Diels-Alder reactions of 2-oxazolidinone dienes in polar solvents using catalysis or non-conventional energy sources

Article abstract of DOI:10.1007/s00706-004-0244-0

The (Z)-N-substituted 4-methylene-5-propylidene-2-oxazolidinone dienes were prepared by a one-step synthesis, starting from 2,3-hexanedione and isocyanates. Diels-Alder cycloadditions of these dienes were carried out in the presence of the dienophiles methyl vinyl ketone, methyl propiolate, and a captodative olefin, under conditions such as solvents of high polarity, Lewis acid catalysis, and non-conventional energy sources. The reactions carried out either with mixtures of H₂O/MeOH or under BF₃· Et₂O catalysis yielded the highest regio- and stereoselectivities. The use of ionic liquids, microwaves, and ultrasound did not significantly increase the selectivity. Springer-Verlag 2004.

Full text of DOI:10.1007/s00706-004-0244-0

Monatshefte f€ur Chemie 136, 177–192 (2005)
DOI 10.1007/s00706-004-0244-0
Diels-Alder Reactions of 2-Oxazolidinone
Dienes in Polar Solvents Using Catalysis
ꢀ
or Non-conventional Energy Sources
1
1;2
1
´
Aydee Fuentes , Rafael Martınez-Palou , Hugo A. Jimenez-Vazquez ,
Francisco Delgado , Alicia Reyes , and Joaquın Tamariz
´
´
´
1
1
1;#
´
1
´ ´
ꢀ
Departamento de Quımica Organica, Escuela Nacional de Ciencias Biologicas,
ꢀ
Instituto Politecnico Nacional. Prol. Carpio y Plan de Ayala,
11340 Mexico, D.F., Mexico
ꢀ
2
´
ꢀ
Programa de Ingenierıa Molecular, Instituto Mexicano del Petroleo,
´ ´
Eje Lazaro Cardenas 152, 07730 Mexico, D.F., Mexico
ꢀ
Received May 13, 2004; accepted (revised) July 22, 2004
Published online November 24, 2004 # Springer-Verlag 2004
Summary. The (Z)-N-substituted 4-methylene-5-propylidene-2-oxazolidinone dienes were prepared
by a one-step synthesis, starting from 2,3-hexanedione and isocyanates. Diels-Alder cycloadditions of
these dienes were carried out in the presence of the dienophiles methyl vinyl ketone, methyl propiolate,
and a captodative olefin, under conditions such as solvents of high polarity, Lewis acid catalysis, and
non-conventional energy sources. The reactions carried out either with mixtures of H2O=MeOH or
under BF3 ꢁ Et2O catalysis yielded the highest regio- and stereoselectivities. The use of ionic liquids,
microwaves, and ultrasound did not significantly increase the selectivity.
Keywords. Cycloadditions; Solvent effect; Ionic liquids; Microwaves; Ab initio calculations.
Introduction
The reactivity of 1,3-conjugated dienes in Diels-Alder cycloadditions depends on
diverse factors, mainly associated with the electron demand of their substituents [1].
However, other factors such as conformational and geometrical effects [2–3], the 1,4-
distance [1a], and steric hindrance [4] in the diene could accelerate or reduce the
reaction rate and, consequently, modify the regio- and stereoselectivity of the process
[5]. Although the structural features of the cycloaddends seem to be directly involved
in controlling reactivity and selectivity, external variables can improve reactivity and
selectivity, such as Lewis acid catalysis [1e, 6], highly polar and ionic liquid solvents
[7], and non-conventional energy sources such as microwaves and ultrasound [8].
ꢀ
Dedicated to Professor Louis S. Hegedus on the occasion of his 60th birthday
#
Corresponding author. E-mail: jtamariz@woodward.encb.ipn.mx

178
A. Fuentes et al.
Scheme 1
We have developed a tandem methodology to prepare the novel N-substituted
exo-2-oxazolidinone dienes 1 and 2 [9], which involves base-assisted condensa-
tion of ꢀ-diketones 3 and isocyanates 4 in the presence of a dehydrating agent
(Scheme 1). The use of the unsymmetrical ꢀ-diketone 3b under similar reaction
conditions, furnished dienes 2 as a single regio- and stereoisomer [9b]. The selec-
tivity was explained in terms of thermodynamic control in the formation of the
enolate of 3b. This methodology has been successfully applied to the synthesis of
1,4-dimethyl substituted dienes [10], and also of those analogues containing the
dienic moiety in a six-membered ring [11]. Moreover, dienes 1 and 2 have shown
to be useful synthons in the preparation of carbazoles [12], leading to the total
synthesis of the natural alkaloid mukonine [13].
Herein, we describe the preparation of new N-substituted (Z)-4-methylene-5-
propylidene-2-oxazolidines 5 with the aim of further extending the versatility of
the method of preparation of these exo-heterocyclic dienes. Besides evaluating the
effect of the ethyl substituent in Diels-Alder additions, we have also assessed the
influence of solvent polarity, catalysis, and energy sources on the reactivity and
selectivity of these reactions.
Results and Discussion
Table 1 summarizes the reaction conditions and yields for the preparation of dienes
5a–5h. The optimum reaction conditions also corresponded to those applied to the
Table 1. Reaction conditions and yields in the preparation of diene 5a
Entry
R0
Diene
Yield %b
1
2
3
4
5
6
7
8
Ph
5a
5b
5c
5d
5e
5f
40
C6H4-4-Cl
C6H4-3-Cl
C6H4-2-Cl
C6H4-4-Me
C6H4-3-Me
C6H4-2-Me
C6H4-2-I
60
31
24
60
43
5g
5h
20
traces
a
Dioxane as solvent, with Et3N (2.0 mol eq) as the base and Li2CO3 (1.2 mol eq) as additive at rt,
b
except in entry 8 (25, 60, 100ꢂC); the reaction time was 12 h; After column chromatography and
c
recrystallization; Ref. [14]

