Push-pull alkenes from cyclic ketene-N,N′-acetals: a wide span of double bond lengths and twist angles

Article abstract of DOI:10.1016/j.tet.2010.02.071

Push-pull alkenes can be quickly accessed by cyclic ketene-N,N′-acetal chemistry. A number of push-pull structures with a wide span of double bond lengths and twist angles were synthesized from the reactions of (1) N,N′-dimethyl cyclic ketene-N,N′-acetals with isocyanates, (2) the products from (1) with isocyanates, (3) 2-methylimidazoline and 2-methyl-1,4,5,6-tetrahydropyrimidine with diacid chlorides, (4) 2-methylimidazoline, and 1,2-dimethyl-1,4,5,6-tetrahydropyrimidine with benzoyl chlorides, and (5) 1,2-dimethylimidazoline and 1,2-dimethyl-1,4,5,6-tetrahydropyrimidine with aryl isocyanates. These reactions proceed under very mild conditions and give moderate to excellent yields. X-ray crystallographic analysis of eight pxush-pull alkenes indicates that the central double bond lengths and twists are sensitive to the ring sizes (5 or 6), ring structures (fused or non-fused), electron donating and withdrawing strengths of pushing and pulling portions, respectively, number of electron pushing or pulling groups and substituent steric effects.

Full text of DOI:10.1016/j.tet.2010.02.071

Tetrahedron 66 (2010) 2919–2927
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Push–pull alkenes from cyclic ketene-N,N0-acetals: a wide span of double bond
lengths and twist angles
Guozhong Ye a, Sabornie Chatterjee a, Min Li b, Aihua Zhou c, Yingquan Song a, Bobby Lloyd Barker a,
,
Chunlong Chen d, Debbie J. Beard a, William P. Henry a, Charles U. Pittman a
*
a Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA
b Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA
c Pharmacy School, University of Wisconsin-Madison, Madison, WI 53706, USA
d Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA
a r t i c l e i n f o
a b s t r a c t
Push–pull alkenes can be quickly accessed by cyclic ketene-N,N0-acetal chemistry. A number of push–pull
structures with a wide span of double bond lengths and twist angles were synthesized from the reactions
of (1) N,N0-dimethyl cyclic ketene-N,N0-acetals with isocyanates, (2) the products from (1) with iso-
cyanates, (3) 2-methylimidazoline and 2-methyl-1,4,5,6-tetrahydropyrimidine with diacid chlorides, (4)
2-methylimidazoline, and 1,2-dimethyl-1,4,5,6-tetrahydropyrimidine with benzoyl chlorides, and (5) 1,2-
dimethylimidazoline and 1,2-dimethyl-1,4,5,6-tetrahydropyrimidine with aryl isocyanates. These
reactions proceed under very mild conditions and give moderate to excellent yields. X-ray crystallo-
graphic analysis of eight pxush–pull alkenes indicates that the central double bond lengths and twists
are sensitive to the ring sizes (5 or 6), ring structures (fused or non-fused), electron donating and
withdrawing strengths of pushing and pulling portions, respectively, number of electron pushing or
pulling groups and substituent steric effects.
Article history:
Received 8 December 2009
Received in revised form 8 February 2010
Accepted 19 February 2010
Available online 25 February 2010
Keywords:
Ketene acetal
Push–pull alkene
Double bond elongation
Double bond twist
X-ray crystallography
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
from the push–pull effect, which reduces the double bond order by
intramolecular charge transfer.
Push–pull alkenes are substituted alkenes with electron-
donating groups (push) and electron-withdrawing groups (pull)
These unusual effects stimulated extensive research on push–
pull alkenes including their synthesis, characterization, and appli-
cations7–19 along with theoretical investigations to understand,
quantify, and predict these effects.20–27
at both ends of the alkene double bond (D–p-A). Intramolecular
charge transfer in the D– -A framework occurs with profound
p
consequences on the molecular structure and properties. These
include central double bond elongation,1 facile out-of-plane rota-
tion,2 strong charge-transfer absorption bands,3 large dipole mo-
ments,4 huge 13C chemical shift differences5 between the alkene
carbons and high hyperpolarizabilities, which are prerequisite for
organic nonlinear optical materials.6 For an example, the lowered
barrier to rotation about the central double bond (Scheme 1) results
Ketene acetals (Scheme 2) are an important class of neutral
carbon nucleophiles. They combine two electron-donating groups
(N, O, or S) at one end of a double bond. The lone-pair electrons on
both heteroatoms delocalize to the C]C bond. As result, the double
bond is polarized and exhibits significant nucleophilicity at the b-
carbon. Ketene acetal chemistry has been increasingly studied and
applied in organic synthesis28,29 and polymer synthesis30 since the
pioneering work by McElvain and co-workers in 1930–1940s.31
Scheme 1. LoweringofthebarriertorotationaboutC]Cbondduetothepush–pulleffect.
Scheme 2. Basic structure of ketene acetals and origin of their nucleophilicity.
Ketene acetal reactivities vary with many structural factors. The
selection of heteroatoms is central to the ketene acetal’s nucleo-
philicity.32 Cyclic ketene-N,N0-acetals are more reactive than their
* Corresponding author. Tel.: þ1 662 325 7616; fax: þ1 662 325 7611.
E-mail address: cpittman@chemistry.msstate.edu (C.U. Pittman).
0040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.02.071

2920
G. Ye et al. / Tetrahedron 66 (2010) 2919–2927
acyclic analogs.33 The reactivity of cyclic ketene-O,O-acetals in
cationic 1,2-vinyl addition polymerizations is modulated by sub-
stituent effects.30a–c Finally, the combination of substituent and
ring-size effects allow diverse reaction paths to exist from in situ
generated ketene-N,N0-acetals.34
Table 1
Reactions of N,N0-dimethyl cyclic ketene-N,N0-acetals with isocyanates
Ketene acetals are ideal building blocks for push–pull alkenes
because pushing function (D) and nucleophilic double bond (p) are
already present in their structures. Moreover, ketene acetals’ tun-
able nucleophilicity also makes them appealing for synthesizing
push–pull alkenes. Ideally, a large pool of push–pull alkenes with
various magnitudes of push–pull activity can be accessed by al-
tering the heteroatoms (six combinations available from N, O, and
S), varying substituents, selecting cyclic/acyclic structures, and
changing ring-size of ketene acetals. However, studies of such cyclic
ketene acetal (CKA) conversions are sparse due to their handling
difficulties.35
Substrate
Isocyanate
Product
Yield (%)a
90
Pittman et al. reported the reactions of cyclic ketene-N,X-acetals
(X¼O,S) with aryl isocyanates and isothiocyanates.36 Push–pull
alkenes were synthesized in excellent yields at very mild condi-
tions (Scheme 3). In this report, the conversion of N,N0-dimethyl
cyclic ketene-N,N0-acetals 1 and 2 (Scheme 4) to push–pull alkenes
via reactions with isocyanates will be demonstrated. The push–pull
alkene structures from cyclic ketene-N,N0-acetals generated in situ
from cyclic amidines 3–6 will also be discussed. The detailed syn-
theses of starting materials 1–6 can be found in our previous
reports.34b,35
93
94
95
84
81
89
93
Scheme 3. push–pull alkenes from cyclic ketene-N,X-acetals (X¼O, S).
