Tetrathiomolybdate assisted epoxide ring opening with masked thiolates and selenoates: Multistep reactions in one pot

Article abstract of DOI:10.1021/jo0263418

Tetrathiomolybdate provides an easy access to β-hydroxy disulfides, β-hydroxy sulfides, and selenides from epoxides in a tandem, multistep process in one pot. This strategy has been utilized effectively in the construction of thiabicylo[3.2.2]nonane derivative 24.

Full text of DOI:10.1021/jo0263418

Tetr a th iom olybd a te Assisted Ep oxid e Rin g Op en in g w ith Ma sk ed
Th iola tes a n d Selen oa tes: Mu ltistep Rea ction s in On e P ot
Naduthambi Devan, Perali Ramu Sridhar, Kandikere Ramaiah Prabhu, and
Srinivasan Chandrasekaran*,†
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, Karnataka, India
scn@orgchem.iisc.ernet.in
Received August 22, 2002
Tetrathiomolybdate provides an easy access to â-hydroxy disulfides, â-hydroxy sulfides, and
selenides from epoxides in a tandem, multistep process in one pot. This strategy has been utilized
effectively in the construction of thiabicylo[3.2.2]nonane derivative 24.
TABLE 1. Rea ction of Ep oxid es w ith
Tetr a th iom olybd a te (1)
Ring opening of epoxides by thiols is a widely used
reaction for the synthesis of â-hydroxy thio derivatives
which are useful intermediates in natural product syn-
thesis.1 The most common protocols employ opening of
the epoxide with thiol in the presence of acid or base2,3
and also in the presence of ZnI2,4 CoCl2,5 lanthanide
trichlorides,6 zinc or magnesium tartrates,7 etc. Recent
years have witnessed a large number of papers on the
asymmetric version of epoxide opening by thiols assisted
by chiral catalysts.8
The tandem reactions involving multistep, one-pot
transformations of substrates to products are currently
of interest because of potential applications in organic
synthesis.9 Additionally, disulfide/diselenide bond cleav-
age could lead to interesting products in adding nucleo-
philic sulfur10/selenium11 species to a variety of organic
substrates. The cleavage of disulfide/diselenide bonds
under neutral conditions would enhance the utility of
such reactions, particularly in tandem reactions.
Earlier we demonstrated that benzyltriethylammo-
nium tetrathiomolybdate,12 [C6H5CH2NEt3]2MoS4, 1, is
a useful reagent that effects a tandem sulfur transfer
reduction-Michael addition in one pot.13 In continuation
of our investigation into the utility of 1 in organic
synthesis,12-14 we report herein a convenient strategy to
open an epoxide ring with 1 (1 equiv; CH3CN:EtOH, 1:1;
28 °C; under sonication) to provide â-hydroxy disulfides15
(Table 1).
Additionally we disclose our results on the cleavage of
disulfide/diselenide bonds assisted by 1 and the use of
masked thiolate/selenoate for the synthesis of â-hydroxy
sulfides/â-hydroxy selenides involving multistep, tandem
reactions in one pot (Scheme 1).
* Author to whom correspondence should be addressed. Tele-
phone: +91-80-394 2404. Fax: +91-80-360 0529. E-mail: scn@
orgchem.iisc.ernet.in.
† Honorary Professor, J awaharlal Nehru Center for Advanced
Scientific Research, Bangalore 560 064, India.
(1) (a) Corey, E. J .; Clark, D. A.; Goto, G. Marfat, A.; Mioskowski,
C.; Samuelsson, B.; Hammarstro¨m, S. J . Am. Chem. Soc. 1980, 102,
1436 and 3663. (b) Adams, H.; Bell, R.; Cheung, Y.; J ones, D. N.;
Tomkinson, N. C. O. Tetrahedron: Asymm. 1999, 10, 4129.
(2) Peach, M. E. In The Chemistry of the Thiol Group; Patai, S., Ed;
J ohn Wiley & Sons: New York, 1974; Part 3, p 771.
(3) Chini, M.; Crotti, P.; Giovani, E.; Macchia, F.; Pineschi, M.
Synlett 1992, 303, and references therein.
(4) (a) Guindon, Y.; Young, R. N.; Frenette, R. Synth. Commun.
1981, 11, 391. (b) de Pomar, J . C. J .; Soderquist, J . A. Tetrahedron
Lett. 1998, 39, 4409.