Diels-Alder Reactions of 2-Oxazolidinone Dienes
179
Scheme 2
preparation of dienes 2 [9b]: 2,3-hexanedione (3c), Et3N as the base, and Li2CO3 as
the dehydrating agent, in dry dioxane, at room temperature, and a reaction time of
12 h give dienes 5 in fair yields [14]. The lowest yields obtained corresponded to
the ortho-substituted isocyanates (Table 1, entries 4 and 7). For the o-iodoisocya-
nate 4h, the reaction did not take place satisfactorily, even after heating to high
temperature. The analysis of the crude showed the presence of starting materials
and traces of diene 5h. Since it is likely that the reaction outcome depends on the
electron-donating effect of the substituent of the isocyanate [9], this low reactivity
for 5h could be associated with steric interactions of the ortho-substituent of 4h at
the cyclization step leading to the heterocycle [11].
1
As expected from the preparation of dienes 1 and 2, the H NMR analysis of
the crude mixtures of dienes 5 did not give evidence of the presence of any other
isomer, thereby, the reaction was highly regio- and stereoselective.
Thermal Diels-Alder addition of diene 5c to methyl vinyl ketone (6) yielded a
mixture of the endo=exo stereoisomeric adducts 7a=8a (Scheme 2). As shown in
Table 2, the reaction conditions were similar to those employed in the reaction with
the methyl diene 2c (R0 ¼ C6H4-m-Cl) [9b], indicating a comparable reactivity.
However, the selectivity was significantly reduced, giving the endo isomer as the
major one (Table 2, entries 1 and 2). These results suggest that the ethyl group
attached to the diene frame might have an effect on controlling the approach of the
cycloaddends at the endo transition state. Considering that this effect would have
mainly steric and electronic components, it does not seem to affect the regioselec-
tivity, since only the ortho isomer was observed. This is in contrast with dienes 1
which furnish a mixture of regioisomers ortho=meta in 8:2 ratio [9b].
In order to evaluate non-conventional energy sources on the reactivity and
selectivity of this process, we investigated the use of microwaves (MW) at different
temperatures (Table 2, entries 3–5) [8a]. It was found that the higher the tempera-
ture, the shorter the reaction time, maintaining, however, a quite similar endo
selectivity. Nevertheless, the stereoselectivity was increased with respect to the
thermal conditions (entry 1), with analogous yields.
The polarity of the solvent was modified as well, shifting to highly polar
mixtures of MeOH=H2O (Table 2, entries 6–8). Under these conditions, the reac-
tion took place at lower temperatures (25ꢂC) than those required for the reaction in
xylene. Even though the reaction times increased, the endo stereoselectivity and
the yields were largely increased as well. Upon varying the water proportion in the
solvent mixture, the shorter reaction time and the best endo selectivity were found

180
A. Fuentes et al.
Table 2. Diels-Alder cycloadditions of dienes 2a, 2c, 5b, 5c, and 5f with olefins 6, 9, and 12a
Entry Diene Olefinb Energy Catalyst
sourcec
Solvent
T=ꢂC t=h
Products Yield=%d
(ratio)
1
2
5c
2c
5c
5c
5c
5c
5c
5c
5c
5c
5c
5c
5c
5c
5c
5c
5f
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
9
9
9
9
9
9
9
heat
heat
MW
MW
MW
–
–
m-xylene
m-xylene
m-xylene
m-xylene
m-xylene
120
130
35
1.5
7a:8a
(72:28)
7b:8b
(96:4)
7a:8a
(87:13)
7a:8a
(88:12)
7a:8a
(88:12)
7a:8a
(94:6)
7a:8a
(93:7)
7a:8a
(91:9)
7a:8a
(92:8)
7a:8a
(92:8)
7a:8a
(88:12)
7a:8a
(87:13)
7a:8a
(87:13)
7a:8a
(91:9)
72
90e
81
76
75
98
97
92
80
86
78
74
76
86
96
73
84
90e
59e
75
96
77
–
1.0
4.0
1.0
0.5
3
–
4
–
60
5
–
90
6
–
MeOH:H2O
(9:1)
25
12
7
–
–
MeOH:H2O
(1:1)
25
12
72
12
8
–
–
MeOH:H2O
(1:9)
25
9
)))
–
MeOH:H2O
(9:1)
50
10
11
12
13
14
15
16
17
18
19
20
21
22
MW
MW
MW
MW
–
–
MeOH:H2O
(9:1)
35
2.0
–
m-xylene:[mpim][Br]
(27:1)
60
1.7
1.2
1.7
0.5
–
m-xylene:[mpim][Br]
(27:1)
90
–
m-xylene:[ppy][Br]
(27:1)
60
AlCl3
CH2Cl2
ꢃ78
ꢃ78
120
120
130
120
60
–
BF3 ꢁ Et2O CH2Cl2
0.08 7a:8a
(98:2)
heat
heat
heat
heat
MW
–
–
–
–
–
–
–
–
m-xylene
m-xylene
m-xylene
m-xylene
m-xylene
8.0
6.0
1.0
6.0
8.5
10a:11a
(85:15)
10b:11b
(90:10)
10c:11c
(93:7)
2a
1a
5c
5c
5c
10d:11d
(6:4)
10a:11a
(88:12)
10a:11a
(98:2)
MeOH:H2O
(9:1)
25
408
19.5
)))
MeOH:H2O
(9:1)
50
10a:11a
(88:12)
(continued)

Diels-Alder Reactions of 2-Oxazolidinone Dienes
Table 2 (continued)
181
Entry
Diene
Olefinb
Energy
sourcec
Catalyst
Solvent
T=ꢂC
t=h
Products
(ratio)
Yield=%d
23
24
25
5c
5b
2a
9
12
12
MW
heat
heat
–
–
–
m-xylene:[ppy][Br]
(27:1)
60
120
120
6.5
8.0
8.0
10a:11a
(85:15)
13a:14a
(86:14)
13b:14b
(80:20)
38f
72
70
m-xylene
m-xylene
a
All under N2 atmosphere; thermal trials in the presence of 1–2% hydroquinone; b 2.0 mol equiv with olefins 6 and 9,
c
d
and 1.5 mol equiv with olefin 12; At 200 W for MW; Corresponding to the isomeric mixtures after column
e
chromatography; See Ref. [9b]; Starting material was recovered (37%)
f
for solvent mixtures containing up to 50% of water. The use of ultrasound and
MW as energy sources with these polar solvents did not significantly change the
endo=exo selectivity, although with the latter the reaction times were largely
reduced (Table 2, entries 6, 9, and 10).
Recently, the use of ionic liquids has attracted widespread interest due to the
enhancement of the reactivity and selectivity in a large number of reactions [7].
Two new ionic liquids were prepared, 3-methyl-1-n-pentyl-3H-imidazol-1-ium
bromide ([mpim][Br]) and 1-n-pentylpyridinium bromide ([ppy][Br]), using n-pen-
tyl bromide with 1-methylimidazole or pyridine under MW irradiation at normal
pressure, which are more gentle conditions than those previously reported [7h].
They were employed as co-solvents, along with m-xylene under MW irradiation
(Table 2, entries 11–13). However, there was no change with respect to those
experiments carried out in neat m-xylene (entries 3–5).
The most relevant results were obtained when the reactions were carried out
under Lewis acid catalysis at low temperature (Table 2, entries 14 and 15). Alu-
minum chloride in methylene chloride was very efficient, since the reaction took
place in 30 min with high stereoselectivity. This result was improved even more by
using BF3 ꢁ Et2O in methylene chloride, obtaining almost exclusively the endo
isomer in high yield and in only 5 min.
The unsymmetrical dienophile methyl propiolate (9) was also evaluated under
thermal conditions with dienes 5c and 5f (Scheme 3), providing 10a as the major
Scheme 3

