, 1H, H–C(4)),
1.46 (s, 3H, Me–C(3)). 13C NMR (100.6 MHz, CDCl3):
4.1.16. {(1Z)-1-[Chloro(difluoro)methyl]-3-mthylbuta-
1,3-dienyl}benzene ((Z)-14b). Using olefination procedure
1, yield 1.5 g, (40%). Purification by FC (pentane, RfZ
0.64). Light yellow oil. UV (CH3CN): 281 (1990), 225
(2900). IR (film): 3061, 2926, 2856, 1636, 1607, 1497,
3
3
1
1445, 1382, 1264, 1229, 1161, 1115. H NMR (400 MHz,

1530
S. R. Dubbaka, P. Vogel / Tetrahedron 61 (2005) 1523–1530
10. (a) Julia, M.; Paris, J. Tetrahedron Lett. 1973, 4833.
´
(b) Kocienski, P. J.; Lythgoe, B.; Ruston, S. J. Chem. Soc.,
CDCl3): 7.48–7.28 (m, 5H, H–C(Ph)), 6.82 (br.s, 1H,
H–C(2)), 5.15 (br.s, 2H, H–C(4)), 1.91 (s, 3H, Me–C(3)).
13C NMR (100.6 MHz, CDCl3): 144.1, 139.8, 133.7, 130.9,
128.6, 127.8, 127.6, 123.9, 122.2, 21.3. CI-MS (NH3): 246
(6, [MCNH4]C), 213 (21, [MCH]C), 228 (43, [MC%]), 209
(48), 193 (73), 177 (86), 143 (100), 128 (77), 115 (16), 91
(10). HRMS (MALDI-TOF): (C12H11F2ClNaC), calcd
251.0415; found: 251.0482.
Perkin Trans. 1 1978, 829.
11. Corey, E. J.; Durst, T. J. Am. Chem. Soc. 1968, 90, 5548.
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13. Kagabu, S.; Hara, K.; Takahashi J. Chem. Soc., Chem.
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14. Kagabu, S.; Shimizu, C.; Takahashi, J.; Hara, K.; Koketsu, M.;
Ishida, M. Bull. Soc. Chim. Fr. 1992, 129, 435.
15. (a) Dubbaka, S. R.; Vogel, P. Chem. Eur. J 2004, in press.
(b) Kasahara, A.; Izumi, T.; Kudou, N.; Azami, H.;
Yamamato, S. Chem. Ind. (London) 1988, 51. (c) Miura, M.;
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Acknowledgements
This work was supported by the Swiss National Science
Foundation (Grant No. 2000-20100002/1). We are also
´
grateful to Mr. M. Rey and F. Sepulveda for their technical
assistance and to Prof. M. Schlosser for hosting us in his
laboratories.
16. (a) Dubbaka, S. R.; Vogel, P. J. Am. Chem. Soc. 2003, 125,
15292. (b) Dubbaka, S. R.; Steunenberg, P.; Vogel, P. Synlett
2003, 123.
17. Dubbaka, S. R.; Vogel, P. Org. Lett. 2004, 6, 95.
18. Dubbaka, S. R.; Vogel, P. Adv. Synth. Catal. 2004, in press.
19. Nader, B. S.; Cordova, J. A.; Reese, K. E.; Powell, C. L.
J. Org. Chem. 1994, 59, 2898.
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´
6. Blakemore, P. R.; Cole, W. J.; Kocienski, P. J.; Morley, A.
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