Sequential synthesis of (E)-α,β-unsaturated primary amides with complete stereoselectivity

Article abstract of DOI:10.1021/jo100320j

A novel, efficient, and totally stereoselective synthesis of (E)-α,β-unsaturated primary amides is reported. This process is consistent with a SmI₂-mediated sequential reaction of an unmasked samarium chloroacetamide enolate with an aldehyde, followed by a β-elimination to produce (E)-α,β-unsaturated primary amides in good yields.

Full text of DOI:10.1021/jo100320j

pubs.acs.org/joc
TABLE 1. Synthesis of 2-Chloro-3-hydroxyamides 3
Sequential Synthesis of (E )-r,β-Unsaturated
Primary Amides with Complete Stereoselectivity†
ꢀ
ꢀ
Jose M. Concellon,* Humberto Rodrıguez-Solla,*
ꢀ
Carmen Concellon, Carmen Simal, and Noemı Alvaredo
´
ꢀ
ꢀ
Departamento de Quımica Organica e Inorganica,
´
entry
3
R
dr
yield (%)
Facultad de Quımica, Universidad de Oviedo,
1
2
3
3a
3b
3c
n-C7H15
c-Hex
PhCH2CH2
1.3:1
1:1
1.4:1
70
74
61
ꢀ
´
Julian Claverıa 8, 33071 Oviedo, Spain
jmcg@uniovi.es
Received February 19, 2010
metalation of iodoacetic and dibromoacetic acids, respec-
tively, with SmI2.4 The addition reaction of the acetic acid
derived samarium enolate to aldehydes allowed accessing
3-hydroxy carboxylic acids in an efficient manner. More-
over, bromoacetic acid derived samarium enolates were used
in the highly stereoselective preparation of (E)-R,β-unsatu-
rated carboxylic acids through a sequential process involving
an aldol-type reaction followed by an elimination step.
Taking into account these precedents, generalization of this
methodology to the preparation of Sm enolates derived from
primary amides would undoubtedly be of interest.
A novel, efficient, and totally stereoselective synthesis of
(E)-R,β-unsaturated primary amides is reported. This
process is consistent with a SmI2-mediated sequential
reaction of an unmasked samarium chloroacetamide
enolate with an aldehyde, followed by a β-elimination
to produce (E)-R,β-unsaturated primary amides in good
yields.
In this paper we describe a new, easy, and efficient pre-
paration of R,β-unsaturated primary amides through a
sequential reaction involving a novel samarium enolate of
a primary amide.
In early experiments, dichloroacetamide 2 was reacted
with SmI2 in the presence of the corresponding aldehyde 1.
In all cases 2-chloro-3-hydroxyamides 3 were obtained in
moderate to high yields. The best results were obtained
by generating SmI2, in situ (from samarium powder and
diiodomethane)5 instead of using a previously generated
samarium diiodide solution (Table 1). Accordingly, diiodo-
methane (2.5 equiv) was added dropwise to a suspension of
the corresponding aldehyde 1 (1.0 equiv), dichloroacetamide
2 (1.0 equiv), and samarium powder (2.5 equiv) in 25 mL of
dry THF at room temperature. The reaction mixtures were
vigorously stirred for 3.5 h before they were hydrolyzed.
Compounds 3 were obtained as a mixture of diastereoi-
somers with diastereoisomeric ratios (dr) ranging from 1:1 to
1.4:1 as determined by 1H NMR spectroscopy on the crude
reaction products (Table 1).
Enolates derived from lithium, magnesium, and other
classical metals are certainly useful reagents. The broad use
of enolates in organic synthesis is a consequence of their ready
preparation and ability to promote C-C bond-forming reac-
tions with high selectivity and under mild conditions. As a
major drawback, in some cases the applicability of enolates is
limited by the basicity of these compounds. For this reason,
reactions of metallic enolates with substrates containing
functional groups with acidic hydrogens (unless protected)
cannot be carried out. In the same way, lithium or magnesium
enolates derived from primary amides are not available.
Accordingly, if these species are required in synthesis trans-
formations, two general strategies are followed: (i) carrying
out reactions with protected amide enolates, followed by
deprotection of the amide function, and (ii) alternatively,
preparing dianionic species from primary amides. However,
this later strategy finds prompt limitations associated with the
low solubility of such species in organic solvents.1
(4) Reviews of synthetic applications of SmI2: (a) Soderquist, J. A.