182
A. Fuentes et al.
regioisomer (Table 2, entries 16 and 17). Interestingly, both reactivity and regio-
selectivity were found to be higher in the case of the m-tolyl derivative 5f. Moreover,
the latter showed a regioselectivity quite similar to that obtained with the methyl
diene 2a [9b] (Table 2, entries 17 and 18), indicating that the presence of the ethyl
group in dienes 5 seems to affect (steric effect) only slightly the orientation of the
addition. However, the contribution of the electronic effect of this group to the
regioselectivity control of these dienes is significant, since, unlike dienes 5, almost
no selectivity was observed with the unsubstituted dienes 1 (e.g. entry 19) [9b].
Unexpectedly, even though the reaction temperature was lower [1a], no signifi-
cant increase in regioselectivity was observed for the reaction of diene 5c neither
when MW were applied, nor when a ionic liquid was added to the reaction mixture as
a co-solvent (Table 2, entries 20 and 23). In contrast, the effect of a major change in
the polarity of the solvent on the regioselectivity for this cycloaddition was clearly
apparent. Thus, a high ortho regioselectivity was observed when the reaction between
diene 5c and 9 was carried out in a mixture of MeOH=H2O (9:1) (Table 2, entry 21).
The reaction time was shortened by using ultrasound in combination with this solvent
mixture, although the regioselectivity decreased (entry 22).
Captodative olefin 12 (PNB ¼ p-nitrobenzoyloxy) proved to be a highly reac-
tive and selective dienophile in Diels-Alder cycloadditions, in spite of its highly
hindered quaternary center [15]. This olefin exhibited good stereo- and regioselec-
tivity with dienes 5b and 2a under thermal reaction conditions (Table 2, entries 24
and 25) since a single regioisomer was isolated, and high stereoisomeric ratios of
13=14 were recorded (Scheme 2). The comparable proportion of endo=exo adducts
between these dienes would support a minimal contribution of steric factors in the
control of the stereoselectivity of the additions, while an electronic effect might
seem rather significant in determining the regioselectivity.
1
The structures of the major isomers were established by H NMR experiments
(COSY, HETCOR, and NOE) correlating the cyclohexene protons as previously
described [9b]. For adduct 10b, the assignment was confirmed by single-crystal
X-ray diffraction (Fig. 1).
The relative reactivity and selectivity in the addition of dienes 5 to unsymmet-
rical dienophiles under thermal conditions with respect to unsubstituted dienes 1
[9b] confirms the expected higher reactivity and regioselectivity of dienes 5 due to
the presence of the ethyl group [1c, 16]. Moreover, the methyl-substituted dienes 2
Fig. 1. X-Ray structure of 10b

Diels-Alder Reactions of 2-Oxazolidinone Dienes
183
proved to be more regioselective and stereoselective than dienes 5 (Table 2). This
may be associated to an enhancement of the steric effect by the ethyl group.
Considering these facts, the comparable high stereoselectivity shown by dienes
2a and 5b in the cycloaddition with the captodative olefin 12 is difficult to explain,
since a number of possible interactions at the transition state would modulate the
stability of the major endo-adducts 13a and 13b obtained, most likely, by kinetic
control. This endo preference at the transition state, displayed not only by olefin 12
but also by methyl vinyl ketone (6), seems to be associated with electronic effects
rather than with steric control. This hypothesis is supported by the fact that in all
the additions, the more crowded ortho regioisomer is obtained, regardless which
substituent is in the diene or in the dienophile.
FMO theory has been successfully used to explain the regioselectivity of Diels-
Alder cycloadditions of dienes 1 and 2 [9b]. Therefore, we have also used this model
to correlate the energies and coefficients of the frontier molecular orbitals of dienes
5 with those of olefins 6, 9, and 12 [17]. Frontier orbitals of diene 5a were calculated
using the ab initio HF method. Geometries were optimized by the AM1 semiempi-
rical method [18] and employed as the starting point for optimization, using the
ꢀ
3-21G and 6-31G basis sets [19]. The orbital eigenvalues derived from these calcu-
lations are shown in Table 3, and compared with those previously calculated for the
unsubstituted and methyl-substituted dienes 1a and 2a [9b]. The normal electron
demand interaction [20], HOMO-diene=LUMO-dienophile, is largely preferred
(1.8–3.1 eV) for all the additions. The energy value of the HOMO for diene 5a is
ꢀ
higher than those of dienes 1a and 2a, at both 3-21G and 6-31G levels. As
expected, the difference is more significant with respect to diene 1a, whose dienic
system does not have an electron-donor substituent. Consequently, dienes 5 should
be more reactive and selective in [4 þ 2] cycloadditions than dienes 1 and 2.
ꢀ
Table 3. Ab initio RHF=3-21G, and 6-31G frontier molecular orbitals of dienes 1a, 2a, and 5a, and
dienophiles 6, 9, and 12
Compound
FMO
E=eV
ꢀ
3-21G
6-31G
1aa
1aa
2aa
2aa
5a
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
ꢃ8.9855
2.8161
ꢃ8.8342
2.9470
ꢃ8.7227
3.0093
ꢃ8.5804
3.1448
ꢃ8.7006
2.9957
ꢃ8.5648
3.1152
5a
6b
ꢃ10.5391
2.9002
ꢃ10.4895
2.9222
6b
9a
ꢃ11.6713
3.0719
ꢃ11.4648
3.2972
9a
12c
12c
ꢃ11.0460
2.2417
ꢃ11.0123
2.4588
a
See Ref. [9b]; b Of the most stable s-cis conformation, see Ref. [9b]; c Of the most stable nonplanar
s-trans conformation, see Ref. [22]

184
A. Fuentes et al.
We can see from Table 2 that, even though the HOMO of diene 5a is less stable
than that of diene 2a (Table 3), the regioselectivity was slightly lower in the reac-
tion of dienes 5c and 5f with olefin 9, in comparison with diene 2a. Nevertheless,
the dienes 5 were, indeed, more reactive and regioselective than diene 1a toward
the same olefin.
The regiochemistry observed in the cycloadditions between dienes 5 and die-
nophiles 6, 9, and 12 [21] can be accounted for by the interactions of the appro-
priate largest orbital coefficients. In order to evaluate this, we calculated the
ꢀ
relative magnitudes of the HOMO and LUMO coefficients at the 6-31G level
for the structurally most simple diene 5a (Table 4). As expected from calculations
of diene 2a [9b], the coefficient at the diene terminus (C4) is larger than that at
carbon C-1 (C1) in the HOMO. According to the coefficient differences (DCi ¼
C4 ꢃ C1), whose values are identical for both dienes, 2a and 5a, one can predict no
difference in regioselectivity. Possibly, a marginal difference in the steric contribu-
tion of the alkyl groups may explain the slightly better regiochemistry found for
diene 2a with respect to 5f with olefin 9. The same ortho regioselectivity can be
predicted for dienes 2a and 5b with dienophile 12, since the largest LUMO coeffi-
cient of this olefin is located at the unsubstituted carbon C-1 [22].
The endo stereoselectivity shown by the ethyl and methyl-substituted dienes 5c,
5b, and 2c in the addition to dienophiles 6 and 12, cannot be explained simply
on the basis of steric or electrostatic effects [23]. The preferred quasi-orthogonal
conformation of the N-phenyl or N-aryl ring of the dienes [9b] would preclude, in
principle, the endo approach of the dienophile, unless a supplementary factor could
stabilize it. Secondary orbital interactions (SOI) at the transition state (TS) have
been traditionally invoked to account for the endo selectivity [20, 24]. Instead of
ꢀ
Table 4. Ab initio RHF=6-31G coefficients (Ci) of the frontier molecular orbitals for dienes 2a and
5a, and dienophiles 6, 9, and 12a
Comp HOMO
LUMO
b
b
C1
C2
C3
C4
DCi
C1
C2
C3
C4
DCi
1ac
2a
5a
6c
0.259
0.176 ꢃ0.217 ꢃ0.334 0.075 ꢃ0.269
0.250
0.235
0.247 ꢃ0.262 0.019
0.254 ꢃ0.259 0.027
ꢃ0.268 ꢃ0.208
ꢃ0.266 ꢃ0.207
ꢃ0.346 ꢃ0.367
ꢃ0.375 ꢃ0.394
0.204
0.203
0.033
0.034
0.024
0.328 0.060 ꢃ0.286
0.326 0.060
0.221 0.021
0.192 0.019
0.168 0.003
0.281 ꢃ0.235 ꢃ0.249
0.311 ꢃ0.207 ꢃ0.281
0.289 ꢃ0.183 ꢃ0.339
0.294 ꢃ0.239 ꢃ0.289
0.256 0.025
0.255 0.104
0.279 0.106
0.280 0.055
9c
12c,d ꢃ0.359 ꢃ0.356
a
0
These are the values of the 2pz coefficients, the relative 2pz contributions and their DCi are
analogous; C4 ꢃ C1 for the dienes and C1 ꢃ C2 for the dienophiles; Ref. [10]; Ref. [22]
b
c
d

