Formation of P-Ylide under Neutral and Metal-Free Conditions: Transformation of Aziridines and Epoxides to Conjugated Dienes in the Presence of Phosphine

Article abstract of DOI:10.1021/jo0354286

A general approach to formation of the P-ylide from the reaction of aziridines or epoxides with organophosphine under neutral and metal-free conditions is realized. Conjugated diene derivatives based on this kind of P-ylide were prepared in a facile and c

Full text of DOI:10.1021/jo0354286

F or m a tion of P -Ylid e u n d er Neu tr a l a n d Meta l-F r ee Con d ition s:
Tr a n sfor m a tion of Azir id in es a n d Ep oxid es to Con ju ga ted Dien es
In th e P r esen ce of P h osp h in e
Ren-Hua Fan,† Xue-Long Hou,*,†,‡ and Li-Xin Dai‡
State Key Laboratory of Organometallic Chemistry and Shanghai-Hong Kong J oint Laboratory in
Chemical Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Road, Shanghai 200032, China
xlhou@mail.sioc.ac.cn
Received September 30, 2003
A general approach to formation of the P-ylide from the reaction of aziridines or epoxides with
organophosphine under neutral and metal-free conditions is realized. Conjugated diene derivatives
based on this kind of P-ylide were prepared in a facile and convenient way.
Ever since the discovery of the Wittig reaction, one of
the most important reactions in organic chemistry, about
50 years ago,1 ylide reactions have become powerful tools
in organic synthesis, especially for the synthesis of
alkenes and small-ring compounds.2 However, there are
several drawbacks associated with ylide reactions. For
example, difficulty is encountered sometimes in the
formation of the salt and the use of strong base is
necessary to form an ylide. Many efforts have been made
to find new procedures to overcome the difficulties in the
formation of salt.3 Aggarwal developed an efficient salt
formation procedure successfully using a metal-catalyzed
carbene transfer method,3d,f-j and Tang realized catalytic
Wittig-type reactions in an elegant way.3k,l,n Very re-
cently, Lu provided an excellent example of a catalytic
C-P ylide reaction.3p However, there is still a need to
develop new ylide formation methodology under more
practical conditions.
typical methods for the construction of conjugated dienes,
but strong bases were usually necessary. The conjugated
dienes could also be prepared from coupling reactions,5g-i
but metal compounds and harsh reaction conditions were
needed. Recently, small-ring heterocyclic compounds such
as epoxides and aziridines have become very popular in
organic synthesis not only as building blocks but also as
synthetic intermediates.6 Although a variety of transfor-
mations for epoxides or aziridines are well documented,
attention focuses mainly on their direct transformations,
especially the nucleophilic ring-opening reaction.7,8 It
remains a challenge to find a new transformation pattern
of these three-membered heterocyclic compounds to make
(3) (a) Oda, R.; Kawabata, T.; Tanimoto S. Tetrahedron Lett. 1964,
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Shi, L. L.; Yang, J . H. Tetrahedron Lett. 1987, 28, 2159. (d) Shi, L. L.;
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On the other hand, the conjugated dienes as subunits
appeared in many natural and synthetic biologically
active compounds, and they also served as the versatile
intermediates in organic synthesis.4 All of these facts
have promoted many studies on their construction.5
Wittig reactions5a-c or elimination reactions5d-f are the
* Corresponding author.
† Shanghai-Hong Kong J oint Laboratory in Chemical Synthesis.
‡ State Key Laboratory of Organometallic Chemistry.
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10.1021/jo0354286 CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/07/2004
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689

Fan et al.
SCHEME 1. P r op osed In ter m ed ia te
them more useful synthetic intermediates that fully
deserve a prominent place in the arsenal of the organic
chemist. In the course of our studies on the synthesis of
aziridines and epoxides and their applications in organic
synthesis,9 we developed a novel phosphine-mediated
ring-opening reaction of various aziridines and epoxides
with a wide range of nucleophiles. The mechanism
studies showed that the P-ylide would be a possible
intermediate. We describe here the investigations on the
novel ylide formation procedure from aziridines or ep-
oxides under neutral and metal-free conditions and its
use in the formation of dienes, as well as important
aspects of the reaction mechanism.