Scheme 4. Substrates for push–pull alkene synthesis.
2. Results and discussion
Cyclic ketene-N,N-acetals 1 and 2 readily react with two
equivalents of isocyanates in THF at rt (Table 1). After stirring for
3 h, tetrasubstituted push–pull alkenes 7a–d and 8a–d were iso-
lated simply by pouring the reaction mixture into hexane. Excellent
yields were achieved for most of the examples (Table 1). The
electron-pushing five or six-membered cyclic ketene-N,N0-acetal
moieties interact directly with the two amide groups in these
products.
A mechanism in accord with the formation of 7a is shown in
a
Yields are based on substrate 1 or 2.
Scheme 5. Nucleophilic attack of the
equivalent of phenyl isocyanate gives zwitterion 9, which rapidly
transfers the acidic hydrogen from the -carbon to produce mono-
b-carbon in 1 on the first
b
adduct 10 with regeneration of ketene acetal double bond.
Compound 10 then reacts with the second equivalent of phenyl
isocyanate to generate zwitterion 11. Proton transfer in 11 finally
gives structure 7a.
The isolated products 7a–d and 8a–d were further treated with
excess isocyanate to probe the reactivities of these tetrasubstituted
polarized alkenes (Table 2). Trisisocyanate adducts 12a and 12b
were obtained from the six-membered systems 8a and 8b, re-
spectively. However, 7a–d and 8c–d did not form analogous tri-
sadducts using the isocyanates examined herein.
The single crystal structure of 7d was determined and shown in
Figure 1. A remarkable feature of this structure is the elongated
central carbon–carbon double bond (C14–C16: 1.45 Å), which is
substantially longer than the normal carbon–carbon double bond

G. Ye et al. / Tetrahedron 66 (2010) 2919–2927
2921
Ph
O
The short enamine carbon–nitrogen bonds (N1–C16: 1.33 Å;
N2–C16: 1.32 Å) and the near planar configurations (vs pyramidal)
of N1 and N2 suggest N1–C16 and N2–C16 bonds have consider-
able double bond character. N1 and N2 are sp2-hybridized. An
intramolecular hydrogen-bond exists in the crystal (O1/H–N3),
which further stabilizes the observed molecular geometry in the
crystal.
The crystal structure of 8a was also obtained (Fig. 2). Its central
carbon–carbon double bond (C8–C16: 1.47 Å) is slightly longer
(w0.02 Å) than that in 7d. The length of the central double bond
was demonstrated to be a reliable parameter for quantifying push–
pull effect in push–pull alkenes based on NBO analysis (the longer
the double bond, the stronger the push–pull effect).25 The longer
C]C bond length of 8a versus 7d helps to explain why the push–
pull alkenes containing six-membered cyclic ketene-N,N0-acetal
donors (8a and 8b) each underwent a further reaction with iso-
cyanates (phenyl and p-tolyl, respectively), while those having five-
membered cyclic ketene-N,N0-acetal donors (7a and 7b) did not.
Ph
O
NH
C
N
C
O
O
Ph
C
H
C
N
Ph
H
N
N
N
N
N
N
N
1
9
10
Ph
O
Ph
O
Ph
O
H
NH
N
N
N
O
N
H
C
Ph
N
H
N
N
11
7a
Scheme 5. A mechanism consistent with the formation of push–pull alkene 7a.
Table 2
Reactions of push–pull alkenes with isocyanates
Substrate
Ra
Product
Yield(%)
7a
7b
7c
7d
8a
8b
8c
8d
Ph
No reaction
No reaction
No reaction
No reaction
12a
12b
No reaction
No reaction
d
d
d
d
32
37
d
d
p-CH3C6H4
p-MeOC6H4
p-FC6H4
Ph
p-CH3C6H4
CH2]CH–CH2–
t-Bu
a
The R group in push–pull reagent alkenes and R group in isocyanates were the
same for each reaction reported here.
of ethylene (1.34 Å).37 In addition to the longer central carbon–
carbon double bond, a concomitant twisting of this double bond
occurs (twist angle38 56.72ꢀ). This is the synergistic outcome of (1)
Figure 2. Crystal structure of 8a. Disordered CHCl3 molecules are present and not
shown for clarity. CCDC: 719324
steric repulsion between the N-substituents and the
b-carbon-
substituents, and (2) the push–pull effect, which reduces the
C14–C16 bond order, making it easier to twist out-of-plane. The
extent of twisting reflects a compromise between the relaxation of
the steric hindrance (energy drop) and the deviation of the alkene
substituents from the ideal co-planar geometry (energy rise).
The central carbon–carbon double bond in 8a has a larger twist
angle (67.44ꢀ) than that in 7d (56.72ꢀ). This enhanced twist is due
to stronger steric repulsions between donor and accepter portions
in 8a versus that in 7d. This larger twist angle is closer to the to-
tally zwitterionic geometry (90ꢀ twist, Scheme 6). Thus, the
minimum energy geometry, 8a, has more zwitterionic character
than the five-membered analog. In other words, the acceptor
portion of a six-membered push–pull alkene is more negatively
charged and is more nucleophilic than the acceptor portions of the
five-membered analog. This explains the higher reactivities of 8a
and 8b toward adding a third equivalent of isocyanate versus 7a
and 7b.
