Preferential formation of cis-4,5-dihydrooxazole derivatives via photoinduced electron transfer-initiated cyclization of N-acyl-α-dehydroarylalanine alkyl esters

Article abstract of DOI:10.1016/j.tet.2007.08.089

The alkyl, aryl, and acyl substituent effects on the photoinduced electron transfer-initiated cyclization reaction of the title compounds (1) were investigated in polar solvents from mechanistic and synthetic points of view. The irradiation of (Z)-1 in me

Full text of DOI:10.1016/j.tet.2007.08.089

Tetrahedron 63 (2007) 11267–11281
Preferential formation of cis-4,5-dihydrooxazole derivatives
via photoinduced electron transfer-initiated cyclization of
N-acyl-a-dehydroarylalanine alkyl esters
Kei Maekawa,a Norikazu Hishikawa,a Kanji Kubo,b Tetsutaro Igarashia and
Tadamitsu Sakuraia,
*
aDepartment of Material and Life Chemistry, Faculty of Engineering, Kanagawa University, Kanagawa-ku,
Yokohama 221-8686, Japan
bSchool of Dentistry, Health Sciences University of Hokkaido, Kanazawa, Ishikari-Tobetsu, Hokkaido 061-0293, Japan
Received 6 July 2007; revised 25 August 2007; accepted 29 August 2007
Available online 1 September 2007
Abstract—The alkyl, aryl, and acyl substituent effects on the photoinduced electron transfer-initiated cyclization reaction of the title com-
pounds (1) were investigated in polar solvents from mechanistic and synthetic points of view. The irradiation of (Z)-1 in methanol containing
triethylamine (TEA) was found to quantitatively give cis- and trans-4,5-dihydrooxazole derivatives (cis-2 and trans-2). In addition to ther-
modynamic considerations for electron transfer and fluorescence quenching in the presence of TEA, acyl and aryl substituent effects on the
emission intensity and photoreactivity of 1 confirmed the involvement of consecutive electron transfer reactions that form (E)-arylmethylene
radical anion and (E)-N-acyl radical anion intermediates. It was also confirmed that the cyclization of the latter intermediate eventually leads
to 2. On the basis of the finding that the selectivity for cis-2 is greatly increased with increasing the steric bulkiness of alkyl and aryl substit-
uents in 1, it was concluded that steric hindrance of these substituents toward hydrogen shift in the cyclized biradical intermediate, precursor
of 2, is responsible for the kinetically controlled hydrogen shift in this intermediate. A product composition analysis showed that the protic
polar solvent, methanol, of hydrogen-bonding solvation ability is a most suitable solvent for the photocyclization reactions examined.
Ó 2007 Published by Elsevier Ltd.
1. Introduction
N-acetyl-a-dehydroalaninamides was found to proceed effi-
ciently affording 3,4-dihydrobenzoquinolinone derivatives
in high selectivity.5 This finding demonstrates the synthetic
utility of PET-initiated cyclization of N-acyl-a-dehydro-
amino acids. In these photocyclization reactions the corre-
sponding isoquinoline and 4,5-dihydrooxazole derivatives
were obtained as minor products. Replacement of acetyl
by benzoyl as the N-acyl group resulted in an enhancement
in selectivity for the dihydrobenzoquinolinone derivatives
without any detectable formation of substituted isoquino-
lines, owing to large steric hindrance of the benzoyl group.
In addition, the structural transformation of N-benzoyl-
a-dehydroalaninamides into the corresponding alanine alkyl
esters makes it impossible to generate the dihydrobenzoqui-
nolinone derivatives through the PET-initiated cyclization.
Accordingly, we were led to predict that N-aroyl-a-dehydro-
naphthylalanine alkyl esters in the excited singlet state un-
dergo one-electron reduction in the presence of tertiary
amine to selectively give the corresponding cis- and trans-
4,5-dihydrooxazole derivatives. Because there is no photo-
chemical synthetic route to dihydrooxazole derivatives
though several routes to these derivatives are known,6 the
above prediction stimulated us to explore PET reactions
of N-aroyl-a-dehydroarylalanine alkyl esters, hoping to
Excited-state chemistry for organic compounds has contin-
ued to contribute to the development of new synthetic
methods that enable the construction of various hetero
atom-containing ring systems.1 In recent years much atten-
tion is being devoted to studies regarding photoinduced elec-
tron transfer (PET) reactions, owing to the fact that many of
these reactions initiated by an electron transfer (ET) give
pharmaceutically useful heterocyclic compounds in high
chemical and quantum yields.1,2 In previous studies we
found that N-acetyl-a-dehydrophenylalaninamides in polar
solvents undergo a novel photocyclization giving isoquino-
line and 1-azetine derivatives in satisfactory yields but
with low efficiency.3 The use of N-acetyl-a-dehydrophenyl-
alanine alkyl esters as the starting a-dehydroamino acid de-
rivatives enabled the selective formation of the
corresponding isoquinolines by their photocyclization.4 In-
terestingly, one-electron reduction of 1-naphthyl-substituted
Key words: Amino acids and derivatives; Photochemistry; Electron transfer;
Dihydrooxazoles; Substituent and solvent effects.
* Corresponding author. Fax: +81 45 491 7915; e-mail: sakurt01@
kanagawa-u.ac.jp
0040–4020/$ - see front matter Ó 2007 Published by Elsevier Ltd.
doi:10.1016/j.tet.2007.08.089

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K. Maekawa et al. / Tetrahedron 63 (2007) 11267–11281
develop an efficient and selective phototransformation
method enabling the construction of a pharmaceutically im-
portant dihydrooxazole ring.6h,7 To this end we designed and
synthesized (Z)-N-(4-substituted benzoyl)- and (Z)-N-piva-
loyl-a-dehydroarylalanine alkyl esters [(Z)-1a–q] (Chart 1)
and examined substituent, solvent, and triethylamine
(TEA) concentration effects on the photoreactivity of 1
and the product composition.
NMR yields at their >99% conversions but (Z)-1q under-
went a secondary photodecomposition to give unknown
products at relatively high conversions of this starting ester
derivative (>50%).
OMe
O
OMe
O
N
N
O
O
H
H
h
R1O
O
(Z)-1a
+
+
H
H
O
TEA/MeOH
N
H
R2
cis-2a
trans-2a
(Z)-1a–f
1a (R1= Me, R2= H); 1b (R1= Et, R2= H);
1c (R1= i-Pr, R2= H); 1d (R1=t-Bu, R2= H);
1e (R1= Me, R2= OMe); 1f (R1= Me, R2= CN);
O
NH
OMe
O
MeO
O
O
t-BuO
O
O
(E)-1a
N
H
N
H
R
Scheme 1.
Ar
(Z)-1g
(Z)-1h–q
2.2. Inter- and intramolecular electron transfer and
kinetically controlled reactions
1h (Ar= 4-MeOC6H4, R= Ph); 1i (Ar= 4-MeC6H4, R= Ph);
1j (Ar= R = Ph); 1k (Ar= 4-ClC6H4, R= Ph);
1l (Ar= 4-NCC6H4, R= Ph); 1m (Ar= 2-MeC6H4, R= Ph);
1n (Ar= 2,6-Me2C6H3, R= Ph);1o (Ar= 2-naphthyl, R= Ph);
1p (Ar= 1-naphthyl, R= t-Bu); 1q (Ar= 9-anthryl, R= Ph)
The finding that the photoproducts 2a–p are stable enough
such that they undergo only negligible decomposition under
the irradiation conditions employed made it possible to mon-
itor the reactions by means of 1H NMR spectroscopy, as typ-
ically shown in Table 1. The result obtained for (Z)-1a
demonstrates the rapid production of (E)-1a and the
Chart 1.
2. Results and discussion
2.1. Product analysis and composition
N1
C15
C16
C12
C11
O3
C2
C14
C13
C9
O2
C17
C20
C1
The starting (Z)-isomers were prepared in reasonable yields
(30–90%) by the ring-opening reactions of (Z)-2-aryl-4-(1-
arylmethylene)-5(4H)-oxazolones with the corresponding
alcohols in the presence of TEA.4,8 After a nitrogen-
C8
C7
C10
C18
O1
C19
C21
saturated methanol solution of (Z)-1a (4.0ꢀ10ꢁ3 mol dmꢁ3
,
C6
500 mL) containing TEA (0.10 mol dmꢁ3) was irradiated
with Pyrex-filtered light (>280 nm) from a 400 W high-
pressure Hg lamp for 5 h at room temperature (conversion,
100%), the reaction mixture obtained was subjected to pre-
parative thin layer chromatography over silica gel (eluent:
EtOAc–hexane or EtOAc–CHCl3). This chromatography
allowed us to isolate cis- and trans-4-methoxycarbonyl-5-
(1-naphthyl)-2-phenyl-4,5-dihydrooxazoles (cis-2a, 30%;
trans-2a, 58% yield) having the vicinal coupling constants
(J4,5) of 10.3 and 6.9 Hz in DMSO-d6, respectively, as shown
in Scheme 1. The (E)-isomer of 1a was isolated indepen-
dently from the reaction mixture (40% yield), which was
irradiated for 0.5 h under the same conditions (conversion,
16.5%), by similar workup (Scheme 1). The structure of
the isolated products was determined based on their spectro-
scopic and physical properties. In addition, X-ray analysis of
cis-2a- and trans-2a-derived single crystals provided defin-
itive evidence for the structure of these two isomers
(Fig. 1).9 The irradiation of (Z)-1b–p in nitrogen-saturated
methanol under the same conditions gave the corresponding
C5
C3
C4
cis-2a
C21
O3
O2
C20
C15
C14
O1
C9
C16
C11
C1
C13
C8
C7
C17
C10
C12
N1
C2
C18
C19
C5
C6
C3
C4
trans-2a
1
cis- and trans-dihydrooxazole isomers in quantitative H
Figure 1. Crystal structures of cis-2a and trans-2a.

K. Maekawa et al. / Tetrahedron 63 (2007) 11267–11281
11269
Table 1. Relation between irradiation time and composition of each com-
pound obtained by the irradiation of (Z)-1a in MeOH containing TEA at
room temperature
Table 3. Effects of TEA concentration on the conversion of 1a and compo-
sition of each compound, obtained by the 3 h irradiation of (Z)-1a in MeOH
at room temperature
Irradiation time (h)
Composition (%)
cis-2a/trans-2a
TEA
Conversion (%)
Composition (%)
cis-2a/
trans-2a
(mol dmꢁ3
)
(Z)-1a (E)-1a cis-2a trans-2a
(Z)-1a (E)-1a cis-2a trans-2a
0
0.5
1
2
3
100
0
0
0
5.0
10.5
23.6
38.6
63.1
—
0
<0.1
48.7
70.4
74.6
80.2
>35.5 >65.4 <0.1 <0.1
17.8 33.5 38.8
10.2 19.5 48.2 22.2
8.8 16.6 43.9 30.7
7.0 12.8 36.4 43.8
—
30.1 53.4
24.4 44.1
14.2 25.4
5.6
0
11.5
21.0
36.9
46.0
36.9
2.3
2.0
1.6
1.2
0.6
0.01
0.05
0.10
0.20
9.9
3.9
2.2
1.4
0.8
9.8
0
5
with an increase in TEA concentration while negligible
amounts of the products are observed without TEA. This
is consistent with the participation of a PET mechanism.
The data of Table 3 also show that the composition ratio of
cis-2a to trans-2a decreases with increasing TEA concentra-
tion, substantiating the occurrence of TEA-catalyzed isom-
erization to the trans-isomer, as already described above.
Thus, we are able to preferentially obtain either of these
two isomers by controlling the amine concentration and
also to mainly generate the trans-isomer by allowing the re-
action mixture to stand in the presence of a tertiary amine
having high basicity. Because more evidence are required
in support of the PET mechanism for the cyclization of
N-acyl-a-dehydroarylalanine alkyl esters, we attempted to
quench the fluorescence of (Z)-1q showing relatively strong
emission by TEA and then to estimate free energy change
(DGet) in electron transfer from TEA to the singlet ex-
cited-state 1a, the fluorescence of which was too weak to
be detected. As clearly shown in Figure 2, fluorescence aris-
ing from the anthrylmethylene chromophore in 1q was
quenched by TEA according to the Stern–Volmer equation:
I0/I¼1+1.57[TEA], where I and I0 are the fluorescence
intensities of 1q with and without TEA, respectively. Be-
cause singlet–singlet energy transfer from the excited-state
anthryl group to TEA is highly endothermic (UV spectral
analysis), the above observation substantiates an ET emis-
sion quenching. Furthermore, the simplified Weller equa-
tion: DGet/kJ molꢁ1¼96.5 (EoxꢁEred)ꢁES,11 where Eox,
Ered, and ES refer to the oxidation potential of TEA
subsequent increase in compositions for cis-2a and trans-2a
with the decrease of (E)- and (Z)-isomer compositions, being
consistent with the mechanism in which either of these iso-
mers serves as a precursor of the products. Since the bond
formed between the N-benzoyl carbonyl oxygen and the ole-
finic carbon in the naphthylmethylene moiety constructs
a 4,5-dihydrooxazole ring, we suggest that the (E)-isomer
serves as a precursor of 2. Inspection of the data given in
Table 1 also revealed that the composition ratio of
cis-2a to trans-2a decreases with irradiation time. The
finding that the heat of formation for cis-2a (DHf¼
ꢁ232.8 kJ molꢁ1) is greater than that (ꢁ237.0 kJ molꢁ1
)
for trans-2a indicates the occurrence of TEA-catalyzed
isomerization to the thermodynamically more stable trans-
isomer.10 This was substantiated by the observation that
on allowing a deuterated methanol (CD3OD) solution of
cis-2a (4.0ꢀ10ꢁ3 mol dmꢁ3, 1.0 mL) containing TEA
(0.10 mol dmꢁ3) to stand at room temperature, the isomeri-
zation to the trans-isomer slowly proceeds to furnish a 9:1
equilibrium mixture of trans-2a and cis-2a, respectively,
after 7 days (Table 2). Additionally, a deuterium atom was
incorporated at the 4-position on the oxazole ring when
the equilibrium was established, confirming the existence
of a carbanion intermediate in the isomerization process.
In order to examine the effect of TEA concentration on
the photoreactivity of 1a and the product compositions,
nitrogen-saturated methanol solutions of (Z)-1a (4.0ꢀ
10ꢁ3 mol dmꢁ3, 10 mL), containing varying amounts of
TEA were irradiated in parallel for 3 h on a merry-
go-round-type irradiation equipment using the same filter
1.4
1.3
1.2
1.1
1.0
1
and light source. H NMR spectral analysis of the reaction
mixture obtained after usual workup gave the data collected
in Table 3, which demonstrate that the reactivity is increased
Table 2. Relation between reaction time and composition (%) of each com-
pound obtained by the hydrogen–deuterium exchange reaction of cis-2a at
room temperature
Ph
Ph
Ph
OMe
O
OMe
O
OMe
O
N
N
N
O
O
O
TEA
CD3OD
H
D
D
H
H
H
+
cis-2a
cis-2a-d1
trans-2a-d1
0
0.04
0.08
[TEA] (mol dm–3
0.12
0.16
Compound
Time (h)
3
)
0
0.5
1
2
5
24
0
168
Figure 2. Stern–Volmer plot for the fluorescence quenching of (Z)-1q
(1.0ꢀ10ꢁ4 mol dmꢁ3) by TEA in N2-saturated MeCN at room temperature.
Wavelengths for excitation and for recording emission intensities are 384
and 465 nm, respectively.
cis-2a
cis-2a-d1
trans-2a-d1
100 63.7 46.3 27.1 15.8
5.4
0
0
0
26.2 38.1 50.7 56.6 60.2 37.7 12.2
10.1 15.6 22.2 27.6 34.4 62.3 87.8