In our previous publications,9i-k we found that organo-
phosphine10 could attack aziridines and epoxides to form
the corresponding betaine intermediate A1.10b We sup-
posed that an ylide intermediate A2 might be formed
from the betaine A1 after an inter- or intra-hydrogen
shift reaction (Scheme 1).10a,11 If it is possible, an umpol-
ung would occur so that the electrophilic carbon atom of
aziridines or epoxides would become a nucleophilic
center.
To prove the existence of the ylide intermediate A2,
benzaldehyde was introduced into the mixture of aziri-
dine 1a with tributylphosphine in CH3CN. After 48 h at
reflux, aziridine 1a disappeared but no corresponding
expected allylic amine was obtained. To our surprise, an
unexpected conjugated diene compound was isolated in
43% yield even in the absence of any base (eq 1). After
considerable experimentation in various reaction condi-
tions, we found that t-BuOH was the optimal solvent,
giving the highest yield (75%). It is noted that the
reaction could also proceed in water at 40 °C (48% yield)
or in solvent-free conditions at 100 °C (53% yield). When
PPh3 was used instead of PBu3, the yield of the reaction
1a with benzaldehyde decreased to 55%.
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To explore the generality of this transformation, a
variety of substrates and carbonyl compounds were
examined. As shown in Table 1, not only the aziridines
but also the epoxides (entries 5, 8, and 10), both easily
derived from acyclic or cyclic precursors, were suitable
substrates. All carbonyl compounds we tested, aromatic
and aliphatic, aldehydes and ketones, worked well in
these diene formation reactions. The yields of dienes from
the aziridines or epoxides derived from acyclic precursors
are higher than those from other substrates derived from
cyclic precursors. The reactions gave predominantly
conjugated dienes with (E,E)- or (E)-configuration. The
stereochemistry of products was determined by 1H NMR.
In our reactions, the conjugated dienes were formed
in a neutral and metal-free conditions. The yields in some
reactions were not high but acceptable compared with
those in other methods. It is worth noting that, because
most of the epoxides or aziridines in Table could be
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690 J . Org. Chem., Vol. 69, No. 3, 2004

Formation of P-Ylide under Neutral and Metal-Free Conditions
TABLE 1. Tr a n sfor m a tion of Azir id in es or Ep oxid es to th e Con ju ga ted Dien es in th e P r esen ce of Tr ibu tylp h osp h in ea
a
b
Bu3P was purified by distillation from CuI, and the reactions were carried out at reflux in t-BuOH under Ar. Isolated yields based
on aziridines or epoxides. c Ratios of isomers were determined at 300 MHz by 1H NMR.
simply prepared from the corresponding alkenes in one
step,7 this approach offered a two-step transformation
path from alkenes to the conjugated diene compounds
in reasonable total yield. For example, compound 2a B
was a compound isolated from natural product reticuli-
termes flavipes12 and can be obtained in 82% yield from
the reaction of aziridine 1a (entry 2), while aziridine 1a
was prepared from octadec-1-ene in 87% yield.
To understand the reaction pathway, aziridine 1d and
Bu3P were mixed in a 1:1.1 ratio in C6D6, and the mixture
(12) Howard, R. W.; McDaniel, C. A.; Blomquist, G. J . J . Chem. Ecol.
1978, 4, 233.
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Fan et al.
SCHEME 2. P or p osed Mech a n ism of F or m a tion of
Con ju ga ted Dien e Com p ou n d s fr om Azir id in es or
Ep oxid es in th e P r esen ce of P h osp h in e
under appropriate conditions (eq 5 and 6).
was stirred at room temperature until the disappearance
of aziridine 1d . The 31P NMR spectrum showed a new
signal at +34.4 ppm, in addition to the signal of Bu3P at
-31.4 ppm. The new signal was consistent with those of
similar phosphonium salts.13 With the addition of 120 mol
% PhCHO, the mixture was stirred at reflux for 48 h and
gave rise to the conjugated diene 2d A in 30% yield (eq
2). On the other hand, a phosphonium salt 3, which is
prepared from the reaction of aziridine 1d with tribu-
tylphosphine, was subjected to PhCHO in the presence
of 110 mol % NaH under the same reaction conditions to
give 2d A in 32% yield (eq 3). When PPh3 was used
instead of PBu3 in the reaction of aziridine 1a with
benzaldehyde, Ph3PO and TsNH2 were separated from
the reaction mixture. These experiments showed that a
phosphonium salt should be an intermediate in the
reaction and that the diene product might be formed via
a Wittig reaction and elimination.
