Polymer-supported tertiary amine in organic synthesis: A useful reagent in the conversion of alkenes to carbonyl compounds via the corresponding ozonides

Article abstract of DOI:10.1016/S0040-4020(02)01558-2

Polymer-supported tertiary amine 6 is useful to workup the ozonolytic reaction of alkenes. The corresponding carbonyl products can be isolated in excellent yields by simple filtration. For some substituted alkenes, carboxylic acids were formed as the major products and can be isolated from the polymeric resin by washing with 1M HCl followed by extraction with organic solvent. In all cases, simply washing with aqueous NaOH can regenerate the polymeric amine 6. The reactivity of the regenerated polymeric amine is as effective as that of the freshly prepared one.

Full text of DOI:10.1016/S0040-4020(02)01558-2

Tetrahedron 59 (2003) 493–498
Polymer-supported tertiary amine in organic synthesis:
a useful reagent in the conversion of alkenes to carbonyl
compounds via the corresponding ozonides
a,b,* a
Yung-Son Hon
and Kun-Chan Wu
a
Department of Chemistry and Biochemistry, National Chung Cheng University, Chia-Yi 621, Taiwan, ROC
Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan, ROC
b
Received 20 August 2002; revised 4 November 2002; accepted 28 November 2002
Dedicated to Professor Chun-Chen Liao of National Hsing Hua University on the occasion of his 60th birthday
Abstract—Polymer-supported tertiary amine 6 is useful to workup the ozonolytic reaction of alkenes. The corresponding carbonyl products
can be isolated in excellent yields by simple filtration. For some substituted alkenes, carboxylic acids were formed as the major products and
can be isolated from the polymeric resin by washing with 1 M HCl followed by extraction with organic solvent. In all cases, simply washing
with aqueous NaOH can regenerate the polymeric amine 6. The reactivity of the regenerated polymeric amine is as effective as that of the
freshly prepared one. q 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
phosphine oxide 1a) can be regenerated using polymethyl-
3b–e
3
a
hydrosiloxane (PMHS) or trichlorosilane
under high
A variety of reduction methods, such as palladium catalyzed
hydrogenation, zinc in acetic acid, Ph P and Me S, are
temperature reaction conditions (Scheme 1). Recently,
soluble matrixes such as poly(ethyleneglycol) (PEG) are
receiving increasing attention both for combinatorial
synthesis and as reagent supports. The PEG–triarylphos-
phine derivative 2 has been applied to the liquid-phase
3
2
1
employed to workup the ozonolysis of alkenes. Among
them, Ph P is one of the most widely used reducing reagents
4
3
for this purpose. However, one often faces problems in the
purification of the carbonyl product from the unreacted
Ph P, especially if the polarities of the carbonyl product and
organic synthesis (LPOS) for a redox reaction involving
5
ozonide decomposition. The liquid-phase reagent 2 gave a
better yield than the Merrifield resin-bound triarylphos-
3
Ph PO are very similar. In order to overcome this problem,
3
5
polystyryldiphenylphosphine resin 1 has been used for the
reduction of ozonides to the corresponding carbonyl
phine derivative 1 in the ozonide reduction. However, the
regeneration of the polymer-supported phosphine 2 from
2
compounds. The advantages of such supported reagents
include the simplicity in the workup procedure and in the
purification process. The oxidized polymer reagent (i.e.
phosphine oxide 2a was achieved with freshly prepared
6
alane (AlH ) in THF in only 75% yield (Scheme 1). The
3
incomplete polymer recovery is attributed to problems
Scheme 1.
Keywords: polystyryldiphenylphosphine; polymer-bound piperidine; poly(ethyleneglycol); ozonide; ozonolysis.
*
Corresponding author. Address: Department of Chemistry and Biochemistry, National Chung Cheng University, Chia-Yi 621, Taiwan, ROC. Tel.: þ886-5-
2720411x6262; fax: þ886-5-2721040; e-mail: cheysh@ccunix.ccu.edu.tw
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)01558-2

494
Y.-S. Hon, K.-C. Wu / Tetrahedron 59 (2003) 493–498
Scheme 2.
associated with the byproduct aluminum salts that prevent
precipitation or polymer-backbone hydrolysis during the
reduction process.
polymer-bound amines have not been utilized to workup the
ozonolytic reactions. In this report, an extremely useful
methodology using the polymer-bound amines as bases to
quench the ozonides via E1cb mechanism is described.