O
O
O
O
O
O
RHN
NHR
RHN
NHR
RHN
NR
N
N
N
N
N
N
VS
( )n
( )n
A
( )n
B
C
Scheme 6. The dependence of the charge separation on the geometry of a push–pull
alkene. A: resonance structure showing no charge separation; B: resonance structure
showing partial charge separation and a partial central double bond (central double
bond may be twisted depending on steric factors); C: structure showing full charge
separation in the 90ꢀ twisted geometry where the central bond is actually a single
Figure 1. Crystal structure of 7d. A water molecule is not shown for clarity. The
thermal ellipsoids are drawn with 50% probability. CCDC: 689417.
bond because no conjugation (
portions.
p-bonding) is possible between donor and acceptor

2922
G. Ye et al. / Tetrahedron 66 (2010) 2919–2927
The crystal structure of the trisadduct 12a was determined
is also less twisted (twist angle: 25.34ꢀ) versus those of 7d (56.72ꢀ),
8a (67.44ꢀ), and 12a (75.52ꢀ) because one N-methyl group is
replaced by oxygen atom. This dramatically reduced the magnitude
of steric repulsions.
(Fig. 3). Its central carbon–carbon double bond length (C8–C9:
1.47 Å) is comparable with that in 8a, but this double bond is fur-
ther twisted (twist angle: 75.52ꢀ) compared to 8a due to 12a0s
larger acceptor groups. Two intramolecular hydrogen-bonds exist
in the 12a crystal structure (O3/H-N4 and O3/H-N1).
We explored the possibility of forming push–pull structures
from in situ generated N-acyl cyclic ketene-N,N0-acetals. 1,8-Naph-
thyridinetetraones were synthesized from 2-methylimidazoline 3
and 2-methyl-1,4,5,6-tetrahydropyrimidine 4 via cyclic ketene-
N,N0-acetal intermediates (Scheme 7) and reported previously.39
The diverse reactivities of in situ generated ketene acetals were
further demonstrated by reactions of 3–6 with aroyl chlorides
(Scheme 8).34 Structures 14,15, 20, and 23 should also exhibit push–
pull character based on their ketene acetal precursors’ nucleophi-
licity. The carbonyl groups present on the ring nitrogens in 14,15, 20,
and 23 weaken the electron-donating capability in these com-
pounds versus those in 7, 8, and 12. Since one crystal structure is
available for each type of these products, a direct comparison of
these weaker push–pull alkenes with the preceding stronger ones
can be made at this point.
O
O
OO
O
O
O
O
R
R
Cl
Cl
N
R
R
R
R
Cl
Cl
R
R
O
R
R
N
NH
( )
1, 2
Et3N, MeCN
N
O
N
N
)
( )
(
1, 2
1, 2
3
4
or
14
, n=1
15, n=2
Scheme 7. Synthesis of 1,8-naphthyridinetetraones.
Figure 3. Crystal structure of 12a. The thermal ellipsoids are drawn with 50% proba-
bility. CCDC: 689418.
O
The five-membered ring N,O-analog, 13 (Fig. 4), was also syn-
thesized according to a literature method.36 The crystal structure of
13 was determined in order to compare with those of 7d, 8a, and
12a. The central double bond length in 13 (C6–C7: 1.39 Å) is con-
siderably shorter than those in 7d (1.45 Å), 8a (1.47 Å), and 12a
(1.47 Å). This was attributed to the weaker electron-donating ability
of the ketene-N,O-acetal versus the stronger donation by ketene-
N,N0-acetals. This agrees with a recent computational analysis of
ketene-N,N0- versus -N,O- versus -N,S- versus -O,O- versus -S,S-
versus -O,S-acetals.32 The central carbon–carbon double bond of 13
O
O
Ar
O
O
H
N
Ar
Ar
N
N
Ar
Ar
N
O
21
22
4
)
5
(from )
(from
Ar
R3
N
Ar
O
O
Ar
O
Ar
O
O
O
R1
R2
N
N
N
N
N
(
)
n
Ar
Ar
16
3
)
(n=1, R1=R2=ArCO, R3=H) (from
17 (n=1, R1=R3=ArCO, R2=Me) (from 4)
18
20
(from 3)
23
(from 6)
5
)
(n=2, R1=R2=ArCO, R3=H) (from
19 (n=2, R1=R3=ArCO, R2=Me) (from 6)
ArCOCl
Et3N, THF
3, n=1, R=H
4
R
, n=1, R=Me
5, n=2, R=H
6, n=2, R=Me
N
N
(
)
n
Scheme 8. Diverse reactivities of cyclic ketene-N,N0-acetals generated in situ by
reactions of cyclic amidines with aroyl chlorides.
The structure of ketene acetal 22a (not a push–pull species)
(Fig. 5) serves as good reference for the push–pull structures of 20a
(Ar¼Ph) (Fig. 6), 23a (Ar¼Ph) (Fig. 7), and 14a (R¼Pr) (Fig. 8). The
C6–C21 double bond in 22a (Ar¼m-MeOPh) (Fig. 5) is 1.33 Å, which
is a normal C]C double bond length.
The two N-benzoyl groups in 20a (Fig. 6) diminished the
pushing ability of two nitrogen atoms. Furthermore, only one
pulling function (phenyl keto group on C7) is present in 20a. Thus,
a weak push–pull activity is expected. The C6–C7 (1.35 Å) length in
20a is only slightly longer than the corresponding double bond in
22a. The twist angle is smaller (14.63ꢀ) compared to preceding
stronger push–pull alkenes.
Figure 4. Crystal structure of 13. The thermal ellipsoids are drawn with 50% proba-
bility. CCDC: 719325

G. Ye et al. / Tetrahedron 66 (2010) 2919–2927
2923
Figure 5. Crystal structure of 22a. CCDC: 753288.
Figure 8. Crystal structure of 14a. CCDC: 684834.