Diels-Alder Reactions of 2-Oxazolidinone Dienes
185
these classical SOI, Garcia et al. observed, in particular for the Lewis acid-cata-
lyzed Diels-Alder reaction, additional SOI for the energetically preferred endo TS,
which are localized between the terminal carbon atom C-1 of the diene and the
carbon atom of the carbonyl group, as a [4 þ 3] or a three-center orbital interaction
[25]. Moreover, recently, a bispericyclic transition state (BTS) leading to a merged
[4 þ 2] and [2 þ 4] approach appears to rationalize the stabilizing interactions that
favor the endo selectivity [26]. While for our addends classical SOI would imply a
supplementary interaction between coefficients of carbon atoms C-2 of the diene
and C-3 of the dienophile, a BTS would suggest a more favorable secondary
interaction between the carbon atom C-3 of the diene and the oxygen atom O-4
of the dienophile [27]. The [4 þ 3] approach would involve a three-center interac-
tion between the carbon atom C-1 of the diene and the carbon atoms C-2 and C-3
of the olefin. Whatever the main secondary interaction is, the HOMO coefficients
in carbons C-2 and C-3 of the diene, and the LUMO coefficients C3 and C4 in the
dienophile (Table 4), are relatively large to take advantage of these probable endo
stabilizing interactions. Calculation of the TSs for the processes herein described
were undertaken, in order to obtain a further insight on the main interactions
controlling the endo stereoselectivity, and they will be disclosed elsewhere.
Conclusions
We describe the synthesis of the ethyl substituted dienes 5c–5g in moderate yields
following the one-pot reaction between the ꢀ-diketone 3c and the corresponding
isocyanates 4. The Diels-Alder additions of dienes 5 with the unsymmetric olefins
6 and 9 under thermal conditions proved to be highly regio- and stereoselective. In
the case of dienophile 12, dienes 5a and 2a also showed high stereo- and regio-
selectivity. The electron-releasing effect of the alkyl group in dienes 2 and 5 seems
to be the cause for the increase in the reactivity and selectivity of these dienes with
respect to dienes 1, as suggested by FMO calculations. These calculations also
explain the regioselectivity obtained in these cycloadditions. In contrast, steric
hindrance of the bulkier substituent in dienes 5 does not seem to have a significant
effect on the regio- and stereoselectivities. Microwave irradiation enhanced the
stereo- and regioselectivity of the cycloadditions between diene 5c and dienophiles
6 and 9 with respect to the thermal trials. However, the best improvement of both
selectivities was reached by increasing the polarity of the solvent, by using mix-
tures of water=methanol, and by using Lewis acid catalysis.
Experimental
Melting points were determined with an Electrothermal capillary melting point apparatus. IR spectra
1
were recorded on a Perkin-Elmer 1600 spectrophotometer. H (300 and 200 MHz) and 13C (75.4 and
50MHz) NMR spectra were recorded on Varian Mercury-300 and Mercury-200 instruments, in CDCl3,
DMSO-d6, or acetone-d6 as solvents and TMS as internal standard. Mass spectra (MS) and high-
resolution mass spectra (HRMS) were obtained, in electron impact (EI) (70eV) mode, on Hewlett-
Packard 5971A and Jeol JMS-AX 505 HA spectrometers. X-Ray data were collected on a Siemens P4
diffractometer. Microwave and ultrasound irradiations were carried out in a mono-mode microwave oven
(600 W, 2.45 GHz) Mod. MIC-I from SEV (Mexico) [28], and with a Branson ultrasonic cleaner.
Elemental analyses (C, H, N, S) were performed by M-H-W Laboratories (Phoenix, AZ); their results