Vedejs observed the formation of vinyl phosphonium
salt in the reaction of cycloheptene oxide and PPh2Me.14
Schweizer and co-workers reported some reactions of
VTB salt (VTB: vinyl triphenylphosphonium bromide),
in which an equilibrium between VTB salt and ylide
intermediate was observed (eq 7).15 It seems that the
structure of the possible intermediate A2 is similar to
that of the ylide intermediate in VTB salt reaction. It is
possible that intermediate A2 might convert into inter-
mediate A4 via a retro-VTB salt reaction and provide an
anion HX-, acting as base in formation of A5 (eq 8), which
then reacts with carbonyl compound to give diene as the
final product.3e To prove this possibility, phosphonium
salt 6, structurally similar to intermediate A4, was
treated with PhCHO in t-BuOH in the presence of 100
mol % TsNHNa or NaOH. Indeed, diene 7 was obtained
in moderate yield (eq 9).
On the basis of the above results, we supposed that
the reaction might proceed via the route depicted in
Scheme 2: phosphine attack on the aziridine or epoxide
to form phosphonium salt A1, followed by conversion into
ylide intermediate A2 after an inter- or intra-hydrogen
shift reaction.11 The ylide intermediate then reacts with
carbonyl compound followed by elimination of H2X to
form the final diene product.
According to the clues we have found, a plausible
reaction mechanism is proposed (Scheme 3). Phosphine
attacked the aziridine or epoxide to form a phosphonium
salt intermediate A1, which followed a hydrogen shift to
give rise to ylide A2. Then, a retro-VTB reaction occurred
to produce A4 and a HX- ion. HX- acted as a base to
deprotonate A4 to form A5 and the corresponding XH2.
A5 would isomerize to an allyl ylide intermediate A6,
If this mechanism were correct, allylic alcohol 4 and
amine 5, structurally similar to the intermediate A3,
would provide the corresponding diene product under the
reaction conditions. However, allylic alcohol 4 and amine
5 failed to be converted into the desired product (eq 4)
even when they were added to the reaction system
containing epoxide 1c or aziridine 1b and benzaldehyde
(14) Vedejs, E.; Snoble, K. A. J .; Fuchs, P. L. J . Org. Chem. 1973,
38, 1178.
(13) Albright, T. A.; Freeman, W. J .; Schweizer, E. E. J . Am. Chem.
Soc. 1975, 97, 2942.
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1966, 31, 467.
692 J . Org. Chem., Vol. 69, No. 3, 2004

Formation of P-Ylide under Neutral and Metal-Free Conditions
(M+, 0.7), 264 (5), 242 (12), 91 (100). Anal. Calcd for C25H48
C, 86.12; H, 13.88. Found: C, 86.17; H, 13.89.
:
SCHEME 3. P la u sible Mech a n ism of F or m a tion of
Dien es fr om Azir id in es or Ep oxid es via a n Allyl
Ylid e Rou te
1,1-P en ta m eth ylen e-1,3-n on a d eca d ien e (2a C). 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ ) 0.82-0.91 (m, 3H), 1.06-
1.45 (m, 26H), 1.54-1.57 (m, 6H), 2.04-2.27 (m, 6H), 5.35 [dt,
J ) 10.5, 7.3 Hz, 1H, (Z)], 5.59 [dt, J ) 14.5, 7.1 Hz, 1H, (E)],
5.74 [d, J ) 10.9 Hz, 1H, (E)], 6.02 [d, J ) 11.2 Hz, 1H, (Z)],
6.17-6.32 (m, 1H). IR (film): ν˜ ) 3026, 2955, 1465, 1447, 963
cm-1. EI-MS: m/z (%) 332 (27) [M+], 189 (40), 135 (60), 92
(100). Anal. Calcd for C25H44: C, 86.67; H, 13.33. Found: C,
86.39; H, 13.59.
