Chemical reactivity of dihydropyrazine derivatives. Cycloaddition behavior toward ketenes.

Abstract

The cycloaddition behavior of dihydropyrazines toward ketenes was investigated using single-crystal X-ray structures of the cycloadducts and density functional theory (DFT) calculation data. The reaction proceeds via a stepwise pathway involving an orientation complex prior to formation of the betaine intermediate. This is followed by electrocyclization to afford the 1 : 1 and 1 : 2 adducts bearing beta-lactam ring(s).

Full text

Page 1

Dihydropyrazines (DHPs), which are derived from amino-
sugars, exhibit various properties such as specific DNA
strand-breakage activity,1—3)facile dimerization,4—6)unique
ESR spectral behavior,7—9)in vitro, induction of apoptosis10)
and mutagenesis11,12)in vivo. It is thought that all these phe-
nomena originate from the two natural characteristics of
DHPs, i.e., their high chemical reactivity and radical genera-
tion ability.4—9)
As exemplified in Chart 1, less substituted DHPs such as
5,6-dimethyl-2,3-dihydropyrazine exhibit high chemical re-
activity and readily transform to dimeric heterocyclic com-
pounds via aldol condensation or pericyclic ene reaction
(Chart 1).5,6)
In order to clarify the inherent chemical reactivity of
DHPs, we studied the cycloaddition behavior of DHPs as
diazadienes or imines towards ketenes. The results are dis-
cussed in detail on the basis of molecular orbital (MO) calcu-
lations on the cycloaddition pathways and the single crystal
X-ray analyses of the cycloadducts.
Results and Discussion
First, reactions of relatively stable 2,3-diphenyl DHP de-
rivatives (1a—c)13)with ketenes (2a—c) were performed
(Chart 2).
The reaction of 1a with 2a provided the 1:1 adduct (3aa)
and two stereoisomeric 1:2 adducts (anti 4aa and syn 4aa).
The yields and product ratios for the reactions with 1a—c
and 2a—c are summarized in Table 1.
846Vol. 57, No. 8
Chemical Reactivity of Dihydropyrazine Derivatives.
Behavior toward Ketenes
Cycloaddition
Kazuhide NAKAHARA, Koki YAMAGUCHI, Yasuyuki YOSHITAKE, Tadatoshi YAMAGUCHI, and
Kazunobu HARANO*
Faculty of Pharmaceutical Sciences, Sojo University; 4–22–1 Ikeda, Kumamoto 860–0082, Japan.
Received April 9, 2009; accepted May 7, 2009; published online May 12, 2009
The cycloaddition behavior of dihydropyrazines toward ketenes was investigated using single-crystal X-ray
structures of the cycloadducts and density functional theory (DFT) calculation data. The reaction proceeds via a
stepwise pathway involving an orientation complex prior to formation of the betaine intermediate. This is fol-
lowed by electrocyclization to afford the 1:1 and 1:2 adducts bearing b b-lactam ring(s).
Key words
dihydropyrazine; ketene; imine; cycloaddition; density functional theory; X-ray analysis
Chem. Pharm. Bull. 57(8) 846—852 (2009)
© 2009 Pharmaceutical Society of Japan
∗ To whom correspondence should be addressed. e-mail: harano@ph.sojo-u.ac.jp
Chart 1
Chart 2