0
treating DOWEX SBR resin (in hydroxide form, 4) with
an excess of 3,3 -thiodipropionate (5) in methanol, was
Polymer-bound 3,3 -thiodipropionic acid (3), made by
0
shown to be quite effective in the reductive quenching of the
2. Results and discussions
7
ozonolysis reactions. The oxidation product 5a could be
washed out of the resin 3a with acid. The resin 4a was first
Polystyrene-derived resin 6 was prepared from chloro-
methyl polystyrene (2% cross-linked Merrifield peptide
resin; chloromethylated styrene/divinylbenzene copolymer;
0
acid to give polymer-bound 3,3 -thiodipropionic acid (3)
treated with base and then with fresh 3,3 -thiodipropionic
0
200–400 mesh, contained 2.0–2.5 mmol Cl/g) by treatment
with piperidine (4.2 mol equiv.) in refluxing benzene for
(
Scheme 2).
1
1
1
1 h in excellent yield. Each gram of resin 6 contains
We have demonstrated that the E1cb mechanism is the
major process in the reaction of tertiary amines and
approximately 2.0 mmol of nitrogen as estimated from
elemental analysis. Resin 7 was prepared by treatment with
pyrrolidine in excellent yield under similar conditions. It
also contains approximately 2.0 mmol of nitrogen per gram.
8
ozonides. Based on this chemical knowledge, several
9
useful synthetic methodologies have been developed. The
use of polymer-bound amines as scavenger reagent in the
purification step of solution-phase reactions has recently
Since the ozonide formation from terminal olefins is
excellent, it is feasible to carry out the ozonolysis followed
by reaction with polymer-bound amine in the same flask.
The ozonide 9a, derived from 4-phenyl-1-butene (9)
1
0
gained much attention. To the best of our knowledge, the
Table 1. Reaction of the ozonide 9a with polymer reagents
(
1 mol equiv.) in CH Cl at 2788C was subsequently
2 2
Entry Polymer reagent (2 mol equiv amine) Time (h) 9b (%) 9c (%)
treated with the polymer-bound piperidine 6 (2.0 mol equiv.
of piperidine moieties) and the suspension solution was then
slowly warmed to room temperature and stirred at ambient
temperature for 8 h. The filtrate was concentrated in vacuo
to give the desired product 9b in an almost pure form.
Simple filtration through the short path silica gel column
gave analytically pure 3-phenylpropanal (9b) in 94% yield
1
2
3
8
8
9
94
71
80
0
20
13
Amberlite (IRA-400 (OH) 8
Scheme 3.

Y.-S. Hon, K.-C. Wu / Tetrahedron 59 (2003) 493–498
495
Table 2. The efficacy of the regenerated polymer 6 in quenching the
ozonolysis reaction
(Scheme 3). The first regenerated polymer-bound amine 6
was employed to react with ozonide 9a under similar
conditions. The aldehyde 9b was isolated in 93% yield and
the resin 6 was recovered in quantitative yield by our
standard protocol (entry 1, Table 2). The results as shown in
Table 2 also indicate that the reactivity of the polymer-
bound piperidine 6 was excellent even after the fourth
regeneration. The yield of the aldehyde 9b formation was
consistently above 90%. In other words, the polymer-bound
piperidine 6 can be recovered using mild condition in high
yield and the recovered polymeric amine can be used
repeatedly without loss of reactivity.
Entry
The nth recovered polymer (n¼)
Time (h)
Product 9b (%)
1
2
3
4
1
2
3
4
8
8
8
8
93
92
94
93
(
entry 1, Table 1). Under similar condition, ozonide 9a
was treated with polymer-bound pyrrolidine 7 to obtain
-phenylpropanal (9b) in 71% yield (entry 2). The lower
3
yield of the aldehyde 9b formation led us to isolate the other
possible product from the resin. The insoluble resin was
acidified with 1 M HCl and the filtrate was extracted
with CH Cl . The organic layer was washed with brine,
Encouraged by the results as shown in Table 2, we proceed
to study the scope of this methodology in quenching the
ozonolytic reaction for different alkenes. The simple
terminal alkenes 9–11 gave the corresponding aldehydes
9b–11b in excellent yields (entries 1–3, Table 3). This
indicates that the polymeric amine abstracts the less
substituted ozonide ring proton predominately. The yield
is slightly lower in entry 2 probably due to the volatility of
heptanal (10b). On the other hand, the benzyl acrylate 12
gave oxalic acid monobenyl ester (12c) instead of oxoacetic
acid benzyl ester (12b) in excellent yields (entry 4). This
result indicates that the polymeric amine predominately
abstracts the more substituted ozonide ring proton, which is
geminal to the carbobenzyloxy group. The electron-with-
drawing property of the carbobenzyloxy group is more
important than its steric effect in directing the reaction. The
1,1-disubstituted alkenes 13–16 gave the corresponding
ketones in excellent yields (entries 5–8).