Aldrichimica Acta 1991, 24, 15–23. (b) Molander, G. A. Chem. Rev. 1992,
92, 29–68. (c) Molander, G. A. In Comprehensive Organic Synthesis; Trost, B.
M., Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 1, pp 251-282.
(d) Molander, G. A. In Organic Reactions; Paquette, L. A., Ed.; John Wiley:
New York, 1994; pp 211-367. (e) Molander, G. A.; Harris, C. R. Chem. Rev.
1996, 96, 307–338. (f) Molander, G. A.; Harris, C. R. Tetrahedron 1998, 54,
3321–3354. (g) Krief, A.; Laval, A. M. Chem. Rev. 1999, 99, 745–777. (h) Steel,
P. G. J. Chem. Soc., Perkin Trans. 1 2001, 2727–2751. (i) Kagan, H. B.
Previously, we have reported the preparation of acetic2
and bromoacetic3 acid derived samarium enolates by
ꢀ
Tetrahedron 2003, 59, 10351–10372. (j) Concellon, J. M.; Rodríguez-Solla, H.
Chem. Soc. Rev. 2004, 33, 599–609. (k) Jung, D. Y.; Kim, Y. H. Synlett 2005,
†
ꢀ
ꢀ
ꢀ
In Memoriam of Prof. Jose M. Concellon, who passed away on March 13,
3019–3032. (l) Concellon, J. M.; Rodríguez-Solla, H. Eur. J. Org. Chem. 2006,
2010.
1613–1625. (m) Gopalaiah, K.; Kagan, H. B. New J. Chem. 2008, 32, 607–637.
(n) Nicolaou, K. C.; Ellery, S. P.; Chen, J. S. Angew. Chem., Int. Ed. 2009, 48,
7140–7165.
(1) For reviews of dianions of carboxylic acids, see: (a) Petragnani, N.;
Yonashiro, M. Synthesis 1982, 521–578. (b) Thompson, C. M.; Green,
D. L. C. Tetrahedron 1991, 47, 4223–4285.
ꢀ
(5) For other reactions promoted by in situ generated SmI2, see: Concellon,
ꢀ
ꢀ
ꢀ
J. M.; Rodrıguez-Solla, H.; Huerta, M.; Perez-Andres, J. A. Eur. J. Org.
ꢀ
(2) Concellon, J. M.; Concellon, C. J. Org. Chem. 2006, 71, 4428–4432.
ꢀ
(3) Concellon, J. M.; Concellon, C. J. Org. Chem. 2006, 71, 1728–1731.
ꢀ
Chem. 2002, 1839–1847.
DOI: 10.1021/jo100320j Published on Web 04/08/2010
r
J. Org. Chem. 2010, 75, 3451–3453 3451
2010 American Chemical Society

ꢀ
Concellon et al.
JOCNote
TABLE 2. Synthesis of (E)-R,β-Unsaturated Amides 4
SCHEME 1. Mechanistic Proposal
entry
4
R
E/Z
yield (%)
1
2
4
5
6
7
4a
4b
4c
4d
4e
4f
n-C7H15
s-Bu
c-Hex
PhCH2CH2
t-BuCH2CH(Me)CH2
(Z)-EtCHdCH(CH2)5
>98/2
>98/2
>98/2
>98/2
>98/2
>98/2
60
56
61
53
56
64
As opposed to classical multistep reactions, sequential
processes are synthetically advantageous because isolation
of intermediates is not necessary. Considering our success in
the preparation of compounds 3, we contemplated the
possibility of carrying out a one-pot sequential process from
dichloroacetamide 2 directed toward the synthesis of unpro-
tected R,β-unsaturated primary amides. In consequence, a
mixture of aldehyde 1 (1.0 equiv), dichloroacetamide 2 (1.0
equiv), and samarium powder (6.0 equiv) in THF (25 mL)
was treated with CH2I2 (6.0 equiv) and stirred for 12 h
at room temperature. Rewardingly, (E)-R,β-unsaturated
amides 4 were obtained, with total E-stereoselectivity and in
good yields (Table 2). Byproducts generated from the hydro-
lysis of enolate intermediates by the amide acidic protons were
not detected.
