One-Step Synthesis and Highly Regio- and Stereoselective Diels-Alder Cycloadditions of Novel exo-2-Oxazolidinone Dienes

Article abstract of DOI:10.1021/jo962403g

An improved synthesis of exo-heterocyclic dienes like TV-substituted 4,5-dimethylene 2-oxazolidinones 1, by a one-step method from diacetyl and isocyanates, was described. This highly convergent synthetic strategy has been successfully used for the prepar

Full text of DOI:10.1021/jo962403g

J . Org. Chem. 1997, 62, 4105-4115
4105
On e-Step Syn th esis a n d High ly Regio- a n d Ster eoselective
Diels-Ald er Cycloa d d ition s of Novel exo-2-Oxa zolid in on e Dien es
Ashis B. Mandal, Adriana Go´mez, Gonzalo Trujillo, Francisco Me´ndez,† Hugo A. J ime´nez,
Mar´ıa de J esu´s Rosales,‡ Rafael Mart´ınez, Francisco Delgado, and J oaqu´ın Tamariz*
Department of Organic Chemistry, Escuela Nacional de Ciencias Biolo´gicas, I.P.N. Prol. Carpio y Plan de
Ayala, 11340 Me´xico, D.F., Mexico, Department of Chemistry, Universidad Auto´noma
Metropolitana-Iztapalapa, P.O. 55-534, 09340 Me´xico, D.F., Mexico, and Department of Chemistry,
CINVESTAV, I.P.N., P.O. 14-740, 07000 Me´xico, D.F., Mexico
Received December 30, 1996X
An improved synthesis of exo-heterocyclic dienes like N-substituted 4,5-dimethylene 2-oxazolidinones
1, by a one-step method from diacetyl and isocyanates, was described. This highly convergent
synthetic strategy has been successfully used for the preparation of novel (Z)-5-ethylidene-4-
methylene analogs 4 in fair yields. Both dienes 1 and 4 undergo efficient addition of symmetric
dienophiles in thermal Diels-Alder reactions, inasmuch as they react stereo- and regioselectively
with the unsymmetric olefins methyl vinyl ketone (MVK) and methyl propiolate. This regioselec-
tivity was greatly improved by using Lewis acid catalysts (TiCl4, AlCl3). The nitrogen atom of the
2-oxazolidinone ring seems to control the orientation of the dienophile approach. These results
have been rationalized in terms of the frontier molecular orbital theory by ab initio calculations.
For dienes 4, addition of MVK gave the endo isomer as the major product. Dimerization of dienes
4 was also highly regio-, chemo-, and stereoselective, giving only isomer 17. This reaction furnished
a second product, which corresponded to dienes 18 obtained by the [1,5] sigmatropic rearrangement
of 4. The structure of the dienes and main products was established by NMR experiments and
X-ray diffraction analysis.
Exocyclic dienes have attracted special attention due
to their high reactivity in cycloaddition reactions and
because of their synthetic potential.1 Particularly, the
preparation and study of exo-heterocyclic dienes have
been the subject of numerous investigations.2 Recently,
we described a one-pot synthesis of the novel N-substi-
tuted 4,5-dimethylene 2-oxazolidinones 1 from diacetyl
(2a ) and the corresponding isocyanate 3 (eq 1).3 Dienes
1 proved to be, for the most part, stable, and they undergo
Diels-Alder cycloadditions in good yields. This prelimi-
nary work has now been extended to the preparation of
the substituted analogs 4, in order to evaluate the general
applicability and selectivity of our methodology. We have
also studied the reactivity and selectivity of these dienes
in Diels-Alder cycloadditions.
Herein, we report full details of the optimized method
for the preparation of dienes 1 and the highly selective
synthesis of new unsymmetric dienes 4 through the same
methodology, but with 2,3-pentanedione (2b) as starting
material. The behavior of both dienes 1 and 4 in Diels-
Alder cycloadditions with symmetric and unsymmetric
dienophiles is also presented. We disclose MO calcula-
tions as well, which rationalize the regioselectivity
observed in these processes.
Resu lts
Table 1 summarizes reaction conditions and yields for
the preparation of dienes 1a -h . For the N-arylated
derivatives, it was found that the optimum reaction
conditions corresponded to a reaction time of 24 h, with
reaction mixtures diluted to a concentration of about 0.1
g/mL in dry dioxane. In addition, the stirring time of
the mixture of 2a , Et3N, and Li2CO3 at room temperature
was also extended in order to increase the concentration
of the enolate of 2a . Even though dienes 1 were prepared
in fair yields, these were even better than those usually
obtained by multistep synthetic approaches for similar
exo-heterocyclic dienes.2 The reaction was likely depend-
ent on the substituent of the isocyanate, because aryl
isocyanates with electron-withdrawing groups on the
N-benzene ring, such as the nitro, gave no reaction.
Dienes 4a -j were prepared under similar conditions
using 2,3-pentanedione (2b) as the starting R-diketone.
The reaction time was shorter (12 h) for these compounds,
and the yields were comparable to those of dienes 1, with
the exception of derivatives 4g-j. The low yields ob-
† Universidad Auto´noma Metropolitana-Iztapalapa.
‡ CINVESTAV.
X Abstract published in Advance ACS Abstracts, May 1, 1997.
(1) (a) Fringuelli, F.; Taticchi, A. Dienes in the Diels-Alder Reaction,
J ohn Wiley: New York, 1990; pp 125-147. (b) Martin, N; Seoane, C.;
Hanack, M. Org. Prep. Proc. Int. 1991, 23, 237. (c) Wiersum, U. E.
Aldrichimica Acta 1981, 14, 53. (d) Toda, F.; Garratt, P. Chem. Rev.
1992, 92, 1685. (e) Ando, K.; Takayama, H. Heterocycles 1994, 37, 1417.
(f) Charlton, J . L.; Alauddin, M. M. Tetrahedron 1987, 43, 2873.
(2) For recent examples: (a) Ando, K.; Kankake, M.; Suzuki, T.;
Takayama, H. Tetrahedron 1995, 51, 129. (b) Sha, C.-K.; Lee, R.-S.;
Wang, Y. Tetrahedron 1995, 51, 193. (c) Van der Baan, J . L.; van der
Heide, T. A. J .; van der Louw, J .; Klumpp, G. W. Synlett 1995, 1. (d)
Leung, M.-k.; Trahanovsky, W. S. J . Am. Chem. Soc. 1995, 117, 841.
(e) Chou, C.-H.; Trahanovsky, W. S. J . Org. Chem. 1995, 60, 5449. (f)
Chung, W.-S.; Lin, W.-J .; Liu, W.-D.; Chen, L.-G. J . Chem. Soc., Chem.
Commun. 1995, 2537. (g) Tome´, A. C.; Cavaleiro, J . A. S.; Storr, R. C.
Tetrahedron 1996, 52, 1735. (h) White, L. A.; Storr, R. C. Tetrahedron
1996, 52, 3117. (i) Tso, H.-H.; Chang, Y.-M.; Tsay, H. Synth. Commun.
1996, 26, 569. (j) Tso, H.-H.; Chandrasekharam, M. Tetrahedron Lett.
1996, 37, 4189. (k) Carly, P. R.; Cappelle, S. L.; Campernolle, F.;
Hoornaert, G. J . Tetrahedron 1996, 52, 11889. (l) Chou, T.-s.; Wang,
C.-T. Tetrahedron 1996, 52, 12459. (m) Kappe, C. O.; Padwa, A. J .
Org. Chem. 1996, 61, 6166, and references cited therein.
(3) Herna´ndez, R.; Sa´nchez, J . M.; Go´mez, A.; Trujillo, G.; Aboytes,
R.; Zepeda, G.; Bates, R. W.; Tamariz, J . Heterocycles 1993, 36, 1951.
S0022-3263(96)02403-6 CCC: $14.00 © 1997 American Chemical Society

4106 J . Org. Chem., Vol. 62, No. 12, 1997
Mandal et al.
Ta ble 1. Im p r oved Rea ction Con d ition s a n d Yield s in
th e P r ep a r a tion of Dien es 1 a n d 4a
Ta ble 2. 1H NMR Da ta of th e Vin ylic P r oton s of Dien es
1a -h a n d 4a -j
entry
R′
diene
yield (%)b
mp (°C)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Ph
1a
1b
1c
1d
1e
1f
1g
1h
4a
4b
4c
4d
4e
4f
54
70
40
45
45
40
60
69
53
54
47
40
42
38
27
18
23
26
75-76
106-107
57-dec
oil
79-80
54-55
88-89
oil
83-84
78-79
82-83
92-93
80-81
83-84
79-80
78-79
81-82
oil
C6H4-p-Cl
C6H4-m-Cl
C6H4-o-Me
C6H4-p-Me
C6H4-m-Me
C6H4-o-Br
(CH2)2Cl
δ (ppm)
H-6aa or CH3
H-6b
H-7ac
H-7bc
b
Ph
diene
C6H4-p-Cl
C6H4-m-Cl
C6H4-p-Me
C6H4-m-Me
C6H4-m-MeO
C6H4-o-Cl
C6H4-o-Br
C6H4-o-Me
(CH2)2Cl
1a
1b
1c
1d
1e
1f
1g
1h
4a
4b
4c
4d
4e
4f
4.93
4.93
4.95
4.94
4.90
4.92
4.96
4.89
1.86
1.86
1.86
1.85
1.86
1.86
1.87
1.90
1.86
1.82
4.98a
4.99a
5.00a
4.98a
4.95a
4.97a
5.00a
4.95a
5.42d
5.43d
5.43d
5.40d
5.40d
5.43d
5.43d
5.44d
5.41d
5.39d
4.35
4.35
4.40
4.01
4.30
4.34
4.01
4.42
4.18
4.17
4.21
4.15
4.16
4.22
3.85
3.82
3.83
4.23
4.76
4.78
4.80
4.72
4.72
4.75
4.76
4.79
4.58
4.59
4.61
4.55
4.56
4.58
4.57
4.59
4.53
4.61
4g
4h
4i
4j
a
Dioxane as the solvent, with Et3N (1.5 mol equiv for dienes 1
and 2.0 mol equiv for dienes 4) as the base and Li2CO3 (1.2 mol
eq) as the additive at rt, except in entry 8 (60 °C). The reaction
time was 24 h for dienes 1, except diene 1h (4 h), and 12 h for
b
dienes 4. After column chromatography and recrystallization.
