An efficient double 1,2-addition reaction of 2,3-allenoates with allyl magnesium chloride

Article abstract of DOI:10.1021/jo801809j

(Chemical Equation Presented) In this paper, it was reported that double 1,2-addition reaction of 2,3-allenoates with allyl magnesium chloride at room temperature in the absence of any transition metal catalyst provides an efficient method for the synthes

Full text of DOI:10.1021/jo801809j

SCHEME 1
An Efficient Double 1,2-Addition Reaction of
2,3-Allenoates with Allyl Magnesium Chloride
Bo Chen, Zhan Lu, Guobi Chai, Chunling Fu, and
Shengming Ma*
Laboratory of Molecular Recognition and Synthesis,
Department of Chemistry, Zhejiang UniVersity,
Hangzhou 310027, Zhejiang, P. R. China
masm@mail.sioc.ac.cn
magnesium chloride affords the double allylation products, that
is, synthetically useful 2,3-allenols.2
ReceiVed August 12, 2008
Recently, we have realized an iron-catalyzed highly regio-
and stereoselective conjugate addition reaction of 2,3-allenoates
with alkyl, aryl or alkenyl Grignard reagents providing an
efficient route to ꢀ,γ-unsaturated alkenoates with excellent
stereoselectivity.3 However, when we tested the similar iron-
catalyzed reaction of ethyl 4-phenyl-2-(n-propyl)- 2,3-butadi-
enoate 1a with allyl magnesium chloride in a solution of toluene
at -78 °C, the double 1,2-addition product, i.e., tertiary 2,3-
allenol 2a, was formed in 85% isolated yield (eq 1). The
formation of the 1,4-addition product was not observed.
In this paper, it was reported that double 1,2-addition reaction
of 2,3-allenoates with allyl magnesium chloride at room
temperature in the absence of any transition metal catalyst
provides an efficient method for the synthesis of tertiary
R-allenols. The optically active allenol could be prepared
from the reaction of the optically active 2,3-allenoate without
obvious racemization of the axial chirality. Under different
reaction conditions, cyclization reactions of R-allenol 2i
prepared have been studied for the synthesis of different 2,5-
dihydrofuran derivatives.
Subsequently, the 1,2-addition reaction of ethyl 4-(p-bro-
mophenyl)-2-propyl-2,3-butadienoate 1b with allyl magnesium
chloride was conducted at -78 °C in the absence of the iron
catalyst to afford the similar alcohol product 2b (entry 1, Table
1). The reaction could also take place in toluene, THF, or Et2O
to afford 2b in very similar yields. Considering the operational
simplicity, safety, and the yield for the formation of 2b (entries
2-4, Table 1), we defined the reaction of 0.4 mmol of 2,3-
allenoates with 3 equiv of allyl magnesium chloride in a solution
of THF (5 mL) at room temperature as the standard (entry 3,
Table 1).
During the last 20 years, due to the high reactivity, substitu-
ent-loading capability and the stereochemistry involved in the
chemistry of allenes, many chemists focus their attention on
the development of new reactions of functionalized allenes.1
On the basis of these developments, allenes have become
powerful starting materials in organic chemistry. Thus, new
methods for efficient synthesis of allenes are of current interest.
Due to the possible 1,2-addition, 1,4-addition, and migration
of the carbon-carbon double bond, the reaction of 2,3-allenoates
with main group organometallic reagents may provide different
products (Scheme 1). Herein, we wish to report our recent
observation that the reaction of 2,3-allenoates with allyl
(2) For some typical reports on the application of 2,3-allenols, see: (a) Nikam,
S. S.; Chu, K. H.; Wang, K. K. J. Org. Chem. 1986, 51, 745. (b) Marshall,
J. A.; Wang, X. J. J. Org. Chem. 1990, 55, 2995. (c) Marshall, J. A.; Wang,
X. J. J. Org. Chem. 1991, 56, 4913. (d) Marshall, J. A.; Pinney, K. G. J. Org.
Chem. 1993, 58, 7180. (e) Marshall, J. A.; Sehon, C. A. J. Org. Chem. 1995,
60, 5966. (f) Marshall, J. A.; Yu, R. H.; Perkins, J. F. J. Org. Chem. 1995, 60,
5550. (g) Hoffmann-Ro¨der, A.; Krause, N. Org. Lett. 2001, 3, 2537. (h) Lan,
Y.; Hammond, G. B. Org. Lett. 2002, 4, 2437. (i) Yoneda, E.; Kaneko, T.; Zhang,
S. W.; Onitsuka, K.; Takahashi, S. Org. Lett. 2000, 2, 441. (j) Alcaide, B.;
Almendros, P.; Rodr´ıguez, R. Chem.-Eur. J. 2005, 11, 5708. (k) Alcaide, B.;
Almendros, P.; Mart´ınez del Campo, T. Angew. Chem., Int. Ed. 2006, 45, 4501.