A single crystal of 14a (R¼Pr) was obtained by slow evaporation
of pentane into a THF solution of 14a. Two independent molecules
of 14a with associated THF molecules were found in the crystal
structure (See Supplementary data: 14a.cif), one of which is num-
bered and shown in Figure 8. Some important bond lengths are also
shown for convenient comparison. The amide carbon–nitrogen
bonds (C3–N3: 1.40 Å, C7–N4: 1.38 Å) are longer than a normal
value of 1.32 Å.40 The central C]C bonds (C6–C10:1.38 Å) is longer
than 1.33 Å in 22a. The enamine carbon–nitrogen single bonds
(C6–N3:1.36 Å, C6–N4:1.36 Å) have considerable double bond
character. The b-keto carbon–carbon single bonds (C5–C10: 1.44 Å,
C9–C10: 1.45 Å) also have significant double bond character, and
they are even shorter than corresponding C1–C2 single bond in
acrolein (1.47 Å).20,41 The near-perfect coplanarity of the three
fused rings indicates a well conjugated
p-bond system exists
among the amide and methylenedione functions. The C6–C10 bond
twist is very small (0.90ꢀ). The elongation of amide carbon–nitro-
gen bonds and the shortening of
b-keto carbon–carbon single
bonds, indicate a considerable amount of nitrogen lone-pair
electron density is withdrawn from the amide moiety into the
methylenedione moiety. In other words, 14a exhibits significant
push–pull electronic character.
Figure 6. Crystal structure of 20a. CCDC: 753287.
In contrast, push–pull alkene 23a (Fig. 7) has a much longer and
more twisted C]C bond (C8–C16: 1.42 Å; twist angle: 39.54ꢀ) than
that in 20a. The replacement of N-benzoyl group with N-methyl
group (which is a stronger pushing function) and an additional
Finally, we investigated the reactions of amidines 3–6 with
isocyanates hoping to produce another type of push–pull systems.
A literature search showed that 2-methylimidazoline 3 reacts with
aryl isocyanates to give pyrimidinedione42 structure 25 (Scheme 9)
presumably through push–pull intermediate 24. However, no
crystal data was ever obtained for 24 or 25. We herein extend this
chemistry to 1,2-dimethylimidazoline 5 and their six-membered
analogs 4 and 6 in order to (1) explore the scope of this pyr-
imidinedione formation reaction and (2) investigate push–pull
character in the pyrimidinedione products.
pulling function (phenyl keto group on the
b-carbon) enhanced
23a’s push–pull activity compared to 20a. Both the phenyl groups
attached to C7 and C9 are pointing away from the hexahydropyr-
imidine ring to avoid steric hindrance with hexahydropyrimidine
ring’s N-substituents.
Ar
HN
Ar
HN
O
O
Ar
HN
Ar
O
N
ArNCO
THF
O
O
N
NH
- ArNH2
N
N
Ar
O
NH
NH
N
3
H
R=Ph, p-tolyl, 1-naphthyl
25
24
Scheme 9. Reactions of 2-methylimidazoline 3 with aryl isocyanates.
Compound 5 and 6 each reacted with an excess of several
different aryl isocyanates in refluxing acetonitrile for 12 h. Pyr-
imidinedione derivatives 26a–h were isolated in moderate yields
(Table 3). These reactions are similar to those of 3 as shown in
Scheme 9. Unexpectedly, no reaction took place (monitored by
TLC) when 2-methyl-1,4,5,6-tetrahydropyrimidine
4
was
Figure 7. Crystal structure of 23a. CCDC: 682487.
treated with phenyl isocyanate, 4-methoxyphenyl isocyanate or

2924
G. Ye et al. / Tetrahedron 66 (2010) 2919–2927
Table 3
Table 3 (continued )
Reactions of 4–6 with isocyanates
Entry Substrate Isocyanate
Product
Yield (%)
Entry Substrate Isocyanate
Product
Yield (%)
H
NCO
O
N
N
NCO
N
N
NH
NH
1
2
No reaction
d
N
O
N
N
10
74
O
N
4
6
NCO
26f
No reaction
No reaction
d
O
4
O
H
OMe
NCO
O
N
N
N
NCO
N
N
O
N
N
N
N
11
63
O
O
N
N
NH
NH
3
4
d
6
OMe
NCO
26g
4
F
F
NCO
F
H
O
N
N
Complex mixture
d
12
76
O
N
4
6
NO2
F
26h
H
O
N
N
N
4-fluorophenyl isocyanate under the same conditions (Table 3,
entries 1–3). The reaction of 4 with the more electrophilic iso-
cyanate, 4-nitrophenyl isocyanate, gave a very complex mixture
with many spots on the TLC plate (entry 4). No major product is
formed and thus no further isolation was performed. Since 22 was
formed from two ArCOCl equivalents and 4 when aroyl chlorides
were used as the electrophile, the lack of reactivity of 4 with aryl
isocyanates is puzzling.
Crystals of 26e were obtained by slow evaporization of pentane
into a chloroform solution of 26e. The crystal structure was then
obtained and it is shown in Figure 9 with selected bond lengths. 26e
shares several characteristics with the crystal structure of 14a
(Fig. 8): (1) The amide carbon–nitrogen bonds (N3–C21: 1.40 Å, N3–
NCO
N
N
O
O
N
N
N
5
65
O
O
5
26a
H
O
N
N
N
NCO
N
N
6
70
5
26b
O
O
H
NCO
OMe
O
N
N
N
O
N
7
8
9
62
73
71
O
N
5
26c
F
F
H
NCO
F
O
N
N
N
N
O
N
N
N
O
5
26d
H
O
N
N
NCO
N
O
N
O
N
6
26e
Figure 9. Crystal structure of 26e. CCDC: 755872.

G. Ye et al. / Tetrahedron 66 (2010) 2919–2927
2925
C9: 1.38 Å, N2–C9: 1.38 Å, N4–C7: 1.36 Å) are longer than a normal
amide carbon–nitrogen value of 1.32 Å;40 (2) the carbon–carbon
double bond (C8–C10: 1.39 Å) is longer than 1.34 Å in ethylene;37
(3) C10–N2 (1.38 Å) and C10–N1 (1.35 Å) have considerable dou-
ble bond character. These bond length changes, especially the
elongation of the central carbon–carbon double bond (C8–C10),
indicate that 26e has significant push–pull electronic character. The
reduced C8–C10 double bond character facilitates the observed
twisting of the C8–C10 central double bond (twist angle: 19.11ꢀ) by
the repulsion between O3 and the N1-methyl group.
ylidene)-N1,N3-diphenylmalonamide 7a (315 mg, 90%). Compound
7a can be further purified by flash chromatography (silica gel, hex-
ane–acetone, 1:1/pure acetone). Compounds 7b–d, 8a–d were
similarly prepared.
