Zirconium(IV) chloride-catalyzed synthesis of 1,5-benzodiazepine derivatives

Article abstract of DOI:10.1139/V07-019

Zirconium tetrachloride efficiently catalyzes the cyclocondensation reaction of o-phenylenediamine and a ketone in refluxing 1,2-dichloroethane to afford the corresponding 2,3-dihydro-1H-1,5-benzodiazepine in high yield. The formation of specific regioiso

Full text of DOI:10.1139/V07-019

1
84
Zirconium(IV) chloride-catalyzed synthesis of
,5-benzodiazepine derivatives
1
K. Srinivasa Reddy, Ch. Venkateshwar Reddy, M. Mahesh, K. Rosi Reddy,
P.V.K. Raju, and V.V. Narayana Reddy
Abstract: Zirconium tetrachloride efficiently catalyzes the cyclocondensation reaction of o-phenylenediamine and a
ketone in refluxing 1,2-dichloroethane to afford the corresponding 2,3-dihydro-1H-1,5-benzodiazepine in high yield.
The formation of specific regioisomers and their structural elucidation are reported for the first time.
1
Key words: zirconium tetrachloride, o-phenylenediamines, ketones, 1,5-benzodiazepines, H NMR, regioisomers.
Résumé : Le tétrachlorure de zirconium catalyse d’une façon efficace la réaction de cyclocondensation de l’o-phénylè-
nediamine avec les cétones, dans du 1,2-dichloroéthane au reflux, pour conduire à la formation des 2,3-dihydro-1H-1,5-
benzodiazépines, avec un rendement élevé. On rapporte pour la première fois la formation de régioisomères spécifiques
ainsi que l’élucidation de leurs structures.
1
Mots-clés : tétrachlorure de zirconium, o-phénylènediamines, cétones, 1,5-benzodiazépines, RMN du H, régioisomères.
[
Traduit par la Rédaction] Reddy et al. 188
Introduction
occurrence of several side reactions. Therefore, there is still
a need to develop new catalytic methods for the synthesis of
these biologically active compounds.
Recently, zirconium(IV) chloride has received consider-
able attention as an efficient Lewis acid catalyst for various
organic transformations such as Biginelli’s reaction (17),
electrophilic amination of activated arenes (18),
thioacetylation of acetals, (19) deoxygenation of
heterocyclic-N-oxides (20), reduction of nitro-compounds
Benzodiazepines are important pharmaceutical com-
pounds that are frequently used as prescribed drugs for com-
bating central nervous system (CNS)-related diseases mainly
because of their anticonvulsant, hypnotic, and other proper-
ties (1, 2). They are also used as intermediates in the manu-
facture of commercially important cationic dyes for acrylic
fibers (3) and also used as starting materials for the prepara-
tion of fused ring compounds such as triazolo- (4),
oxadiazolo- (5), oxazino- (6), or furano-benzodiazepines (7).
More interestingly, the related diazapines showed unusual
and novel pharmacological activity as HIV-1 reverse tran-
scriptase inhibitors, as evidenced by niverperin (8). Because
of their versatile biological activity and commercial utility,
the synthesis of these compounds has acquired greater atten-
tion in synthetic organic chemistry and led to the develop-
ment of new synthetic strategies. Generally, these
compounds are prepared by cyclocondensation of o-
phenylenediamine with α,β-unsaturated carbonyl compounds
(
21), and conversion of carbonyl compounds to 1,3-
oxathiolanes (22). To continue our interest in zirconium(IV)
chloride as a catalyst for novel synthetic methodologies (17,
2
3), we envisaged its use and effect in the cyclocondensation
of ketones with o-phenylenediamine.