The total stereoselective outcome of the reaction was
established on crude reaction mixtures by GC-MS analysis
and 1H NMR spectroscopy, revealing the presence of a single
stereoisomer. The relative E-configuration of amides 4 was
also assigned by 1H NMR spectroscopy measuring the value
of the coupling constants (3JH,H) for the olefinic protons.
R,β-Unsaturated amides 4 could be prepared from alipha-
tic (linear, branched or cyclic) aldehydes 1. However, in our
hands, aromatic amides (4, R = Ar) were never isolated.6 To
the best of our knowledge, this sequential C-C bond for-
mation/elimination reaction constitutes the first example
described in the literature in which primary R,β-unsaturated
amides were obtained using unprotected amides as starting
materials and avoiding protection/deprotection protocols.
To explain the observed results, we propose a mechanism
similar to those previously considered for other SmI2-pro-
moted olefination reactions. In this way, dichloroacetamide
2 reacts with SmI2 to generate the samarium enolate 5
(Scheme 1).7 The addition reaction of 5 to aldehyde 1 affords
the corresponding 2-chloro-3-oxy amide 6. Metalation of 6
with an additional 2 equiv of samarium diiodide gives access
to enolate intermediate 7-70, which would undergo a sponta-
neous elimination reaction, rendering after workup the
corresponding R,β-unsaturated primary amide 4.
lar arguments used to justify the formation of acetic2 and
bromoacetic3 acid derived samarium enolates. Therefore, we
reckon that once dichloroacetamide 2 has reacted and the
corresponding enolate is formed, the equilibrium 5 T 50 is
fully displaced favoring the tautomeric form ClCHdC(NH2)-
OSmI2 (50) as a consequence of the high oxophilic character of
Sm(III) ions (Scheme 1). This tautomer 50 is capable of
coexisting in the presence of a NH2 group.
Further evidence reinforcing this argument is the prepara-
tion of β-hydroxy samarium enolates, previously obtained
by treating the corresponding 2-chloro-3-hydroxyester8 or
amides9 with SmI2.
We assume that the elimination process takes place through
a cyclic six-membered ring transition state 7,10 guided by
coordination of the SmIII center with the oxygen atom of
the alcoholate function11 (Scheme 2). Two conformations, I
and II, are feasible for intermediate 7. Conformer I is pre-
sumably more stable than II, as the R group adopts a pseudo-
equatorialorientation, thusavoiding unfavorable interactions
with the samarium coordination sphere. Elimination from I
renders (E)-R,β-unsaturated amides 4. Our group and others
have previously proposed similar chairlike transition states to
ꢀ
ꢀ
ꢀ
(8) Concellon, J. M.; Perez-Andres, J. A.; Rodrıguez-Solla, H. Angew.
Chem., Int. Ed. 2000, 39, 2773–2775.
ꢀ
ꢀ
ꢀ
(9) Concellon, J. M.; Perez-Andres, J. A.; Rodrıguez-Solla, H. Chem.;
Eur. J. 2001, 7, 3062–3068.
(10) A similar model involving a chairlike transition state has been
proposed to explain the selectivity observed in other reactions with SmI2:
(a) Molander, G. A.; Etter, J. B.; Zinke, P. W. J. Am. Chem. Soc. 1987, 109,
453–463. (b) Urban, D.; Skrydstrup, T.; Beau, J. M. J. Org. Chem. 1998, 63,
2507–2516. (c) Prasad, E.; Flowers, R. A., II J. Am. Chem. Soc. 2002, 124,
6357–6381. (d) Enemrke, R. J.; Larsen, J.; Hjollund, G. H.; Skrydstrup, T.;
Daasbjerg, K. Organometallics 2005, 24, 1252–1262. (e) Davis, T. A.;
Chopade, P. R.; Hilmersson, G.; Flowers, R. A. Org. Lett. 2005, 7, 119–122.
(11) For a discussion of the highly oxophilic character of Sm(III) species,
see: Molander, G. A. In Comprehensive Organic Synthesis; Trost, B. M.,
Fleming, I., Schreiber, S. L., Eds.; Pergamon: Cambridge, 1991; Vol. 1, p 252.
The formation of the samarium enolate 5 (presenting two
NH2 acidic protons) could be explained on the basis of simi-
(6) When aromatic aldehydes were employed as starting materials, pro-
ducts derived from their pinacol coupling were isolated instead of the
corresponding R,β-unsaturated amides 4.