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K. Maekawa et al. / Tetrahedron 63 (2007) 11267–11281
(0.76 V vs Ag/AgCl in MeCN), the reduction potential of 1a
(ꢁ2.26 V vs Ag/AgCl in MeCN), and the first singlet excita-
tion energy of 1a (368 kJ molꢁ1 in MeCN), respectively,
which allowed us to estimate the free energy change
(Z)-1e
(DGet) for an ET from TEA to singlet 1a as ꢁ77 kJ molꢁ1
.
These findings are in full agreement with the involvement
of an ET in the primary process of the photocyclization reac-
tion of 1. In connection with the PET mechanism for this cy-
clization reaction, it is significant to confirm whether TEA
acts as an ET catalyst during the reaction. For this confirma-
tion a CD3OD solution of (Z)-1a (0.025 mol dmꢁ3) contain-
(Z)-1a
ing TEA (0.10 mol dmꢁ3
) and 1,4-dioxane (internal
standard, 0.10 mol dmꢁ3) was irradiated with Pyrex-filtered
light from a 450 W high-pressure Hg lamp for 4–8 h at room
temperature and subjected to H NMR spectral analysis.
(Z)-1f
1
360
380
400
420
440
460
480
500
Wavelength (nm)
This analysis showed no sign of a change in the amine con-
centration and, hence, TEAwas proved to be an ET catalyst.
Figure 4. Fluorescence spectra of (Z)-1a, (Z)-1e, and (Z)-1f
(1.0ꢀ10ꢁ4 mol dmꢁ3) in N2-saturated MeOH at room temperature. Excita-
tion wavelength is 320 nm.
Before we propose the PET mechanism by which the cycli-
zation reaction of 1 proceeds to give cis-2 in preference to
trans-2, we should discuss the possibility of a direct ET
from TEA to the N-acyl moiety in the excited-state (E)-1.
In Figure 3 are shown UVabsorption spectra of (Z)-1a, benz-
amide, 4-methoxybenzamide, and 4-cyanobenzamide
(1.0ꢀ10ꢁ4 mol dmꢁ3) in methanol. Clearly, benzamide
and its derivatives exhibit negligible absorptions at wave-
lengths longer than 300 nm whereas there is a strong absorp-
tion at 320 nm in the spectrum of 1a. It is, thus, very likely
that an electron from TEA is not directly shifted to the acyl
moiety but preferentially to the naphthylmethylene moiety.
This consideration strongly suggests that the initially formed
naphthylmethylene radical anion intramolecularly reduces
the acyl group to give the corresponding acyl radical anion,
the nucleophilicity of which is highly enhanced. If so, the
higher electron-withdrawing ability of a substituent attached
to the para-position on the N-benzoyl benzene ring is
expected to reduce the fluorescence intensity of (Z)-1 to
a greater extent. As Figure 4 demonstrates, the finding that
its emission intensity is lowered in the order of 1e
(R2¼OMe)>1a (R2¼H)>1f (R2¼CN) is consistent with
the above expectation and, hence, confirms the formation
of the acyl radical anion intermediate in a stepwise manner.
On the basis of the considerations described above, we were
led to propose Scheme 2 that explains the quantitative for-
mation of cis- and trans-dihydrooxazole derivatives (cis-2
and trans-2). In addition to the principle of least motion,12
molecular modeling of (Z)-1 and (E)-1 reveals that the latter
isomer adopts a most suitable conformation for the cycliza-
tion reaction that eventually gives 2. The previous finding
that the triplet-sensitized reaction of (Z)-N-acyl-a-dehydro-
phenylalanine derivatives gives only the corresponding
(E)-isomers without forming any cyclized products is in
perfect agreement with the involvement of the singlet ex-
cited-state 1 in the ET process.3,4 As already suggested, an
ET from TEA to the singlet excited-state (E)-1 results in
an initial formation of the (E)-isomer-derived arylmethylene
ꢂ
radical anion (E)-1Aꢁ , which activates the N-acyl carbonyl
oxygen through an intramolecular ET. Accordingly, we may
assume a dynamic equilibrium between these two radical
ꢂ
ꢂ
anion intermediates (E)-1Aꢁ and (E)-1Bꢁ (see Scheme
2). In addition, the observation of the TEA-catalyzed isom-
erization of 2 allows us to propose the cyclized radical anion/
TEA radical cation pair intermediate I formed by the
nucleophilic attack of N-acyl carbonyl oxygen upon the
ꢂ
olefinic carbon in the (E)-1-derived radical anion (E)-1Bꢁ .
There are two possible routes for a back ET in the radical ion
pair I. One is the transfer of an unpaired electron from the
2-position on the dihydrooxazole ring to the TEA radical
cation, which gives a zwitterion intermediate. The other is
the transfer of one of the paired electrons from the 4-position
on the oxazole ring to the TEA radical cation, as shown in
Scheme 2. Very recently we found that the PET-initiated
cyclization of bulky chiral auxiliary-substituted N-benzoyl-
a-dehydronaphthylalanine alkyl esters affords the corre-
sponding cis- and trans-4,5-dihydrooxazole derivatives in
high diastereomeric excess (de).13 In this study tertiary
amine and temperature effects on the de value for each oxa-
zole derivative provided a piece of evidence in support of
a biradical intermediate. It is, thus, reasonable to consider
that a back ET to the TEA radical cation in I regenerates
TEA with the appearance of the biradical intermediate II.
Owing to steric hindrance of a bulky aryl group bonded to
the oxazole ring at its 5-position, hydrogen shift in II pro-
ceeds from the opposite side of this group to afford cis-2
2.0
R= CN
R= OMe
H2N
R
1.5
1.0
0.5
0
O
(Z)-1a
R= H
250
300
350
400
Wavelength (nm)
Figure 3. UV absorption spectra of (Z)-1a and 4-substituted benzamides
(1.0ꢀ10ꢁ4 mol dmꢁ3) in MeOH at room temperature.

K. Maekawa et al. / Tetrahedron 63 (2007) 11267–11281
11271
R1O
+
O
O
N
H
Ar
TEA
h
Electron transfer (ET)
TEA
Back ET
Back ET
+
(Z)-1*
(Z)-1
TEA
(Z)-1
+
/TEA
(Z)-1
O
NH
Ar
+
O
h
TEA
ET
TEA
+
(E)-1
(E)-1*
(E)-1
TEA
OR1
+
(E)-1A /TEA
Ar
+
Ar
TEA
Ar
TEA
NH
NH
O
NH
O
O
O
O
+
O
Back ET
Cyclization
H shift
TEA
OR1
OR1
OR1
+
(E)-1B /
TEA
I
II
Ar
O
Ar
OR1
O
OR1
N
N
O
O
H
H
H
H
+
cis-2
trans-2
Scheme 2.
in preference to trans-2, and hence the observed hydrogen
shift can be regarded as a kinetically controlled process.
On the other hand, an ET to the singlet excited-state (Z)-1
should take place to give the (Z)-1 radical anion/TEA radical
cation pair intermediate, in competition with an ET to the
(E)-isomer. However, the nucleophilic attack of N-acyl car-
bonyl oxygen upon the olefinic carbon in this intermediate
undergoes a great steric hindrance of the aryl group, so
that a return ET should proceed exclusively in the (Z)-iso-
mer-derived radical ion pair to regenerate (Z)-1 and TEA
(Scheme 2).
electron-donating methoxy or an electron-withdrawing cya-
no group at the 4-position on the N-benzoyl benzene ring
lowers the excited-state reactivity of 1a to some extent.
This suggests the delocalization of an electron migrated to
the N-aroyl moiety, namely, a decrease in the nucleophilicity
of the N-aroyl carbonyl oxygen. Furthermore, the finding
that the selectivity of cis-2 decreases with increasing elec-
tron-withdrawing ability of the aryl group attached to the ox-
azole ring (2e>2a>2f) provides additional evidence for the
Table 4. Substituent effects on the conversion of 1 and selectivity of cis-2,
obtained by the irradiation of (Z)-1 in MeOH or EtOH at room temperature
2.3. Substituent effects in N-aroyl-a-dehydroarylalanine
alkyl esters
Compound Irradiation Conversiona Composition (%) Selectivityb
time (h)
(%)
(%)
cis-2
trans-2
For the purpose of exploring the substituent effects on the
photoreactivity of 1, the selectivity of cis-2 and the extent
to which the cis-isomer is isomerized to the corresponding
trans-isomer, a nitrogen-saturated methanol or ethanol solu-
tion of 1b–g (4.0ꢀ10ꢁ3 mol dmꢁ3, 500 mL) being irradiated
in the presence of TEA (0.10 mol dmꢁ3) for a given period of
time was subjected to 1H NMR spectral analysis in DMSO-
d6 (Table 4). Interestingly, the introduction of a bulky alkyl
substituent such as an isopropyl or a tert-butyl group into the
alkoxycarbonyl moiety of (Z)-1 produced the corresponding
cis-isomer in high selectivity and, in addition, considerably
suppressed the isomerization induced by TEA as a base. The
suppression of this isomerization was also achieved by
employing the less polar protic solvent, ethanol. Both the
steric bulkiness of the alkyl group and the polarity of a given
protic solvent are considered to exert great effects on the
abstraction of a proton at the 4-position on the oxazole
ring by TEA. On the other hand, the presence of an
1ac
1bd
1cc
1dc
1ec
1fc
0.5
5
0.5
5
0.5
5
0.5
5
0.5
5
0.5
5
0.5
5
16.5
100
18.9
100
18.9
100
20.0
100
12.6
92.7
9.0
75.4
8.7
11.5
36.9
13.2
68.3
16.5
80.9
18.4
91.5
9.8
60.6
4.6
13.5
5.0
5.0
63.1
5.7
31.7
2.4
19.1
1.6
8.5
2.8
32.1
4.4
61.9
3.7
70
37
70
68
87
81
92
92
78
65
51
18
57
28
1gc
73.1
20.6
52.5
a
Conversion was estimated by subtracting the sum of composition for (Z)-1
and (E)-1 from 100.
Selectivity was estimated by dividing composition for cis-2 by the sum of
composition for cis-2 ans trans-2.
In MeOH.
b
c
d
In EtOH. This solvent was used because of the occurrence of TEA-
catalyzed ester exchange reaction in MeOH.

11272
K. Maekawa et al. / Tetrahedron 63 (2007) 11267–11281
TEA-catalyzed isomerization of the cis-isomer to the trans.
Inspection of the data in Table 4 also confirms that replace-
ment of the 1-naphthyl group in 1a by the phenyl (1g) still
enables selective phototransformation into 2, though it re-
sults in a decrease in the excited-state reactivity of 1 as
well as in the selectivity of cis-2. It is very likely that the for-
mer decrease is due to the lower ability of the benzylidene
group to accept an electron from TEA and the less bulky
phenyl group is responsible for the latter decrease.
the electronic effect of a given substituent as one of the chief
determining factors. The enhanced p-electron density of the
benzene ring may slow down the relative rate for hydrogen
shift taking place at the side of this ring owing to electronic
repulsion. Interestingly, upon the introduction of methyl
group at either or both of the ortho-positions of the benzene
ring, the cis-selectivity was dramatically increased. For ex-
ample, the tert-butyl ester derivative 1n having two methyl
groups at the ortho-positions underwent PET-initiated cycli-
zation reaction to afford cis-2n selectively. This result con-
forms to that obtained for the 9-anthryl-substituted ester
derivative 1q and also a change in aryl substituent from
1-naphthyl (1d) to 2-naphthyl (1o) resulted in a great
decrease in the cis-selectivity (92%/79%; the latter value
is comparable to that of cis-2i with a 4-tolyl group). In addi-
tion, a comparison of the selectivity of cis-2d with that of
cis-2p confirmed that the cis-selectivity is not practically
influenced by the N-acyl substituent. Therefore, the consid-
erations described above led us to conclude that the selectiv-
ity of cis-2 carrying a tert-butoxycarbonyl group is
controlled not only by the p-electron density of a substituted
phenyl group attached at the 5-position on the dihydro-
oxazole ring but also by the steric bulkiness around the
ortho-positions of this phenyl group.
2.4. Substituent effects in N-acyl-a-dehydroarylalanine
tert-butyl esters
The bulky tert-butoxycarbonyl group in cis-2 was already
shown to suppress the TEA-catalyzed isomerization of this
isomer to a considerable extent even at 5 h irradiation (Table
4). It is thus possible to discuss quantitatively the substituent
effects on the selectivity of the cis-4,5-dihydrooxazole iso-
mer and to shed much light on factors, which influence not
only the cis-selectivity but also the photoreactivity of 1. In
Table 5 are summarized the conversion of 1, the compo-
sitions of cis-2 and trans-2, and the selectivity of cis-2,
obtained by the irradiation of (Z)-1h–q in nitrogen-saturated
methanol and also included the results for (Z)-1d, for com-
parison. First of all, our attention is directed to the effect
of substituents (introduced into the phenyl group) on the
selectivity of cis-2. It is clear from the data in Table 5 that
the tert-butyl ester derivatives 1h–n are only subject to the
very little TEA-catalyzed isomerization reactions except 1l
bearing the cyano group. Because the para-substituents
attached to the benzene ring should exert negligible steric ef-
fects on hydrogen shift in the biradical intermediate II, the
finding that the selectivity of cis-2 is lowered in the order
of MeO(1h)>Me(1i)>H(1j)>Cl(1k)>CN(1l) substantiates
Next we concentrate our attention on the more detailed sub-
stituent effects on the photoreactivity of 1 in the presence of
TEA. As shown in Scheme 2, inter- and intramolecular ET
ꢂ
ꢂ
reactions giving the radical anions (E)-1Aꢁ and (E)-1Bꢁ ,
respectively, are regarded as reversible processes, so that
the photoreactivity (i.e., the conversion) of 1 may be deter-
mined by the overall efficiency of these two processes, as al-
ready suggested. An increase in the stability of the initially
ꢂ
formed radical anion (E)-1Aꢁ is considered to cause a
decrease in the reactivity of this intermediate and, hence, to
ꢂ
shift the equilibrium between (E)-1Aꢁ and (E)-1Bꢁꢂ to the
side of the former radical anion. Additionally, the enhanced
stability of the N-acyl radical anion (E)-1Bꢁꢂ should acceler-
ate an intramolecular ET to the acyl moiety but diminish the
nucleophilicity of the acyl carbonyl oxygen. These two ef-
fects, which are compensated by each other, are exemplified
in the N-acyl substituent effect on the photoreactivity of 1a,
1e, and 1f. Accordingly, the finding [that the reactivity of
1h–l possessing a 4-substituted phenyl group is small and
is increased in the following order: 4-OMeC6H4 (1h)<4-
MeC6H4 (1i)<Ph (1j)<4-CNC6H4 (1l)<4-ClC6H4 (1k)] re-
vealed that the reactivity of these derivatives is determined
by the balance of consecutive inter- and intramolecular ET
reaction efficiencies. The difference in reactivity between
1d and 1p having the N-benzoyl and N-pivaloyl groups, re-
spectively, is also interpreted in a similar way. On the other
hand, the importance of efficiency for the cyclization pro-
cess yielding the intermediate I is found in the fact that the
2-naphthyl-substituted derivative 1o possesses a higher
photoreactivity than that of the 1-naphthyl-substituted 1d
despite almost the same electron-accepting abilities of the
1-naphthyl and 2-naphthyl groups. In other words, the latter
group exerts a less steric effect on the above-mentioned
cyclization process.
Table 5. Substituent effects on the conversion of 1 and selectivity of cis-2,
obtained by the irradiation of (Z)-1 in MeOH at room temperature
Compound Irradiation Conversiona Composition (%) Selectivityb
time (h)
(%)
(%)
cis-2
trans-2
1d
1h
1i
0.5
3
0.5
3
0.5
3
0.5
3
0.5
3
0.5
3
0.5
3
0.5
3
20.0
75.0
5.1
30.9
6.3
37.1
6.6
40.8
9.5
57.6
7.4
42.2
4.9
29.8
2.1
18.4
68.9
4.1
24.5
5.0
29.0
5.1
31.6
7.1
42.5
5.0
23.0
4.7
28.5
2.1
1.6
6.1
1.0
6.4
1.3
8.1
1.5
9.2
2.4
15.1
2.4
19.2
0.2
1.3
0
92
92
80
79
79
78
77
77
75
74
68
55
96
96
100
100
79
1j
1k
1l
1m
1n
1o
1p
1q
14.0
33.3
100
12.4
50.7
7.4
14.0
26.2
78.1
11.6
46.9
7.4
0
0.5
3
0.5
3
0.5
3
7.1
21.9
0.8
3.8
0
78
94
93
100
100
21.1
21.1
0
a
Conversion was estimated by subtracting the sum of composition for (Z)-1
and (E)-1 from 100.
Selectivity was estimated by dividing composition for cis-2 by the sum of
composition for cis-2 ans trans-2.
As already stated, the introduction of a bulky 2,6-dimethyl-
pheny or a 9-anthryl group enabled the selective transforma-
tion into the corresponding cis-4,5-dihydrooxazole
b

