A practical method for the synthesis of 2-alkynylpropenals

Article abstract of DOI:10.1021/jo047869a

(Chemical Equation Presented) A general method for the preparation of 2-alkynyl acroleins is described beginning with vinyl iodide 5 and involving a combination of Sonogashira coupling and Dess-Martin oxidation. Critical to the success of this approach is

Full text of DOI:10.1021/jo047869a

In 2000, Funk and co-workers described an ingenious
method for the in situ generation of sensitive 2-substi-
tuted acroleins involving the thermal [4+2] cyclorever-
sion of 5-substituted 4H-1,3-dioxins.5 Attempted appli-
cation of this protocol to the preparation of a 2-alkynyl
acrolein derivative led only to the isolation of the corre-
sponding hetero-Diels-Alder dimer, but the unstable
enynal could be trapped in situ by carrying out the
thermolysis in the presence of excess Danishefsky’s diene
(eq 1).5 Unfortunately, these conditions are not compat-
A Practical Method for the Synthesis of
2-Alkynylpropenals
Charnsak Thongsornkleeb and Rick L. Danheiser*
Department of Chemistry, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139
danheisr@mit.edu
Received December 1, 2004
A general method for the preparation of 2-alkynyl acroleins
is described beginning with vinyl iodide 5 and involving a
combination of Sonogashira coupling and Dess-Martin
oxidation. Critical to the success of this approach is the use
of a special workup procedure for the oxidation step. The
resultant enynals participate in a variety of addition reac-
tions including aldol condensations and reactions with
organolithium compounds.
ible with our projected use of 2-alkynylpropenals, which
involves aldol condensation of the aldehydes at low
temperature with lithium enolates. The elevated tem-
peratures required in the Funk protocol, as well as the
fact that 1 equiv of acetone is generated as a byproduct
of the retro-Diels-Alder reaction, obviously set severe
constraints on the range of applications possible for
2-alkynyl acroleins produced under these conditions. We
therefore undertook an investigation of alternative meth-
ods for the preparation of these R,â-unsaturated alde-
hydes that would be compatible with a broader range of
subsequent transformations.
Scheme 1 outlines our approach to the synthesis of
2-alkynylpropenals. Sonogashira coupling6 of 2-iodo-2-
propenol7 (5) with a wide range of acetylenes was
expected to provide convenient access to enynyl alcohols
of type 7,8 which would then be oxidized to furnish the
desired aldehydes. In the event, standard oxidation
protocols provided the unstable acroleins in poor yield
due to the propensity of these R,â-unsaturated aldehydes
to undergo Diels-Alder dimerization and polymerization.
Success was finally achieved by employing a modified
Dess-Martin oxidation9,10 protocol, which avoids an
In connection with our studies on the total synthesis
of glycinoeclepin A, we required a practical and efficient
method for the preparation of R,â-unsaturated aldehydes
of type 1 bearing alkynyl substituents at the C-2 position
(“R-alkynyl acroleins”). Remarkably, only one example of
the isolation and characterization of an aldehyde of this
class has previously been reported in the literature.1
Acrolein derivatives of this type are expected to undergo
facile dimerization via hetero-Diels-Alder [4+2] cyclo-
addition,2,3 and also should be exceptionally prone to
polymerization via radical pathways and in the presence
of nucleophiles. On the other hand, the multiple func-
tional groups incorporated in these compounds suggest
that they should serve as valuable synthetic building
blocks in a number of applications. Unfortunately, the
anticipated sensitivity of these enynals limits the range
of methods potentially applicable for their preparation,
and to date the synthetic utility of this class of aldehydes
remains unrealized.4
(4) For a review of methods for the synthesis of acrolein and its
R-substituted derivatives, see: Keiko, N. A.; Voronkov, M. G. Russ.
Chem. Rev. 1993, 62, 751.
(5) Fearnley, S. P.; Funk, R. L.; Gregg, R. J. Tetrahedron 2000, 56,
10275.
(6) For recent reviews on palladium-catalyzed alkynylation, see: (a)
Sonogashira, K. Sonogashira Alkyne Synthesis. In Handbook of
Organopalladium Chemistry for Organic Synthesis; Negishi, E., de
Meijere, A., Eds.; Wiley-Interscience: New York, 2002; Vol. I, p 493.
