Comparison of Conventional and Microwave Assisted Reactions Giving Aromatic Oxazolidines

Article abstract of DOI:10.1007/s00706-003-0020-6

By reaction of aromatic aldehydes with (-)-ephedrine aromatic 1,3-oxazolidines can be obtained. The reaction was carried out either at conventional conditions or by microwave heating. The different diastereomeric ratios were determined by means of ^1<

Full text of DOI:10.1007/s00706-003-0020-6

Monatshefte f€ur Chemie 134, 1105–1111 (2003)
DOI 10.1007/s00706-003-0020-6
Comparison of Conventional and Microwave
Assisted Reactions Giving Aromatic
Oxazolidines
ꢀ
Florian Diwischek, Eberhard Heller, and Ulrike Holzgrabe
Institute of Pharmacy and Food Chemistry, Am Hubland, University of W€urzburg,
D-97074 W€urzburg, Germany
Received December 19, 2002; accepted December 23, 2002
Published online June 2, 2003 # Springer-Verlag 2003
Summary. By reaction of aromatic aldehydes with (-)-ephedrine aromatic 1,3-oxazolidines can be
obtained. The reaction was carried out either at conventional conditions or by microwave heating. The
1
different diastereomeric ratios were determined by means of H NMR spectroscopy.
Keywords. Oxazolidines; Microwave chemistry; Diastereomeric ratio.
Introduction
Microwaves are increasingly used in chemical synthesis. Compared to conventional
heating, microwave dielectric heating exhibits the following thermal differences [1]:
The introduction of microwave energy into a reaction mixture which contains at least
one compound capable of absorbing microwaves strongly, leads to much higher
heating rates. Microwave energy is introduced into a chemical reaction immediately
and without direct contact between energy source and solution. In addition, chemi-
cals do not interact equally with the microwave frequency applied. This may lead to
a different temperature profile of the reaction and, therefore, may result in an alter-
native distribution of products compared to conventional heating. Application of
closed reaction tubes allows heating up to temperatures much higher than the con-
ventional boiling point of the solvent used. The higher temperature may lead to a
large acceleration of reactions. Even though non-thermal effects (effects specially
inherent to microwaves and not caused by different temperature regimes) were
initially claimed, it now seems to be generally accepted that the different tempera-
ture regime caused by microwave heating is the main responsible factor to any
acceleration observed [2]. However, the subject of non-thermal effects in microwave
heating is still intensively discussed [3, 4].
ꢀ
Corresponding author. E-mail: u.holzgrabe@pharmazie.uni-wuerzburg.de

1106
F. Diwischek et al.
As a test reaction for the comparison of a conventional reaction setup with a
microwave-assisted one, the condensation of carbonyl compounds with amino
alcohols giving diastereomeric oxazolidines was studied.
Results and Discussion
Conventional synthesis routes to oxazolidines have been reported often in literature
[5–8]. Neelakantan [9] has found a remarkable diastereoselective oxazolidine for-
mation by conversion of aromatic aldehydes with (-)-ephedrine. Neelakantan’s
stereochemical assignments [10] have been a matter of discussion [11]. In contrast
to Neelakantan the resulting major isomer a represents the (S)-configuration at C2
and the second diastereomer b (2R) does not exceed 10% in the isolated product
mixture (see Fig. 1) [12]. The resulting diastereomers of the oxazolidines can be
used as chiral auxiliaries for stereoselective synthesis [12–14].
Kuhnert et al. have investigated the formation of 1,3-oxazolidines with focused
microwaves under thermodynamic control [15]. In contrast to their investigation
we compared the diastereomeric ratios of the classical way of oxazolidine synthe-
sis with the one in which we used microwave heating for different periods of times
and at different temperatures.
The reaction mechanism is characterized by the initial formation of an iminium
ion formed by ephedrine and the aldehyde, which reacts to the oxazolidine by an
intramolecular addition of the hydroxy group onto the iminium ion. Studying the
oxazolidine formation with different aromatic aldehydes Agami et al. [16] have
observed for benzaldehyde and 4-methoxybenzaldehyde the immediate formation
of isomer a as the major product. For 4-cyano- and 4-nitrobenzaldehydes a ratio of
a:b ¼ 1:1 was found by means of NMR spectroscopy at the beginning of the reac-
tion in chloroform. However, at the end of the reaction isomer a was the major
product. Therefore, isomer a is supposed to be the thermodynamically controlled
and isomer b the kinetically controlled product. This is supported by results
obtained by Beckett [17] who has investigated the reaction of (-)-ephedrine and
acetaldehyde and has also observed the formation of both diastereomers at the
beginning of the reaction and, subsequently, the conversion of the (2R) into the
(2S) isomer. Performing the reaction of 4-cyano- and 4-nitrobenzaldehyde in
methanol, isomer b could be only observed at the beginning of the reaction, which
also resulted in a as the predominating isomer in the end. Finally, even isomer b
could be isolated (Ar ¼ 4-C6H4Br) after the reaction being carried out in aqueous
ethanol under carefully controlled conditions [16].
