Thermal transformation of arylamidoximes in the presence of phosphorus ylides. Unexpected formation of 3-aryl-5-arylamino-1,2,4-oxadiazoles

Article abstract of DOI:10.1002/jhet.5570410404

The unexpected formation of 3-aryl-5-arylamino-1,2,4-oxadiazoles took place, when arylamidoximes reacted thermally with ethoxycarbonylmethylene(triphenyl)phosphorane. Furoxans, nitriles, ureas were also isolated suggesting aryl cyanide oxides as intermedi

Full text of DOI:10.1002/jhet.5570410404

Jul-Aug 2004
499
Thermal Transformation of Arylamidoximes in the Presence of
Phosphorus Ylides. Unexpected Formation of 3-Aryl-5-arylamino-
1,2,4-oxadiazoles
Demetrios N. Nicolaidesa*, Konstantinos E. Litinasa, Ioannis Vrasidasa and
Konstantina C. Fylaktakidoub
a
Laboratory of Organic Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki
54124, Greece
b
Molecular Biology and Genetics Department, Democritus University of Thrace, Alexandroupolis 68100, Greece
Received July 14, 2003
The unexpected formation of 3-aryl-5-arylamino-1,2,4-oxadiazoles took place, when arylamidoximes
reacted thermally with ethoxycarbonylmethylene(triphenyl)phosphorane. Furoxans, nitriles, ureas were also
isolated suggesting aryl cyanide oxides as intermediates. 3-Aryl-5-arylamino-1,2,4-oxadiazoles were
formed via an aryl migration from the carbon atom to the nitrogen atom of the amidoxime, and the structure
was further proved from the X-ray crystal structure of the N-(4-bromobenzoyl) derivative.
J. Heterocyclic Chem., 41, 499 (2004).
Introduction.
from 2, ii) reaction [8] of nitrile oxide formed with starting
amidoxime, iii) reaction [9] between the nitrile formed and
the starting amidoxime. The last mechanism is supported
by the fact that diarylfuroxans, always formed by further
dimerization of nitrile oxides [10], has not been isolated in
the reactions studied. In contrast arylnitriles were either
isolated or identified in the reaction mixture.
Our major scientific interests in the chemistry of ami-
doximes [11,12] and phosphorus ylides [13,14] prompted
us to investigate the reactivity of phosphorus ylides
towards amidoximes, taking into account our finding [15],
where an O-alcylation or "Wittig type" olefination pro-
ceeded on some oximes C=NOH upon heating with phos-
phorus ylides.
Thermal transformations of arylamidoximes was the
subject of interest in a number of scientific papers. For
example, when arylamidoximes were heated with car-
boxylic acids [1-3], 3,5-bis-aryl-1,2,4-oxadiazoles 9 and
arylamides 8 were produced, while 3-aryl-5-methyl-1,2,4-
oxadiazoles [4] 4 were additionally isolated, when glacial
acetic acid was used. Leandri and Rebora [5] reported the
thermal transformation of neat benzamidoxime 2b at 170
°C to benzonitrile 6b, benzamide 8b, 3,5-diphenyl-1,2,4-
triazole, 2,4,6-triphenyl-1,3,5-triazine, 3,5-diphenyl-1,2,4-
oxadiazole and gaseous products (N2, N2O, NH3, H2O).
Srivastava et al. [6] later reported that compounds of the
type 9 and 4 were isolated in moderate yields from thermal
reactions of arylamidoximes 2 in acetic acid solutions.
Interestingly, diaryloxadiazoles 9 were also formed upon
heating at controlled temperatures in the absence of acetic
acid showing that the presence of a carboxylic acid was not
critical for the above transformation. Concerning the latest
steps of the mechanism for the formation of bis-aryloxadi-
azoles three possibilities were suggested: i) reaction [7]
between arylnitrile and arylnitrile oxide originally formed
The reactions studied and the products obtained, as well
as the proposed mechanism (Schemes 1-4) are expected to
enlighten further the thermal transformation of ami-
doximes.
Results and Discussion.