2
2
dried over anhydrous Na SO , and concentrated to give
4
2
3
-phenylpropanoic acid (9c), which was then purified
through a short path silica gel column. The yield of pure
c is 20%. Using a similar procedure, the ozonide 9a was
9
2
treated with Amberlite (IRA-400, OH form) 8 to obtain
-phenylpropanal (9b) in 80% yield and carboxylic acid 9c
3
in 13% yield (entry 3). The data as shown in Table 1 indicate
that the polymer-bound piperidine 6 is the most regioselec-
tive reagent in the reaction with monosubstituted ozonide.
The polymer-bound piperidine 6 reacted with ozonide 9a to
give the insoluble piperidinium formate 6a. After filtration,
the insoluble resin was treated with aqueous NaOH, water
and methanol in sequence. The regenerated resin 6 was
isolated in almost quantitative yield after drying in vacuo
2 2
Table 3. The alkenes undergo ozonolysis at 2788C in CH Cl followed by treatement with polymer amine 6 at room temperature
a
Entry
Starting material
Time (h)
Product
Yield (%)
1
2
3
4
5
6
Ph(CH
Me(CH
THPO(CH
PhCH CCHvCH
Ph(Me)CvCH
2
)
2
CHvCH
CHvCH
CHvCH
2
9
8
8
8
7
9
10
Ph(CH
Me(CH
THPO(CH
2
PhCH CCO H
2
)
2
CHO
CHO
CHO
9b
94
85
94
91
92
95
2
)
5
2
10
11
12
13
14
2
)
5
10b
11b
12c
13b
14b
2
)
4
2
2 4
)
2
O
2
2
2
O
2
2
PhCOMe
7
8
9
THPOCH (Me)CvCH
2
2
15
16
17
8
8
24
THPOCH
2
COMe
CCOMe
15b
16b
17b
93
96
28
PhCH C(Me)CvCH
2
O
2
2
PhCH
2
O
2
OHC(CH
2
)
4
CHO
0
b
OHC(CH
2
)
4
CO
CO
CHO
2
Me
17c
17c
34
14
23
0
0
b
MeO
2
C(CH
2
)
4
2
Me
1
1
0
1
18
19
24
24
OHC(CH
2
)
3
18b
0
b
OHC(CH
2
)
3
CO
CO
CHO
2
Me
18c
18c
59
14
56
0
0
b
MeO
2
C(CH
2
)
3
2
Me
PhCO(CH
2
)
4
19b
0
0
b
b
PhCO(CH
PhCO(CH
2
)
)
4
3
CO
CO
2
Me
Me
19c
20c
39
91
12
20
8
2
2
a
Isolated yields are reported.
The product was isolated as its methyl ester by treatment with diazomethane during the workup.
b

496
Y.-S. Hon, K.-C. Wu / Tetrahedron 59 (2003) 493–498
Scheme 4.
A cyclic alkene, cyclohexene (17), was treated with ozone
in CH Cl at 2788C followed by reaction with polymer-
3. Conclusions
2
2
bound amine 6. After the reaction was complete, adipalde-
hyde (17b) can only be isolated from the filtrate in 28%
yield. Therefore, we tried to isolate the other products from
the resin. The insoluble resin was acidified with 1 M HCl
and the aqueous filtrate was extracted with CH Cl . The
The polymer-supported piperidine 6 is useful to workup the
ozonolytic reaction of alkenes. For simple terminal alkenes,
the aldehydes can be isolated in good yields by simple
filtration. For benzyl acrylate, carboxylic acids were formed
as the major products and can be isolated from the
polymeric resin by washing with 1 M HCl. The filtrate
was then extracted with organic solvent. For 1,1-disubsti-
tuted alkenes, the corresponding ketones were formed in
excellent yields. For cyclic alkenes, the product distribution
depends on the ring size and its substitution pattern. The
used resin can simply be washed with base to regenerate the
polymeric amine. The reactivity of the regenerated
polymeric amine is as good as that of the freshly prepared
one. In general, polymeric piperidine 6 is a much superior
reagent in comparison with those literature methods as
shown in Schemes 1 and 2 in its efficiency and simplicity to
workup the ozonolytic reaction.