Page 2

The structures of the adducts were established by MS, IR
and NMR spectral data, showing the formation of a b-lactam
ring. The 1H-NMR spectrum of 3aa exhibited a methoxy sig-
nal at 3.50ppm, methylene signals at 3.21—4.17ppm, a me-
thine signal at 4.63ppm and aromatic proton signals at
7.34—7.85ppm. The 13C-NMR spectrum exhibited seven
non-aromatic carbon signals, suggesting the presence of one
methyl (d 59.3), two methylenes (d 37.0, 45.2), one methine
(d 89.1), one quaternary (d 63.7) and two sp2carbons (d
168.4, 171.0). The IR spectrum showed the characteristic ab-
sorption of a b-lactam carbonyl at 1769cm?1. The structure
of the adduct (3aa) was confirmed by comparison of the
spectral features with those of the analogous 1:1 adducts
(3ba, 3ac) whose structures were firmly established by single
crystal X-ray analysis (see Fig. 2, Table 2).
Both 1:2 adducts showed similar spectral behaviors as ob-
served in the 1:1 adducts. Comparison of the 1H-NMR spec-
tra of the 1:2 adducts afforded a clue to the determination
of the syn/anti orientation. In the syn orientation, the two
phenyl rings are in a face-to-face disposition which agrees
with the observation that the phenyl hydrogens are shifted
upfield (6.91—7.26ppm) in comparison with those of the
anti adduct (7.25—7.52ppm), whereas the methine proton
[?CH(OMe)–CO–] of the anti adduct resonates at ca.
0.7ppm higher field than that of the syn adduct. The assign-
ment of the 1H-NMR spectra of the 1:2 adducts (anti 4aa
and syn 4aa) is shown in Fig. 1. The structure of syn 4aa was
confirmed by single crystal X-ray analysis (see Fig. 2, Table
2).
It is noted that the reaction of 1a—c with diphenylketene
(2c) gave only the 1:1 adduct. The reaction of the mono-
methyl-substituted DHP (1b) with 2a—c gave mixtures of
the stereoisomeric cycloadducts. Even in the reaction of 1b
with 2c, four isomers were recognized, indicating that the re-
action was affected by site selectivity in addition to the steric
effect. Of those, the structure of 3bc was established by X-
ray analysis (see Fig. 2, Table 2).
The ketene cycloaddition reaction has attracted much at-
tention from both synthetic and theoretical chemists because
the [2?2] cycloaddition products of cyclic dienes and
ketenes are known to be formed via [3,3]-sigmatropic re-
arrangement of the [4?2] cycloadduct in which the carbonyl
double bond of the ketene acts as a dienophile (Chart 3, entry
b). This finding has caused a revolution in ketene chem-
istry.14)
In this connection, we previously reported another cy-
cloaddition mechanism for the reaction of dihydropyridines
with ketenes on the basis of MO calculations,15)in which the
lactam ring formation does not proceed via a sequential peri-
cyclic reaction mechanism but takes place by a stepwise re-
action mechanism (Chart 3, entry c).
August 2009 847
Table 1.
razine (1) with Ketene (2)
Reaction Product (%) and Ratio (anti 4:syn 4) for Dihydropy-
Product (%)Product ratio
Starting substance Reaction time
34
(anti 4:syn 4)
1a 2a
2b
2c
2a
2b
2c
2a
2b
2c
40.1 (3aa)
—
90.3 (3ac)
2.3a)(3ba)
21.5 (3bb)
33.3 (3bc)
38.8 (3ca)
42.5 (3cb)
79.3 (3cc)
52.6 (4aa)
90.2 (4ab)
—
36.7a)(4ba)
58.6a)(4bb)
—
30.2 (4ca)
54.9 (4cb)
—
3:2
7:2
—
—
—
—
2:1
5:1
—
21.5h
24h
18h
17h
24h
17h
16h
24h
22h
1b
1c
a) Mixture of stereoisomers.
Fig. 1. 500MHz 1H-NMR Spectral Data for 1:2 Adducts
Fig. 2.ORTEP Drawings of the Cycloadducts (3ba, 3ac, 3bc, syn 4aa)
Table 2. Crystal and Analysis Data for the Adducts
Crystal data
3ba3ac 3bc
syn 4aa
Formula
Crystal system
Lattice parameters
a (Å)
b (Å)
c (Å)
b (°)
V (Å3)
Space group
Z
Density (calcd.)
Density (obsd.)
Unique data used
R
Rw
Goodness of fit
CCDC number
C20H20N2O2
Monoclinic
C30H24N2O
Monoclinic
C31H26N2O
Monoclinic
C22H24N2O5
Orthorhombic
19.243(7)
8.149(4)
10.861(3)
99.00
1682(1)
P21/n
4
1.265
1.266
3868
0.095
0.166
1.000
717626
11.656(5)
17.754(7)
10.810(3)
90.99(3)
2236(1)
P21/n
4
1.272
1.272
5117
0.083
0.108
1.001
717627
11.930(3)
18.068(7)
10.863(3)
94.09(2)
2335(1)
P21/n
4
1.259
1.258
5347
0.088
0.089
1.270
717628
14.549(5)
19.032(5)
14.218(5)
3937(2)
Pbca
8
1.338
1.335
4526
0.072
0.116
1.002
717629