Fig. 1. Oxazolidine diastereomers

Microwave Assisted Reactions Giving Oxazolidines
1107
Scheme 1. Synthesis of oxazolidines 1–7; conventional method: 25ꢁC, 23–48 h; microwave assisted
reaction: 100, 120ꢁC, 5, 10, 30 min
The aim of this investigation was to find out whether differences in the isomeric
ratio occur when the reaction is carried out under conventional conditions com-
pared to using microwaves at different temperatures and various periods of time.
Benzaldehydes of different substitution pattern (R ¼ H, 4-CH3, 4-NMe2, 4-OMe,
4-NO2, 2-NO2, 2-Cl; 8–14) were considered in this study. The progress of the
reaction was monitored by TLC and the isomeric ratio was determined by means
of NMR spectroscopy.
In order to evaluate the isomeric ratio the 1H NMR spectra were first assigned. The integral of the
signals of H2 of both isomers, which are well separated (e.g. ꢀ ¼ 4.62ppm versus 5.26 ppm for 1a
and 1b) was used as a measure of the content of each isomer in the reaction mixture. In the case of
2-(2-nitrophenyl)-5-phenyl-3,4-dimethyloxazolidine signals of H2 and H5 were overlapping. Thus,
the integrals of H4 signals were evaluated to measure the diastereomeric ratio.
For each aromatic aldehyde at least one conventional and four microwave
experiments were executed. The results are reported in Table 1. The most remark-
able finding is the shortening of the reaction time from 1–2 days to minutes, which
was observed for all examples. Close inspection of the data obtained from micro-
wave heating revealed the diastereomeric ratio partially to be dependent on the
temperature and the time of reaction.
Concerning the comparison between conventional and microwave reactions it
can be pointed out that for the formation of oxazolidines 1–4 the same or nearly the
same diastereomeric ratios were achieved but in much shorter reaction times in the
microwave oven. The corresponding aldehydes are characterised by either an elec-
tron donating group (9–11) or no substitution (8).
The most striking differences in the stereoisomeric ratio of the products
between the conventional and the microwave assisted experiments were ob-
tained in the case of aromatic aldehydes substituted with electron withdrawing
groups. For 2-(4-nitrophenyl)-5-phenyl-3,4-dimethyloxazolidine (5a) and (5b) a
higher selectivity is achieved by the conventional method. Increasing time and
temperature in the microwave assisted experiments the isomeric ratio of

1108
F. Diwischek et al.
Table 1. Isomeric ratio of oxazolidine diastereomers (2S,4S,5R)-(1a–7a):(2R,4S,5R)-(1b–7b) in
dependance on the kind and time of heating (CW ¼ conventional way; MW ¼ microwave-assisted
reaction); the ratio was determined by integration of the 1H NMR spectra; for 5a and 5b the integrals of
the H4 signals were used, in all other cases the signal of H2
Kind of heating
CW
MW
MW
MW
MW
MW
Time=min
Temp=ꢁC
2880
25
5
10
5
10
30
120
–
100
100
120
120
1
2
3
4
5
6
7
92:8
92:8
93:7
93:7
89:11
92:8
93:7
90:10
81:19
65:35
85:15
97:3
91:9
93:7
91:9
82:18
88:12
83:17
89:11
92:8
93:7
90:10
83:17
77:23
92:8
97:3
92:8
93:7
92:8
87:13
>98:<2
94:6
92:8
93:7
93:7
–
ꢀ
93:7
84:16
–
>98:<2
–
ꢀ
Reaction time 1380 min
5a:5b increases but does not reach the selectivity obtained conventionally. For
2-(2-nitrophenyl)-5-phenyl-3,4-dimethyloxazolidine (6a) and (6b) a diastereoselec-
tive reaction takes place at 120ꢁC in the microwave oven, whereas the conventional
experiment results in an isomeric mixture. For 2-(2-chlorophenyl)-5-phenyl-3,4-
dimethyloxazolidine (7a) and (7b) the result resembles the findings for 5, a
diastereoselective reaction can be observed conventionally. Applying higher tem-
peratures in the microwave oven the diastereoselectivity is enhanced, but again
does not reach the conventionally obtained selectivity.
