Cerium(IV) ammonium nitrate mediated addition of thiocyanate and azide to styrenes: Expeditious routes to phenacyl thiocyanates and phenacyl azides

Article abstract of DOI:10.1016/S0040-4020(00)00675-X

An efficient synthesis of phenacyl thiocyanates and phenacyl azides is described here. Styrenes react with ammonium thiocyanate and sodium azide in the presence of cerium(IV) ammonium nitrate under an oxygen atmosphere to afford phenacyl thiocyanates and phenacyl azides respectively in good yields. (C) 2000 Published by Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4020(00)00675-X

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 7607–7611
Cerium(IV) Ammonium Nitrate Mediated Addition of
Thiocyanate and Azide to Styrenes: Expeditious Routes to
Phenacyl Thiocyanates and Phenacyl Azides
*
Vijay Nair, Latha G. Nair, Tesmol G. George and Anu Augustine
Organic Chemistry Division, Regional Research Laboratory (CSIR), Trivandrum 695019, India
This paper is dedicated with respectful regards to Professor Josef Fried on the occasion of his 85th birthday
Received 4 October 1999; revised 11 July 2000; accepted 27 July 2000
Abstract—An efficient synthesis of phenacyl thiocyanates and phenacyl azides is described here. Styrenes react with ammonium thio-
cyanate and sodium azide in the presence of cerium(IV) ammonium nitrate under an oxygen atmosphere to afford phenacyl thiocyanates and
phenacyl azides respectively in good yields. ᭧ 2000 Published by Elsevier Science Ltd.
Introduction
Results and Discussion
In view of the current interest in the synthetic applications
of cerium(IV) ammonium nitrate (CAN)1,2 we have been
exploring its potential use in carbon–heteroatom bond
formation. Recently we have reported a very efficient
CAN mediated addition of thiocyanate to styrenes and elec-
tron rich aromatics leading to the formation of 1,2-dithio-
cyanates and aryl thiocyanates respectively.3 Subsequently
we encountered a novel procedure for the synthesis of
phenacyl thiocyanates directly from styrenes, using ammo-
nium thiocyanate mediated by CAN. This strategy was
found applicable to the synthesis of phenacyl azides also;
these results are reported here. It is noteworthy that recently,
iodine(III) reagents have been used for the thiocyanation of
alkenes4,5 and for the conversion of enol silyl ethers to a-
thiocyanato ketones.6
Our initial experiment involved the reaction of 4-methyl-
styrene with ammonium thiocyanate in the presence of CAN
in methanol, which led to an inseparable mixture of 1,2-
dithiocyanate 2 and phenacyl thiocyanate 3a in 1:2 ratio.
With the reasonable assumption that oxygen takes part in
the reaction leading to the phenacyl thiocyanate, the
reaction of 1a with CAN (2.3 equiv.) and ammonium
thiocyanate (1.1 equiv.) was conducted in an atmosphere
of oxygen. In this case the phenacyl thiocyanate was
formed in 70% yield and no dithiocyanate was observed
(Scheme 1).
Various substituted styrenes when subjected to similar
experimental conditions afforded the corresponding
phenacyl thiocyanates (Scheme 2, Table 1).
Scheme 1.
Keywords: cerium(IV) ammonium nitrate; phenacyl thiocyanate; phenacyl azide; styrene.
* Corresponding author. Tel.: ϩ91-471-490406; fax: ϩ91-471-491712; e-mail: gvn@csrrltrd.ren.nic.in
0040–4020/00/$ - see front matter ᭧ 2000 Published by Elsevier Science Ltd.
PII: S0040-4020(00)00675-X

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V. Nair et al. / Tetrahedron 56 (2000) 7607–7611
Scheme 2.