Page 3

Based on the results for dihydropyridines, we considered
that the reaction of a ketene with an a,b-unsaturated cyclic
imine does not fall into the new category but involves initial
nucleophilic attack of the imine nitrogen on the ketene sp-hy-
bridized carbon, followed by 4p electrocyclic ring closure to
give the [2?2] cycloaddition product. This reaction behavior
is easily explained by frontier molecular orbital (FMO) the-
ory.16,17)The highest occupied molecular orbital (HOMO) of
the dihydropyridine localizes on the lone pair of the nitrogen
atom, 0.35eV higher than p-NHOMO, and the resulting aza-
diene readily undergoes conrotatory electrocyclic ring clo-
sure (Chart 4).
To confirm the reaction mechanism for the reaction of
DHP and ketene, we calculated possible transition-state (TS)
structures using the density functional theory (DFT) method
at the B3LYP/6-31G (d) level.18)The TS geometries of [4?2]
cycloadditions using parent molecules [DHP as 4p diene to-
wards ketene (C?C or C?O) as 2p dienophile], the interact-
ing bond distances and energies (Hartree) are depicted in Fig.
3. As shown in Fig. 3, [4?2] cycloadditions are energetically
unfavorable19)in comparison with the reaction barrier for the
intermediate formation in the stepwise cyclization reaction
(see also Fig. 4).
The energy profile for the model reaction of 1 with 2 is de-
picted in Fig. 4. The reaction is found to proceed via a be-
taine intermediate followed by electrocyclization.
The energy profile for the reaction of 1a with 2a is de-
picted in Fig. 5. The betaine intermediate is more stable than
that for the model reaction using the parent molecules.
The 1:2 cycloadduct formation pathway is also calculated
(Fig. 6). The energy profile is essentially the same as that for
1:1 cycloadduct formation. The calculations indicate that
the syn approach of the methoxy group with respect to the
lactam ring is energetically favorable, inconsistent with the
experimental results. Close inspection of the reaction path-
way between GS and TS1 suggests the presence of an orien-
tation complex (OC)20—25)in which the reactants loosely
combine with each other as compared with the intermediate
(IM). The calculation indicates that the syn OC is more sta-
ble than the anti OC. The reaction barrier based on the anti
OC is lower than that for the syn OC, implying dominant for-
mation of the anti adduct.
848 Vol. 57, No. 8
Chart 3
Chart 4
Fig. 3.
[4?2]p Cycloadditions
B3LYP/6-31G (d) Calculated Transition Structures for Possible
Fig. 4.B3LYP/6-31G (d) Simulation for Model Reaction Pathway for DHP and Ketene Using Parent Addends