4.2.1. 2-(1,3-Dimethylimidazolidin-2-ylidene)-N1,N3-diphenylmalo-
namide (7a). Yield 90%; Rf¼0.29 (acetone); white solid; mp 206–
208 ꢀC; 1H NMR (300 MHz, CDCl3):
d (ppm) 11.63 (br, 1H), 7.50–6.93
(m,10H), 3.29 (s, 4H), 2.88 (s, 6H). Only one NH peak can be located;
13C NMR (75 MHz, CDCl3):
d (ppm) 168.9, 166.4, 140.2, 128.5, 121.8,
The pyrimidinedione ring (N2–C9–N3–C21–C8–C10) takes on
a roughly planar geometry, indicating good conjugation exists in
the ring system. The repulsion between O3 and N1-methyl group
119.5, 71.1, 48.3, 35.6; IR (KBr, cmꢁ1): 3306, 3025, 2921, 1633, 1494,
1428; HRMS (ESI, [MþNa]þ): calcd for C20H22N4NaO2, 373.1640;
found, 373.1626.
turns the exocyclic b-keto group (C7–O3) out of the optimal planar
geometry for conjugation with pyrimidinedione plane (O3–C7–
C8–C10: 21.95ꢀ). Therefore, the exocyclic carbon–carbon single
bond (C7–C8: 1.48 Å) is longer than the endocyclic carbon–carbon
(C21–C8: 1.43 Å). The N4-phenyl group points away from the
fused bicycle’s structure to (1) avoid the steric hindrance with
N1-methyl group and C21–O2 carbonyl group and to (2) allow
intramolecular hydrogen-bonding between O2 and N4-hydrogen
(O2–H6: 1.94 Å).
4.2.2. 2-(1,3-Dimethylimidazolidin-2-ylidene)-N1,N3-di-p-tolylmalo-
namide (7b). Yield 93%; Rf¼0.30 (acetone); white solid; mp 203–
205 ꢀC; 1H NMR (300 MHz, CDCl3):
d (ppm) 11.35 (br, 1H), 7.34–7.04
(m, 8H), 3.44 (s, 4H), 2.95 (s, 6H), 2.28 (s, 6H). Only one NH peak can
be located; 13C NMR (75 MHz, CDCl3):
d (ppm) 169.5, 166.3, 137.6,
129.1, 119.9, 71.0, 48.6, 36.1, 20.6; IR (KBr, cmꢁ1): 3270, 2919, 1638,
1614, 1587, 1563, 1513, 1486; HRMS (ESI, [MþNa]þ): calcd for
C22H26N4NaO2, 401.1953; found, 401.1942.
3. Conclusions
4.2.3. 2-(1,3-Dimethylimidazolidin-2-ylidene)-N1,N3-bis(4-methoxy-
phenyl)malonamide (7c). Yield 94%; Rf¼0.17 (acetone); white solid;
A variety of push–pull alkene structures were obtained from
cyclic ketene-N,N0-acetals or their in situ generated analogs. A wide
range of push–pull double bond lengths and twist angles is acces-
sible by varying ring sizes (5 or 6), ring structures (fused or non-
fused), electron donating or withdrawing strength of substituents,
number of electron pushing or pulling units and substituent steric
effects. The push–pull activity can be controlled by a number of
tunable factors synthetically accessible through cyclic ketene-
N,N0-acetal chemistry.
mp 194–196 ꢀC; 1H NMR (300 MHz, CDCl3):
d (ppm) 11.53 (br, 1H),
7.38–6.80 (m, 8H), 3.75 (s, 6H), 3.43 (s, 4H), 2.95 (s, 6H). Only one
NH peak can be located; 13C NMR (75 MHz, CDCl3):
d (ppm) 169.4,
166.4, 154.9, 133.4, 121.5, 113.8, 70.5, 55.4, 48.5, 36.0; IR (KBr, cmꢁ1):
3285, 2951,1634,1588,1557,1509,1471; HRMS (ESI, [MþH]þ): calcd
for C22H26N4O4, 411.2032; found, 411.2031.
4.2.4. 2-(1,3-Dimethylimidazolidin-2-ylidene)-N1,N3-bis(4-fluo-
rophenyl)malonamide (7d). Yield 95%; Rf¼0.28 (acetone); white
solid; mp 170–172 ꢀC; 1H NMR (300 MHz, CDCl3):
d (ppm) 11.80 (br,
4. Experimental
1H), 7.43–6.93 (m, 8H), 3.60 (s, 4H), 3.02 (s, 6H). Only one NH peak
exists; 13C NMR (75 MHz, CDCl3):
d (ppm) 169.6, 166.2, 158.3 (d,
4.1. General methods
1JCF¼239.1 Hz), 136.0, 115.2 (d, 2JCF¼22.1 Hz), 121.4, 70.5, 48.7, 36.3;
IR (KBr, cmꢁ1): 3366, 3278, 2933, 1615, 1593, 1615, 1593, 1573, 1503;
HRMS (ESI, [MþH]þ): calcd for C20H21F2N4O2, 387.1633; found,
387.1633.
Melting points were recorded with a Mel-Temp apparatus and
were uncorrected. The FTIR spectra were recorded on a Thermo
Nicolet spectrometer as films on KBr plates. The 1H and 13C NMR
spectra were recorded using a Bruker model AMX-300 300 MHz
spectrometer operating at 300 MHz for proton and 75 MHz for
carbon. Chemical shifts were reported in parts per million down-
field from Me4Si, which was used as the internal standard for all
NMR spectra. Splitting patterns are designed as ‘s, d, t, q, and m’;
these symbols indicate ‘singlet, doublet, triplet, quartet, and mul-
tiplet’, respectively. All reactions were carried out under nitrogen.
Tetrahydrofuran (THF) was distilled from Na/benzophenone under
nitrogen. Acetonitrile and triethylamine were distilled from cal-
cium hydride under nitrogen. All other commercially obtained re-
agents were used as received. The silica gel used for the column
chromatography was purchased from Aldrich Company (70–
230 mesh, 60 Å).
4.2.5. 2-(1,3-Dimethyltetrahydropyrimidin-2(1H)-ylidene)-N1,N3-di-
phenylmalonamide (8a). Yield 84%; Rf¼0.20 (acetone); white solid;
mp 193–195 ꢀC; 1H NMR (300 MHz, CDCl3):
d (ppm) 11.55 (br, 1H),
7.61–6.97 (m, 10H), 6.07 (br, 1H), 3.38 (br, 4H), 3.28 (s, 6H), 2.02 (br,
2H); 13C NMR (75 MHz, CDCl3):
d (ppm) 166.1, 165.8, 140.5, 128.6,
119.6, 112.1, 75.9, 47.9, 41.9, 20.7; IR (KBr, cmꢁ1): 3289, 3053, 2926,
1609, 1591, 1573, 1486, 1428; HRMS (ESI, [MþNa]þ): calcd for
C21H24N4NaO2, 387.1797; found, 387.1790.