Conclusion
The experiments revealed a strong effect on the formation of oxazolidines when
using microwaves. A tremendous shortening of reaction time was observed in all
cases. The strongest differences in diastereomeric ratios between conventional
method and the microwave assisted reaction could be found when using aromatic
aldehydes substituted with electron withdrawing groups.
Experimental
Microwave reactions were carried out in a Milestone MLS-Ethos 1600 using a quartz glass vessel
(length 15 cm, diameter 2 cm) equipped with a 20bar excess pressure valve, which was procured from
Mikrowellen-Laborsysteme (MWS).
1H NMR (400.13 MHz) and 13C NMR (100.61MHz) spectra were recorded on a Bruker Avance
400 MHz spectrometer, equipped with XWIN-NMR software running on a Microsoft Windows PC.
CDCl3 was applied as solvent and the center of the signal of CDCl3 was used as an internal reference.
Thin layer chromatography was performed using aluminium oxide 60 F254 neutral plastic sheets.
General
The following commercially available aromatic aldehydes were used: benzaldehyde (8), 4-methylbenz-
aldehyde (9), 4-(dimethylamino)benzaldehyde (10), 4-methoxy-benzaldehyde (11), 4-nitrobenzalde-
hyde (12), 2-nitrobenzaldehyde (13), 2-chloro-benzaldehyde (14).

Microwave Assisted Reactions Giving Oxazolidines
1109
Preparation of Oxazolidines via the Conventional Route [16]
The aromatic aldehyde (1.2mmol) and 1.2 mmol of (-)-ephedrine were dissolved in 15cm3 of CHCl3
˚
and 1.0g of molecular sieve (4A) was added. The solution was stirred over a period of 23–48h at
25ꢁC. To isolate the oxazolidine the mixture was filtered and the solvent was evaporated in vacuo.
Preparation of Oxazolidines via Microwave Heating
The aromatic aldehyde (1.2mmol), 1.2 mmol of (-)-ephedrine, 15cm3 of CHCl3, and molecular sieve
˚
4 A (1.0g) were added to a sealed tube and placed in the microwave oven. The solution was heated in
two minutes to a temperature of 100 or 120ꢁC, kept at this temperature for 5, 10, or 30min and then
cooled down to 40ꢁC within 10min by additional cooling. The mixture was filtered and the solvent was
removed in vacuo to give the oxazolidine.
3,4-Dimethyl-2,5-diphenyloxazolidine (1)
(2S,4S,5R)-3,4-Dimethyl-2,5-diphenyloxazolidine (1a, C17H19NO): 1H NMR: ꢀ ¼ 7.62–7.14 (m,
10Harom), 5.08 (d, J ¼ 8.1 Hz, H5), 4.62 (s, H2), 2.86–2.93 (m, H4), 2.12 (s, NCH3), 0.72 (d, J ¼ 6.6 Hz,
CHCH3) ppm; 13C NMR: ꢀ ¼ 139.8, 138.1, 129.1, 128.5, 128.3, 128.1, 127.8, 126.0 (Carom), 98.9
(C2), 82.5 (C5), 64.0 (C4), 35.7 (NCH3), 15.5 (CHCH3) ppm.
1
(2R,4S,5R)-3,4-Dimethyl-2,5-diphenyloxazolidine (1b, C17H19NO): H NMR: ꢀ ¼ 7.62–7.14 (m,
10Harom), 5.51 (d, J ¼ 5.3 Hz, H5), 5.26 (s, H2), 3.68–3.60 (m, H4), 2.19 (s, NCH3), 0.65 (d,
J ¼ 6.6 Hz, CHCH3) ppm; 13C NMR: ꢀ ¼ 141.1, 138.2, 129.8, 128.0, 127.9, 127.6, 127.1, 126.2
(Carom), 95.3 (C2), 82.5 (C5), 61.7 (C4), 33.5 (NCH3), 9.0 (CHCH3) ppm.