2
2
organic layer was then treated with CH N and we were able
2
2
0
to isolate methyl 6-oxohexanoate (17c ) and dimethyl
0
0
hexanedioate (17c ) in 34 and 14% yields, respectively
entry 9, Table 3). The result of this reaction can be
rationalized as follows. Both mono-ozonide 17a and
(
0
polymeric ozonide 17a are formed after the ozonolysis
of cyclohexene (17) (Scheme 4). The deprotonation of
mono-ozonide 17a with polymeric amine 6 followed by
treatment with CH N results in the formation of aldehyde-
2
2
0
ester 17c under our workup procedure. The deprotonation
processes of the polymeric ozonide 17a are more
0
complicated. The deprotonation of its H and H followed
a
c
by treatment with CH N result in the formation of the
2
2
0
protons H and H followed by treatment with CH N result
aldehyde-ester 17c . The deprotonation of ozonide ring
b
c
2 2
4. Experimental
00
in the formation of the di-ester 17c . The deprotonation of
ozonide ring protons H and H result in the formation of
a
d
All reactions were carried out under nitrogen. Unless
otherwise noted, materials were obtained from commercial
suppliers and used without further purification. Melting
points were determined using a Thomas-Hoover melting
the di-aldehyde 17b (Scheme 4). A similar rationale can also
be applied to the case of cyclopentene (18). The cyclo-
pentene gave glutaraldehyde (18b) (23%), methyl 5-oxo-
0
00
1
13
pentanoate (18c ) (59%), and dimethyl glutarate (18c )
14%), respectively (entry 10).
point apparatus and are uncorrected. The H- and C NMR
spectra were recorded on a Bruker Avance DPX400
spectrometer, and chemical shifts were given in ppm
downfield from tetramethylsilane (TMS). IR spectra were
taken with a Perkin–Elmer 682 spectrophotometer and only
noteworthy absorptions were listed. Mass spectra were
measured on a VG Trio-2000GC/MS spectrometer by
electronic impact at 70 eV (unless otherwise indicated).
High Resolution Mass Spectroscopy (HRMS) was measured
on a JEOL JMS-HX 110 (National Hsing-Hua University)
or VG-11-250J (Academia Sinica) Mass Spectrometer.
(
We found that the ozonolysis of 1-phenylcyclohexene (19)
in CH Cl gave a mixture of keto-aldehyde 19b (56%) and
ozonide 19a. The ozonide 19a can be converted to the
2
2
0
corresponding keto-ester 19c in 39% yield in the same flask
(
entry 11). Interestingly, the ozonolysis of 1-phenylcyclo-
pentene (20) gave ozonide 20a in excellent yield. The
ozonide 20a can be converted to the corresponding keto-
ester 20c in 91% yield (entry 12).
0

Y.-S. Hon, K.-C. Wu / Tetrahedron 59 (2003) 493–498
497
The products obtained in this study, such as 3-phenylpro-
panal (9b), heptanal (10b), acetophenone (13b), a-tetralone
EtOAc/hexane (1/8) was employed to give 9b (107.1 mg,
94% yield) as a colorless oil.
00
14b), adipaldehyde (17b), dimethyl adipate (17c ), glutar-
aldehyde (18b), dimethyl glutarate (18c ), are commercial
(
00
4.4. General procedure for the reaction of ozonide with
polymer-supported amine to give the carboxylic acid
product
available. Their spectra are identical to those as shown in the
1
2,13
Aldrich handbook.
yloxy)-1-hexene (11), 5-(tetrahydropyran-2-yloxy)penta-
The products, 6-(tetrahydropyran-2-
1
4
1
5
16
nal (11b), oxalic acid monobenzyl ester (12c), 1-(tetra-
1
In a 50 mL two-neck flask, equipped with a magnetic stirrer,
a drying tube and a gas dispersing tube (with porous fritted
tip), were placed 20 mL of CH Cl and 1-phenyl-1-
7
hydropyran-2-yloxy)propan-2-one (15b), 2-oxopropionic
acid benzyl ester (16b), methyl 5-oxoheptanoate (17c ),
1
8
0
-oxopentanoic acid methyl ester (18c ), 6-oxo-6-phenyl-
19
2
2
0
hexanoic acid methyl ester (19c ), 5-oxo-5-phenylpenta-
20
5
cyclopentene 20 (207.1 mg, 1.44 mmol). A stream of
ozone was bubbled through the solution at 2788C. Ozone
treatment was terminated when the mixture assumed a blue
color. A stream of nitrogen removed the excess ozone. To
the solution was added polymer-supported amine 6 (1.4 g,
2.9 mmol). The reaction was warmed slowly to room
temperature and stirred for 8 h. The reaction mixture was
filtered through sintered glass. The resin was collected and
washed with 1 M HCl. The filtrate was extracted with ethyl
acetate. The organic layer was concentrated to about 5 mL.
