An efficient method for the synthesis of enol ethers and enecarbamates. Total syntheses of isoindolobenzazepine alkaloids, lennoxamine and chilenine

Article abstract of DOI:10.1039/b706087d

An efficient method for the synthesis of enol ethers and enecarbamates has been developed based on catalytic hydrosilane reduction of α-phosphonoxy enol ethers and α-phosphonoxy enecarbamates. This method has been applied to the total syntheses of two iso

Full text of DOI:10.1039/b706087d

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An efficient method for the synthesis of enol ethers and enecarbamates. Total
syntheses of isoindolobenzazepine alkaloids, lennoxamine and chilenine†
Haruhiko Fuwa* and Makoto Sasaki*
Received 23rd April 2007, Accepted 8th May 2007
First published as an Advance Article on the web 16th May 2007
DOI: 10.1039/b706087d
An efficient method for the synthesis of enol ethers and
enecarbamates has been developed based on catalytic hy-
drosilane reduction of a-phosphonoxy enol ethers and a-
phosphonoxy enecarbamates. This method has been applied
to the total syntheses of two isoindolobenzazepine alkaloids,
lennoxamine and chilenine.
Enol ethers and enecarbamates play significant roles in organic
synthesis as useful reactants in a variety of fundamental trans-
formations including the Heck reaction,1 radical cyclisation,2
olefin metathesis,3 Diels–Alder cycloaddition,4 and oxidation.5
We have recently found that a-phosphonoxy enol ethers and a-
phosphonoxy enecarbamates act as versatile reactants in several
palladium(0)-catalysed transformations and we have developed
efficient strategies for the synthesis of marine polyether natural
products6 and nitrogen-containing heterocycles.7 In this communi-
cation, we describe a palladium(0)-catalysed hydrosilane reduction
of a-phosphonoxy enol ethers and a-phosphonoxy enecarbamates
as a new, mild method for the synthesis of enol ethers and enecar-
bamates. Its application to the total syntheses of ( )-lennoxa-
mine and ( )-chilenine, isoindolobenzazepine alkaloids, is also
demonstrated.
Scheme 1 Concept of the present work.
It is well known that enol triflate (e.g., 1) can be reduced with
As a model substrate, we used a-phosphonoxy enol ether
n-Bu3SnH and a catalytic amount of Pd(PPh3)4 to give an olefin
7, which was derived from the corresponding propionate by
(e.g., 2) (Scheme 1, eqn 1).8 However, instability of enol triflates is
treatment with LDA followed by (PhO)2P(O)Cl. Reduction of
7 was screened with several hydrosilanes in the presence of a
palladium catalyst under various conditions, and the results are
summarised in Table 1. Treatment of 7 with 5 equiv. of Et3SiH
sometimes problematic, and it is generally difficult to synthesise
a-heteroatom substituted acyclic enol triflates.6a In addition, their
preparation requires expensive reagents such as trifluoromethane-
and catalytic Pd(PPh3)4 in DMF at 60 ◦C provided (Z)-enol ether
sulfonic anhydride or N-phenyl bis(trifluoromethanesulfonimide).
On the other hand, the phosphate counterpart is much easier
8a along with its (E)-isomer 8b in 86% combined yield (8a–8b =
to handle due to its stability and can be prepared by the less
ca. 2 : 1) (entry 1). Unfortunately, stereochemical scrambling of
expensive diphenylphosphoryl chloride.‡ Greene and co-workers
the double bond was significant in this case. We then surveyed
have reported that treatment of acyclic a-phosphonoxy enol ether
several reaction conditions. Among the hydrosilanes examined,
Me2PhSiH provided the best result (entry 2). Thus, exposure
3 with Et3Al and Pd(PPh3)4 gave enol ether 4 in moderate yield
(eqn 2).9a This method has been applied to the synthesis of a
of 7 to 5 equiv. of Me2PhSiH and 10 mol% of Pd(PPh3)4 in
cyclic enecarbamate.9b On the other hand, we envisioned that
DMF at 50 ◦C gave enol ethers 8a,b in 89% yield as a ca.