Results and discussion
In this paper, we wish to present the results of a facile
one-pot method for the synthesis of 1,5-benzodiazepines
(
9), β-haloketones (10), or ketones, involving combinations
of Lewis acids and transition-metal salts like BF -etherate
3
catalysed by ZrCl ,and also to report, for the first time, the
4
(
11), NaBH4 (12), polyphosphoric acid or (SiO ) (13),
2
unequivocal structure elucidation of the regioisomers formed
in this reaction. Typically, 5 mmols of o-phenylenediamine
were reacted with 10 mmols of ketone in 1,2-dichloroethane
under refluxing condition for 30–180 min (Scheme 1). The
MgO/POCl (14), Yb(OTf) (15), and Al O /P O under mi-
3
3
2
3
2
5
crowave conditions (16). However, many of these methods
suffer from drawbacks, such as drastic reaction conditions,
cumbersome workup procedures, low yields, and co-
cyclocondensation is catalyzed by 10 mol% of ZrCl to af-
4
ford the corresponding 1,5-benzodiazepine derivatives in
good to excellent yields. The effects of different solvents
such as acetonitrile, tetrahydrofuran, ethanol, dichloro-
methane, and 1,2-dichloroethane on the yield of 1,5-
benzodiazepine in presence of 10 mol% of catalyst was stud-
ied, and it was observed that 1,2-dichloroethane was the sol-
vent of choice in terms of yield and reaction time. Regarding
the optimum quantity of the catalyst, it was found that 10
Received 20 September 2006. Accepted 22 February 2007.
Published on the NRC Research Press Web site at
http://canjchem.nrc.ca on 21 March 2007.
K.S. Reddy, Ch. V. Reddy, M. Mahesh, K.R. Reddy,
1
P.V.K. Raju, and V.V.N. Reddy. Organic Chemistry
Division II, Indian Institute of Chemical Technology,
Hyderabad 500007, India.
mol% of ZrCl is necessary to promote the reaction. How-
4
1Corresponding author (e-mail: vvnreddy1@rediffmail.com).
ever, no reaction was observed in the absence of the catalyst,
Can. J. Chem. 85: 184–188 (2007)
doi:10.1139/V07-019
© 2007 NRC Canada

Reddy et al.
185
Table 1. Zirconium(IV) chloride-catalyzed formation of 1,5-benzodiazepines.
Producta
R
R1
R2
Yield (%)b
Time (min)
Mp (°C)b
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
a
H
H
H
H
H
H
H
H
CH3
CH3
CH3
CH3
NO2
NO2
NO2
OCH3
OCH3
H
H
H
H
H
H
H
H
CH3
CH3
CH3
CH3
H
H
H
H
H
C H
4-(Me)–C H
4-(Cl)–C H
4-(Br)–C H
4-(OMe)–C H
4-(NO )–C H
4
CH3
90
96
94
97
89
92
96
88
92
97
92
95
70
72
80
85
85
30
60
55
50
65
60
30
90
45
60
45
60
180
180
180
150
150
107–108
88–89
6
5
6
4
133–135
145–146
120–121
132–133
119–120
117–118
118–120
127–128
146–147
120–122
144–145
173–174
160–162
120–121
115–118
6
4
6
4
6
4
2
6
CH –CH–(CH )
2
3 2
C H
6
5
4-(NO )–C H
4
4-(Cl)–C H
4-(Me)–C H
2
6
6
4
6
4
C H
6
5
4-(Cl)–C H
6 4
4-(Br)–C H
6
4
C H
6
5
4-(Cl)–C H
6
4
1
All products were characterized by H NMR, IR, and mass spectroscopy.
Isolated and unoptimized yields, and melting points are uncorrected.
b
Scheme 1.
benzodiazapine derivatives. Although the formation of 7- or
8
4
4
-substituted 1,5-benzodiazepines from the corresponding
-substituted o-phenylenediamines (e.g., 4-chloro, 4-nitro,
-methyl, and 4-benzoyl) and acetone, acetophenone, or
cyclohexanone was previously reported, the structure confir-
mation of the products has not been reported, and there ex-
ists ambiguity of the regioisomers (24). Similar to this is the
case of the formation of a mixture of regioisomers in the ra-
tio of 60:40, corresponding to 7- and 8- substituted 1,5-
benzodiazapine derivatives obtained from 4-chloro and
even after prolonged reaction time (>18 h), under the present
reaction conditions.