Table 1. Synthesis of phenacyl thiocyanates
Mn(OAc)3 leading to the corresponding diazides.11,12 In
Entry
Substrate
R
Product
Yielda (%)
this context, it may be recalled that iodosobenzene diace-
tate–azidotrimethylsilane13 or CrO3–TMSN314 combination
has been used in the conversion of styrenes to phenacyl
azides. The formation of phenacyl azides from styrenes
and azide mediated by CAN, however, has not been
reported previously. In a procedure analogous to the one
leading to phenacyl thiocyanates, the reaction of styrene
1
2
3
4
1b
1c
1d
1e
OMe
OAc
Cl
3b
3c
3d
3e
53
68
65
57
NHAc
a Isolated yield.
Scheme 3.
As expected, when the experiment was conducted under a
deoxygenated atmosphere, the dithiocyanate was formed as
the only isolable product and the formation of phenacyl
thiocyanate was not observed. In the reaction of thiocyanate
with 1-vinylnaphthalene 4, although the major product
was the expected keto thiocyanate 5, a small amount of
the b-methoxy thiocyanate 6 was also obtained (Scheme 3).
1f and sodium azide in presence of CAN in methanol in
oxygen atmosphere afforded phenacyl azide 7f in 85%
yield. Similar results were obtained with other styrenes
also (Scheme 4, Table 2).
A mechanistic rationalization of the process leading to
phenacyl thiocyanate or azide may be presented as follows
(Scheme 5). Oxidation of the anion (thiocyanate or azide) by
CAN would lead to the corresponding radical. Conceivably
addition of the latter to the styrene will result in the benzylic
radical A. This can trap oxygen2b,15 to form the peroxy
radical B, eventually leading to the hydroperoxide C by
abstraction of a hydrogen from the solvent. Oxidative clea-
vage of C by CAN will result in the corresponding phenacyl
thiocyanate or azide D.
Prompted by the success of the above reaction, it was
decided to explore the possibility of synthesizing phenacyl
azides using a similar strategy. It is noteworthy that in his
pioneering work in 1971 Trahanovsky reported that CAN
mediated addition of azide to alkenes resulted in the forma-
tion of 1-azido-2-nitrates and diazides.7,8 Subsequent work
on CAN mediated azidation of glycals and triisopropylsilyl
enol ethers is also well documented.9,10 Mention may also
be made of the azidation of alkenes and glycals using
In conclusion, we have uncovered a facile process for
the synthesis of phenacyl thiocyanates and phenacyl azides.
a-Thiocyanatoketones have been conventionally prepared
from a-haloketones via nucleophilic displacement with
thiocyanate or by the ring opening of epoxide using thio-
cyanate anion followed by oxidation.16,17 These methods,
however, often require drastic reaction conditions leading
to low yields of products. Phenacyl azides are generally
prepared from the corresponding bromides obtainable
from acetophenone, by displacement with azide.18,19 The
instability and lachrymatory property of phenacyl bromides
make this procedure inconvenient.
Scheme 4.
Table 2. Synthesis of phenacyl azides
Entry
Substrate
R
Product
Yielda (%)
Both the phenacyl azides and thiocyanates are useful inter-
mediates in organic synthesis. A recent example of the use
of phenacyl azides in synthesis involves their conversion to
formyl oxazoles by Vilsmeir reaction.20 The thiocyanato
group is an important functional group since it can be trans-
formed into a number of other functionalities.21,22 Its
presence in several biologically active natural products is
also noteworthy.23,24 a-Thiocyanato ketones25 serve as
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
Me
7a
7b
7c
7d
7e
7f
76
95
75
69
68
85
OMe
OAc
Cl
NHAc
H
a Isolated yield.

V. Nair et al. / Tetrahedron 56 (2000) 7607–7611
7609
Scheme 5.
intermediates in the synthesis of several types of hetero-
cycles.26
Jˆ7.7 Hz, ArH), 4.73 (s, 2H, COCH2), 2.45 (s, 3H, CH3);
13C NMR (CDCl3): d 190.10, 145.77, 131.51, 129.72,
128.51, 111.63, 42.98, 21.76; Anal. Calcd for C10H9NOS:
C, 62.82; H, 4.71; N, 7.32; S, 16.75. Found: C, 62.77; H,
4.78; N, 7.39; S, 16.82.
