A convenient method for the synthesis of 2,5-difunctionalized phospholes bearing ester groups

Article abstract of DOI:10.1021/jo0605748

Symmetrically and unsymmetrically 2,5-difunctionalized phospholes bearing ester groups were prepared in a one-pot procedure from the corresponding diynes and dichloro(phenyl)phosphine via titanacyclopentadienes. The observed optical properties of the func

Full text of DOI:10.1021/jo0605748

catalysts. However, the number of phospholes bearing multiple
carbonyl functionalities at the R-positions is still limited.3
Burgada et al. and Tebby et al. independently reported the
synthesis of phosphole-2,3,4,5-tetraesters by sequential addi-
tion-elimination reactions of trialkyl phosphites or dialkyl
phosphonites with 2 equiv of dimethyl acetylenedicarboxylate.4,5
Mathey and co-workers prepared a phosphole-2,5-dicarboxylic
acid and its dimethyl ester via dilithiation of the corresponding
2,5-dibromophosphole.6 Another interesting approach reported
by Mathey’s group involves potassium phospholides as key
intermediates, from which several phosphole-2,5-dicarboxylic
acids and esters are prepared via sequential [1,5]-sigmatropy
of the functional groups.7 Although these methods have their
own synthetic merits, there is ample room for developing an
efficient synthetic approach to this class of compounds. Here
we report a new, convenient method for the synthesis of
1-phenylphosphole derivatives bearing one or two ester groups
at the R-positions via titanacycles. The crystal structure,
reactions, and optical properties of the newly prepared 2,5-
difunctionalized phospholes are also described.
A Convenient Method for the Synthesis of
2,5-Difunctionalized Phospholes Bearing Ester
Groups
Yoshihiro Matano,* Tooru Miyajima, Takashi Nakabuchi,
Yuichiro Matsutani, and Hiroshi Imahori
Department of Molecular Engineering, Graduate School of
Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510,
Japan
matano@scl.kyoto-u.ac.jp
ReceiVed March 15, 2006
To obtain the target phosphole derivatives, we set out to use
titanacyclopentadienes as key intermediates because this class
of compounds is known as versatile precursors for various
carbocyclic and heterocyclic compounds.8,9 Quite recently,
Tomita reported the synthesis of phosphole-containing polymers
via titanacyclopentadienes, generated from phenylene-bridged
diynes and Ti(II) reagents.10 To our knowledge, however, Ti-
Symmetrically and unsymmetrically 2,5-difunctionalized
phospholes bearing ester groups were prepared in a one-pot
procedure from the corresponding diynes and dichloro-
(phenyl)phosphine via titanacyclopentadienes. The observed
optical properties of the functionalized phospholes show that
the π-conjugative push-pull interaction between the 2- and
5-substituents plays an important role in controlling the light-
emitting efficiency.
(3) Phospholes bearing one carbonyl functional group at the 2- or
5-position have been reported. (a) Mathey, F. Tetrahedron Lett. 1973, 3255.
(b) Mathey, F. Tetrahedron 1974, 30, 3127. (c) Mathey, F. Tetrahedron
1976, 32, 2395. (d) Santini, C. C.; Mathey, F. J. Org. Chem. 1985, 50,
467. (e) Deschamps, E.; Mathey, F. Bull. Soc. Chim. Fr. 1992, 129, 486.
(f) Deschamps, E.; Mathey, F. Bull. Soc. Chim. Fr. 1996, 133, 541. (g)
Niemi, T.-A.; Coe, P. L.; Till, S. J. J. Chem. Soc., Perkin Trans. 1 2000,
1519. (h) Carmichael, D.; Klankermayer, J.; Richard, L.; Seeboth, N. Chem.
Commun. 2004, 1144.
(4) (a) Burgada, R.; Leroux, Y.; El Khoshnieh, Y. O. Tetrahedron Lett.
1981, 22, 3533. (b) Burgada, R.; El Khoshnieh, Y. O.; Leroux, Y.
Tetrahedron 1985, 41, 1223.
(5) (a) Tebby, J. C.; Willetts, S. E.; Griffiths, D. V. J. Chem. Soc., Chem.