(b) Negishi, E.; Xu, C. Palladium-Catalyzed Alkynylation with Al-
kynylmetals and Alkynyl Electrophiles. In Handbook of Organopal-
ladium Chemistry for Organic Synthesis; Negishi, E., de Meijere, A.,
Eds.; Wiley-Interscience: New York, 2002; Vol. I, p 531. (c) Negishi,
E.; Anastasia, L. Chem. Rev. 2003, 103, 1979.
(1) Dreiding has reported that vapor phase pyrolysis of propargyl
propiolate at 500 °C affords a complex mixture of products from which
2-ethynylpropenal was isolated in 12% yield. See: Bilinski, V.; Dre-
iding, A. S.; Hollenstein, H. Helv. Chim. Acta 1983, 66, 2322.
(2) For reviews of the hetero-Diels-Alder reaction, see: (a) Tietze,
L. F.; Kettschau, G. Top. Curr. Chem. 1997, 189, 1. (b) Boger, D. L.;
Weinreb, S. M. Hetero Diels-Alder Methodology in Organic Synthesis;
Academic Press: San Diego, CA, 1987.
(7) Available by reaction of propargyl alcohol with chlorotrimeth-
ylsilane and sodium iodide according to the procedure of: Kamiya, N.;
Chikami, Y.; Ishii, Y. Synlett 1990, 675.
(3) For examples, see: (a) Schulz, H.; Wagner, H. Angew. Chem.
1950, 29, 105. (b) Laitalainen, T.; Kuronen, P.; Hesso, A. Org. Prep.
Proced. Int. 1993, 25, 597.
(8) Nicolaou has reported that Sonogashira coupling of (trimethyl-
silyl)acetylene with 5 furnishes 7a in near quantitative yield. See:
Nicolaou, K. C.; Koide, K. Tetrahedron Lett. 1997, 38, 3667.
10.1021/jo047869a CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/10/2005
2364
J. Org. Chem. 2005, 70, 2364-2367

SCHEME 1
SCHEME 2
aqueous workup procedure. A stratagem for precipitating
iodinane byproducts by diluting with pentane was intro-
duced recently by Wavrin and Viala,11 and we found that
with some modification this protocol can be employed for
the efficient formation of highly sensitive 2-alkynylpro-
penals. Specifically, oxidation is first carried out using
1.1-1.3 equiv of Dess-Martin periodinane in CH2Cl2 at
0 °C. The reaction mixture is then cooled to -78 °C and
diluted with an equal volume of pentane, and then excess
poly(4-vinylpyridine) is added to sequester the acetic acid
generated in the reaction. The reaction mixture is filtered
under a positive pressure of argon through a jacketed
plug of silica gel cooled at -78 °C, and then concentrated
with the aid of toluene to remove remaining traces of
acetic acid via azeotropic distillation. Final concentration
to dryness is conducted at 0.05 mmHg and -78 °C. The
desired aldehydes 1a-d are obtained in 41-78% overall
yield and in greater than 95% purity as determined by
Scheme 2 to furnish the expected enynyl alcohols 10 and
11 in good overall yield from 7a, the precursor to the
alkynyl acrolein. Moderately stabilized organolithium
compounds react in a like manner. For example, addition
of the lithium derivative of methyl phenyl sulfone to
enynal 1a occurs smoothly to afford the expected â-hy-
droxy sulfone 12 in 61% yield (eq 4).
In summary, the combination of Sonogashira coupling
and Dess-Martin oxidation provides a convenient method
for the preparation of a wide range of 2-alkynyl acroleins.
Critical to the success of this approach is the use of a
modified workup procedure for the oxidation step, in
which byproducts are separated by precipitation with
pentane, trapping with poly(4-vinylpyridine), and azeo-
tropic distillation with toluene.
1
IR and H NMR analysis.
As expected, 2-alkynylpropenals 1a-d readily undergo
dimerization via hetero-Diels-Alder cycloaddition. For
example, NMR analysis of a solution of aldehyde 1a in
CDCl3 (0.4 M) showed traces of dimer formation within
minutes at room temperature, and after standing over-
night, more than half of the starting material was
converted to the hetero-Diels-Alder cycloadduct 8. On
Experimental Section
General Procedure for the Preparation of Enynyl Al-
cohols: 2-Methylene-4-(triisopropylsilyl)but-3-yn-1-ol (7b).