Experimental
40-Methoxy-2-thiocyanatoacetophenone (3b). Reaction of
4-methoxystyrene 1b (0.136 g, 1 mmol) and ammonium
thiocyanate (0.086 g, 1.1 mmol) with CAN (1.26 g,
2.3 mmol) in methanol under an oxygen atmosphere
afforded 3b as colorless crystals (0.110 g, 53%). It was
recrystallized from CHCl3–hexane; mp 122–125ЊC (lit.,27
121ЊC); IR (KBr)nmax: 3063, 2982, 2840, 2160 (SCN), 1674
NMR spectra were recorded on a Bruker—300 MHz NMR
spectrometer. Chemical shifts are reported (d) relative to
TMS (1H) and CDCl3 (13C) as the internal standards.
Elemental analyses were performed on a Hewlett–Packard
185-B CHN analyser. Mass spectra were recorded under EI/
HRMS (at 5000 resolution) using Auto Spec. M mass spec-
trometer. Melting points were recorded on Bu¨chi melting
point apparatus and were uncorrected. Column chromato-
graphy was performed on silica gel (100–200 mesh).
Solvents were distilled prior to use. The CAN used for the
reactions was purchased from Aldrich Co. and was used
without purification. Substituted styrenes were prepared
from the corresponding aldehydes via Wittig olefination
reaction.
(CO), 1600 cmϪ1
.
40-Acetoxy-2-thiocyanatoacetophenone (3c). Reaction of
4-acetoxystyrene 1c (0.162 g, 1 mmol) and ammonium thio-
cyanate (.086 g, 1.1 mmol) with CAN (1.26 g, 2.3 mmol) in
methanol under an oxygen atmosphere afforded 3c as a
colorless solid (0.160 g, 68%) and recrystallized from
CHCl3–hexane; mp 99–102ЊC; IR (KBr)nmax: 2935, 2160,
1
1762, 1688, 1550 cmϪ1; H NMR (CDCl3): d 7.97 (d, 2H,
General procedure for the synthesis of phenacyl thio-
cyanates
Jˆ8.6 Hz, ArH), 7.27 (d, 2H, Jˆ8.6 Hz, ArH), 4.72 (s, 2H,
COCH2), 2.34 (s, 3H, CH3); 13C NMR (CDCl3): d 189.32,
168.16, 155.52, 131.39, 130.06, 122.33, 111.35, 42.79,
21.02. Anal. Calcd for C11H9NO3S: C, 56.16; H, 3.86;
N, 5.96; S, 13.60. Found: C, 55.92; H, 3.89; N, 5.81; S,
13.48.
To a solution of styrene (1 mmol) and ammonium thio-
cyanate (1.1 mmol) in methanol saturated with oxygen, an
oxygenated solution of CAN (2.3 mmol) in methanol was
added dropwise at ice temperature, while the reaction
mixture was continuously being purged with oxygen.
After 30 min, the reaction mixture was diluted with distilled
water (50 mL), extracted with dichloromethane (5×20 mL),
washed with saturated brine, and dried over anhydrous
sodium sulfate. The residue obtained after the removal of
solvent was purified using a silica gel column to afford the
corresponding phenacyl thiocyanates.
40-Chloro-2-thiocyanatoacetophenone (3d). A mixture of
4-chlorostyrene 1d (0.138 g, 1 mmol) and ammonium thio-
cyanate (0.086 g, 1.1 mmol) in methanol was treated with a
solution of CAN (1.26 g, 2.3 mmol) in methanol under an
oxygen atmosphere to afford 3d as colorless crystals
(0.138 g, 65%). It was recrystallized from CHCl3–hexane;
mp 132–135ЊC (lit.,27135ЊC). IR (KBr)nmax: 3097, 2987,
2153, 1667, 1580, 1485 cmϪ1
.
