Selective and efficient transformation of N-(4-substituted benzoyl)-α-dehydroarylalanine alkyl esters into 4,5-dihydrooxazole derivatives via photoinduced electron transfer

Article abstract of DOI:10.1016/j.tetlet.2004.03.042

The irradiation of N-(4-substituted benzoyl)-α-dehydroarylalanine alkyl esters (1) in methanol containing triethylamine (TEA) was found to quantitatively give cis- and trans-4,5-dihydrooxazole derivatives (2), which were described as being formed via electron transfer from TEA to the excited-state (E)-1 followed by kinetically-controlled cyclization of the (E)-1-derived anion radical. A product composition analysis showed that the cis-2/trans-2 composition ratio is greatly varied depending on the stereoelectronic properties of the substituents, the polarity of protic solvents and the concentration of TEA.

Full text of DOI:10.1016/j.tetlet.2004.03.042

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 3663–3667
Selective and efficient transformation of N-(4-substituted benzoyl)-
a-dehydroarylalanine alkyl esters into 4,5-dihydrooxazole
derivatives via photoinduced electron transfer
Kei Maekawa,a Takahiro Sasaki,a Kanji Kubo,b Tetsutaro Igarashia and
Tadamitsu Sakuraia,*
aDepartment of Applied Chemistry, Faculty of Engineering, Kanagawa University, Kanagawa-ku, Yokohama 221-8686, Japan
bInstitute for Materials Chemistry and Engineering, Kyushu University, Kasugakoen, Kasuga 816-8580, Japan
Received 19 January 2004; revised 4 March 2004; accepted 5 March 2004
Abstract—The irradiation of N-(4-substituted benzoyl)-a-dehydroarylalanine alkyl esters (1) in methanol containing triethylamine
(TEA) was found to quantitatively give cis- and trans-4,5-dihydrooxazole derivatives (2), which were described as being formed via
electron transfer from TEA to the excited-state (E)-1 followed by kinetically-controlled cyclization of the (E)-1-derived anion
radical. A product composition analysis showed that the cis-2/trans-2 composition ratio is greatly varied depending on the stereo-
electronic properties of the substituents, the polarity of protic solvents and the concentration of TEA.
Ó 2004 Elsevier Ltd. All rights reserved.
Excited-state chemistry has continued to contribute to
the development of new synthetic methods that enable
the construction of a variety of hetero atom-containing
rings.1 In recent years much attention is being devoted
to studies regarding photoinduced electron transfer
reactions, owing to the fact that many of these reactions
initiated by an electron transfer give products 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
isoquinoline and 1-azetine derivatives in satisfactory
yields but with low efficiency.3 The use of N-acetyl-a-
dehydrophenylalanine alkyl esters as the starting a-
dehydroamino acid derivatives enabled the selective
formation of the corresponding isoquinolines by their
photocyclization.4 In addition, one-electron reduction of
1-naphthyl-substituted a-dehydroalaninamides was found
to proceed efficiently affording 3,4-dihydrobenzoquin-
olinone derivatives in high selectivity.5 The latter finding
demonstrates the synthetic utility of electron transfer-
initiated photocyclization of N-acyl-a-dehydroamino
acids and, hence, it stimulated us to explore photoin-
duced electron transfer reactions of N-acyl-a-dehydro-
alanine alkyl esters, hoping to develop an efficient and
highly selective phototransformation process of syn-
thetic utility. To this end we designed and synthesized
N-(4-substituted benzoyl)-a-dehydroarylalanine alkyl
esters (1a–g) and examined the effects of the substituent
and triethylamine (TEA) concentration on the reactivity
of 1a and the photoproduct composition.
R1O
O
O
N
H
R2
(Z)-1a (R1= Me, R2= H); (Z)-1b (R1= Et, R2= H);
(Z)-1c (R1= i-Pr, R2= H); (Z)-1d (R1= t-Bu, R2= H);
(Z)-1e (R1= Me, R2= OMe); (Z)-1f (R1= Me, R2= CN)
MeO
O
O
N
H
Keywords: Amino acids and derivatives; Photochemistry; Electron
transfer; Dihydrooxazoles; Substituent effects.
