A new route to papaverine analogs via photocyclization of substituted N-acyl-α-dehydrophenylalaninamides

Article abstract of DOI:10.3987/com-03-9801

(Z)-N-Phenylacetyl-α-dehydro(3,4-dimethoxyphenyl)alaninamide derivatives [(Z)-1] were prepared in satisfactory yields, starting from 3,4-dimethoxybenzaldehyde. On irradiation in methanol, regioselective photocyclization of these derivatives proceeded to g

Full text of DOI:10.3987/com-03-9801

HETEROCYCLES, Vol. 60, No. 8, 2003
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HETEROCYCLES, Vol. 60, No. 8, 2003, pp. 1779 - 1786
Received, 7th May, 2003, Accepted, 23rd June, 2003, Published online, 24th June, 2003
A
NEW
ROUTE
TO
PAPAVERINE
ANALOGS
VIA
PHOTOCYCLIZATION OF SUBSTITUTED N-ACYL-α-DEHYDRO-
PHENYLALANINAMIDES
Hideki Hoshina, Kei Maekawa, Keisuke Taie, Tetsutaro Igarashi, and
Tadamitsu Sakurai*
Department of Applied Chemistry, Faculty of Engineering, Kanagawa University,
Kanagawa-ku, Yokohama 221-8686, Japan
Abstract–(Z)-N-Phenylacetyl-α-dehydro(3,4-dimethoxyphenyl)alaninamide
derivatives [(Z)-1] were prepared in satisfactory yields, starting from 3,4-
dimethoxybenzaldehyde.
On irradiation in methanol, regioselective
photocyclization of these derivatives proceeded to give papaverine analogs in good
yields. Substituents introduced into (Z)-1 were found to exert only a minor effect
on the conversion of the starting isomers (1) as well as on the selectivity of the
analogs which were formed along with 1-azetines.
Photochemistry has continued to contribute to the development of efficient and selective transformations of
organic materials into pharmaceutically important heterocyclic compounds.1 It is well-known that
numerous biomolecules and natural products contain various types of heterocyclic compounds as their
principal constituents. We have been interested in exploring the excited-state reactivities of α-
2
dehydroamino acid derivatives, one of the main constituents of some antibiotics, as well as in demonstrating
the synthetic utility of these photochemical reactions. Taking into account that α-dehydroamino acid-
derived products may possess high biological activities, we embarked on a systematic study regarding
photochemical reactivities of aryl-substituted α-dehydroalanine derivatives and discovered novel
photocyclization reactions forming pharmaceutically useful products.3,4 One of the important findings is
that the photocyclization of substituted (Z)-N-acetyl-α-dehydrophenylalaninamides in methanol and
acetonitrile constitutes a useful method for constructing the isoquinoline skeleton.3 Very recently, we found
that the introduction of a methoxy group at the meta position on the styryl benzene ring accomplishes
regioselective photocyclization to give 6-methoxyisoquinoline derivative in a reasonable yield without
forming any 8-methoxy derivative.5 These findings stimulated us to photochemically synthesize analogs of
papaverine, one of the isoquinoline alkaloids, as an extension of our study on the photocyclization of N-
acyl-α-dehydroamino acid derivatives. To this end we designed and synthesized substituted (Z)-N-
phenylacetyl-α-dehydrophenylalaninamides (1a–h), hoping to establish a new route to analogs of
pharmaceutically important papaverine.
The starting (Z)-isomers (1a–h) were prepared in high yields (>80%) by the ring opening reactions of (Z)-