40-Methyl-2-thiocyanatoacetophenone (3a). A mixture of
4-methylstyrene 1a (0.118 g, 1 mmol) and ammonium thio-
cyanate (0.086 g, 1.1 mmol) in methanol (5 mL) was treated
with CAN (1.26 g, 2.3 mmol) in methanol (10 mL) in an
atmosphere of oxygen. The reaction mixture was worked
up as usual and the residue on purification afforded 3a as
a colorless crystalline solid (0.134 g, 70%) which was
recrystallized from CHCl3–hexane, mp 105–107ЊC; IR
40-Acetamido-2-thiocyanatoacetophenone (3e). Reaction
of 1e (0.162 g, 1mmol) and ammonium thiocyanate
(0.086 g, 1.1 mmol) with CAN (1.26 g, 2.3 mmol) in
methanol under an oxygen atmosphere afforded 3e as
pale yellow powder (0.133 g, 57%) which was recrystal-
lized from acetone–hexane. Mp 192–194ЊC (lit.,28
193ЊC). IR (KBr)nmax: 3306, 2153, 1688, 1640, 1600,
1
(KBr)nmax: 2985, 2153 (SCN), 1678 (CO), 1605 cmϪ1; H
1539 cmϪ1
.
NMR (CDCl3): d 7.82 (d, 2H, Jˆ7.7 Hz, ArH), 7.31 (d, 2H,

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V. Nair et al. / Tetrahedron 56 (2000) 7607–7611
1-(1-Naphthyl)-2-thiocyanatoethanone (5) and 1-methoxy-
1-(1-naphthyl)-2-thiocyanatoethane (6)
an oxygen atmosphere afforded 7c as a colorless oil
(0.165 g, 75%). It was recrystallized at 0ЊC from CH2Cl2–
hexane; mp 54–56ЊC; IR (KBr) nmax: 2921, 2099, 1755,
1
To a stirred mixture of 1-vinylnaphthalene 4 (0.154 g,
1 mmol) and ammonium thiocyanate (0.086 g, 1.1 mmol)
in methanol (5 mL), CAN (1.26 g, 2.3 mmol) in methanol
(10 mL) was added dropwise. The reaction mixture on work
up and purification afforded 5 as a colorless solid (0.136 g,
60%) and 6 (.024 g, 10%) as a yellow viscous liquid.
1694, 1607, 1499, 1371, 1196 cmϪ1; H NMR (CDCl3,
300 MHz): d 7.89 (d, 2H, Jˆ8.5 Hz, ArH), 7.18 (d, 2H,
Jˆ8.5 Hz, ArH), 4.48 (s, 2H, COCH2N3), 2.28 (s, 3H,
CH3); 13C NMR (CDCl3, 75 MHz): d 191.76, 168.37,
155.04, 131.89, 129.58, 122.20, 54.71, 21.10; Anal. Calcd
for C10H9N3O3: C, 55.07; H, 4.00. Found: C, 54.79; H, 4.14.
Data of 1-(1-naphthyl)-2-thiocyanatoethanone (5). Mp
2-Azido-40-chloroacetophenone (7d). Reaction of 4-
chlorostyrene 1d (0.138g, 1 mmol) and sodium azide
(0.097 g, 1.5 mmol) with CAN (1.26 g, 2.3 mmol) in
methanol under an oxygen atmosphere afforded 7d
(0.135 g, 69%) as a colorless solid. It was recrystallized
from CH2Cl2–hexane; mp 68–70ЊC (lit.,18 67–70ЊC); IR
1
89–90ЊC; IR (KBr)nmax: 2916, 2155, 1669 cmϪ1. H NMR
(CDCl3): d 8.80 (d, 1H, Jˆ8.5 Hz, ArH), 8.08 (d, 1H,
Jˆ8.2 Hz, ArH), 7.94–7.87 (m, 2H, ArH), 7.67–7.49 (m,
3H, ArH), 4.81 (s, 2H, CH2SCN); 13C NMR (CDCl3): d
193.14, 135.24, 134.03, 131.03, 130.33, 129.81, 129.17,
128.70, 127.12, 125.66, 124.18, 111.71, 45.04; Anal.