A solution of p-tolylamidoxime 2a and an equimolar
amount of e t h o x y c a r b o n y l m e t h y l e n e ( t r i p h e n y l ) -

500
D. N. Nicolaides, K. E. Litinas, I. Vrasidas and K. C. Fylaktakidou
Vol. 41
phosphorane 1 in toluene was heated under reflux for two
days (Method A) and the complicated reaction mixture
was then subjected to column chromatography to give
N,N-bis(4-methylphenyl)urea 7a (5.5%), 4-methylbenz-
amide 8a (5%) and a product 5a (19%, 28% on the basis
of 2a consumed) constructed from two molecules of ami-
doxime 2a less an ammonia and a water molecule, as it is
1,2,4-oxadiazol-5-yl]benzamide 23b (60%) respectively
were obtained. The suggested structures of compounds 5a,
21, 23a and 23b were unequivocally established by X-ray
crystallography of compound 23b (see Figure 1 for
ORTEP structure) [18].
1
indicated by its analytical and spectral data (MS, H-
NMR, 13C-NMR). The recorded 1H-NMR spectrum of the
product showed two absorptions for its methyl substituents
at δ 2.33 and 2.40 ppm, while the 13C-NMR spectrum also
revealed a non-symmetric structure of the product in ques-
tion. The recorded mass spectrum showed a molecular ion
(M+) m/z = 265. On the basis of these data we suggested
for the product 5 a the structure of N, 3 - b i s ( 4 -
methylphenyl)-1,2,4-oxadiazol-5-amine (Scheme 1).
When a melted mixture of equimolar amounts of 1 and
2a was heated at 135-140°C for 4,5 h (Method B) and the
mixture separated as above compounds 5a (29%), 7a
(10%) and 8a (9%) were obtained again, along with 3,4-
bis(4-methyphenyl)-furoxan 3a[16] (2%) and 5-methyl-3-
(4-methylphenyl)-1,2,4-oxadiazole 4a [6] (2.5%). 3.5-
Bis(4-methylphenyl)-1,2,4-oxadiazole 9a [6], expected
from the thermal transformation of 2a, was separated in
traces (0.4%) from the reaction mixture.
Similar products 5b (15%, Method A), 5c (17%, Method
A), 5d (9%, Method B) and 5e (13%, Method B) were also
obtained when amidoximes 2b, 2c, 2d, 2e were treated
with ylide 1. In contrast to the above reactions, from the
treatment of amidoxime 2f with ylide 1 at ~150° for 4 h
and separation of the complicated reaction mixture by col-
umn chromatography only 4-nitrobenzonitrile 6f [6] (6%)
and compounds 4f (11%) were isolated. Efforts for separa-
tion or detection of oxadiazole 5f among the reaction prod-
ucts were unsuccessful.
Finally, treatment of N-phenyl-4-methoxybenzamidoxi-
me 10 with ylide 1 at ~180° for 3 h afforded the known
[17] 2-(4-methoxyphenyl)-1H-benzimidazole 11 (17%)
(Scheme 2).
Treatment of compound 5a with methyl iodide/potas-
sium carbonate afforded N- m e t h y l - N, 3 - b i s ( 4 -
methylphenyl)-1,2,4-oxadiazol-5-amine 21 (63%), while
by treating of 5a with acetic anhydride 22a or with 4-bro-
mobenzoic anhydride 22b N-(4-methylphenyl)-N-[3-(4-
methylphenyl)-1,2,4-oxadiazol-5-yl]acetamide 23a (78%)
and 4-bromo-N-(4-methylphenyl-N-[3-(4-methylphenyl)-
Figure 1. ORTEP presentation of benzamide 23b.
The observed aryl migration from the carbon to the
nitrogen atom during the formation of 5 from 2 can be
explained by supposing the intermediate formation of the
corresponding aryl-isocyanate 28 via the oxazirene inter-
mediate 27 as it is depicted in Scheme 4. An oxazirene
intermediate of type 27 has also been suggested [19] to
be formed during the known transformation of nitrile
oxides to isocyanates, in addition to or instead of their
dimerization [10] to the corresponding furoxans 3.
Although the direct formation of 27 from the inter-
mediate 24 seems also possible, the isolation of the
furoxan 3a is strong evidence for the prior formation of
the corresponding nitrile oxide 26. Further attack [20] of
amidoxime 2 to isocyanate 2 8, followed by elimination
of water from the intermediate 29 such formed can
account for the formation of the unexpected 1,2,4-oxadi-
azoles 5 obtained. The suggested mechanism in Scheme
4 also explains the formation of the other products (6 - 8)
obtained.