4g
4h
4i
4j
a
b
As a doublet, ca. J ) 3.6 Hz. As a doublet, ca. J ) 7.3 Hz.
c As doublets, ca. J ) 3.0 Hz. As a quartet, ca. J ) 7.3 Hz.
d
H
7a,5 as expected from the almost orthogonal position of
the aryl ring plane with respect to the bis-methylene
plane (Figure 1), could account for the diamagnetic shift
of this proton. However, this factor should be discarded,
because a similar chemical shift pattern for the dienic
protons was observed for the N-alkyl derivative 1h . It
could be argued6 that there is a common electron-
releasing effect from the nitrogen atom lone-pair, present
in all the dienes as an enamide system. The signal
assignment was done by NOE experiments (Figure 1).
Interestingly, in the preparation of N-tolyl derivatives
4d and 4e, 5-ethylidene-4-hydroxy-4-methyl-2-oxazolidi-
nones 5a and 5b were also isolated, respectively, in low
yields. The structure of these compounds suggests that
they may be involved as the last precursors in the
mechanism of formation of dienes 4 by dehydration (vide
infra).
F igu r e 1. Structure of diene 4c and NOEs observed upon
irradiation of protons in diene 4a . The figure was constructed
from the X-ray crystallographic data of 4c.
tained for some of the latter could be associated with
steric interactions between the ortho-substituent of the
aryl isocyanate and the more crowded diketone 2b. It is
1
noteworthy that H NMR analysis of the crude mixtures
did not give evidence of the presence of any other isomer.
Therefore, the reaction was highly regio- and stereo-
selective.
Diene 4c was isolated as colorless crystals (acetone/
hexane, 9:1) and its structure was unambiguously es-
tablished by X-ray diffraction analysis.4 It can be ob-
served that the aromatic ring is almost perpendicular to
the diene plane, with the oxazolidinone ring being planar
due to the sp2 configuration of the nitrogen (Figure 1).
1H NMR spectra of dienes 4 showed chemical shifts for
the vinylic protons similar to those in dienes 1, along with
the H6 signal at low-field as a quartet (Table 2). The
most shielded proton corresponded to H7a. The assign-
ment was done by NOE experiments, indicating that the
methyl group was located on methylene C6, vicinal to the
oxygen, and with Z configuration, as shown by the X-ray
structure.
The thermal Diels-Alder reaction of dienes 1a and 4a
with the symmetrical dienophile dimethyl acetylenedi-
carboxylate (6) (Scheme 1) provided the corresponding
adducts 7 and 8 (Table 3, entries 1 and 4). The reaction
of N-phenylmaleimide (9) with dienes 1a and 1b also took
place readily at 180 °C to give adducts 10a and 10b,
respectively.
In dienes 1, the most shielded proton also corresponded
to H7a, with H6b being at the lowest field. A shielding
magnetic effect of the aryl ring on the spatial region of
(4) The authors have deposited the atomic coordinates for this
structure with the Cambridge Crystallographic Data Centre. The
coordinates can be obtained, on request, from the Director, Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ,
UK.
(5) Abraham, R. J .; Fisher, J .; Loftus, P. Introduction to NMR
Spectroscopy; J ohn Wiley & Sons: Chichester, 1988; pp 19-22.
As illustrated in Scheme 2, the cycloadditions of dienes
1a , 1b, 4a , 4c, and 4d with unsymmetrical olefins such
(6) Pretsch, E.; Clerc, T.; Seibl, J .; Simon, W. Tables of Spectral Data
for Structure Determination of Organic Compounds; Springer-Verlag:
Berlin, 1989; pp 215-216.

Diels-Alder Cycloadditions of exo-2-Oxazolidinone Dienes
J . Org. Chem., Vol. 62, No. 12, 1997 4107
Sch em e 1
respect to the exo-diene plane, as shown by the X-ray
structure of diene 4c.
The structures of the major isomers were established
1
by H NMR spectroscopy. In the case of isomers 13 and
15, double irradiation, COSY, and NOE experiments
were carried out in order to correlate the cyclohexene
protons. For example, for 13c the signals of two vicinal
CH and methylene groups were observed (Figure 2). The
coupling constants for proton H5â (13a , J ) 13.8, 10.6,
9.3, and 5.8 Hz; 13c, J ) 14.0, 12.1, 10.2, and 6.4 Hz)
suggest that the conformational equilibrium of the half-
chair is shifted toward maintaining the equatorial posi-
tion of the acetyl group on carbon C6 (Figure 3). For
isomers 13c, 13e, and 13g, the methyl group on carbon
C7 would be in a pseudoaxial position. In cycloadducts
15, the carboxymethyl group was also readily located by
NOE experiments, as shown in Figure 2, because an
enhancement of the signals of vinylic and aromatic
protons was observed when the bis-allylic protons C4 were
irradiated.
Ta ble 3. Diels-Ald er Cycloa d d ition s of Dien es 1a , 1b,
a n d 4a w ith Sym m etr ic Olefin s 6 a n d 9a
This assignment was confirmed by single-crystal X-ray
diffraction of adduct 13c,4 which could be recrystallized
from mixtures of acetone/hexane to give colorless crystals.
The cyclohexene ring exhibits a half-chair conformation
with the acetyl and methyl groups in the equatorial and
pseudo-axial positions, respectively, as anticipated by 1H
NMR spectroscopy (Figure 3). The axial conformation
of H6 gives rise to a spatial proximity to the syn H4R, in
agreement with the NOE effect displayed when the
former is irradiated.
entry
diene
olefinb
T (°C) t (h) product
yield (%)c
1
2
3
4
1a
1a
1b
4a
6
9
9
6
180
180
180
130
1.0
1.0
1.0
1.0
7
56
75
79
85
10a
10b
8
a
All under N2 atmosphere. Thermal trials in the presence of
b
1-2% hydroquinone. 3.0 mol equiv with dienes 1a and 1b, and
2.0 mol equiv with diene 4a . c Corresponding to the major isomer
after column chromatography and recrystallization.
In contrast with the dimerization of diene 1a , in which
a mixture of all possible adducts was obtained,3 the
thermal (xylene, 120 °C, 10 h) dimerization of dienes 4a
and 4c turned out to be highly regio-, chemo-, and
stereoselective, inasmuch as it afforded only the corre-
sponding isomeric dimers 17a and 17b, as determined
from 1H NMR of the crude reaction mixtures (Scheme
3). The structures of dimers 17 were established by
double irradiation and NOE experiments as depicted in
Figure 4. The enhancement observed in the signals
attributed to protons H7R and H5R, when the vinylic
proton H13 was irradiated, indicates their spatial proxim-
ity. Consequently, the methyl group C8 and proton H4â
are expected to have an interaction with the N-phenyl
protons in the spiro oxazolidinone ring in C6. Indeed,
these enhancements were observed, supporting the rela-
tive configuration shown, which supposes a preferential
exo transition state, with respect to the dienophilic π
system, in this process.
Interestingly, besides dimers 17a and 17b, a second
product was present in the crude reaction mixtures. This
product was isolated and characterized, proving to be
dienes 18a and 18b, respectively. Scheme 3 shows the
dimer 17/diene 18 ratios and yields obtained for the
thermal treatment of dienes 4a and 4c. Considering the
conditions of the reaction, these products were probably
formed by a [1,5] sigmatropic rearrangement of the
protons attached to the methyl group. In order to
determine a possible equilibrium between the isomeric
species 4 and 18, they were separately heated to 120 °C,
at high dilution in order to minimize the dimerization.
Thus, diene 4a was totally converted to 18a , but the
latter remained unchanged. This suggests the higher
stability of isomer 18a with the endo-cyclic half-dienic
moiety, in comparison with the exo-cyclic diene system
of 4a . The s-trans conformation of dienes 18 was
established by NOE experiments.
as methyl vinyl ketone (11) and methyl propiolate (12)
were carried out either thermally or catalytically to
provide the corresponding adducts with significant se-
lectivity (Table 4). Thus, thermal (180 °C) cycloaddition
of 1a to 11 furnished a 4:1 ratio of regioisomers 13a /14a .
The use of catalysts such as TiCl4 and AlCl3 at low
temperature and shorter reaction times improved this
selectivity (entries 2 and 3). BF3‚Et2O was in all cases
the least efficient catalyst. Analogous results were
obtained in the reaction with 12 (entries 5-11), regio-
isomer 15a being the major product. As expected,7 by
decreasing the reaction temperature, the regioselectivity
was much higher (entries 5-8). In order to evaluate the
effect of the substituent in the aromatic ring on the
selectivity, diene 1b was added under similar conditions
to dienophiles 11 and 12, showing no difference with
respect to diene 1a (entries 12-19).
Substituted dienes 4a , 4c, and 4d proved to be much
more selective, since the thermal (130 °C) cycloaddition
of these compounds with 11 stereoselectively furnished
isomers endo/exo 13c/13d , 13e/13f, and 13g/13h , respec-
tively, in high yields (Table 4, entries 20, 22, and 24).
None of the corresponding regioisomers 14c-h were
detected by 1H NMR in the crude mixtures. Comparable
selectivities were exhibited by the thermal reaction with
12 (entries 21 and 23), to give regioisomers 15 as the
major products. No significant effect can then be at-
tributed to the functional group in the aromatic ring,
suggesting an absence or a very small electronic pertur-
bation of the latter on the nitrogen, which could cause a
change in the polarization of the diene orbitals. This
inhibition of the electronic effects of the substituted
benzene ring could be rationalized by the fact that the
aryl ring is in an almost orthogonal conformation with
(7) Sauer, J .; Sustmann, R. Angew. Chem., Int. Ed. Engl. 1980, 19,
779.

4108 J . Org. Chem., Vol. 62, No. 12, 1997
Mandal et al.
Sch em e 2
Ta ble 4. Diels-Ald er Cycloa d d ition s of Dien es 1a , 1b, 4a , 4c, a n d 4d w ith Olefin s 11 a n d 12a
entry
diene
olefinb
solvent
catalystc
T (°C)
t (h)
products (ratio)
yieldd (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
1b
4a
4a
4c
4c
4d
11
11
11
11
12
12
12
12
12
12
12
11
11
11
11
12
12
12
12
11
12
11
12
11
xylene
CH2Cl2
CH2Cl2
CH2Cl2
xylene
xylene
xylene
xylene
CH2Cl2
CH2Cl2
CH2Cl2
xylene
CH2Cl2
CH2Cl2
CH2Cl2
xylene
CH2Cl2
CH2Cl2
CH2Cl2
xylene
xylene
xylene
xylene
xylene
180
-78
-78
-78
180
160
120
60
-78
-78
-78
180
-78
-78
-78
180
-78
-78
-78
130
130
130
130
130
1.0
0.5
0.5
0.5
1.0
1.0
6.0
18.0
0.5
0.5
0.5
1.0
0.5
0.5
0.5
1.0
0.5
0.5
0.5
1.0
1.0
1.0
1.0
1.0
13a /14a (8:2)
13a /14a (95:5)
13a /14a (9:1)
13a /14a (8:2)
15a /16a (1:1)
15a /16a (6:4)
15a /16a (6:4)
15a /16a (7:3)
15a /16a (92:8)
15a /16a (89:11)
15a /16a (57:43)
13b/14b (8:2)
13b/14b (95:5)
13b/14b (9:1)
13b/14b (7:3)
15b/16b (1:1)
15b/16b (9:1)
15b/16b (9:1)
15b/16b (6:4)
13c/13d (96:4)
15c/16c (93:7)
13e/13f (96:4)
15d /16d (92:8)
13g/13h (9:1)
63
70
68
60
47e
44e
59e
60e
68
61
58
46
72
65
64
40e
72
53
57e
90
90
91
92
88
TiCl4
AlCl3
BF3‚Et2O
TiCl4
AlCl3
BF3‚Et2O
TiCl4
AlCl3
BF3‚Et2O
TiCl4
AlCl3
BF3‚Et2O
a
b
All under N2 atmosphere. Thermal trials in the presence of 1-2% hydroquinone. 3.0 mol equiv with dienes 1a and 1b, and 2.0 mol
equiv with dienes 4a , 4c, and 4d . c 5.0 mol equiv. Corresponding to the major isomer after column chromatography and recrystallization.
d
e Of the pure isomeric mixture.