K. Maekawa et al. / Tetrahedron 63 (2007) 11267–11281
11273
Table 6. Relation between irradiation time and composition of each
compound obtained by the irradiation of (Z)-1a in MeCN containing TEA
at room temperature
derivative. This finding suggests that hydrogen shift (in the
intermediate II) giving trans-2 meets with large steric hin-
drance exerted by both the aryl and tert-butoxycarbonyl
groups and, hence, becomes a kinetically controlled process.
The bulky aryl group should also exert a great steric hin-
drance toward the nucleophilic attack of the N-acyl carbonyl
Irradiation
time (h)
Composition (%)
cis-2a/trans-2a
(Z)-1a (E)-1a cis-2a trans-2a 3a
ꢂ
0
0.5
1
2
3
100
0
0
7.1
14.8
30.2
42.2
44.1
0
2.6
5.2
11.6
17.9
25.2
0
0
0
0
—
oxygen on the olefinic carbon in (E)-1Bꢁ . Therefore, the
41.3 49.0
36.4 43.6
26.3 31.9
16.7 20.5
2.7
2.8
2.6
fact (that as in the case of 1n with a 2,6-dimethylphenyl
group, the photoreactivity of 9-anthryl-substituted 1q is
much lower than that of 1-naphthyl-substituted 1d despite
the larger electron-accepting ability of the former derivative)
confirms that the bulky aryl substituent exerts a much greater
steric hindrance toward the above nucleophilic attack caus-
ing a substantial decrease in the photoreactivity. While the
prolonged irradiation of 1q is responsible for undesirable
side reactions, the PET-initiated cyclization of 1n proceeded
without undergoing any side reactions to selectively produce
cis-2n.
2.7 2.4
30.7 1.8
5
0
0
3. Conclusions
As already described in Section 1, there are several synthetic
routes to 4,5-dihydrooxazole derivatives but any convenient
photochemical route to these derivatives is not yet developed.
The procedure for preparing the starting (Z)-N-acyl-a-dehy-
droarylalanine alkyl esters (Z)-1 with various types of alkyl,
aryl, and acyl substituents is simple and readily applicable to
their related compounds. The PET reactions of 1, except
9-anthryl-substituted tert-butyl ester derivative, proceed effi-
ciently in the presence of TEA (which is readily evacuated
from the reaction mixture) to quantitatively give the corre-
sponding cis- and trans-4,5-dihydrooxazoles without any un-
dergoing undesirable side reactions. These two product
compositions can be controlled by utilizing the steric and
electronic effects of alkyl and aryl substituents introduced
into (Z)-1, in addition to the amine basicity and the solvent
property. Therefore, the PET-initiated cyclization reaction
of 1 constitutes a novel photochemical method for the con-
struction of a pharmaceutically useful dihydrooxazole ring.
2.5. Solvent effects
In a previous study we found that the change in solvent from
methanol to acetonitrile affects the photoreactivity of
N-acyl-a-dehydronaphthylalaninamide derivatives as well
as the selectivity of the corresponding 3,4-dihydrobenzo[f]-
quinolinones as major products.5 The hydrogen-bonding
solvation ability of methanol was verified to play a major
role in this solvent effect. In order to explore the role of hy-
drogen-bonding solvation in our PET-initiated cyclization
forming 4,5-dihydrooxazoles in quantitative yields, the
methyl ester derivative (Z)-1a and acetonitrile were chosen
as typical N-acyl-a-dehydronaphthylalanine alkyl ester and
aprotic polar solvent, respectively. Product analysis in aceto-
nitrile gave the unexpected result that the prolonged irradia-
tion induces mainly the side reaction of cis-2a to afford N-
benzoyl-1-naphthylalanine methyl ester (3a), as shown in
Scheme 3. This byproduct was not obtained without TEA
and also in methanol containing TEA, so that the role of
the protic polar solvent is considered to inhibit the dihy-
drooxazole ring-opening reaction through hydrogen-bond-
ing solvation of the cis-2a-derived radical anion. In Table
6 are summarized the dependence of the product composi-
tion and the selectivity of cis-2a on irradiation time. Though
the use of acetonitrile caused a side reaction of cis-2a, it sup-
pressed TEA-catalyzed isomerization into trans-2a accom-
panied by a slight increase in the cis-selectivity as well as
by a minor decrease in the photoreactivity. Taking into ac-
count the fact that the presence of the bulky alkoxycarbonyl
group prevents the cis-isomer from isomerizing into the
trans-isomer, we propose that methanol is a more suitable
solvent for the reaction.
4. Experimental
4.1. General
1H and 13C NMR, and IR spectra were taken with a JEOL
JNM-A500 spectrometer and a SHIMADZU PRESTIGE-
21 infrared spectrometer, respectively. Chemical shifts
were determined using tetramethylsilane as an internal stan-
dard. UVabsorption and fluorescence spectra were measured
at room temperature with a HITACHI U-3300 spectropho-
tometer and a HITACHI F-4500 spectrofluorimeter, respec-
tively. A cell with a 10-mm pathlength was used. Elemental
analysis was performed on a PERKIN–ELMER PE2400 se-
ries II CHNS/O analyzer. Oxidation and reduction potentials
were measured with a YANACO P-1100 polarographic
analyzer. Methanol, ethanol, and acetonitrile were purified
according to the standard procedures14 and distilled just be-
fore their use. TEA was fractionally distilled from sodium
hydroxide. All other reagents used were obtained from
commercial sources and of the highest grade available.
O
NH
OMe
4.2. General procedure for the synthesis of (Z)-4-
(1-naphthylmethylene)-2-(4-substituted phenyl)-5(4H)-
oxazolones, (Z)-2-phenyl-4-(substituted benzylidene)-
5(4H)-oxazolones, (Z)-4-(2-naphthylmethylene)-2-phen-
yl-5(4H)-oxazolone, (Z)-2-(tert-butyl)-4-(1-naphthyl-
methylene)-5(4H)-oxazolone, and (Z)-4-
h
(Z)-1a
+
+
+
cis-2a
trans-2a
O
TEA/MeCN
3a
(E)-1a
Scheme 3.
(9-anthrylmethylene)-2-phenyl-5(4H)-oxazolone

11274
K. Maekawa et al. / Tetrahedron 63 (2007) 11267–11281
N-(4-Substituted benzoyl)glycine or trimethylacetylglycine
(50 mmol), 1-naphthaldehyde, substituted benzaldehyde,
2-naphthaldehyde or anthracene-9-carbaldehyde (50 mmol),
and sodium acetate (20 mmol) were added to acetic anhydride
(10–30 mL) and the resulting mixture was heated at 65–
75 ꢃC for 1–2 h [N-(4-substituted benzoyl)glycine] or 6 h
(N-trimethylacetylglycine) with stirring. The mixture was
cooled with ice and the solid separated out was collected
by filtration with suction and washed with water, a small
amount of cold ethanol, and then with dry hexane. After
the crude product was air-dried at room temperature, it
was recrystallized from hexane–chloroform to give yellow
crystals (40–80%).
4.2.8. (Z)-4-(4-Cyanobenzylidene)-2-phenyl-5(4H)-ox-
azolone. Mp 215.0–216.0 ꢃC. IR (KBr): 2222, 1796, 1661,
1
1292 cmꢁ1. H NMR (500 MHz, CDCl3): d 7.19 (1H, s),
7.57 (2H, dd, J¼7.4, 8.0 Hz), 7.67 (1H, dd, J¼7.4,
7.4 Hz), 7.75 (2H, d, J¼8.6 Hz), 8.20 (2H, d, J¼8.0 Hz),
8.30 (2H, d, J¼8.6 Hz).
4.2.9. (Z)-4-(2-Methylbenzylidene)-2-phenyl-5(4H)-ox-
azolone. Mp 134.0–135.0 ꢃC. IR (KBr): 1788, 1643,
1
1200 cmꢁ1. H NMR (500 MHz, CDCl3): d 2.53 (3H, s),
7.26 (1H, d, J¼9.2 Hz), 7.33–7.36 (2H, m), 7.53 (2H, dd,
J¼7.4, 8.0 Hz), 7.55 (1H, s), 7.61 (1H, dd, J¼7.4, 7.4 Hz),
8.18 (2H, d, J¼8.0 Hz), 8.81 (1H, d, J¼9.7 Hz).
4.2.1. (Z)-4-(1-Naphthylmethylene)-2-phenyl-5(4H)-oxa-
zolone. Mp 166.0–167.0 ꢃC. IR (KBr): 1797, 1647,
4.2.10. (Z)-4-(2,6-Dimethylbenzylidene)-2-phenyl-5(4H)-
oxazolone. Mp 112.0–113.0 ꢃC. IR (KBr): 1798, 1663,
1
1
1167 cmꢁ1. H NMR (500 MHz, CDCl3): d 7.54 (2H, dd,
1171 cmꢁ1. H NMR (500 MHz, CDCl3): d 2.36 (6H, s),
J¼7.3, 7.6 Hz), 7.55 (1H, dd, J¼8.6, 8.6 Hz), 7.62 (1H,
dd, J¼7.3, 7.3 Hz), 7.63 (1H, dd, J¼8.6, 8.6 Hz), 7.64
(1H, dd, J¼6.7, 8.6 Hz), 7.90 (1H, d, J¼8.6 Hz), 7.97 (1H,
d, J¼8.6 Hz), 8.13 (1H, s), 8.21 (2H, d, J¼7.6 Hz), 8.31
(1H, d, J¼8.6 Hz), 9.03 (1H, d, J¼6.7 Hz).
7.12 (2H, d, J¼7.4 Hz), 7.23 (1H, dd, J¼7.4, 7.4 Hz), 7.48
(2H, dd, J¼7.4, 8.0 Hz), 7.53 (1H, s), 7.59 (1H, dd, J¼8.0,
8.0 Hz), 8.09 (2H, d, J¼7.4 Hz).
4.2.11. (Z)-4-(2-Naphthylmethylene)-2-phenyl-5(4H)-ox-
azolone. Mp 142.0–143.0 ꢃC. IR (KBr): 1797, 1626,
1
4.2.2. (Z)-2-(4-Methoxyphenyl)-4-(1-naphthylmethy-
lene)-5(4H)-oxazolone. Mp 207.0–208.0 ꢃC. IR (KBr):
1167 cmꢁ1. H NMR (500 MHz, CDCl3): d 7.51 (1H, dd,
J¼7.9, 7.9 Hz), 7.54 (1H, dd, J¼7.9, 7.9 Hz), 7.54 (2H,
dd, J¼7.9, 7.9 Hz), 7.61 (1H, dd, J¼7.9, 7.9 Hz), 7.83
(1H, d, J¼7.9 Hz), 7.89 (1H, d, J¼8.5 Hz), 7.91 (1H, d,
J¼7.9 Hz), 8.19 (2H, d, J¼7.9 Hz), 8.43 (1H, s), 8.49 (1H,
d, J¼8.5 Hz).
1788, 1644, 1170 cmꢁ1 1H NMR (500 MHz, CDCl3):
.
d 3.91 (3H, s), 7.03 (2H, d, J¼8.8 Hz), 7.54–7.66 (3H, m),
7.90 (1H, d, J¼7.9 Hz), 7.95 (1H, d, J¼7.9 Hz), 8.07 (1H,
s), 8.16 (2H, d, J¼8.8 Hz), 8.32 (1H, d, J¼8.5 Hz), 9.02
(1H, d, J¼7.3 Hz).
4.2.12. (Z)-2-(tert-Butyl)-4-(1-naphthylmethylene)-
5(4H)-oxazolone. Mp 89.0–90.0 ꢃC. IR (KBr): 1794,
4.2.3. (Z)-2-(4-Cyanophenyl)-4-(1-naphthylmethylene)-
5(4H)-oxazolone. Mp 237.0–238.0 ꢃC. IR (KBr): 2236,
1652, 1647, 1150 cmꢁ1 1H NMR (500 MHz, CDCl3):
.
1794, 1641, 1164 cmꢁ1
.
1H NMR (500 MHz, CDCl3):
d 1.41 (9H, s), 7.54 (1H, dd, J¼6.9, 8.2 Hz), 7.60 (1H, dd,
J¼7.6, 8.2 Hz), 7.61 (1H, dd, J¼6.9, 8.9 Hz), 7.89 (1H, d,
J¼8.2 Hz), 7.94 (1H, d, J¼8.2 Hz), 8.04 (1H, s), 8.27 (1H,
d, J¼8.9 Hz), 8.89 (1H, d, J¼7.6 Hz).
d 7.59 (1H, dd, J¼7.3, 7.9 Hz), 7.64–7.68 (2H, m), 7.84
(2H, d, J¼8.2 Hz), 7.93 (1H, d, J¼7.9 Hz), 8.02 (1H, d,
J¼7.9 Hz), 8.25 (1H, s), 8.31 (2H, d, J¼8.2 Hz), 8.32 (1H,
d, J¼8.5 Hz), 9.00 (1H, d, J¼7.3 Hz).
4.2.13. (Z)-4-(9-Anthrylmethylene)-2-phenyl-5(4H)-ox-
azolone. Mp 231.0–233.0 ꢃC. IR (KBr): 1796, 1659,
4.2.4. (Z)-4-(4-Methoxybenzylidene)-2-phenyl-5(4H)-ox-
azolone. Mp 155.0–156.0 ꢃC. IR (KBr): 1773, 1650,
1
1186 cmꢁ1. H NMR (500 MHz, CDCl3): d 7.42 (2H, dd,
1
1267 cmꢁ1. H NMR (500 MHz, CDCl3): d 3.89 (3H, s),
J¼7.4, 8.0 Hz), 7.50–7.56 (5H, m), 7.99 (2H, d,
7.00 (2H, d, J¼7.5 Hz), 7.22 (1H, s), 7.52 (2H, dd, J¼6.9,
J¼7.4 Hz), 8.06–8.13 (4H, m), 8.35 (1H, s), 8.57 (1H, s).
7.5 Hz), 7.59 (1H, dd, J¼6.9, 6.9 Hz), 8.16–8.20 (4H, m).
4.3. General procedure for the synthesis of alkyl (Z)-2-
(4-substituted benzoylamino)-3-(1-naphthyl)-2-prope-
noates [(Z)-1a–f], methyl (Z)-2-benzoylamino-3-phenyl-
2-propenoate [(Z)-1g], tert-butyl (Z)-3-aryl-2-benzoyl-
amino-2-propenoates [(Z)-1h–o,q], and tert-butyl (Z)-3-
(1-naphthyl)-2-pivaloylamino-2-propenoate [(Z)-1p]
4.2.5. (Z)-4-(4-Methylbenzylidene)-2-phenyl-5(4H)-ox-
azolone. Mp 137.0–138.0 ꢃC. IR (KBr): 1797, 1653,
1
1296 cmꢁ1. H NMR (500 MHz, CDCl3): d 2.37 (3H, s),
7.29 (1H, s), 7.33 (2H, d, J¼7.9 Hz), 7.62 (2H, dd, J¼7.3,
7.3 Hz), 7.70 (1H, dd, J¼7.3, 7.3 Hz), 8.10 (2H, d,
J¼7.3 Hz), 8.17 (2H, d, J¼7.9 Hz).
(Z)-4-(1-Naphthylmethylene)-2-(4-substituted phenyl)-5(4H)-
oxazolone (for 1a–f, 10 mmol), (Z)-2-phenyl-4-(substituted
benzylidene)-5(4H)-oxazolone (for 1g–n, 10 mmol), (Z)-4-
(2-naphthylmethylene)-2-phenyl-5(4H)-oxazolone (for 1o,
10 mmol), (Z)-2-tert-butyl-4-(1-naphthylmethylene)-5(4H)-
oxazolone (for 1p, 10 mmol), or (Z)-4-(9-anthrylmethylene)-
2-phenyl-5(4H)-oxazolone (for 1q, 10 mmol) were added to
a given alcohol (60 mL) containing TEA or DBU (5.0 mmol)
andtheresultingsolutionwasrefluxedfor1–2 h. Afterremoval
of the solvent under reduced pressure, the reaction mixture
obtained was dissolved in chloroform (50–100 mL) and then
washed twice with hydrochloric acid (3 mol dmꢁ3, 50 mL),
4.2.6. (Z)-4-Benzylidene-2-phenyl-5(4H)-oxazolone. Mp
162.0–163.0 ꢃC. IR (KBr): 1779, 1659, 1266 cmꢁ1 1H
.
NMR (500 MHz, CDCl3): d 7.25 (1H, s), 7.47 (2H, dd,
J¼7.4, 8.0 Hz), 7.49 (1H, dd, J¼7.4, 7.4 Hz), 7.54 (2H,
dd, J¼7.2, 7.4 Hz), 7.62 (1H, dd, J¼7.4, 7.4 Hz), 8.19
(2H, d, J¼8.0 Hz), 8.21 (2H, d, J¼7.2 Hz).
4.2.7. (Z)-4-(4-Chlorobenzylidene)-2-phenyl-5(4H)-ox-
azolone. Mp 192.5–193.5 ꢃC. IR (KBr): 1797, 1770, 1656,
1
1236 cmꢁ1. H NMR (500 MHz, CDCl3): d 7.19 (1H, s),
7.43–7.63 (5H, m), 8.13–8.21 (4H, m).