Commun. 1981, 420. (b) Caesar, J. C.; Griffiths, D. V.; Tebby, J. C.; Willetts,
S. E. J. Chem. Soc., Perkin Trans. 1 1984, 1627. (c) Caesar, J. C.; Griffiths,
D. V.; Tebby, J. C. Phosphorus, Sulfur Silicon Relat. Elem. 1987, 29, 123.
(d) Caesar, J. C.; Griffiths, D. V.; Tebby, J. C. J. Chem. Soc., Perkin Trans.
1 1988, 175.
The chemistry of phospholes has been the subject of
numerous studies because of their utility as optical materials
and P ligands.1 It is well-known that the HOMO and LUMO
energies and their energy gap, namely, optical and electrochemi-
cal properties, of phospholes are strongly dependent on the
nature of the 2- and 5-substituents (R-substituents).2 The
coordinating ability of the phosphorus center of σ3-phospholes
is also influenced electronically and sterically by the R-substit-
uents. In these respects, functionalization of the phosphole ring
at the R-positions is a highly promising approach to design
unexploited classes of phosphole-containing materials and
(6) (a) Deschamps, EÄ .; Mathey, F. C.R. Acad. Sci. Paris Ser. IIc 1998,
715. (b) Deschamps, EÄ .; Richard, L.; Mathey, F. Angew. Chem., Int. Ed.
Engl. 1994, 33, 1158.
(7) (a) Holand, S.; Jeanjean, M.; Mathey, F. Angew. Chem., Int. Ed. Engl.
1997, 36, 98. (b) Holand, S.; Maigrot, N.; Charrier, C.; Mathey, F.
Organometallics 1998, 17, 2996. (c) Toullec, P.; Mathey, F. Synlett 2001,
1977. (d) Melaimi, M.; Ricard, L.; Mathey, F.; Le Floch, P. Org. Lett. 2002,
4, 1245.
(1) For reviews, see: (a) Quin, L. D. The Heterocyclic Chemistry of
Phosphorus; Wiley: New York, 1981. (b) Mathey, F. Chem. ReV. 1988,
88, 429. (c) Mathey, F. J. Organomet. Chem. 1990, 400, 149. (d) Quin, L.
D. In ComprehensiVe Heterocyclic Chemistry; Katritzky, A. R., Rees, C.
W., Scriven, E. F. V., Eds.; Elsevier: Oxford, 1996; Vol. 2. (e) Mathey,
F.; Mercier, F. C.R. Acad. Sci. Paris, Ser. IIb 1997, 701. (f) Hissler, M.;
Dyer, P. W.; Re´au, R. Coord. Chem. ReV. 2003, 244, 1. (g) Mathey, F.
Angew. Chem., Int. Ed. 2003, 42, 1578. (h) Mathey, F. Acc. Chem. Res.
2004, 37, 954. (i) Hissler, M.; Lescop, C.; Re´au, R. C.R. Chimie 2005, 8,
1186.
(2) (a) Hay, C.; LeVilain, D.; Deborde, V.; Toupet, L.; Re´au, R. Chem.
Commun. 1999, 345. (b) Hay, C.; Fischmeister, C.; Hissler, M.; Toupet,
L.; Re´au, R. Angew. Chem., Int. Ed. 2000, 10, 1812. (c) Hay, C.; Hissler,
M.; Fischmeister, C.; Rault-Berthelot, J.; Toupet, L.; Nyulaszi, L.; Re´au,
R. Chem.sEur. J. 2001, 7, 4222. (d) Delaere, D.; Nguyen, M. T.;
Vanquickenborne, L. G. Phys. Chem. Chem. Phys. 2002, 4, 1522. (e)
Delaere, D.; Nguyen, M. T.; Vanquickenborne, L. G. J. Phys. Chem. A
2003, 107, 838. (f) Su, H.-C.; Fadhel, O.; Yang, C.-J.; Cho, T.-Y.; Fave,
C.; Hissler, M.; Wu, C.-C.; Re´au, R. J. Am. Chem. Soc. 2006, 128, 983.
(8) For example, see: (a) Ohff, A.; Pulst, S.; Lefeber, C.; Peulecke, N.;
Arndt, P.; Burkalov, V. V.; Rosenthal, U. Synlett 1996, 111. (b) Whitby,
R. J. Transition Met. Org. Synth. 1997, 133. (c) Sato, F.; Urabe, H.;
Okamoto, S. Pure Appl. Chem. 1999, 71, 1511. (d) Siebeneicher, H.; Doye,
S. J. Prakt. Chem. 2000, 342, 102. (e) Sato, F.; Urabe, H.; Okamoto, S.
Chem. ReV. 2000, 100, 2835. (f) Mikami, K.; Matsumoto, Y.; Shiono, T.