(5) Iqbal, J .; Pandey, A.; Shukla, A.; Srivastava, R. R.; Tripathi, S.
Tetrahedron 1990, 46, 6423.
(6) Vougioukas, A. E.; Kagan, H. B Tetrahedron Lett. 1987, 28, 6065.
(7) Yamashita, H. Bull. Chem. Soc. J pn. 1988, 61, 1213.
(8) Selected references for asymmetric version of epoxide opening
by thiols: (a) Iida, T.; Yamamoto, N.; Sasai, H.; Shibasaki, M. J . Am.
Chem. Soc. 1997, 119, 4783. (b) Wu, M. H.; J acobsen, E. N. J . Org.
Chem. 1998, 63, 5252.
(9) (a) Ikeda, S. Acc. Chem. Res. 2000, 33, 511, and references
therein. (b) Son, S. U.; Park, K. H.; Seo, H.; Chung, Y. K.; Lee, S.-G.
Chem. Commun. 2001, 2440, and references therein.
(10) Kondo, T.; Uenoyama, S.; Fujita, K.; Mitsudo, T. J . Am. Chem.
Soc. 1999, 121, 482.
It has earlier been shown that disulfide bonds are
cleaved in the presence of 1 involving an induced redox
(12) (a) Ramesha, A. R.; Chandrasekaran, S. Synth. Commun. 1992,
22, 3277. (b) Ramesha, A. R.; Chandrasekaran, S. J . Org. Chem. 1994,
59, 1354.
(13) Prabhu, K. R.; Sivanand, P.; Chandrasekaran, S. Angew. Chem.,
Int. Ed. 2000, 39, 4316.
(14) Prabhu, K. R.; Chandrasekaran, S. Chem. Commun. 1997, 1021.
(15) Reactions were performed under sonochemical conditions (ul-
trasonic cleaning bath, 25 kHz). Without sonication it took a longer
time (22-70 h) to produce a lower yield (30-60%) of the products.
(11) (a) Sakakibara, M.; Katsumata, K.; Watanabe, Y.; Toru, T.;
Ueno, Y. Synthesis 1992, 377. (b) Engman, L.; Gupta, V. J . Org. Chem.
1997, 62, 157, and references therein.
10.1021/jo0263418 CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/28/2002
J . Org. Chem. 2002, 67, 9417-9420
9417

Devan et al.
TABLE 3. Mu ltistep Rea ction s w ith
Tetr a th iom olybd a te (1) in On e P ot
SCHEME 1
TABLE 2. Disu lfid e/Diselen id e Bon d Clea va ge-Ep oxid e
Op en in g Assisted by Tetr a th iom olybd a te (1)
a
All compounds exhibited expected spectral and analytical data.
Isolated yields.
b
Reaction of benzyl bromide 17 with 2 and 1 (2 equiv; CH3-
CN:EtOH, 1:1; 28 °C; 2 h) proceeded smoothly to furnish
the corresponding 13a in good yield (81%, entry 1, Table
3). As expected, under identical experimental conditions,
1 reacted with benzyl thiocyanate 18 and 2 to produce
the corresponding 13a (87%, entry 2, Table 3). The
reaction of 1 with benzyl thiocyanate generated in situ
(from 17 and potassium thiocyanate) and 2, afforded the
13a (73%, Table 3). When benzyl alcohol 19 was activated
with DCC/CuCl17 and treated with 1 followed by the
addition of 2, 13a was obtained (73%, Table 3).
Employing a similar strategy, 13b was obtained from
the corresponding bromide or alcohol precursor in a four-/
five-step tandem reaction in one pot. Thus, 17 on treat-
ment with KSeCN followed by the addition of 1 (2 equiv)
and 2 furnished the corresponding 13b in 60% yield
(entry 5, Table 3). In the reaction of benzyl alcohol,
activation of alcohol (DCC/CuCl) followed by treatment
with KSeCN, 1 (2 equiv), and 2 resulted in the formation
of 13b in 56% yield (entry 6, Table 3). The noteworthy
feature of this methodology is the five-step reactions
(activation of alcohol, formation of selenocyanate, dis-
elenide formation, cleavage of diselenide bond, and
epoxide ring opening) are performed in tandem in one
pot using tetrathiomolybdate as the key reagent.18
The one-pot intramolecular reactions of ω-halo-ep-
oxides and ω-hydroxy epoxides with 1 are presented in
Table 3 (entries 7-10). When epichlorohydrin 20 was
treated with 1 (2 equiv; CH3CN:EtOH, 1:1; 28 °C; 3 h),
the four membered â-hydroxy sufide 21 was isolated in
excellent yield. Hydroxy epoxide 22 on activation with
DCC/CuCl followed by treatment with excess 1 under-
went a tandem reaction to produce the same four
a
All compounds exhibited expected spectral and analytical data.