F igu r e 3. Half-chair conformation of cyclohexene moiety of
adducts 13a (R ) H) and 13c (R ) Me).
F igu r e 2. NOE percentages shown in compounds 13c and
15a .
Considering these results, one can propose a reaction
mechanism for this synthetic pathway (Scheme 4). In
the case of dienes 4, the thermodynamically more stable
enolate 19b, with respect to 20b and 21, adds to the
isocyanate to give intermediate 22b. The ring closure
would give rise to oxazolidinone 23b by addition of the
carbamide to the carbonyl group. Protonation and
dehydration of this intermediate would provide the
desired dienes 4, probably via precursors 5 and 25b. The
Discu ssion
The one-pot reaction method for the preparation of
dienes 1a -h was improved by modifying some reaction
parameters. This method also proved to be applicable
to the synthesis of novel substituted dienes 4a -j, as the
formation of these compounds was highly selective.

Diels-Alder Cycloadditions of exo-2-Oxazolidinone Dienes
J . Org. Chem., Vol. 62, No. 12, 1997 4109
similar mechanism could be postulated for the formation
of dienes 1 (Scheme 4).
The cyclization step seems to be the rate-determining
one. Considering that this process would depend on the
nucleophilicity of the carbamide anion 22, one might
expect that electron-withdrawing groups in the N-aryl
ring restrain the addition to the carbonyl group. Indeed,
the reaction with nitroaryl isocyanates and diacetyl (2a )
failed to give the corresponding dienes.3
The stereoselectivity in the formation of dienes 4 would
depend on the stability of the primarily formed enolate
19b, this being even more stable than the E enolate 21.
This is probably due to repulsive van der Waals interac-
tions generated between the vinylic methyl and the acetyl
groups.
F igu r e 4. NOEs observed upon irradiation of protons in
compound 17a .
Sch em e 3
It was also possible to assess the relative reactivity and
selectivity in the addition of dienes 1 and 4 to sym-
metrical and unsymmetrical dienophiles under both
thermal and Lewis acid conditions. Thus, the tempera-
ture used in the reaction between diene 4a and olefin 6
was lower than that used in the reaction with the
unsubstituted diene 1a , obtaining similar yields within
the same reaction time. This confirms the expected
higher reactivity of diene 4a due to the presence of the
methyl group.8
Moreover, substituted dienes 4a and 4c proved to be
much more regioselective than dienes 1, since the ther-
mal cycloaddition of the former compounds with 11
stereoselectively furnished the endo/exo isomers 13c/13d
and 13e/13f in high yields (Table 4). An analogous
comparison can be made with the regioselectivities
exhibited with olefin 12 (Table 4). The enhancement of
the regioselectivity shown by dienes 4 could also be a
consequence of the polarization of the π system toward
the C7 terminus, due to the electron-donating effect of
the methyl group on C6. Bond distances in the X-ray
structure of 4c (Figure 1) would reveal this polarization
and the delocalization of the lone-pair of the heteroatoms
of the 2-oxazolidinone ring through the dienic π system.
The shortening of the C5dC6 (1.28(1) Å) double bond in
comparison with C4dC7 (1.34(1) Å) suggests a larger
electron-donating effect of the lone-pair of the nitrogen
atom with respect to that of the oxygen. In addition, the
shortness of the O1-C2 (1.329(9) Å; average length, 1.350
Å9) bond reflects electronic delocalization of the lone-pair
of the oxygen O1 toward the carbonyl group rather than
the dienic moiety. The above would be further supported
by an O1-C5 (1.419(8) Å, average length: 1.354 Å9) bond
longer than expected, as indeed is shown by the crystal
structure.
Sch em e 4
No significant effect on the regioselectivity could be
detected by changing the functional group in the N-
aromatic ring (Table 4), suggesting the absence of or a
very small electronic perturbation of the latter on the
nitrogen atom, which would polarize the diene orbitals
differently. This could be explained by the fact that the
aryl ring molecular π orbitals cannot efficiently overlap
the nitrogen lone-pair, because they are almost orthogo-
nal, as shown by X-ray structure of diene 4c (Figure 1).
The high stereoselectivity displayed by the additions
of dienes 4 to 11, giving as major isomers 13c, 13e, and
13g, indicates an endo preference at the transition state
isolation of 4-hydroxy-2-oxazolidinones 5a and 5b would
support this mechanism. The dehydration step would
be favored by the presence of drying agents, such as Li2-
CO3 or MgSO4. In fact, the yield of the reaction dramati-
(8) (a) Houk, K. N. Acc. Chem. Res. 1975, 8, 361. (b) Sustmann, R.
Tetrahedron Lett. 1971, 2721.
(9) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J . Chem. Soc. Perkin Trans. 2 1987, S1.
cally decreases in the absence of these additives.3
A

4110 J . Org. Chem., Vol. 62, No. 12, 1997
Mandal et al.
two levels, 6-31G** providing the less stable HOMO set;
however, in the case of LUMOs, the 6-31G* level yielded
the highest energies. A similar trend is shown by the
FMOs of olefins 11 and 12. From the energy diagrams
(only 6-31G** is illustrated, Figure 6) it appears that the
normal electron demand interaction,14 HOMO-diene/
LUMO-dienophile, is largely preferred (1.47-2.88 eV)
(Table 6). The higher reactivity shown by diene 4a in
comparison with 1a could account for a more energetic
HOMO of the former with respect to the latter by 0.244
eV. Figure 6 would also explain the origin of the larger
regioselectivity found for 11 with both dienes, since the
energy gap of HOMO-diene/LUMO-11 is smaller than
that of HOMO-diene/LUMO-12 (0.367 eV) due to the
more stable LUMO of 11.
F igu r e 5. Possible Diels-Alder transition state for the endo
cycloaddition between dienes 4 and dienophile 11, showing the
secondary orbital interaction between C5 and CdO.
Ta ble 5. Ab in itio RHF /3-21G, 6-31G*, a n d 6-31G**
F r on tier Molecu la r Or bita ls of Dien es 1a a n d 4a a n d
Olefin s 11 a n d 12
Table 7 lists the relative magnitudes for the HOMO
and LUMO coefficients calculated at the 6-31G** level.
On the basis of coefficient differences for the proper
frontier orbital interaction between diene and dienophile,
regioselectivity could then be estimated. It can be
observed that the relative magnitude of the coefficient
of the diene terminus C4 is bigger than that of carbon C1
in the HOMO of both dienes 1a and 4a . This polarization
of the π-system could be caused by the lower electro-
negativity and higher releasing capacity of the lone-pair
nitrogen atom, as compared with the oxygen atom of the
heterocycle. A significant coefficient on the nitrogen
atom in the HOMO would support this preferential
contribution. Therefore, in order to rationalize the
presence of adducts 13 and 15 as the major regioisomers
when the addition takes place with olefins 11 and 12,
respectively, the main interactions to be expected are
those between carbon C4 of dienes 1a and 4a with carbon
C1 of dienophiles 11 and 12. This was confirmed by the
RHF calculations, assuming that the largest FMO coef-
ficients will become bonded preferentially at the transi-
tion state.11,14 The largest LUMO coefficients of the
dienophiles are located on the unsubstituted carbon C1
(Table 7), as depicted in Figure 7. Similar predictions
were deduced from the calculations at the 3-21G and
6-31G* levels.
Our calculations also show large coefficients on the
diene internal carbons C4 and C5. Particularly, the C4
coefficient could participate in stabilizing the endo tran-
sition state by secondary orbital interactions,15 with the
large coefficients in the carbonyl group of the dienophile
(Figure 5).16
The self-addition of dienes 4, which yielded only one
of the eight possible adducts, could be controlled by steric
effects.17 The interaction between the diene and the
methyl group of the C5dC6 double bond of the dienophile
would prevent addition to this substituted terminus. The
preferential cycloaddition to the methylene on C4 would
be restrained in the orientation of the cycloaddends,
yielding the meta regioisomer (position of the methyl
group relative to the spiro quaternary carbon), because
of the possible destabilizing interactions between the
3-21G
6-31G*
6-31G**
compound
FMO
E (eV)
E (eV)
E (eV)
1a
1a
4a
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
-8.9855
2.8161
-8.7227
3.0093
-10.5391
2.9002
-8.8342
2.9470
-8.5804
3.1448
-10.4895
2.9222
-8.8051
2.9065
-8.5610
3.1035
-10.4868
2.9217
4a
11a
11a
12b
12
-11.6713
-11.4648
-11.4621
3.0719
3.2972
3.2891
a
b
Of the most stable s-cis conformation. For levels 6-31G* and
6-31G**, the gaps for dienophile 12 were calculated with the
NHOMO, since its HOMO does not have any pz contribution.
(Figure 5). The dienophile seems not to be perturbed by
the position of the aryl ring out of the plane of the
exocyclic diene. The endo preference also suggests a
minimal contribution from possible electrostatic repul-
sions between the lone-pairs of the heteroatoms, i.e.,
oxygen and nitrogen, and the π electrons of the acetyl
group.10
The regioselectivity cannot be explained simply on the
basis of the steric effect exerted by the N-aryl ring of the
diene component. Therefore, this could be correlated
instead to the energies and coefficients of the frontier
molecular orbitals.11 Frontier molecular orbitals (FMO)
of dienes 1a and 4a and methyl vinyl ketone (11) and
methyl propiolate (12) were calculated using an ab initio
RHF procedure. Geometries were fully optimized by the
AM1 semiempirical method12 and employed as the start-
ing point for optimization, using 3-21G, 6-31G*, and
6-31G** basis sets.13 The energy levels derived from
these calculations are summarized in Table 5. The
energies for both HOMO and LUMO of dienes 1a and
4a are lower at the 3-21G level with respect to the other
(10) (a) McCarrick, M. A.; Wu, Y.-D.; Houk, K. N. J . Org. Chem.