Sci. Synth. 2003, 2, 457. (g) Rosenthal, U.; Arndt, P.; Baumann, W.;
Burlakov, V. V.; Spannenberg, A. J. Organomet. Chem. 2003, 670, 84.
(9) For the syntheses of 2,5-disubstituted phospholes via zirconacyclo-
pentadienes (Fagan-Nugent method), see: (a) Fagan, P. J.; Nugent, W. A.
J. Am. Chem. Soc. 1988, 110, 2310. (b) Douglas, T.; Theopold, K. H. Angew.
Chem., Int. Ed. Engl. 1989, 28, 1367. (c) Fagan, P. J.; Nugent, W. A.;
Calabrese, J. C. J. Am. Chem. Soc. 1994, 116, 1880. (d) Mao, S. S. H.;
Tilley, T. D. Macromolecules 1997, 30, 5566. (e) Morisaki, Y.; Aiki, Y.;
Chujo, Y. Macromolecules 2003, 36, 2594. See also ref 2.
10.1021/jo0605748 CCC: $33.50 © 2006 American Chemical Society
Published on Web 06/22/2006
5792
J. Org. Chem. 2006, 71, 5792-5795

TABLE 1. Synthesis of 2,5-Difunctionalized Phospholes 4
SCHEME 1. Synthesis of Phosphole 4a
(II)-mediated cyclization of diynes bearing carbonyl groups at
the terminal sp carbons to the corresponding 2,5-difunctionalized
heteroles has not been examined so far. Scheme 1 depicts the
synthesis of 1-phenylphosphole-2,5-dicarboxylic acid diethyl
ester 4a. Reaction of diethyl deca-2,8-diynedioate (1a) with (η2-
propene)Ti(O-i-Pr)2, generated in situ from Ti(O-i-Pr)4 and 2
equiv of i-PrMgCl as previously reported by Sato and co-
workers,11 followed by treatment with dichloro(phenyl)phos-
phine (3) afforded 4a in 31% isolated yield. When the reaction
was quenched by 1 M HCl without adding 3, bis(ethoxycarbo-
nylmethylene)cyclohexane (5)12 was obtained as the hydrolyzed
product. This result suggests that 2,5-bis(ethoxycarbonyl)-
titanacyclopentadiene (2a) is formed by treatment of 1a with
(η2-propene)Ti(O-i-Pr)2 at -50 °C.13 It should be noted here
that the ester groups are compatible with the reaction conditions
employed.14
Other symmetrically 2,5-difunctionalized phospholes 4b-d
were prepared from the corresponding diynedioates 1b-d
according to a similar procedure (Table 1, entries 2-4).
Unsymmetrical diynes 1e-g bearing an aryl group at one
terminal and an ethoxycarbonyl group at the other were also
converted to unsymmetrically 2,5-difunctionalized phospholes
4e-g (entries 5-7). Attempts to convert 9-anthryl derivative
1h to phosphole have been unsuccessful, probably due to steric
reasons (entry 8). Although the isolated yields of 4a-g are
moderate, the present approach is convenient in that the desired
phospholes are prepared from readily available substrates in a
few steps.15 Another practical advantage is that the symmetrical
derivatives 4a-d can be purified by simple recrystallization of
the crude reaction mixture from MeOH without performing
column chromatography. More importantly, it has been found
that the Sato and Urabe titanacycle method is applicable to the
synthesis of heteroles bearing one or two R-ester groups.
Compounds 4a-g are air-stable solids and have been fully
characterized by 1H, 13C, and 31P NMR, MS, IR, and elemental
analyses. As was observed for the related bicyclic 1-phenyl-
2,5-diarylphospholes,2c the 31P and 13C chemical shifts of the
phosphole ring are strongly influenced by the fused carbocycle.
Thus, the 31P NMR peaks of the 3,4-C3-bridged derivatives
4b-g were observed at much lower fields (δ ) +32.0-40.5)
than that of the 3,4-C4-bridged derivative 4a (δ ) +8.7). The
â-carbons (C3 and C4 carbons) are much more deshielded than
the R-carbons (C2 and C5 carbons) for 4b-g, whereas there is
a slight difference in their chemical shifts for 4a. These data
imply that the fused carbocycle constrains the structural
modification of the phosphole ring to some extent. On the other
hand, the carbonyl carbons of 4a-g were observed at almost
2
the same field (δ ) 163.8-165.1) with relatively large JP-C
values of 15.6-21.7 Hz.