Page 4

The reaction barrier of the electrocyclization process is
lower than that for 1:1 adduct formation. This may be due to
the difference in the stabilization energy between the ground
states of the reactants.
As the reaction may take place in part via direct cycloaddi-
tion without the OC formation, the product ratio does not
necessarily correspond to the calculation prediction.
During the course of the cyclization reaction, a change of
color in the reaction mixture was observed. The MOS-F
CNDO/S calculations26)suggested the presence of a charge-
transfer complex, presumably due to the OC and IM struc-
tures (see Fig. 7, Table 3).
August 2009849
Fig. 5.Reaction Profile for Cycloaddition of 1a with 2a Calculated by the B3LYP/6-31G (d) Method
Fig. 6.Energy Diagram for the Reaction of 2a and 3aa
Fig. 7.Possible OC and IM in the Reaction of 1a with 2c Used for the MOS-F CNDO/S Calculation

Page 5

In summary, DHP shows a high cycloaddition reactivity
toward ketenes, in which a stepwise reaction takes place via
nucleophilic attack of the nitrogen lone pair of DHP on the
ketene central carbon, followed by electrocyclization of the
azadiene.
Experimental
Melting points are uncorrected. The IR spectra were obtained with a Hi-
tachi 270-30 spectrophotometer. 1H- and 13C-NMR spectra were obtained
with JEOL JNM-AL 300 (300MHz) and JNM-A 500 (500MHz) spectrome-
ters using tetramethylsilane as an internal standard. Mass spectra were ob-
tained using a JMS-DX303HF instrument.
Materials
The DHPs (1a—c) were prepared according to the litera-
ture.1,27)The ketenes (2a—c) were also synthesized by established meth-
ods.28,29)
General Procedure
A solution of methoxyacetyl chloride (0.43g,
4mmol) in CH2Cl2(5.5ml) was added dropwise to a solution of 2,3-dihy-
dropyrazine derivative (2mmol) in CH2Cl2(7.0ml) containing triethylamine
(0.55ml). After stirring at room temperature overnight, aqueous NaHCO3
was added to neutralize the reaction mixture. The products were extracted
with CH2Cl2three times and dried over anhydrous MgSO4. Evaporation of
the solvent gave crude products, which were purified by chromatography on
silica gel. Crystallization from n-hexane gave pure cycloadducts.
3aa: Colorless prisms. mp 126—128°C. IR (KBr) cm?1: 1769 (C?O).
1H-NMR (500MHz; CDCl3) d: 3.21 (1H, m, CH), 3.35—3.41 (1H, m, CH),
3.46—3.52 (1H, m, CH), 3.50 (3H, s, OCH3), 4.13—4.17 (1H, m, CH), 4.63