A 50-mL, one-necked, round-bottomed flask fitted with a rubber
septum and argon inlet needle was charged with (Ph3P)4Pd
(0.089 g, 0.077 mmol), CuI (0.042 g, 0.22 mmol), and 10 mL of
THF. A solution of the vinyl iodide 5 (0.464 g, 2.52 mmol) in 3
mL of THF was added via cannula over 2 min (the flask was
rinsed with 2 mL of THF) and triethylamine (0.46 mL, 0.33 g,
3.28 mmol) was then added via syringe over 30 s. The resulting
yellow mixture was stirred at room temperature for 3 min, and
then (triisopropylsilyl)acetylene (0.74 mL, 0.60 g, 3.28 mmol)
was added via syringe over 2 min. The reaction mixture was
stirred at room temperature for 11.5 h and then heated at reflux
for 9 h. The reaction mixture was allowed to cool to room
temperature and then filtered through 10 g of silica gel with
the aid of three 15-mL portions of Et2O. Concentration of the
filtrate afforded 0.689 g of a brown oil. Column chromatography
on 35 g of silica gel (gradient elution with 0-10% EtOAc-
hexane) provided 0.336 g (56%) of enyne 7b as a yellow oil: IR
the other hand, the 2-alkynyl R,â-unsaturated aldehydes
produced in this fashion can be taken up in THF and
employed in a number of subsequent useful synthetic
transformations. For example, aldol condensation with
the lithium enolate derivative of pinacolone provides the
desired â-hydroxy ketones 9a-d in good to excellent yield
(eq 3).
(9) (a) Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48, 4155. (b)
Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991, 113, 7277.
(10) For reviews of oxidations using the Dess-Martin periodinane,
see: (a) Boeckman, R. K.; Shao, P.; Mullins, J. J. In Organic Syntheses;
Wiley & Sons: New York, 2004; Collect. Vol. X, p 696. (b) Tohma, H.;
Kita, Y. Adv. Synth. Catal. 2004, 346, 111.
In a similar fashion, addition of organo- and alkynyl-
lithium compounds proceeds smoothly as outlined in
(11) Wavrin, L.; Viala, J. Synthesis 2002, 326.
J. Org. Chem, Vol. 70, No. 6, 2005 2365

(film) 3356, 2942, 2865, 2145, and 1617 cm-1
;
1H NMR (500
of silica gel (gradient elution with 0-10% EtOAc-hexane)
afforded 0.242 g (94%) of 9a as a pale yellow oil: IR (film) 3473,
MHz, CDCl3) δ 5.54 (dd, J ) 1.5, 10.5 Hz, 2H), 4.12 (td, J ) 1.5,
7.0 Hz, 2H), 1.68 (dt, J ) 1.5, 7.0 Hz, 1H), and 1.09 (app s, 21H);
13C NMR (125 MHz, CDCl3) δ 131.7, 120.8, 104.7, 93.0, 65.5,
18.8, and 11.4; HRMS-EI m/z [M]+ calcd for C14H26OSi 238.1747,
found 238.1741.
2964, 2906, 2873, 2143, 1705, and 1625 cm-1 1H NMR (500
;
MHz, CDCl3) δ 5.69 (t, J ) 1.5 Hz, 1H), 5.55 (t, J ) 1.5 Hz, 1H),
4.50-4.52 (m, 1H), 3.51 (d, J ) 5.0 Hz, 1H), 3.02 (dd, J ) 3.0,
17.5 Hz, 1H), 2.79 (dd, J ) 8.5, 17.5 Hz, 1H), 1.16 (s, 9H), and
0.20 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 217.3, 132.8, 122.1,
103.0, 96.9, 70.0, 44.7, 41.9, 26.3, and 0.1; HRMS-ESI m/z [M +
H]+ calcd for C14H24O2Si 253.1618, found 253.1625.
1,6-Bis(trimethylsilyl)-3-hydroxy-4-methylenehexa-1,5-
diyne (10). A 50-mL, two-necked, round-bottomed flask fitted
with an argon inlet adapter and rubber septum was charged
with 12 mL of THF and trimethylsilylacetylene (0.26 mL, 0.182
g, 1.85 mmol) and cooled at -78 °C while 0.67 mL of n-BuLi
solution (2.50 M in hexane, 1.68 mmol) was added dropwise via
syringe over 30 s. The resulting pale yellow solution was stirred
at -78 °C for 1 h.