Calcd for C13H9NOS: C, 68.70; H, 3.99; N, 6.16; S, 14.11.
Found: C, 68.81; H, 3.95; N, 6.09; S, 14.15.
(KBr) nmax: 2903, 2100, 1693, 1217 cmϪ1
.
2-Azido-40-acetamidoacetophenone (7e). Reaction of 4-
acetamidostyrene 1e (0.161 g, 1 mmol) and sodium azide
(0.097 g, 1.5 mmol) with CAN (1.26 g, 2.3 mmol) in
methanol under an oxygen atmosphere afforded 7e
(0.150 g, 68%) as colorless powder, which got decomposed
at 165–166ЊC (lit.,29 167ЊC); IR (KBr) nmax: 3335, 3195,
Data of 1-methoxy-1-(1-naphthyl)-2-thiocyanatoethane
1
(6). IR (CH2Cl2)nmax: 2934, 2830, 2149, 1103; H NMR
(CDCl3): d 8.06 (d, 1H, Jˆ8.1 Hz, ArH), 7.86–7.78 (m,
2H, ArH), 7.57–7.43 (m, 4H, ArH), 5.15 (dd, 1H, Jˆ3.3,
9.0 Hz, CHOMe), 3.35 (s, 3H, OMe), 3.30–3.18 (m, 2H,
CH2SCN). 13C NMR (CDCl3): d 133.90, 133.49, 130.52,
129.20, 129.11, 126.71, 125.85, 125.30, 124.24, 122.07,
112.19, 79.58, 57.36, 40.00; HRMS Calcd for
C14H13NOS: 243.0717. Found: 243.0725.
2113, 1675, 1653, 1596, 1535 cmϪ1
.
2-Azidoacetophenone (7f). Reaction of styrene 1f (0.104 g,
1 mmol) and sodium azide (0.097 g, 1.5 mmol) with CAN
(1.26 g, 2.3 mmol) in methanol under an oxygen atmo-
sphere afforded 7f 30 as a pale yellow viscous liquid,
(0.136 g, 85%). IR (CH2Cl2) nmax: 3063, 2921, 2106,
General procedure for the synthesis of phenacyl azides
1694, 1593, 1452, 1283 cmϪ1
.
To a solution of styrene 1a–f (1 mmol) and sodium azide
(1.5 mmol) in methanol saturated with oxygen, an
oxygenated solution of CAN (2.3 mmol) in methanol was
added dropwise at ice temperature, while the reaction
mixture was continuously being purged with oxygen.
After 30–45 min, the reaction mixture was diluted with
distilled water (50 mL), extracted with dichloromethane
(5×20 mL), washed with saturated brine, and dried over
anhydrous sodium sulfate. The residue obtained after the
removal of solvent was purified on a silica gel column to
afford the corresponding phenacyl azide 7a–f.
Acknowledgements
L. G. N., T. G. G. and A. A. thank CSIR, New Delhi for the
award of research fellowships and Dr P. Shanmugam and
Ms Soumini Mathew for NMR spectra.
References
2-Azido-40-methylacetophenone (7a). Reaction of 4-
methylstyrene 1a (0.118 g, 1 mmol) and sodium azide
(0.097 g, 1.5 mmol) with CAN (1.26 g, 2.3 mmol) in
methanol (10 mL) under an oxygen atmosphere afforded
7a (0.133 g, 76%) as a colorless solid. It was recrystallized
from CH2Cl2–hexane; mp 60–61ЊC (lit.,18 59–62ЊC); IR
1. (a) Baciocchi, E.; Casu, A.; Ruzziconi, R. Tetrahedron Lett.