(1H, s, CH), 7.34—7.85 (10H, m, aromatic-H). 13C-NMR (125MHz;
CDCl3) d: 37.0 (C2), 45.2 (C3), 59.3 (C7-OMe), 63.7 (C6), 89.1 (C7),
127.5, 128.3, 128.4, 128.6, 128.7, 130.9, 134.8, 136.0 (aromatic-C), 168.4
(C?N), 171.0 (C?O). EI-MS (m/z): 306 (M??1). Anal. Calcd for
C19H18N2O2: C, 74.49; H, 5.92; N, 9.14. Found: C, 74.33; H, 5.70; N, 9.14.
anti 4aa: Colorless prisms. mp 212—214°C. IR (KBr) cm?1: 1760
(C?O), 1740 (C?O). 1H-NMR (500MHz; CDCl3) d: 3.07 (6H, s, OCH3),
3.44 (2H, d, J?7.9Hz, CH2), 4.10 (2H, d, J?7.9Hz, CH2), 4.32 (2H, s, CH),
7.40—7.52 (10H, m, aromatic-H). 13C-NMR (125MHz; CDCl3) d: 36.6
(C6, C7), 58.2 (C3-OMe, C10-OMe), 71.3 (C1, C2), 90.5 (C3, C10),
127.22, 127.61, 127.75, 127.92, 128.11, 128.51, 137.11 (aromatic-C),
165.69 (C?O). FAB-MS (m/z): 379 (M??1). Anal. Calcd for C22H22N2O4:
C, 69.83; H, 5.86; N, 7.40. Found: C, 69.58; H, 5.90; N, 7.22.
syn 4aa: Colorless prisms. mp 248—251°C. IR (KBr) cm?1: 1749
(C?O). 1H-NMR (500MHz; CDCl3) d: 3.26 (6H, s, OCH3), 3.52 (2H, d,
J?7.3Hz, CH2), 4.17 (2H, d, J?7.3Hz, CH2), 5.01 (2H, s, CH), 6.91—7.26
(10H, m, aromatic-H). 13C-NMR (125MHz; CDCl3) d: 37.9 (C6, C7), 58.1
(C3-OMe, C10-OMe), 70.7 (C1, C2), 88.0 (C3, C10), 127.62, 127.72,
127.78, 127.93, 136.51 (aromatic-C), 168.97 (C?O). EI-MS (m/z): 378
(M?). Anal. Calcd for C22H22N2O4: C, 69.83; H, 5.86; N, 7.40. Found: C,
69.95; H, 5.68; N, 7.50.
anti 4ab: Colorless prisms. mp 222—224°C. IR (KBr) cm?1: 1760
(C?O). 1H-NMR (500MHz; CDCl3) d: 3.56 (2H, d, J?7.9Hz, CH2), 4.20
(2H, d, J?7.9Hz, CH2), 5.11 (2H, s, CH), 6.62—7.40 (20H, m, aromatic-
H). 13C-NMR (125MHz; CDCl3) d: 36.7 (C6, C7), 71.8 (C1, C2), 87.1 (C3,
C10), 116.46, 122.68, 128.68, 128.76, 129.35, 136.37, 156.93 (aromatic-C),
164.9 (C?O). EI-MS (m/z): 502 (M?). Anal. Calcd for C32H26N2O4: C,
76.48; H, 5.21; N, 5.57. Found: C, 76.33; H, 5.18; N, 5.59.
syn 4ab: Colorless prisms. mp 118—120°C. IR (KBr) cm?1: 1754
(C?O). 1H-NMR (500MHz; CDCl3) d: 3.62 (2H, d, J?7.3Hz, CH2), 4.29
(2H, d, J?7.3Hz, CH2), 5.61 (2H, s, CH), 6.91—7.34 (20H, m, aromatic-
H). 13C-NMR (125MHz; CDCl3) d: 38.1 (C6, C7), 71.2 (C1, C2), 84.7 (C3,
C10), 116.4, 123.1, 127.4, 127.9, 128.2, 129.7, 135.9, 157.0 (aromatic-C),
168.0 (C?O). FAB-MS (m/z): 503 (M??1). HR-MS Calcd for C32H27N2O4
(M??H): 503.1971. Found: 503.2018.
3ac: Colorless prisms. mp 174—176°C.30)IR (KBr) cm?1: 3100—2800
(C–H), 1752 (C?O), 1601 (C?N), 1574, 1495 (Ph), 1446 (C?N). 1H-NMR