(l) Deng, Y.; Li, J.; Ma, S. Chem.-Eur. J. 2008, 14, 4263. (m) Deng, Y.; Yu,
Y.; Ma, S. J. Org. Chem. 2008, 73, 585. (n) Ma, S.; Zhao, S. J. Am. Chem. Soc.
1999, 121, 7943. (o) Kang, S. K.; Yamaguchi, T.; Pyun, S. J.; Lee, Y. T.; Baik,
T. G. Tetrahedron Lett. 1998, 39, 2127. (p) Ma, S.; Zhao, S. J. Am. Chem. Soc.
2001, 123, 5578. (q) Deng, Y.; Jin, X.; Ma, S. J. Org. Chem. 2007, 72, 5901.
(r) Lu, Z.; Ma, S. AdV. Synth. Catal. 2007, 349, 1225. (s) Park, C.; Lee, P. H.
Org. Lett. 2008, 10, 3359. (t) Ma, S.; Gu, Z. J. Am. Chem. Soc. 2005, 127,
6182. (u) Fu, C.; Li, J.; Ma, S. Chem. Commun. 2005, 4119. (v) Li, J.; Fu, C.;
Chen, G.; Chai, G.; Ma, S. AdV. Synth. Catal. 2008, 350, 1376.
* To whom correspondence should be addressed. Fax: (+86) 21-64167510.
(1) For reviews on the chemistry of allenes, see: (a) Zimmer, R.; Dinesh,
C. U.; Nandanan, E.; Khan, F. A. Chem. ReV. 2000, 100, 3067. (b) Marshall,
J. A. Chem. ReV. 2000, 100, 3163. (c) Hashmi, A. S. K. Angew. Chem., Int. Ed.
2000, 39, 3590. (d) Bates, R.; Satcharoen, V. Chem. Soc. ReV. 2002, 31, 12. (e)
Sydnes, L. K. Chem. ReV. 2003, 103, 1133. (f) Ma, S. Acc. Chem. Res. 2003,
36, 701. (g) Brandsma, L.; Nedolya, N. A. Synthesis 2004, 735. (h) Tius, M. A.
Acc. Chem. Res. 2003, 36, 284. (i) Wei, L.-L.; Xiong, H.; Hsung, R. P. Acc.
Chem. Res. 2003, 36, 773. (j) Lu, X.; Zhang, C.; Xu, Z. Acc. Chem. Res. 2001,
34, 535. (k) Wang, K. K. Chem. ReV. 1996, 96, 207. (l) Ma, S. Chem. ReV.
2005, 105, 2829. (m) Ma, S. Aldrichim. Acta 2007, 40, 91. For a recent
monograph on the chemistry of allenes, see: (n) Krause, N.; Hashmi, A. S. K.
Modern Allene Chemistry; Wiley-VCH: Weinheim, 2004. For a recent review
on the synthesis of allenes, see: (o) Brummond, K. M.; DeForrest, J. E. Synthesis
2007, 795.
(3) (a) Lu, Z.; Chai, G.; Ma, S. J. Am. Chem. Soc. 2007, 129, 14546. (b) Lu,
Z.; Chai, G.; Zhang, X.; Ma, S. Org. Lett. 2008, 10, 3517.
9486 J. Org. Chem. 2008, 73, 9486–9489
10.1021/jo801809j CCC: $40.75 2008 American Chemical Society
Published on Web 11/04/2008

TABLE 1. Effect of Temperature and Solvents on the Reaction of
2,3-Allenoate 1b with Allyl Magnesium Chloride
entry
solvent
T (°C)
time (min)
NMR yield of 2b (%)
1
2
3
4
toluene
toluene
THF
-78
rt
rt
51
52
33
26
86
90
91
89
Et2O
rt
In addition, optically active allenol (R)-2c can be prepared
in 90% isolated yield from optically active (R)-1c4 without
obvious racemization of the axial chirality (eq 3).