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HETEROCYCLES, Vol. 60, No. 8, 2003
2-substituted benzyl-4-methoxy-substituted benzylidene-5(4H)-oxazolones with butylamine (1a,c–h)
5
R
1a
1b
1c
1d
1e
1f
1g
1h
3
4
O
NH
R
1
O
R : OMe OMe
OMe OMe
OMe OMe
OMe OMe OMe OMe
H OMe OMe OMe
2
R : OMe OMe
N
H
R
3
R : OMe OMe
OMe
H
H
OMe Cl
OMe Cl
Cl
H
F
4
R : OMe OMe
H
H
2
R
5
1
R : Bu
CH Ph Bu
2
Bu
Bu
Bu
Bu
Bu
(Z)-1a–h
R
or benzylamine (1b) in the presence of triethylamine.6 The synthesis of these oxazolones was achieved by
the Knoevenagel-type condensation and ring closure reactions between methoxy-substituted benzaldehydes
6
and N-substituted phenylacetylglycines, though their yields were not so high (20–30%). After a nitrogen
purged methanol solution of 1a (1.0×10–3 mol dm–3) was irradiated with Pyrex-filtered light (>280 nm) from
a 400 W high pressure Hg lamp for 360 min at room temperature, the product mixture obtained was subjected
to column or preparative thin layer chromatography over silica gel (ethyl acetate-chloroform), which allowed
us to isolate papaverine analog (2a, 46%). In addition, (E)-1a could be isolated in 33% yield from the
mixture obtained by the 60 min irradiation of the same solution.
The structures of 2-(3,4-
dimethoxybenzyl)-substituted oxazolone, (Z)-1a, (E)-1a, and 2a were determined based on their
spectroscopic and physical properties.7 A 1H NMR spectral analysis of the product mixture showed that
there is formation of trans-1-azetine [trans-3a; δ= 4.37 and 5.45 ppm, J= 6.7 Hz (azetine ring-proton
signals)] and cis-3a [δ= 4.82 and 5.69 ppm, J= 10.4 Hz (azetine ring-proton signals)], as predicted from the
previous studies (Scheme 1).3,4 However, any attempts to isolate these azetine derivatives were
unsuccessful, probably because of the more decreased stability of 3a as compared to that of 1-azetines
previously isolated.
MeO
OMe
O
NHBu
O
N
MeO
MeO
NHBu
hν
(Z)-1a
(E)-1a
N
OMe
MeO
OMe
OMe
2a
3a (trans and cis)
Scheme 1
Since the photocyclization of (Z)-1a accompanies side reactions to some extent, the sum of composition for
(Z)-1a, (E)-1a, 2a, and 3a was regarded as 100% for estimating the compositions of these compounds

HETEROCYCLES, Vol. 60, No. 8, 2003
1781
based on the area ratio of given 1H NMR signals (Table 1). The result obtained for (Z)-1a demonstrates the
rapid production of (E)-1a and the subsequent increase in a composition for 2a and 3a with the decrease of
(Z)- and (E)-isomer compositions. In addition, the use of (E)-1a (1.0×10–3 mol dm–3) as the starting
isomer gave almost the same isomer composition as that derived from (Z)-1a at the early stage of the reaction
(Table 1), confirming that the rate of the photoisomerization (Path A in Scheme 2) is much faster than that
Table 1. Relation between irradiation time and composition (%) of each
compound obtained by the irradiation of (Z)-1a and (E)-1a in methanol
Irradiation time (min)
Compound
0
30
60
90
120
240
360
3.5
(Z)-1a 100
39.6
38.6
36.2
34.1
18.4
(0)a) (38.9)a) (38.0)a)
57.3 53.5
(100)a) (58.0)a) (54.5)a)
1.8 4.6
(0)a) (1.5)a) (4.0)a)
0.8 2.0
(0)a) (1.0)a) (2.0)a)
0.5 1.3
(0)a) (0.6)a) (1.5)a)
(E)-1a
2a
0
50.2
8.0
3.2
2.4
46.4
12.5
4.4
25.9
38.0
12.0
5.7
4.3
63.9
19.6
8.7
0
trans-3a
cis-3a
0
0
2.6
a) Composition obtained by the irradiation of (E)-1a.
5
5
CONHR
CONHR
N
H
NH
O
hν
1) H shift
Cyclization
Path B
(Z)-1a–h
(Z)-1a–h*
H
2) –H O
2
2
2
3
R
R
1
1
R
R
4
4
Path A
R
R
3
3
R
R
3
4
4
R
R
R
R
2a–h
Path A
H
N
O
N
R
5
5
CONHR
CONHR
hν
1) H abstraction
Cyclization
Path B
H
(E)-1a–h
(E)-1a–h*
2) –H O
2
2
2
R
R
1
1
R
( 3a–h )
Scheme 2