K. Maekawa et al. / Tetrahedron 63 (2007) 11267–11281
11275
and finally dried over sodium sulfate. Evaporation of
chloroform in vacuo gave the crystalline solid, which was
recrystallized from ethyl acetate–hexane affording colorless
crystals (30–90%). Before recrystallization some of the
crude alkyl 2-acylamino-3-aryl-2-propenoates were chroma-
tographed on a silica gel column with ethyl acetate–
chloroform or ethyl acetate–hexane as an eluent for
preliminary purification.
d 27.6 (3C), 80.7, 124.0, 125.4, 126.0, 126.5 (2C), 127.5
(2C), 128.2 (2C), 128.4, 128.6, 128.9, 130.4, 130.7, 130.9,
131.5, 133.1, 133.6, 163.7, 166.5. Anal. Calcd (found) for
C24H23NO3: C, 77.19 (77.35); H, 6.21 (6.29); N, 3.75%
(3.74%).
4.3.5. Methyl (Z)-2-(4-methoxybenzoylamino)-3-(1-
naphthyl)-2-propenoate [(Z)-1e]. Mp 137.0–138.0 ꢃC. IR
(KBr): 3275, 1717, 1639 cmꢁ1 1H NMR (500 MHz,
.
4.3.1. Methyl (Z)-2-benzoylamino-3-(1-naphthyl)-2-pro-
penoate [(Z)-1a]. Mp 118.0–118.5 ꢃC. IR (KBr): 3200,
DMSO-d6): d 3.79 (3H, s), 3.80 (3H, s), 6.99 (2H, d,
J¼8.6 Hz), 7.49 (1H, dd, J¼6.9, 8.6 Hz), 7.55–7.59 (2H,
m), 7.67 (1H, d, J¼6.9 Hz), 7.84 (2H, d, J¼8.6 Hz), 7.87
(1H, s), 7.92 (1H, d, J¼8.6 Hz), 7.96–7.98 (1H, m), 8.02–
8.04 (1H, m), 9.84 (1H, s). 13C NMR (125 MHz, DMSO-
d6) d 52.2, 55.3, 113.5 (2C), 124.0, 125.28, 125.34, 126.1,
126.5, 126.6, 128.4, 129.0, 129.1, 129.2, 129.5 (2C),
130.5, 130.9, 133.1, 162.0, 165.4, 165.7. Anal. Calcd
(found) for C22H19NO4: C, 73.12 (73.01); H, 5.30 (5.16);
N, 3.88% (3.81%).
1
1732, 1632 cmꢁ1. H NMR (500 MHz, DMSO-d6): d 3.80
(3H, s), 7.46 (2H, dd, J¼7.6, 7.6 Hz), 7.51 (1H, dd, J¼7.6,
8.9 Hz), 7.55 (1H, dd, J¼7.6, 7.6 Hz), 7.58 (1H, dd,
J¼6.9, 6.9 Hz), 7.59 (1H, dd, J¼6.9, 7.6 Hz), 7.69 (1H, d,
J¼7.6 Hz), 7.84 (2H, d, J¼7.6 Hz), 7.90 (1H, s), 7.94 (1H,
d, J¼8.9 Hz), 7.97 (1H, d, J¼6.9 Hz), 8.05 (1H, d,
J¼7.6 Hz), 10.0 (1H, s). 13C NMR (125 MHz, DMSO-d6)
d 52.3, 124.1, 125.4, 126.2, 126.62, 126.65, 127.6 (2C),
128.3 (2C), 128.5, 128.9, 129.2, 129.9, 130.5, 131.0,
131.8, 133.15, 133.22, 165.3, 166.4. Anal. Calcd (found)
for C21H17NO3: C, 76.12 (75.88); H, 5.17 (4.98); N,
4.23% (4.19%).
4.3.6. Methyl (Z)-2-(4-cyanobenzoylamino)-3-(1-naph-
thyl)-2-propenoate [(Z)-1f]. Mp 166.5–167.5 ꢃC. IR
1
(KBr): 3325, 2230, 1709, 1626 cmꢁ1. H NMR (500 MHz,
DMSO-d6): d 3.80 (3H, s), 7.51 (1H, dd, J¼7.4, 8.0 Hz),
7.55–7.61 (2H, m), 7.67 (1H, d, J¼7.4 Hz), 7.94–7.99
(7H, m), 8.02 (1H, d, J¼8.0 Hz), 10.27 (1H, s). 13C NMR
(125 MHz, DMSO-d6) d 52.4, 114.2, 118.2, 124.1, 125.5,
126.3, 126.68, 126.73, 128.4 (2C), 128.6, 129.4, 130.3,
130.7 (2C), 130.9, 132.5 (2C), 133.2, 137.1, 165.0 (2C).
Anal. Calcd (found) for C22H16N2O3: C, 74.15 (74.12); H,
4.59 (4.62); N, 7.86% (7.86%).
4.3.2. Ethyl (Z)-2-benzoylamino-3-(1-naphthyl)-2-prope-
noate [(Z)-1b]. Mp 108.0–109.0 ꢃC. IR (KBr): 3250, 1720,
1644 cmꢁ1. 1H NMR (500 MHz, DMSO-d6): d 1.29 (3H, t,
J¼6.9 Hz), 4.26 (2H, q, J¼6.9 Hz), 7.46 (2H, dd, J¼7.6,
7.6 Hz), 7.51 (1H, dd, J¼7.6, 7.6 Hz), 7.55 (1H, dd,
J¼7.6, 7.6 Hz), 7.57 (1H, dd, J¼6.9, 6.9 Hz), 7.59 (1H,
dd, J¼6.9, 7.6 Hz), 7.70 (1H, d, J¼7.6 Hz), 7.84 (2H, d,
J¼7.6 Hz), 7.90 (1H, s), 7.94 (1H, d, J¼7.6 Hz), 7.97 (1H,
d, J¼6.9 Hz), 8.05 (1H, d, J¼7.6 Hz), 9.98 (1H, s). 13C
NMR (125 MHz, DMSO-d6) d 14.1, 61.0, 124.1, 125.4,
126.2, 126.7 (2C), 127.6 (2C), 128.4 (2C), 128.5, 128.2,
129.3, 129.6, 130.5, 131.0, 131.8, 133.2, 133.4, 164.8,
166.5. Anal. Calcd (found) for C22H19NO3: C, 76.50
(76.18); H, 5.54 (5.46); N, 4.06% (3.99%).
4.3.7. Methyl (Z)-2-benzoylamino-3-phenyl-2-prope-
noate [(Z)-1g]. Mp 139.0–140.0 ꢃC. IR (KBr): 3333, 1715,
1
1643 cmꢁ1. H NMR (500 MHz, DMSO-d6): d 3.74 (3H,
s), 7.37 (1H, dd, J¼7.4, 7.4 Hz), 7.41 (2H, dd, J¼6.9,
7.4 Hz), 7.43 (1H, s), 7.54 (2H, dd, J¼7.4, 7.4 Hz), 7.61
(1H, dd, J¼7.4, 7.4 Hz), 7.68 (2H, d, J¼6.9 Hz), 7.99 (2H,
d, J¼7.4 Hz), 10.11 (1H, s). 13C NMR (125 MHz, DMSO-
d6) d 52.2, 126.5, 127.6 (2C), 128.4 (2C), 128.6 (2C),
129.5, 129.8 (2C), 131.8, 133.10, 133.12, 133.3, 165.4,
166.0. Anal. Calcd (found) for C17H15NO3: C, 72.58
(72.43); H, 5.37 (5.46); N, 4.98% (5.10%).
4.3.3. Isopropyl (Z)-2-benzoylamino-3-(1-naphthyl)-2-
propenoate [(Z)-1c]. Mp 89.0–90.0 ꢃC. IR (KBr): 3240,
1
1721, 1638 cmꢁ1. H NMR (500 MHz, DMSO-d6): d 1.28
(6H, d, J¼6.2 Hz), 5.06 (1H, q, J¼6.2 Hz), 7.46 (2H, dd,
J¼7.6, 8.2 Hz), 7.51 (1H, dd, J¼6.9, 8.2 Hz), 7.55 (1H,
dd, J¼6.9, 8.6 Hz), 7.56 (1H, dd, J¼6.9, 7.6 Hz), 7.59
(1H, dd, J¼8.2, 8.2 Hz), 7.70 (1H, d, J¼6.9 Hz), 7.82 (2H,
d, J¼7.6 Hz), 7.85 (1H, s), 7.93 (1H, d, J¼8.2 Hz), 7.97
(1H, d, J¼8.6 Hz), 8.02 (1H, d, J¼7.6 Hz), 9.96 (1H, s).
13C NMR (125 MHz, DMSO-d6) d 21.6 (2C), 68.5, 124.1,
125.4, 126.2, 126.6, 126.7, 127.5 (2C), 128.3 (2C), 128.5,
129.07, 129.11, 129.7, 130.6, 130.9, 131.7, 133.2, 133.5,
164.3, 166.5. Anal. Calcd (found) for C23H21NO3: C,
76.86 (76.82); H, 5.89 (5.62); N, 3.90% (3.81%).
4.3.8. tert-Butyl (Z)-2-benzoylamino-3-(4-methoxy-
phenyl)-2-propenoate [(Z)-1h]. Mp 155.0–156.0 ꢃC. IR
(KBr): 3304, 1715, 1647 cmꢁ1 1H NMR (500 MHz,
.
DMSO-d6): d 1.44 (9H, s), 3.76 (3H, s), 6.96 (2H, d,
J¼9.2 Hz), 7.31 (1H, s), 7.53 (2H, dd, J¼7.4, 8.0 Hz), 7.59
(1H, dd, J¼7.4, 7.4 Hz), 7.63 (2H, d, J¼9.2 Hz), 7.96 (2H,
d, J¼8.0 Hz), 9.86 (1H, s). 13C NMR (125 MHz, DMSO-
d6): d 27.7 (3C), 55.2, 80.3, 114.1 (2C), 125.8, 126.1 (2C),
127.6 (2C), 128.4 (2C), 131.58, 131.63, 132.4, 133.8,
160.1, 164.2, 166.2. Anal. Calcd (found) for C21H23NO4:
C, 71.37 (71.01); H, 6.56 (6.58); N, 3.96% (3.94%).
4.3.4. tert-Butyl (Z)-2-benzoylamino-3-(1-naphthyl)-2-
propenoate [(Z)-1d]. Mp 120.0–121.0 ꢃC. IR (KBr): 3289,
1
1709, 1638 cmꢁ1. H NMR (500 MHz, DMSO-d6): d 1.50
4.3.9. tert-Butyl (Z)-2-benzoylamino-3-(4-methylphenyl)-
2-propenoate [(Z)-1i]. Mp 150.0–151.0 ꢃC. IR (KBr): 3298,
(9H, s), 7.45 (2H, dd, J¼7.6, 8.2 Hz), 7.50 (1H, dd, J¼6.9,
8.2 Hz), 7.54 (1H, dd, J¼7.6, 7.6 Hz), 7.58 (1H, dd,
J¼6.9, 8.2 Hz), 7.59 (1H, dd, J¼6.9, 7.6 Hz), 7.68 (1H, d,
J¼6.9 Hz), 7.81 (2H, d, J¼8.2 Hz), 7.82 (1H, s), 7.92 (1H,
d, J¼8.2 Hz), 7.96 (1H, d, J¼7.6 Hz), 8.01 (1H, d,
J¼8.2 Hz), 9.88 (1H, s). 13C NMR (125 MHz, DMSO-d6)
1
1718, 1641 cmꢁ1. H NMR (500 MHz, DMSO-d6): d 1.44
(9H, s), 2.29 (3H, s), 7.20 (2H, d, J¼7.5 Hz), 7.30 (1H, s),
7.51–7.56 (4H, m), 7.59 (1H, dd, J¼7.5, 7.5 Hz), 7.96 (2H,
d, J¼7.5 Hz), 9.91 (1H, s). 13C NMR (125 MHz, DMSO-
d6): d 20.9, 27.6 (3C), 80.5, 127.3, 127.6 (2C), 128.4 (2C),

