Reactions of nitroso hetero-Diels-Alder cycloadducts with azides: Stereoselective formation of triazolines and aziridines

Article abstract of DOI:10.1021/jo0701987

(Chemical Equation Presented) The addition of azides to acylnitroso hetero-Diels-Alder cycloadducts derived from cyclopentadiene affords exo-triazolines in excellent yield. The reaction is greatly affected by the level of alkene strain, while sterically d

Full text of DOI:10.1021/jo0701987

results in amino alcohols,5 which are suitable intermediates for
the synthesis of natural products,6 carbocyclic nucleosides,7 and
other important biologically active molecules.8 The C-O bond
can be cleaved through metal-mediated reactions in the presence
of nucleophiles9 or electrophiles,10 yielding 1,4-benzodiaz-
epines11 and other useful synthetic intermediates. The N-acyl
group can also be cleaved under relatively mild conditions
(where R ) alkyl or aryl).12 While extensive chemistry has been
developed that capitalizes on the strained nature of 1, modifica-
tions of the olefin have been mainly limited to epoxidation,6c
dihydroxylation,13 and oxidative cleavage.14
Reactions of Nitroso Hetero-Diels-Alder
Cycloadducts with Azides: Stereoselective
Formation of Triazolines and Aziridines
Brian S. Bodnar† and Marvin J. Miller*,†,‡
Department of Chemistry and Biochemistry, UniVersity of Notre
Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana
46556, and Leibniz-Institute for Natural Product Research and
Infection Biology, Hans Kno¨ll Institute, Beutenbergstrasse 11a,
07745 Jena, Germany
mmiller1@nd.edu
ReceiVed February 2, 2007
In an effort to expand upon the versatility of 1, we were
interested in selective functionalization of the olefin to produce
new structural features. Encouraged by a recent report highlight-
ing the addition of nitrile oxides to 1,15 we wish to report on
studies regarding the reactivity of 1 with azides to form
triazolines and their subsequent transformation to aziridines.
Intermolecular [3 + 2] cycloaddition reactions of azides to
strained bicyclic alkenes are well-documented in the literature.16
Examples include additions to norbornene, 2-azabicyclo[2.2.1]-
hept-5-en-3-one (ABH) 2 to afford 2′-3′-epimino carbocyclic
The addition of azides to acylnitroso hetero-Diels-Alder
cycloadducts derived from cyclopentadiene affords exo-
triazolines in excellent yield. The reaction is greatly affected
by the level of alkene strain, while sterically demanding
azides do not hinder the reaction. Conversion of the
triazolines to aziridines is also described.
(5) (a) Shireman, B. T.; Miller, M. J. Tetrahedron Lett. 2000, 41, 9537-
9540. (b) Keck, G. E.; Wager, T. T.; McHardy, S. F. Tetrahedron 1999,
55, 11755-11772. (c) Keck, G. E.; McHardy, S. F.; Wager, T. T.
Tetrahedron Lett. 1995, 36, 7419-7422.
(6) (a) Li, F.; Warshakoon, N. C.; Miller, M. J. J. Org. Chem. 2004, 69,
8836-8841. (b) Malpass, J. R.; Hemmings, D. A.; Wallis, A. L.; Fletcher,
S. R.; Patel, S. J. Chem. Soc., Perkin Trans. 1 2001, 1044-1050. (c) Justice,
D. E.; Malpass, J. R. Tetrahedron 1996, 52, 11977-11994.
(7) (a) Li, F.; Brogan, J. B.; Gage, J. L.; Zhang, D.; Miller, M. J. J. Org.
Chem. 2004, 69, 4538-4540. (b) Kim, K.-H.; Miller, M. J. Tetrahedron
Lett. 2003, 44, 4571-4573. (c) Li, H.; Miller, M. J. J. Org. Chem. 1999,
64, 9289-9293.
The nitroso hetero-Diels-Alder (HDA) reaction provides a
useful method for incorporating 1,4-aminoalcohol moieties into
a carbon framework in a diastereoselective fashion.1 Stereose-
lective nitroso HDA reactions2 have also provided access to
new classes of synthetically useful molecules. While a variety
of nitroso species have been investigated for use in HDA
reactions,3 our research has largely focused on acyl- and
carboxylnitroso species due to their ease of synthesis and high
reactivity. Hydroxamic acids and N-hydroxy carbamates, when
oxidized in the presence of cyclic dienes, afford hetero-Diels-
Alder adducts 1.1a,4 Reduction of the N-O bond of 1
(8) (a) Jiang, M. X.-W.; Warshakoon, N. C.; Miller, M. J. J. Org. Chem.