TABLE 2. Reaction of 2,3-Allenoates with Allyl Magnesium
Chloride
1
entry
R1
R2
R3
time(min) isolated yield of 2 (%)
1
2
3
4
5
6
7
8
9
Ph
p-BrC6H4
Ph
p-ClC6H4
p-FC6H4
p-MeOC6H4
Ph
H
H
H
H
H
H
H
H
H
H
n-Pr (1a)
n-Pr (1b)
Me (1c)
Me (1d)
Me (1e)
Me (1f)
Et (1g)
Me (1h)
Bn (1i)
Bn (1i)
3
33
20
4
19
3
4
4
8
4
3
6
33
3
2
35
3
2
94 (2a)
95 (2b)
85 (2c)
90 (2d)
88 (2e)
79 (2f)
94 (2g)
86 (2h)
100 (2i)
97 (2i)
83 (2j)
97 (2k)
97 (2l)
91 (2m)
90 (2n)
60 (2o)
94 (2p)
63 (2q)
64 (2r)
The reaction may also proceed with 2-methylallyl magnesium
chloride5a (eq 4).
n-C4H9
n-C4H9
10a n-C4H9
11
12
13
14
15
16
17
18a
19b
Ph
n-C7H15
Ph
Ph
Ph
Et
(CH2)5
H
Ph H (1j)
H (1k)
H
Me Me (1l)
Et Me (1m)
Ph Me (1n)
Et Me (1o)
Me (1p)
However, the reaction of 1i with 1-buten-3-yl, 2-butenyl,
3-phenyl-2-propenyl5b magnesium chlorides all afforded com-
plicated mixtures probably due to the issue of regioselectivity.
Finally, it should be noted that under the standard reaction
conditions, the reaction of 2-benzylocta-2,3-dienoate 1i with
vinyl magnesium chloride in THF is complicated; the corre-
sponding reaction with methyl or phenyl magnesium chloride
in THF at room temperature yielded the related conjugated
addition products.3
H
H
Me (1q)
H (1r)
H
20
a The reaction was run with 2 mmol of 2,3-allenoate. b The allenoate
1r was methyl 2,3-butadienoate.
Under different cyclization conditions,6 the prepared 4-allyl-
5-benzyl-1,5,6-undecatrien-4-ol 2i can easily be converted to
different 2,5-dihydrofuran derivatives in 76∼99% yields (Scheme
2). Due to the presence of two allyl groups, 2,5-dihydrofuran 7
may undergo a RCM reaction7 to form an extra five-membered
ring to afford bicyclic compound 8 in 86% yield (Scheme 2).
In conclusion, we have established a protocol for the efficient
and general preparation of tertiary 1,1-diallyl substituted-2,3-
allenols in high yields via the reaction of 2,3-allenoates with
allyl magnesium chlorides at room tempreture. The optically
active allenol may be prepared from the reaction of the optically
active 2,3-allenoate with allyl magnesium chloride without
obvious racemization of the axial chirality. The products have
been applied to the synthesis of 2,5-dihydrofuran derivatives.
Under the optimized reaction conditions, a wide range of the
substrates were tested and the results are summarized in Table
2. The data shown in Table 2 indicated that the reactions are
fast, efficient, and general (Table 2) at room temperature in the
absence of any transition metal catalyst: R1 and R2 can be aryl,
alkyl, and Bn in 2,4-disubstituted 2,3-allenoates (entries 1-10,
Table 2); 4,4-Diphenylbutadienoate 1j and undeca-2,3-dienoate
1k may also react smoothly to afford the corresponding alcohols
2j and 2k in 83% and 97% yields, respectively (entries 11 and
12, Table 2). The fully substituted 2,3-allenoates 1l-1q all
reacted with allyl magnesium chloride smoothly (entries 13-17,
Table 2). However, the reaction of 4-unsubstituted ethyl
2-methyl-2,3-butadienoate 1q and methyl 2,3-butadienoate 1r
with allyl magnesium chloride afforded the 1,2,6-heptatrien-4-
ols 2q and 2r in much lower yields (63% and 64%, respectively)
(entries 18 and 19, Table 2).
(4) Ma, S.; Wu, S. Chem. Commun. 2001, 441.