(500MHz; CDCl3) d: 3.22—3.27 (1H, m, CH), 3.47—3.54 (1H, m, CH),
3.91—3.96 (1H, m, CH), 4.27—4.33 (1H, m, CH), 6.79—7.53 (20H, m,
aromatic-H). 13C-NMR (125MHz; CDCl3) d: 34.8 (C2), 45.8 (C3), 68.9
(C6), 78.6 (C7), 127.0, 127.3, 127.4, 127.7, 127.8, 128.0, 128.3, 128.9,
129.2, 129.6, 135.6, 137.8, 138.2, 138.4 (aromatic-C), 169.84 (C?N),
170.51 (C?O). FAB-MS (m/z): 429 (M??1). Anal. Calcd for C30H24N2O:
C, 84.08; H, 5.65; N, 6.54. Found: C, 84.18; H, 5.55; N, 6.54.
3ba: Colorless prisms. mp 189—191°C. IR (KBr) cm?1: 1750 (C?O).
1H-NMR (500MHz; CDCl3) d: 1.29 (3H, d, J?7.3Hz, CH3), 2.55 (1H, dd,
J?9.1, 13.4Hz, CH), 3.62 (3H, s, OCH3), 3.86 (1H, dd, J?6.7, 13.4Hz,
CH), 4.08—4.12 (1H, m, CH), 4.74 (1H, s, CH), 7.25—7.80 (10H, m, aro-
matic-H). 13C-NMR (125MHz; CDCl3) d: 20.8 (C3-Me), 40.7 (C2), 51.8
(C3), 59.9 (C7-OMe), 61.5 (C6), 90.7 (C7), 127.5, 127.7, 128.0, 128.4,
128.6, 129.8, 130.5, 135.6, 136.2 (aromatic-C), 163.5 (C?N), 166.0 (C?O).
EI-MS (m/z): 320 (M?). Anal. Calcd for C20H20N2O2: C, 74.98; H, 6.29; N,
8.74. Found: C, 74.97; H, 6.25; N, 8.63.
3ba-1: Recognized as a minor product in the 1H-NMR spectrum of 3ba.
1H-NMR (300MHz; CDCl3) d: 1.78 (3H, d, J?6.6Hz, CH3), 3.29 (3H, s,
OCH3), 3.60 (1H, m, CH), 3.92—3.96 (2H, m, CH), 5.03 (1H, s, CH),
6.86—7.25 (10H, m, aromatic-H).
4ba: Colorless prisms. mp 158—160°C. IR (KBr) cm?1: 1752 (C?O).
1H-NMR (300MHz; CDCl3) d: 1.75 (3H, d, J?6.7Hz, CH3), 3.07—3.18
(6H, m, OCH3), 3.09 (1H, s, CH), 3.84 (1H, m, CH), 4.10 (1H, dd, J?4.4,
12.8Hz, CH), 4.19 (1H, s, CH), 4.43 (1H, s, CH), 7.25—7.52 (10H, m, aro-
matic-H). 13C-NMR (125MHz; CDCl3) d: 16.2 (C7-Me), 44.6 (C6), 49.5
(C7), 58.2 (C3-OMe), 58.3 (C10-OMe), 70.5 (C1), 74.2 (C2), 85.5 (C3),
90.5 (C10), 127.0, 127.2, 127.5, 127.7, 127.8, 127.9, 128.0, 128.4, 128.7,
128.8, 129.1, 129.8, 130.5, 137.2, 137.3 (aromatic-C), 165.4 (C?O), 165.7
(C?O). EI-MS (m/z): 392 (M?). Anal. Calcd for C23H24N2O4: C, 70.39; H,
6.16; N, 7.14. Found: C, 70.11; H, 6.12; N, 7.10.
4ba-1: Recognized as a minor product in the 1H-NMR spectrum of 4ba.
1H-NMR (300MHz; CDCl3) d: 1.46 (3H, d, J? 6.4Hz, CH3), 3.00—3.09
(1H, m, CH), 3.29 (6H, s, OCH3), 4.05—4.12 (1H, m, CH), 4.18—4.24 (1H,
m, CH), 4.90 (1H, s, CH), 4.97 (1H, s, CH), 6.86—7.25 (10H, m, aromatic-
H).
3bb: Colorless prisms. mp 164—166°C. IR (KBr) cm?1: (C?O). 1H-
NMR (500MHz; CDCl3) d: 1.46—1.47 (3H, s, CH3), 3.16—3.17 (1H, m,
CH), 3.30—3.31 (1H, m, CH), 3.51—3.52 (1H, m, CH), 5.30—5.31 (1H, s,