1993, 58, 3330. (b) Kakushima, M. Can. J . Chem. 1979, 57, 2564.
(11) (a) Fukui, K. Acc. Chem. Res. 1971, 4, 57. (b) Alston, P. V.;
Ottenbrite, R. M.; Shillady, D. D. J . Org. Chem. 1973, 38, 4075. (c)
Eisenstein, O.; Lefour, J . M.; Anh, N. T.; Hudson, R. F. Tetrahedron
1977, 33, 523. (d) Houk, K. N. J . Am. Chem. Soc. 1973, 95, 4092. (e)
Alston, P. V.; Ottenbrite, R. M.; Gu¨ner, O. F.; Shillady, D. D.
Tetrahedron 1986, 42, 4403.
(12) Dewar, M. J . S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J . J . P.
J . Am. Chem. Soc. 1985, 107, 3902.
(13) RHF/3-21G and 6-31G* calculations were performed using
MacSpartan,13a and at the 6-31G** level with Gaussian 94.13b (a)
MacSpartan, v 1.0, Wave Function Inc., 18401 Von Karman, Suite 370,
Irvine, CA 92715. (b) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Gill,
P. M. W.; J ohnson, B. G.; Robb, M. A.; Cheeseman, J . R.; Keith, T.;
Petersson, G. A.; Montgomery, J . A.; Raghavachari, K.; Al-Laham, M.
A.; Zakrzewski, V. G.; Ortiz, J . V.; Foresman, J . B.; Cioslowski, J .;
Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala,
P. Y.; Chen, W.; Wong, M. W.; Andres, J . L.; Reploge, E. S.; Gomperts,
R.; Martin, R. L.; Fox, D. J .; Binkley, J . S.; Defrees, D. J .; Baker, J .;
Stewart, J . P.; Head-Gordon, M.; Gonzalez, C.; Pople, J . A. Gaussian,
Inc., Pittsburgh, PA, 1995.
(14) Fleming, I. Frontier Orbitals and Organic Chemical Reactions;
Wiley: New York, 1975.
(15) (a) Houk, K. N. Tetrahedron Lett. 1970, 2621. (b) Gu¨ner, O. F.;
Ottenbrite, R. M.; Shillady, D. D.; Alston, P. V. J . Org. Chem. 1988,
53, 5348.
(16) Loncharich, R. J .; Brown, F. K.; Houk, K. N. J . Org. Chem. 1989,
54, 1129.
(17) (a) Fox, M. A.; Cardona, R.; Kiwiet, N. J . J . Org. Chem. 1987,
52, 1469. (b) Bachler, V.; Mark, F. Theor. Chim. Acta 1976, 43, 121,
and references cited therein. (c) Tripathy, R.; Franck, R. W.; Onan, K.
D. J . Am. Chem. Soc. 1988, 110, 3257.

Diels-Alder Cycloadditions of exo-2-Oxazolidinone Dienes
Ta ble 6. En er gy Ga p s (eV) of F r on tier Or bita ls for Dien es 1a a n d 4a a n d Dien op h iles 11 a n d 12
11a 12a
J . Org. Chem., Vol. 62, No. 12, 1997 4111
method
diene
HOMO-LUMO
LUMO-HOMO
diff
HOMO-LUMO
LUMO-HOMO
diff
6-31G** b
6-31G** b
6-31G* b
6-31G* b
3-21G
1a
4a
1a
4a
1a
4a
11.727
11.483
11.756
11.502
11.885
11.623
13.393
13.590
13.436
13.634
13.355
13.548
1.666
2.107
1.680
2.132
1.470
1.925
12.094
11.850
12.131
11.877
12.057
11.795
14.368
14.565
14.412
14.610
14.487
14.680
2.274
2.715
2.281
2.733
2.430
2.885
3-21G
a
b
HOMO-diene/LUMO-dienophile and LUMO-diene/HOMO-dienophile. The gaps for dienophile 12 were calculated with the NHOMO,
since its HOMO does not have any pz contribution.
for the increase in the reactivity and selectivity of these
dienes with respect to dienes 1. The highly selective
dimerization of dienes 4 would be controlled by steric
hindrance. The FMO model, using ab initio calculations,
seems to interpret the regioselectivity obtained in our
Diels-Alder cycloadditions. Isomerization of dienes 4 to
dienes 18 via a [1,5] sigmatropic rearrangement would
be controlled by the greater thermodynamic stability of
the latter.
Exp er im en ta l Section
Gen er a l. All air and moisture sensitive reactions were
carried out under nitrogen using oven-dried glassware. Di-
oxane and xylene were freshly distilled from sodium. Li2CO3
was dried overnight at 100 °C before using. Triethylamine
was freshly distilled from NaOH. Further analytical proce-
dures were described elsewhere.19
Gen er a l P r oced u r e for th e P r ep a r a tion of N-Ar yl-4,5-
Dim eth ylen e-2-oxa zolid in on es (1a -g). A solution of 2,3-
butanedione (2a ) (3.0 g, 34 mmol) in dry dioxane (10 mL) and
triethylamine (5.29 g, 52 mmol) were added dropwise to a
magnetically stirred solution of dioxane (10 mL) containing
anhydrous Li2CO3 (2.5 g, 34 mmol) at rt under N2 atmosphere,
F igu r e 6. Ab initio RHF/6-31G** frontier molecular orbital
and the mixture was stirred for 30 min. Then, a solution of
interactions for the Diels-Alder reaction between dienes 1a
the corresponding aryl isocyanate (52 mmol) in dioxane (10
and 4a and olefins 11 and 12.
mL) was added over a period of 30 min and stirring was
continued for 12 h at rt. The mixture was filtered and the
N-phenyl ring of the diene and the bulky substituents of
the dienophile.
solvent removed under vacuo. The residue was purified by
column chromatography over silica gel impregnated with
triethylamine (10%) in hexane (hexane/EtOAc, 95:5) to give
dienes 1a -g.
The enhancement of the regioselectivity of the addi-
tions of 1a to olefins 11 and 12 by the presence of Lewis
acids has been explained as a consequence of complex-
ation of the dienophile with the Lewis acid.18 This
interaction stabilizes the LUMO and increases the rela-
tive magnitude of the eigenvector coefficient on the
unsubstituted terminus of the olefin, leading to a stronger
perturbation and, consequently, to an increase in both
reactivity and selectivity.8a,11
4,5-Dim eth ylen e-N-p h en yl-2-oxa zolid in on e (1a ). Using
the general procedure with 6.23 g of phenyl isocyanate (3a )
gave 3.52 g (54%) of 1a as colorless crystals (CH2Cl2/hexane,
1:3): Rf 0.63 (hexane/EtOAc, 8:2); mp 75-76 °C; IR (KBr) 1745,
1
1510, 1400, 1310 cm-1; H NMR (300 MHz, CDCl3) δ 4.35 (d,
J ) 3.0 Hz, 1H), 4.76 (d, J ) 3.0 Hz, 1H), 4.93 (d, J ) 3.5 Hz,
1H) 4.98 (d, J ) 3.5 Hz, 1H), 7.35-7.55 (m, 5H); 13C NMR (75
MHz, CDCl3) δ 85.2 (C-7), 87.5 (C-6), 127.5 (C-9), 129.3 (C-
11), 130.3 (C-10), 133.6 (C-8), 139.4 (C-4), 149.4 (C-5), 152.9
(C-2); MS (70 eV) 187 (M+, 45), 143 (87), 117 (49), 104 (51), 91
(22), 77 (100), 51 (52). Anal. Calcd for C11H9NO2: C, 70.57;
H, 4.84; N, 7.48. Found: C, 70.39; H, 4.87; N, 7.33.
Con clu sion
In summary, the one-pot reaction between R-diketones
and the corresponding isocyanates, used originally for
preparing dienes 1, proved to be a versatile and efficient
methodology to obtain methyl-substituted dienes 4a -j,
in moderate yields. The geometry and position of the
methyl group on the exocyclic diene was completely
controlled in the process. Dienes 4 proved to be highly
regio- and stereoselective in Diels-Alder additions with
unsymmetric olefins 11 and 12 under thermal conditions.
In the case of dienes 1, the use of Lewis acids drastically
improved the regioselectivity with the same dienophiles.
The methyl group in dienes 4 appears to be the reason
N -(p -C h lo r o p h e n y l)-4,5-d im e t h y le n e -2-o x a zo lid i-
n on e (1b). Using the general procedure with 7.98 g of
p-chlorophenyl isocyanate (3b) gave 5.4 g (70%) of 1b as
colorless crystals (CH2Cl2/hexane, 1:1): Rf 0.68 (hexane/EtOAc,
8:2); mp 106-107 °C; IR (KBr) 1750, 1700, 1490, 1400, 1300
1
cm-1; H NMR (300 MHz, CDCl3) δ 4.35 (d, J ) 3.2 Hz, 1H),
4.78 (d, J ) 3.2 Hz, 1H), 4.93 (d, J ) 3.6 Hz, 1H), 4.99 (d, J )
3.6 Hz, 1H), 7.27-740 (m, 2H), 7.40-7.54 (m, 2H); 13C NMR
(75 MHz, CDCl3) δ 84.7 (C-7), 87.3 (C-6), 128.2, 130.0, 131.5,
134.6, 138.5, 148.6, 152.1; MS (70 eV) 221 (M+, 100), 186 (19),
177 (7), 151 (34), 142 (83), 137 (84), 111 (49), 102 (21), 75 (73),
51 (9). Anal. Calcd for C11H8ClNO2: C, 59.61; H, 3.64.
Found: C, 59.57; H, 3.77.
Gen er a l P r oced u r e for th e P r ep a r a tion of (5Z)-N-
Su b st it u t e d -5-e t h ylid e n e -4-m e t h yle n e -2-oxa zolid in o-
(18) (a) Houk, K. N.; Strozier, R. W. J . Am. Chem. Soc. 1973, 95,
4094. (b) Loncharich, R. J .; Schwartz, T. R.; Houk, K. N. J . Am. Chem.
Soc. 1987, 109, 14.
(19) Ordun˜a, A.; Zepeda, L. G.; Tamariz, J . Synthesis 1993, 375.

4112 J . Org. Chem., Vol. 62, No. 12, 1997
Mandal et al.