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HETEROCYCLES, Vol. 60, No. 8, 2003
of the subsequent photocyclization process (Path B).8 The data in Table 1 also show that selectivity for 2a
[defined as the product-composition ratio, namely, 2/(2 + 3)] is in the range of 60–70% and comparable to
that (69%) for isoquinoline derivative generated by the irradiation of N-acetyl-a-dehydro(4-
methoxyphenyl)alaninamide in methanol.3 This finding strongly suggests that the methoxy-substituted
phenylacetyl carbonyl carbon in the excited-state (Z)- and (E)-isomers possesses the reactivity comparable to
1
that of the acetyl carbonyl carbon. A careful analysis of the H NMR spectrum recorded after 360 min
irradiation revealed negligible formation of 7,8-dimethoxyisoquinoline derivative. In order to confirm
whether the methoxy group introduced at the 3-position on the styryl benzene ring exerts a large steric effect
on the cyclization proceeding at the 2-position, conformational energy of the (Z)-isomer was minimized by
MM2 and PM5 calculations (Figure 1).9 An examination of the energy-minimized conformation revealed
that the 3-methoxy methyl group is directed toward the hydrogen atom attached to the 2-position and, hence,
the cyclization process taking place at this position experiences a great steric hindrance of the methoxy group.
Therefore, we were led to conclude that the 3-methoxy substituent plays a decisive role in constructing the
papaverine skeleton.
H
H
H
H
H
H
H
H
H
Table 2. Relation between irradiation time and
composition (%) of each compound obtained by
the irradiation of (Z)-1a in acetonitrile
O
N
H
O
H
H
Irradiation time (min)
Compound
N
H
H
H
O
1
0
30
60
90
120
H
H
H
H
2
6
H
100 69.7 66.0 62.5 57.3
(Z)-1a
(E)-1a
2a
H
H
H
H
H
3
O
0
0
0
0
30.3 32.0 33.4 35.2
5
H
H
0
0
0
2.0
0
3.5
0.6
0
6.3
1.2
0
O
4
O
trans-3a
cis-3a
H
H
H
H
0
H
Figure 1. Energy-minimized
conformation of (Z)-1a.
In Table 2 are shown solvent effects on the photoreactivity of 1a and the product composition.
A
comparison of the data given in Tables 1 and 2 clearly indicates that the use of aprotic polar solvent,
acetonitrile, lowers the reactivity of each isomer in the excited state whereas the (Z)-isomer composition is
increased by a factor of about 2 at photostationary state in this solvent. It was previously found that
methanol is ab le to fo rm a hydr ogen bond to the amide car bonyl oxy gen of N- acetyl-a -
dehydronaphthylalaninamide derivative in the ground and excited states.4 In addition to the fact that
methanol and acetonitrile have comparable polarities,10 this finding allowed us to propose that hydrogen-
bonding solvation of (Z)- and (E)-isomers in the excited state greatly affects not only the relative rate for the
isomerization but also the rate for the cyclization from a given isomer. Interestingly, the change in solvent

HETEROCYCLES, Vol. 60, No. 8, 2003
1783
from methanol to acetonitrile resulted in an increase of the selectivity for 2a (64→84% at 120 min irradiation)
with a decrease of that for 3a (36→16%). However, on prolonged irradiation (>240 min) in acetonitrile
side reactions took place to an appreciable extent, making methanol better solvent for the reaction.
In order to shed light on the scope and limitations of the photoinduced cyclization of (Z)-1a that provides a
new route to papaverine analog (2a), we investigated substituent effects on the composition of each
compound obtained at a given irradiation time. As demonstrated in Table 3, the removal of the methoxy
group from the 3-position on the styryl benzene ring (1e) had only a small effect on both the conversion of 1a
and the selectivity of 2a. In addition, these conversion and selectivity were only sightly affected also by the
replacement of two methoxy groups on the benzene ring of the phenylacetyl moiety (R3 and R4) by two
chlorine atoms (1f), as well as by the introduction of benzyl group (R5) instead of butyl one (1b). A
comparison of the data for 1c (R3= OMe, R4= H), 1d (R3= R4= H), 1g (R3= Cl, R4= H), and 1h (R3= F,
R4= H) confirmed that the electron-donating methoxy group has a clear tendency to increase the conversion of
1 at a given irradiation time. It is likely that the methoxy group as substituent R3 (1c) results in an
enhancement in the reactivity of the phenylacetyl carbonyl carbon in the excited state (Scheme 2). Thus, the
observed substituent effects allow us to predict that the regioselective photocyclization of (Z)-1 is applicable
to the synthesis of various kinds of papaverine analogs.
Table 3. Substituent effects on the composition of each compound obtained by the
irradiation of (Z)-1a in methanol and the selectivity of 2a.
Composition (%)
Irradiation
time (min)
Selectivity
Compound
2
3a)
of 2 (%)
(Z)-1
(E)-1
1a
1b
1c
1d
1e
1f
60
360
60
38.6
3.5
53.5
4.3
4.6
63.9 (46)b)
2.6
3.3
28.3
2.0
69
67
70
66
62
81
68
75
39.9
4.3
55.5
5.7
360
60
60.7 (45)b)
29.3
3.1
36.7
0.7
55.0
0.9
5.2
68.6 (43)b)
3.7
240
60
29.8
3.3
36.9
3.0
56.1
4.0
360
60
61.5 (48)b)
31.5
0.9
50.0
5.3
47.6
5.7
1.5
54.9 (40)b)
3.8
360
60
34.1
1.7
34.8
2.7
59.7
3.6
360
60
75.6 (50)b)
18.1
1.0
1g
1h
36.5
3.2
59.6
4.2
2.9
62.6 (42)b)
4.2
360
60
30.0
1.9
36.1
2.5
57.8
3.7
360
70.4 (46)b)
23.4
a) The sum of cis- and trans-isomers. b) Isolated yield (%).