CH), 6.94—7.77 (15H, m, aromatic-H). 13C-NMR (125MHz; CDCl3) d:
20.9 (CH3), 44.1 (C2), 50.7 (C3), 63.9 (C5), 86.1 (C8), 117.3, 123.0, 127.8,
128.5, 128.8, 129.1, 129.2, 129.6, 131.1, 134.8, 135.7 (aromatic-C), 157.4
(OPh), 167.6 (C?N), 169.5 (C?O). EI-MS (m/z): 382 (M?). HR-MS Calcd
for C25H23N2O2(M??H): 383.1760. Found: 383.1723.
4bb: Colorless prisms. mp 204—205°C. IR (KBr) cm?1: 1757 (C?O).
1H-NMR (500MHz; CDCl3) d: 1.55 (3H, d, J?6.7Hz, CH3), 3.65 (1H, dd,
J?5.5, 12.8Hz, CH), 3.87 (1H, dd, J?2.4, 12.8Hz, CH), 4.51—4.54 (1H,
m, CH), 5.12 (1H, s, CH), 5.40 (1H, s, CH), 6.56—7.43 (20H, m, aromatic-
H). 13C-NMR (125MHz; CDCl3) d: 19.3 (C7-Me), 42.6 (C6), 43.5 (C7),
71.3 (C1), 73.7 (C2), 87.0 (C3), 87.2 (C10), 116.7, 122.7, 122.8, 128.2,
128.4, 128.6, 128.7, 128.8, 129.3, 129.4, 136.5, 136.9, 156.9, 157.0 (aro-
matic-C), 166.2 (C?O), 166.7 (C?O). EI-MS (m/z): 516 (M?). Anal. Calcd
for C33H28N2O4: C, 76.73; H, 5.46; N, 5.42. Found: C, 76.64; H, 5.26; N,
5.42.
Continued fractional crystallization caused enrichment but the minor
products could not be isolated in pure form. The following products were
recognized in the 1H-NMR spectrum of the enriched fractions whose struc-
tures were determined by comparison of the 1H-NMR data with each other.
4bb-1: 1H-NMR (300MHz; CDCl3) d: 1.81 (3H, d, J?6.8Hz, CH3),
3.21—3.29 (1H, m, CH), 3.93—4.00 (1H, m, CH), 4.17—4.22 (1H, m, CH),
5.02 (1H, s, CH), 5.18 (1H, s, CH), 6.55—7.40 (20H, m, aromatic-H).
4bb-2: 1H-NMR (300MHz; CDCl3) d: 1.41 (3H, d, J?6.6Hz, CH3),
3.06—3.14 (1H, m, CH), 4.13—4.20 (1H, m, CH), 4.31—4.39 (1H, m, CH),
5.48 (1H, s, CH), 5.55 (1H, s, CH), 6.78—7.31 (20H, m, aromatic-H).
4bb-3: 1H-NMR (300MHz; CDCl3) d: 1.84 (3H, d, J?6.6Hz, CH3),
3.70—3.73 (1H, m, CH), 4.01—4.07 (2H, m, CH), 5.61 (1H, s, CH), 5.62
(1H, s, CH), 6.78—7.31 (20H, m, aromatic-H).
3bc: Colorless prisms. mp 184—186°C. IR (KBr) cm?1: 1747 (C?O).
1H-NMR (500MHz; CDCl3) d: 1.29 (3H, d, J?6.1Hz, CH3), 2.79 (1H, dd,
J?10.9, 14.6Hz, CH), 4.09—4.14 (1H, m, CH), 4.27 (1H, dd, J?6.1,
14.6Hz, CH), 6.72—7.53 (20H, m, aromatic-H). 13C-NMR (125MHz;
CDCl3) d: 19.8 (C3-Me), 43.9 (C3), 52.6 (C2), 69.9 (C6), 78.5 (C7), 126.9,
850Vol. 57, No. 8
Table 3.Calculated Absorption Wavelength and Oscillator Strength
State Absorption wavelength (nm) Oscillator strengtha)
OC
TS-SW1
IM
373.33
389.26
854.75
367.23
355.49
482.42
0.00765
0.00489
0.00525
0.00657
0.00134
0.53804TS-SW2
a) MOS-F CNDO/S.