2005, 70, 2824-2827. (b) Lee, W.; Miller, M. J. J. Org. Chem. 2004, 69,
4516-4519.
(9) (a) Surman, M. D.; Mulvihill, M. J.; Miller, M. J. J. Org. Chem.
2002, 67, 4115-4121. (b) Surman, M. D.; Mulvihill, M. J.; Miller, M. J.
Tetrahedron Lett. 2002, 43, 1131-1134. (c) Surman, M. D.; Miller, M. J.
J. Org. Chem. 2001, 66, 2466-2469. (d) Mulvihill, M. J.; Surman, M. D.;
Miller, M. J. J. Org. Chem. 1998, 63, 4874-4875.
(10) Lee, W.; Kim, K.-H.; Surman, M. D.; Miller, M. J. J. Org. Chem.
2003, 68, 139-149.
(11) Surman, M. D.; Mulvihill, M. J.; Miller, M. J. Org. Lett. 2002, 4,
139-141.
(12) Keck, G. E.; Webb, R. R.; Yates, J. B. Tetrahedron 1981, 37, 4007-
4016.
† University of Notre Dame.
‡ Hans Kno¨ll Institute.
(13) Keck, G. E.; Romer, D. R. J. Org. Chem. 1993, 58, 6083-6089.
(14) (a) Shireman, B. T.; Miller, M. J. J. Org. Chem. 2001, 66, 4809-
4813. (b) Nora, G. P.; Miller, M. J.; Moellmann, U. Bioorg. Med. Chem.
Lett. 2006, 16, 3966-3970.
(15) Quadrelli, P.; Mella, M.; Paganoni, P.; Caramella, P. Eur. J. Org.
Chem. 2000, 14, 2613-2620.
(16) (a) Braese, S.; Gil, C.; Knepper, K.; Zimmermann, V. Angew. Chem.,
Int. Ed. 2005, 44, 5188-5240. (b) Shea, K. J.; Kim, J. S. J. Am. Chem.
Soc. 1992, 114, 4846-4855. (c) Becker, K. B.; Hohermuth, M. K. HelV.
Chim. Acta 1979, 62, 2025-2036. (d) Taniguchi, H.; Ikeda, T.; Imoto, E.
Bull. Chem. Soc. Jpn. 1978, 51, 1859-1865. (e) Scheiner, P. Tetrahedron
1968, 24, 2757-2766. (f) Scheiner, P. J. Am. Chem. Soc. 1968, 90, 988-
992. (g) Rolf, H. Angew. Chem., Int. Ed. Engl. 1963, 2, 565-598. (h)
Tarabara, I. N.; Kas’yan, A. O.; Yarovoi, M. Y.; Shishkina, S. V.; Shishkin,
O. V.; Kas’yan, L. I. Russ. J. Org. Chem. 2004, 40, 992-998.
(1) (a) Kirby, G. W. Chem. Soc. ReV. 1977, 6, 1-24. (b) Corrie, J. E.
T.; Kirby, G. W.; Mackinnon, J. W. M. J. Chem. Soc., Perkin Trans. 1985,
1, 883-886. (c) Leach, A. G.; Houk, K. N. J. Org. Chem. 2001, 66, 5192-
5200.
(2) (a) Yamamoto, Y.; Yamamoto, H. J. Am. Chem. Soc. 2004, 126,
4128-4129. (b) Kirby, G. W.; Nazeer, M. Tetrahedron Lett. 1988, 29,
6173-6174. (c) Ritter, A. R.; Miller, M. J. J. Org. Chem. 1994, 59, 4602-
4611.
(3) (a) Miller, C. A.; Batey, R. A. Org. Lett. 2004, 6, 699-702. (b)
Calvet, G.; Dussaussois, M.; Blanchard, N.; Kouklovsky, C. Org. Lett. 2004,
6, 2449-2451. (c) Ware, R. W., Jr.; Day, C. S.; King, S. B. J. Org. Chem.
2002, 67, 6174-6180. (d) Ware, R. W., Jr.; King, S. B. J. Org. Chem.