(7) For recent revisions on Samarium-enolates, see: (a) Rudkin, I. M.;
Miller, L. C.; Procter, D. J. Organomet. Chem. 2008, 34, 19–45. (b) Procter, D.
J.; Flowers, R. A., II; Skrydstrup, T. Organic Synthesis Using Samarium
Diiodide: A Practical Guide; RSC Publishing: Cambridge, 2010.
3452 J. Org. Chem. Vol. 75, No. 10, 2010

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Concellon et al.
JOCNote
explain the relative configuration of olefins obtained by other
SmI2-promoted elimination reactions.
filtered, and concentrated under reduced pressure. Flash chro-
matography on silica gel (hexane/EtOAc 3:1) afforded pure
(E)-R,β-unsaturated amides 4.
In conclusion, we have demonstrated that the sama-
rium enolate formed from dichloroacetamide can be easily
prepared and employed in the synthesis of unmasked (E)-R,β-
unsaturated primary amides. These compounds are obtained
with complete E-stereoselectivity through a sequential two-
step process: an aldol-type and a β-elimination reaction.
(E)-2-Decenamide (4a). 1H NMR (300 MHz, CDCl3) δ 6.86
(dt, J=15.2. 6.9 Hz,1 H), 5.82 (d, J=15.2 Hz, 1 H), 5.43 (br s,
2 H), 2.22-2.15 (m, 2 H), 2.03-1.18 (m, 10 H), 0.78 (t, J =
6.5 Hz, 3 H); 13C NMR (75 MHz, CDCl3) δ 168.1 (C), 146.6
(CH), 122.5 (CH), 32.0 (CH2), 31.7 (CH2), 29.0 (2ꢀCH2), 28.1
(CH2), 22.5 (CH2), 14.0 (CH3); MS (70 eV, EI) m/z (%) 151
([M - H2O]þ, 1), 122 (40), 98 (91), 72 (89), 59 (100); HRMS
(70 eV) calcd for [C10H19NO] 169.1467, found 169.1468; IR
(neat) 3387, 1680, 1258 cm-1; Rf=0.25 (hexane/EtOAc 1:2).
(E)-4-Methyl-2-hexenamide (4b). 1H NMR (300 MHz,
CDCl3) δ 6.74 (dd, J=15.3, 7.7 Hz, 1 H), 5.78 (dd, J=15.3,
3.0, Hz, 1 H), 5.60 (br s, 2 H), 2.22-2.17 (m, 1 H), 1.41-1.37
Experimental Section
Preparation of 2-Chloro-3-hydroxyamides (3). To a stirred
suspension of aldehyde 1 (0.4 mmol), dichloroacetamide 2
(0.4 mmol), and samarium powder (1 mmol) in THF (10 mL)
was added CH2I2 (1 mmol) dropwise at room temperature, and
the mixture was stirred for 3.5 h. The excess of SmI2 was
removed by bubbling a stream of air through the solution.
The crude reaction material was washed with 0.1 N (aq) HCl
(10 mL) and was extracted with DCM (3 ꢀ 10 mL). The
combined organic layers were dried over anhydrous Na2SO4,
filtered, and concentrated under reduced pressure. Flash chro-
matography on silica gel (hexane/EtOAc 3:1) afforded pure
chlorohydrines 3.
(m, 2 H), 1.03 (d, J=6.5 Hz, 3 H), 0.87 (t, J=7.3 Hz, 3 H); 13
C
NMR (75 MHz, CDCl3) δ 168.2 (C), 151.6 (CH), 121.0 (CH),
37.9 (CH), 28.8 (CH2), 19.0 (CH3), 11.6 (CH3); MS (ESIþ) m/z
(%) 127 ([M]þ, 27), 112 (100), 98 (24), 83 (18), 70 (22), 57 (24);
HRMS (ESIþ) calcd for [C7H14NO]þ [M þ H]þ 128.1075, found
128.1070; IR (neat) 3406, 1676, 1458, 1266 cm-1; Rf = 0.32
(EtOAc).