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HETEROCYCLES, Vol. 60, No. 8, 2003
11
Although there are many synthetic routes to papaverine and its analogs, no convenient photochemical route
to the analogs is known. The procedure for preparing the starting 1 is very simple and easily applicable to
its related compounds. In addition, column chromatography on silica gel enables rapid separation of 2, the
Rf value of which is much larger than that of (Z)-1, (E)-1, and 3. The photocyclization of substituted (Z)-
N-phenylacetyl-a-dehydrophenylalaninamides described above, therefore, provides a new synthetic route to
papaverine analogs.
ACKNOWLEDGMENT
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. ‘Synthetic Organic Photochemistry,’ ed. by W. M. Horspool, Plenum Press, New York, 1984.
2. K. Noda, Y. Shimohigashi, and N. Izumiya, ‘The Peptides-Analysis, Synthesis, Biology,’ ed. by E.
Gross and J. Meienhofer, Academic Press, New York, 1983, Vol. 5, pp. 285–339.
3. K. Kubo, S. Yaegashi, K. Sasaki, T. Sakurai, and H. Inoue, Tetrahedron Lett., 1996, 37, 5917; K.
Kubo, M. Koshiba, H. Hoshina, and T. Sakurai, Heterocycles, 1998, 48, 25; H. Hoshina, K. Kubo, A.
Morita, and T. Sakurai, Tetrahedron, 2000, 56, 2941; H. Hoshina, H. Tsuru, K. Kubo, T. Igarashi, and
T. Sakurai, Heterocycles, 2000, 53, 2261.
4. K. Kubo, T. Ishii, T. Sakurai, and M. Makino, Tetrahedron Lett., 1998, 39, 4083; T. Sakurai, Y.
Morioka, K. Maekawa, and K. Kubo, Heterocycles, 2000, 53, 271; K. Maekawa, T. Igarashi, K. Kubo,
and T. Sakurai, Tetrahedron, 2001, 57, 5515; T. Motohashi, K. Maekawa, K. Kubo, T. Igarashi, and T.
Sakurai, Heterocycles, 2002, 57, 269; K. Maekawa, K. Kubo, T. Igarashi, and T. Sakurai,
Heterocycles, 2002, 57, 1591.
5. K. Maekawa, H. Kajiwara, Y. Iseya, T. Igarashi, and T. Sakurai, Heterocycles, 2003, 60, 637.
6. Y. S. Rao and R. Filler, Synthesis, 1975, 749; B. Rzeszotarska, J. Karolak-Wojciechowska, M. A.
Broda, Z. Galdecki, B. Trzezwinska, and A. E. Koziol, Int. J. Peptide Protein Res., 1994, 44, 313.
7. Selected data for (Z)-2-(3,4-dimethoxybenzyl)-4-(3,4-dimethoxybenzylidene)-5(4H)-oxazolone: mp
157.0–158.0 °C (CHCl3-hexane); IR (KBr): 1794, 1770, 1650, 1266 cm–1; 1H NMR (500 MHz, CDCl3):
d 3.85 (3H, s), 3.87 (3H, s), 3.89 (3H, s), 3.91 (2H, s), 3.93 (3H, s), 6.84 (1H, d, J= 8.3 Hz), 6.88
(1H, d, J= 2.0 Hz), 6.92 (1H, dd, J= 2.0, 8.3 Hz), 7.10 (1H, s), 7.46 (1H, dd, J= 2.0, 8.3 Hz), 7.97
13
(1H, d, J= 2.0 Hz); C NMR (CDCl3): d 35.7, 55.8, 56.0 (3C), 110.8, 111.3, 112.6, 113.9, 121.7,
125.4, 126.5, 127.5, 130.4, 132.3, 148.6, 149.1 (2C), 152.0, 166.2, 168.0.
For (Z)-N-butyl-3-(3,4-dimethoxyphenyl)-2-(3,4-dimethoxyphenylacetylamino)-2-propenamide [(Z)-
1
1a]: mp 140.0–141.0 °C (EtOH-hexane); IR (KBr): 3272, 2956, 2836, 1644, 1620 cm–1; H NMR (500
MHz, DMSO-d6): d 0.87 (3H, t, J= 7.3 Hz), 1.27 (2H, tq, J= 7.3, 7.3 Hz), 1.40 (2H, tt, J= 7.3, 7.3 Hz),
3.11 (2H, dt, J= 5.5, 7.3 Hz), 3.55 (2H, s), 3.64 (3H, s), 3.71 (3H, s), 3.74 (3H, s), 3.75 (3H, s),
6.76 (1H, d, J= 8.5 Hz), 6.84 (1H, dd, J= 1.8, 7.9 Hz), 6.90 (1H, d, J= 7.9 Hz), 6.94 (1H, br s), 6.95
(1H, dd, J= 1.8, 8.5 Hz), 6.97 (1H, s), 7.12 (1H, d, J= 1.8 Hz), 7.71 (1H, t, J= 5.5 Hz), 9.43 (1H, s);