a-phosphonoxy enol ethers 5a and a-phosphonoxy enecarba-
mates 5b can be reduced with a combination of hydrosilane
and palladium catalyst under neutral conditions to provide the
5.6 : 1 mixture of stereoisomers. The stereoselectivity of the
reduction was increased by the use of the less bulky Me2PhSiH
and was significantly reduced using the more bulky MePh2SiH
or Ph3SiH (entries 3 and 4). (Me3Si)3SiH was ineffective as a
corresponding enol ethers 6a and enecarbamates 6b, respectively
(eqn 3).10
hydride source (entry 5). An attempt to use Ph2SiH2 resulted in
significant isomerisation of the double bond (entry 6). When the
reaction was performed using Me2PhSiH at room temperature,
isomerisation of the double bond was further suppressed, but
the yield of 8a,b declined slightly (entry 7). Although we have
also attempted the reduction of 7 with n-Bu3SnH or n-Bu3GeH,
these hydride sources proved to be ineffective; we only observed
extensive homocoupling of these reagents under the conditions
Laboratory of Biostructural Chemistry, Graduate School of Life Sciences,
Tohoku University, 1-1 Tsutsumidori-amamiya, Sendai, 981-8555, Japan.
E-mail: hfuwa@bios.tohoku.ac.jp, masasaki@bios.tohoku.ac.jp; Fax: +81-
22-717-8896; Tel: +81-22-717-8895
† Electronic supplementary information (ESI) available: Spectroscopic
data for new compounds and copies of 1H and 13C NMR spectra of
compounds 25 and 34. See DOI: 10.1039/b706087d
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Table 1 Screening of conditions
Entry
Hydrosilane
Cone angle/deg
D (Si–H)/kJ mol−1
Yield (%)
Z–Ea
1
2
3
4
5
6
7b
Et3SiH
132
122
136
145
182
398.0
364.0
359.2
354.8
351.0
377.8
364.0
86
89
78
87
Trace
89
70
2.0 : 1
5.6 : 1
2.5 : 1
1.3 : 1
N/A
1.4 : 1
9.1 : 1
Me2PhSiH
MePh2SiH
Ph3SiH
(Me3Si)3SiH
Ph2SiH2
Me2PhSiH
122
a Ratio of 8a and 8b was determined based on 1H NMR spectra (500 MHz) of a purified mixture of 8a,b. b The reaction was performed at room
temperature.
established for Me2PhSiH. In addition, we examined the use of
additives such as LiCl or CuI, but these additives were found to
inhibit the reaction completely.11 Screening of catalysts including
Pd(PPh3)4, Pd(OAc)2–(2-furyl)3P, Pd(OAc)2–Cy3P, and Pd(OAc)2–
(o-dicyclohexylphosphino)biphenyl revealed that Pd(PPh3)4 was
the catalyst of choice.
We postulated a plausible reaction mechanism of the present
process based on several related literature precedents12 (Scheme 2).
Thus, the present catalytic reaction starts with oxidative addition
of a hydrosilane (R3Si–H) into a palladium(0) complex to form
a R3Si–Pd(II)–H intermediate. Two pathways are possible for
association of the intermediate with Rꢀ–X (e.g., 7) and subsequent
r-bond metathesis. Finally, reductive elimination affords the
product Rꢀ–H (e.g., 8) and regenerates the initial palladium(0)
complex.
degree of isomerisation may also depend on the activity of the
R3Si–Pd(II)–H complex adding to the product Rꢀ–H. We speculate
that the observed isomerisation may be attributed to these two
factors. The bond strength of hydrosilanes is another factor that
might influence the stereochemical outcome, because the catalytic
cycle of the present reaction should start with oxidative insertion of
a palladium(0) catalyst into an Si–H bond. However, as shown in
Table 1, the bond strength of the hydrosilanes14 does not correlate
with the observed stereoselectivity.