Ta ble 7. Ab In itio RHF /6-31G** Ca lcu la tion s of Coefficien ts (Ci) of th e F r on tier Molecu la r Or bita ls for Dien es 1a a n d 4a
a n d Dien op h iles 11 a n d 12a
HOMO
C3
0.164 -0.209 -0.326 -0.242 0.139
0.198 0.320 -0.216 0.150
LUMO
C3
compound
C1
C2
C4
CN
CO
∆Cib
C1
C2
C4
CN
CO
∆Cib
1a
4a
0.246
-0.257 -0.199
0.080 0.263 -0.245 -0.245 0.258 0.068 0.058 0.005
0.063 0.274 -0.222 -0.245 0.248 0.066 0.050 0.026
11
0.346
0.374
0.366 -0.039 -0.221
0.394 -0.034 -0.192
-0.020 0.311 -0.208 -0.280 0.254
-0.020 0.289 -0.184 -0.338 0.278
0.103
0.105
12c
a
b
These are the values of the 2pz coefficients; the relative 3pz contributions and their ∆Ci are analogous. Carbon 4-carbon 1 for the
dienes and carbon 1-carbon 2 for the olefins. c These values are for the NHOMO; the HOMO does not have any pz contribution.
(dd, J ) 165.2, 5.7 Hz, C-10), 130.2 (dd, J ) 168.3, 5.3 Hz,
C-11), 131.7 (m, C-9), 134.6 (m, C-12), 138.9 (br s, C-4), 143.0
(m, C-5), 152.5 (s, C-2); MS (70 eV) 237 (M+ + 2, 33), 235 (M+,
100), 220 (2), 190 (9), 152 (90), 138 (39), 129 (17), 111 (38), 75
(31). Anal. Calcd for C12H10ClNO2: C, 61.16; H, 4.27; N, 5.94.
Found: C, 60.90; H, 4.16; N, 5.79.
(5Z)-N-(m -Ch lor op h en yl)-5-eth ylid en e-4-m eth ylen e-2-
oxa zolid in on e (4c). Using the general procedure with m-
chlorophenyl isocyanate (3c) (0.79 g) gave 0.57 g (47%) of 4c
as colorless crystals (CH2Cl2/hexane, 1:1); Rf 0.45 (hexane/
EtOAc, 9:1); mp 82-83 °C; IR (KBr) 1780, 1700, 1510, 1400
1
cm-1; H NMR (300 MHz, CDCl3) δ 1.86 (d, J ) 7.3 Hz, 3H),
4.21 (d, J ) 3.1 Hz, 1H), 4.61 (d, J ) 3.1 Hz, 1H), 5.43 (q, J )
7.3 Hz, 1H), 7.26 (dt, J ) 7.4, 1.8 Hz, 1H), 7.37-7.48 (m, 3H);
F igu r e 7. Ab initio RHF/6-31G** frontier molecular orbital
13C NMR (75 MHz, CDCl3) δ 10.4, 81.9, 99.6, 125.3, 127.4,
interactions for the Diels-Alder reaction between diene 4a and
olefins 11 and 12.
128.9, 130.7, 134.5, 135.3, 138.8, 143.0, 152.4; MS (70 eV) 237
(M+ + 2, 33), 235 (M+, 100), 191 (5), 190 (13), 154 (37), 152
(91), 138 (12), 129 (11), 111 (41), 75 (31). Anal. Calcd for
n es (4a -j). A solution of 2,3-pentanedione (2b) (0.35 g, 3.5
mmol) in dry dioxane (1 mL) was added dropwise to a
magnetically stirred solution of triethylamine (0.71 g, 7.0
mmol) in dioxane (2 mL) containing anhydrous Li2CO3 (0.31
g, 4.2 mmol) at rt under N2 atmosphere, and the mixture was
stirred for 30 min. Then, a solution of the corresponding aryl
isocyanate (5.2 mmol) in dioxane (2 mL) was added over a
period of 30 min and stirring was continued for 12 h at rt.
The mixture was filtered and solvent removed under vacuo.
The residue was purified by column chromatography over silica
gel impregnated with triethylamine (10%) in hexane (hexane/
EtOAc, 9:1) to give dienes 4a -j.
(5Z)-5-Eth ylid en e-4-m eth ylen e-N-p h en yl-2-oxa zolid i-
n on e (4a ). Using the general procedure with phenyl isocy-
anate (3a ) (0.62 g) gave 0.55 g (53%) of 4a as colorless crystals
(CH2Cl2/hexane, 1:1): Rf 0.5 (hexane/EtOAc, 9:1); mp 83-84
°C; IR (KBr) 1770, 1690, 1480, 1390 cm-1; 1H NMR (300 MHz,
CDCl3) δ 1.86 (dt, J ) 7.3, 0.7 Hz, 3H, CH3C)), 4.18 (d, J )
2.9 Hz, 1H, H-7a), 4.58 (d, J ) 2.9 Hz, 1H, H-7b), 5.42 (q, J )
7.3 Hz, 1H, H-6b), 7.33-7.53 (m, 5H, PhH); 13C NMR (75 MHz,
CDCl3) δ 10.4 (q, J ) 129.0 Hz, C-8), 81.6 (t, J ) 164.5 Hz,
C-7), 99.0 (dq, J ) 158.8, 7.4 Hz, C-6), 127.1 (ddd, J ) 162.9,
7.5, 4.9 Hz, C-10), 128.7 (dt, J ) 161.7, 7.5 Hz, C-12), 129.7
(dd, J ) 162.7, 7.8 Hz, C-11), 133.4 (t, J ) 8.7 Hz, C-9), 139.4
(br s, C-4), 143.3 (m, C-5), 152.7 (s, C-2); MS (70 eV) 201 (M+,
4), 156 (4), 118 (41), 104 (13), 91 (9), 77 (93), 55 (26), 51 (100).
Anal. Calcd for C12H11NO2: C, 71.63; H, 5.50; N, 6.96.
Found: C, 71.55; H, 5.64; N, 6.84.
C12H10ClNO2: C, 61.16; H, 4.27; N, 5.94. Found: C, 61.38; H,
4.55; N, 6.11.
(5Z)-5-E t h ylid en e-4-m et h ylen e-N-p -t olyl-2-oxa zolid i-
n on e (4d ). Using the general procedure with p-tolyl isocy-
anate (3e) (0.69 g) gave a mixture of 4d /5a , which after
purification yielded 0.44 g (40%) of 4d as colorless crystals
(CH2Cl2/hexane, 1:1) and 0.17 g (14%) of 5a as colorless
crystals (CH2Cl2/hexane, 1:1). 4d : Rf 0.5 (hexane/EtOAc, 9:1);
mp 92-93 °C; IR (KBr) 1780, 1690, 1510, 1400 cm-1; 1H NMR
(300 MHz, CDCl3) δ 1.85 (d, J ) 7.3 Hz, 3H), 2.39 (s, 3H),
4.15 (d, J ) 2.9 Hz, 1H), 4.55 (d, J ) 2.9 Hz, 1H), 5.40 (q, J )
7.3 Hz, 1H), 7.18-7.22 (m, 4H); 13C NMR (75 MHz, CDCl3) δ
10.4, 21.2, 81.5, 98.9, 126.9, 130.3, 130.6, 138.8, 139.5, 143.3,
152.9; MS (70 eV) 215 (M+, 90), 200 (9), 170 (80, 132 (100), 91
(46), 65 (28). Anal. Calcd for C13H13NO2: C, 72.54; H, 6.08;
N, 6.50. Found: C, 72.54; H, 6.20; N, 6.45. 5a : Rf 0.4 (hexane/
EtOAc, 4:1); mp 104-105 °C; IR (KBr) 3320, 1770, 1710, 1520,
1390 cm-1; 1H NMR (300 MHz, CDCl3) δ 1.52 (s, 3H), 1.75 (d,
J ) 7.0 Hz, 3H), 2.37 (s, 3 H), 3.08 (s, 1H), 5.10 (q, J ) 7.0 Hz,
1H), 7.21-7.34 (m, 4H); 13H NMR (75 MHz, CDCl3) δ 9.9, 21.1,
25.4, 87.8, 99.6, 127.0, 129.0, 131.0, 137.0, 150.9, 152.9. Anal.
Calcd for C13H15NO3: C, 66.94; H, 6.48; N, 6.00. Found: C,
67.13; H, 6.60; N, 5.96.
Gen er a l P r oced u r es for th e Diels-Ald er Rea ction of
Dien op h iles Dim eth yl Acetylen ed ica r boxyla te (6), N-
P h en ylm a leim id e (9), Meth yl Vin yl Keton e (11), a n d
Meth yl P r op iola te (12) w ith N-Ar yl-4,5-d im eth ylen e-2-
oxa zolid in on es 1a a n d 1b. Meth od A. A mixture of diene
1 (0.8 mmol), dienophile (2.4 mmol), and hydroquinone (0.003
g) in dry xylene (3 mL) was placed in a sealed tube under N2
atmosphere and heated at 180 °C for 1 h. The solvent was
removed under vacuo and the residue purified by column
chromatography (hexane/EtOAc, 4:1) to give the corresponding
adducts.
(5Z)-N-(p-Ch lor op h en yl)-5-eth ylid en e-4-m eth ylen e-2-
oxa zolid in on e (4b). Using the general procedure with
p-chlorophenyl isocyanate (3b) (0.79 g) gave 0.65 g (54%) of
4b as colorless crystals (CH2Cl2/hexane, 1:1): Rf 0.45 (hexane/
EtOAc, 9:1); mp 78-79 °C; IR (KBr) 1780, 1700, 1500, 1390
1
cm-1; H NMR (300 MHz, CDCl3) δ 1.86 (dt, J ) 7.3, 0.7 Hz,
3H), 4.17 (d, J ) 3.1 Hz, 1H), 4.59 (d, J ) 3.1 Hz, 1H), 5.43 (q,
J ) 7.3 Hz, 1H), 7.25-7.32 (m, 2H), 7.42-7.51 (m, 2H); 13C
NMR (75 MHz, CDCl3) δ 10.4 (q, J ) 128.8 Hz, C-8), 81.7 (t,
J ) 164.7 Hz, C-7), 99.5 (dq, J ) 159.1, 7.3 Hz, C-6), 128.0
Meth od B. To a solution of the diene (0.8 mmol) in dry
CH2Cl2 (3 mL) under N2 atmosphere at -78 °C was added the
Lewis acid (AlCl3, TiCl4, or BF3‚Et2O) (4.0 mmol) and dieno-
phile (2.4 mmol) in dry CH2Cl2 (3 mL). After being stirred at

Diels-Alder Cycloadditions of exo-2-Oxazolidinone Dienes
J . Org. Chem., Vol. 62, No. 12, 1997 4113
-78 °C for 30 min, the reaction mixture was diluted with CH2-
Cl2 (20 mL) and washed with aqueous 5% NaHCO3 (2 × 5 mL)
and aqueous 5% NH4Cl (2 × 10 mL), and finally the organic
extract was dried (Na2SO4). The solvent was evaporated and
the residue purified as above.