186
A. Fuentes et al.
were found to be in good agreement (ꢄ0.2%) with the calculated values. TLC analysis was carried
out using E. Merck silica gel 60 F254 coated 0.25 plates visualizing by long- and short-wavelength UV
lamp. Flash column chromatography was performed on 230–400 mesh silica gel (Natland Int.). All air
moisture sensitive reactions were carried out under nitrogen using oven-dried glassware. Dioxane and
m-xylene were freshly distilled over Na, and CH2Cl2 over CaH2, prior to use. Li2CO3 and molecular
sieves were dried overnight at 120ꢂC prior to use. Triethylamine was distilled over NaOH. All other
reagents were used without further purification. Dienes 5a and 5b were prepared as reported [14].
General Procedure for the Synthesis (5Z)-N-Substituted-4-methylene-5-propylidene-
2-oxazolidinones 5c–5g
A solution of 0.4 g of 2,3-hexanedione (3c) (3.5mmol) in 1 cm3 of dry dioxane was added dropwise to
a magnetically stirred solution of 0.71g of triethylamine (7.0mmol) in 2 cm3 of dry dioxane containing
0.31g of anhydrous Li2CO3 (4.2mmol) at rt under N2 and the mixture was stirred for 30min. A
solution of the corresponding aryl isocyanate (5.2mmol) in 2 cm3 of dioxane was added dropwise and
stirring was continued for 12 h at room temperature. The mixture was filtered and the solvent removed
under vacuum. The residue was purified by column chromatography on silica gel impregnated with
triethylamine (10%) in n-hexane (n-hexane:EtOAc, 9:1).
(5Z)-N-(3-Chlorophenyl)-4-methylene-5-propylidene-2-oxazolidinone (5c, C13H12ClNO2)
From 0.79 g of 4c 0.27 g (31%) of 5c as colorless crystals; Rf ¼ 0.59 (n-hexane:EtOAc, 9:1); mp
68–69ꢂC (CH2Cl2:n-hexane, 7:3); IR (KBr): ꢁꢁ¼ 1762, 1691, 1637, 1479, 1391, 1293, 1042cmꢃ1
;
1H NMR (300MHz, CDCl3): ꢂ ¼ 1.14 (t, J ¼ 7.5 Hz, CH3CH2CH¼), 2.32–2.45 (m, CH2CH¼), 4.26
(d, J ¼ 3.1 Hz, H-6), 4.65 (d, J ¼ 3.1 Hz, H-6), 5.43 (t, J ¼ 7.7Hz, H-7), 7.27 (dt, J ¼ 7.4, 1.8 Hz, ArH),
7.37–7.47 (m, 3ArH) ppm; 13C NMR (75.4 MHz, CDCl3): ꢂ ¼ 13.7 (CH3CH2CH¼), 18.6
(CH3CH2CH¼), 82.1 (C-6), 106.5 (C-7), 125.2 (ArH), 127.3 (ArH), 128.9 (ArH), 130.7 (ArH),
134.3 (Ar), 135.2 (Ar), 138.7 (C-4), 141.8 (C-5), 152.1 (C-2) ppm; MS (70 eV): m=z(%) ¼ 251 (Mþþ
2, 6), 249 (Mþ, 14), 190 (7), 154 (45), 152 (100), 125 (17), 111 (75), 90 (26), 75 (94), 55 (98).
(5Z)-N-(2-Chlorophenyl)-4-methylene-5-propylidene-2-oxazolidinone (5d, C13H12ClNO2)
From 0.79 g of 4d 0.21 g (24%) of 5d as colorless crystals; Rf ¼ 0.60 (n-hexane:EtOAc, 9:1); mp
73–74ꢂC (CH2Cl2:n-hexane, 7:3); IR (KBr): ꢁꢁ¼ 1764, 1687, 1630, 1494, 1452, 1383, 1292,
1035 cmꢃ1
;
1H NMR (300 MHz, CDCl3): ꢂ ¼ 1.08 (t, J ¼ 7.5 Hz, CH3CH2CH¼), 2.30–2.43
(m, CH2CH¼), 3.82 (d, J ¼ 3.1 Hz, H-6), 4.56 (d, J ¼ 3.1 Hz, H-6), 5.39 (t, J ¼ 7.7 Hz, H-7),
7.30–7.50 (m, 4ArH) ppm; 13C NMR (75.4 MHz, CDCl3): ꢂ ¼ 13.4 (CH3CH2CH¼), 18.4
(CH3CH2CH¼), 82.1 (C-6), 105.8 (C-7), 128.1 (ArH), 129.0 (Ar), 130.0 (ArH), 130.5 (2ArH),
133.6 (Ar), 138.2 (C-4), 141.9 (C-5), 151.8 (C-2) ppm; MS (70 eV): m=z(%) ¼ 251 (Mþþ2, 2), 249
(Mþ, 8), 190 (4), 154 (45), 152 (100), 125 (17), 111 (55), 90 (26), 75 (78), 55 (51).
(5Z)-4-Methylene-5-propylidene-N-(p-tolyl)-2-oxazolidinone (5e, C14H15NO2)
From 0.69 g of 4e 0.48g (60%) of 5e as colorless crystals; Rf ¼ 0.56 (n-hexane:EtOAc, 9:1); mp 62–63ꢂC
(CH2Cl2:n-hexane, 7:3); IR (KBr): ꢁꢁ¼ 1768, 1497, 1400, 1290, 1040, 836cmꢃ1; 1H NMR (300MHz,
CDCl3): ꢂ ¼ 1.08 (t, J ¼ 7.5 Hz, CH3CH2CH¼), 2.26–2.38 (m, CH2CH¼), 2.39 (s, MeAr), 4.15
(d, J ¼ 2.7 Hz, H-6), 4.56 (d, J ¼ 2.7 Hz, H-6), 5.36 (t, J ¼ 7.7Hz, H-7), 7.22–7.43 (m, 4ArH) ppm;
13C NMR (75.4 MHz, CDCl3): ꢂ ¼ 13.6 (CH3CH2CH¼), 18.5 (CH3CH2CH¼), 21.1 (CH3Ar), 81.6
(C-6), 105.8 (C-7), 126.9 (2ArH), 130.1 (2ArH), 130.4 (Ar), 138.7 (C-4), 139.3 (Ar), 142.4 (C-5),
152.7 (C-2) ppm; MS (70 eV): m=z(%) ¼ 229 (Mþ, 2), 132 (47), 91 (79), 69 (23), 65 (100), 55 (85).