HETEROCYCLES, Vol. 60, No. 8, 2003
1785
13 C NMR (DMSO-d6): d 13.7, 19.5, 31.2, 38.7, 41.9, 55.2, 55.3 (2C), 55.5, 111.2, 111.8, 113.0,
113.1, 121.2, 122.7, 126.7, 127.7, 128.0, 128.2, 147.6, 148.2, 148.5, 149.1, 164.9, 169.9. Anal.
Calcd for C25H32N2O6: C, 65.77; H, 7.06; N, 6.14. Found: C, 65.91; H, 7.03; N, 6.35.
For (E)-N-butyl-3-(3,4-dimethoxyphenyl)-2-(3,4-dimethoxyphenylacetylamino)-2-propenamide [(E)-
1
1a]: mp 139.0–140.0 °C (EtOH-hexane); IR (KBr): 3310, 2956, 1641 cm–1; H NMR (500 MHz,
DMSO-d6): d 0.79 (3H, t, J= 7.3 Hz), 1.13 (2H, tq, J= 7.3, 7.3 Hz), 1.30 (2H, tt, J= 6.7, 7.3 Hz), 3.02
(2H, dt, J= 6.1, 6.7 Hz), 3.47 (2H, s), 3.69 (3H, s), 3.72 (3H, s), 3.73 (3H, s), 3.75 (3H, s), 6.76 (1H,
dd, J= 1.8, 8.5 Hz), 6.80 (1H, s), 6.81 (1H, dd, J= 1.8, 8.5 Hz), 6.84 (1H, d, J= 8.5 Hz), 6.86 (1H, d,
J= 1.8 Hz), 6.88 (1H, d, J= 8.5 Hz), 6.92 (1H, d, J= 1.8 Hz), 7.99 (1H, t, J= 6.1 Hz), 9.63 (1H, s);
13 C NMR (DMSO-d6): d 13.6, 19.5, 30.4, 38.6, 42.2, 55.2, 55.4 (2C), 55.5, 111.5, 111.6, 111.8,
113.1, 115.5, 120.8, 121.1, 128.0, 128.3, 132.2, 147.6, 147.8, 148.2, 148.5, 164.8, 169.1. Anal.
Calcd for C25H32N2O6: C, 65.77; H, 7.06; N, 6.14. Found: C, 65.46; H, 6.84; N, 5.99.
For
3-butylaminocarbonyl-6,7-dimethoxy-1-(3,4-dimethoxybenzyl)isoquinoline
(2a):
mp
1
140.0–140.5 °C (EtOAc); IR (KBr): 3388, 2932, 1743, 1665 cm–1; H NMR (500 MHz, DMSO-d6): d
0.92 (3H, t, J= 7.3 Hz), 1.33 (2H, tq, J= 7.3, 7.3 Hz), 1.54 (2H, tt, J= 6.7, 7.3 Hz), 3.36 (2H, dt, J=
6.1, 6.7 Hz), 3.67 (3H, s), 3.69 (3H, s), 3.92 (6H, s), 4.56 (2H, s), 6.82 (1H, d, J= 7.9 Hz),
6.83–6.85 (1H, m), 7.12 (1H, br s), 7.56 (1H, s), 7.60 (1H, s), 8.25 (1H, s), 8.68 (1H, t, J= 6.1 Hz);
13C NMR (DMSO-d6): d 13.7, 19.6, 31.5, 38.3, 40.4, 55.3, 55.4, 55.77, 55.82, 104.5, 107.1, 111.8,
112.9, 117.2, 120.7, 123.3, 131.6, 133.0, 141.4, 147.2, 148.6, 150.7, 152.5, 157.0, 164.3. Anal.
Calcd for C25H30N2O5: C, 68.47; H, 6.90; N, 6.39. Found: C, 68.14; H, 6.82; N, 6.19.
8. The photocyclization of (Z)-1a in methanol gave the same product distribution as that derived from the