Optimizing the reaction conditions, we extended this pro-
cedure to a number of 1,5-bezodiazepine derivatives, using
4
-benzoyl o-phenolenediamines and acetone (25). At this
point, it is noteworthy to mention that the ZrCl -catalyzed
ZrCl (10 mol%) in 1,2-dichloroethane at reflux tempera-
4
4
reaction between 4-nitro, 4-methoxy o-phenylenediamine
and the acetophenone derivative led to the formation of spe-
cific regioisomer in good yields (3m–q). The formation of
regioisomers is in accordance with the mechanism
ture. The results summarized in Table 1 indicate the scope
and generality of the reaction with respect to the examples
described therein. As shown in Table 1, the reaction of ace-
tone with o-phenylenediamine in the presence of ZrCl af-
4
(
Scheme 2) proposed by Jung and co-workers (13), in which
forded 2,2,4-trimethyl-2,3-dihydro-1H-benzodiazepine (Table 1,
entry 7) in 96% yield. Acetophenone reacted with o-
phenylenediamine under similar reaction conditions to yield
the intramolecular imine–enamine cyclization is favored by
the presence of an electron-withdrawing group (NO ) para to
imine moiety, and para to enamine moiety in case of an elec-
tron-donating group (OCH ). The intermediate formed in this
reaction does not require the activation of amino-
functionality, and as a result, the electronic nature of the
substituent may not have significant effect on the yield; how-
ever, it affected the nature of cyclization, leading to the for-
mation of specific regioisomers. We report detailed structural
2
2
-methyl-2,4-diphenyl-2,3-dihydro-1,5-bezodiazepine (Table 1,
entry 1) in 90% yield. The cyclocondensation of o-phenyl-
enediamine with para-substituted acetophenones and aliphatic
ketones afforded the corresponding 1,5-benzodiazepine de-
rivatives in good yields (88%–97%), indicating that the sub-
stitution on a ketone substrate has little effect on the yield
3
(
Table 1, 3a–i). The cyclocondensation reaction may be pro-
1
elucidation of the regioisomers (3m–q), using H NMR spec-
ceeding via an intramolecular imine–enamine cyclization,
troscopic data and the chemical shifts of the diagnostic pro-
tons along with the coupling constants, in Table 2 for the first
time.
promoted by ZrCl , as depicted in Scheme 2 (13). It may be
4
presumed that the carbonyl group of the ketone coordinates
with ZrCl , forming a complex C (17) for which we have no
4
evidence, yet its formulation seems reasonable. The amine
of o-phenylenediamine attacks the carbonyl group of the
ketone, giving the intermediate dimine A. Then, 1,3-shift of
the hydrogen attached to the methyl group occurs to form an
isomeric enamine B, which on cyclization affords the seven-
membered ring.
In 2-methyl-7-nitro-2,4-di(4′-chlorophenyl)-2,3-dihydro-1H-
1,5-benz-odiazepine (3n, Fig. 1a), the meta coupled doublet
at δ 8.18 ppm (J = 2.4 Hz) is assigned to a proton (6-H)
ortho to the imine nitrogen of benzodiazepines, which is in
the downfield region compared with the ortho coupled dou-
blet at δ 6.76 ppm (9-H) (J = 9.0 Hz), which is ortho to the
amine nitrogen. The ortho and meta coupled doublet of dou-
blet at δ 7.89 ppm (J = 9.0 and 2.4 Hz) is assigned to 8-H.
4
-Substituted o-phenelenediamines were reacted with ace-
tone, as well as acetophenone, to give the corresponding 1,5-
©
2007 NRC Canada

1
86
Can. J. Chem. Vol. 85, 2007
Scheme 2.
1
Table 2. Selected H NMR chemical shifts of compounds 3m, 3n, 3o, 3p, 3q, and 3r.
Compound
C6-H
C8-H/7-H
C9-H
3
3
3
3
3
3
m
δ 8.24
d (J = 2.5 Hz)
δ 8.18
d (J = 2.4 Hz)
δ 8.26
δ 7.92
dd (J = 8.5 and 2.5 Hz)
δ 7.89
dd (J = 9.0 and 2.4 Hz)
δ 7.95
dd (J = 8.9 and 2.4 Hz)
δ 6.52
dd (J = 8.7 and 2.2 Hz)
δ 6.54
dd (J = 8.7 and 2.3 Hz)
δ 6.68
dd (J = 9.0 and 3.0 Hz)
δ 6.76
d (J = 8.4 Hz)
δ 6.76
d (J = 9.0 Hz)
δ 6.81
d (J = 8.9 Hz)
δ 6.28
d (J = 2.2 Hz)
δ 6.26
d (J = 2.3 Hz)
δ 6.76
d (J = 3.0 Hz)
n
o
d (J = 2.4 Hz)
δ 7.19 da
p
Merged
q
δ 7.20 da
Merged
r (N-methyl)
δ 6.90
d (J = 9.0 Hz)
a
Merged with the ab quartet of other aromatic protons.