Diels-Alder Reactions of 2-Oxazolidinone Dienes
187
(5Z)-4-Methylene-5-propylidene-N-(m-tolyl)-2-oxazolidinone (5f, C14H15NO2)
From 0.69 g of 4f 0.34g (43%) of 5f as colorless crystals; Rf ¼ 0.57 (n-hexane:EtOAc, 9:1); mp 58–
59ꢂC (CH2Cl2:n-hexane, 7:3); IR (KBr): ꢁꢁ¼ 1767, 1691, 1637, 1495, 1397, 1293, 1037, 830 cmꢃ1; 1H
NMR (300MHz, CDCl3): ꢂ ¼ 1.09 (t, J ¼ 7.5 Hz, CH3CH2CH¼), 2.26–2.39 (m, CH2CH¼), 2.39 (s,
MeAr), 4.17 (d, J ¼ 2.9 Hz, H-6), 4.58 (d, J ¼ 2.9 Hz, H-6), 5.37 (t, J ¼ 7.7 Hz, H-7), 7.10–7.16 (m,
2ArH), 7.21 (br d, J ¼ 7.6 Hz, 1ArH), 7.36 (t, J ¼ 7.6 Hz, 1ArH) ppm; 13C NMR (75.4 MHz, CDCl3):
ꢂ ¼ 13.7 (CH3CH2CH¼), 18.5 (CH3CH2CH¼), 21.2 (CH3Ar), 81.7 (C-6), 105.9 (C-7), 124.0 (ArH),
127.6 (ArH), 129.4 (2ArH), 133.2 (Ar), 139.3 (C-4), 139.8 (Ar), 142.1 (C-5), 152.7 (C-2) ppm; MS
(70 eV): m=z(%) ¼ 229 (Mþ, 6), 184 (3), 170 (3), 132 (56), 104 (7), 91 (84), 77 (20), 65 (100).
(5Z)-4-Methylene-5-propylidene-N-(o-tolyl)-2-oxazolidinone (5g, C14H15NO2)
From 0.69 g of 4g 0.16g (20%) of 5g as colorless crystals; Rf ¼ 0.56 (n-hexane:EtOAc, 9:1); mp 69–
70ꢂC (CH2Cl2:n-hexane, 7:3); IR (KBr): ꢁꢁ¼ 1767, 1495, 1391, 1032, 966, 759 cmꢃ1
;
1H NMR
(300 MHz, CDCl3): ꢂ ¼ 1.16 (t, J ¼ 7.5 Hz, CH3CH2CH¼), 2.29 (s, MeAr), 2.30–2.45 (m, CH2CH¼),
3.89 (d, J ¼ 2.8 Hz, H-6), 4.58 (d, J ¼ 2.8 Hz, H-6), 5.42 (t, J ¼ 7.7 Hz, H-7), 7.22–7.41 (m, 4ArH)
ppm; 13C NMR (75.4MHz, CDCl3): ꢂ ¼ 13.6 (CH3CH2CH¼), 17.3 (CH3CH2CH¼), 18.5 (CH3Ar),
81.8 (C-6), 106.1 (C-7), 127.3 (ArH), 128.4 (ArH), 129.6 (ArH), 131.3 (Ar), 131.4 (ArH), 139.0 (C-4),
139.8 (Ar), 142.1 (C-5), 152.5 (C-2) ppm; MS (70 eV): m=z(%) ¼ 229 (Mþ, 10), 186 (10), 172 (11),
148 (29), 147 (17), 103 (17), 92 (42), 77 (74), 63 (100), 51 (68).
General Procedure for the Diels-Alder Reactions of Dienophiles 6 and 9
with N-Aryl-4-methylene-5-propylidene-2-oxazolidinones 5c and 5f
Method A. A mixture of 0.4 mmol of 5, 0.92 mmol of 6, or 1.9 mmol of 9, and 3 mg of hydroquinone in
0.4 cm3 of dry m-xylene was placed in a threaded ACE glass pressure tube with a sealed Teflon screw
cap under N2 and in the dark. The mixture was stirred and heated at 120ꢂC for 1.0–8.0 h (see Table 2).
The mixture was filtered, the solvent removed under vacuum, and the residue purified by column
chromatography on silica gel (n-hexane:EtOAc, 4:1) to give the corresponding adducts 7a=8a or
10a–10b=11a–11b.
Method B. The same procedure as method A, with MW irradiation (200W) at 60ꢂC for 1.0–8.5 h (see
Table 2), to give the corresponding adducts 7a=8a or 10a=11a.
Method C. The same procedure as method A, with a mixture of MeOH:H2O (9:1) as the solvent, and
allowing to react at room temperature for 12–408 h (see Table 2). The solvent was removed under
vacuum, and the residue was extracted with 3ꢅ3 cm3 of CH2Cl2. The combined organic extracts were
dried (Na2SO4), the solvent was evaporated under vacuum, and the residue was purified by column
chromatography to give the corresponding adducts 7a=8a or 10a=11a.
Method D. The same procedure as method A, adding 0.028g of [mpim][Br] (0.12 mmol) or 0.027 g of
[ppy][Br] (0.12 mmol), and irradiating with MW (200W) at 60ꢂC for 1.7–6.5 h (see Table 2). The
layers were separated, and the layer containing the ionic liquid was washed with 3ꢅ0.5 cm3 of m-
xylene. The extracts were combined, the solvent was evaporated under vacuum, and the residue was
purified by column chromatography to give the corresponding adducts 7a=8a or 10a=11a.
Method E. To a solution of 0.1 g of 5c (0.4 mmol) in 3 cm3 of dry CH2Cl2 under N2 at ꢃ78ꢂC a solution of
the Lewis acid (AlCl3 or BF3 ꢁ Et2O) (4.0mmol) and 0.168 g of 6 (2.4mmol) in 3 cm3 of dry CH2Cl2 was
slowly added. After being stirred at ꢃ78ꢂC for 5–30 min (see Table 2), the reaction mixture was diluted
with 20cm3 of CH2Cl2, washed with 2ꢅ5 cm3 of aqueous 5% NaHCO3, 2ꢅ10cm3 of aqueous 5%
NH4Cl, and finally the organic extract was dried (Na2SO4). The solvent was evaporated and the residue
purified by column chromatography as described in method A, to give the corresponding adducts 7a=8a.

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A. Fuentes et al.
Method F. The same procedure as method A, with ultrasound irradiation at 50ꢂC for 12.0–19.5 h (see
Table 2), to give the corresponding adducts 7a=8a or 10a=11a.
ꢀ
2-one (7a, C17H18ClNO3)
ꢀ
(6R ,7R )-6-Acetyl-N-(3-chlorophenyl)-7-ethyl-4,5,6,7-tetrahydrobenzoxazol-
According to method B, and after reacting for 1 h, a mixture of 7a:8a (88:12) was separated by column
chromatography on silica gel (12 g, n-hexane:EtOAc, 9:1), affording 0.092g (72%) of 7a as a white
solid. Rf ¼ 0.26 (n-hexane:EtOAc, 7:3); mp 138–139ꢂC (CH2Cl2:n-hexane, 1:1); IR (KBr): ꢁꢁ¼ 1757,
1706, 1588, 1482, 1381, 1184, 1162, 791, 752cmꢃ1
;
1H NMR (300 MHz, CDCl3): ꢂ ¼ 1.06 (t,
J ¼ 7.5 Hz, CH3CH2), 1.32–1.55 (m, CH3CH2), 1.85–2.10 (m, 2H-5), 2.24 (s, CH3CO), 2.27–2.44
(m, 2H-4), 2.94 (ddd, J ¼ 11.6, 5.1, 2.9 Hz, H-6), 3.00–3.10 (m, H-7), 7.24–7.42 (m, 4ArH) ppm; 13
C
NMR (75.4 MHz, CDCl3): ꢂ ¼ 12.3 (CH3CH2), 19.3 (C-5), 20.0 (C-4), 23.5 (CH3CH2), 28.7 (CH3CO),
35.4 (C-7), 51.0 (C-6), 120.6 (C-3a), 123.2 (ArH), 125.3 (ArH), 127.8 (ArH), 130.3 (ArH), 134.9 (Ar),
135.0 (Ar), 137.9 (C-7a), 154.0 (C-2), 207.9 (CH3CO) ppm; MS (70 eV): m=z(%) ¼ 319 (Mþ, 26), 290
(22), 276 (24), 248 (100), 232 (31), 168 (11), 151 (29), 111 (42), 95 (18), 75 (44), 55 (24).
6-Carbomethoxy-3-(3-chlorophenyl)-7-ethyl-4,7-dihydrobenzoxazol-
2-one (10a, C17H16ClNO4)
According to method A, and after heating for 8 h, a mixture of 10a:11a (85:15) was separated by column
chromatography on silica gel (12 g, n-hexane:EtOAc, 95:5), affording 0.083g (62%) of 10a as a white
solid. Rf ¼ 0.36 (n-hexane:EtOAc, 8:2); mp 119–120ꢂC (CH2Cl2:n-hexane, 7:3); IR (KBr): ꢁꢁ¼ 1761,
1725, 1543, 1484, 1401, 1250, 749 cmꢃ1
;
1H NMR (300MHz, CDCl3): ꢂ ¼ 0.79 (t, J ¼ 7.4 Hz,
CH3CH2), 1.75–2.01 (m, CH3CH2), 3.16 (ddd, J ¼ 22.5, 6.8, 4.4 Hz, H-4), 3.29 (ddd, J ¼ 22.5, 7.0,
3.3 Hz, H-4), 3.81 (s, 3H, CO2CH3), 3.79–3.87 (m, H-7), 7.04–7.07 (m, H-5), 7.26–7.30 (m, ArH), 7.30–
7.44 (m, 3ArH) ppm; 13C NMR (75.4 MHz, CDCl3): ꢂ ¼ 9.2 (CH3CH2), 24.1 (CH3CH2), 24.4 (C-4), 34.3
(C-7), 52.1 (CO2CH3), 117.4 (C-3a), 123.1 (ArH), 125.2 (ArH), 128.0 (ArH), 130.5 (ArH), 131.5 (C-6),
134.1 (C-5), 134.8, 134.9, 135.8, 154.2 (C-2), 166.0 (CO2CH3) ppm; MS (70 eV): m=z(%) ¼ 335 (Mþ,
32), 333 (Mþ, 90), 306 (13), 304 (40), 274 (25), 272 (39), 260 (63), 245 (56), 230 (46), 201 (38), 180 (97),
167 (53), 166 (87), 153 (30), 133 (30), 111 (78), 91 (58), 77 (76), 59 (100).
6-Carbomethoxy-7-ethyl-3-(m-tolyl)-4,7-dihydrobenzoxazol-2-one (10b, C18H21NO4)
According to method A, and after heating for 6 h, a mixture of 10b:11b (90:10) was separated by
column chromatography on silica gel (12 g, n-hexane:EtOAc, 90:10), affording 0.08 g (64%) of 10b
as a white solid. Rf ¼ 0.28 (n-hexane:EtOAc, 7:3); mp 95–96ꢂC (CH2Cl2:n-hexane, 7:3); IR (KBr):
1
ꢁꢁ¼ 1761, 1718, 1597, 1491, 1407, 1271, 1220, 744 cmꢃ1; H NMR (300 MHz, CDCl3): ꢂ ¼ 0.79 (t,
J ¼ 7.5 Hz, CH3CH2), 1.76–2.05 (m, CH3CH2), 2.40 (s, CH3Ar), 3.13 (ddd, J ¼ 22.5, 6.9, 4.3 Hz, H-4),
3.24 (ddd, J ¼ 22.5, 7.0, 3.2 Hz, H-4), 3.80 (s, CO2CH3), 3.83–3.95 (m, 1H, H-7), 7.05–7.12 (m, H-5),
7.10–7.42 (m, 4ArH) ppm; 13C NMR (75.4 MHz, CDCl3): ꢂ ¼ 10.1 (CH3CH2), 21.3 (CH3Ar), 23.9
(CH3CH2), 24.3 (C-4), 34.3 (C-7), 52.0 (CO2CH3), 117.8 (C-3a), 122.1 (ArH), 125.6 (ArH), 128.7
(ArH), 129.2 (ArH), 131.4 (C-6), 133.6 (Ar), 134.4 (C-5), 135.4, 139.6, 154.6 (C-2), 166.1 (CO2CH3)
ppm; MS (70 eV): m=z(%) ¼ 313 (Mþ, 62), 283 (48), 252 (69), 240 (40), 225 (40), 210 (26), 180 (72),
165 (20), 152 (10), 133 (30), 107 (28), 91 (100), 65 (88), 59 (68).
General Procedure for the Diels-Alder Reaction of Dienophile 12 with 2a and 5b
A mixture of 5.0 mmol of diene, 0.61g of 12 (2.6mmol), and 3 mg of hydroquinone in 1 cm3 of dry m-
xylene was placed in a threaded ACE glass pressure tube with a sealed Teflon screw cap, under N2 and