11276
K. Maekawa et al. / Tetrahedron 63 (2007) 11267–11281
129.2 (2C), 129.7 (2C), 130.9, 131.7, 132.2, 133.7, 139.1,
164.1, 166.2. Anal. Calcd (found) for C21H23NO3: C, 74.75
(74.33); H, 6.87 (6.53); N, 4.15% (4.37%).
4.3.15. tert-Butyl (Z)-2-benzoylamino-3-(2-naphthyl)-2-
propenoate [(Z)-1o]. Mp 169.0–170.0 ꢃC. IR (KBr): 3323,
1
1707, 1660 cmꢁ1. H NMR (500 MHz, DMSO-d6): d 1.48
(9H, s), 7.49 (1H, s), 7.51–7.56 (4H, m), 7.61 (1H, dd,
J¼7.4, 7.4 Hz), 7.82 (1H, d, J¼8.6 Hz), 7.86–7.91 (3H,
m), 7.98 (2H, d, J¼7.4 Hz), 8.19 (1H, s), 10.1 (1H, s). 13C
NMR (125 MHz, DMSO-d6): d 28.5 (3C), 81.5, 126.9,
127.4, 127.9, 128.3, 128.4 (2C), 128.7, 129.1, 129.2, 129.3
(2C), 131.1, 132.2, 132.6, 132.7, 133.5, 133.8, 134.5,
164.9, 167.2. Anal. Calcd (found) for C24H23NO3: C,
77.19 (77.02); H, 6.21 (6.19); N, 3.75% (3.70%).
4.3.10. tert-Butyl (Z)-2-benzoylamino-3-phenyl-2-prope-
noate [(Z)-1j]. Mp 144.0–145.0 ꢃC. IR (KBr): 3248, 1709,
1
1641 cmꢁ1. H NMR (500 MHz, DMSO-d6): d 1.44 (9H,
s), 7.31 (1H, s), 7.34 (1H, dd, J¼7.5, 7.5 Hz), 7.40 (2H,
dd, J¼7.5, 7.5 Hz), 7.51 (2H, dd, J¼7.5, 7.5 Hz), 7.60
(1H, dd, J¼7.5, 7.5 Hz), 7.65 (2H, d, J¼7.5 Hz), 7.95 (2H,
d, J¼7.5 Hz), 9.98 (1H, s). 13C NMR (125 MHz, DMSO-
d6): d 27.1 (3C), 80.1, 127.1 (2C), 127.7, 127.9 (2C),
128.1 (2C), 128.7, 129.2 (2C), 131.2, 131.4, 133.15,
133.19, 163.5, 165.8. Anal. Calcd (found) for C20H21NO3:
C, 74.28 (74.11); H, 6.55 (6.21); N, 4.33% (4.43%).
4.3.16. tert-Butyl (Z)-3-(1-naphthyl)-2-pivaloylamino-2-
propenoate [(Z)-1p]. Mp 154.0–155.0 ꢃC. IR (KBr): 3271,
1
1703, 1639 cmꢁ1. H NMR (500 MHz, DMSO-d6): d 1.07
(9H, s), 1.48 (9H, s), 7.51 (1H, dd, J¼7.6, 8.2 Hz), 7.56–
7.57 (2H, m), 7.61 (1H, d, J¼7.6 Hz), 7.68 (1H, s), 7.91–
7.97 (3H, m), 8.94 (1H, s). 13C NMR (125 MHz, DMSO-
d6): d 26.9 (3C), 26.9 (3C), 38.1, 80.4, 124.1, 125.3, 126.0,
126.5, 126.8, 127.7, 128.5, 128.9, 130.8 (2C), 131.0,
133.2, 164.1, 177.2. Anal. Calcd (found) for C22H27NO3:
C, 74.76 (74.66); H, 7.70 (7.70); N, 3.96% (4.18%).
4.3.11. tert-Butyl (Z)-2-benzoylamino-3-(4-chloro-
phenyl)-2-propenoate [(Z)-1k]. Mp 171.0–172.0 ꢃC. IR
(KBr): 3281, 1709, 1641 cmꢁ1 1H NMR (500 MHz,
.
DMSO-d6): d 1.44 (9H, s), 7.29 (1H, s), 7.47 (2H, d,
J¼7.4 Hz), 7.53 (2H, dd, J¼7.4, 7.4 Hz), 7.60 (1H,
dd, J¼7.4, 7.4 Hz), 7.66 (2H, d, J¼8.6 Hz), 7.94 (2H, d,
J¼8.6 Hz), 9.98 (1H, s). 13C NMR (125 MHz, DMSO-d6):
d 27.6 (3C), 80.8, 127.6 (2C), 128.4 (2C), 128.6 (2C),
128.8, 130.3, 131.3 (2C), 131.8, 132.7, 133.5, 133.6,
163.9, 166.3. Anal. Calcd (found) for C20H20ClNO3: C,
67.13 (67.20); H, 5.63 (5.34); N, 3.91% (3.99%).
4.3.17. tert-Butyl (Z)-3-(9-anthryl)-2-benzoylamino-2-
propenoate [(Z)-1q]. Mp 166.0–167.0 ꢃC. IR (KBr): 3323,
1
1692, 1670 cmꢁ1. H NMR (500 MHz, DMSO-d6): d 1.54
(9H, s), 7.29 (2H, dd, J¼7.4, 7.4 Hz), 7.41 (1H, dd, J¼7.4,
7.4 Hz), 7.46–7.51 (6H, m), 7.87 (1H, s), 8.04 (2H, d,
J¼7.4 Hz), 8.09 (2H, d, J¼7.4 Hz), 8.61 (1H, s), 9.62 (1H,
s). 13C NMR (125 MHz, DMSO-d6): d 28.0 (3C), 81.1,
125.6 (2C), 125.8, 126.0 (2C), 126.3 (2C), 127.7 (2C),
127.9, 128.0, 128.3 (2C), 129.0 (3C), 131.1 (2C), 131.7,
133.7, 134.1 (2C), 163.7, 166.9. Anal. Calcd (found) for
C28H25NO3: C, 79.41 (79.10); H, 5.95 (5.91); N, 3.31%
(3.22%).
4.3.12. tert-Butyl (Z)-2-benzoylamino-3-(4-cyanophenyl)-
2-propenoate [(Z)-1l]. Mp 160.0–161.0 ꢃC. IR (KBr): 3264,
1
2230, 1709, 1657 cmꢁ1. H NMR (500 MHz, DMSO-d6):
d 1.45 (9H, s), 7.29 (1H, s), 7.53 (2H, dd, J¼7.5, 7.5 Hz),
7.61 (1H, dd, J¼7.5, 7.5 Hz), 7.80 (2H, d, J¼7.5 Hz), 7.86
(2H, d, J¼8.6 Hz), 7.93 (2H, d, J¼8.6 Hz), 10.1 (1H, s).
13C NMR (125 MHz, DMSO-d6): d 19.9, 27.6 (3C), 80.6,
127.1 (2C), 127.4, 127.6 (2C), 128.3 (2C), 129.5, 131.2,
131.6, 133.0, 133.8, 136.0 (2C), 163.5, 166.1. Anal. Calcd
(found) for C21H20N2O3: C, 72.40 (72.16); H, 5.79 (5.45);
N, 8.04% (8.01%).
4.4. General procedure for the irradiation of (Z)-1a–q
In order to examine the dependence of the product distribu-
tion and composition on irradiation time, a nitrogen-satu-
4.3.13. tert-Butyl (Z)-2-benzoylamino-3-(2-methyl-
phenyl)-2-propenoate [(Z)-1m]. Mp 157.0–158.0 ꢃC. IR
rated methanol solution of (Z)-1 (4.0ꢀ10ꢁ3 mol dmꢁ3
,
500 mL) containing TEA (0.10 mol dmꢁ3), placed in a Pyrex
vessel, was irradiated with Pyrex-filtered light from a 400 W
high-pressure Hg lamp at room temperature (internal irradi-
ation). At suitable time intervals, an aliquot (5 mL) of the so-
lution was pipetted off and concentrated to dryness in vacuo.
The resulting residue was dissolved in DMSO-d6 and sub-
(KBr): 3296, 1711, 1643 cmꢁ1 1H NMR (500 MHz,
.
DMSO-d6): d 1.43 (9H, s), 2.31 (3H, s), 7.14 (1H, dd,
J¼6.9, 6.9 Hz), 7.19–7.25 (2H, m), 7.38 (1H, s), 7.46–7.49
(3H, m), 7.55 (1H, dd, J¼7.4, 7.4 Hz), 7.85 (2H, d,
J¼7.4 Hz), 9.83 (1H, s). 13C NMR (125 MHz, DMSO-d6):
d 19.9, 27.9 (3C), 80.9, 126.0, 127.9 (2C), 128.4, 128.7
(2C), 129.0, 129.3, 130.37, 130.44, 131.9, 133.1, 134.0,
137.4, 164.2, 166.7. Anal. Calcd (found) for C21H23NO3:
C, 74.75 (74.43); H, 6.87 (6.79); N, 4.15% (4.18%).
1
jected to H NMR spectral analysis. The composition was
estimated from the area ratio of a given H NMR signal
1
for each compound. The remaining solution was concen-
trated to dryness under reduced pressure and the resulting
residue was subjected to column chromatography over silica
gel (230 mesh, Merck) eluting with chloroform–ethyl ace-
tate or hexane–ethyl acetate. Preparative TLC plate (silica
gel) was also used for isolating the photoproducts.
4.3.14. tert-Butyl (Z)-2-benzoylamino-3-(2,6-dimethyl-
phenyl)-2-propenoate [(Z)-1n]. Mp 120.0–121.0 ꢃC. IR
(KBr): 3362, 1721, 1667 cmꢁ1. 1H NMR (500 MHz, DMSO-
d6): d 1.45 (9H, s), 2.15 (6H, s), 7.01 (2H, d, J¼7.4 Hz), 7.08
(1H, dd, J¼8.0, 8.0 Hz), 7.13 (1H, s), 7.44 (2H, dd, J¼7.4,
8.0 Hz), 7.53 (1H, dd, J¼7.4, 7.4 Hz), 7.74 (2H, d,
J¼7.4 Hz), 9.56 (1H, s). 13C NMR (125 MHz, DMSO-d6):
d 20.8 (2C), 28.5 (3C), 81.5, 128.0 (2C), 128.3, 128.5 (2C),
129.2 (2C), 130.4, 132.1, 132.5, 133.9, 134.7, 136.9 (2C),
164.4, 167.0. Anal. Calcd (found) for C22H25NO3: C, 75.19
(74.82); H, 7.17 (7.34); N, 3.99% (3.83%).
For the purpose of analyzing the effect of TEA concentration
on the photoreactivity of 1a and the product composition,
nitrogen-saturated methanol solutions of (Z)-1a (4.0ꢀ
10ꢁ3 mol dmꢁ3, 10 mL) containing varying amounts of
TEA, placed in sealed Pyrex tubes, were irradiated in
parallel for 3 h with Pyrex-filtered light from a 400 W
high-pressure Hg lamp set in a Pyrex cooling jacket (external