It should be noted here that another catalytic cycle that involves
oxidative addition of 7 into a palladium(0) complex to form a
Rꢀ–Pd(II)–X intermediate, followed by transmetallation with a
hydrosilane, may also be possible. At present, we cannot rule out
the possibility of this catalytic cycle operating in our case, although
Masuda et al. have demonstrated that oxidative addition of a silane
bond into a palladium(0) complex must be the key for the catalytic
silylation of aryl halides with triethoxysilane.12c
We next applied the optimal conditions for the reduction
of a series of a-phosphonoxy enol ethers and a-phosphonoxy
enecarbamates, which were prepared from the corresponding
esters and imides, respectively, by treating them with KHMDS–
(PhO)2P(O)Cl (Table 2). A variety of cyclic and acyclic enol ethers
and enecarbamates could be synthesised in good to excellent yields
according to our method. Reduction of 10a,b (ca. 1 : 1 mixture
of stereoisomers) in DMF at 60 ◦C gave enol ethers 11a (36%)
and 11b (9%), and 10b was recovered in 43% yield (entry 1). In
DMF at 135 ◦C, the reduction proceeded smoothly to provide
a mixture of 11a and 11b in 58% and 19% yields, respectively
(entry 2). These results were attributable to the reactivity difference
between 10a and 10b, the latter of which was less reactive due
to steric hindrance. The vinyl ether 14§ was synthesised from the
corresponding acetate 12 in 82% yield for the two steps (entry 3). In
this case, no potentially competitive intramolecular Heck coupling
was observed. Sterically encumbered 1-adamantyl acetate 15 was
efficiently delivered to vinyl ether 1715 in 63% overall yield (entry
4). The medium-sized lactam 18 was efficiently transformed to the
corresponding cyclic enecarbamate 20 in 68% yield (entry 5). In
the case of 22, a moderate reactivity difference between the aryl
bromide and the a-phosphonoxy enecarbamate functionalities
was observed (entry 6). This selectivity was further enhanced by
Scheme 2 Plausible mechanistic considerations.
Interestingly, it seems that the degree of isomerisation of the
double bond moderately correlates with the steric bulkiness of
hydrosilanes, except for Ph2SiH2.13 The steric bulkiness of the
R group should be detrimental for association of R3Si–Pd(II)–H
with RꢀX and subsequent r-bond metathesis, thereby lowering
the rate of these steps and possibly resulting in unfavourable
isomerisation. It can be considered that re-addition of the R3Si–
Pd(II)–H complex to the product Rꢀ–H followed by elimination
would also cause isomerisation of the double bond. Thus, the
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Table 2 Application to a variety of substrates
Entry
1a
Substrate
Phosphate
Product(s)
Yield (%)
11a: 36; 11b: 9d
2b
3a
11a: 58; 11b: 19
82
4a
63
68
5a
6a
23: 55; 24: 11
7c
23: 73
a Reaction conditions: KHMDS, (PhO)2P(O)Cl, THF–HMPA, −78 ◦C; then Me2PhSiH (5 equiv.), Pd(PPh3)4 (10 mol%), DMF, 60 ◦C. b The reduction
was performed at 135 ◦C. c The reduction was performed at room temperature. d 10b was recovered in 43% yield.
performing the reaction with 1.2 equiv. of Me2PhSiH and 10 mol%
of Pd(PPh3)4 in DMF at room temperature, giving 23 in 73%
yield for the two steps (entry 7). In some cases, a small amount
(ca. 5%) of over-reduced product was also isolated, although the
mechanism underlying its formation is elusive.16
Finally, we applied our methodology to the total syntheses of
( )-lennoxamine (25)17,18 and ( )-chilenine (26),19,20 isoindoloben-
zazepine alkaloids isolated from Chilean barberries Berberis
darwinii and Berberis empetrifolia, respectively. Our synthesis
plan is illustrated in Scheme 3. The enamide 27 was envisioned
as the common intermediate for 25 and 26. The isoindolinone
systems of 25 and 26 were to be constructed by radical cyclisation
and palladium-catalysed cyclisation, respectively. Chemoselective
reduction of a-phosphonoxy enamide 28 would provide 27. This
reaction was planned based on the observed reactivity difference
between the aryl bromide and a-phosphonoxy enecarbamate
functionalities of 22 (Table 2, entry 7).