2000, 65, 8725-8729.
(4) Vogt, P. F.; Miller, M. J. Tetrahedron 1998, 54, 1317-1348.
10.1021/jo0701987 CCC: $37.00 © 2007 American Chemical Society
Published on Web 04/13/2007
J. Org. Chem. 2007, 72, 3929-3932
3929

nucleosides,17 2,3-diazabicyclo[2.2.1]hept-5-enes 3 to afford 1,4-
dihydropyridines,18 as well as many other strained systems.16h,19
However, the addition of azides to bicyclic oxazines such as 1
has not been disclosed in the literature.
TABLE 1. Reaction of 1a with Various Azides
entry
R
conditionsa
productsb
yieldc (%)
1
2
3
4
5
6
7
8
9
Bn
Bn
Add
Bn
n-octyl
cyclopentyl
Ph
A
B
B
C
C
C
C
D
D
D
D
D
5a/6a
5a/6a
5b/6b
5a/6a
5c/6c
5d/6d
5e/6e
5a/6a
5c/6c
5d/6d
5b/6b
5e/6e
99
99
95
88
99
97
97
88
81
86
85
99
Since we wished to probe the role of olefin strain on reactivity
with azides, we chose to subject the olefins of 1a-c as well as
protected amino alcohols 4a-c to reactions with various azides
(Scheme 1). Oxazines 1a-c can be prepared on a large scale
from hydroxylamine hydrochloride in two steps in moderate to
Bn
n-octyl
cyclopentyl
Add
high yields.20 N-O bond reduction with Mo(CO)6 followed
21
10
11
12
by protection of the resulting alcohol yielded monocyclic olefins
Ph
4a-c.22
a A ) neat, rt, 2 days; B ) CHCl3, rt, 4 weeks; C ) CHCl3, reflux, 3
days; D ) PhCH3, reflux, 4 h. b Ratio of 5:6 ) 1.0:1.1 in all cases as
SCHEME 1. Synthesis of Cycloadducts and Protected
Amino Alcohols
1
determined by H NMR. c Isolated yield. d Ad ) 1-adamantyl.
FIGURE 1. Observed W-coupling and 3JCH-coupling of exo-triazolines.
In an effort to ascertain the limitations of this chemistry, we
treated alkene 1a with various organic azides under a number
of reaction conditions (Table 1). The reaction proceeded equally
well refluxing in toluene or chloroform (entries 4 and 8);
however, it was sluggish when stirred in solution at room
temperature (entry 2). The steric bulk of the azide had little
effect on either the regioselectivity or yield of the reaction, with
primary, secondary, tertiary, and aryl azides affording triazolines
5 and 6 in excellent yield (entries 8-12).
The structures of 5 and 6 were assigned based on single-
crystal X-ray diffraction for 5a and 6a and by 2D NMR
experiments for 5b-e and 6b-e. The exo stereochemistry was
confirmed from the observed 4J-coupling (“W-coupling”) of H2
and H3 with H5′, but not with H5. The position of the Boc group
relative to the R group was confirmed by HMBC experiments
based on the observed 3J-coupling between H4 (or H1) and the
carbonyl (Figure 1).26
We were very pleased to find that, when 1a was treated with
benzyl azide,23 the regioisomeric exo-triazolines 5a and 6a were
obtained in quantitative yield after stirring for 2 days neat at
room temperature (eq 1). Similar reactions of 2-azabicyclo[2.2.2]-
hept-5-en-3-ones reportedly required high pressure for the
cycloaddition reaction to occur.17c The exo-specificity of the
reaction is in agreement with what has been reported in the
literature in related reactions of bicyclo[2.2.1]hept-5-ene-2,3-
dicarboximides,16h ABH (2) and derivatives,24 7-oxabicyclo-
[2.2.1]hept-5-en-2-yl derivatives, and 3.18,25
Trimethylsilyl azide has been reported to add to norbornene
systems and other bicyclic olefins to yield triazolines and/or
aziridines directly.18,27 Since we were interested in obtaining
the unsubstituted aziridine 7, 1a was subjected to azidotrim-
ethylsilane (Scheme 2). Under a variety of reaction conditions
(17) (a) Ishikura, M.; Katagiri, N. Rec. Res. DeV. Org. Chem. 2004, 8,
197-205. (b) Ishikura, M.; Murakami, A.; Katagiri, N. Org. Biomol. Chem.