(75 MHz, CDCl3) δ 23.7, 24.7, 52.8, 117.8, 125.8, 128.2, 128.5,
130.1, 133.3, 134.0, 134.4, 155.1, 166.7; MS (70 eV) 271 (M+,
100), 256 (3), 212 (9), 167 (4), 77 (82), 59 (29), 51 (71). Anal.
Calcd for C15H13NO4: C, 66.41; H, 4.83. Found: C, 66.53; H,
4.78.
6-Ace t yl-N -p h e n yl-4,5,6,7-t e t r a h yd r ob e n zoxa zol-2-
on e (13a ). Meth od A. Reaction of 1a (0.15 g) with methyl
vinyl ketone (11) (0.17 g) gave a mixture of 13a /14a (4:1) as a
pale yellow powder, which was purified and recrystallized from
acetone/hexane, 9:1, to yield 0.13 g (63%) of major isomer 13a
as colorless needles: Rf 0.44 (hexane/EtOAc, 6:4); mp 111-
6-(Car boxym eth yl)-N-(p-ch lor oph en yl)-4,7-dih ydr oben -
zoxa zol-2-on e (15b). Meth od A. Reaction of 1b (0.18 g)
with 12 (0.202 g) gave a mixture of 15b/16b (1:1), which was
purified to yield 0.099 g (40%) of 15b/16b (1:1).
Meth od B. Reaction of 1b (0.18 g) with 12 (0.202 g) in the
presence of AlCl3 (0.53 g) gave a mixture of 15b/16b (9:1),
which was purified and recrystallized from acetone/hexane (9:
1) to yield 0.11 g (53%) of 15b.
1
112 °C; IR (KBr) 1750, 1700, 1500, 1400 cm-1; H NMR (300
MHz, CDCl3) δ 1.79 (dddd, J ) 13.8, 10.6, 9.3, 5.8 Hz, 1H),
2.12-2.21 (m, 1H), 2.26 (s, 3H), 2.36-2.51 (m, 2H), 2.67-2.72
(m, 2H), 2.85-2.93 (m, 1H), 7.30-7.40 (m, 3H), 7.40-7.50 (m,
2H); 13C NMR (75 MHz, CDCl3) δ 19.9, 22.8, 24.2, 28.1, 46.7,
120.4, 125.0, 127.6, 129.4, 133.8, 133.9, 154.4, 208.3; MS (70
eV) 257 (M+, 21), 242 (4), 214 (4), 186 (8), 170 (38), 158 (46),
143 (44), 130 (35), 117 (47), 104 (18), 91 (9), 77 (100), 51 (36).
Anal. Calcd for C15H15NO3: C, 70.02; H, 5.88; N, 5.44.
Found: C, 70.16; H, 6.01; N, 5.33.
Meth od B. Reaction of 1a (0.15 g) with 11 (0.17 g) in the
presence of AlCl3 (0.53 g) gave a mixture of 13a /14a (9:1) and
purification by column chromatography and recrystallization
from toluene gave 0.14 g (68%) of 13a .
Reaction of 1a (0.15 g) with 11 (0.17 g) in the presence of
BF3‚Et2O (0.57 g) gave a mixture of 13a /14a (8:2), and
purification as above gave 0.124 g (60%) of 13a .
Reaction of 1a (0.15 g) with 11 (0.17 g) in the presence of
TiCl4 (0.76 g) gave a mixture of 13a /14a (95:5), which was
purified and recrystallized from toluene to yield 0.144 g (70%)
of 13a as colorless needles.
Reaction of 1b (0.18 g) with 12 (0.202 g) in the presence of
BF3‚Et2O (0.57 g) gave a mixture of 15b/16b (6:4), which was
purified to yield 0.118 g (57%) of 15b/16b (6:4).
Reaction of 1b (0.015 g) with 12 (0.202 g) in the presence of
TiCl4 (0.75 g) gave a mixture of 15b/16b (9:1), which was
purified and recrystallized from acetone/hexane (9:1) to yield
0.149 g (72%) of 15b as colorless crystals: Rf 0.34 (hexane/
EtOAc, 1:1); mp 192-194 °C; IR (KBr) 1750, 1710, 1500, 1290
cm-1; 1H NMR (300 MHz, CDCl3) δ 3.19-3.27 (m, 2H), 3.40-
3.47 (m, 2H), 3.82 (s, 3H), 7.03-7.07 (m, 1H), 7.29-7.33 (m,
2H), 7.43-7.47 (m, 2H); 13C NMR (75 MHz, CDCl3) δ 23.7,
24.6, 52.8, 117.4, 127.0, 128.2, 130.3, 133.0, 133.6, 134.1, 134.3,
154.8, 166.6; MS (70 eV) 305 (M+, 31), 303 (100), 272 (32), 228
(3), 200 (3), 171 (3), 111 (5), 75 (15), 63 (19). Anal. Calcd for
C15H12ClNO4: C, 58.93; H, 3.96; N, 4.58. Found: C, 59.00; H,
4.07; N, 4.70.
5,6-Bis(ca r boxym eth yl)-N-p h en yl-4,7-d ih yd r oben zox-
a zol-2-on e (7). Meth od A. Reaction of 1a (0.15 g) with
dimethyl acetylenedicarboxylate (6) (0.34 g) gave a pale yellow
powder, which was recrystallized from acetone/hexane (9:1)
to yield 0.148 g (56%) of 7 as colorless crystals: Rf 0.4 (hexane/
EtOAc, 6:4); mp 149-150 °C; IR (KBr) 1740, 1700, 1450, 1280
cm-1; 1H NMR (300 MHz, CDCl3) δ 3.34-3.42 (m, 2H), 3.51-
6-Acetyl-N-(p-ch lor op h en yl)-4,5,6,7-tetr a h yd r oben zox-
a zol-2-on e (13b). Meth od A. Reaction of 1b (0.18 g) with
11 (0.17 g) gave a mixture of 13b/14b (8:2) as a pale yellow
powder, which was purified and recrystallized from acetone/
hexane (9:1) to yield 0.109 g (46%) of 13b: Rf 0.41 (hexane/
3.56 (m, 2H), 3.83 (s, 3H), 3.79 (s, 3H), 7.30-7.51 (m, 5H); 13
C
EtOAc, 6:4); mp 106-107 °C; IR (KBr) 1762, 1695, 1496 cm-1
;
NMR (75 MHz, CDCl3) δ 25.1, 52.7, 52.8, 116.9, 125.3, 128.2,
129.7, 130.6, 131.6, 131.8, 133.5, 154.3, 166.8, 167.2; MS (70
eV) 327 (M+ - 2, 86), 296 (100), 281 (8), 237 (4), 77 (38), 51
(26). Anal. Calcd for C17H15NO6: C, 62.00; H, 4.59; N, 4.25.
Found: C, 61.91; H, 4.60; N, 4.18.
1H NMR (300 MHz, CDCl3) δ 1.74-1.89 (m, 1H), 2.14-2.25
(m, 1H), 2.29 (s, 3H), 2.34-2.48 (m, 2H), 2.67-2.74 (m, 2H),
2.85-2.94 (m, 1H), 7.29-7.43 (m, 5H); 13C NMR (75 MHz,
CDCl3) δ 19.9, 22.8, 24.1, 28.1, 46.6, 120.1, 126.2, 129.6, 132.5,
133.3, 134.1, 154.1, 207.8. Anal. Calcd for C15H14ClNO3: C,
61.76; H, 4.84; N, 4.80. Found: C, 61.90; H, 4.77; N, 4.67.
Meth od B. Reaction of 1b (0.18 g) with 11 (0.17 g) in the
presence of AlCl3 (0.53 g) gave a mixture of 13b/14b (9:1),
which was purified and recrystallized from acetone/hexane (9:
1) to yield 0.154 g (65%) of 13b.
3N,9N-Dip h en yl-3,4,5,6,7,8,9,10-octa h yd r o-2H-p yr r olo-
[3,4-f]ben zoxa zole-2,8,10-tr ion e (10a ).20 Meth od A. Reac-
tion of 1a (0.15 g) with N-phenylmaleimide (9) (0.415 g) gave
a pale yellow powder, which was recrystallized from toluene
to yield 0.216 g (75%) of 10a as colorless crystals: Rf 0.4
(hexane/EtOAc, 6:4); mp 156-158 °C; IR (KBr) 1750, 1700,
Reaction of 1b (0.18 g) with 11 (0.17 g) in the presence of
BF3‚Et2O (0.57 g) gave a mixture of 13b/14b (7:3), which was
purified and recrystallized from acetone/hexane (9:1) to yield
0.152 (64%) of 13b.
Reaction of 1b (0.18 g) with 11 (0.17 g) in the presence of
TiCl4 (0.76 g) gave a mixture of 13b/14b (95:5), which was
purified and recrystallized from acetone/hexane (9:1) to yield
0.171 g (72%) of 13b.
6-(Ca r boxym eth yl)-N-p h en yl-4,7-d ih yd r oben zoxa zol-
2-on e (15a ). Meth od A. Reaction of 1a (0.15 g) with methyl
propiolate (12) (0.202 g) gave a mixture of 15a /16a (1:1), which
was purified and recrystallized from acetone/hexane (9:1) to
yield 0.102 g (47%) of 15a /16a (1:1).
Meth od B. Reaction of 1a (0.15 g) with 12 (0.202 g) in the
presence of AlCl3 (0.53 g) gave a mixture of 15a /16a (89:11),
which was purified and recrystallized from acetone/hexane (9:
1) to yield 0.132 g (61%) of 15a .
Reaction of 1a (0.15 g) with 12 (0.202 g) in the presence of
BF3‚Et2O (0.57 g) gave a mixture of 15a /16a (57:43), which
was purified and recrystallized from acetone/hexane (9:1) to
yield 0.126 (58%) of 15a /16a (6:4).