3,4-Dimethyl-2-(4-methylphenyl)-5-phenyloxazolidine (2)
(2S,4S,5R)-3,4-Dimethyl-2-(4-methylphenyl)-5-phenyloxazolidine (2a, C18H21NO): 1H NMR: ꢀ ¼
7.65–7.05 (m, 9H, C6H5, C6H4CH3), 5.01 (d, J ¼ 8.1 Hz, H5), 4.55 (s, H2), 2.86–2.76 (m, H4),
2.26 (m, 3H, overlapping hydrogens of (2S) and (2R) isomers, C6H4CH3), 2.13–2.05 (m, 3H, over-
lapping hydrogens of (2S) and (2R) isomers, NCH3), 0.66 (d, J ¼ 6.6 Hz, CHCH3) ppm; 13C NMR:
ꢀ ¼ 139.8, 139.4, 135.3, 129.1, 128.2, 127.9, 127.7, 126.1 (Carom), 99.2 (C2), 82.8 (C5), 64.4 (C4), 36.2
(NCH3), 21.8 (C6H4CH3), 15.5 (CHCH3) ppm.
(2R,4S,5R)-3,4-Dimethyl-2-(4-methylphenyl)-5-phenyloxazolidine (2b, C18H21NO): 1H NMR:
ꢀ ¼ 7.05–7.65 (m, 9H, C6H5, C6H4CH3), 5.46 (d, J ¼ 5.3 Hz, H5), 5.20 (s, H2), 3.52–3.62 (m, H4),
2.26 (m, 3H, overlapping hydrogens of (2S) and (2R) isomers, C6H4CH3), 2.05–2.13 (m, 3H, over-
lapping hydrogens of (2S) and (2R) isomers, NCH3), 0.60 (d, J ¼ 6.8 Hz, CHCH3) ppm; 13C NMR:
ꢀ ¼ 138.4, 138.2, 136.9, 129.6, 128.3, 127.9, 127.4, 126.1 (Carom), 95.7 (C2), 82.5 (C5), 62.1 (C4), 34.0
(NCH3), 21.8 (C6H4CH3), 9.5 (CHCH3) ppm.
2-[4-(Dimethylamino)phenyl]-3,4-dimethyl-5-phenyloxazolidine (3)
(2S,4S,5R)-2-[4-(Dimethylamino)phenyl]-3,4-dimethyl-5-phenyloxazolidine(3a,C19H24N2O):1HNMR:
ꢀ ¼ 7.79–6.72 (m, 9H, C6H5, C6H4NMe2), 5.14 (d, J ¼ 8.3 Hz, H5), 4.65 (s, H2), 2.90–3.05 (m, 7H,
overlapping hydrogens of (2S) and (2R) isomers, N(CH3)2 þ overlapping hydrogen H4), 2.19 (s, NCH3),
0.81 (d, J ¼ 6.3 Hz, CHCH3) ppm; 13C NMR: ꢀ ¼ 151.3 (CNMe2), 140.2, 129.2, 128.0, 127.8, 127.4,
125.6, 112.3 (Carom), 98.0 (C2), 81.0 (C5), 62.9 (C4), 39.5 (N(CH3)2), 34.7 (NCH3), 14.0 (CHCH3) ppm.
1
(2R,4S,5R)-2-[4-(Dimethylamino)phenyl]-3,4-dimethyl-5-phenyloxazolidine (3b, C19H24N2O): H
NMR: ꢀ ¼ 7.79–6.72 (m, 9H, C6H5, C6H4NMe2), 5.59 (d, J ¼ 5.3 Hz, H5), 5.30 (s, H2), 3.70–3.78 (m,
H4), 3.05–2.90 (m, 6H, overlapping hydrogens of (2S) and (2R) isomers, N(CH3)2), 2.28 (s, NCH3),
0.74 (d, J ¼ 6.6 Hz, CHCH3) ppm; 13C NMR: ꢀ ¼ 154.3 (CNMe2), 145.7, 132.0, 128.5, 127.7, 127.0,
125.6, 111.0 (Carom), 98.0 (C2), 81.0 (C5), 60.6 (C4), 39.0 (N(CH3)2), 32.5 (NCH3), 7.8 (CHCH3) ppm.