General Procedure for the Oxidation of Allylic Alcohols
to Enynyl Aldehydes: 2-Methylene-4-(trimethylsilyl)but-
3-ynal (1a). A 50-mL, two-necked, pear-shaped flask fitted with
an argon inlet adapter and a rubber septum was charged with
a solution of the allylic alcohol 7a (0.942 g, 6.11 mmol) in 30
mL of dichloromethane and cooled at 0 °C while Dess-Martin
periodinane (2.856 g, 6.73 mmol) was added in one portion. The
heterogeneous reaction mixture was stirred at 0 °C for 5 min
and then allowed to warm to room temperature. After 20 min,
the reaction mixture was cooled to -78 °C and diluted with 30
mL of pentane, poly(4-vinylpyridine) (3.227 g, 30.69 mmol) was
added in one portion, and the resulting mixture was stirred at
-78 °C for 25 min. A 2.5-cm diameter jacketed column fitted
with a rubber septum at the top and a short needle at the bottom
was charged with a 6-cm plug of silica gel, which was cooled at
-78 °C by filling the jacket with dry ice-acetone. The reaction
mixture was transferred into the column by cannula and filtered
through the silica gel under a positive pressure of argon into a
200-mL recovery flask fitted with a rubber septum with a short
needle as vent. The reaction flask and the column were rinsed
with four 15-mL portions of 4:1 pentane-ether. The filtrate was
concentrated by rotary evaporation at 25 °C (20 mmHg) to a
volume of ca. 3 mL and this solution was then cooled to -78 °C
and diluted with 5 mL of toluene. The resulting solution was
concentrated by rotary evaporation at 25 °C to a volume of ca.
3 mL and the resulting pale yellow solution was cooled to -78
°C and further concentrated at 0.05 mmHg (ca. 15 min) to
furnish aldehyde 1a as a yellow oil. For spectroscopic analysis,
this material was dissolved in ca. 1 mL of CDCl3 and 4-isopro-
pylbenzaldehyde (0.92 mL, 0.90 g, 6.1 mmol) was added as an
internal standard. The resulting solution was transferred to an
NMR tube via cannula under a positive pressure of argon. The
yield of aldehyde 1a was determined by 1H NMR analysis to be
52%: IR (film) 2960, 2928, 2144, 1749, and 1624 cm-1; 1H NMR
(500 MHz, CDCl3) δ 9.48 (s, 1H), 6.63 (d, J ) 1.5 Hz, 1H), 6.40
(d, J ) 1.5 Hz, 1H), and 0.25 (s, 1H); 13C NMR (125 MHz, CDCl3)
δ 190.0, 139.6, 133.1, 101.8, 97.5, and -0.1.
General Procedure for the Aldol Reaction of Enynyl
Aldehydes: 5-Hydroxy-2,2-dimethyl-6-methylene-8-(tri-
methylsilyl)oct-7-yn-3-one (9a). A 25-mL, two-necked, round-
bottomed flask fitted with an argon inlet adapter and a rubber
septum was charged with 12 mL of THF and diisopropylamine
(0.19 mL, 0.13 g, 1.3 mmol) and cooled at 0 °C while 0.50 mL of
n-BuLi solution (2.41 M in hexane, 1.2 mmol) was added
dropwise via syringe over 30 s. The resulting yellow solution
was stirred at 0 °C for 15 min and then cooled to -78 °C.
A 10-mL, one-necked, pear-shaped flask was charged with 1.5
mL of THF and pinacolone (0.12 mL, 0.10 g, 1.0 mmol) and
cooled to -78 °C. The resulting solution was transferred to the
solution of LDA via cannula over 3 min (the flask was rinsed
with 0.5 mL of THF) and the resulting cloudy pale-yellow
mixture was stirred at -78 °C for 2 h.