4.2.6. 2-(1,3-Dimethyltetrahydropyrimidin-2(1H)-ylidene)-N1,N3-di-
p-tolylmalonamide (8b). Yield 81%; Rf¼0.22 (acetone); white solid;
mp 183–185 ꢀC; 1H NMR (300 MHz, CDCl3):
d (ppm) 11.51 (br, 1H),
7.49–7.03 (m, 8H), 6.25 (br, 1H), 3.30 (br, 4H), 3.18 (s, 6H), 2.27 (s,
6H), 1.87 (br, 2H); 13C NMR (75 MHz, CDCl3):
d (ppm) 165.8, 165.8,
4.2. General procedure for synthesis of push–pull structures
7a–d and 8a–d
137.5, 128.9, 119.5, 75.6, 47.6, 41.6, 20.5; IR (KBr, cmꢁ1): 3284, 3206,
2922, 1621, 1590, 1573, 1504; HRMS (ESI, [MþNa]þ): calcd for
C23H28N4NaO2, 415.2110; found, 415.1945.
Phenyl isocyanate (0.24 g, 2 mmol) was added dropwise into
a stirred solution of 1,3-dimethyl-2-methyleneimidazolidine 1
(0.112 g, 1 mmol) in 10 mL of THF under nitrogen at rt. The reaction
mixture was stirred at rt under nitrogen for 3 h and then poured into
n-hexane (80 mL) to precipitate the product. The precipitates were
filtered, washed with dry ethyl acetate and dried at 80 ꢀC/2 mmHg.
The product was identified as 2-(1,3-dimethylimidazolidin-2-
4.2.7. N1,N3-Diallyl-2-(1,3-dimethyltetrahydropyrimidin-2(1H)-yli-
dene)malonamide (8c). Yield 89%; Rf¼0.25 (AcOEt–MeOH, 2:1);
colorless viscous liquid; 1H NMR (300 MHz, CDCl3):
d (ppm) 9.26
(br, 1H), 5.90 (m, 2H), 5.19–5.02 (m, 4H), 3.89 (br, 4H), 3.45 (t,
J¼5.3 Hz, 4H), 3.20 (s, 6H), 2.12 (br, 2H). Only one NH peak can be
located; 13C NMR (75 MHz, CDCl3):
d (ppm) 167.1,166.3,136.3,113.8,

2926
G. Ye et al. / Tetrahedron 66 (2010) 2919–2927
73.6, 47.4, 41.2, 40.8, 20.72; IR (KBr, cmꢁ1): 3358, 2926, 1587, 1504;
HRMS (ESI, [MþNa]þ): calcd for C15H24N4NaO2, 315.1797; found,
315.1779.
CDCl3): d (ppm) 164.6, 161.6, 156.8, 147.8, 138.6, 134.6, 129.4, 128.8,
128.7, 128.5, 123.4, 120.1, 83.8, 51.8, 41.8, 39.3; IR (KBr, cmꢁ1): 3240,
3051, 2970, 1709, 1665, 1624, 1580, 1545, 1496; HRMS (ESI,
[MþNa]þ): calcd for C20H18N4NaO3, 385.1277; found, 385.1256.
4.2.8. N1,N3-Di-tert-butyl-2-(1,3-dimethyltetrahydropyrimidin-
2(1H)-ylidene)malonamide (8d). Yield 93%; Rf¼0.40 (CH2Cl2–AcOEt,
4.4.2. 1-Methyl-5,7-dioxo-N,6-di-p-tolyl-1,2,3,5,6,7-hexahydro-
imidazo[1,2-c]pyrimidine-8-carboxamide (26b). Yield 70%; Rf¼0.29
(1:1 hexane–acetone); white solid; mp 266–268 ꢀC; 1H NMR
1:1); colorless liquid; 1H NMR (300 MHz, CDCl3):
d (ppm) 3.40 (t,
J¼5.7 Hz, 4H), 3.18 (s, 6H), 2.10 (quintet, J¼5.7 Hz, 2H), 1.36 (s, 18H).
No NH peak was located due to the fast exchange of the amide-H
between amide nitrogen and solvent molecules or other hydrogen-
(300 MHz, CDCl3):
J¼8.5 Hz, 2H), 3.87 (t, J¼8.5 Hz, 2H), 3.24 (s, 3H), 2.40 (s, 3H), 2.38
(s, 3H). 13C NMR (75 MHz, CDCl3):
(ppm) 164.7, 161.5, 156.7, 147.9,
d (ppm) 10.62 (s, 1H), 7.47–7.07 (m, 8H), 4.09 (t,
bond acceptors. 13C NMR (75 MHz, CDCl3):
d
(ppm) 168.0, 167.7,
d
75.7, 49.5, 47.8, 41.7, 29.9, 21.6; IR (KBr, cmꢁ1): 3310, 3070, 2965,
2927, 1631, 1530, 1484; HRMS (ESI, [MþNa]þ): calcd for
C17H32N4NaO2, 347.2423; found, 347.2396.
138.7, 136.1, 132.8, 132.0, 130.0, 129.2, 128.2, 120.1, 83.9, 51.8, 41.8,
39.3, 21.2, 20.7; IR (KBr, cmꢁ1): 3221, 3113, 3015, 2938, 1706, 1676,
1581, 1524, 1498; HRMS (ESI, [MþH]þ): calcd for C22H23N4O3,
391.1770; found, 391.1775.
4.3. General procedure for synthesis of push–pull alkenes
12a,b
4.4.3. N,6-Bis(4-methoxyphenyl)-1-methyl-5,7-dioxo-1,2,3,5,6,7-
hexahydroimidazo[1,2-c]pyrimidine-8-carboxamide
(26c). Yield
2-(1,3-Dimethyltetrahydropyrimidin-2(1H)-ylidene)-N1,N3-diph-
enylmalonamide 8a (0.36 g, 1 mmol) was added to a stirred solution
of phenyl isocyanate (0.24 g, 2 mmol) in 20 mL of THF. The reaction
mixture was then stirred for 8 h under nitrogen at rt. The solvent was
then removed by rotary evaporation. The residue was purified by
flash column chromatography (silica gel, hexane–acetone, 4:1/1:2)
to give 12a (154 mg, 32%). Compound 12b was similarly prepared.