1110
F. Diwischek et al.
2-(4-Methoxyphenyl)-3,4-dimethyl-5-phenyloxazolidine (4)
(2S,4S,5R)-2-(4-Methoxyphenyl)-3,4-dimethyl-5-phenyloxazolidine (4a, C18H21NO2): 1H NMR:
ꢀ ¼ 7.78–6.80 (m, 9H, C6H5, C6H4OMe), 5.03 (d, J ¼ 8.3Hz, H5), 4.56 (s, H2), 3.73 (s, OCH3),
2.89–2.81 (m, H4), 2.07 (s, NCH3), 0.69 (d, J ¼ 6.3Hz, CHCH3) ppm; 13C NMR: ꢀ ¼ 160.3 (COMe),
139.8, 131.9, 129.6, 128.0, 127.8, 126.0, 113.6 (Carom), 98.5 (C2), 82.2 (C5), 63.9 (C4), 55.2 (OCH3),
35.6 (NCH3), 15.0 (CHCH3) ppm.
(2R,4S,5R)-2-(4-Methoxyphenyl)-3,4-dimethyl-5-phenyloxazolidine (4b, C18H21NO2): 1H NMR:
ꢀ ¼ 7.78–6.80 (m, 9H, C6H5, C6H4OMe), 5.49 (d, J ¼ 5.3 Hz, H5), 5.32 (s, H2), 3.78 (s, 3H,
OCH3), 2.73–2.68 (m, H4), 2.15 (s, NCH3), 0.62 (d, J ¼ 6.8 Hz, CHCH3) ppm; 13C NMR:
ꢀ ¼ 164.5 (COMe), 141.5, 130.2, 129.8, 128.8, 127.0, 126.1, 114.2 (Carom), 98.4 (C2), 81.8 (C5),
61.5 (C4), 55.5 (OCH3), 33.3 (NCH3), 8.7 (CHCH3) ppm.
3,4-Dimethyl-2-(4-nitrophenyl)-5-phenyloxazolidine (5)
(2S,4S,5R)-3,4-Dimethyl-2-(4-nitrophenyl)-5-phenyloxazolidine (5a, C17H18N2O3): 1H NMR: ꢀ ¼
8.23–7.18 (m, 9H, C6H5, C6H4NO2), 5.10 (d, J ¼ 8.4 Hz, H5), 4.72 (s, H2), 3.01–2.93 (m, H4),
2.14 (s, NCH3), 0.72 (d, J ¼ 6.6Hz, CHCH3) ppm; 13C NMR: ꢀ ¼ 148.5 (CNO2), 145.4, 139.0,
129.2, 128.2, 127.9, 126.1, 123.7 (Carom), 97.3 (C2), 82.9 (C5), 64.0 (C4), 35.8 (NCH3), 15.0 (CHCH3)
ppm.
1
(2R,4S,5R)-3,4-Dimethyl-2-(4-nitrophenyl)-5-phenyloxazolidine (5b, C17H18N2O3): H NMR: ꢀ ¼
8.23–7.18 (m, 9H, C6H5, C6H4NO2), 5.48 (d, J ¼ 5.3 Hz, H5), 5.31 (s, H2), 3.68–3.59 (m, H4), 2.20 (s,
NCH3), 0.66 (d, J ¼ 6.8 Hz, CHCH3) ppm; 13C NMR: ꢀ ¼ 148.2 (CNO2), 147.4, 138.5, 130.4, 128.5,
128.0, 127.4, 123.5 (Carom), 94.1 (C2), 82.4 (C5), 61.7 (C4), 33.5 (NCH3), 9.0 (CHCH3) ppm.
3,4-Dimethyl-2-(2-nitrophenyl)-5-phenyloxazolidine (6)
(2S,4S,5R)-3,4-Dimethyl-2-(2-nitrophenyl)-5-phenyloxazolidine (6a, C17H18N2O3): 1H NMR: ꢀ ¼
8.20–7.10 (m, 9H, C6H5, C6H4NO2), 5.28 (s, H2), 5.12 (d, J ¼ 8.8 Hz, H5), 3.05–2.94 (m, H4),
2.18 (s, NCH3), 0.72–0.66 (m, 3H, overlapping hydrogens of (2S) and (2R) isomers, CHCH3) ppm;
13C NMR: ꢀ ¼ 151.3 (CNO2), 139.6, 133.5, 133.3, 130.3, 130.1, 128.6, 128.1, 126.6, 124.3 (Carom),
92.2 (C2), 82.6 (C5), 63.6 (C4), 36.5 (NCH3), 15.2 (CHCH3) ppm.