The same analogy is applicable to 2-methyl-7-nitro-2,4-di-
phenyl-2,3-dihydro-1H-1,5-benzodiazepine (3m) and 2-methyl-
doublet at δ 6.90 ppm (J = 9.0 Hz), and doublet of doublet at
δ 6.68 ppm (J = 9.0 and 3.0 Hz) that are assigned to 9-H, 6-
H, and 7-H, respectively.
7
-nitro-2,4-di(4′-bromophenyl)-2,3-dihydro-1H-1,5-benzodiazepine
(
3o). However, in case of methoxy substituted benzo-
To the best of our knowledge, 2-methyl-8-methoxy-2,4-
di(4′-chloropheny-l)-2,3-dihydro-1H-1,5-benzodiazepine
(3q) is reported for the first time, and the structures of the
regioisomers have been unequivocally assigned, based on
IR, NMR, and mass spectral data, and finally confirmed by
diazepines, namely 2-methyl-8-methoxy-2,4-diphenyl-2,3-
dihydro-1H-1,5-benzodiazepine (3p) and 2-methyl-8-
methoxy-2,4-di(4′-chlorophenyl)-2,3-dihydro-1H-1,5-benzo-
diazepine (3q) (Table 1), the meta coupled doublet (9-H) at
δ 6.28 ppm (J = 2.2 Hz) is in the most shielded region, and
the ortho-coupled proton (6-H) signal is merged with the
other aromatic-proton signals in the region of δ 7.1–7.2 ppm.
The doublet of doublet at δ 6.52 ppm (7-H) with a coupling
constant of 8.7 and 2.2 Hz, respectively, is assigned to ortho
and meta couplings of 6-H and 9-H protons. This clearly in-
dicates that the shielded proton in this derivative is ortho to
the amine nitrogen. Therefore, structure 3p has been as-
signed for the methoxy-substituted benzodiazapine deriva-
tive (Fig. 1b). This is further confirmed by converting
compound 3p into its N-methyl derivative (1,2-dimethyl-2,4-
diphenyl-2,3-dihydro-1H-1,5-benzodiazepin-8-yl methyl ether
2
X-ray crystallographic analysis.
In conclusion, the results reveal that ZrCl has proved to
4
be an effective catalyst for the synthesis of 2,3-dihydro-1H-
1,5-benzodiazepines. This protocol offers several advan-
tages, such as selectivity, high yields, shorter reaction times,
and simple workup procedure. Moreover, the interesting out-
come of the present study is the isolation of the specific
regioisomers 3m–q in high yields, and their structures have
been unequivocally characterized by spectroscopic data for
the first time.
Experimental
(
3r)) using dimethyl sulfate under phase transfer catalysis
1
(PTC) conditions (26). The H NMR spectrum of the methyl-
NMR, spectra were recorded on Gemini 200, 300, and
ated compound 3r (Fig. 1b) revealed the presence of a meta-
coupled doublet at δ 6.76 ppm (J = 3.0 Hz), ortho-coupled
400 MHz and Avance 600 spectrometers in CDCl , using
TMS as internal standard. IR spectra were recorded on a
3
2
G.Y.S.K. Swamy, K. Ravikumar, V.V.N. Reddy, and K.S. Reddy. Manuscript in preparation.
©
2007 NRC Canada

Reddy et al.
187
Fig. 1.
3H, CH ), 2.92–3.35 (dd, 2H, CH , J = 13.6 Hz), 4.43 (br s,
3
2
1
7
8
H, NH, D O exchangeable), 6.76 (d, 1H, ArH, J = 9.0 Hz),
.13 (d, 2H, ArH, J = 9.2 Hz), 7.18 (d, 2H, ArH, J =
.0 Hz), 7.21 (d, 2H, ArH, J = 8.5 Hz), 7.41 (d, 2H, ArH, J
2
=
8.2 Hz), 7.89 (dd, 1H, ArH, J = 9.0 and 2.4 Hz), 8.18(d,
1
3
1
4
1
H, ArH, J = 2.4Hz). C NMR (75 MHz, CDCl ) δ: 31.1,
3
3.9, 69.1, 119.2, 122.5, 126.7, 127.4, 128.3, 128.4, 128.6,
33.5, 134.7, 136.7, 137.6, 140.7, 143.9, 144.3, 165.7. MS–
+
FAB m/z (%): 426 ([M + H] , 15), 391 (25), 274 (15), 154
(
55), 136 (45), 107 (28), 91 (60), 77 (50), 57 (100). HR-
+
ESIMS: ([MNa] ) m/z calcd. for C H N O Cl : 448.0595;
2
2
18
3
2
2
found: 448.0595.