K. Maekawa et al. / Tetrahedron 63 (2007) 11267–11281
11277
irradiation). Parallel irradiation of the solutions was carried
out at room temperature on a merry-go-round-type irradia-
tion equipment immersed into a water bath (RIKO model
RH400-10W). After the irradiation, each solution was con-
centrated to dryness in vacuo. The resulting residue was dis-
solved in DMSO-d6 and subjected to 1H NMR spectral
analysis.
J¼8.6 Hz), 8.04 (1H, d, J¼6.3 Hz), 8.05 (2H, d,
J¼7.4 Hz). 13C NMR (125 MHz, DMSO-d6): d 53.3, 75.8,
81.2, 123.3, 123.7, 126.1, 126.8, 126.9, 127.4, 128.9 (2C),
129.5 (2C), 129.6, 129.7, 130.0, 133.0, 134.1, 135.0,
165.1, 171.5. Anal. Calcd (found) for C21H17NO3: C,
76.12 (76.14); H, 5.17 (5.21); N, 4.23% (4.09%).
4.4.5. cis-4-Ethoxycarbonyl-5-(1-naphthyl)-2-phenyl-
4,5-dihydrooxazole (cis-2b). Mp 100.0–101.0 ꢃC. IR
Physical and spectroscopic data of cis-4,5-dihydrooxazoles
(cis-2), trans-4,5-dihydrooxazoles (trans-2), (E)-1, and 1-
naphthylalanine derivative (3a) isolated are as follows.
1
(KBr): 1738, 1651 cmꢁ1. H NMR (500 MHz, DMSO-d6):
d 0.12 (3H, dd, J¼6.9, 6.9 Hz), 3.09 (1H, dq, J¼6.9,
10.3 Hz), 3.33 (1H, dq, J¼6.9, 10.3 Hz), 5.60 (1H, d,
J¼10.3 Hz), 6.88 (1H, d, J¼10.3 Hz), 7.50 (1H,
dd, J¼6.9, 7.4 Hz), 7.52 (1H, d, J¼7.4 Hz), 7.58 (1H, dd,
J¼8.0, 9.2 Hz), 7.59 (2H, dd, J¼7.4, 7.4 Hz), 7.61 (1H,
dd, J¼8.0, 9.2 Hz), 7.67 (1H, dd, J¼7.4, 7.4 Hz), 7.91
(1H, d, J¼6.9 Hz), 7.97 (1H, d, J¼8.0 Hz), 8.08 (2H, d,
J¼7.4 Hz), 8.12 (1H, d, J¼8.0 Hz). 13C NMR (125 MHz,
DMSO-d6): d 12.5, 59.7, 72.4, 80.1, 123.0, 123.8, 125.1,
125.9, 126.2, 126.7, 128.2 (2C), 128.3, 128.4, 128.9 (2C),
129.9, 132.1, 132.3, 132.9, 165.1, 168.6. Anal. Calcd
(found) for C22H19NO3: C, 76.50 (75.92); H, 5.54 (5.81);
N, 4.06% (3.94%).
4.4.1. Methyl (E)-2-benzoylamino-3-(1-naphthyl)-2-pro-
penoate [(E)-1a]. Mp 156.0–157.0 ꢃC. IR (KBr): 3240,
1
1736, 1632 cmꢁ1. H NMR (500 MHz, DMSO-d6): d 3.47
(3H, s), 7.32 (1H, d, J¼7.4 Hz), 7.34 (1H, s), 7.49 (1H, dd,
J¼7.4, 8.0 Hz), 7.55–7.61 (2H, m), 7.57 (2H, dd, J¼7.4,
8.6 Hz), 7.64 (1H, dd, J¼7.4, 7.4 Hz), 7.90 (1H, d,
J¼8.0 Hz), 7.95–7.98 (1H, m), 8.00 (2H, d, J¼8.6 Hz),
8.02–8.04 (1H, m), 10.7 (1H, s). 13C NMR (125 MHz,
DMSO-d6): d 51.7, 120.0, 124.3, 125.5, 125.7, 126.2,
126.5, 127.8 (2C), 128.1, 128.5, 128.6 (2C), 131.0, 131.1,
131.6, 132.2, 132.8, 133.1, 165.09, 165.10. Anal. Calcd
(found) for C21H17NO3: C, 76.12 (76.02); H, 5.17 (5.26);
N, 4.23% (4.09%).
4.4.6. trans-4-Ethoxycarbonyl-5-(1-naphthyl)-2-phenyl-
4,5-dihydrooxazole (trans-2b). Oily liquid. IR (NaCl):
1
4.4.2. tert-Butyl (E)-2-benzoylamino-3-(1-naphthyl)-2-
propenoate [(E)-1d]. Mp 163.0–164.0 ꢃC. IR (KBr):
1734, 1645 cmꢁ1. H NMR (500 MHz, DMSO-d6): d 1.28
(3H, dd, J¼6.9, 6.9 Hz), 4.24 (1H, dq, J¼6.9, 10.3 Hz),
4.30 (1H, dq, J¼6.9, 10.3 Hz), 4.83 (1H, d, J¼6.3 Hz),
6.69 (1H, d, J¼6.3 Hz), 7.52 (1H, dd, J¼7.4, 8.0 Hz), 7.53
(1H, d, J¼8.0 Hz), 7.58 (2H, dd, J¼7.4, 8.0 Hz), 7.61–
7.63 (2H, m), 7.67 (1H, dd, J¼7.4, 7.4 Hz), 7.98 (1H, d,
J¼7.4 Hz), 8.00 (1H, d, J¼8.6 Hz), 8.06 (2H, d,
J¼8.0 Hz), 8.04 (1H, d, J¼9.7 Hz). 13C NMR (125 MHz,
DMSO-d6): d 14.0, 61.5, 75.4, 80.6, 122.7, 123.0, 125.5,
126.2, 126.3, 126.8, 128.3 (2C), 128.95 (2C), 129.00,
129.04, 129.3, 132.4, 133.5, 134.6, 164.5, 170.5. Anal.
Calcd (found) for C22H19NO3: C, 76.50 (76.34); H, 5.54
(5.36); N, 4.06% (4.36%).
1
3198, 1724, 1628 cmꢁ1. H NMR (500 MHz, DMSO-d6):
d 1.02 (9H, s), 7.24 (1H, s), 7.35 (1H, d, J¼6.9 Hz), 7.50
(1H, dd, J¼6.9, 8.2 Hz), 7.54–7.57 (2H, m), 7.56 (2H, dd,
J¼7.6, 7.6 Hz), 7.63 (1H, dd, J¼7.6, 7.6 Hz), 7.90 (1H, d,
J¼8.2 Hz), 7.90–8.10 (2H, m), 7.98 (2H, d, J¼7.6 Hz),
10.51 (1H, s). 13C NMR (125 MHz, DMSO-d6): d 27.0
(3C), 80.4, 120.1, 124.8, 125.2, 125.99, 126.01, 126.2,
127.69, 127.72 (2C), 128.2, 128.5 (2C), 131.2, 132.0,
132.5, 132.85, 132.94, 133.2, 163.0, 165.0. Anal. Calcd
(found) for C24H23NO3: C, 77.19 (77.09); H, 6.21 (6.32);
N, 3.75% (3.73%).
4.4.3. cis-4-Methoxycarbonyl-5-(1-naphthyl)-2-phenyl-
4,5-dihydrooxazole (cis-2a). Mp 133.0–134.0 ꢃC. IR
4.4.7. cis-4-Isopropoxycarbonyl-5-(1-naphthyl)-2-phen-
yl-4,5-dihydrooxazole (cis-2c). Mp 132.5–133.5 ꢃC. IR
1
1
(KBr): 1742, 1646 cmꢁ1. H NMR (500 MHz, DMSO-d6):
(KBr): 1730, 1653 cmꢁ1. H NMR (500 MHz, DMSO-d6):
d 2.72 (3H, s), 5.62 (1H, d, J¼10.3 Hz), 6.88 (1H, d,
J¼10.3 Hz), 7.51–7.52 (2H, m), 7.57 (1H, dd, J¼7.4,
7.4 Hz), 7.59 (2H, dd, J¼7.4, 7.7 Hz), 7.60 (1H, dd,
J¼7.4, 8.7 Hz), 7.67 (1H, dd, J¼7.4, 7.4 Hz), 7.90–7.92
(1H, m), 7.96 (1H, d, J¼7.4 Hz), 8.08 (2H, d, J¼7.7 Hz),
8.10 (1H, d, J¼8.7 Hz). 13C NMR (125 MHz, DMSO-d6):
d 50.7, 72.7, 80.0, 122.9, 123.6, 125.1, 125.9, 126.2,
126.4, 128.2 (2C), 128.3, 128.4, 128.9 (2C), 129.7, 132.0,
132.3, 132.8, 165.2, 169.0. Anal. Calcd (found) for
C21H17NO3: C, 76.12 (75.89); H, 5.17 (5.20); N, 4.23%
(4.48%).
d ꢁ0.15 (3H, d, J¼6.3 Hz), 0.69 (3H, d, J¼6.3 Hz), 4.07
(1H, qq, J¼6.3, 6.3 Hz), 5.56 (1H, d, J¼10.3 Hz),
6.84 (1H, d, J¼10.3 Hz), 7.50 (1H, dd, J¼7.4, 7.4 Hz),
7.54 (1H, d, J¼7.4 Hz), 7.57 (1H, dd, J¼7.4, 8.0 Hz), 7.59
(2H, dd, J¼7.4, 8.0 Hz), 7.61 (1H, dd, J¼7.4, 8.0 Hz),
7.67 (1H, dd, J¼7.4, 7.4 Hz), 7.90 (1H, d, J¼7.4 Hz), 7.96
(1H, d, J¼8.0 Hz), 8.07 (2H, d, J¼8.0 Hz), 8.11 (1H, d,
J¼8.0 Hz). 13C NMR (125 MHz, DMSO-d6): d 19.5, 21.0,
67.4, 72.3, 80.2, 123.1, 123.9, 125.2, 125.9, 126.2, 126.7,
128.2 (2C), 128.3, 128.4, 128.9 (2C), 130.0, 132.0, 132.3,
133.0, 165.0, 168.1. Anal. Calcd (found) for C23H21NO3:
C, 76.86 (76.86); H, 5.89 (6.12); N, 3.90% (3.79%).
4.4.4. trans-4-Methoxycarbonyl-5-(1-naphthyl)-2-phen-
yl-4,5-dihydrooxazole (trans-2a). Mp 108.0–109.0 ꢃC. IR
4.4.8. trans-4-Isopropoxycarbonyl-5-(1-naphthyl)-2-
phenyl-4,5-dihydrooxazole (trans-2c). Oily liquid. IR
1
(KBr): 1742, 1640 cmꢁ1. H NMR (500 MHz, DMSO-d6):
d 3.81 (3H, s), 4.89 (1H, d, J¼6.9 Hz), 6.70 (1H, d,
J¼6.9 Hz), 7.52 (1H, d, J¼7.4 Hz), 7.54 (1H, dd, J¼6.9,
7.4 Hz), 7.57 (2H, dd, J¼7.4, 7.4 Hz), 7.62 (1H, dd,
J¼8.6, 9.2 Hz), 7.62 (1H, dd, J¼6.3, 9.2 Hz), 7.67 (1H,
dd, J¼7.4, 7.4 Hz), 7.98 (1H, d, J¼6.9 Hz), 8.00 (1H, d,
(NaCl): 1734, 1647 cmꢁ1 1H NMR (500 MHz, DMSO-
.
d6): d 1.27 (3H, d, J¼6.3 Hz), 1.31 (3H, d, J¼5.7 Hz),
4.74 (1H, d, J¼6.9 Hz), 5.10 (1H, qq, J¼5.7, 6.3 Hz), 6.66
(1H, d, J¼6.9 Hz), 7.51 (1H, d, J¼6.3 Hz), 7.54 (1H, dd,
J¼6.3, 8.6 Hz), 7.58 (2H, dd, J¼7.4, 7.4 Hz), 7.60–7.62

11278
K. Maekawa et al. / Tetrahedron 63 (2007) 11267–11281
(2H, m), 7.67 (1H, dd, J¼7.4, 7.4 Hz), 7.97 (1H, d,
J¼8.6 Hz), 7.97 (1H, d, J¼8.6 Hz), 8.04 (1H, d,
J¼8.3 Hz), 8.06 (2H, d, J¼7.4 Hz). 13C NMR (125 MHz,
DMSO-d6): d 21.4, 21.5, 69.2, 75.7, 80.7, 122.7, 122.9,
125.5, 126.2, 126.4, 126.7, 128.3 (2C), 128.97 (3C),
129.05, 129.2, 132.4, 133.5, 134.7, 164.5, 170.0. Anal.
Calcd (found) for C23H21NO3: C, 76.86 (76.82); H, 5.89
(5.98); N, 3.90% (3.79%).
J¼8.2 Hz). 13C NMR (125 MHz, DMSO-d6): d 52.7, 55.5,
75.2, 80.2, 113.8, 114.3 (2C), 118.6, 122.7, 123.0, 125.5,
126.2, 126.8, 129.0, 130.2 (2C), 131.5, 133.5, 134.6,
162.4, 164.3, 171.2. Anal. Calcd (found) for C22H19NO4:
C, 73.12 (73.23); H, 5.30 (5.23); N, 3.88% (3.56%).
4.4.13. cis-2-(4-Cyanophenyl)-4-methoxycarbonyl-5-(1-
naphthyl)-4,5-dihydrooxazole (cis-2f). Mp 172.0–
173.0 ꢃC. IR (KBr): 2229, 1745, 1650 cmꢁ1 1H NMR
.
4.4.9. cis-4-tert-Butoxycarbonyl-5-(1-naphthyl)-2-phen-
yl-4,5-dihydrooxazole (cis-2d). Mp 144.0–145.0 ꢃC. IR
(500 MHz, DMSO-d6): d 2.72 (3H, s), 5.69 (1H, d,
J¼10.3 Hz), 6.94 (1H, d, J¼10.3 Hz), 7.41 (1H, dd, J¼7.4,
7.4 Hz), 7.46 (1H, d, J¼7.4 Hz), 7.59 (1H, dd, J¼8.6,
8.6 Hz), 7.61 (1H, dd, J¼8.6, 8.6 Hz), 7.81 (1H, d,
J¼7.4 Hz), 8.07 (2H, d, J¼8.0 Hz), 8.09 (1H, d, J¼8.6 Hz),
8.15 (1H, d, J¼8.6 Hz), 8.23 (2H, d, J¼8.0 Hz). 13C NMR
(125 MHz, DMSO-d6): d 51.3, 73.2, 81.0, 115.0, 118.6,
123.4, 123.9, 125.5, 126.3, 126.7, 128.8, 129.0, 129.4 (2C),
130.0, 131.1, 132.0, 133.2, 133.4 (2C), 164.5, 169.1. Anal.
Calcd (found) for C22H16N2O3: C, 74.15 (74.35); H, 4.53
(4.46); N, 7.86% (7.93%).
1
(KBr): 1737, 1649 cmꢁ1. H NMR (500 MHz, DMSO-d6):
d 0.50 (9H, s), 5.48 (1H, d, J¼10.3 Hz), 6.81 (1H, d,
J¼10.3 Hz), 7.51 (1H, d, J¼8.2 Hz), 7.54 (1H, dd, J¼7.6,
8.2 Hz), 7.58 (1H, dd, J¼7.6, 7.6 Hz), 7.58 (2H, dd,
J¼6.9, 7.6 Hz), 7.62 (1H, dd, J¼7.6, 8.2 Hz), 7.66 (1H,
dd, J¼7.6, 7.6 Hz), 7.93 (1H, d, J¼7.6 Hz), 7.98 (1H, d,
J¼7.6 Hz), 8.07 (2H, d, J¼6.9 Hz), 8.11 (1H, d,
J¼8.2 Hz). 13C NMR (125 MHz, DMSO-d6): d 26.4 (3C),
72.5, 80.0, 80.2, 123.1, 124.0, 125.2, 125.9, 126.1, 126.8,
128.1 (2C), 128.31, 128.33, 128.8 (2C), 130.2, 132.1,
132.3, 133.0, 164.8, 167.6. Anal. Calcd (found) for
C24H23NO3: C, 77.19 (77.30); H, 6.21 (6.03); N, 3.75%
(3.65%).
4.4.14. trans-2-(4-Cyanophenyl)-4-methoxycarbonyl-5-
(1-naphthyl)-4,5-dihydrooxazole (trans-2f)ꢁ. 1Mp 173.0–
174.0 ꢃC. IR (KBr): 2230, 1740, 1639 cm
.
1H NMR
(500 MHz, DMSO-d6): d 3.81 (3H, s), 5.00 (1H, d,
J¼6.9 Hz), 6.77 (1H, d, J¼6.9 Hz), 7.52–7.55 (2H, m), 7.61
(1H, dd, J¼6.3, 8.2 Hz), 7.64 (1H, dd, J¼6.3, 9.2 Hz),
7.97–8.00 (1H, m), 8.01 (1H, d, J¼8.2 Hz), 8.04 (1H, d,
J¼9.2 Hz), 8.04 (2H, d, J¼8.2 Hz), 8.19 (2H, d, J¼8.2 Hz).
13C NMR (125 MHz, DMSO-d6): d 52.8, 75.1, 81.0, 114.6,
118.1, 122.7, 123.3, 125.5, 126.2, 126.9, 128.98, 129.00
(2C), 129.2, 129.4, 130.4, 133.0 (2C), 133.5, 133.9, 163.3,
170.6. Anal. Calcd (found) for C22H16N2O3: C, 74.15
(73.83); H, 4.53 (4.16); N, 7.86% (7.77%).
4.4.10. trans-4-tert-Butoxycarbonyl-5-(1-naphthyl)-2-
phenyl-4,5-dihydrooxazole (trans-2d). Oily liquid. IR
1
(NaCl): 1728, 1647 cmꢁ1. H NMR (500 MHz, DMSO-d6):
d 1.51 (9H, s), 4.65 (1H, d, J¼6.2 Hz), 6.61 (1H, d,
J¼6.2 Hz), 7.50 (1H, d, J¼6.9 Hz), 7.54 (1H, dd, J¼6.9,
6.9 Hz), 7.58 (2H, dd, J¼7.6, 7.6 Hz), 7.60–7.63 (2H, m),
7.67 (1H, dd, J¼7.6, 7.6 Hz), 7.97 (1H, d, J¼6.9 Hz), 7.97
(1H, d, J¼6.9 Hz), 8.03–8.06 (1H, m), 8.06 (2H, d,
J¼7.6 Hz). 13C NMR (125 MHz, DMSO-d6): d 27.6 (3C),
76.4, 80.8, 82.1, 122.7, 122.8, 125.5, 126.2, 126.4, 126.6,
128.3 (2C), 128.92, 128.94 (2C), 129.0, 129.1, 132.4, 133.5,
134.7, 164.4, 169.6. Anal. Calcd (found) for C24H23NO3: C,
77.19 (76.94); H, 6.21 (5.96); N, 3.75% (3.57%).
4.4.15. cis-2,5-Diphenyl-4-methoxycarbonyl-4,5-dihy-
drooxazole (cis-2g). Mp 141.0–142.0 ꢃC. IR (KBr): 1748,
1649 cmꢁ1. 1H NMR (500 MHz, DMSO-d6): d 3.12 (3H, s),
5.38 (1H, d, J¼10.3 Hz), 6.13 (1H, d, J¼10.3 Hz), 7.25
(2H, d, J¼6.9 Hz), 7.33 (1H, dd, J¼7.4, 7.4 Hz), 7.37 (2H,
dd, J¼6.9, 7.4 Hz), 7.56 (2H, dd, J¼7.4, 7.4 Hz), 7.65 (1H,
dd, J¼7.4, 7.4 Hz), 8.00 (2H, d, J¼7.4 Hz). 13C NMR
(125 MHz, DMSO-d6): d 51.2, 73.2, 82.1, 126.1 (2C),
126.6, 128.1 (2C), 128.2 (2C), 128.4, 128.8 (2C), 132.2,
136.1, 165.0, 169.2. Anal. Calcd (found) for C17H15NO3: C,
72.58 (72.52); H, 5.37 (5.46); N, 4.98% (4.89%).
4.4.11. cis-4-Methoxycarbonyl-2-(4-methoxyphenyl)-5-
(1-naphthyl)-4,5-dihydrooxazole (cis-2e). Mp 158.0–
159.0 ꢃC. IR (KBr): 1744, 1651 cmꢁ1
.
1H NMR
(500 MHz, DMSO-d6): d 2.71 (3H, s), 3.86 (3H, s), 5.56
(1H, d, J¼10.3 Hz), 6.84 (1H, d, J¼10.3 Hz), 7.12 (2H, d,
J¼8.9 Hz), 7.49 (1H, d, J¼7.6 Hz), 7.51 (1H, dd, J¼6.9,
7.6 Hz), 7.56 (1H, dd, J¼7.6, 8.2 Hz), 7.59 (1H, dd,
J¼7.6, 8.2 Hz), 7.90 (1H, d, J¼6.9 Hz), 7.96 (1H,
d, J¼8.2 Hz), 8.01 (2H, d, J¼8.9 Hz), 8.09 (1H, d,
J¼8.2 Hz). 13C NMR (125 MHz, DMSO-d6): d 50.7, 55.5,
72.7, 79.8, 114.2 (2C), 118.9, 122.9, 123.6, 125.1, 125.8,
126.1, 128.3, 128.4, 129.7, 130.7 (2C), 132.1, 132.8,
162.3, 165.0, 169.2. Anal. Calcd (found) for C22H19NO4:
C, 73.12 (72.75); H, 5.30 (5.38); N, 3.88% (3.74%).
4.4.16. trans-2,5-Diphenyl-4-methoxycarbonyl-4,5-dihy-
drooxazole (trans-2g). Oily liquid. IR (NaCl): 1744,
1631 cmꢁ1. 1H NMR (500 MHz, DMSO-d6): d 3.76 (3H, s),
4.83 (1H, d, J¼7.4 Hz), 5.29 (1H, d, J¼7.4 Hz), 7.40 (2H,
d, J¼6.9 Hz), 7.39 (1H, dd, J¼6.9, 6.9 Hz), 7.44 (2H, dd,
J¼6.9, 6.9 Hz), 7.54 (2H, dd, J¼7.4, 7.4 Hz), 7.63 (1H, dd,
J¼7.4, 7.4 Hz), 7.98 (2H, d, J¼7.4 Hz). 13C NMR
(125 MHz, DMSO-d6): d 52.5, 75.8, 82.7, 125.8 (2C),
126.4, 128.2 (2C), 128.7, 128.8 (2C), 128.9 (2C), 132.3,
139.4, 164.2, 170.8. Anal. Calcd (found) for C17H15NO3: C,
72.58 (72.46); H, 5.37 (5.35); N, 4.98% (4.92%).
4.4.12. trans-4-Methoxycarbonyl-2-(4-methoxyphenyl)-
5-(1-naphthyl)-4,5-dihydrooxazole (trans-2e). Mp 116.5–
117.5 ꢃC. IR (KBr): 1719, 1647 cmꢁ1
.
1H NMR
(500 MHz, DMSO-d6): d 3.80 (3H, s), 3.85 (3H, s), 4.82
(1H, d, J¼6.9 Hz), 6.64 (1H, d, J¼6.9 Hz), 7.10 (2H, d,
J¼8.2 Hz), 7.49 (1H, d, J¼6.9 Hz), 7.53 (1H, dd, J¼6.9,
8.2 Hz), 7.60 (1H, dd, J¼6.9, 7.9 Hz), 7.63 (1H, dd,
J¼6.9, 8.2 Hz), 7.97 (1H, d, J¼8.2 Hz), 7.97 (1H, d,
J¼7.9 Hz), 7.99 (2H, d, J¼8.2 Hz), 8.03 (1H, d,
4.4.17. cis-4-tert-Butoxycarbonyl-5-(4-methoxyphenyl)-
2-phenyl-4,5-dihydrooxazole (cis-2h). Mp 96.0–97.0 ꢃC.
1
IR (KBr): 1736, 1655 cmꢁ1. H NMR (500 MHz, DMSO-
d6): d 0.99 (9H, s), 3.73 (3H, s), 5.18 (1H, d, J¼10.9 Hz),