1-P h en ylh ep t-1, 3-d ien e (2bA).16 1H NMR (300 MHz,
CDCl3, 25 °C, TMS): δ ) 0.84-0.95 (m, 3H), 1.41-1.54 (m,
2H), 2.10-2.18 [m, 2H, (E, E)], 2.25-2.33 [m, 2H, (E, Z)], 5.55
[dt, J ) 10.9, 7.7 Hz, 1H, (E, Z)], 5.84 [dt, J ) 14.7, 7.0 Hz,
1H, (E, E)], 6.14-6.27 (m, 1H), 6.45 [d, J ) 15.6 Hz, 1H, (E,
E)], 6.54 [d, J ) 15.4 Hz, 1H, (E, Z)], 6.77 [dd, J ) 15.7, 10.4
Hz, 1H, (E, E)], 7.09 [dd, J ) 15.6, 11.1 Hz, 1H, (E, Z)], 7.17-
7.32 (m, 5H). IR (film): ν˜ ) 3061, 3026, 2960, 1598, 1493, 1379,
987, 964 cm-1. EI-MS: m/z (%): 172 (57) [M+], 143 (49), 129
(100).
which reacted with carbonyl compound to produce con-
jugated diene as the final product.3e
From the above studies on the mechanism of the
transformation of aziridines or epoxides into the conju-
gated dienes, we found a novel ylide formation procedure
under metal-free and neutral conditions. And because the
phosphonium salt is formed in situ, it would provide a
chance to realize the ylide reaction in a catalytic mode.
In conclusion, a general approach to formation of
P-ylide under metal-free and neutral conditions is real-
ized. Conjugated diene derivatives were prepared from
aziridines or epoxides in the presence of organophosphine
based upon this kind of P-ylide in a facile and convenient
way. Studies on the further applications of this novel
methodology to ylide and its catalytic and asymmetric
version are in progress.
3-Ben zylid en e-cycloh exen e (2d A).17 1H NMR (300 MHz,
CDCl3, 25 °C, TMS): δ ) 1.69-1.88 (m, 2H), 2.16-2.25 (m,
2H), 2.43-2.48 (m, 1H), 2.65-2.70 (m, 1H), 5.89-5.97 (m, 1H),
6.20-6.22 [m, 1H, (E)], 6.23-6.25 [m, 1H, (Z)], 6.28 [br, 1H,
(E)], 6.64 [dt, J ) 10.2, 2.0 Hz, 1H, (Z)]. EI-MS: m/z (%) 170
(M+, 100), 155 (34), 141 (37), 91 (45).
Ben zylid en cycloh ep t-2-en (2gA).18 1H NMR (300 MHz,
CDCl3, 25 °C, TMS): δ ) 1.76-1.84 (m, 4H), 2.26-2.35 (m,
2H), 2.47-2.52 [m, 2H, (E)], 2.61-2.65 [m, 2H, (Z)], 5.78 [dt,
J ) 11.1, 5.7 Hz, 1H, (Z)], 5.91 [dt, J ) 11.1, 5.1 Hz, 1H, (E)],
6.20 [dd, J ) 12, 1.2 Hz, 1H, (Z)], 6.30 [s, 1H, (E)], 6.30-6.37
[m, 1H, (Z)], 6.39-6.41 [m, 1H, (E)], 7.15-7.35 (m, 5H).
3-Cycloh exylid en e-cycloh exen e (2fC). 1H NMR (300
MHz, CDCl3, 25 °C, TMS): δ ) 1.48-1.76 (m, 8H), 2.15-2.35
(m, 8H), 5.86-5.89 (m, 1H), 6.62 (s, 1H); ν˜ ) 3073, 3040, 2972,
1623, 1443 cm-1. EI-MS: m/z (%) 162 (0.4) [M+], 131 (14), 117
(10). Anal. δ ) C12H18: C, 88.89; H, 11.11. Found: C, 88.93;
H, 10.82.
Exp er im en ta l Section
Gen er a l Exp er im en ta l Con d ition s. All reactions were
performed under an atmosphere of either dry argon or nitrogen
using oven-dried glassware. Solvents were distilled under an
atmosphere of nitrogen before use. The commercially available
reagents were used as received without further purification.
Melting points are uncorrected. 1H NMR spectra were recorded
on a 300 MHz spectrometer, and the chemical shifts were
referenced to CHCl3 (δ 7.27) in CDCl3. Chemical shifts are
given in parts per million relative to tetramethylsilane as an
Mech a n ism Stu d y: P r ep a r a tion of P h osp h on iu m Sa lt
3. To a suspension of Bu3P (0.28 mL, 1.1 mmol) in ethanol (10
mL) was added 1d (252 mg, 1 mmol), giving a clear solution
after a few minutes. The mixture was then stirred for 36 h at
room temperature after which addition of 60% aqueous per-
chloric acid (3 mL) and cooling at 0 °C gave a solid product.