62%; Rf¼0.48 (hexane–acetone, 1:2); white solid; mp¼216–218 ꢀC;
1H NMR (600 MHz, CDCl3):
d (ppm) 10.56 (s, 1H), 7.50–6.82 (m, 8H),
4.11 (t, J¼8.5 Hz, 2H), 3.89 (t, J¼8.5 Hz, 2H), 3.83 (s, 3H), 3.77 (s, 3H),
3.26 (s, 3H); 13C NMR (125 MHz, CDCl3):
d
(ppm) 164.8, 161.5, 159.5,
156.6, 155.7, 148.1, 131.9, 129.5, 127.1, 121.7, 114.7, 113.9, 83.8, 55.4,
55.4, 51.8, 41.9, 39.3; IR (KBr, cmꢁ1): 3228, 3057, 2935, 1706, 1667,
1576, 1539, 1510, 1497; HRMS (ESI, [MþH]þ): calcd for C22H23N4O5,
423.1668; found, 423.1661.
4.3.1. 2-(1,3-Dimethyltetrahydropyrimidin-2(1H)-ylidene)-N1,N3-di-
phenyl-N1-(phenylcarbamoyl)malonamide (12a). Yield 32%; Rf¼0.62
(acetone); white solid; mp 173–175 ꢀC; 1H NMR (300 MHz, CDCl3):
4.4.4. N,6-Bis(4-fluorophenyl)-1-methyl-5,7-dioxo-1,2,3,5,6,7-hex-
ahydroimidazo[1,2-c]pyrimidine-8-carboxamide (26d). Yield 73%;
Rf¼0.33 (hexane–acetone, 1:1); white solid; mp 295–297 ꢀC; 1H
d
(ppm) 11.53 (s, 1H), 10.12 (s, 1H), 7.68–6.97 (m, 15H), 3.22 (m, 2H),
3.03 (m, 2H), 2.90 (s, 6H), 1.86 (m, 2H); 13C NMR (75 MHz, CDCl3):
(ppm) 167.7, 165.6, 163.1, 152.9, 139.7, 139.5, 138.6, 128.7, 128.6,
NMR (300 MHz, CDCl3):
d (ppm) 10.71 (s, 1H), 7.65–7.04 (m, 8H),
d
4.24 (t, J¼8.5 Hz, 2H), 4.04 (t, J¼8.5 Hz, 2H), 3.36 (s, 3H); 13C NMR
1
128.3, 127.8, 126.4, 123.0, 122.2, 119.9, 119.5, 84.5, 46.9, 41.7, 18.9; IR
(KBr, cmꢁ1): 3193, 3023, 2939, 1693, 1633, 1605, 1563, 1519; HRMS
(ESI, [MþNa]þ): calcd for C28H29N5NaO3, 506.2168; found,
506.2150.
(75 MHz, CDCl3):
d
(ppm) 164.6, 162.5 (d, JCF¼248.5 Hz), 161.5,
158.9 (d, 1JCF¼242.5 Hz), 156.8, 147.8, 134.6 (d, 4JCF¼1.9 Hz), 130.4 (d,
3JCF¼8.8 Hz), 121.7 (d, 3JCF¼7.6 Hz), 116.4 (d, 2JCF¼22.9 Hz), 115.3 (d,
2JCF¼22.2 Hz), 83.6, 51.9, 41.9, 39.4; IR (KBr, cmꢁ1): 3245, 3065,
2938, 1707, 1670, 1584, 1550, 1500; HRMS (ESI, [MþNa]þ): calcd for
C20H16F2N4NaO3, 421.1088; found, 421.1070.
4.3.2. 2-(1,3-Dimethyltetrahydropyrimidin-2(1H)-ylidene)-N1,N3-di-
p-tolyl-N1-(p-tolylcarbamoyl)malonamide (12b). Yield 37%; Rf¼0.80
(acetone); white solid; mp 168–170 ꢀC; 1H NMR (300 MHz, CDCl3):
4.4.5. 1-Methyl-6,8-dioxo-N,7-diphenyl-2,3,4,6,7,8-hexahydro-1H-
pyrimido[1,6-a]pyrimidine-9-carboxamide (26e). Yield 71%; Rf¼0.17
(AcOEt); white solid; mp 263–265 ꢀC; 1H NMR (300 MHz, CDCl3):
d
(ppm) 11.45 (s, 1H), 10.01 (s, 1H), 7.55–7.08 (m, 12H), 3.20 (m, 2H),
3.02 (m, 2H), 2.88 (s, 6H), 2.33 (s, 3H), 2.29 (s, 3H), 2.28 (s, 3H), 1.91
(m, 1H), 1.78 (m, 1H); 13C NMR (75 MHz, CDCl3):
(ppm) 167.7,
d
d (ppm) 10.67 (s, 1H), 7.62–7.00 (m, 10H), 3.99 (br, 2H), 3.48 (t,
165.5, 163.4, 153.1, 137.2, 137.0, 136.1, 136.1, 132.3, 131.4, 129.1, 129.0,
128.7, 127.7, 119.8, 119.5, 84.1, 46.9, 41.6, 20.8, 20.6, 20.6, 18.9; IR
(KBr, cmꢁ1): 3193, 3022, 2917, 1688, 1682, 1626, 1598, 1563, 1504;
HRMS (ESI, [MþH]þ): calcd for C31H36N5O3, 526.2818; found,
526.2821.
J¼6.0 Hz, 2H), 3.17 (s, 3H), 2.15 (quintet, J¼6.0 Hz, 2H); 13C NMR
(75 MHz, CDCl3):
d (ppm) 163.3, 161.8, 156.2, 149.2, 138.9, 135.1,
129.3, 128.6, 128.6, 128.3, 123.1, 119.8, 86.1, 50.1, 44.3, 41.4, 20.7; IR
(KBr, cmꢁ1): 3234, 3060, 2958, 1713, 1666, 1614, 1590, 1573, 1543,
1504; HRMS (ESI, [MþNa]þ): calcd for C21H20N4NaO3, 399.1433;
found, 399.1414.