The synthesis started with treatment of 3,4-methylenedi-
oxyphenylacetic acid 29 with thionyl chloride followed by coupling
with aminoacetaldehyde dimethylacetal to provide amide 30 in
good yield (Scheme 4). The Pomeranz–Fritsch-type cyclisation21
of 30 afforded enecarbamate 31 in 59% yield. Hydrogenation
of the double bond within 31 gave lactam 32 in 83% yield.
Despite several attempts, N-acylation of 32 under the standard
Scheme 3 Synthesis plan toward lennoxamine and chilenine.
conditions (n-BuLi or LiN(SiMe3)2, HMPA, THF; then 2-bromo-
5,6-dimethoxybenzoyl chloride18b) met with failure. However,
treatment of 32 with 2-bromo-5,6-dimethoxybenzoyl ch◦loride in
˚
the presence of 4 A molecular sieves (ClCH2CH2Cl, 65 C) gave
imide 33 in a near-quantitative yield. After conversion of 33 into
a-phosphonoxy enamide 28, chemoselective hydrosilane reduction
was executed by exposing 28 to Me2PhSiH and Pd(PPh3)4 catalyst
◦
in DMF at 80 C, affording enamide 2718i in 61% yield for the
two steps. Finally, according to Funk et al.’s procedure, radical
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the present method to the total synthesis of natural products is
currently under investigation.
Acknowledgements
This work was supported in part by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science,
and Technology (MEXT), Japan.
Notes and references
‡ The prices of these reagents (reagent grade) in the 2007 Aldrich catalogue
are: trifluoromethanesulfonic anhydride = 6500 yen per 10 g (184 yen
per mmol); N-phenyl bis(trifluoromethanesulfonimide) = 9100 yen per
5 g (650 yen per mmol); diphenylphosphoryl chloride (96% purity) =
6000 yen per 100 g (16 yen per mmol).
§ Representative experimental procedures. The synthesis of compound 14.
To a solution of acetate 12 (46.6 mg, 0.147 mmol) in THF (4 mL) were
added HMPA (0.127 mL, 0.730 mmol) and (PhO)2P(O)Cl (0.152 mL,
0.733 mmol). The resultant mixture was cooled to −78 ◦C and treated with
KHMDS (0.5 M solution in toluene, 0.88 mL, 0.44 mmol). After being
stirred at −78 ◦C for 0.5 h, the reaction mixture was quenched with 3%
NH4OH, diluted with Et2O, and allowed to warm to room temperature
over 20 min. The resultant mixture was extracted with EtOAc, washed
with brine, dried over Na2SO4, filtered, and concentrated under reduced
pressure to provide crude a-phosphonoxy enol ether 13, which was passed
through a short pad of florisil column and used immediately in the next
reaction. To a solution of 13 in DMF (2 mL) were added Me2PhSiH
(0.112 mL, 0.731 mmol) and Pd(PPh3)4 (16.9 mg, 0.0146 mmol). After
being stirred at 60 ◦C for 1 h, the reaction mixture was cooled to room
temperature, diluted with EtOAc, washed with H2O and brine, dried over
Na2SO4, filtered, and concentrated under reduced pressure. Purification
of the residue by flash chromatography on silica gel (5 to 10% diethyl
ether/hexane) gave 14 (36.1 mg, 82%) as a colourless oil.
Scheme 4 Reagents and conditions: (a) SOCl2, reflux; (b) aminoacetalde-
hyde dimethylacetal, pyridine, CH2Cl2, room temperature, 99%; (c) con-
centrated HCl, AcOH, room temperature, 59%; (d) H2, 10% Pd/C, MeOH,
˚
room temperature, 83%; (e) 2-bromo-5,6-dimethoxybenzoyl chloride, 4 A
molecular sieves, ClCH2CH2Cl, 65 ◦C, 99%; (f) KHMDS, (PhO)2P(O)Cl,
THF–HMPA, −78 ◦C; (g) Me2PhSiH, Pd(PPh3)4, DMF, 80 ◦C, 61% (two
steps); (h) n-Bu3SnH, AIBN, benzene, 105 ◦C, 67% (+ debrominated 27,
25%); (i) Pd(PPh3)4, KOAc, n-Bu4NCl, DMF, 110 ◦C, 84% (+ recovered
27, 9%).