After a recrystallization from CH2Cl2, a good crystal was
obtained in 46% yield.
internal standard. IR spectra were measured in cm-1
.
Gen er a l P r oced u r e for Rea ction of Azir id in e or Ep -
oxid e w ith Ca r bon yl Com p ou n d in th e P r esen ce of
P h osp h in e. To a stirred solution of aziridine or epoxide 1 (0.5
mmol) and carbonyl compound (0.55 mmol) in corresponding
solvent (2.0 mL) was added phosphine (0.6 mmol) under argon,
and the resulting mixture was stirred at corresponding tem-
perature for 48-60 h. The solvent was removed in a vacuum,
and the crude product was purified by flash column chroma-
tography to provide the corresponding product.
1
Ch a r a cter iza tion of 3. H NMR (300 MHz, CDCl3, 25 °C,
TMS): δ ) 0.99-1.03 (m, 11H), 1.10-1.37 (m, 3H), 1.55-1.63
(m, 13H), 1.74-1.77 (m, 1H), 1.91-1.93 (m, 1H), 2.30-2.40
(m, 6H), 2.42 (s, 3H), 2.93-2.98 (m, 1H), 3.41-3.46 (m, 1H),
7.30 (d, J ) 8.0 Hz, 2H), 7.78 (d, J ) 8.2 Hz, 2H). 31P NMR
(162 MHz, CDCl3, 25 °C, 85% H3PO4): δ ) 35.46. EI-MS m/z
(%): 283 (100), 253 (6). Anal. Calcd for C25H45NO6PS: C, 54.19;
H, 8.19; N, 2.53. Found: C, 54.13; H, 7.93; N, 2.38.
Rea ction of P h osp h on iu m Sa lt 3 w ith P h CHO. To a
stirred solution of 3 (224 mg, 0.4 mmol) in t-BuOH (2.0 mL)
was added PhCHO (0.05 mL, 0.5 mmol) under argon, and the
resulting mixture was stirred at reflux for 24 h. No reaction
took place as detected by TLC.
Rea ction of P h osp h on iu m Sa lt 3 w ith P h CHO in th e
P r esen ce of Na H. To a stirred solution of 3 (224 mg, 0.4
mmol) in t-BuOH (2.0 mL) was added NaH (11 mg, 0.5 mmol)
under argon, and the resulting mixture was stirred at room
temperature for 1 h. Then, PhCHO (0.05 mL, 0.5 mmol) was
added, and the mixture was stirred at reflux for 24 h.
Compound 2d A was isolated in 32% yield.
1
1-P h en yln on a d eca -1,3-d ien e (2a A). H NMR (300 MHz,
CDCl3, 25 °C, TMS): δ ) 0.81 0.95 (m, 3H), 1.06-1.44 (m,
26H), 2.10-2.18 [m, 2H, (E, E)], 2.25-2.30 [m, 2H, (E, Z)],
5.52 [dt, J ) 10.5, 7.6 Hz, 1H, (E, Z)], 5.83 [dt, J ) 14.8, 7.0
Hz, 1H, (E, E)], 6.12-6.25 (m, 1H), 6.44 [d, J ) 15.9 Hz, 1H,
(E, E)], 6.53 [d, J ) 15.6 Hz, 1H, (E, Z)], 6.76 [dd, J ) 15.4,
10.6 Hz, 1H, (E, E)], 7.07 [dd, J ) 15.6, 11.2 Hz, 1H, (E, Z)],
7.16-7.29 (m, 5H). IR (film): ν˜ ) 3025, 2955, 1596, 1496, 1378,
985, 945 cm-1. EI-MS: m/z (%) 340 (20) [M+], 143 (49), 129
(100). Anal. Calcd for C25H40: C, 88.16; H, 11.65. Found: C,
87.93; H, 11.44.
P en ta cosa -7,9-d ien e (2a B). 1H NMR (300 MHz, CDCl3,
25 °C, TMS): δ ) 0.81-0.91 (M, 6H), 1.05-1.19 (M, 34H),
2.00-2.16 (m, 4H), 5.29 [dt, J ) 11.1, 7.6 Hz, 1H, (E,Z)], 5.39-
5.49 [m, 1H, (Z, E and Z, Z)], 5.51-5.59 (m, 1H), 5.65 [dt, J )
14.4, 6.6 Hz, 1H, (E,E)], 5.90-5.98 (m, 1H), 6.00-6.04 [m, 1H,
(E, E)], 6.23-6.34 [m, 1H, (E, Z; Z, E and Z, Z)]; IR (film): ν˜ )
3016, 2957, 1466, 1378, 953, 946 cm-1. EI-MS: m/z (%) 348
(16) Ousset, J . B.; Mioskowski, C.; Solladie, G. Synth. Commun.