(2R,4S,5R)-3,4-Dimethyl-2-(2-nitrophenyl)-5-phenyloxazolidine (6b, C17H18N2O3): 1H NMR:
ꢀ ¼ 8.20–7.10 (m, 9H, C6H5, C6H4NO2), 5.94 (s, H2), 5.32 (d, J ¼ 5.8 Hz, H5), 3.60–3.51 (m, H4),
2.28 (s, NCH3), 0.72–0.66 (m, 3H, overlapping hydrogens of (2S) and (2R) isomers, CHCH3) ppm; 13
C
NMR: ꢀ ¼ 150.8 (CNO2), 139.4, 135.6, 133.0, 129.8, 129.6, 128.3, 128.2, 126.6, 124.2 (Carom), 90.7
(C2), 81.8 (C5), 61.0 (C4), 34.4 (NCH3), 9.7 (CHCH3) ppm.
2-(2-Chlorophenyl)-3,4-dimethyl-5-phenyloxazolidine (7)
(2S,4S,5R)-2-(2-Chlorophenyl)-3,4-dimethyl-5-phenyloxazolidine (7a, C17H18ClNO): 1H NMR: ꢀ ¼
7.12–7.90 (m, 9H, C6H5, C6H4Cl), 5.19 (s, H2), 5.10 (d, J ¼ 8.32Hz, H5), 2.92–2.98 (m, H4), 2.14
(s, NCH3), 0.66–0.72 (m, 3H, overlapping hydrogens of (2S) and (2R) isomers, CHCH3) ppm;
13C NMR: ꢀ ¼ 139.5 (C–Cl), 135.2, 129.9, 129.6, 129.4, 127.9, 127.8, 127.6, 127.1, 126.1 (Carom),
93.8 (C2), 82.7 (C5), 63.7 (C4), 35.8(NCH3), 15.0 (CHCH3) ppm.
(2R,4S,5R)-2-(2-Chlorophenyl)-3,4-dimethyl-5-phenyloxazolidine (7b, C17H18ClNO): 1H NMR:
ꢀ ¼ 7.90–7.12 (m, 9H, C6H5, C6H4Cl), 5.81 (s, H2), 5.47 (d, J ¼ 5.0 Hz, H5), 3.63–3.55 (m, H4),
2.23 (s, NCH3), 0.72–0.66 (m, 3H, overlapping hydrogens of (2S) and (2R) isomers, CHCH3) ppm; 13
C
NMR: ꢀ ¼ 139.0 (C–Cl), 137.0, 134.0, 129.9, 129.4, 129.1, 128.0, 126.9, 125.8, 127.8 (Carom), 90.9
(C2), 81.9 (C5), 61.9 (C4), 33.7 (NCH3), 9.4 (CHCH3) ppm.

Microwave Assisted Reactions Giving Oxazolidines
1111
Acknowledgement
Thanks are due to Dr. G. Lautenschl€ager and his team from Mikrowellen-Laborsysteme (MWS;
Leutkirch=Germany) for special equipment and technical support and the Fonds der Chemischen
Industrie for financial support.
References
[1] Gabriel C, Gabriel S, Grant EH, Halstead BSJ, Mingos DMP (1998) Chem Soc Rev 27: 213
[2] Galema S (1997) Chem Soc Rev 26: 233
[3] Kuhnert N (2002) Angew Chem 114: 1943
[4] Strauss CR (2002) Angew Chem 114: 3741
[5] Soliman S, Abdine H, El-Nenaey S (1975) Aust J Chem 28: 49
[6] Beckett A, Jones G, Hollingsbee D (1978) J Pharm Pharmacol 30: 15
[7] Bergmann E (1953) Chem Rev 53: 309
[8] Pfanz H, Kirchner G (1958) Liebigs Ann Chem 614: 149
[9] Neelankantan L (1971) J Org Chem 36: 2256
[10] Neelankantan L, Molin-Case J (1971) J Org Chem 36: 2261
[11] Just G, Potvin P, Uggowitzer P, Bird P (1983) J Org Chem 48: 2923
[12] Berlan J, Besace Y, Pourcelot G, Cresson P (1983) J Organomet Chem 256: 181
[13] Brunner H, Miehling W (1984) Monatsh Chemie 115: 1237
[14] Real S, Kronenthal D, Wu H (1993) Tetrahedron Lett 34: 8063
[15] Kuhnert N, Danks TN (2001) Green Chem 3: 68
[16] Agami C, Rizk T (1985) Tetrahedron 41: 537
[17] Beckett A, Jones G (1977) Tetrahedron 33: 3313
[18] Soliman S (1973) Can J Pharm Sci 8: 132