Diels-Alder Reactions of 2-Oxazolidinone Dienes
189
in the dark. The mixture was stirred and heated at 120ꢂC for 8.0 h. The mixture was filtered, the solvent
removed under vacuum, and the residue purified by column chromatography on silica gel (n-
hexane:EtOAc, 4:1) to give the corresponding adducts.
ꢀ
6-yl 4-nitrobenzoate (13a, C24H21ClN2O7)
ꢀ
(6R ,7R )-6-Acetyl-3-(4-chlorophenyl)-7-ethyl-2-oxo-2,3,4,5,6,7-hexahydrobenzoxazol-
Using the general procedure with 1.25g of 5b gave a mixture of 13a:14a (86:14) which was separated
and purified by column chromatography and recrystallization (CH2Cl2:n-hexane, 1:1), to yield 1.41g
(58%) of 13a as a white solid. Rf ¼ 0.27 (n-hexane:EtOAc, 7:3); mp 149–150ꢂC (CH2Cl2:n-hexane,
7:3); IR (KBr): ꢁꢁ¼ 1766, 1725, 1528, 1497, 1282, 1092, 830, 743 cmꢃ1; 1H NMR (300MHz, CDCl3):
ꢂ ¼ 1.30 (t, J ¼ 7.4 Hz, CH3CH2), 1.30–1.66 (m, CH3CH2), 2.00–2.20 (m, H-5), 2.25 (s, CH3CO),
2.30–2.50 (m, 2H-4), 2.74–2.90 (m, H-5), 3.05 (dd, J ¼ 9.2, 4.6 Hz, H-7), 7.20–7.44 (m, 4ArH), 8.12–
8.37 (m, 4ArH) ppm; 13C NMR (75.4 MHz, CDCl3): ꢂ ¼ 11.9 (CH3CH2), 17.4 (C-4), 22.4 (C-5), 24.0
(CH3CH2), 24.9 (CH3CO), 41.9 (C-7), 87.8 (C-6), 119.9 (C-3a), 123.9 (ArH), 126.2 (ArH), 129.7
(ArH), 130.9 (ArH), 132.1 (Ar), 133.8 (Ar), 133.9 (C-7a), 135.8 (Ar), 151.2 (Ar), 154.7 (C-2), 163.6
(ArCO2), 203.3 (CH3CO) ppm; MS (70 eV): m=z(%) ¼ 484 (Mþ, 2), 317 (100), 274 (55), 246 (31), 164
(6), 150 (23), 104 (18), 77 (16), 43 (78).
ꢀ
6-yl 4-nitrobenzoate (13b, C23H20N2O7)
ꢀ
(6R ,7R )-6-Acetyl-7-methyl-2-oxo-3-phenyl-2,3,4,5,6,7-hexahydrobenzoxazol-
Using the general procedure with 1.18 g of 2a gave a mixture of 13b:14b (80:20) which was separated
and purified by column chromatography and recrystallization (CH2Cl2:n-hexane, 7:3), to yield 1.13g
(60%) of 13b as a white solid. Rf ¼ 0.33 (n-hexane:EtOAc, 7:3); mp 145–146ꢂC; IR (KBr): ꢁꢁ¼ 1767,
1718, 1522, 1397, 1353, 1277, 1081, 715 cmꢃ1; 1H NMR (300MHz, CDCl3): ꢂ ¼ 1.16 (d, J ¼ 7.0 Hz,
CH3C-7), 2.20–2.45 (m, 2H-4, H-5), 2.25 (s, CH3CO), 2.70–2.84 (m, H-5), 3.25 (br q, J ¼ 7.0 Hz, H-
7), 7.28–7.53 (m, 5PhH), 8.20–8.42 (m, 4ArH) ppm; 13C NMR (75.4 MHz, CDCl3): ꢂ ¼ 14.9 (CH3C-
7), 17.1 (C-4), 21.8 (C-5), 24.8 (CH3CO), 34.9 (C-7), 88.0 (C-6), 119.4 (C-3a), 123.9 (ArH), 124.8
(PhH), 127.8 (PhH), 129.5 (PhH), 131.0 (ArH), 131.5 (Ph), 133.9 (C-7a), 135.8 (Ar), 151.1 (Ar), 154.3
(C-2), 163.7 (ArCO2), 203.1 (CH3CO) ppm; MS (70 eV): m=z(%) ¼ 436 (Mþ, 6), 269 (100), 254 (31),
226 (96), 201 (14), 150 (70), 120 (13), 77 (15).
3-Methyl-1-n-pentyl-3H-imidazol-1-ium bromide ([mpim][Br])
A stirred mixture of 2.05g of 1-methylimidazol (0.05 mol) and 4.53g of 1-bromopentane (0.03 mol)
was submitted to MW irradiation (100W) at 80ꢂC for 40 s. The mixture was washed with 2ꢅ10cm3
of Et2O and 2ꢅ10cm3 of EtOAc, and dried under vacuum at 80ꢂC for 4 h, giving 5.7 g (98%)
of [mpim][Br] as a colorless oily liquid. IR (CH2Cl2): ꢁꢁ¼ 3079, 2955, 2864, 1628, 1569, 1462,
1168 cmꢃ1
;
1H NMR (200MHz, CDCl3): ꢂ ¼ 0.68 (t, J ¼ 6.6 Hz, CH3(CH2)4N), 1.00–1.25 (m,
4CH3(CH2)4N), 1.61–1.80 (m, CH3(CH2)4N), 3.90 (s, CH3N), 4.12 (t, J ¼ 7.3 Hz, CH3(CH2)3CH2N),
7.37 (br s, 1imid-H), 7.49 (br s, 1imid-H), 9.90 (br s, 1imid-H) ppm; 13C NMR (75.4 MHz, CDCl3):
ꢂ ¼ 13.5 (CH3CH2)4N), 21.6 (CH3(CH2)4N), 27.8 (CH3(CH2)4N), 29.5 (CH3(CH2)4N), 36.2 (CH3),
49.5 (CH2N), 121.5 (imid-H), 123.1 (imid-H), 136.1 (imid-H); HRMS (FAB, Mþ-Br) (mNBA): calcd
for C9H17N2 153.1392, found 153.1386.
1-n-Pentylpyridinium bromide ([ppy][Br])
Following the method used for the preparation of [mpim][Br] 1.97g of pyridine (0.025mol), 4.53 g of
1-bromopentane (0.03 mol), and irradiating (100W) at 100ꢂC for 5 min gave 4.3 g (75%) of [ppy][Br]