b
Isolated yields.
reaction.13,16 The results of tandem cleavage of disulfide
bonds assisted by 1 followed by epoxide ring opening
to provide â-hydroxy sulfides are summarized in Table
2. Thus, in the reaction of disulfide 10a or 12a with
1 (1 equiv) followed by the addition of epoxide 2
(CH3CN:EtOH, 1:1; 28 °C; 4 h) the corresponding â-hy-
droxy sulfide 11a or 13a was obtained as the only product
in very good yield. Treatment of epoxide 4, 6, or 8 and
diphenyl disulfide 10a with 1 under similar conditions
gave the â-hydroxy sulfides 14a , 15a , and 16a , respec-
tively.
The fact that 1 cleaves the disulfide bond under
appropriate reaction conditions13,16 prompted us to ex-
plore the possibility of cleavage of diselenides in situ,
which can be taken advantage of in the ring opening of
epoxides. Accordingly, epoxide 2 was treated with di-
phenyl diselenide 10b in the presence of 1 (1 equiv;
CH3CN:EtOH, 1:1; 28 °C; 5 h), and the â-hydroxy selenide
11b was obtained in 82% yield. The reaction of 10b with
epoxides 4, 6, and 8 in the presence of 1 also gave the
corresponding 14b, 15b, and 16b, respectively, in very
good yield. Under similar reaction conditions 2 and 12b
in the presence of 1 afforded 13b (Table 2).
Epoxide ring opening reactions using disulfides or
diselenides assisted by 1 present an opportunity to
explore tandem, multistep reactions with 1 in one pot.
Therefore, it was of interest to study the reaction of
epoxides with 1 and precursors of disulfides or diselenides
(for example, alkyl halides or tosylates; Scheme 1).
(17) Sinha, S.; Ilankumaran, P.; Chandrasekaran, S. Tetrahedron
1999, 55, 14769.
(16) Coyle, C. L.; Harmer, M. A.; George, G. N.; Daage, M.; Stiefel,
E. I. Inorg. Chem. 1990, 29, 14, and references therein.
(18) In an incomplete reaction dibenzyl diselenide can be isolated
(10-15%), indicating its intermediacy in the reaction.
9418 J . Org. Chem., Vol. 67, No. 26, 2002

Tetrathiomolybdate Assisted Multistep Reactions
molybdate (1; 1.1 mmol) at once. The reaction mixture was
subjected to sonication using an ultrasonic cleaning bath (20
kHz) for 3.5-6 h (Table 1). The solvent was removed under
reduced pressure, and the black residue was extracted with
dichloromethane:diethyl ether (1:5, 10 mL × 3) and filtered
through a Celite pad. The filtrate was concentrated, and the
residue was purified by chromatography on silica gel using
ethyl acetate:hexane (1:15). The products 3,22 5,22 7,22 and 922
exhibited expected analytical and spectral data (reaction of
epoxides 6 and 8 with 1 resulted in the formation of 7 and 9,
respectively, as single regioisomers).
SCHEME 2
SCHEME 3
Disu lfid e/Diselen id e Bon d Clea va ge-Ep oxid e Rin g
Op en in g Assisted by Tetr a th iom olybd a te (1) (Gen er a l
P r oced u r e). To a well-stirred solution of disulfide (10a or 12a ,
0.5 mmol) or diselenide (10b or 12b, 0.5 mmol) in CH3CN (2
mL) was added 1 (1.2 mmol). The reaction mixture was stirred
at room temperature (26 °C) for 2 h, and then a solution of
epoxide (2 or 4 or 6 or 8, 1 mmol) in EtOH (2 mL) was added
and stirred at room temperature (26 °C) for the time indicated
in Table 2. The solvent was removed under vacuum. The black
residue was extracted with dichloromethane:diethyl ether
(1:5, 10 mL × 3) and filtered through a Celite pad. The filtrate
was concentrated, and the residue was purified by chroma-
tography on silica gel using ethyl acetate:hexane (1:15). The
products 11a ,4 11b ,23 13a ,7 13b ,23 14a ,4 14b ,24 15a ,4 15b ,23
16a ,5 and 16b23 exhibited expected analytical and spectral
data.