2
1
-Methyl-7-nitro-2,4-di(4′-bromophenyl)-2,3-dihydro-1H-
,5-benzodiazepine (3o)
–
1
Yellow solid; mp 160–162 °C. IR (KBr) (cm ): 3500,
1
1
3
1
7
2
500, 1550, 1310. H NMR (400 MHz, CDCl ) δ: 1.74 (s,
3
H, CH ), 2.94–3.35 (dd, 2H, CH , J = 13.6 Hz), 4.50 (br s,
3
2
H, NH, D O exchangeable), 6.81 (d, 1H, ArH, J = 8.9 Hz),
2
.2–7.43 (m, 8H, ArH), 7.95 (dd, 1H, ArH, J = 8.9 and
Nicolet 740 FTIR spectrometer, and Mass spectra were re-
corded on a VG Micro Mass 7070H. The melting points
were determined in open glass capillaries on a Metler FP 51
melting point apparatus and are uncorrected. All reactions
were carried out using reagent-grade solvents, and the re-
agents were purchased from local suppliers except ZrCl4,
which was purchased from Merck Limited, Mumbai.
1
3
.4 Hz), 8.26 (d, 1H, ArH, J = 2.4 Hz). C NMR
(
150 MHz, CDCl ) δ: 31.0, 43.9, 69.2, 119.3, 121.7, 122.6,
3
1
1
2
25.2, 127.1, 127.3, 128.5, 131.5, 131.6, 134.8, 138.1,
40.8, 143.8, 144.8, 165.8. MS–FAB m/z (%): 516 ([MH +
+
] , 80), 391 (45), 319 (40), 154 (100), 136 (85). HR-
+
ESIMS: ([MH] ) m/z calcd. for C H N O Br : 513.9765;
22 18
3
2
2
found: 513.9780.
General procedure for the ZrCl -catalyzed synthesis of
4
2
-Methyl-8-methoxy-2,4-diphenyl-2,3-dihydro-1H-1,5-
1
,5-benzodiazepines (3)
A mixture, containing o-phenylenediamine (5 mmol),
benzodiazepine (3p)
–
1
Yellow solid; mp 120–121 °C. IR (KBr) (cm ): 3350,
ketone (10 mmol), and ZrCl4 (10 mol%) in 1,2-
dichloroethane (15 mL), was refluxed for an appropriate pe-
riod of time, as mentioned in Table 1. After completion of
the reaction, as indicated by TLC, the reaction mixture was
cooled to room temperature and washed with water (3 ×
1
2
950, 1620, 1490, 1250. H NMR (300 MHz, CDCl ) δ: 1.74
3
(
(
(
s, 3H, CH ), 2.94–3.15 (dd, 2H, CH , J = 12.87 Hz), 3.54
br s, 1H, NH, D O exchangeable), 3.80 (s, 3H, OCH ), 6.28
d, 1H, ArH, J = 2.2 Hz), 6.52 (dd, 1H, ArH, J = 8.7 and
3
2
2
3
2
.2 Hz), 7.10–7.25 (m, 5H, ArH), 7.51 (m, 6H, ArH). 13
C
1
0 mL). The organic layer was dried over anhyd. Na SO ,
2 4
NMR (75 MHz, CDCl ) δ: 30.1, 43.6, 55.4, 71.5, 105.6,
3
and the solvent was removed under reduced pressure. The
crude mixture was purified by column chromatography on
silica gel (100–200 mesh), using hexane – ethyl acetate
1
1
3
06.7, 125.4, 126.9, 127.0, 128.0, 128.3, 129.3, 130.8,
33.0, 139.5, 140.1, 147.7, 158.2, 165.1. MS–FAB m/z (%):
+
43 ([M + H] , 100), 224 (85), 209 (20), 154 (40), 136 (50).
(
98:2) as an eluent to afford the pure products.