Oxidation of allylic alcohol 7a (0.129 g, 0.84 mmol) with Dess-
Martin periodinane (0.394 g, 0.92 mmol) in 4 mL of CH2Cl2 was
carried out according to the General Procedure to furnish
aldehyde 1a (estimated yield of 0.44 mmol based on previous
experiments). This material (not allowed to warm above -78
°C once solvent was removed) was dissolved in 3 mL of THF,
and then transferred via cannula over 3 min into the solution
of lithium (trimethylsilyl)acetylide prepared as described above.
The resulting clear yellow solution was allowed to stir at -78
°C for 15 min and then treated with 5 mL of saturated aq NH4-
Cl solution. The resulting mixture was diluted with 10 mL of
Et2O and 5 mL of water, and the aqueous phase was separated
and extracted with three 10-mL portions of Et2O. The combined
organic phases were washed with 20 mL of saturated NaCl
solution, dried over MgSO4, filtered, and concentrated to afford
0.118 g of yellow oil. Column chromatography on 15 g of silica
gel (gradient elution with 0-5% EtOAc-hexane) afforded 0.081
g (38% overall from 7a) of alcohol 10 as a pale yellow oil: IR
(film) 3356, 2961, 2178, 2149, and 1616 cm-1 1H NMR (500
;
MHz, CDCl3) δ 5.74 (t, J ) 1.5 Hz, 1H), 5.59 (t, J ) 1.5 Hz, 1H),
4.84 (dt, J ) 1.0, 7.5 Hz, 1H), 2.23 (d, J ) 2.0 Hz, 1H), 0.22 (s,
9H), and 0.20 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 131.4, 123.1,
103.1, 101.6, 97.7, 91.9, 65.3, 0.05, and -0.04; HRMS-ESI m/z
[M + Na]+ calcd for C13H22OSi2 273.1101, found 273.1107.
1-Phenyl-1-hydroxy-2-methylene-4-(trimethylsilyl)-3-
butyne (11). A 50-mL, two-necked, round-bottomed flask fitted
with an argon inlet adapter and a rubber septum was charged
with 30 mL of THF and 1.24 mL of PhLi solution (1.80 M in
hexane, 2.23 mmol) and cooled at -78 °C.
Oxidation of allylic alcohol 7a (0.149 g, 0.97 mmol) with Dess-
Martin periodinane (0.459 g, 1.07 mmol) in 5 mL of CH2Cl2 was
carried out according to the General Procedure to furnish
aldehyde 1a (estimated yield of 1.0 mmol based on previous
experiments). This material (not allowed to warm above -78
°C once solvent was removed) was dissolved in 3 mL of THF,
and then transferred via cannula over 3 min into the solution
of PhLi described above. The resulting clear yellow solution was
allowed to stir at -78 °C for 1 h and then treated dropwise with
5 mL of saturated aq NH4Cl solution. The resulting mixture was
diluted with 10 mL of Et2O and 5 mL of water, and the aqueous
phase was separated and extracted with three 10-mL portions
of Et2O. The combined organic phases were washed with 20 mL
of saturated NaCl solution, dried over MgSO4, filtered, and
concentrated to afford 0.249 g of yellow oil. Column chromatog-
raphy on 25 g of silica gel (gradient elution with 0-5% EtOAc-
hexane) afforded 0.089 g (40% overall from 7a) of alcohol 11 as
a pale yellow oil: IR (film) 3385, 3088, 3065, 3031, and 2146
Oxidation of allylic alcohol 7a (0.423 g, 2.74 mmol) with Dess-
Martin periodinane (1.284 g, 3.027 mmol) in 27 mL of CH2Cl2
was carried out according to the General Procedure to furnish
aldehyde 1a (estimated yield of 1.5 mmol based on previous
experiments). This material (not allowed to warm above -78
°C once solvent was removed) was dissolved in 3 mL of THF
and then transferred via cannula over 2 min into the solution
of lithium enolate prepared as described above. The resulting
clear, yellow solution was stirred at -78 °C for 2 h and then
treated dropwise with 1 mL of half-saturated aq NH4Cl solution
(pre-cooled at 0 °C). The resulting mixture was diluted with 15
mL of Et2O and 10 mL of water, and the aqueous phase was
separated and extracted with three 7-mL portions of Et2O. The
combined organic phases were washed with 20 mL of saturated
NaCl solution, dried over MgSO4, filtered, and concentrated to
afford 0.406 g of a yellow oil. Column chromatography on 20 g
cm-1 1H NMR (500 MHz, CDCl3) δ 7.41-7.43 (m, 2H), 7.34-
;
7.38 (m, 2H), 7.31 (tt, J ) 2.5, 7.0 Hz, 1H), 5.63 (t, J ) 1.5 Hz,
1H), 5.57 (dd, J ) 1.0, 1.5 Hz, 1H), 5.22 (d, J ) 5.0 Hz, 1H),
2.29 (d, J ) 5.0 Hz, 1H), and 0.14 (s, 9H); 13C NMR (125 MHz,
CDCl3) δ 141.4, 134.4, 128.5, 128.1, 126.8, 121.9, 102.6, 98.0,
76.4, and -0.04; HRMS-ESI m/z [M + Na]+ calcd for C14H18OSi
253.1019, found 253.1024.