2003, 1, 452-453. (c) Ishikura, M.; Kudo, S.; Hino, A.; Ohnuki, N.;
Katagiri, N. Heterocycles 2000, 53, 1499-1504.
SCHEME 2. Reaction of 1a to Yield Aziridines Directly
(18) Stout, D. M.; Takaya, T.; Meyers, A. I. J. Org. Chem. 1975, 40,
563-569.
(19) Huenig, S.; Kraft, P. Heterocycles 1995, 40, 639-652.
(20) Zhang, D.; Sueling, C.; Miller, M. J. J. Org. Chem. 1998, 63, 885-
888.
(21) Cicchi, S.; Goti, A.; Brandi, A.; Guarna, A.; De Sarlo, F.
Tetrahedron Lett. 1990, 31, 3351-3354.
(neat, 25-80 °C; PhCH3, 25-110 °C), only decomposition of
the alkene was observed. Tosyl azide28 has also been reported
3930 J. Org. Chem., Vol. 72, No. 10, 2007

SCHEME 3. Reaction of Other Alkenes with Benzyl Azide
TABLE 2. Photolysis of Triazolines 5a and 6a
to afford aziridines directly,17b and upon treatment of 1a with
tosyl azide, we were able to obtain aziridine 8 in good yield.
Substantial rate increases were reported for [3 + 2] cycload-
ditions (especially “Click chemistry”29 reactions) when both
reactants were “floated” on water compared to reactions that
were stirred with or without solvent.30 We found no significant
increase in the rate of reaction when 1a was treated with benzyl
azide “on water” or in a concentrated solution of toluene “on
water” when compared to stirring the two reagents neat at room
temperature.
The effect of alkene strain on the formation of triazolines
was also studied. When the bicyclo[2.2.2] cycloadduct 1b was
treated with 1.4 equiv of benzyl azide neat at room temperature,
no reaction was observed. When 1b was treated with a large
excess of benzyl azide in refluxing toluene, however, a mixture
of triazolines 9-12 was obtained in moderate yield (Scheme
3). In contrast to bicyclo[2.2.1] cycloadduct 1a, endo-triazolines
11 and 12 were formed along with the exo-triazolines 9 and
10. The influence of ring strain on reactivity was especially
evident when bicyclo[2.2.4] cycloadduct 1c and monocyclic
alkenes 4a-c were subjected to the same conditions. When
heated neat or in toluene, only trace amounts of triazoline
products were observed.
entry
isomer
solvent
filter
timea (h)
yieldb (%)
1
2
3
4
5
5a
6a
5a
6a
5a
CHCl3
CHCl3
CH3CN
CH3CN
CH3CN
quartz
quartz
vycor
vycor
pyrex
1.5
3.5
2
65
78
75
67
93
3
20
a Reactions were monitored by TLC and/or 1H NMR. b Isolated yield.
SCHEME 4. Structural Elaboration of Triazolines 5a
and 6a
In the course of this study, we found that triazolines 5a and
6a were stable to temperatures up to 120 °C and did not
decompose readily in ambient light. When irradiated below
∼300 nm, 5a and 6a readily underwent photolytic conversion
to aziridine 13 in 2-4 h (Table 2). At wavelengths higher than
300 nm, the time required for the reaction to proceed to 100%
conversion increased greatly (entry 5).
(22) (a) Li, F.; Miller, M. J. J. Org. Chem. 2006, 71, 5221-5227. (b)
Sirisoma, N. S.; Woster, P. M. Tetrahedron Lett. 1998, 39, 1489-1492.
(23) Alvarez, S. G.; Alvarez, M. T. Synthesis 1997, 413-414.
(24) Malpass, J. R.; Belkacemi, D.; Griffith, G. A.; Robertson, M. D.
ARKIVOC 2002, 164-174.
(25) Reymond, J.-L.; Vogel, P. Tetrahedron Lett. 1988, 29, 3695-3698.
(26) For a full description of the assignment of stereochemical relation-
ships and the position of the R and Boc groups using 2D NMR experiments,
see the Supporting Information.