The structure of diester 4b was further elucidated by X-ray
crystallographic analysis (Figure 1).16 The phosphorus center
adopts a distorted trigonal pyramidal geometry with a mean
C-P-C angle of 98.1°. The average CR-Câ bond length of
1.36 Å is much shorter than the Câ-Câ bond length of 1.456-
(2) Å, and the Bird index was calculated to be 25.6,17 indicating
the low aromaticity of the phosphole ring in 4b. Two carbonyl
groups lie on almost the same plane of the phosphole ring,
probably due to the π-conjugation with the adjacent CR-Câ
double bonds.
(10) (a) Tomita, I.; Ueda, M. Macromol. Symp. 2004, 209, 217. (b)
Tomita, I. Polym. Prep. 2004, 45, 415.
(11) (a) Urabe, H.; Suzuki, K.; Sato, F. J. Am. Chem. Soc. 1997, 119,
10014. (b) Urabe, H.; Takeda, T.; Hideura, D.; Sato, F. J. Am. Chem. Soc.
1997, 119, 11295.
(12) Kawasaki, T.; Saito, S.; Yamamoto, Y. J. Org. Chem. 2002, 67,
4911.
(13) In this reaction, the temperature should be kept at -50 °C during
the formation of the titanacycle. Below or above this temperature, the yield
of 4a decreased significantly.
Compounds 4 are potential precursors for other 2,5-difunc-
tionalized phospholes. Some chemical transformations at the
ester groups and/or the phosphorus center of 4b and 4g are
summarized in Scheme 2. Reaction of 4b with an excess amount
(14) (a) Boymond, L.; Rottla¨nder, M.; Cahiez, G.; Knochel P. Angew.
Chem., Int. Ed. 1998, 37, 1701. (b) Ren, H.; Krasovskiy, A.; Knochel, P.
Org. Lett. 2004, 6, 4215. (c) Knochel, P.; Dohle, W.; Gommermann, N.;
Kneisel, F. F.; Kopp, F.; Korn, T.; Sapountzis, I.; Vu, V. A. Angew. Chem.,
Int. Ed. 2003, 42, 4302.
(15) Compounds 1a-h were prepared from the corresponding terminal-
free diynes in one or two steps.
(16) Formula C19H21O4P, space group P21/a, a ) 9.350(3) Å, b ) 20.265-
(6) Å, c ) 9.951(3) Å, â ) 113.1422(11)°, V ) 1733.7(10) Å3, Z ) 4, Dc
) 1.319 g cm-3, 13 544 observed and 218 variables, Rw ) 0.1221, R )
0.0503 (I > 2.00σ(I)), GOF ) 1.16.
(17) (a) Bird, C. W. Tetrahedron 1985, 41, 1409. (b) Bird, C. W.
Tetrahedron 1990, 46, 5697.
J. Org. Chem, Vol. 71, No. 15, 2006 5793

TABLE 2. Absorption and Fluorescence Spectra of
2,5-Difunctionalized Phospholesa
phosphole
λ
max/nm (log ꢀ)
λ
em/nmb
4b
4e
4f
4g
9
329 (3.92)
355 (4.12)
357 (4.18)
382 (4.17)
390 (4.03)
375 (3.87)
c
453
d
475
c
10
c
a Measured in THF at 25 °C. b Excited at 365 nm. c No emission was
detected. d A weak and broad band was observed at around 585 nm.