(5) (a) Lipshutz, B. H.; Hackmann, C. J. Org. Chem. 1994, 59, 7437. (b)
Baker, K. V.; Brown, J. M.; Hughes, N.; Skarnulis, A. J.; Sexton, A. J. Org.
Chem. 1991, 56, 698.
(6) (a) Ma, S.; Gao, W. Tetrahedron Lett. 2000, 41, 8933. (b) Fu, C.; Ma, S.
Eur. J. Org. Chem. 2005, 3942. (c) Olsson, L. I.; Claesson, A. Synthesis 1979,
743.
The reaction of diethyl 2,9-dimethyl-2,3,7,8-decatetraenedio-
ate 3 with 6 equiv of allyl magnesium chloride afforded the
tetraallyl-substituted bis-allenol 4 in 82% yield (eq 2).
(7) Grubbs, R. H.; Tumas, W. Science 1989, 243, 907.
J. Org. Chem. Vol. 73, No. 23, 2008 9487

SCHEME 2
as monitored by TLC, the mixture was then quenched with a
saturated aqueous solution of Na2S2O3 to remove the excess I2. This
mixture was extracted with ether (3 × 25 mL), washed with an
aqueous solution of NaCl, and dried over Na2SO4. Concentration
and column chromatography on silica gel (First: petroleum ether,
then: petroleum ether/ethyl acetate ) 40:1) afforded 5 (74.4 mg,
89%): oil. 1H NMR (300 MHz, CDCl3) 7.40-7.18 (m, 5 H),
5.72-5.54 (m, 2 H), 5.02-4.84 (m, 4 H), 4.65-4.59 (m, 1 H),
3.55 (d, J ) 15.0 Hz, 1 H), 3.46 (dd, J1 ) 15.0 Hz, J2 ) 1.4 Hz,
1 H), 2.26 (ddt, J1 ) 14.4 Hz, J2 ) 6.9 Hz, J3 ) 1.4 Hz, 1 H),
2.18-2.07 (m, 3 H), 1.94-1.82 (m, 1 H), 1.56-1.28 (m, 5 H),
0.92 (t, J ) 7.1 Hz, 3 H); 13C NMR (CDCl3, 75 MHz) 145.9, 137.4,
133.5, 133.2, 128.9, 128.4, 126.7, 117.8, 96.5, 93.0, 87.8, 43.9,
43.1, 35.5, 34.9, 27.0, 22.7, 14.1; MS (m/z) 381 ((M - C3H5)+,
56.03), 254 ((M - C3H5-I)+, 100); IR (neat) 3075, 3027, 2956,
Due to the synthetic potential of the 2,3-allenols, this reaction
may be useful in organic synthesis. Further studies in this area
are being conducted in our laboratory.
Experimental Section
Materials. Toluene, Et2O, and THF were distilled from Na/
benzophenone. 2,3-Allenoates were prepared according to the
known procedure.8 Allyl magnesium chloride (1.7 M solution in
THF) used in this study was purchased from Acros Organics.
1-Buten-3-yl magnesium chloride and 2-butenyl magnesium chlo-
ride (0.5 M solution in THF) used in this study were purchased
from Aldrich Organics. 2-Methylallyl magnesium chloride (0.4 M
solution in THF) and 3-phenyl-2-propenyl magnesium chloride (0.4
M solution in Et2O) used in this study were prepared according to
the known procedures.5 Other commercially available chemicals
were purchased and used without additional purification unless
noted otherwise.
2930, 2858, 1640, 1601, 1494, 1454, 1432, 1331, 1100 cm-1
;
Elemental analysis: Calcd for C21H27IO: C, 59.72; H, 6.44; Found:
C, 59.85; H, 6.40.
(2) Synthesis of 2,2,4-Triallyl-3-benzyl-5-butyl-2,5-dihydrofu-
ran (6). A solution of 2i (89.0 mg, 0.3 mmol), allyl bromide (188.0
mg, 1.5 mmol, 5 equiv), and PdCl2 (3.0 mg, 0.015 mmol) in DMA
(1 mL) was stirred at rt. When the reaction was complete as
monitored by TLC, this mixture was added with ether (60 mL).
The organic layer was washed subsequently with diluted HCl (1%,
aq.), NaHCO3 (sat. aq.), brine, and dried over anhydrous Na2SO4.