Syn th esis of 2R,3R-Ep oxy-5R-(1-(br om om eth yl)vin yl)-
2-m eth ylcycloh exa n on e (23).25 9-Bromocarvone 1 (1 g, 4.34
mmol) was dissolved in methanol (1 mL), and the solution was
cooled to 0 °C. Hydrogen peroxide (30%, 1.4 mL) was added
slowly with stirring to the cooled solution (0.5 h) followed by
the addition of 6 M NaOH (0.37 mL), ensuring that internal
temperature was always <0 °C. The reaction mixture was
allowed to stir at 0 °C for an additional 16 h, then poured into
water (10 mL), and saturated NaCl. The reaction mixture was
extracted with Et2O (4 × 10 mL), and the combined organic
extract was washed with brine and then dried (MgSO4). After
filtration, the solvent was evaporated and the crude residue
was purified by column chromatography on silica gel using
3% EtOAc:hexane as solvent to afford 23 as a colorless liquid.
Yield: 0.651 g (62%). IR: 1709, 1639, 1051 cm-1. 1H NMR (300
MHz, CDCl3): δ 5.26 (s, 1H), 4.98 (s, 1H), 3.94 (s, 2H), 3.44
(dd, J ) 0.9 Hz, J ) 3 Hz, 1H), 2.97-3.02 (m, 1H), 2.65 (ddd,
J ) 1.5 Hz, J ) 4.8 Hz, J ) 17.7 Hz, 1H), 2.41-2.49 (m, 1H),
2.03 (dd, J ) 11.7 Hz, J ) 17.7 Hz, 1H), 1.94 (ddd, J ) 1.2 Hz,
J ) 11.4 Hz, J ) 14.7 Hz, 1H), 1.39 (s, 3H). 13C NMR (75 MHz,
CDCl3): δ 206.1, 148.5, 117.2, 62.5, 60.3, 43.6, 36.5, 33.0, 30.7,
16.7. Low-resolution MS: m/z: 245 (M+). Anal. Calcd for
membered 2119 (73%). The application of this methodol-
ogy has been demonstrated in the synthesis of the
2-thiabicyclo[3.2.2]nonane skeleton (entries 9 and 10,
Table 3). Interestingly, when ω-bromo epoxide 23 was
treated with an excess of tetrathiomolybdate, the only
product formed in the reaction turned out to be the
2-thiabicylo[3.2.2]nonane derivative 24 in excellent yield
(90%). A similar result was achieved in the reaction of
ω-hydroxy epoxide 25, which was activated by DCC/CuCl
and treated with 1 (2 equiv) at room temperature to
furnish the bicyclic product 24 in 72% yield (Table 3).
A plausible reaction mechanism is proposed in Scheme
2. It is believed that alkylation of the metal-sulfur bond
with opening of the epoxide ring is the key step, which
triggers an internal redox reaction with oxidation of the
ligand to the disulfide and the concomitant reduction of
the metal center, leading to the formation of dihydroxy
disulfide and sulfide cluster of molybdenum.20
Further, the cleavage of disulfide by 1 is an induced
internal redox reaction, which is thoroughly investigated
by Stiefel,16 and the thiolate generated in situ further
reacts with the epoxide to generate the corresponding
â-hydroxy sulfide (Scheme 3).21
In summary, tetrathiomolybdate 1 provides an easy
access to â-hydroxy disulfides, â-hydroxy sulfides, and
selenides from epoxides in a tandem, multistep process
in one pot. Additionally, interesting examples of tandem
reactions to construct the 2-thiabicyclo[3.2.2]nonane
skeleton utilizing 1 makes this methodology attractive.
C
10H13BrO2: C, 49.00; H, 5.35. Found: C, 49.18; H, 5.49.