K. Maekawa et al. / Tetrahedron 63 (2007) 11267–11281
11279
6.02 (1H, d, J¼10.9 Hz), 6.93 (2H, d, J¼8.6 Hz), 7.20 (2H,
d, J¼8.6 Hz), 7.54 (2H, dd, J¼7.4, 8.0 Hz), 7.62 (1H, dd,
J¼7.4, 7.4 Hz), 7.96 (2H, d, J¼8.0 Hz). 13C NMR
(125 MHz, DMSO-d6): d 27.0 (3C), 55.2, 73.1, 80.4, 82.0,
113.6 (2C), 126.9, 128.1 (2C), 128.2 (2C), 128.8 (2C),
131.6, 132.1, 159.4, 164.5, 167.8. Anal. Calcd (found) for
C21H23NO4: C, 71.37 (71.29); H, 6.56 (6.42); N, 3.96%
(3.77%).
7.38–7.40 (3H, m), 7.44 (2H, dd, J¼6.9, 7.6 Hz), 7.53
(2H, dd, J¼7.4, 7.4 Hz), 7.62 (1H, dd, J¼7.4, 7.4 Hz),
7.97 (2H, d, J¼7.4 Hz). 13C NMR (125 MHz, DMSO-d6):
d 27.6 (3C), 76.8, 81.7, 82.9, 125.7 (2C), 126.5, 128.2
(2C), 128.6, 128.8 (2C), 128.9 (2C), 132.2, 139.6, 164.0,
169.4. Anal. Calcd (found) for C20H21NO3: C, 74.28
(74.23); H, 6.55 (6.25); N, 4.33% (4.46%).
4.4.23. cis-4-tert-Butoxycarbonyl-5-(4-chlorophenyl)-2-
phenyl-4,5-dihydrooxazole (cis-2k). Mp 100.0–101.0 ꢃC.
4.4.18. trans-4-tert-Butoxycarbonyl-5-(4-methoxy-
phenyl)-2-phenyl-4,5-dihydrooxazole (trans-2h). Mp
1
IR (KBr): 1746, 1643 cmꢁ1. H NMR (500 MHz, DMSO-
105.0–106.0 ꢃC. IR (KBr): 1726, 1638 cmꢁ1
.
1H NMR
d6): d 1.00 (9H, s), 5.22 (1H, d, J¼10.9 Hz), 6.09 (1H,
d, J¼10.9 Hz), 7.32 (2H, d, J¼8.6 Hz), 7.46 (2H, d,
J¼8.6 Hz), 7.55 (2H, dd, J¼7.4, 7.4 Hz), 7.63 (1H, dd,
J¼7.4, 7.4 Hz), 7.98 (2H, d, J¼7.4 Hz). 13C NMR
(125 MHz, DMSO-d6): d 27.8 (3C), 74.0, 81.5, 82.3,
127.5, 128.9 (2C), 129.0 (2C), 129.5 (2C), 129.6 (2C),
133.0, 133.8, 136.3, 165.4, 168.4. Anal. Calcd (found) for
C20H20ClNO3: C, 67.13 (66.99); H, 5.63 (5.30); N, 3.91%
(3.96%).
(500 MHz, DMSO-d6): d 1.46 (9H, s), 3.76 (3H, s), 4.66
(1H, d, J¼7.4 Hz), 5.74 (1H, d, J¼7.4 Hz), 6.98 (2H, d,
J¼8.6 Hz), 7.33 (2H, d, J¼8.6 Hz), 7.52 (2H, dd, J¼7.4,
8.0 Hz), 7.61 (1H, dd, J¼7.4, 7.4 Hz), 7.95 (2H, d,
J¼8.0 Hz). 13C NMR (125 MHz, DMSO-d6): d 27.6 (3C),
55.2, 76.6, 81.6, 83.0, 114.3 (2C), 126.7, 127.6 (2C), 128.1
(2C), 128.8 (2C), 131.4, 132.1, 159.5, 163.9, 169.6. Anal.
Calcd (found) for C21H23NO4: C, 71.37 (70.97); H, 6.56
(6.47); N, 3.96% (3.82%).
4.4.24. trans-4-tert-Butoxycarbonyl-5-(4-chlorophenyl)-
2-phenyl-4,5-dihydrooxazole (trans-2k). Mp 88.0–
4.4.19. cis-4-tert-Butoxycarbonyl-2-phenyl-5-(4-methyl-
phenyl)-4,5-dihydrooxazole (cis-2i). Mp 97.0–98.0 ꢃC. IR
1
89.0 ꢃC. IR (KBr): 1732, 1641 cmꢁ1. H NMR (500 MHz,
1
(KBr): 1740, 1659 cmꢁ1. H NMR (500 MHz, DMSO-d6):
DMSO-d6): d 1.47 (9H, s), 4.68 (1H, d, J¼7.4 Hz), 5.84
(1H, d, J¼7.4 Hz), 7.42 (2H, d, J¼8.6 Hz), 7.50 (2H, d,
J¼8.6 Hz), 7.54 (2H, dd, J¼7.4, 8.0 Hz), 7.63 (1H, dd,
J¼8.0, 8.0 Hz), 7.97 (2H, d, J¼7.4 Hz). 13C NMR
(125 MHz, DMSO-d6): d 27.2 (3C), 76.3, 81.4, 81.7,
126.0, 127.3 (2C), 127.8 (2C), 128.4 (2C), 128.5 (2C),
131.8, 132.8, 138.1, 163.5, 168.9. Anal. Calcd (found) for
C20H20ClNO3: C, 67.13 (67.12); H, 5.63 (5.38); N, 3.91%
(3.92%).
d 0.96 (9H, s), 2.28 (3H, s), 5.20 (1H, d, J¼10.9 Hz), 6.02
(1H, d, J¼10.9 Hz), 7.15–7.19 (4H, m), 7.54 (2H, dd,
J¼7.4, 8.0 Hz), 7.63 (1H, dd, J¼7.4, 7.4 Hz), 7.97 (2H, d,
J¼8.0 Hz). 13C NMR (125 MHz, DMSO-d6): d 20.7, 26.9
(3C), 73.1, 80.4, 82.2, 126.7 (2C), 126.8, 128.1 (2C),
128.6 (2C), 128.8 (2C), 132.1, 133.5, 137.7, 164.5, 167.7.
Anal. Calcd (found) for C21H23NO3: C, 74.75 (74.70); H,
6.87 (6.71); N, 4.15% (4.12%).
4.4.20. trans-4-tert-Butoxycarbonyl-2-phenyl-5-(4-meth-
ylphenyl)-4,5-dihydrooxazole (trans-2i). Mp 98.0–
4.4.25. cis-4-tert-Butoxycarbonyl-5-(4-cyanophenyl)-2-
phenyl-4,5-dihydrooxazole (cis-2l). Mp 158.0–159.0 ꢃC.
1
1
99.0 ꢃC. IR (KBr): 1728, 1636 cmꢁ1. H NMR (500 MHz,
IR (KBr): 2226, 1742, 1647 cmꢁ1. H NMR (500 MHz,
DMSO-d6): d 1.45 (9H, s), 2.30 (3H, s), 4.62 (1H, d,
J¼7.4 Hz), 5.74 (1H, d, J¼7.4 Hz), 7.22–7.27 (4H, m),
7.52 (2H, dd, J¼7.4, 8.0 Hz), 7.61 (1H, dd, J¼7.4,
7.4 Hz), 7.94 (2H, d, J¼8.0 Hz). 13C NMR (125 MHz,
DMSO-d6): d 27.6, 30.7 (3C), 76.7, 81.6, 83.0, 125.8 (2C),
126.6, 128.2 (2C), 128.8 (2C), 129.4 (2C), 132.2, 136.6,
138.1, 164.0, 169.5. Anal. Calcd (found) for C21H23NO3:
C, 74.75 (74.41); H, 6.87 (6.62); N, 4.15% (4.70%).
DMSO-d6): d 0.98 (9H, s), 5.28 (1H, d, J¼10.9 Hz), 6.19
(1H, d, J¼10.9 Hz), 7.52 (2H, d, J¼8.0 Hz), 7.55 (2H, dd,
J¼7.4, 8.6 Hz), 7.65 (1H, dd, J¼7.4, 7.4 Hz), 7.89 (2H, d,
J¼8.6 Hz), 7.99 (2H, d, J¼8.0 Hz). 13C NMR (125 MHz,
DMSO-d6): d 26.9 (3C), 73.2, 80.9, 81.3, 111.0, 118.6,
126.4, 127.7 (2C), 128.2 (2C), 128.9 (2C), 132.2 (2C),
132.3, 141.9, 164.6, 167.4. Anal. Calcd (found) for
C21H20N2O3: C, 72.40 (72.75); H, 5.79 (5.80); N, 8.04%
(7.99%).
4.4.21. cis-4-tert-Butoxycarbonyl-2,5-diphenyl-4,5-dihy-
drooxazole (cis-2j). Mp 108.0–109.0 ꢃC. IR (KBr): 1748,
4.4.26. trans-4-tert-Butoxycarbonyl-5-(4-cyanophenyl)-
2-phenyl-4,5-dihydrooxazole (trans-2l). Oily liquid. IR
1
1653 cmꢁ1. H NMR (500 MHz, DMSO-d6): d 0.95 (9H,
s), 5.22 (1H, d, J¼10.9 Hz), 6.06 (1H, d, J¼10.9 Hz), 7.29
(2H, d, J¼6.9 Hz), 7.33 (1H, dd, J¼6.9, 6.9 Hz), 7.38 (2H,
dd, J¼6.9, 6.9 Hz), 7.54 (2H, dd, J¼7.4, 7.4 Hz), 7.63
(1H, dd, J¼7.4, 7.4 Hz), 7.98 (2H, d, J¼7.4 Hz). 13C
NMR (125 MHz, DMSO-d6): d 26.9 (3C), 73.1, 80.4, 82.2,
126.7 (2C), 126.8, 128.07 (2C), 128.10 (2C), 128.3, 128.8
(2C), 132.1, 136.5, 164.6, 167.6. Anal. Calcd (found) for
C20H21NO3: C, 74.28 (73.94); H, 6.55 (6.21); N, 4.33%
(4.39%).
(NaCl): 2230, 1724, 1641 cmꢁ1 1H NMR (500 MHz,
.
DMSO-d6): d 1.48 (9H, s), 4.71 (1H, d, J¼7.4 Hz), 5.95
(1H, d, J¼7.4 Hz), 7.55 (2H, dd, J¼7.5, 7.5 Hz), 7.59 (2H,
d, J¼8.6 Hz), 7.64 (1H, dd, J¼7.4, 7.4 Hz), 7.92 (2H, d,
J¼8.6 Hz), 7.99 (2H, d, J¼7.4 Hz). 13C NMR (125 MHz,
DMSO-d6): d 27.6 (3C), 76.6, 81.85, 81.94, 111.3, 118.5,
126.3, 126.6 (2C), 128.2 (2C), 128.9 (2C), 132.9 (2C),
132.4, 144.9, 163.5, 168.9. Anal. Calcd (found) for
C21H20N2O3: C, 72.40 (72.38); H, 5.79 (5.70); N, 8.04%
(7.97%).
4.4.22. trans-4-tert-Butoxycarbonyl-2,5-diphenyl-4,5-di-
hydrooxazole (trans-2j). Mp 89.0–90.0 ꢃC. IR (KBr):
4.4.27. cis-4-tert-Butoxycarbonyl-2-phenyl-5-(2-methyl-
phenyl)-4,5-dihydrooxazole (cis-2m). Oily liquid. IR
1
1734, 1643 cmꢁ1. H NMR (500 MHz, DMSO-d6): d 1.47
(9H, s), 4.66 (1H, d, J¼7.4 Hz), 5.88 (1H, d, J¼7.4 Hz),
(NaCl): 1744, 1649 cmꢁ1 1H NMR (500 MHz, DMSO-
.