1 Selected recent reviews on the Heck reaction: (a) J. P. Knowles and A.
Whiting, Org. Biomol. Chem., 2007, 5, 31; (b) A. B. Dounay and L. E.
Overman, Chem. Rev., 2003, 103, 2945; (c) I. P. Beletskaya and A. V.
Cheprakov, Chem. Rev., 2000, 100, 3009; (d) J. T. Link, Org. React.,
2002, 60, 157.
2 Selected recent reviews on radical reactions: (a) K. C. Majumdar, P. K.
Basu and P. P. Mukhopadhyay, Tetrahedron, 2005, 61, 10603; (b) K. C.
Majumdar, P. K. Basu and P. P. Mukhopadhyay, Tetrahedron, 2004,
60, 6239; (c) M. P. Sibi, S. Manyem and J. Zimmerman, Chem. Rev.,
2003, 103, 3263; (d) T. R. Rheault and M. P. Sibi, Synthesis, 2003, 803;
(e) A. Gansa¨uer and H. Bluhm, Chem. Rev., 2000, 100, 2771; (f) C.
Chatgilialoglu, D. Crich, M. Komatsu and I. Ryu, Chem. Rev., 1999,
99, 1991.
3 Selected recent reviews on olefin metathesis: (a) K. C. Nicolaou, P. G.
Bulger and D. Sarlah, Angew. Chem., Int. Ed., 2005, 44, 4490; (b) A.
Fu¨rstner, Angew. Chem., Int. Ed., 2000, 39, 3012; (c) R. H. Grubbs and
S. Chang, Tetrahedron, 1998, 54, 4413.
4 Selected recent reviews on Diels–Alder cycloaddition: (a) K. C.
Nicolaou, S. A. Snyder, T. Montagnon and G. Vassilikogiannakis,
Angew. Chem., Int. Ed., 2002, 41, 1668; (b) E. J. Corey, Angew. Chem.,
Int. Ed., 2002, 41, 1650.
cyclisation of◦27 under tin hydride conditions (n-Bu3SnH, AIBN,
benzene, 105 C)18i,22 furnished ( )-lennoxamine 25 in 67% yield
along with 25% yield of debrominated 27. The spectroscopic data
(1H, 13C NMR, HRMS) of synthetic 25 were in full accordance
with those reported in the literature.17 Thus, the total synthesis of
( )-lennoxamine was accomplished in seven steps (20% overall
yield) from commercially available 29. On the other hand,
palladium-catalysed cyclisation of 27 was smoothly achieved using
10 mol% of Pd(PPh3)4, KOAc, and n-Bu4NCl in DMF at 110 ◦C
for 14 h,23 affording dehydrolennoxamine 34 in 84% yield along
with 9% of recovered 27. Since the spectroscopic data (1H, 13C
NMR, HRMS) of 34 matched the reported data,18k and since 34
has already been converted to 26 by Danishefsky and Fang,20a
the present synthesis constitutes the formal total synthesis of
( )-chilenine 26 (seven steps, 25% overall yield from 29).
5 For a recent review on the synthesis of enol ethers and enamides, see:
J. R. Dehli, J. Legros and C. Bolm, Chem. Commun., 2005, 973.
6 (a) M. Sasaki, H. Fuwa, M. Ishikawa and K. Tachibana, Org. Lett.,
1999, 1, 1075; (b) M. Sasaki, M. Ishikawa, H. Fuwa and K. Tachibana,
Tetrahedron, 2002, 58, 1883; (c) H. Fuwa, M. Sasaki, M. Satake and
K. Tachibana, Org. Lett., 2002, 4, 2981; (d) H. Fuwa, N. Kainuma, K.
Tachibana and M. Sasaki, J. Am. Chem. Soc., 2002, 124, 14983; (e) H.
Fuwa and M. Sasaki, Synlett, 2004, 1851; (f) C. Tsukano, M. Ebine
and M. Sasaki, J. Am. Chem. Soc., 2005, 127, 4326; (g) H. Fuwa, M.