Ta n d em Rea ction of 2R,3R-Ep oxy-5R-(1-(br om om eth -
Exp er im en ta l Section
yl)vin yl)-2-m eth ylcycloh exa n on e (23) w ith Tetr a th io-
m olybd a te (1). To a well-stirred solution of 2R,3R-epoxy-5R-
(1-(bromomethyl)vinyl)-2-methylcyclohexanone (23) (0.15 g,
0.61 mmol) in acetonitrile and ethanol mixture (1:1, 5 mL) was
added benzyltriethylammonium tetrathiomolybdate (1) (0.77
g, 1.27 mmol) and stirred at room temperature (28 °C) for 7 h
under argon atmosphere. The solvent was evaporated under
reduced pressure, and the black residue was extracted with
dichloromethane:diethyl ether (1:5, 10 mL × 3) and filtered
through a Celite pad. The filtrate was concentrated and the
residue purified by chromatography on silica gel eluting with
ethyl acetate:hexane, (1:15) to obtain pure compound 24 as a
Gen er a l Meth od s. All reactions are performed in an oven-
dried apparatus. Reaction mixtures were stirred magnetically
unless otherwise stated. The products were characterized by
NMR, IR, FTIR, and GCMS. Commercial grade solvents were
distilled prior to use. Chloroform, dichloromethane, and ac-
etonitrile were initially dried over phosphorus pentoxide and
stored over 4 Å molecular sieves. IH and 13C NMR spectra were
recorded at 300 and 75 MHz, respectively. Coupling constants
are reported in hertz. Infrared (IR) spectra were measured as
CHCl3 solution or as thin film.
Rea ction of Ep oxid es w ith Tetr a th iom olybd a te
(Gen er a l P r oced u r e). To a well-stirred solution of epoxide
(1 mmol) in CH3CN:EtOH (1:1, 2 mL) was added tetrathio-
1
colorless liquid (0.11 g, 90%). IR (neat): 3392, 1706,1639 cm-1
.
1H NMR (300 MHz, CDCl3): δ 4.80 (s, 1H), 4.78 (s, 1H), 4.27
(19) (a) Dittmer, D. C.; Christy, M. E.; Takashina, N.; Henion, R.
S.; Balquist, J . M. J . Org. Chem. 1971, 36, 1324. (b) Searles, S.; Hays,
H. R.; Lutz, E. F. J . Org. Chem. 1962, 27, 2828.
(20) Boorman, P. M.; Wang, M.; Parvez, M. J . Chem. Soc., Chem.
Commun. 1995, 999.
(21) It is important to recognize that free thiol/selenol are not
generated in the cleavage of disulfides/diselenides mediated by 1.
(22) (a) Kolar, A. J .; Olsen, R. K. J . Org. Chem. 1971, 36, 591. (b)
Mousseron, M. Bull. Soc. Chim. Fr. 1948, 84.
(23) Posner, G. H.; Rogers, D. Z. J . Am. Chem. Soc. 1977, 99, 8208.
(24) Sharpless, K. B.; Gordon, K. M.; Lauer, R. F.; Patrick, D. W.;
Singer, S. P.; Young, M. W. Chem. Scr. 1975, 8A, 9.
(25) Srikrishna, A.; Hemamalini, P. Indian J . Chem., Sect. B 1990,
29, 201.
J . Org. Chem, Vol. 67, No. 26, 2002 9419

Devan et al.
(d, 1H, J ) 6.6 Hz), 3.47 (d, 1H, J ) 15.1 Hz), 3.26 (d, 1H, J
) 15.1 Hz), 3.05-2.95 (m, 1H), 2.75 (dd, 1H, J ) 18.3, 5.5 Hz),
2.52 (dd, 1H, J ) 15.4, 7 Hz), 2.41 (d, 1H, J ) 18.4 Hz), 2.20
(dd, 1H, J ) 15.4 Hz, 8.8 Hz), 2.04 (bs, OH), 1.45 (s, 3H). 13C
NMR (75 MHz, CDCl3): δ 207.2, 150.0, 110.8, 75.6, 53.9, 41.9,
37.3, 36.4, 33.9, 19.7. Low-resolution MS (EI): m/z: 198 (M+),
180, 165, 151. HRMS (EI). m/z: Calcd for C10H14O2S: 198.0714.
Found: 198.0708.
cm-1. 1H NMR (300 MHz, CDCl3): δ 5.16 (s, 1H), 5.01 (s, 1H),
4.42 (s, 2H), 3.44 (dd, 1H, J ) 0.9 Hz, J ) 2.1 Hz), 2.84-2.90
(m, 1H), 2.65 (ddd, 1H, J ) 0.9 Hz, J ) 3.6 Hz, J ) 12.9 Hz,
1H), 2.38-2.42 (m, 1H), 2.07 (dd, 1H, J ) 8.7 Hz, J ) 13.2
Hz), 1.93 (ddd, 1H, J ) 0.9 Hz, J ) 8.1 Hz, J ) 11.1 Hz, 1H),
1.37 (s, 3H). 13C NMR (75 MHz, CDCl3): δ 207.35, 146.96,
116.91, 80.52, 62.93, 60.46, 43.34, 33,37, 30.26, 16.61. Low-
resolution MS (CI): m/z: 183 (M+ + 1). Since this compound
is unstable, CHN analysis or HRMS data could not be
collected.