1983, 13, 1193.
(17) Takaki, K.; Yasumura, M.; Negoro, K. J . Org. Chem. 1983, 48,
54.
(18) Vedejs, E.; Fuchs, P. L. J . Am. Chem. Soc. 1971, 93, 4070.
J . Org. Chem, Vol. 69, No. 3, 2004 693

Fan et al.
Test of Elim in a tion Rea ction of Allylic Alcoh ol 4 a n d
Am in e 5. To a stirred solution of 4 or 5 (1 mmol) in t-BuOH
(2.0 mL) was added Bu3P (0.28 mL, 1.1 mmol) under argon,
and the resulting mixture was stirred at reflux for 24 h. No
reaction took place as detected by TLC.
Test of Elim in a tion Rea ction of Allylic Alcoh ol 4 a n d
Am in e 5 in th e Rea ction System of Ep oxid e 1c or
Azir id in e 1b w ith P h CHO in th e P r esen ce of Bu 3P . To a
mixture of 1c or 1b (1 mmol) with PhCHO (0.11 mL, 1.1 mmol)
and Bu3P (0.28 mL, 1.1 mmol) in t-BuOH (2.0 mL) was added
4 or 5 (1 mmol) under argon, and the resulting mixture was
stirred at reflux for 24 h. The corresponding diene compound
2bA, 4, or 5 was recovered in >90% yield.
Rea ction of P h osp h on iu m Sa lt 6 w ith P h CHO in th e
P r esen ce of TsNHNa or Na OH. To a stirred solution of 6
(392 mg, 1 mmol) in t-BuOH (2.0 mL) was added TsNHNa or
NaOH (1.1 mmol) under argon, and the resulting mixture was
stirred at room temperature for 1 h. Then, PhCHO (0.1 mL, 1
mmol) was added, and the mixture was stirred at reflux for
24 h. 1-Phenyl-1,3-butadiene 7 was isolated in 46 and 52%
yields, respectively. 1H NMR of 7 (300 MHz, CDCl3, 25 °C,
TMS): δ ) 5.18 (d, J ) 10.8 Hz, 1H), 5.34 (d, J ) 16.9 Hz,
1H), 6.44-6.58 (m, 2H), 6.78 (dd, J ) 15.7, 10.7 Hz, 1H), 7.21-
7.45 (m, 5H).20
P r ep a r a tion of P h osp h on iu m Sa lt 6.19 2-Methylvinyl-
triphenylphosphonium bromide 6 was prepared by stirring a
solution of 5 g of salt allyltriphenylphosphonium bromide
in40 mL of dry pyridine containing 6 drops of triethylamine
at room temperature for 18 h. After removal of solvent,
purification was effected by adding 50 mL of boiling benzene,
adding enough methylene chloride to effect solution, cooling
to room temperature, and precipitating the salt with 20 mL
Ack n ow led gm en t. We are grateful for the financial
support from National Natural Sciences Foundation of
China, the Major Basic Research Development Program
(Grant G2000077506), National Outstanding Youth
Fund, Chinese Academy of Sciences and Science and
Technology Commission of Shanghai Municipality. R.H.F.
gratefully acknowledges Hong Kong Croucher Founda-
tion for a Studentship.
1
of ethyl acetate. Yield: 88%. H NMR of 6 (300 MHz, CDCl3,
25 °C, TMS): δ ) 2.35-2.39 (m, 3H), 2.58 (br, 1H), 6.59-6.75
J O0354286
(m, 1H), 7.56-7.90 (m, 15H). 31P NMR: δ ) 19.36.
(19) Tsukamoto, M.; Iio, H.; Tokoroyama, T. Tetrahedron Lett. 1987,
28, 4561.
(20) Avery, T. D.; Taylor, D. K.; Tiekink, E. R. T. J . Org. Chem. 2000,
65, 5531.
694 J . Org. Chem., Vol. 69, No. 3, 2004