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A. Fuentes et al.
as a colorless oily liquid. IR (CH2Cl2): ꢁꢁ¼ 3046, 2956, 2932, 2864, 1632, 1488, 1171, 776, 684 cmꢃ1
;
1H NMR (200 MHz, CDCl3): ꢂ ¼ 0.73 (t, J ¼ 6.6 Hz, CH3(CH2)4N), 1.12–1.32 (m, 4CH3(CH2)4N),
1.85–2.02 (m, CH3(CH2)4N), 4.80 (t, J ¼ 7.3 Hz, CH3(CH2)3CH2N), 8.07 (t, J ¼ 7.2 Hz, 2pyr-H),
8.38–8.53 (m, 1pyr-H), 9.30 (d, J ¼ 6.2 Hz, 2pyr-H) ppm; 13C NMR (75.4 MHz, CDCl3): ꢂ ¼ 13.7
(CH3CH2)4N, 21.9 (CH3(CH2)4N), 27.8 (CH3(CH2)4N), 31.4 (CH3(CH2)4N), 61.6 (CH2N), 128.1
(pyr-H), 144.4 (pyr-H), 144.9 (pyr-H) ppm; HRMS (FAB, Mþ-Br) (mNBA): calcd for C10H16N
150.1283, found 150.1276.
Single-Crystal X-ray Crystallography
A single-crystal of adduct 10b was obtained by recrystallization from CH2Cl2:n-hexane (7:3) as white
crystals. These were mounted in glass fibers. Crystallographic measurements were performed on a
Table 5. Crystallographic data of compound 10b
Crystal
10b
Empirical formula
Formula mass
Colour
C18H19NO4
313.34
white, prism
0.44 ꢅ 0.50 ꢅ 0.56
Monoclinic
P21=c
Crystal dimensions=mm
Crystal system
Space group
Z
4
˚
a=A
˚
b=A
6.9818 (7)
10.3225 (9)
22.655 (2)
90
˚
c=A
ꢀ=ꢂ
ꢃ=ꢂ
90.961 (8)
90
ꢄ=ꢂ
Collection ranges
ꢃ1 ¼ h ¼ 8; ꢃ1 ¼ k ¼ 12;
ꢃ26 ¼ l ¼ 26
293 (2)
Temperature=K
3
˚
Volume=A
Dcalcd=Mg mꢃ3
1632.5 (3)
1.275
˚
Radiation
Mo Kꢀ (ꢂ ¼ 71073 A)
Absorption coeff. (ꢅ)=mmꢃ1
Absorption correction
F (000)
0.090
None
664
ꢆ range for data collection=ꢂ
Observed reflections
Independent reflections
Data=restraints=parameters
Maximum shift=error
Goodness-of-fit on F2
Final R indices [I>2ꢇ(I)]
R indices (all data)
1.80 to 25.00
2091
2873 (Rint ¼ 0.0157)
2835=0=211
0.00
1.021
R1 ¼ 0.0436, wR2 ¼ 0.1066
R1 ¼ 0.0653, wR2 ¼ 0.1266
N=a
Absolute structure parameter
Extinction coefficient
˚
Largest fidd. peak and hole=e A
N=a
0.208 and ꢃ0.148
ꢃ3

Diels-Alder Reactions of 2-Oxazolidinone Dienes
191
˚
Siemens P4 diffractometer using Mo Kꢀ radiation (graphite crystal monochromator, ꢈ ¼ 0.71073 A),
and at room temperature. Three standard reflections were monitored periodically; they showed no
change during data collection. Unit cell parameters were obtained from least-squares refinement of 26
reflections in the range 2<2Y<20ꢂ. Intensities were corrected for Lorentz and polarization effects. No
absorption correction was applied. Anisotropic temperature factors were introduced for all non-hydro-
gen atoms. Hydrogen atoms were placed in idealized positions and their atomic coordinates refined.
Unit weights were used in the refinement. Structures were solved using the SHELXTL [29] program on
a personal computer. Data for 10b are summarized in Table 5. Further crystallographic data (excluding
structure factors) have been deposited with the Cambridge Crystallographic Data Centre, as supple-
mentary publication number CCDC-232603. Copies of the data can be obtained, free of charge, on
application to the CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK [fax: þ44-(0)1223-336 033 or
e-mail: deposit@ccdc.cam.ac.uk].
Acknowledgments
We thank F. Labarrios for his help in spectrometric measurements. J.T. is grateful to CGPI=IPN
(Grants 916510, 200410, 20030147, and 20040123) and CONACYT (Grant 1203-E9203 and
43508-Q) for financial support. R.M. thanks CGPI=IPN and PIFI=IPN for scholarships, and the Ludwig
K. Hellweg Foundation for a complementary scholarship. H.A.J.V. acknowledges support from
ꢀ
CONACYT (Grant 3251P), and thanks IPN’s Programa de Contratacion de Personal de Excelencia.
H.A.J.V., F.D., and J.T. are fellows of the EDI=IPN and COFAA=IPN programs.
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