Ta n d em R ea ct ion of 2R,3R-E p oxy-5R-(1-(h yd r oxy-
m eth yl)vin yl)-2-m eth ylcycloh exa n on e (25) w ith Tetr a -
th iom olybd a te (1). A solution of 2R,3R-epoxy-5R-(1-(hy-
droxymethyl)vinyl)-2-methylcyclohexanone (25) (0.100 g, 0.55
mmol), DCC (0.126 g, 0.6 mmol), and CuCl (4 mg, 6 mol %)
was stirred under a solvent free condition at room temperature
(28 °C). After the completion of the reaction (8 h, TLC), a
solution of 1 (0.67 g, 1.1 mmol) in CH3CN/EtOH (4 mL, 1:1)
was added at once at room temperature, and the reaction
mixture was stirred at room temperature for 4 h. The solvent
was removed under vacuum, and the black residue was
extracted with dichloromethane:diethyl ether (1:5, 10 mL ×
3) and filtered through a Celite pad. The filtrate was concen-
trated, and purification on a silica gel column (ethyl acetate:
hexane, 1:15) afforded the pure compound 24 as a colorless
liquid (0.078 g, 72%).
P r ep a r a tion of 9-Hyd r oxyca r von e.26 A solution of 9-bro-
mocarvone (1.15 g, 5 mmol) and 15% aqueous HMPA (25 mL)
was refluxed at 120 °C for 30 min. The reaction mixture was
cooled to room temperature, diluted with water, extracted with
diethyl ether (3 × 20 mL), and dried (MgSO4). The solvent was
evaporated under reduced pressure, and the residue was
purified by column chromatography on silica gel to furnish
9-hydroxycarvone as a colorless liquid. Yield: 0.747 g, (90%).
1H NMR (300 MHz, CDCl3): δ 6.74-6.78 (m, 1H), 5.15 (s, 1H),
4.95 (s. 1H), 4.15 (s, 2H), 2.90-2.2 (m, 5H), 2.0 (bs, 1H), 1.80
(s, 3H). 13C NMR (75 MHz, CDCl3): δ 199.6, 150.2, 144.6,
135.4, 110.3, 64.7, 43.2, 38.2, 31.5, 15.6. Low-resolution MS
(EI): m/z: 166 (M+).
P r ep a r a tion of 2R,3R-Ep oxy-5R-(1-(h yd r oxym eth yl)-
vin yl)-2-m eth ylcycloh exa n on e (25). 9-Hydroxycarvone (0.5
g, 3 mmol) was dissolved in methanol (0.5 mL), and the
solution was cooled to 0 °C. Hydrogen peroxide (30%, 1 mL)
was added slowly with stirring to the cooled solution (0.5 h).
A solution of 6 M NaOH (0.26 mL) was added ensuring that
the internal temperature was always <0 °C. The reaction
mixture was allowed to stir at 0 °C for an additional 16 h and
then poured into water (5 mL) and saturated NaCl. The
reaction mixture was extracted with EtOAc (4 × 5 mL), and
the combined organic extract was washed with brine and then
dried (MgSO4). After filtration, the solvent was evaporated and
the crude residue was purified by column chromatography on
silica gel using 1% MeOH/CHCl3 as solvent to afford 25 as a
colorless liquid. Yield: 0.256 g (47%). IR: 3340, 1706, 1639
Ack n ow led gm en t. The authors thank the Volks-
wagen Foundation, Germany, for financial support of
this investigation. P.R.S. thanks CSIR, New Delhi, for
a research fellowship.
Su p p or tin g In for m a tion Ava ila ble: Characterization
1
data (including H and 13C NMR spectra) for compounds 23-
25. This material is available free of charge via the Internet
at http://pubs.acs.org.
(26) Weinges, K.; Schwarz, G. Liebigs Ann. Chem. 1993, 7, 811.
J O0263418
9420 J . Org. Chem., Vol. 67, No. 26, 2002