To the concentrated solution was added CH N until the
0
noic acid methyl ester (20c ), are known in the literature.
21
0
22
4.1. Procedure for the preparation of crossed-linked
poly-[1-(4-vinylbenzyl)piperidine] (6)
A suspension of chloromethylated 2% cross-linked-poly-
2
styrene (contains 2.0–2.5 mmol Cl /g, 200–400 mesh,
4
.0 g) and 1.6 mL of piperidine (16.6 mmol) in 50 mL of
benzene was heated at reflux for 11 h. The reaction mixture
was cooled down to room temperature and filtered through a
fine-porosity Corning glass frit (4–5.5 mm) under vacuum.
The resin was washed sequentially with anhydrous benzene,
CH Cl , H O and MeOH. The washed polymer was dried in
2
2
solution turned yellow. The solution was concentrated and
purified by silica gel column chromatography by elution
0
with EtOAc/hexane (1/1) to give 20c (270 mg, 91% yield)
as a colorless oil.
2
2
2
vacuum at 608C for 5 h to give a white powder resin 6
3.8 g) in 95% yield. Each gram of resin 6 contains
approximately 2.0 mmol of nitrogen as estimated from
elemental analysis.
(
4.5. General procedure for the regeneration of the
polymer-supported amine
The polymer-bound piperidine 6 reacted with ozonide to
give the corresponding carbonyl compound and the
insoluble piperidinium formate 6a. After filtration, the
insoluble resin was treated with aqueous NaOH, water and
methanol in sequence. The regenerated resin 6 was isolated
in almost quantitative yield after drying in vacuo.
4.2. Procedure for the preparation of crossed-linked
poly-[1-(4-vinylbenzyl)pyrrolidine] (7)
A suspension of chloromethylated 2% cross-linked-poly-
2
styrene (contains 2.0–2.5 mmol Cl /g, 200–400 mesh,
.0 g) and 1.7 mL of piperidine (21.1 mmol) in 50 mL of
4
benzene was heated at reflux for 11 h. The reaction mixture
was cooled to room temperature and filtered through a fine-
porosity Corning glass frit (4–5.5 mm) under vacuum. The
resin was washed sequentially with anhydrous benzene,
CH Cl , H O and MeOH. The washed polymer was dried in
Acknowledgements
We are grateful to the National Science Council, National
Chung Cheng University, and Academia Sinica for the
financial support.
2
2
2
vacuum at 608C for 5 h to give a white powder resin 7
3.8 g) in 96% yield. Each gram of resin 7 contains
(
approximately 2.0 mmol of nitrogen as estimated from
elemental analysis.
References
4.3. General procedure for the reaction of ozonide with
polymer-supported amine to give carbonyl product
1. (a) Bailey, P. S. Ozonation in Organic Chemistry; Academic:
New York, 1978, Vol. 1; 1982, Vol. 2. (b) Razumovskii, S. D.;
Zaiikov, G. E. Ozone and its Reactions with Organic
Compounds; Elsevier: The Netherlands, 1984.
In a 50 mL two-neck flask, equipped with a magnetic stirrer,
a drying tube and a gas dispersing tube (with porous fritted
tip), were placed 20 mL of CH Cl and 1-phenyl-1-
cyclopentene 9 (100 mg, 0.8 mmol). A stream of ozone
was bubbled through the solution at 2788C. Ozone
treatment was terminated when the mixtures assumed a
blue color. A stream of nitrogen removed excess ozone. To
the solution was added polymer-supported amine 6
2
2
2. Ferraboschi, P.; Gambero, C.; Azadani, M. N.; Santaniello, E.
Synth. Commun. 1986, 16, 667–672.
3. (a) Lipowitz, J.; Bowman, S. A. J. Org. Chem. 1973, 38,
162–165. (b) Horner, L.; Balzer, W. D. Tetrahedron Lett.
1965, 3, 1157–1158. (c) Cremer, S. E.; Chovat, R. J. J. Org.
Chem. 1967, 32, 4066–4070. (d) Regen, S. L.; Lee, D. P.