Deoxygenation of nitrile oxide 26 by triphenylphos-
phine produced can afford [19] nitrile 6 and further
amide 8 from the later, while hydrolysis of the iso-
cyanate 28 and further reaction of the amine such formed
with 28 can lead to the formation of the corresponding
urea 7. It is obvious that the formation of nitrile oxide 2 6
is the key intermediate in the reactions studied and that it
does not lead to the formation of 3,5-bisaryl-1,2,4-oxadi-
azoles 9. Compounds 5 were formed thermally only in
the presence of the ylide 1, since when amidoxime 2a
was heated at 130-140° for 5 h only the formation of
compound 9 a was observed. It must be noticed that com-
pound 5a was also formed, when amidoxime 2a was

Jul-Aug 2004
Thermal Transformation of Arylamidoximes in the Presence of Phosphorus Ylides
501
heated under reflux with ylide Ph3P=C(CH2Ph)CO2Et in
toluene, as well as when only 1/10 equivalent of the
ylide 1 was used for the reaction at ~180°, as it was indi-
cated by tlc examinations of the above reaction mixtures.
In an effort to exclude the participation of 1 in the reac-
tion as a simple base, amidoxime 2a was treated with n-
butyl lithium in dry ether under an argon atmosphere for
16 h at r.t. and with 4-dimethylaminopyridine under
reflux in toluene for 48 h. In both cases compound 5 was
not detected by tlc examination, while starting material
was found.
oxadiazoles 5 proceeds thermally but only in the pres-
ence of an ylide, likely acting initially as a base for the
abstraction of the hydrogen atom from the =NOH
group. The phosphonium salt 25 so formed can act fur-
ther as an acid for the abstraction of ammonia from the
intermediate anion 24 to give nitrile oxide 26. It is of
interest to note that treatment of benzamide oximes with
arylcyclohexylcarbodiimide also affords, as main prod-
ucts, 3-aryl-5-arylamino-1,2,4-oxadiazoles, but with
participation of the aryl-substituent of the diimide as the
arylamino-substituent in the oxadiazole product formed
[23,24].
The formation of compound 4a can be explained by the
assumption that ylide 1 is partly thermally hydrolysed [21]
to ethyl acetate, which reacts further [22] with the ami-
doxime 2a used.
EXPERIMENTAL
Melting points were determined on a Kofler hot-stage appara-
tus and are uncorrected. IR spectra were obtained with a Perkin-
Elmer 1310 spectrophotometer as Nujol mulls except otherwise
stated. Nmr spectra were recorded on a Bruker AM 300 (300
MHz, and 75 MHz for H and C respectively) using deuteri-
ochloroform as solvent and TMS as an internal standard.
Coupling constants (J) are reported in Hz. Mass spectra were
determined on a VG-250 spectrometer at 70 eV under Electron
The analytical and spectral data of all new compounds
are given in the experimental part and correspond well the
structures suggested for them, while the known com-
pounds gave data identical to those reported in the litera-
ture for them.
1
13
From the experimental data presented we can con-
clude that the transformation of amidoximes 2 to 1,2,4-

502
D. N. Nicolaides, K. E. Litinas, I. Vrasidas and K. C. Fylaktakidou
Vol. 41
13
2H), 8.10 (brs, 1H, exchanged by D O); C nmr: δ 21.3, 21.4,
Impact (EI) conditions, or on a Perkin Elmer API 100 Sciex
Simple quadrupole under Electrospray Ionization (ESI) condi-
tions. High resolution mass spectra (hrms) were recorded on an
Ionspec mass spectrometer under Matrix-Assisted Laser
Desorption-Ionization Fourier Transform Mass Spectrometer
(MALDI-FTMS) conditions with 2,5-dihydroxybenzoic acid
(DHB) as the matrix. Microanalyses were performed on a Perkin-
Elmer 2400-II Element analyzer. Silica gel N° 60, Merck A.G.
has been used for column chromatographies. Earlier reported
procedures were used for the preparation of amidoximes [1,25].
2
115.5, 119.1, 124.5, 124.9, 126.9, 127.9, 128.6, 129.2, 131.8,
+
136.8, 138.5, 139.4, 168.2, 168.4; ms (EI): m/z 265 (46, M ), 158
(11), 132 (98), 131 (100), 117 (18), 91 (44).
Anal. Calcd. for C H N O: C: 72.42, H: 5.70, N: 15.85.
16 15 3
Found C: 72.17, H: 5.68, N: 15.93.
N,3-Bis(2-methylphenyl)-1,2,4-oxadiazol-5-amine (5d).