Evaporation and column chromatography on silica gel (First:
petroleum ether, then: petroleum ether/ethyl acetate ) 100/1)
Synthesis of 2,3-Allenols via the Reaction of 2,3-Allenoates in
THF with Allyl Magnesium Chloride in THF. Synthesis of
4-Allyl-1-phenyl-3-(n-propyl)-1,2,6-heptatrien-4-ol (2a). To a solu-
tion of 1a (92.5 mg, 0.4 mmol) in THF (5 mL) in a dry Schlenk
tube under a nitrogen atmosphere at room temperature was added
a solution of allyl magnesium chloride in THF (0.7 mL, 1.7 M,
1.2 mmol, 3 equiv) by a syringe at rt. The reaction was monitored
by TLC. After 3 min, the reaction mixture was quenched slowly
with saturated NH4Cl (1 mL) at rt and extracted with ether (60
mL). The organic layer was washed subsequently with diluted HCl
(1%, aq.), NaHCO3 (sat. aq.), brine, and dried over anhydrous
Na2SO4. Evaporation and column chromatography on silica gel
(eluent: petroleum ether/ethyl acetate ) 100/1) afforded 2a (101.8
1
afforded the desired product 6 (76.8 mg, 76%): liquid. H NMR
(300 MHz, CDCl3) 7.32-7.13 (m, 5 H), 5.82-5.60 (m, 3 H),
5.13-4.85 (m, 6 H), 4.72-4.60 (m, 1 H), 3.43 (d, J ) 15.9 Hz, 1
H), 3.30 (d, J ) 15.9 Hz, 1 H), 3.02-2.90 (m, 1 H), 2.67 (dd, J )
15.6 Hz, J ) 7.2 Hz, 1 H), 2.40-2.28 (m, 1 H), 2.22-2.08 (m, 3
H), 1.74-1.56 (m, 1 H), 1.50-1.23 (m, 5 H), 0.91 (t, J ) 6.9 Hz,
3 H); 13C NMR (CDCl3, 75 MHz) 138.9, 136.1, 134.7, 134.6, 134.4,
134.3, 128.8, 128.3, 126.2, 117.2, 117.0, 116.2, 92.3, 85.4, 44.2,
43.1, 34.5, 31.3, 30.1, 27.7, 22.9, 14.1; MS (m/z) 295 ((M - C3H5)+,
100); IR (neat) 3075, 3027, 3007, 2957, 2929, 2858, 1830, 1639,
1
mg, 94%): oil. H NMR (300 MHz, CDCl3) 7.33-7.26 (m, 4 H),
7.28-7.20 (m, 1 H), 6.28 (t, J ) 3.3 Hz, 1 H), 5.92-5.70 (m, 2
H), 5.15-5.00 (m, 4 H), 2.45-2.25 (m, 4 H), 2.10-1.90 (m, 3 H),
1.50-1.33 (m, 2 H), 0.86 (t, J ) 7.4 Hz, 3 H); 13C NMR (CDCl3,
75 MHz) 200.7, 134.9, 133.8, 133.5, 128.6, 126.9, 126.6, 118.60,
118.58, 114.6, 99.4, 74.7, 44.1, 43.8, 29.9, 21.1, 14.2; MS (m/z)
268 (M+, 2.17), 250 ((M-H2O)+, 6.24), 227 ((M-C3H5)+, 41.00),
41 (C3H5+, 100); IR (neat) 3560, 3475, 3076, 3030, 2958, 2931,
1603, 1495, 1454, 1432, 1377, 1335, 1259, 1076, 1016 cm-1
;
Elemental analysis: Calcd for C24H32O: C, 85.66; H, 9.58; Found:
C, 85.63; H, 9.57.
2872, 1943, 1639, 1598, 1496, 1459, 1436, 1340, 1028 cm-1
HRMS calcd for C19H24O (M+) 268.1827, found 268.1830.
;
(3) Synthesis of 2,2-Diallyl-3-benzyl-5-butyl-2,5-dihydrofuran
(7). A solution of 2i (58.9 mg, 0.2 mmol) and AgNO3 (6.9 mg,
0.04 mmol, 20 mol%) in CH2Cl2 (2 mL) was stirred at rt. When
the reaction was complete as monitored by TLC, ether (10 mL)
was added to quench the reaction. Concentration and column
chromatography on silica gel (First: petroleum ether, then: petro-
leum ether/ethyl acetate ) 100:1) afforded 7 (58.5 mg, 99%): liquid.