(E)-3-Cyclohexylacrilamide (4c). 1H NMR (300 MHz,
CDCl3) δ 6.80 (dd, J=15.5, 6.8 Hz, 1 H), 5.76 (d, J=15.5 Hz,
1 H), 5.53 (br s, 2 H), 2.16-2.04 (m, 1 H), 1.97-1.65 (m, 5 H),
1.31-1.08 (m, 5 H); 13C NMR (75 MHz, CDCl3) δ 166.7 (C),
151.5 (CH), 120.1 (CH), 40.2 (CH), 31.8 (2ꢀCH2), 25.9 (CH2),
25.7 (2ꢀCH2); MS (70 eV, EI) m/z (%) 153 ([M]þ, 100), 136 (50),
109 (35); HRMS (ESIþ) calcd for [C9H16NO]þ [M þ H]þ
2-Chloro-3-hydroxydecanamide (3a). 1H NMR (300 MHz,
CDCl3) δ 6.69 (br s, 1 H), 6.60 (br s, 1 H), 6.22 (br s, 1 H),
6.13 (br s, 1 H), 4.35 (d, J=2.3 Hz, 1 H), 4.26 (d, J=5.7 Hz, 1 H),
4.20-4.16 (m, 1 H), 4.06-4.00 (m, 1 H), 3.30 (br s, 1 H), 2.81
(br s, 1 H), 1.67-1.46 (m, 4 H), 1.42-1.17 (m, 20 H), 0.86 (t, J=
6.3 Hz, 6 H); 13C NMR (75 MHz, CDCl3) δ 171.0 (C), 170.7 (C),
72.8 (CH), 71.9 (CH), 63.6 (CH), 62.3 (CH), 33.6 (CH2), 32.9
(CH2), 31.7 (2ꢀCH2), 29.3 (CH2), 29.2 (CH2), 29.0 (2ꢀCH2),
25.5 (CH2), 25.2 (CH2), 22.5 (2 ꢀ CH2), 14.0 (2 ꢀ CH3); MS
(70 eV, EI) m/z (%) 186 ([Mþ - Cl], 12), 122 (23), 95 (33), 93
(100), 55 (40); HRMS (70 eV) calcd for [C10H20ClNO2 - H2O]
154.1231, found 154.1226; IR (neat) 3363, 1676, 1406 cm-1
;
Rf=0.50 (EtOAc).
(E)-5-Phenyl-2-pentenamide (4d). 1H NMR (300 MHz, CDCl3)
δ 7.50-7.08 (m, 5 H), 6.91 (dt, J=15.3, 6.9 Hz, 1 H), 5.85 (d, J=
15.3 Hz 1 H), 5.56 (br s, 2 H), 2.79 (t, J=7.7 Hz, 2 H), 2.54 (c, J=
7.3 Hz, 2 H); 13C NMR (75 MHz, CDCl3) δ 167.7 (C), 145.1
(CH), 140.8 (C), 128.5 (2xCH), 128.4 (2ꢀCH), 126.1 (CH), 123.1
(CH); 34.4(CH2), 33.7 (CH2); MS(70eV, EI) m/z (%) 175 ([M]þ,
2), 130 (9), 91 (100), 77 (3), 44 (5); HRMS (ESIþ) calcd for
[C11H14NO]þ [M þ H]þ 176.1075, found 176.1070; IR (neat)
3435, 1643, 1453, 1265 cm-1; Rf=0.35 (hexane/EtOAc 1:5).
203.1077, found 203.1086; IR (neat) 3383, 1663, 1081, 723 cm-1
;
Rf=0.30 (hexane/EtOAc 1:1).