cyclization of substituted N-acetyl-a-dehydrophenylalaninamides,3 thus allowing us to discuss the rate of
the isomerization relative to the rate of the cyclization on the basis of the same reaction scheme as that
previously proposed.
9. MM2 and PM5 calculations were accomplished by using CAChe 5.0 for Windows available from Fujitsu
Ltd, 2002. N. L. Allinger, J. Am. Chem. Soc., 1977, 99, 8127; N. L. Allinger, Y. H. Yuh, and J.-
H. Lii, J. Am. Chem. Soc., 1989, 111, 8551.
10. J. A. Riddick, W. B. Bunger, and T. K. Sakano, ‘Organic Solvents,’ 4th edn., Wiley, Chichester,
1986.
11. W. E. Kreighbaum, W. F. Kavanaugh, W. T. Comer, and D. Deitchman, J. Med. Chem., 1972, 15,
1131; E. C. Taylor and S. F. Martin, J. Am. Chem. Soc., 1972, 94, 2874; M. P. Cava, I. Noguchi, and
K. T. Buck, J. Org. Chem., 1973, 38, 2394; E. C. Taylor and S. F. Martin, J. Am. Chem. Soc., 1974,
96, 8095; E. Prudhommeaux, G. Ernouf, O. Foussard-Blanpin, and C. Viel, Eur. J. Med. Chem., 1975,
10, 19; S. Kano, Y. Tanaka, and S. Hibino, Heterocycles, 1980, 14, 39; D. Bhattacharjee and F. D.
Popp, J. Pharm. Sci., 1980, 69, 120; A. Brossi, K. C. Rice, C. P. Mak, J. Reden, A. E. Jacobson, Y.
Nimitkitpaisan, P. Skolnick, and J. Daly, J. Med. Chem., 1980, 23, 648; A. P. Kozikowski and A.
Ames, J. Org. Chem., 1980, 45, 2548; J. R. Falck, S. Manna, and C. Mioskowski, J. Org. Chem.,
1981, 46, 3742; K. A. Walker, M. R. Boots, J. F. Stubbins, M. E. Rogers, and C. W. Davis, J. Med.
Chem., 1983, 26, 174; J. B. Hendrickson and C. Rodriguez, J. Org. Chem., 1983, 48, 3344; L. S.

1786
HETEROCYCLES, Vol. 60, No. 8, 2003
Trifonov and A. S. Orahovats, Tetrahedron Lett., 1985, 26, 3159; L. Kovacs and P. Kerekes, Acta
Chim. Hung., 1985, 120, 103; P. R. Huddleston, J. M. Barker, B. Stickland, M. L. Wood, and L. H.
M. Guindi, J. Chem. Res. (S), 1988, 240; C. T. Goralski, R. C. Krauss, B. M. Williams, D. R. Henton,
and D. T. Brown, Ind. Eng. Chem. Res., 1989, 28, 221; R. Hirsenkorn, Tetrahedron Lett., 1991, 32,
1775; R. Suau, R. Rico-Gomez, F. A. Souto-Bachiller, L. de los A. Rodriguez-Rodriguez, and M. L.
Ruiz, Tetrahedron Lett., 1995, 36, 2653.