4.4. General procedure for synthesis of push–pull alkenes
26a–h
4.4.6. 1-Methyl-6,8-dioxo-N,7-di-p-tolyl-2,3,4,6,7,8-hexahydro-1H-
pyrimido[1,6-a]pyrimidine-9-carboxamide (26f). Yield 74%; Rf¼0.33
(hexane–acetone, 1:1); white solid; mp 270–272 ꢀC; 1H NMR
1,2-Dimethylimidazoline 5 (0.098 g, 1 mmol) was dissolved in
10 mL of MeCN and then added dropwise into a stirred solution of
phenyl isocyanate (0.48 g, 4 mmol) in 10 mL of MeCN under ni-
trogen at rt. The above solution was then refluxed for 12 h and the
solvent was removed by rotary evaporation. The residue was pu-
rified by flash column chromatography (silica gel, hexane–ethyl
acetate, 1:1) to give 26a (235 mg, 65%). Compounds 26b–h were
similarly prepared.
(300 MHz, CDCl3):
d (ppm) 10.63 (s, 1H), 7.50–7.06 (m, 8H), 4.03
(br, 2H), 3.52 (t, J¼6.0 Hz, 2H), 3.20 (s, 3H), 2.40 (s, 3H), 2.29 (s,
3H), 2.20 (quintet, J¼6.0 Hz, 2H); 13C NMR (75 MHz, CDCl3):
d
(ppm) 163.4, 161.8, 156.2, 149.4, 138.6, 136.4, 132.6, 132.5, 130.0,
129.13, 128.0, 119.9, 86.3, 50.1, 44.3, 41.4, 21.1, 20.9, 20.7; IR (KBr,
cmꢁ1): 3288, 3024, 2954, 1714, 1665, 1610, 1595, 1562, 1538, 1505;
HRMS (ESI, [MþH]þ): calcd for C23H25N4O3, 405.1927; found,
405.1917.
4.4.1. 1-Methyl-5,7-dioxo-N,6-diphenyl-1,2,3,5,6,7-hexahydro-
imidazo[1,2-c]pyrimidine-8-carboxamide (26a). Yield 65%; Rf¼0.18
(CH2Cl2–AcOEt, 1:1); white solid; mp 291–293 ꢀC; 1H NMR
4.4.7. N,7-Bis(4-methoxyphenyl)-1-methyl-6,8-dioxo-2,3,4,6,7,8-
hexahydro-1H-pyrimido[1,6-a]pyrimidine-9-carboxamide
(26g). Yield 63%; Rf¼0.16 (hexane–acetone, 1:1); white solid; mp
(300 MHz, CDCl3):
d (ppm) 10.67 (s, 1H), 7.60–7.01 (m, 10H), 4.12 (t,
J¼8.5 Hz, 2H), 3.90 (t, J¼8.5 Hz, 2H), 3.26 (s, 3H); 13C NMR (75 MHz,
263–265 ꢀC; 1H NMR (600 MHz, CDCl3):
d (ppm) 10.57 (s, 1H), 7.52–

G. Ye et al. / Tetrahedron 66 (2010) 2919–2927
2927
18. Schubert, H.; Bast, I.; Regitz, M. Synthesis 1983, 661.
19. Lloyd, D.; McNab, H. Angew. Chem. 1976, 88, 496.
20. Rattananakin, P.; Pittman, C. U., Jr.; Collier, W. E.; Saebo, S. Struct. Chem. 2007,
18, 399 and references therein.
6.80 (m, 8H), 3.99 (br, 2H), 3.82 (s, 3H), 3.76 (s, 3H), 3.47 (t, J¼6.0 Hz,
2H), 3.16 (s, 3H), 2.14 (quintet, J¼6.0 Hz, 2H); 13C NMR (125 MHz,
CDCl3):
d (ppm) 163.5, 161.7, 159.3, 156.0, 155.5, 149.5, 132.2, 129.2,
21. Kleinpeter, E.; Klod, S.; Rudorf, W.-D. J. Org. Chem. 2004, 69, 4317.
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127.7, 121.3, 114.6, 113.7, 86.1, 55.3, 50.0, 44.2, 41.4, 20.8; IR (KBr,
cmꢁ1): 3328, 2967, 1688, 1650, 1633, 1603, 1580, 1543, 1511, 1502;
HRMS (ESI, [MþNa]þ): calcd for C23H24N4NaO5, 459.1644; found,
459.1612.
25. (a) Kleinpeter, E.; Schulenburg, A. Tetrahedron Lett. 2005, 46, 5995; (b) Klein-
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ahydro-1H-pyrimido[1,6-a]pyrimidine-9-carboxamide (26h). Yield
76%; Rf¼0.32 (hexane–acetone, 1:1); white solid; mp 258–260 ꢀC;
26. Matus, M. H.; Contreras, R.; Cedillo, A.; Galvan, M. J. Chem. Phys. 2003, 119, 4112.
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Acta 2005, 88, 1589; (d) Roberts, R. M.; Corse, J.; Boschan, R.; Seymour, D.;
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1H NMR (600 MHz, CDCl3):
d (ppm) 10.64 (s, 1H), 7.57–6.95 (m, 8H),
4.00 (br, 2H), 3.50 (t, J¼6.0 Hz, 2H), 3.18 (s, 3H), 2.17 (quintet,
J¼6.0 Hz, 2H); 13C NMR (125 MHz, CDCl3):
d (ppm) 163.3, 162.3
(d, 1JCF¼248.3 Hz), 161.7, 158.6 (d, 1JCF¼242.0 Hz), 156.2, 149.2, 135.0
4
4
3
(d, JCF¼1.9 Hz), 130.9 (d, JCF¼2.5 Hz), 130.1 (d, JCF¼8.6 Hz), 121.3
3
2
2
(d, JCF¼7.6 Hz), 116.2 (d, JCF¼22.8 Hz,), 115.1 (d, JCF¼22.2 Hz),
85.8, 50.1, 44.4, 41.5, 20.7; IR (KBr, cmꢁ1): 3284, 2977, 2881, 1697,
1668, 1623, 1604, 1570, 1542, 1499; HRMS (ESI, [MþNa]þ): calcd for
C21H18F2N4NaO3, 435.1245; found, 435.1212.
Acknowledgements
The authors acknowledge the educational and general funds of
Mississippi State University for partial financial support of this work.
Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2010.02.071.
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