Spectroscopic data
-Methyl-7-nitro-2,4-diphenyl-2,3-dihydro-1H-1,5-
+
HR-ESIMS: ([MH] ) m/z calcd. for C H N O: 343.1810;
found: 343.1818.
2
3
23
2
2
1
-Methyl-8-methoxy-2,4-di(4′-chlorophenyl)-2,3-dihydro-
H-1,5-benzodiazepine (3q)
2
benzodiazepine (3m)
–
1
–
1
Yellow solid; mp 115–118 °C. IR (KBr) (cm ): 3350,
Yellow solid; mp 144–145 °C. IR (KBr) (cm ): 3300,
1
1
3000, 1610, 1490, 1230. H NMR (300 MHz, CDCl ) δ: 1.72
1
3
651. H NMR (200 MHz, CDCl ) δ: 1.80 (s, 3H, CH ),
3
3
3
(
s, 3H, CH ), 2.85–3.11 (dd, 2H, CH , J = 13.7 Hz),
.05–3.37 (dd, 2H, CH , J = 12.6 Hz), 4.40 (br s, 1H, NH,
3 2
.51(br s, 1H, NH, D O exchangeable), 3.80 (s, 3H, OCH ),
2 3
.26 (d, 1H, ArH, J = 2.3 Hz), 6.54 (dd, 1H, ArH, J = 8.7
2
3
6
D O exchangeable), 6.76 (d, 1H, ArH, J = 8.4 Hz), 7.1–7.4
2
(
m, 8H, ArH), 7.6 (d, 2H, ArH, J = 6.74 Hz), 7.92 (dd, 1H,
and 2.3 Hz), 7.20 (m, 5H, ArH), 7.51 (d, 4H, ArH, J =
ArH, J = 8.5 and 2.5 Hz), 8.24 (d, 1H, ArH, J = 2.50 Hz).
1
3
1
3
9.3 Hz). C NMR (75 MHz, CDCl ) δ: 30.2, 43.3, 55.4,
C NMR (75 MHz, CDCl ) δ: 30.7, 44.2, 69.0, 119.0,
3
3
7
1
1.1, 105.6, 107.0, 127.0, 128.0, 128.2, 128.3, 130.9, 132.7,
1
1
(
(
22.3, 125.0, 127.0, 127.4, 128.1, 128.4, 130.0, 134.6,
33.0, 135.6, 138.3, 139.0, 145.8, 158.4, 163.3. MS–FAB
39.4, 140.1, 144.4, 146.0, 167.2. MS–FAB m/z (%): 357
+
+
m/z (%): 411 ([M + H] , 100), 258 (85), 154 (45), 57 (30).
[M] , 30), 345 (10), 282 (20), 241 (100), 194 (10), 130
+
+
HR-ESIMS: ([MH] ) m/z calcd. for C H N OCl :
4
25), 119 (30), 78 (10), 57 (50). HR-ESIMS: ([MH] ) m/z
23 21
2
2
11.1030; found: 411.1029.
calcd. for C H N O : 358.1555; found: 358.1566.
2
2
20
3
2
2
1
-Methyl-7-nitro-2,4-di(4′-chlorophenyl)-2,3-dihydro-1H-
Acknowledgements
,5-benzodiazepine (3n)
–
1
Yellow solid; mp 173–174 °C. IR (KBr) (cm ): 3500,
K.S.R. and K.R.R thank the Director of the Indian Insti-
tute of Chemical Technology (IICT) for financial support.
1
1
600, 1550, 1300. H NMR (200 MHz, CDCl ) δ: 1.73 (s,
3
©
2007 NRC Canada

1
88
Can. J. Chem. Vol. 85, 2007
Ch.V.R. and M.M. thank the Council of Scientific and In-
dustrial Research (CSIR), New Delhi, for the awards of fel-
lowship.
13. D.I. Jung, T.W. Choi, Y.Y. Kim, I.S. Kim, Y.M. Park, Y.G.
Lee, and D.H. Jung. Synth. Commun. 29, 1941 (1999).
1
1
1
4. M.S. Balkrishna and B.A. Kaboudin. Tetrahedron Lett. 42,
127 (2001).
5. M. Curini, F. Epifano, M.C. Marcotullio, and O. Rosati. Tetra-
hedron Lett. 42, 3193 (2001).
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