THF (Table 2). The absorption maximum due to the π-π*
transition of the extended conjugated system of thienyl ester
4g (λmax ) 382 nm) is red shifted relative to those of phenyl
and pyridyl esters 4e,f (λmax ) 355 and 357 nm, respectively)
and diester 4b (λmax ) 329 nm). The functionalization at
phosphorus from σ3 to σ4 (9 vs 4g) causes a slight red shift,
whereas the replacement of the ester group by the hydroxy-
methyl group (10 vs 9) causes a blue shift. Significant substituent
effects were also observed in the fluorescence spectra. The σ3-
types of phenyl and thienyl esters 4e and 4g are moderately
fluorescent,19 wherein 4g exhibits an emission band at a longer
wavelength (λmax ) 475 nm) than 4e (λmax ) 453 nm). In
contrast, the other phospholes examined are weakly fluorescent
(4f) or nonfluorescent (4b, 9, 10). These data clearly show that
the electronic properties of the R-substituents and the phosphorus
center affect the degree of π-conjugation with the 1,3-diene unit
of the phosphole ring. More importantly, the π-conjugative
push-pull (charge-transfer) interaction between the electron-
donating 5-aryl group and the electron-withdrawing 2-ester
group plays an important role in controlling the HOMO-LUMO
gap of 2,5-difunctionalized phospholes and their light-emitting
efficiencies.20 To compare the polarity in the excited state with
that in the ground state, the solvatochromism of the absorption
and fluorescence spectra of 4g was also examined. Thus, when
the solvent was changed from cyclohexane to N,N-dimethyl-
formamide (DMF), the emission maxima of 4g displayed a
larger bathochromic shift (∆λem ) 1263 cm-1) than the
absorption maxima (∆λmax ) 478 cm-1).21 A Kamlet-Taft
analysis22 for the fluorescence spectra gave a linear solvation
energy relationship (LSER) versus the solvatochromic parameter
π* [νem ) 21.80-1.29π* (103 cm-1)] with an R value of 0.973
(n ) 11). Although a reliable LSER could not be given from
the absorption spectra, it is apparent that the polarity of the
excited state of 4g is larger than that of the ground state.
In summary, we have demonstrated for the first time that the
diynes bearing one or two terminal ester groups are successfully
transformed to the corresponding 2,5-difunctionalized phos-
pholes via titanacyclopentadiene intermediates. A practical
FIGURE 1. ORTEP diagram of 4b (30% probability ellipsoids).
Selected bond lengths (Å) and bond angles (deg): P1-C1, 1.820(2);
P1-C7, 1.8200(19); P1-C14, 1.8417(17); C1-C2, 1.357(2); C2-C6,
1.456(2); C6-C7, 1.359(3); C1-C8, 1.475(2); C7-C11, 1.478(2); O1-
C8, 1.211(2); O2-C9, 1.459(2); C1-P1-C7, 89.88(9); C1-P1-C14,
102.12(8); C7-P1-C14, 102.16(8).
SCHEME 2. Functionalization of Phospholes 4b,g
of diisobutylaluminum hydride (DIBAH) in hexane gave 2,5-
bis(hydroxymethyl)phosphole 6 as a moderately air-sensitive
substance.18 The subsequent treatment of 6 with elemental sulfur
(S8) afforded the corresponding phosphine sulfide 7 as an air-
stable solid in 63% total yield (from 4b). Treatment of 4b with
4 equiv of MeMgBr in THF, followed by addition of S8, gave
tertiary diol 8 in 40% yield. The diols 7 and 8 are potential
precursors for phosphole-containing macrocyclic compounds.
When treated with excess S8 in CH2Cl2 at room temperature
for 5 days, 4g was converted to the phosphine sulfide 9 in 35%
yield. Under the same conditions, diester 4b was recovered
quantitatively. The thienyl ester 4g also reacted with DIBAH/
S8 to give primary alcohol 10 in 83% yield. The 31P chemical
shifts of the σ4-phospholes 7-10 were observed at fields lower
(δ ) +66.2-68.9) than those of the σ3-phospholes 4b and 4g.
To examine the substituent effects on the optical properties
of the 2,5-difunctionalized phospholes, UV-vis absorption and
fluorescence spectra of 4b, 4e-g, 9, and 10 were measured in
(18) Compound 6 was characterized by 1H NMR and mass spectrometry.
(19) Fluorescence quantum yields of 4e and 4g relative to quinine sulfate
were determined to be 0.074 and 0.088%, respectively.
(20) Rea´u and co-workers have extensively studied the optical properties
of 2,5-diarylphosphole derivatives and disclosed the important role of an
intramolecular charge-transfer interaction enforced by the presence of
different types of aryl substituents, such as thienyl and pyridyl groups.
See: (a) Fave, C.; Hissler, M.; Se´ne´chal, K.; Ledoux, I.; Zyss, J.; Re´au, R.
Chem. Commun. 2002, 1674. (b) Hay, C.; Sauthier, M.; Deborde, V.; Hissler,
M.; Toupet, L.; Rea´u, R. J. Organomet. Chem. 2002, 643-644, 494.