* Corresponding author. Fax: +81-45-491-7915; e-mail: tsakurai@
kamome.cc.kanagawa-u.ac.jp
(Z)-1g
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.03.042

3664
K. Maekawa et al. / Tetrahedron Letters 45 (2004) 3663–3667
The starting (Z)-isomers [(Z)-1a–g] were prepared in
good yields (42–71%) 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;6
The finding that the photocyclization proceeds cleanly
without forming any other products allows us to mon-
itor the reaction by means of H NMR spectroscopy
1
and then to examine the effect of TEA concentration on
the reactivity of 1a and the composition ratio of cis-2a
and trans-2a. For this examination, deaerated methanol
solutions of (Z)-1a (4.0 ꢀ 10ꢁ3 mol dmꢁ3, 10 mL), which
contain varying amounts of TEA were irradiated in
parallel for 3 h on a merry-go-round apparatus using the
After
a nitrogen-saturated methanol solution of
(Z)-1a (4.0 ꢀ 10ꢁ3 mol dmꢁ3, 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 preparative thin-layer
chromatography over silica gel (eluent: EtOAc–hexane
or EtOAc–CHCl3), which allowed us to isolate cis- and
trans-4-methoxycarbonyl-5-(1-naphthyl)-2-phenyl-4,5-
dihydrooxazoles (cis-2a, 30%; trans-2a, 58% yield) hav-
ing 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 independently from
the reaction mixture, which was irradiated for 0.5 h
under the same conditions (conversion, 16.5%), by
similar workup (Scheme 1). The structures of the iso-
lated products were determined based on their spectro-
scopic and physical properties.7 In addition, an X-ray
analysis of cis-2a-derived single crystal provided defini-
tive evidence for the structure of the cis-isomer with
larger J4;5 value (Fig. 1).8
1
same filter and light source. H NMR spectral analysis
of the reaction mixture obtained after usual workup
gave the results collected in Table 1, which demonstrate
that the reactivity is increased with an increase in TEA
concentration while negligible amounts of the products
are observed without TEA. This is consistent with the
participation of an electron transfer mechanism. The
data of Table 1 also show that the composition ratio of
cis-2a to trans-2a decreases with increasing TEA con-
centration, suggesting the occurrence of TEA-catalyzed
isomerization to the thermodynamically more stable
trans-isomer. This suggestion was substantiated by the
observation that on allowing a deuterated methanol
solution of cis-2a (4.0 ꢀ 10ꢁ3 mol dmꢁ3, 1.0 mL) con-
taining TEA (0.10 mol dmꢁ3) to stand at room temper-
ature, the isomerization to the trans-isomer slowly
proceeds to furnish a 9:1 equilibrium mixture of trans-2a
and cis-2a, respectively, after 7 days. 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. Thus, we are allowed to
preferentially obtain either of these two isomers by
controlling the amine concentration and also to mainly
generate the trans-isomer by allowing the reaction mix-
ture to stand in the presence of a tertiary amine having
high basicity.
N
O
H
OMe
O
hν
(Z)-1a
MeOH/TEA
H
cis-2a
In order to explore the substituent effects on the reac-
tivity of the excited-state 1a and the extent to which cis-
2a is isomerized to trans-2a, an oxygen-free methanol or
ethanol solution of 1b–g (4.0 ꢀ 10ꢁ3 mol dmꢁ3, 500 mL)
O
NH
being irradiated in the presence of TEA (0.10 mol dmꢁ3
)
N
O
H
OMe
O
OMe
1
for a given period of time was subjected to H NMR
spectral analysis in DMSO-d6, which gave the results
collected in Table 2. 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 selectiv-
ity and, in addition, considerably suppressed the iso-
merization induced by TEA. The suppression of this
isomerization was also achieved by employing the less
polar protic solvent, ethanol. It is very likely that the
abstraction of the proton at the 4-position by TEA is
subject to large steric and solvent polarity effects. On the
other hand, the presence of an electron-donating meth-
oxy or an electron-withdrawing cyano group at the
para-position on the N-benzoyl benzene ring both low-
ers the excited-state reactivity of 1a, suggesting that if we
accept an electron transfer mechanism, the anion radical
generated is delocalized into the N-acyl moiety. Fur-
thermore, the finding that the isomer ratio of cis-2 to
trans-2 decreases with increasing electron-withdrawing
ability of the aryl group attached to the oxazole ring
O
H
trans-2a
(E)-1a
Scheme 1.
Figure 1. ORTEP drawing of cis-2a.