1-Phenylsulfonyl-2-hydroxy-3-methylene-5-(trimethyl-
silyl)-4-pentyne (12). A 25-mL, two-necked, round-bottomed
flask fitted with an argon inlet adapter and a rubber septum
2366 J. Org. Chem., Vol. 70, No. 6, 2005

was charged with phenyl methyl sulfone (0.095 g, 0.60 mmol)
and 5 mL of THF and cooled at -78 °C while 0.28 mL of n-BuLi
solution (2.40 M in hexane, 0.67 mmol) was added dropwise via
syringe over 30 s. The resulting yellow solution was stirred at
-78 °C for 1 h.
and 1307 cm-1; 1H NMR (500 MHz, CDCl3) δ 7.95-7.98 (m, 2H),
7.71 (tt, J ) 1.5, 8.0 Hz, 1H), 7.60-7.63 (m, 2H), 5.72 (t, J ) 1.5
Hz, 1H), 5.56 (t, J ) 1.0 Hz, 1H), 4.54-4.58 (m, 1H), 3.61 (dd,
J ) 2.0, 14.5 Hz, 1H), 3.46 (d, J ) 3.0 Hz, 1H), 3.31 (dd, J )
9.5, 15.0 Hz, 1H), and 0.14 (s, 9H); 13C NMR (125 MHz, CDCl3)
δ 139.1, 134.3, 130.5, 129.6, 128.2, 123.2, 101.4, 97.9, 68.4, 61.2,
and -0.07; HRMS-ESI m/z [M + Na]+ calcd for C15H20O3SSi
331.0795, found 331.0801.
Oxidation of allylic alcohol 7a (0.198 g, 1.28 mmol) with Dess-
Martin periodinane (0.659 g, 1.54 mmol) in 5 mL of CH2Cl2 was
carried out according to the General Procedure to furnish
aldehyde 1a (estimated yield of 0.7 mmol based on previous
experiments). This material (not allowed to warm above -78
°C once solvent was removed) was dissolved in 3 mL of THF,
and then transferred via cannula over 5 min into the solution
of lithiated sulfone prepared as described above. The resulting
clear yellow solution was allowed to stir at -78 °C for 1.7 h and
then treated dropwise with 10 mL of half-saturated aq NH4Cl
solution (pre-cooled at 0 °C). The resulting mixture was allowed
to warm to room temperature over 15 min and then diluted with
15 mL of Et2O and 10 mL of water. The aqueous phase was
separated and extracted with four 10-mL portions of Et2O, and
the combined organic phases were washed with 30 mL of
saturated NaCl solution, dried over MgSO4, filtered, and con-
centrated to afford 0.248 g of an orange-yellow oil. Column
chromatography on 13 g of silica gel (gradient elution with 10-
40% EtOAc-hexane) afforded 0.114 g (61%) of alcohol 12 as a
white solid: mp 122-123 °C; IR (film) 3492, 3067, 2960, 2147,
Acknowledgment. We thank the National Insti-
tutes of Health (GM 28273), Pharmacia, and Merck
Research Laboratories for generous financial support.
C.T. was supported in part by The Institute for the
Promotion of Teaching Science and Technology, Thai-
land.
Supporting Information Available: Experimental pro-
cedures and characterization data for compounds 7c,d, 1b-
d, and 9b-d and1H NMR spectra for compounds 1a-d, 7b-
d, 9a-d, and 10-12. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO047869A
J. Org. Chem, Vol. 70, No. 6, 2005 2367