J. Org. Chem. 1975, 40, 1669–1670. (e) Heitz, W.; Michels,
R. Justus Liebigs Ann. Chem. 1973, 227–230.
(
760 mg, 1.5 mmol). The reaction was warmed slowly to
room temperature and stirred for 8 h. The reaction mixture
was filtered through sintered glass and the filtrate was
concentrated to give 3-phenylpropanal (9b) in an almost
pure form. In order to obtain the analytically pure product,
short silica gel column chromatography by elution with
4. (a) Geckeler, K. E. Adv. Polym. Sci. 1995, 121, 31–79.
(b) Wentworth, Jr. P.; Vandersteen, A. M.; Janda, K. D. Chem.
Commun. 1997, 759–760. (c) Wipf, P.; Venkatraman, S.
Tetrahedron Lett. 1996, 37, 4659–4663. (d) Harris, J. H.; Liu,

498
Y.-S. Hon, K.-C. Wu / Tetrahedron 59 (2003) 493–498
1
3
1
Y.; Chai, S.; Andrews, M. D.; Vederas, J. C. J. Org. Chem.
998, 63, 2407–2409.
. Sieber, F.; Wentworth, Jr., P.; Toker, J. D.; Wentworth, A. D.;
12. Pouchert, C. J.; Behnke, J. The Aldrich Library of C and H
FT-NMR Spectra. Aldrich Chemical: Milwaukee, WI, 1992.
13. Pouchert, C. J. The Aldrich Library of Infrared Spectra.
Aldrich Chemical: Milwaukee, WI, 1981.
1
5
Metz, W. A.; Reed, N. N.; Janda, K. D. J. Org. Chem. 1999,
6
4, 5188–5192.
. Griffin, S.; Heath, L.; Wyatt, P. Tetrahedron Lett. 1998, 39,
405–4406.
. Appell, R. B.; Tomlinson, I. A.; Hill, I. Synth. Commun. 1995,
5, 3589–3595.
. Hon, Y. S.; Lin, S. W.; Lu, L.; Chen, Y. J. Tetrahedron 1995,
1, 5019–5034.
. (a) Hon, Y. S.; Yan, J. L. Tetrahedron 1998, 54, 8525–8542.
14. Nishida, A.; Kawahara, N.; Nishida, M.; Yonemitsu, O.
Tetrahedron 1996, 52, 9713–9734.
6
7
8
9
1
5. Clasby, M. C.; Craig, D.; Slawin, A. M. Z.; White, A. J.; P, .;
Williams, D. J. Tetrahedron 1995, 51, 1509–1532.
6. Bartel, K. J. Chem. Soc. C 1971, 3766–3769.
4
1
1
2
7. Daub, G. W.; Edwards, J. P.; Allen, J. W.; Maxey, C. T.;
Wells, M. S.; Goldstein, A. S.; Dibley, M. J. J. Org. Chem.
5
1
8. Yamamoto, Y.; Maruyama, K.; Komatsu, T.; Ito, W. J. Org.
997, 62, 1976–1985.
1
1
2
2
2
(
b) Hon, Y. S.; Yan, J. L. Tetrahedron 1997, 53, 5217–5232.
c) Hon, Y. S.; Chang, F. J.; Lu, L.; Lin, S. W. Tetrahedron
Chem. 1986, 51, 886–891.
9. Gannett, P. M.; Nagel, D. L.; Reilly, P. J.; Lawson, T.; Sharpe,
J.; Toth, B. J. Org. Chem. 1988, 53, 1064–1071.
(
1998, 54, 5233–5246.
1
1
0. (a) Booth, J. R.; Hodges, J. C. J. Am. Chem. Soc. 1997, 119,
0. Goese, M.; Elsenelch, W.; Kupfer, E.; Stohler, P.; Weber, W.;
Leuenberger, H. G. J. Org. Chem. 2001, 66, 4673–4678.
1. Bhatt, M. V.; Ravindranathan, M.; Somayaji, V.; Rao, G. V.
J. Org. Chem. 1984, 49, 3170–3173.
4882–4886. (b) Creswell, M. W. Tetrahedron 1988, 54,
3983–3995. (c) Blackburn, G. Tetrahedron Lett. 1998, 39,
3635–3638.
1. Metelko, M.; Zupan, M. Synth. Commun. 1988, 18,
821–1831.
2. Baba, A.; Yasuba, M.; Ohigashi, N.; Shibata, I. J. Org. Chem.
1
1
999, 64, 2180–2181.