This compound was obtained from the reaction between 1 and
2d by the Method B (heating at 135-140 °C, 4 h) (23 mg, 9%);
m.p. 130-132 °C (light yellow crystals from ethyl
Reactions of Ylide 1 with Arylamidoximes 2a-f, 10.
-1
1
acetate/hexane); ir: 3260, 3030, 1630, 1590 cm ; H nmr: δ 2.34
(s, 3H), 2.63 (s, 3H), 7.10 (d, 1H, J=7.4 Hz), 7.20-7.41 (m, 6H),
General Procedure.
13
7.93 (d, 1H, J=7.0 Hz), 7.97 (d, 1H, J=8.4 Hz); C nmr: δ 17.8,
22.0, 120.1, 124.7, 125.8, 127.2, 127.3, 129.9, 130.3, 130.4,
130.8, 131.2, 135.3, 138.1, 167.9, 169.2; ms (EI): m/z 265 (45,
+
M ), 159 (85), 131 (87), 118 (62), 106 (88), 105 (92), 91 (82), 90
(100).
A mixture of ylide 1 (0.696 g, 2 mmoles) and the appropriate
arylamidoxime 2a-f, 10 (2 mmoles) in toluene (10 ml) was
heated under reflux for 4.5 h - 3 days (Method A) or was melted
and heated at 135-180° for 1.5-4.5 h (Method B). The reaction
mixture was then separated by column chromatography (silica
gel, hexane/ethyl acetate 8:1 or dichloromethane/ethyl acetate
6:1 only in the case of the reaction of 10).
Anal. Calcd. for C H N O: C: 72.42, H: 5.70, N: 15.85.
16 15 3
Found C: 72.20, H: 5.95, N: 15.55.
3,4-Bis(4-methylphenyl)-1,2,5-oxadiazole-N-oxide (3a).
N,3-Bis(4-methoxyphenyl)-1,2,4-oxadiazole-5-amine (5e).
This compound was obtained as white crystals from the reac-
tion between 1 and 2a after heating the melted mixture at 135-
145° for 4 h (Method B), (6 mg, 2 %) m.p. 112-114°C (lit [16]
m.p. 113-114°C).
This compound was obtained from the reaction between 1 and
2e according to Method B (heating at 170 °C, 4 h) (38 mg, 13 %)
m.p. 198-200 °C (yellow crystals from ethyl acetate/hexane); ir:
3280, 3040, 1640, 1590 cm ; H nmr: δ 3.82 (s, 3H), 3.86 (s,
3H), 6.93 (d, 2H, J=9.0 Hz), 6.96 (d, 2H, J=8.9 Hz), 7.26 (brs,
13
1H), 7.44 (d, 2H, J=9.0 Hz), 7.98 (d, 2H, J=8.9 Hz); C nmr: δ
55.4, 55.6, 114.1, 114.7, 120.6, 121.5, 128.9, 130.2, 160.0, 161.8,
+
168.4, 171.4,; ms (EI): m/z 297 (M , 57%), 148 (100), 133 (87),
-1
1
5-Methyl-3-(4-methylphenyl)-1,2,4-oxadiazole (4a).
This compound was obtained as white crystals by method B
from the reaction between 1 and 2a, eluted after 3a (6 mg, 2.5
%), m.p. 73-74° C (lit [6] m.p. 74°C).
122 (50), 106 (62).
Anal. Calcd. for C
Found C: 64.35, H: 5.39, N: 13.95.
H N O : C; 64.62, H; 5.09, N: 14.14.
16 15 3 3
N,3-Bis(4-methylphenyl)-1,2,4-oxadiazol-5-amine (5a).
This compound was obtained from the reaction between 1 and
2a, eluted after compounds 3a, 4a ( 50 mg, 28 % by the method
A, or 75 mg, 29 % by the method B), m.p. 189-191° C (light yel-
low crystals from chloroform/hexane); ir: 3283, 3025, 1631,
2-(4-Methoxyphenyl)-1H-benzimidazole (11).
This compound was obtained as white crystals from the reac-
tion between 1 and 10 according to Method B (heating at 180 °C,
3h) (78 mg, 17 %), m.p. 226-227 °C (lit [17] m.p. 227 °C).