(21) Solvatochromism of the absorption and emission bands (λmax, λem
)
of 4g (nm): cyclohexane (379, 457); Et3N (380, 465); Et2O (379, 468);
CCl4 (383, 464); EtOAc (386, 473); THF (382, 475); CHCl3 (385, 477);
acetone (381, 478); MeCN (380, 482); CH2Cl2 (382, 479); DMF (384, 485).
(22) Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. J.
Org. Chem. 1983, 48, 2877.
5794 J. Org. Chem., Vol. 71, No. 15, 2006

yellow solid. When the reaction was quenched without adding 3,
cyclic olefin 5 was obtained as the major product.
advantage of the present method is that the starting substrates
are readily accessible from commercially available materials.
Furthermore, the present method is potentially applicable to the
synthesis of other heterocyclic compounds bearing R-ester
groups. The observed optical properties of newly prepared 2,5-
difunctionalized phospholes demonstrate that a combination of
the 2-ester group and the electron-donating 5-aryl group
contributes to the efficient π-conjugative interaction with the
phosphole ring.
1
4a: Mp 71-72 °C; H NMR (CDCl3) δ 1.10 (t, 6H, J ) 6.8
Hz), 1.76 (br, 4H), 3.03 (br, 2H), 3.21 (br, 2H), 4.11 (br, 4H), 7.25-
7.35 (m, 5H); 13C{1H} NMR (CDCl3) δ 14.1, 22.2, 29.0, 60.3, 128.3
3
4
2
(d, JP-C ) 8.7 Hz), 129.8 (d, JP-C ) 1.9 Hz), 134.0 (d, JP-C
)
)
2
2
19.9 Hz), 136.9, 158.8 (d, JP-C ) 10.6 Hz), 164.8 (d, JP-C
18.8 Hz); 31P{1H} NMR (CDCl3) δ + 8.7; IR (KBr) νmax 1701,
1696 (CdO) cm-1; MS (MALDI-TOF) m/z 358 (M+). Anal. Calcd
for C12H23O4P: C, 67.03; H, 6.47; P, 8.64. Found: C, 67.21; H,
6.51; P, 8.44. The ipso carbon of the phenyl group could not be
detected clearly in the 13C NMR.
Experimental Section
5. (1E,2E)-Isomer: 1H NMR (CDCl3) δ 1.29 (t, 6H, J ) 6.8
Hz), 1.74 (m, 4H), 2.98 (m, 4H), 4.15 (q, 4H, J ) 6.8 Hz), 5.83 (s,
2H). (1E,2Z)-Isomer: 1H NMR (CDCl3) δ 1.23 (t, 6H, J ) 6.8
Hz), 1.74 (m, 4H), 2.37 (m, 4H), 4.21 (q, 4H, J ) 6.8 Hz), 5.61 (s,
1H), 5.66 (s, 1H).
To a mixture of 1a (1.25 g, 5.0 mmol), Ti(O-i-Pr)4 (1.5 mL, 5.0
mmol), and Et2O (75 mL) was slowly added an ether solution of
i-PrMgCl (2.0 M × 5.0 mL, 10 mmol) at -78 °C, and the resulting
mixture was stirred for 2 h at -50 °C. Dichloro(phenyl)phosphine
3 (0.68 mL, 5.0 mmol) was then added to the mixture at this
temperature, and the resulting suspension was allowed to warm to
room temperature. After stirring for an additional 2 h at room
temperature, a saturated NH4Cl solution (30 mL) was poured into
the reaction mixture, and insoluble substances were filtered off
through a Celite bed. The filtrate was separated, and the aqueous
phase was extracted with Et2O (15 mL × 2). The combined organic
extracts were washed with brine (50 mL), dried over MgSO4, and
concentrated in vacuo to give an oily residue, which was crystallized
from cold MeOH at -78 °C to afford 4a (560 mg, 31%) as a pale
Acknowledgment. This work was partially supported by a
Grant-in-Aid (No. 17350018) from the Ministry of Education,
Culture, Sports, Science, and Technology of Japan.
Supporting Information Available: Experimental details, full
spectroscopic data, and the CIF file of 4b. This material is available
free of charge via the Internet at http://pubs.acs.org.
JO0605748
J. Org. Chem, Vol. 71, No. 15, 2006 5795