1H NMR (300 MHz, CDCl3) 7.40-7.14 (m, 5 H), 5.94-5.75 (m,
2 H), 5.15-4.98 (m, 5 H), 4.62-4.54 (m, 1 H), 3.23-3.10 (m, 2
Synthesis of 2,5-Dihydrofuran Derivatives via Cyclization Reac-
tions of r-Allenol 2i. (1) Synthesis of 2,2-Diallyl-3-benzyl-5-butyl-
4-iodo-2,5-dihydrofuran (5). A solution of 2i (58.9 mg, 0.2 mmol)
and I2 (131.0 mg, 0.5 mmol, 2.5 equiv) in CH3CN (2 mL) and
H2O (0.13 mL) was stirred at rt. When the reaction was complete
(8) (a) Buono, G. Synthesis 1981, 872. (b) Buono, G. Tetrahedron Lett. 1972,
13, 3257.
9488 J. Org. Chem. Vol. 73, No. 23, 2008

H), 2.52-2.27 (m, 4 H), 1.55-1.45 (m, 1 H), 1.40-1.20 (m, 5 H),
0.86 (t, J ) 6.9 Hz, 3 H); 13C NMR (CDCl3, 75 MHz) 143.7, 138.7,
134.4, 134.1, 129.4, 128.3, 126.4, 126.2, 117.1, 91.7, 84.8, 43.4,
43.2, 36.6, 33.4, 27.9, 22.8, 14.0; MS (m/z) 255 ((M - C3H5)+,
100); IR (neat) 3073, 3028, 2956, 2928, 2858, 1640, 1605, 1496,
1454, 1432, 1118, 1029 cm-1; Elemental analysis: Calcd for
C21H28O: C, 85.08; H, 9.52; Found: C, 85.11; H, 9.54.
(4) Synthesis of 4-Benyl-2-(n-butyl)-1-oxaspiro[4.4]nona-3,7-
diene (8). The Grubbs II catalyst (9.2 mg, 0.01mmol) was added
to a solution of 7 (41.8 mg, 0.2 mmol, 5 mol%) in CH2Cl2 (4 mL)
under a N2 atmosphere. After being stirred under reflux conditions
for 2 h as monitored by TLC, the resulting solution was concentrated
and purified by flash column chromatography on silica gel
(petroleum ether/Et2O ) 100:1) to give 8 (32.5 mg, 86%): liquid.
1H NMR (300 MHz, CDCl3) 7.35-7.28 (m, 2 H), 7.27-7.18 (m,
3 H), 5.75-5.68 (m, 2 H), 5.15-5.10 (m, 1 H), 4.75-4.65 (m, 1
H), 3.28-3.23 (m, 2 H), 2.67-2.44 (m, 4 H), 1.54-1.40 (m, 2 H),
1.38-1.20 (m, 4 H), 0.87 (t, J ) 6.8 Hz, 3 H); 13C NMR (CDCl3,
75 MHz) 144.5, 139.0, 129.1, 128.7, 128.5, 128.3, 126.2, 125.1,
96.5, 83.5, 45.3, 43.9, 36.4, 33.2, 27.3, 22.8, 14.0; MS (m/z) 268
(M+, 34.32), 211 ((M - C4H9)+, 100); IR (neat) 3060, 3028, 2956,
2928, 2857, 1659, 1614, 1603, 1496, 1454, 1466, 1431, 1378, 1331,
1300, 1260, 1211, 1113, 1085, 1030 cm-1; Elemental analysis:
Calcd for C19H24O: C, 85.03; H, 9.01; Found: C, 85.06; H, 9.04.
Acknowledgment. Financial support from National Natural
Science Foundation of China (20732005) and the Major State
Basic Research Development Program (2006CB806105) is
greatly appreciated. Shengming Ma is a Qiu Shi Adjunct
Professor at Zhejiang University. We thank Mr. Xinpeng Jiang
and Mr. Guofei Chen in this group for reproducing the results
presented in entries 5, 9, 12, and 18 in Table 2.
Supporting Information Available: Experimental details and
1H/13C NMR spectra of all the products. This material is
available free of charge via the Internet at http://pubs.acs.org.
JO801809J
J. Org. Chem. Vol. 73, No. 23, 2008 9489