K. Maekawa et al. / Tetrahedron Letters 45 (2004) 3663–3667
3665
Table 1. Effects of TEA concentration on the conversion of 1a and the composition of each compound, obtained by the 3 h irradiation of (Z)-1a in
methanol at room temperature
TEA (mol dmꢁ3
)
Conversion (%)
Composition (%)
cis-2a/trans-2a
(Z)-1
(E)-1
cis-2a
trans-2a
0
<0.1
48.7
70.4
74.6
80.2
>35.5
17.8
10.2
8.8
>65.4
33.5
19.5
16.6
12.8
<0.1
38.8
48.2
43.9
36.4
<0.1
9.9
––
0.01
0.05
0.10
0.20
3.9
2.2
1.4
0.8
22.2
30.7
43.8
7.0
Table 2. Substituent effects on the conversion of 1 and the composition of each compound, obtained by the irradiation of (Z)-1 in methanol or
ethanol at room temperature
Compound
Irradiation time Conversion (%)
(h)
Composition (%)
cis-2/trans-2
(Z)-1
(E)-1
cis-2
trans-2
1aa
1bb
1ca
1da
1ea
1fa
0.5
5
16.5
100
18.9
100
18.9
100
19.7
100
30.1
0
53.4
0
11.5
36.9
13.2
68.3
16.5
80.9
18.1
88.5
9.8
5.0
63.1
5.7
2.3
0.6
2.3
2.2
6.9
4.2
11.3
7.7
3.5
1.9
1.0
0.2
1.4
0.4
0.5
5
29.8
0
51.3
0
31.7
2.4
0.5
5
25.2
0
55.9
0
19.1
1.6
0.5
5
25.5
0
54.8
0
11.5
2.8
0.5
5
12.6
92.7
9.0
33.5
2.9
47.9
12.3
75.9
18.1
53.9
4.4
43.1
12.4
15.4
8.8
60.6
4.6
32.1
4.4
0.5
5
75.4
8.7
13.5
5.0
61.9
3.7
1ga
0.5
5
73.1
20.6
52.5
a In methanol.
b In ethanol. This solvent was used because of the occurrence of TEA-catalyzed ester exchange reaction in methanol.
(2e > 2a > 2f) provides additional evidence for the
occurrence of TEA-induced isomerization of the cis-
isomer. An inspection of the data in Table 2 confirms
that replacement of the 1-naphthyl group in 1a by the
phenyl (1g) still enables selective phototransformation
into 2g, though it is responsible for the decreased
excited-state reactivity as well as for the enhanced
isomerization rate.
that the TEA concentration remains constant during the
irradiation,11 we were led to propose Scheme 2 that
explains the formation of dihydrooxazole derivatives
(2a–g).
Because the para-substituted benzoyl chromophores
exhibit only very little absorption in the range of 300–
400 nm, an electron from TEA should at first migrate to
the naphthylmethylene moiety and then should be
delocalized into the N-acyl moiety. This interpretation is
substantiated by the finding that the fluorescence
intensity of 1 is decreased in the following order: 1e
(R2 ¼ OMe) > 1a (R2 ¼ H) > 1f (R2 ¼ CN, virtually
nonfluorescent). The observation of the TEA-catalyzed
isomerization of 2 allows us to assume the anion radical
intermediate I formed by the nucleophilic attack of
N-acyl carbonyl oxygen upon the olefinic carbon in the
(E)-1-derived anion radical (Scheme 2). Back electron
transfer to the TEA cation radical followed by hydrogen
shift in II affords cis-2 and trans-2. The presence of a
cyano group as the substituent R2 is considered to lower
the nucleophilicity of the acyl carbonyl oxygen through
stabilization of the anion radical delocalized into the
acyl moiety, as already described. It is very likely that
the electron-donating methoxy substituent in 1e sup-
presses delocalization of the anion radical into the acyl
moiety to induce a decrease in reactivity of the carbonyl
oxygen. The fact that the thermodynamically less stable
cis-isomer is preferentially formed leads us to propose
In addition to the principle of least motion,9 molecular
modeling of (Z)-1a and (E)-1a reveals that the latter
isomer adopts a most suitable conformation for the
cyclization reaction that eventually gives 2a. The previ-
ous findings that the triplet-sensitized reaction of (Z)-N-
acyl-a-dehydrophenylalanine derivatives gives only the
corresponding (E)-isomers without forming any cyclized
products strongly suggest the involvement of singlet
excited-state 1a in the electron transfer process.3;4
Although the very weak fluorescence of 1a made it
extremely difficult to measure emission quenching in the
presence of TEA, the simplified Weller equation:
DGet=kJ molꢁ1 ¼ 96:5ðEox ꢁ EredÞ ꢁ ES,10 where Eox, Ered
and ES refer to the oxidation potential of TEA (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
excitation energy of 1a (368 kJ molꢁ1 in MeCN),
respectively, allowed us to estimate the free energy
change (DGet) for electron transfer from TEA to singlet
1a as ꢁ77 kJ molꢁ1. Thus, taking into account the fact

3666
K. Maekawa et al. / Tetrahedron Letters 45 (2004) 3663–3667
hν
Electron transfer (ET)
Back ET
Ar
(Z)-1
(E)-1
(Z)-1*
(E)-1*
(Z)-1 TEA
(Z)-1 /TEA
TEA
Ar
TEA
NH
hν
ET
O
NH
O
TEA
O
O
OR1
TEA
OR1
TEA
H
TEA
H
N
H
OR1
H
OR1
O
OR1
Ar
Ar
OR1
Ar
Back ET
N
N
Ar
O
N
H shift
O
O
O
O
O
O
H
H
H
H
cis-2
trans-2
I
II
Scheme 2.