Reaction of 1a (0.15 g) with 12 (0.202 g) in the presence of
TiCl4 (0.76 g) gave a mixture of 15a /16a (92:8), which was
purified and recrystallized from acetone/hexane (9:1) to yield
0.147 (68%) of 15a as colorless needles: Rf 0.38 (hexane/EtOAc,
1:1); mp 175-176 °C; IR (KBr) 1750, 1700, 1500 cm-1; 1H NMR
(300 MHz, CDCl3) δ 3.20-3.27 (m, 2H), 3.40-3.47 (m, 2H),
3.82 (s, 3H), 7.02-7.08 (m, 1H), 7.31-7.51 (m, 5H); 13C NMR
1600, 1500, 1390, 1170 cm-1 1H NMR (300 MHz, CDCl3) δ
;
2.83 (ddt, J ) 17.2, 8.4, 2.4 Hz, 1H), 3.01 (ddt, J ) 17.2, 3.5,
1.9 Hz, 1H), 3.06-3.11 (m, 2H), 3.49 (ddd, J ) 8.7, 8.4, 3.5
Hz, 1H), 3.58 (dt, J ) 8.7, 6.5 Hz, 1H), 7.25-7.52 (m, 10H);
13C NMR (75 MHz, CDCl3) δ 19.0, 20.0, 37.8, 37.9, 118.4, 125.4,
126.1, 128.2, 128.9, 129.2, 129.6, 131.5, 131.9, 133.3, 154.1,
176.9, 177.0. Anal. Calcd for C21H16N2O4: C, 69.99; H, 4.48;
N, 7.77. Found: C, 69.78; H, 4.61; N, 7.62.
3N-(p-Ch lor op h en yl)-9N-p h en yl-3,4,5,6,7,8,9,10-octa h y-
d r o-2H -p yr r olo[3,4-f]b en zoxa zole-2,8,10-t r ion e
(10b ).
Meth od A. Reaction of 1b (0.18 g) with 9 (0.415 g) gave a
pale yellow powder, which was recrystallized from acetone/
hexane (9:1) to yield 0.253 g (79%) of 10b as colorless
crystals: Rf 0.34 (hexane/EtOAc, 1:1); mp 237-238 °C; IR
(KBr) 1763, 1708, 1500, 1390 cm-1; 1H NMR (300 MHz, CDCl3)
δ 2.82 (ddt, J ) 16.9, 8.3, 2.3 Hz, 1H), 2.99 (ddt, J ) 16.9, 3.2,
1.8 Hz, 1H), 3.08 (m, 2H), 3.49 (ddd, J ) 8.7, 8.3, 3.2 Hz, 1H),
3.58 (dt, J ) 8.6, 6.5 Hz, 1H), 7.30 (m, 4H), 7.40-7.55 (m, 6H).
Anal. Calcd for C21H15ClN2O4: C, 63.89, H, 3.83, N, 7.09.
Found: C, 63.67; H, 3.98; N, 6.83.
Gen er a l P r oced u r e for th e Diels-Ald er Rea ction of
Dien op h iles Meth yl Vin yl Keton e (11), Meth yl P r op i-
ola te (12), a n d Dim eth yl Acetylen ed ica r boxyla te (6) w ith
Dien es 4. A mixture of diene 4 (0.8 mmol), dienophile (1.6
mmol), and hydroquinone (0.002 g) in dry xylene (3 mL) was
(20) See Scheme 1 for the numbering scheme used.

4114 J . Org. Chem., Vol. 62, No. 12, 1997
Mandal et al.
placed in a sealed tube under N2 atmosphere and heated at
130 °C for 1 h. The solvent was removed under vacuo and
the residue purified by column chromatography (hexane/
EtOAc, 4:1) to give the corresponding adducts.
137.3, 154.5, 166.1; MS (70 eV) 285 (M+, 100), 270 (36), 254
(11), 238 (24), 226 (57), 211 (54), 167 (21), 107 (26), 77 (40), 59
(23). Anal. Calcd for C16H15NO4: C, 67.36; H, 5.30; N, 4.90.
Found: C, 67.32; H, 5.12; N, 4.87.
(6R*,7R*)-6-Acetyl-7-m eth yl-N-p h en yl-4,5,6,7-tetr a h y-
d r ob en zoxa zol-2-on e (13c) a n d (6R*,7S*)-6-Acet yl-7-
m e t h yl-N -p h e n yl-4,5,6,7-t e t r a h yd r ob e n zoxa zol-2-on e
(13d ). Using the general procedure with 4a (0.16 g) and
methyl vinyl ketone (11) (0.11 g) gave a mixture of 13c/13d
(96:4), which was purified and recrystallized from acetone/
hexane (1:1) as colorless crystals to yield 0.19 g (90%) of 13c:
Rf 0.45 (hexane/EtOAc, 7:3); mp 132-133 °C; IR (KBr) 1760,
6-Ca r bom eth oxy-N-(m -ch lor op h en yl)-7-m eth yl-4,7-d i-
h yd r oben zoxa zol-2-on e (15d ) a n d 5-Ca r bom eth oxy-N-(m -
ch lor op h e n yl)-7-m e t h yl-4,7-d ih yd r ob e n zoxa zol-2-on e
(16d ). Using the general procedure with 4c (0.204 g) and 12
(0.134 g) gave a mixture of 15d /16d (92:8), which was purified
and recrystallized from acetone/hexane (1:1) to yield 0.25 g
(92%) of 15d as colorless crystals: Rf 0.4 (hexane/EtOAc, 7:3);
mp 145-146 °C; IR (KBr) 1760, 1730, 1710, 1480, 1250 cm-1
;
1700, 1500, 1390 cm-1; H NMR (300 MHz, CDCl3) δ 1.06 (d,
1H NMR (300 MHz, CDCl3) δ 1.40 (d, J ) 6.7 Hz, 3H), 3.17
(ddd, J ) 22.4, 7.1, 4.2 Hz, 1H), 3.28 (ddd, J ) 22.4, 7.0, 3.3
Hz, 1H), 3.70-3.80 (m, 1H), 3.81 (s, 3H), 6.98 (ddd, J ) 4.2,
3.3, 1.2 Hz, 1H), 7.28 (ddd, J ) 7.7, 1.8, 1.5 Hz, 1H), 7.35 (ddd,
J ) 8.1, 1.8, 1.5 Hz, 1H), 7.38-7.44 (m, 2H); 13C NMR (75
MHz, CDCl3) δ 19.1, 24.1, 28.8, 52.0, 115.9, 123.2, 125.3, 128.0,
130.5, 132.6, 133.5, 135.1, 135.2, 137.7, 154.1, 165.9; MS (70
eV) 321 (M+ + 2, 33), 319 (M+, 100), 304 (33), 272 (22), 235
(6), 260 (59), 245 (44), 166 (15), 109 (32), 107 (37), 75 (24), 59
(59). Anal. Calcd for C16H14ClNO4: C, 60.10; H, 4.41; N, 4.38.
Found: C, 60.23; H, 4.60; N, 4.35.
1
J ) 6.8 Hz, 3H), 1.84 (dddd, J ) 14.0, 12.1, 10.2, 6.4 Hz, 1H),
1.98 (dddd, J ) 14.0, 5.2, 2.8, 2.5 Hz, 1H), 2.23 (s, 3H), 2.28-
2.39 (m, 2H), 2.96 (ddd, J ) 12.1, 5.4, 2.8 Hz, 1H), 3.21-3.28
(m, 1H), 7.32-7.48 (m, 5H). Signals attributed to minor
isomer 13d : 0.86 (d, J ) 6.8 Hz), 2.22 (s). 13C NMR (75 MHz,
CDCl3) δ 14.2, 18.6, 19.9, 28.3, 28.7, 50.9, 120.5, 125.0, 127.6,
129.4, 133.8, 137.9, 154.4, 208.0; MS (70 eV) 271 (M+, 70), 256
(4), 228 (11), 214 (14), 201 (100), 184 (10), 156 (7), 118 (10),
104 (4), 77 (17). Anal. Calcd for C16H17NO3: C, 70.83; H, 6.31;
N, 5.16. Found: C, 70.82; H, 6.14; N, 5.10.
5,6-Dicar bom eth oxy-7-m eth yl-N-ph en yl-4,7-dih ydr oben -
zoxa zol-2-on e (8). Using the general procedure with 4a (0.16
g) and dimethyl acetylenedicarboxylate (6) (0.23 g) gave 0.23
g (85%) of 8 as colorless crystals (acetone/hexane, 1:1): Rf 0.40
(hexane/EtOAc, 7:3); mp 175-176 °C; IR (KBr) 1770, 1720,
(6R*,7R*)-6-Acetyl-N-(m-ch lor oph en yl)-7-m eth yl-4,5,6,7-
tetr ah ydr oben zoxazol-2-on e (13e) an d (6R*,7S*)-6-Acetyl-
N-(m -ch lor op h en yl)-7-m eth yl-4,5,6,7-tetr a h yd r oben zox-
a zol-2-on e (13f). Using the general procedure with 4c (0.204
g) and 11 (0.11 g) gave a mixture of 13e/13f (96:4), which was
purified and crystallized from acetone/hexane (1:1) to yield
0.236 (91%) of 13e as colorless crystals: Rf 0.4 (hexane/EtOAc,
7:3); mp 143-144 °C; IR (KBr) 1760, 1710, 1480 cm-1; 1H NMR
(300 MHz, CDCl3) δ 1.06 (d, J ) 6.8 Hz, 3H), 1.85 (dddd, J )
14.0, 12.1, 10.2, 6.4 Hz, 1H), 2.00 (dddd, J ) 14.0, 5.2, 2.7, 2.5
Hz, 1H), 2.23 (s, 3H), 2.28-2.41 (m, 2H), 2.96 (ddd, J ) 12.1,
5.3, 2.7 Hz, 1H), 3.20-3.31 (m, 1H), 7.26 (ddd, J ) 7.8, 2.22,
1.3 Hz, 1H), 7.32 (ddd, J ) 7.5, 2.0, 1.3 Hz, 1H), 7.36-7.41
(m, 2H). Signals attributed to minor isomer 13f: 0.83 (d, J )
6.8 Hz), 2.21 (s). 13C NMR (75 MHz, CDCl3) δ 14.2, 18.7, 20.3,
28.3, 28.7, 50.9, 120.2, 123.1, 125.2, 127.8, 130.4, 135.0, 135.1,
138.4, 154.0, 207.7; MS (70 eV) 307 (M+ + 2, 33), 305 (M+,
92), 290 (24), 262 (59), 248 (43), 235 (86), 218 (100), 152 (36),
111 (53), 75 (33), 43 (86). Anal. Calcd for C16H16ClNO3: C,
62.85; H, 5.26; N, 4.58. Found: C, 63.08; H, 5.49; N, 4.62.
(6R*,7R*)-6-Acet yl-7-m et h yl-N-p -t olyl-4,5,6,7-t et r a h y-
d r ob en zoxa zol-2-on e (13g) a n d (6R*,7S*)-6-Acet yl-7-
m eth yl-N-p-tolyl-4,5,6,7-tetr ah ydr oben zoxazol-2-on e (13h ).