11280
K. Maekawa et al. / Tetrahedron 63 (2007) 11267–11281
1
d6): d 0.92 (9H, s), 2.37 (3H, s), 5.24 (1H, d, J¼11.0 Hz),
6.20 (1H, d, J¼11.0 Hz), 7.17–7.25 (4H, m), 7.56 (2H, dd,
J¼6.9, 7.6 Hz), 7.64 (1H, dd, J¼6.9, 6.9 Hz), 8.00 (2H, d,
J¼7.6 Hz). 13C NMR (125 MHz, DMSO-d6): d 19.1, 27.0
(3C), 72.0, 80.4, 80.7, 125.7, 125.9 (2C), 127.0, 128.2,
128.3 (2C), 129.0, 130.1, 132.3, 134.9, 135.5, 164.9,
167.9. Anal. Calcd (found) for C21H23NO3: C, 74.75
(74.70); H, 6.87 (6.82); N, 4.15% (4.03%).
(KBr): 1726, 1651 cmꢁ1. H NMR (500 MHz, DMSO-d6):
d 0.49 (9H, s), 1.33 (9H, s), 5.18 (1H, d, J¼10.3 Hz), 6.52
(1H, d, J¼10.3 Hz), 7.42 (1H, d, J¼6.9 Hz), 7.50 (1H,
dd, J¼7.4, 7.4 Hz), 7.52–7.58 (2H, m), 7.87 (1H, d,
J¼8.0 Hz), 7.94 (1H, d, J¼7.4 Hz), 8.02 (1H, d, J¼7.4 Hz).
13C NMR (125 MHz, DMSO-d6): d 26.4 (3C), 27.6 (3C),
33.2, 72.1, 79.6, 79.8, 122.9, 127.0, 125.2, 125.8, 126.0,
128.1, 128.3, 130.2, 132.7, 132.9, 167.7, 175.1. Anal. Calcd
(found) for C22H27NO3: C, 74.51 (74.46); H, 7.89 (7.70); N,
4.06% (3.96%).
4.4.28. trans-4-tert-Butoxycarbonyl-2-phenyl-5-(2-meth-
ylphenyl)-4,5-dihydrooxazole (trans-2m). Oily liquid. IR
(NaCl): 1734, 1641 cmꢁ1. 1H NMR (500 MHz, DMSO-d6):
d 1.47 (9H, s), 2.33 (3H, s), 4.54 (1H, d, J¼6.9 Hz), 6.03
(1H, d, J¼6.9 Hz), 7.18 (1H, d, J¼7.6 Hz), 7.21–7.27 (3H,
m), 7.55 (2H, dd, J¼7.6, 7.6 Hz), 7.64 (1H, dd, J¼7.6,
7.6 Hz), 8.00 (2H, d, J¼7.6 Hz). 13C NMR (125 MHz,
DMSO-d6): d 18.6, 27.6 (3C), 76.3, 80.8, 81.9, 124.8,
126.38, 126.41, 128.2 (2C), 128.3, 128.9 (2C), 130.8,
132.3, 134.5, 137.5, 164.3, 169.7. Anal. Calcd (found) for
C21H23NO3: C, 74.75 (75.11); H, 6.87 (6.87); N, 4.15%
(4.17%).
4.4.33. trans-4-tert-Butoxycarbonyl-2-tert-butyl-5-(1-
naphthyl)-4,5-dihydrooxazole (trans-2p). Any attempts
to isolate trans-2p were unsuccessful owing to its poor yield.
The formation of the trans-isomer was deduced from the
chemical shift and vicinal coupling constant [d 4.30 (1H,
d, J¼6.6 Hz), 6.29 (1H, d, J¼6.6 Hz)] of the dihydrooxazole
ring-proton signals in DMSO-d6.
4.4.34. cis-5-(9-Anthryl)-4-tert-butoxycarbonyl-2-phen-
yl-4,5-dihydrooxazole (cis-2q). Any attempts to isolate
cis-2q were not fruitful owing to its low product composition
as well as the difficulty in separating this isomer from (Z)-
1q, (E)-1q, and certain amounts of by-products. The forma-
tion of the cis-isomer was deduced based on the chemical
shift and vicinal coupling constant [d 5.71 (1H, d,
J¼10.0 Hz)] of the dihydrooxazole ring-proton signals in
DMSO-d6. The other ring-proton signal showed a pro-
nounced overlap with aromatic proton signals, which ap-
peared in the range of 7.2–7.8 ppm.
4.4.29. cis-4-tert-Butoxycarbonyl-2-phenyl-5-(2,6-dime-
thylphenyl)-4,5-dihydrooxazole (cis-2n). Oily liquid. IR
(NaCl): 1734, 1655 cmꢁ1 1H NMR (500 MHz, DMSO-
.
d6): d 0.98 (9H, s), 5.28 (1H, d, J¼10.9 Hz), 6.19 (1H, d,
J¼10.9 Hz), 7.52 (2H, d, J¼8.0 Hz), 7.55 (2H, dd, J¼7.4,
8.6 Hz), 7.65 (1H, dd, J¼7.4, 7.4 Hz), 7.89 (2H, d,
J¼8.6 Hz), 7.99 (2H, d, J¼8.0 Hz). 13C NMR (125 MHz,
DMSO-d6): d 20.4, 26.7 (3C), 72.0, 80.2, 80.4 (2C), 126.9
(2C), 127.9 (2C), 128.0 (2C), 128.9 (2C), 130.1, 132.1,
136.6, 136.9, 164.1, 168.2. Anal. Calcd (found) for
C22H25NO3: C, 75.19 (75.33); H, 7.15 (7.32); N, 3.99%
(3.89%).
4.4.35. Methyl 2-benzoylamino-3-(1-naphthyl)propa-
noate (3a). Mp 110.0–111.0 ꢃC. IR (KBr): 3369, 1757,
1639 cmꢁ1. 1H NMR (500 MHz, DMSO-d6): d 3.65 (3H, s),
4.81 (1H, ddd, J¼7.6, 8.9, 10.3 Hz), 5.52 (1H, dd, J¼8.9,
10.3 Hz), 5.73 (1H, dd, J¼10.3, 10.3 Hz), 7.42 (1H,
dd, J¼7.6, 7.6 Hz), 7.45 (2H, dd, J¼7.6, 7.6 Hz), 7.49 (1H,
d, J¼6.9 Hz), 7.53 (1H, dd, J¼5.5, 8.2 Hz), 7.54 (1H, dd,
J¼5.5, 8.9 Hz), 7.61 (1H, dd, J¼6.9, 8.2 Hz), 7.78 (2H, d,
J¼7.6 Hz), 7.81 (1H, d, J¼8.2 Hz), 7.94 (1H, d, J¼8.2 Hz),
8.16 (1H, d, J¼8.9 Hz), 8.96 (1H, d, J¼7.6 Hz). 13C NMR
(125 MHz, DMSO-d6): d 34.0, 52.5, 53.9, 123.6, 125.8,
126.1, 126.8, 127.72, 127.75 (2C), 127.8, 128.7 (2C), 129.2,
131.8, 131.9, 133.8, 133.96, 133.97, 166.9, 172.6.
4.4.30. cis-4-tert-Butoxycarbonyl-5-(2-naphthyl)-2-phen-
yl-4,5-dihydrooxazole (cis-2o). Mp 142.0–143.0 ꢃC. IR
1
(KBr): 1736, 1655 cmꢁ1. H NMR (500 MHz, DMSO-d6):
d 0.77 (9H, s), 5.29 (1H, d, J¼11.0 Hz), 6.24 (1H, d,
J¼11.0 Hz), 7.38 (1H, d, J¼8.2 Hz), 7.50–7.53 (2H, m),
7.55 (2H, dd, J¼7.6, 7.6 Hz), 7.64 (1H, dd, J¼7.6,
7.6 Hz), 7.87 (1H, s), 7.89–7.93 (3H, m), 8.02 (2H, d,
J¼7.6 Hz). 13C NMR (125 MHz, DMSO-d6): d 26.8 (3C),
73.3, 80.4, 82.4, 124.4, 125.7, 126.3, 126.5, 126.8, 127.5,
127.7, 127.8, 128.2 (2C), 128.8 (2C), 132.1, 132.4, 132.8,
133.9, 164.7, 167.7. Anal. Calcd (found) for C24H23NO3:
C, 77.19 (76.83); H, 6.21 (6.40); N, 3.75% (3.76%).
4.5. Control experiment
In order to investigate whether the concentration of TEA is
changed during irradiation, an NMR tube was employed in-
stead of a Pyrex vessel. A CD3OD solution of (Z)-1a
4.4.31. trans-4-tert-Butoxycarbonyl-5-(2-naphthyl)-2-
phenyl-4,5-dihydrooxazole (trans-2o). Mp 103.0–
104.0 ꢃC. IR (KBr): 1730, 1638 cmꢁ1
.
1H NMR
(0.025 mol dmꢁ3, 1 mL) containing TEA (0.10 mol dmꢁ3
)
(500 MHz, DMSO-d6): d 1.47 (9H, s), 4.77 (1H, d,
J¼7.6 Hz), 5.97 (1H, d, J¼7.6 Hz), 7.47 (1H, d,
J¼8.2 Hz), 7.52–7.55 (4H, m), 7.62 (1H, dd, J¼7.6,
7.6 Hz), 7.92–8.01 (6H, m). 13C NMR (125 MHz, DMSO-
d6): d 27.6 (3C), 76.7, 81.7, 83.2, 123.3, 125.0, 126.57
(2C), 126.64, 127.6, 128.0, 128.2 (2C), 128.8 (2C), 128.9,
132.2, 132.6, 132.8, 136.8, 164.0, 169.5. Anal. Calcd
(found) for C24H23NO3: C, 77.19 (77.19); H, 6.21 (6.43);
N, 3.75% (3.87%).
and 1,4-dioxane (0.10 mol dmꢁ3) was placed in an NMR
tube, which was sealed after the solution was saturated
with nitrogen. This CD3OD solution was irradiated for 4–
8 h at room temperature with Pyrex-filtered light from
a 450 W high-pressure Hg lamp mounted in a homemade
lamp house and subjected to 1H NMR spectral analysis.
4.6. X-ray crystallographic analysis of cis-2a and
trans-2a
4.4.32. cis-4-tert-Butoxycarbonyl-2-tert-butyl-5-(1-naph-
thyl)-4,5-dihydrooxazole (cis-2p). Mp 129.0–130.0 ꢃC. IR
A colorless crystal (of the molecular formula C21H17NO3)
having approximate dimensions of 0.35ꢀ0.23ꢀ0.20 mm

K. Maekawa et al. / Tetrahedron 63 (2007) 11267–11281
11281
Schneider, S.; Jaeger, W.; Gedeck, P.; Gahr, M. J. Am. Chem.
Soc. 1995, 117, 660–669.
3. Hoshina, H.; Kubo, K.; Morita, A.; Sakurai, T. Tetrahedron
2000, 56, 2941–2951.
4. Hoshina, H.; Tsuru, H.; Kubo, K.; Igarashi, T.; Sakurai, T.
Heterocycles 2000, 53, 2261–2274.
5. (a) Kubo, K.; Ishii, Y.; Sakurai, T.; Makino, M. Tetrahedron
Lett. 1998, 39, 4083–4086; (b) Maekawa, K.; Igarashi, T.;
Kubo, K.; Sakurai, T. Tetrahedron 2001, 57, 5515–5526; (c)
Motohashi, T.; Maekawa, K.; Kubo, K.; Igarashi, T.; Sakurai,
T. Heterocycles 2002, 57, 269–292.
(cis-2a) or 0.18ꢀ0.18ꢀ0.10 mm (trans-2a) was mounted on
a glass fiber in a random orientation. Preliminary examina-
tion and data collection were performed with Mo Ka radia-
˚
tion (l¼0.71069 A) on a Rigaku RAXIS-RAPID equipped
with an imaging plate. Data collection and cell refinement:
MSC/AFC diffractometer control. Data reduction: teXsan
for windows version 1.06.15 Structure solution: SIR92.16 Re-
finement: SHELXL97.17
Crystal data for cis-2a. C21H17NO3, fw¼331.36; colorless
˚
˚
prism, space group P21/c; a¼8.2216(4) A, b¼18.288(1) A,
c¼10.9309(7) A, a¼90.00ꢃ, b¼94.889(3) , g¼90.00ꢃ,
ꢃ
˚
6. (a) Overman, L. E.; Tsuboi, S.; Angle, S. J. Org. Chem. 1979,
€
V¼1637.55(18) A ; Z¼4; Dcalcd¼1.344 g cmꢁ3; R¼0.0468,
3
˚
44, 2323–2325; (b) Burger, K.; Huber, E.; Schontag, W.;
wR(F2)¼0.1573.
Ottlinger, R. J. Chem. Soc., Chem. Commun. 1983, 945–947;
(c) Ibarra, C. A.; Cereceda, J. A.; Ortiz, P.; Vicente, A.;
Quiroga, M. L. Tetrahedron Lett. 1985, 26, 243–246; (d)
Roberto, D.; Alper, H. J. Chem. Soc., Chem. Commun. 1987,
212–213; (e) Minozzi, F.; Venturello, P. J. Chem. Soc., Chem.
Commun. 1987, 1255–1256; (f) Hassner, A.; Amarasekara,
A. S.; Andisik, D. J. Org. Chem. 1988, 53, 27–30; (g) Wipf,
P.; Miller, C. P. Tetrahedron Lett. 1992, 33, 907–910; (h)
Einsiedel, J.; Schoerner, C.; Gmeiner, P. Tetrahedron 2003,
59, 3403–3407.
Crystal data for trans-2a. C21H17NO3, fw¼331.36; colorless
˚
˚
ꢃ
prism, space group P1; a¼9.031(8) A, b¼9.133(8) A,
ꢃ
ꢃ
˚
c¼10.526(9) A, a¼86.57(5) , b¼80.10(5) , g¼75.19(5) ,
V¼826(1) A ; Z¼2; Dcalcd¼1.33 g cmꢁ3
;
R¼0.153,
3
˚
wR(F2)¼ 0.464.
Crystallographic data (excluding these structure factors) for
cis-2a and trans-2a described in this paper have been depos-
ited with the Cambridge Crystallographic Data Centre as
supplementary publication numbers CCDC 228004 and
CCDC 264885, respectively. Copies of the data can be ob-
tained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (fax: +44 (0) 1223
336033 or e-mail: deposit@ccdc.cam.ac.uk).
7. Einsiedel, J.; H€ubner, H.; Gmeiner, P. Bioorg. Med. Chem. Lett.
2001, 11, 2533–2536.
8. (a) Rao, Y. S.; Filler, R. Synthesis 1975, 749–764; (b)
Rzeszotarska, B.; Karolak-Wojciechowska, J.; Broda, M. A.;
Galdecki, Z.; Trzezwinska, B.; Koziol, A. E. Int. J. Pept.
Protein Res. 1994, 44, 313–319.
9. Cambridge Crystallographic Data Centre, Mercury 1.2: Crystal
structure visualization and exploration made easy, 12 Union
Road, Cambridge CB2 1EZ, UK, 2004.
Acknowledgements
10. MM2 and PM5 calculations were accomplished by using
CAChe 5.0 for Windows available from Fujitsu Ltd, 2002.
11. (a) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259–271; (b)
Rehm, D.; Weller, A. Z. Phys. Chem. 1970, 69, 183–200.
12. Gilbert, A.; Baggott, J. Essentials of Molecular
Photochemistry; Blackwell Scientific Publications: Oxford,
1991; 256–263.
This research was partially supported by a ‘Scientific Fron-
tier Research Project’ from the Ministry of Education,
Sports, Culture, Science and Technology, Japan.
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