3. Hoshina, H.; Kubo, K.; Morita, A.; Sakurai, T. Tetrahe-
dron 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. Tetra-
hedron 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.
6. (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;
that electrostatic attraction between the anion radical I
and the TEA cation radical, as well as hydrogen bond-
ing solvation of I (by methanol) and II (by methanol
and TEA), renders an electron transfer-initiated
cyclization a kinetically-controlled process. Since elec-
trostatic and hydrogen bonding interactions in the
cis-configurations may undergo less steric hindrance,
hydrogen shift from cis-II should be accelerated to a
much greater extent, as compared to that from trans-II,
resulting in a preferential formation of the cis-isomers.
Although there are several synthetic routes to dihydro-
oxazole derivatives, no convenient photochemical route
to these derivatives is known.12 The procedure for pre-
paring the starting a-dehydroamino acids (Z)-1a–g is
very simple and easily applicable to their related com-
pounds. The photoinduced electron transfer reaction
of (Z)-N-(4-substituted benzoyl)-a-dehydroarylalanine
alkyl esters described above, therefore, constitutes a
novel photochemical method for the construction of a
dihydrooxazole ring.
Data for (Z)-1a. Mp 118.0–118.5 °C. IR (KBr): 3200,
1
1732, 1632 cmꢁ1. H NMR (600 MHz, DMSO-d6): d 3.80
(3H, s), 7.46 (2H, dd, J ¼ 7:6, 7.6 Hz), 7.51 (1H, dd,
J ¼ 7:6, 8.9 Hz), 7.55 (1H, dd, J ¼ 7:6, 7.6 Hz), 7.58 (1H,
dd, J ¼ 6:9, 6.9 Hz), 7.59 (1H, dd, J ¼ 6:9, 7.6 Hz), 7.69
(1H, d, J ¼ 7:6 Hz), 7.84 (2H, d, J ¼ 7:6 Hz), 7.90 (1H, s),
7.94 (1H, d, J ¼ 8:9 Hz), 7.97 (1H, d, J ¼ 6:9 Hz), 8.05
(1H, d, J ¼ 7:6 Hz), 10.0 (1H, s). 13C NMR (150 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%).
7. Data for (E)-1a. Mp 156.0–157.0 °C. IR (KBr): 3240,
1
1736, 1632 cmꢁ1. H NMR (600 MHz, DMSO-d6): d 3.47
Acknowledgements
(3H, s), 7.32 (1H, d, J ¼ 7:4 Hz), 7.34 (1H, s), 7.49 (1H,
dd, J ¼ 7:4, 8.0 Hz), 7.55–7.61 (2H, m), 7.57 (2H, dd,
J ¼ 7:4, 8.6 Hz), 7.64 (1H, dd, J ¼ 7:4, 7.4 Hz), 7.90 (1H,
d, J ¼ 8:0 Hz), 7.95–7.98 (1H, m), 8.00 (2H, d,
J ¼ 8:6 Hz), 8.02–8.04 (1H, m), 10.7 (1H, s). 13C NMR
(150 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%).
This research was partially supported by a ‘High-Tech
Research Project’ from the Ministry of Education,
Sports, Culture, Science and Technology, Japan.
References and notes
1. Mariano, P. S.; Stavinoha, J. L. In Synthetic Organic
Photochemistry; Horspool, W. M., Ed.; Plenum: New
York, 1984; pp 145–257.
2. (a) Lewis, F. D.; Bassani, D. M.; Reddy, G. D. J. Org.
Chem. 1993, 58, 6390–6393; (b) Lewis, F. D.; Reddy, G.
D.; Bassani, D. M.; Schneider, S.; Gahr, M. J. Am. Chem.