Surprisingly, N-O bond reduction of triazolines 5a and 6a
proceeded cleanly to afford triazolines 14 and 15 without
decomposition of the triazoline ring (Scheme 4). Triazoline 14
was cleanly transformed to aziridine 16 upon irradiation in
acetonitrile; however, a complex mixture of products was
observed when triazoline 15 was subjected to the same
conditions. The reason for this result is unclear at this time and
will be investigated further.
(27) Peterson, W. R., Jr.; Arkles, B.; Washburne, S. S. J. Organomet.
Chem. 1976, 121, 285-291.
(28) Pollex, A.; Hiersemann, M. Org. Lett. 2005, 7, 5705-5708.
(29) (a) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int.
Ed. 2001, 40, 2004-2021. (b) Kolb, H. C.; Sharpless, K. B. Drug DiscoVery
Today 2003, 8, 1128-1137.
In summary, the addition of azides to cycloadduct 1a affords
a mixture of exo-triazolines in good to excellent yield. The steric
bulk of the azide does not appear to play a significant role in
the course of the reaction; however, the reactivity of the alkene
(30) (a) Narayan, S.; Muldoon, J.; Finn, M. G.; Fokin, V. V.; Kolb, H.
C.; Sharpless, K. B. Angew. Chem., Int. Ed. 2005, 44, 3275-3279. (b) Klijn,
J. E.; Engberts, J. B. F. N. Nature 2005, 435, 746-747.
J. Org. Chem, Vol. 72, No. 10, 2007 3931

4.72 (d, J ) 14.5 Hz, 1H), 4.09 (s, 1H), 3.55 (d, J ) 9.8 Hz, 1H),
1.70 (dt, J ) 11.4, 1.5 Hz, 1H), 1.45 (d, J ) 11.5 Hz, 1H), 1.39 (s,
9H) ppm; 13C NMR (125 MHz, CDCl3) δ 156.4, 135.6, 128.8,
128.24, 128.17, 84.6, 82.6, 79.9, 61.2, 59.3, 53.4, 32.4, 27.9 ppm;
HRMS (FAB), m/z (M + H) calcd for C17H23N4O3+, 331.1770;
obsd, 331.1745. 6a: mp ) 100-101 °C; λmax ) 255 nm; 1H NMR
(500 MHz, CDCl3) δ 7.35-7.24 (m, 5H), 4.92 (d, J ) 14.7 Hz,
1H), 4.85 (s, 1H), 4.84 (d, J ) 9.9 Hz, 1H), 4.61 (d, J ) 14.7 Hz,
1H), 4.09 (s, 1H), 3.50 (d, J ) 9.9 Hz, 1H), 1.68 (d, J ) 11.5 Hz,
1H), 1.45-1.43 (m, 10H) ppm; 13C NMR (125 MHz, CDCl3) δ
156.4, 135.6, 128.8, 128.3, 128.2, 83.7, 82.7, 79.6, 61.5, 59.9, 53.6,
was diminished significantly in cycloadducts derived from larger
cyclic dienes (1b and 1c) or in monocyclic alkenes (4a-c).
Experimental Section
General Procedure for the Synthesis of Triazolines Using
Method A. The alkene (1 mmol) and azide (1.5 mmol) were
combined and stirred neat at 25 °C in a single-necked round-
bottomed flask. The progress of the reaction was monitored by TLC
1
or H NMR for the disappearance of the alkene, and the crude
material was chromatographed through silica gel 60 (230-400
mesh).
32.3, 27.9 ppm; HRMS (FAB), m/z (M + H) calcd for C17H23N4O3+
,
331.1770; obsd, 331.1753.
General Procedure for the Synthesis of Triazolines Using
Method B. The alkene (1 mmol) and azide (1.5 mmol) were
dissolved in 10 mL of CHCl3 in a single-necked round-bottomed
flask and stirred at 25 °C. The progress of the reaction was
monitored by TLC or 1H NMR for the disappearance of the alkene
(typically 4 weeks), and the crude material was chromatographed
through silica.