Ebine and M. Sasaki, J. Am. Chem. Soc., 2006, 128, 9648; (h) H. Fuwa,
M. Ebine, A. J. Bourdelais, D. G. Baden and M. Sasaki, J. Am. Chem.
Soc., 2006, 128, 16989.
7 (a) H. Fuwa, A. Kaneko, Y. Sugimoto, T. Tomita, T. Iwatsubo and M.
Sasaki, Heterocycles, 2006, 70, 101; (b) H. Fuwa and M. Sasaki, Chem.
Commun., 2007, DOI: 10.1039/b704374k.
8 Reported methods for catalytic hydrosilane reduction of enol
triflates: (a) W. J. Scott and J. K. Stille, J. Am. Chem. Soc., 1986, 108,
In conclusion, we have developed a new method for the synthesis
of enol ethers and enecarbamates based on a catalytic hydrosilane
reduction of a-phosphonoxy enol ethers and enecarbamates,
respectively. The present method is applicable to the synthesis
of a variety of cyclic and acyclic enol ethers and enecarbamates.
In addition, we found that this reaction displays an interest-
ing functional group selectivity between aryl bromides and a-
phosphonoxy enecarbamates. The short and highly efficient total
syntheses of isoindolobenzoazepine alkaloids ( )-lennoxamine
and ( )-chilenine have been accomplished based on the catalytic
hydrosilane reduction of the a-phosphonoxy enamide 28 and sub-
sequent radical cyclisation and palladium-catalysed cyclisation,
respectively, as the key transformations. Further application of
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3033; (b) H. Kotsuki, P. K. Datta, H. Hayakawa and H. Suenaga,
Synthesis, 1995, 3148.
9 (a) F. Charbonnier, A. Moyano and A. E. Greene, J. Org. Chem., 1987,
52, 2303; (b) K. C. Nicolaou, G.-Q. Shi, K. Namoto and F. Bernal,
Chem. Commun., 1998, 1757.
10 Other reported methods for reduction of enol phosphates: (a) S. C.
Welch and M. E. Walters, J. Org. Chem., 1978, 43, 2715; (b) T. Ishihara,
T. Maekawa, Y. Yamasaki and T. Ando, J. Org. Chem., 1987, 52,
300.
11 For selected papers discussing the effects of inorganic salts as additives
in palladium(0)-catalysed reactions: (a) W. J. Scott, G. T. Krisp and
J. K. Stille, J. Am. Chem. Soc., 1984, 106, 4630; (b) E. Piers, R. W.
Friesen and B. A. Keay, J. Chem. Soc., Chem. Commun., 1985, 809;
(c) R. A. Gibbs and U. Krishnan, Tetrahedron Lett., 1994, 35, 2509;
(d) V. Farina, S. Kapadia, B. Krishnan, C. Wang and L. S. Liebeskind,
J. Org. Chem., 1994, 59, 5905.
12 (a) A. Kunai, T. Sakurai, E. Toyoda, M. Ishikawa and Y. Yamamoto,
Organometallics, 1994, 13, 3233; (b) R. Boukherroub, C. Chatgilialoglu
and G. Manuel, Organometallics, 1996, 15, 1508; (c) M. Murata, K.
Suzuki, S. Watanabe and Y. Masuda, J. Org. Chem., 1997, 62, 8569;
(d) A. S. Manoso and P. DeShong, J. Org. Chem., 2001, 66, 7449; (e) Y.
Yamanoi, J. Org. Chem., 2005, 70, 9607.
13 L. A. Dakin, P. C. Ong, J. S. Panek, R. J. Staples and P. Stavropoulos,
Organometallics, 2000, 19, 2896.
14 (a) T. I. Drozdova, E. T. Denisov, A. F. Shestakov and N. S.
Emel’yanova, Kinet. Catal., 2006, 47, 106; (b) M. Ballestri, C. Chat-
gilialoglu, M. Guerra, A. Guerrini, M. Lucarini and G. Seconi, J. Chem.
Soc., Perkin Trans. 2, 1993, 421.
(c) D. L. Comins, S. Schilling and Y. Zhang, Org. Lett., 2005, 7, 95; (d) T.
Taniguchi, K. Iwasaki, M. Uchiyama, O. Tamura and H. Ishibashi, Org.