-1
1
1578, 1513 cm ; H-nmr: δ 2.34 (s, 3H), 2.41 (s, 3H), 7.17 (d,
2H, J=8.0 Hz), 7.26 (d, 2H, J=8.3 Hz), 7.39 (d, 2H, J=8.3 Hz),
13
7.94 (d, 2H, J=8.0 Hz), 7.82 (brs, 1H, exchanged by D O); C-
5-Methyl-3-(4-nitrophenyl)-1,2,4-oxadiazole (4f).
2
nmr: δ 20.8, 21.6, 118.6, 124.4, 127.3, 129.4, 129.9, 133.7, 134.5,
This compound was obtained as light yellow crystals from the
reaction between 1 and 2f according to method B (heating at 150
°C, 4 h) (44 mg,11 %), m.p. 136-138 °C (lit [6] m.p. 138 °C).
+
141.3, 168.2, 168.4; ms (EI): m/z 265 (55, M ), 159 (12), 132
(100), 118 (8), 106 (16), 91 (55); For C H N O hrms Calcd.
16 16 3
266.1288; Found 266.1290.
N-Methyl-N, 3-bis(4-methylphenyl)-1, 2, 4-oxadiazol-5-amine
(21).
3,5-Bis(4-methylphenyl)-1,2,4-oxadiazole (9a).
This compound was obtained as light yellow crystals by
Method B from the reaction between 1 and 2a eluted after 3a and
before 4a (1 mg, 0.4 %) ; m.p. 133-134 °C (lit [6] mp. 134 °C).
A mixture of compound 5a (41 mg, 0.15 mmole), methyl
iodide (8 drops, excess) and potassium carbonate (54 mg, 0.39
mmole) in DMF (4 ml) was stirred at room temperature for 24 h
and then extracted first with water and then with ether. The
organic layers dried over sodium sulphate and concentrated to
give product 21 (27 mg, 63 %), m.p. 87-88 °C (yellow crystals
N,3-Diphenyl-1,2,4-ozadiazol-5-amine (5b).
This compound was obtained as white crystals from the reac-
tion between 1 and 2b according to method A (benzene, reflux 3
days) ( 36 mg, 15 %); m.p. 165-167 °C (lit [26] m.p. 168 °C).
1
from ethyl acetate/hexane); ir: 3020, 1580, 1500; H nmr: δ 2.38
(s, 3H), 2.39 (s, 3H), 3.61 (s, 3H), 7.23 (d, 2H, J=8.3 Hz), 7.24 (d,
2H, J=8.1 Hz), 7.33 (d, 2H, J=8.3 Hz), 7.90 (d, 2H, J=8.1 Hz);
13
C nmr: δ 21.0, 21.5, 39.5, 123.8, 124.9, 127.2, 127.3, 129.9,
136.3, 139.8, 140.9, 168.6, 171.1; ms (EI): m/z 279 (M , 92%),
159 (16), 148 (100), 120 (53), 91 (34).
N,3-Bis(3-methylphenyl)-1,2,4-oxadiazol-5-amine (5c).
This compound was obtained from the reaction between 1 and
2c according to Method A (toluene, reflux 3 days) (44 mg, 17%),
m.p. 115-116 °C (yellow crystals from ethyl acetate/hexane); ir:
+
-1
1
3280, 3040, 1640, 1595 cm ; H nmr: δ 2.33 (s, 3H), 2.38 (s,
3H), 6.95 (d, 1H, J=7.2 Hz), 7.22-7.42 (m, 5H), 7.83-7.92 (m,
Anal. Calcd. For C H N O: C: 73.08, H: 6.1 4, N: 15.05.
17 17 3
Found C: 72.85, H: 6.20, N: 14.79.

Jul-Aug 2004
Thermal Transformation of Arylamidoximes in the Presence of Phosphorus Ylides
503
Springer-Verlag, N. York 1971, p.130.
[8] A. Tomazic, M. Tisler and B. Stanovnik, Tetrahedron, 37,
1787 (1981).
[9] R. M. Srivastava, M. N. Ramos and L. M. M. Silva, Gazz.
Chim. Ital., 113, 845 (1983).
[10] J . - P. Gilbert, R. Jacqier, C. Petrus and F. Petrus,
Tetrahedron Lett., 755 (1974).
[11] D. N. Nicolaides and E. A. Varella, In The chemistry of
Amidoximes; Patai, S., Ed. Supplement B. The chemistry of acid
derivatives, Vol. 2, Part 2. John Wiley & Sons: Chichester, England,
1992; pp. 875-966.