General Procedure for Photolysis of Triazolines. tert-Butyl
(1R,2â,4â,5R)-3-Benzyl-6-oxa-3,7-diazatricyclo[3.2.1.02,4]octane-
7-carboxylate (13). 5a (333 mg, 1.01 mmol) was dissolved in 300
mL of degassed CH3CN and transferred to a 450 mL photochemical
reaction vessel. The solution was irradiated in an immersion-well
reactor under a stream of Ar with a Hanovia 450 W mercury lamp
equipped with a Vycor filter sleeve. Reaction progress was
monitored by TLC and 1H NMR for the disappearance of 5a. After
3 h, the reaction was concentrated, and the crude material was
chromatographed through 30 g of silica using a gradient consisting
of 100% CH2Cl2 to 95% CH2Cl2/EtOAc to afford 13 as a colorless
oil (205 mg, 67% yield): 1H NMR (500 MHz, CDCl3) δ 7.35-
7.27 (m, 5H), 4.79 (m, 1H), 4.62 (m, 1H), 3.40 (d, J ) 13.5 Hz,
1H), 3.35 (d, J ) 14.0 Hz, 1H), 2.29-2.22 (m, 3H), 1.50 (s, 9H),
1.39 (d, J ) 10.5 Hz, 1H) ppm; 13C NMR (125 MHz, CDCl3) δ
157.5, 138.6, 128.4, 127.7, 127.3, 82.3, 79.9, 60.7, 59.1, 37.1, 36.7,
General Procedure for the Synthesis of Triazolines Using
Method C. The alkene (1 mmol) and azide (1.5 mmol) were
dissolved in 10 mL of CHCl3 in a single-necked round-bottomed
flask fitted with a condenser and heated to reflux in an oil bath
(oil temp ) 80 °C). The progress of the reaction was monitored
by TLC or 1H NMR for the disappearance of the alkene (typically
3 days), and the crude material was chromatographed through silica.
General Procedure for the Synthesis of Triazolines Using
Method D. The alkene (1 mmol) and azide (1.5 mmol) were
dissolved in 10 mL of PhCH3 in a single-necked round-bottomed
flask fitted with a condenser and heated to reflux in an oil bath
(oil temp ) 125 °C). The progress of the reaction was monitored
by TLC or 1H NMR for the disappearance of the alkene (typically
3-4 h), and the crude material was chromatographed through silica.
tert-Butyl (3aR,4â,7â,7aR)-1-Benzyl-3a,4,7,7a-tetrahydro-4,7-
methano[1,2,3]triazole[4,5-d][1,2]oxazine-6(1H)-carboxylate (5a)
and tert-Butyl (3aR,4â,7â,7aR)-3-Benzyl-3a,4,7,7a-tetrahydro-
4,7-methano[1,2,3]triazole[4,5-d][1,2]oxazine-6(3H)-carboxy-
late (6a). Prepared following the general procedure for the synthesis
of triazolines using Method A. Cycloadduct 1a (198 mg, 1.01
mmol) and benzyl azide (211 mg, 1.58 mmol) were reacted for 48
h. The brown crude material was chromatographed through 15 g
of silica using a solvent gradient of 100% CH2Cl2 to 98% CH2-
Cl2/EtOAc to afford triazoline 6a (159 mg, 48% yield) as a white
solid, then 85% CH2Cl2/EtOAc to afford triazoline 5a (170 mg,
51% yield) as a white solid (99% total combined yield). Analytical
and X-ray crystallographic samples of 5a and 6a were prepared by
recrystallization from EtOAc/hexanes. 5a: mp ) 104-105 °C; λmax
29.4, 28.2 ppm; MS (FAB) m/z 303 (M + H), 247, 203, 171
+
(100%); HRMS (FAB) m/z (M + H) calcd for C17H23N2O3
303.1709; obsd, 303.1712.
,
Acknowledgment. Support from the NIH (AI054193 and
GM 68012) is gratefully acknowledged. The authors would also
like to thank Jaroslav Zajicek for assistance with 2D NMR
experiments, Bill Boggess and Nonka Sevova for mass spec-
troscopic analyses, and Bruce Noll for X-ray crystallographic
data. M.J.M. gratefully acknowledges the Hans Kno¨ll Institute
and the University of Notre Dame for support of a sabbatical
leave.
Supporting Information Available: Experimental details and
characterization data for triazolines 5a-e, 6a-e, 9-12, and 14-
16, including 1D and 2D NMR spectra and X-ray crystallographic
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
1
) 255 nm; H NMR (500 MHz, CDCl3) δ 7.37-7.27 (m, 5H),
4.92 (s, 1H), 4.90 (d, J ) 14.5 Hz, 1H), 4.82 (d, J ) 9.8 Hz, 1H),
JO0701987
3932 J. Org. Chem., Vol. 72, No. 10, 2007