Lett., 2005, 7, 4389; (e) T. Honda and Y. Sakamaki, Tetrahedron Lett.,
2005, 46, 6823; (f) P. Sahakitpichan and S. Ruchirawat, Tetrahedron,
2004, 60, 4169; (g) G. Kim, J. H. Kim, W.-J. Kim and Y. A. Kim,
Tetrahedron Lett., 2003, 44, 8207; (h) Y. Koseki, S. Katsura, S. Kusano,
H. Sataka, H. Sato, Y. Monzene and T. Nagasaka, Heterocycles, 2003,
59, 527; (i) J. R. Fuchs and R. L. Funk, Org. Lett., 2001, 3, 3923; (j) A.
Couture, E. Deniau, P. Grandclaudon and C. Hoarau, Tetrahedron,
2000, 56, 1491; (k) S. Ruchirawat and P. Sahakitpichan, Tetrahedron
Lett., 2000, 41, 8007; (l) Y. Koseki, S. Kusano, H. Sakata and T.
Nagasaka, Tetrahedron Lett., 1999, 40, 2169; (m) H. Ishibashi, H.
Kawanami and M. Ikeda, J. Chem. Soc., Perkin Trans. 1, 1997, 817;
(n) G. Rodriguez, M. M. Cid, C. Saa, L. Castedo and D. Dom´ınguez,
J. Org. Chem., 1996, 61, 2780; (o) Y. Koseki and T. Nagasaka, Chem.
Pharm. Bull., 1995, 43, 1604; (p) C. J. Moody and G. J. Warrellow,
J. Chem. Soc., Perkin Trans. 1, 1990, 2929; (q) C. J. Moody and G. J.
Warrellow, Tetrahedron Lett., 1987, 28, 6089; (r) E. Napolitano, G.
Spinelli, R. Fiaschi and A. Marsili, J. Chem. Soc., Perkin Trans. 1, 1986,
785; (s) S. Teitel, W. Klo¨tzer, J. Borgese and A. Brossi, Can. J. Chem.,
1972, 50, 2022.
19 V. Fajardo, V. Elango, B. K. Cassels and M. Shamma, Tetrahedron
Lett., 1982, 23, 39.
20 (a) F. Fang and S. J. Danishefsky, Tetrahedron Lett., 1989, 30, 2747;
(b) C. R. Dorn, F. J. Koszyk and G. R. Lenz, J. Org. Chem., 1984,
49, 2642; (c) H. Ishibashi, H. Kawanami, H. Iriyama and M. Ikeda,
Tetrahedron Lett., 1995, 36, 6733; (d) H. Yoda, A. Nakahama, T.
Koketsu and K. Takabe, Tetrahedron Lett., 2002, 43, 4667; (e) H.
Yoda, K.-i. Inoue, Y. Ujihara, N. Mase and K. Takabe, Tetrahedron
Lett., 2003, 44, 9057. See also: ref. 18e, 18h, 18l and 18m.
15 I. F. Marko and G. R. Evans, Bull. Soc. Chim. Belg., 1994, 103, 295.
16 Hydrogenolysis of enol triflates by palladium on carbon has been
reported: V. B. Jigajinni and R. H. Wightmann, Tetrahedron Lett.,
1982, 23, 117.
17 E. Valencia, A. J. Freyer, M. Shamma and V. Fajardo, Tetrahedron
Lett., 1984, 25, 599.
21 For reviews on the Pomeranz–Fritsch cyclisation: (a) J. M. Bobbitt and
A. J. Bourque, Heterocycles, 1987, 25, 601; (b) M. D. Rozwadowska,
Heterocycles, 1994, 39, 903.
22 W. Zhang and G. Pugh, Tetrahedron, 2003, 59, 3009.
18 (a) T. Suzuki, K. Takabe and H. Yoda, Synlett, 2006, 3407; (b) S.
Couty, B. Liegault, C. Meyer and J. Cossy, Tetrahedron, 2006, 62, 3882;
23 For a related example: D. L. Comins, S. P. Joseph and Y.-M. Zhang,
Tetrahedron Lett., 1996, 37, 793.
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