Soc. 1994, 116, 597–605; (c) Lewis, F. D.; Bassani, D. M.;
Burch, E. L.; Cohen, B. E.; Engleman, J. A.; Reddy, G.
D.; Schneider, S.; Jaeger, W.; Gedeck, P.; Gahr, M. J. Am.
Chem. Soc. 1995, 117, 660–669.
Data for cis-2a. Mp 133.0–134.0 °C. IR (KBr): 1742,
1646 cmꢁ1 1H NMR (600 MHz, DMSO-d6): d 2.72 (3H,
.
s), 5.62 (1H, d, J ¼ 10:3 Hz), 6.88 (1H, d, J ¼ 10:3 Hz),
7.51–7.52 (2H, m), 7.57 (1H, dd, J ¼ 7:4, 7.4 Hz), 7.59
(2H, dd, J ¼ 7:4, 7.7 Hz), 7.60 (1H, dd, J ¼ 7:4, 8.7 Hz),
7.67 (1H, dd, J ¼ 7:4, 7.4 Hz), 7.90–7.92 (1H, m), 7.96
(1H, d, J ¼ 7:4 Hz), 8.08 (2H, d, J ¼ 7:7 Hz), 8.10 (1H, d,
J ¼ 8:7 Hz). 13C NMR (150 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,

K. Maekawa et al. / Tetrahedron Letters 45 (2004) 3663–3667
3667
169.0. Anal. Calcd (found) for C21H17NO3: C, 76.12
(75.89); H, 5.17 (5.20); N, 4.23% (4.48%).
CB2 1EZ, UK (fax: +44(0)-1223-336033 or e-mail:
deposit@ccdc.cam.ac.uk).
Data for trans-2a. Mp 108.0–109.0 °C. IR (KBr): 1742,
9. Gilbert, A.; Baggott, J. Essentials of Molecular Photo-
chemistry; Blackwell Scientific Publications: Oxford, 1991;
pp 256–263.
10. (a) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259–271;
(b) Rehm, D.; Weller, A. Z. Phys. Chem. 1970, 69, 183–
200.
11. A 1H NMR spectral analysis of the reaction mixture
obtained by the irradiation of a CD3OD solution of
(Z)-1a (4.0 ꢀ 10ꢁ3 mol dmꢁ3) containing TEA (1.0 ꢀ 10ꢁ2
mol dmꢁ3) and 1,4-dioxane (internal standard, 1.0 ꢀ 10ꢁ2
mol dmꢁ3) for a given period of time showed no sign of a
change in this amine concentration during the reaction.
12. (a) Overman, L. E.; Tsuboi, S.; Angle, S. J. Org. Chem.
1
1640 cmꢁ1. H NMR (600 MHz, DMSO-d6): d 3.81 (3H,
s), 4.89 (1H, d, J ¼ 6:9 Hz), 6.70 (1H, d, J ¼ 6:9 Hz), 7.52
(1H, d, J ¼ 7:4 Hz), 7.54 (1H, dd, J ¼ 6:9, 7.4 Hz), 7.57
(2H, dd, J ¼ 7:4, 7.4 Hz), 7.62 (1H, dd, J ¼ 8:6, 9.2 Hz),
7.62 (1H, dd, J ¼ 6:3, 9.2 Hz), 7.67 (1H, dd, J ¼ 7:4,
7.4 Hz), 7.98 (1H, d, J ¼ 6:9 Hz), 8.00 (1H, d, J ¼ 8:6 Hz),
8.04 (1H, d, J ¼ 6:3 Hz), 8.05 (2H, d, J ¼ 7:4 Hz). 13C
NMR (150 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%).
€
8. Crystal data for cis-2a: C21H17NO3, fw ¼ 331.36; colour-
1979, 44, 2323–2325; (b) Burger, K.; Huber, E.; Schontag,
less prism, 0.35 ꢀ 0.23 ꢀ 0.20 mm, monoclinic, space
W.; 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.
ꢀ
ꢀ
group P21/c; a ¼ 8:2216ð4Þ A, b ¼ 18:288ð1Þ A, c ¼
ꢀ
10:9309ð7Þ A, a ¼ 90:00°, b ¼ 94:889ð3Þ°, c ¼ 90:00°,
V ¼ 1637:55ð18Þ A ; Z ¼ 4; Dcalc ¼ 1:344 g cmꢁ3
;
R ¼
3
ꢀ
0:0468, wRðF 2Þ ¼ 0:1573. Crystallographic data of cis-2a
have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication number CCDC
228004. Copies of the data can be obtained, free of charge,
on application to CCDC, 12 Union Road, Cambridge