Using the general procedure with 4d (0.17 g) and 11 (0.11 g)
gave a mixture of 13g/13h (9:1), which was purified and
recrystallized from acetone/hexane (1:1) to yield 0.198 g (88%)
of 13g as colorless crystals: Rf 0.45 (hexane/EtOAc, 7:3); mp
1
1410, 1270 cm-1; H NMR (300 MHz, CDCl3) δ 1.39 (d, J )
7.0 Hz, 3H), 3.23 (dd, J ) 21.7, 7.8 Hz, 1H), 3.46 (dd, J ) 21.7,
8.0 Hz, 1H), 3.76 (s, 3H), 3.77-3.84 (m, 1H), 3.85 (s, 3H), 7.34-
7.51 (m, 5H); 13C NMR (75 MHz, CDCl3) δ 17.6, 24.4, 31.1,
52.6, 116.3, 125.3, 127.4, 128.2, 129.7, 133.6, 134.7, 140.4,
154.3, 166.3, 167.5; MS (70 eV) 343 (M+, 27), 312 (7), 284 (74),
252 (100), 225 (55), 180 (31), 121 (26), 77 (47), 59 (38). Anal.
Calcd for C18H17NO6: C, 62.98; H, 4.99; N, 4.08. Found: C,
62.79; H, 5.06; N, 4.12.
Gen er a l P r oced u r e for th e Dim er iza tion a n d Isom er -
iza tion of Dien es 4a a n d 4c. A solution of diene 4a or 4c
(0.5 mmol) in dry xylene (2 mL) was placed in a sealed tube
under N2 atmosphere and heated at 120 °C for 10 h. The
solvent was removed under vacuo and the residue purified by
column chromatography.
Dim er iza tion a n d Isom er iza tion of Dien e 4a . Using
the general procedure with diene 4a (0.10 g), the reaction gave
three products: unreacted 4a (0.02 g, 20%), dimer 17a (0.05
g, 50%) as a colorless amorphous solid (CHCl3/hexane), and
4-methyl-N-phenyl-5-vinyl-1,3-oxazolin-2-one (18a ) (0.16 g,
16%) as colorless crystals (acetone/hexane). 17a : Rf 0.4 (hex-
ane/EtOAc, 4:1); mp 243-244 °C; IR (KBr) 1790, 1750, 1700,
1500, 1250 cm-1; 1H NMR (300 MHz, CDCl3) δ 1.38-1.52 (m,
1H), 1.53 (d, J ) 7.1 Hz, 3H), 1.80 (d, J ) 6.9 Hz, 3H), 1.98
(ddd, J ) 17.0, 7.6, 2.6 Hz, 1H), 2.11 (ddd, J ) 14.5, 10.7, 7.7
Hz, 1H), 2.38 (dd, J ) 14.5, 6.5 Hz, 1H), 3.04-3.08 (m, 1H),
4.70 (q, J ) 6.9 Hz, 1H), 6.78-6.82 (m, 2H), 7.22-7.39 (m,
8H); 13C NMR (75 MHz, CDCl3) δ 10.1, 10.2, 18.3, 34.2, 40.0,
67.4, 97.2, 119.7, 125.2, 127.9, 128.8, 129.3, 129.4, 133.2, 134.1,
134.2, 136.4, 150.9, 154.3; MS (70 eV) 402 (M+, 6), 215 (13),
201 (100), 156 (7), 118 (18), 91 (5), 77 (14), 51 (4). Anal. Calcd
for C24H22N2O4: C, 71.63; H, 5.51; N, 6.96. Found: C, 71.44;
H, 5.30; N, 6.84. 18a : Rf 0.4 (hexane/EtOAc, 9:1); mp 90-91
°C; IR (KBr) 1750, 1500, 1380 cm-1; 1NMR (300 MHz, CDCl3)
δ 1.97 (s, 3H), 5.19 (dm, J ) 11.3 Hz, 1H), 5.58 (dm, J ) 17.5
Hz, 1H), 6.33 (dd, J ) 17.5, 11.3 Hz, 1H), 7.26-7.51 (m, 5H);
13C NMR (75 MHz, CDCl3) δ 8.9, 112.5, 119.8, 120.6, 127.0,
128.6, 129.6, 133.4, 134.8, 152.8; MS (70 eV) 201 (M+, 6), 118
1
140-141 °C; IR (KBr) 1760, 1700, 1510, 1390 cm-1; H NMR
(300 MHz, CDCl3) δ 1.05 (d, J ) 6.8 Hz, 3H), 1.83 (dddd, J )
13.9, 12.0, 9.9, 6.7 Hz, 1H), 1.96 (dddd, J ) 13.9, 5.3, 2.9, 2.5
Hz, 1H), 2.22 (s, 3H), 2.25-2.33 (m, 2H), 2.37 (s, 3H), 2.95
(ddd, J ) 12.0, 5.3, 2.9 Hz, 1H), 3.24 (m, 1H), 7.18-7.27 (m,
4H). Signals attributed to minor isomer 13h : 0.85 (d, J )
6.8 Hz), 2.23 (s); 13C NMR (75 MHz, CDCl3) δ 14.3, 18.8, 19.9,
21.0, 28.4, 28.6, 51.1, 120.7, 125.1, 130.0, 131.4, 137.8, 137.9,
154.6, 207.7; MS (70 eV) 285 (M+, 75), 270 (7), 242 (16), 228
(20), 215 (100), 198 (29), 132 (33), 91 (31), 43 (17). Anal. Calcd
for C17H19NO3: C, 71.56; H, 6.70; N, 4.90. Found: C, 71.45;
H, 6.49; N, 4.78.
6-Ca r b om et h oxy-7-m et h yl-N-p h en yl-4,7-d ih yd r ob en -
zoxa zol-2-on e (15c) a n d 5-Ca r bom eth oxy-7-m eth yl-N-
p h en yl-4,7-d ih yd r oben zoxa zol-2-on e (16c). Using the gen-
eral procedure with 4a (0.16 g) and methyl propiolate (12)
(0.134 g) gave a mixture of 15c/16c (93:7), which was purified
and recrystallized from acetone/hexane (1:1) to yield 0.204 g
(90%) of 15c as colorless crystals: Rf 0.40 (hexane/EtOAc, 7:3);
(23), 77 (36), 64 (7), 53 (14), 51 (100). Anal. Calcd for C12H11
-
NO2: C, 71.63; H, 5.51; N, 6.96. Found: C, 71.53; H, 5.65; N,
6.99.
mp 180-181 °C; IR (KBr) 1760, 1720, 1700, 1500, 1250 cm-1
;
Dim er iza tion a n d Isom er iza tion of Dien e 4c. Using the
general procedure with diene 4c (0.117 g), the reaction gave
three products: unreacted 4c (0.03 g, 26%), dimer 17b (0.053
g, 45%) as colorless crystals (CH2Cl2/hexane), and 4-methyl-
N-(m-chlorophenyl)-5-vinyl-1,3-oxazolin-2-one (18b): (0.029 g,
25%) as colorless crystals (CH2Cl2/hexane). 17b: Rf 0.37
(hexane/EtOAc, 4:1); mp 288-289 °C; IR (KBr) 1780, 1750,
1H NMR (300 MHz, CDCl3) δ 1.4 (d, J ) 6.7 Hz, 3H), 3.15
(ddd, J ) 22.5, 7.1, 4.1 Hz, 1H), 3.26 (ddd, J ) 22.5, 7.1, 3.3
Hz, 1H), 3.73-3.78 (m, 1H), 3.81 (s, 3H), 6.97 (m, 1H), 7.34-
7.50 (m, 5H). Signals attributed to minor isomer 16c: 1.33
(d, J ) 6.7 Hz), 3.76 (s). 13C NMR (75 MHz, CDCl3) δ 19.2,
24.0, 28.8, 52.0, 116.3, 125.2, 127.9, 129.6, 132.9, 133.5, 133.9,

Diels-Alder Cycloadditions of exo-2-Oxazolidinone Dienes
J . Org. Chem., Vol. 62, No. 12, 1997 4115
1
1710, 1480, 1240 cm-1; H NMR (300 MHz, CDCl3) δ 1.37-
spectrometric measurements. J .T. acknowledges DEPI/
IPN (Grant 916510) and CONACYT (Grant 1203-
E9203) for financial support. A.B.M. thanks CONACYT
for a posdoctoral fellowship awarded (Ca´tedra Patrimo-
nial Nivel II, 940240). R.M. thanks SEP for a fellowship
(Programa MUTIS). H.A.J . acknowledges support from
CONACYT (Fondo de Repatriacio´n, 960082).
1.50 (m, 1H), 1.51 (d, J ) 7.1 Hz, 3H), 1.80 (d, J ) 6.9 Hz,
3H), 2.05-2.21 (m, 2H), 2.34-2.44 (m, 1H), 3.03-3.12 (m, 1H),
4.72 (q, J ) 6.9 Hz, 1H), 6.81-6.88 (m, 2H), 7.16 (ddd, J )
7.9, 2.1, 1.2 Hz, 1H), 7.23-7.33 (m, 4H), 7.39 (ddd, J ) 8.1,
1.9, 1.2 Hz, 1H). Anal. Calcd for C24H20Cl2N2O4: C, 61.16;
H, 4.28; N, 5.94. Found: C, 61.34; H, 4.40; N, 5.95. 18b: Rf
0.4 (hexane/EtOAc, 9:1); mp 136-137 °C; IR (KBr) 1740, 1480,
1380 cm-1; 1H NMR (300 MHz, CDCl3) δ 1.99 (s, 3H), 5.21 (dm,
J ) 11.3 Hz, 1H), 5.59 (dm, J ) 17.3 Hz, 1H), 6.32 (dd, J )
17.3, 11.3 Hz, 1H), 7.22 (dt, J ) 6.9, 2.1 Hz, 1H), 7.26-7.45
(m, 3H); 13C NMR (75 MHz, CDCl3) δ 8.9, 113.0, 119.7, 120.1,
125.1, 127.2, 128.9, 130.5, 134.6, 135.1, 135.2, 153.4. MS (70
eV) 235 (M+, 11), 152 (37), 111 (38), 75 (89), 63 (44), 55 (96),
51 (100). Anal. Calcd for C12H10ClNO2: C, 61.16; H, 4.28; N,
5.94. Found: C, 61.30; H, 4.40; N, 5.85.
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
acquisition data and ORTEP structures of 4c and 13c, relative
magnitudes of the FMO coefficients calculated for 1a , 4a , 11,
and 12 at 3-21G, 6-31G*, and 6-31G** levels, and details of
the preparation of dienes 1c-h , 4e-j, and 5b, along with
complete spectroscopic data of these products (10 pages). This
material is contained in libraries on microfiche, immediately
follows this article in the microfilm version of the journal, and
can be ordered from the ACS; see any current masthead page
for ordering information.
Ack n ow led gm en t. We are grateful to Dr. Gerardo
Zepeda and Dr. Roderick W. Bates for helpful com-
ments. We thank Fernando Labarrios for his help in
J O962403G