2-Chloro-3-cyclohexyl-3-hydroxypropanamide (3b). 1H NMR
(300 MHz, CDCl3) δ 6.66 (br 1 H), 6.64 (br 1 H), 5.34 (apparent
s, 1 H), 5.08 (dd, J=8.2, 6.7 Hz, 1 H), 1.91-0.90 (m, 11 H); 13
C
NMR (75 MHz, CDCl3) δ 164.7 (C), 78.5 (CH), 66.2 (CH), 42.1
(CH), 29.6 (CH2), 27.8 (CH2), 26.0 (CH2), 25.5 (2ꢀCH2); MS (70
eV, EI) m/z (%) 187 ([Mþ-H2O], 15), 186 (100), 150 (79), 93 (15),
79 (20), 67 (10); HRMS (70 eV) calcd for [C9H16ClNO2 - H2O]
1
(E)-5,7,7-Trimethyl-2-octenamide (4e). H NMR (300 MHz,
CDCl3) δ 6.90-6.80 (m, 1 H), 5.84 (dt, J=16.4, 1.5 Hz, 1 H),
5.54 (br s, 2 H), 2.16-2.14 (m, 1 H), 2.08-2.0 (m, 1 H),
1.98-1.64 (m, 2 H), 1.07 (dd, J=14.0, 6.5 Hz, 1 H), 0.94 (d,
J=6.5 Hz, 3 H), 0.89 (s, 9 H); 13C NMR (75 MHz, CDCl3) δ
167.8 (C), 145.4 (CH), 123.7 (CH), 50.5 (CH2), 41.8 (CH2), 31.0
(C), 29.9 (3ꢀCH3), 29.0 (CH), 22.4 (CH3); MS (ESIþ) m/z (%)
184 ([MþH]þ, 100), 142 (3), 128 (2), 102 (1); HRMS (ESIþ) calcd
for [C11H22NO]þ [M þ H]þ 184.1701, found 184.1696; IR (neat)
3055, 1686, 1422, 1264 cm-1; Rf=0.35 (hexane/EtOAc 1:2).
(2E,9Z)-Dodeca-2,9-dienamide (4f). 1H NMR (300 MHz, CDCl3)
δ 6.85 (dt, J=15.3, 6.9 Hz, 1 H), 5.81 (d, J=15.3, 1 H), 5.58 (br s,
2 H), 5.40-5.25 (m, 2 H), 2.22-2.15 (m, 2 H), 2.07-1.96 (m, 4 H),
1.47-1.25 (m, 6 H), 0.95 (t, J=7.5 Hz, 3H); 13C NMR (75 MHz,
CDCl3) δ 167.9 (C), 146.4 (CH), 131.7 (CH), 128.9 (CH), 122.6
(CH), 31.9 (CH2), 29.4 (CH2); 28.7 (CH2), 28.0 (CH2), 26.8 (CH2),
20.4 (CH2), 14.3 (CH3); MS (70 eV, EI) m/z (%) 195 ([M]þ, 100),
174 (9), 153 (5), 109 (15); HRMS (ESIþ) calcd for [C12H22NO]þ
[M þ H]þ 196.1701, found 196.1697; IR (neat) 3220, 1681, 1265,
1250 cm-1; Rf=0.80 (hexane/EtOAc 1:5).
187.0734, found 187.0739; IR (neat) 3370, 1670, 1099, 740 cm-1
;
Rf=0.26 (hexane/EtOAc 3:1).
2-Chloro-3-hydroxy-5-phenylpentanamide (3c). 1H NMR
(300 MHz, CDCl3) δ 7.30-7.20 (m, 7 H), 5.85 (apparent s, 1 H),
5.38-5.30 (m, 1 H), 3.56 (d, J=3.8 Hz, 1 H), 2.90-2.70 (m, 2 H),
2.17-1.95 (m, 2 H); 13C NMR (75 MHz, CDCl3) δ 164.7 (C),
140.4 (C), 128.6 (2ꢀCH), 128.4 (2ꢀCH), 126.3 (CH), 74.8 (CH),
65.9 (CH), 36.3 (CH2), 30.7 (CH2); MS (70 eV, EI) m/z (%)
174 ([Mþ - Cl - H2O], 2), 131 (100), 103 (27), 77 (10); HRMS
(70 eV) calcd for [C11H14ClNO2 - Cl - H2O] 174.0919, found
174.0926; IR (neat) 3352, 1675, 1109, 766, cm-1; Rf = 0.39
(hexane/EtOAc 3:1).
Synthesis of (E)-r,β-Unsaturated Amides (4). To a stirred
suspension of aldehyde 1 (0.4 mmol), dichloroacetamide 2
(0.4 mmol), and samarium powder (2.4 mmol) in THF (25 mL)
was added CH2I2 (2.4 mmol) dropwise at room temperature.
After the mixture stirred vigorously for 12 h, the excess of SmI2
was removed by bubbling a stream of air through the solution.
The crude reaction material was washed with 0.1 N (aq) HCl
(10 mL) and was extracted with dichloromethane (3ꢀ10 mL).
The combined organic layers were dried over anhydrous Na2SO4,
1
Supporting Information Available: Copies of H and 13C
NMR spectra for compounds 3 and 4. This material is available
free of charge via the Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 75, No. 10, 2010 3453