[12] D. N. Nicolaides, K. C. Fylaktakidou, K. E. Litinas and D.
Hadjipavlou-Litina, Eur. J. Med. Chem., 33, 715 (1998).
[13] D. N. Nicolaides, K. E. Litinas, D. A. Lefkaditis, S. G.
Adamopoulos, C. P. Raptopoulou and A. Terzis, J. Chem. Soc., Perkin
Trans. 1, 2107 (1994).
N-(4-Methylphenyl)-N-[3-(4-methylphenyl)-1,2,4-oxadiazol-5-
yl]acetamide (23a).
A mixture of compound 5a (100 mg, 0.377 mmole), acetic
anhydride (0.1 ml, 0.108 g, 1.06 mmoles) and pyridine (3 drops) in
dry benzene (3 ml) was heated under reflux for 22 h and the solu-
tion was concentrated in a rotary evaporator to give compound 23a
(90 mg, 78 %), m.p. 113-115 °C (yellow crystals from chloro-
-1
1
form); ir: 3020, 1700, 1580, 1560 cm ; H nmr: δ 2.41 (s, 3H),
2.43 (s, 3H), 2.57 (s, 3H), 7.19 (d, 2H, J=8.2 Hz), 7.26 (d, 2H,
13
J=8.0 Hz), 7.31 (d, 2H, J=8.0 Hz), 7.91 (d, 2H, J=8.2 Hz);
nmr: δ 21.3, 21.6, 25.0, 123.8, 127.3, 128.0, 129.5, 130.5, 135.1,
C
+
139.5, 141.7, 167.5, 168.9, 170.1; ms (EI): m/z 307 (M , 63%),
265 (69), 222 (53), 159 (58), 133 (100), 131 (88), 117 (83), 91 (86).
Anal. Calcd. For C
Found C: 70.25, H: 5.40, N: 13.6
H N O : C: 70.33, H: 5.58, N: 13.68.
18 17 3 2
[14] D. R. Gautam, K. E. Litinas, K. C. Fylaktakidou and D. N.
Nicolaides, J. Heterocyclic Chem., 40, 399 (2003).
[15] G. Papageorgiou, D. N. Nicolaides and J. Stephanidou-
Stephanatou, J. Liebigs Ann. Chem., 397 (1989).
[16] R. H. Wiley and B. J. Wakefield, J. Org. Chem., 25, 546
(1960).
[17] T. Bacchetti and A. Alemagna, Atti Accad. Nazl. Lincei,
Rend., Classe Sci. Fis., Mat. E Nat., 28, 824 (1960); Chem. Abstr. 56,
7304c (1960).
[18] Crystallographic data (excluding structure factors) for com-
pound 23b have been deposited with the Cambridge Crystallographic
Data as Supplementary publication number CCDC 207973. Copies of
the data can be obtained 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].
[19] Ref. [7], p.65.
[20] R. Buyle, F. Eloy and R. Lenaers, Helvetica Chimica Acta,
46, 1073 (1963).
4-Bromo-N-(4-methylphenyl)-N-[3-(4-methylphenyl)-1,2,4-oxa-
diazol-5-yl]benzamide (23b).
A mixture of compound 5a (2 mg, 0.0075 mmole), triethyl-
amine (0.75 mg, 0.0075 mmole), 4-dimethylaminopyridine (0.05
mg, 0.0004 mmole) and 4-bromobenzoylchloride (1.6 mg,
0.0075 mmole) in dichloromethane (2 ml) was heated under
reflux for 20 h. The mixture was concentrated in a rotary evapo-
rator and the residue was separated by column chromatography
(silica gel, hexane/ethyl acetate to give compound 23b (2 mg
60%), m.p. 151-152 °C (yellow crystals from ethyl
-1
1
acetate/hexane); ir: 3025, 1702, 1584, 1561, 1508 cm ; H nmr:
δ 2.36 (s, 6H), 7.15-7.23 (m, 6H), 7.46 (s, 4H), 7.79 (d, 2H, J=7.9
13
Hz); C nmr: δ 21.2, 21.6, 123.6, 127.0, 127.2, 127.3, 129.4,
130.4, 130.5, 131.7, 132.6, 136.1, 139.0, 141.8, 168.6, 168.9,
+
169.8; ms (ESI): m/z 450/448 (M+H) , 472/470 (M+Na) .
+
REFERENCES AND NOTES
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th
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