1989, 30, 3707. (b) Baciocchi, E.; Casu, A.; Ruzziconi, R. Synlett
1990, 679 and references cited therein. (c) Linker, T.;
Sommermann, T.; Kahlenberg, F. J. Am. Chem. Soc. 1997, 119,
9377. (d) Citterio, A.; Sebastiano, R.; Carvaryal, M. C. J. Org.
Chem. 1991, 56, 5335. (e) Narasaka, K.; Okauchi, T.; Tanaka,
K.; Murakami, M. Chem. Lett. 1992, 2099.
(KBr)nmax: 2903, 2101, 1690, 1227 cmϪ1
.
2. (a) Nair, V.; Mathew, J.; Radhakrishnan, K. V. J. Chem. Soc.,
Perkin Trans. 1 1996, 1487. (b) Nair, V.; Nair, L. G.; Mathew, J.
Tetrahedron Lett. 1998, 39, 2801. (c) Nair, V.; Mathew, J.;
Kanakamma, P. P.; Panicker, S. B.; Sheeba, V.; Zeena, S.;
Eigendorf, G. Tetrahedron Lett. 1997, 38, 2191. (d) Nair, V.;
Mathew, J.; Prabhakaran, J. Chem. Soc. Rev. 1997, 127 and
references cited therein.
3. (a) Nair, V.; Nair, L. G. Tetrahedron Lett. 1998, 39, 4585.
(b) Nair, V.; George, T. G.; Nair, L. G.; Panicker, S. B.
Tetrahedron Lett. 1999, 40, 1195.
2-Azido-40-methoxy-acetophenone (7b). Reaction of 4-
methoxystyrene 1b (0.136 g, 1 mmol) and sodium azide
(0.097 g, 1.5 mmol) with CAN (1.26 g, 2.3 mmol) in
methanol under an oxygen atmosphere afforded 7b as color-
less solid, (0.181 g, 95%). It was recrystallized from
CH2Cl2–hexane; mp 71–72ЊC (lit.,18 71–73ЊC); IR (KBr)
nmax: 2903, 2843, 2126, 1684, 1602, 1240 cmϪ1
.
2-Azido-40-acetoxyacetophenone (7c). Reaction of 4-acet-
oxystyrene 1c (0.162 g, 1 mmol) and sodium azide (0.097 g,
1.5 mmol) with CAN (1.26 g, 2.3 mmol) in methanol under
4. De Mico, A.; Margarita, R.; Mariani, A.; Piancatelli, G. J. Chem.
Soc., Chem. Commun. 1997, 1237.

V. Nair et al. / Tetrahedron 56 (2000) 7607–7611
7611
5. Bruno, M.; Margarita, R.; Parlanti, L.; Piancatelli, G.; Trifoni,
M. Tetrahedron Lett. 1998, 39, 3847.
19. Cowper, R. M.; Davidson, L. H. Organic Synthesis Collective,
Volume 2, Wiley: New York, 1943; p 480.
6. Prakash, O.; Rani, N.; Sharma, V.; Moriarty, R. M. Synlett
1997, 1255.
7. Trahanovsky, W. S.; Robbins, M. D. J. Am. Chem. Soc. 1971,
93, 5256.
8. Hansen, R. S.; Trahanovsky, W. S. J. Org. Chem. 1974, 39, 570.
9. Lemieux, R. U.; Ratcliffe, R. M. Can. J. Chem. 1979, 57, 1244.
10. Magnus, P.; Barth, L. Tetrahedron Lett. 1992, 33, 2777.
11. Fristad, W. E.; Brandvold, T. A.; Peterson, J. R.; Thompson,
S. R. J. Org. Chem. 1985, 50, 3647.
12. Snider, B. B.; Lin, H. Synth. Commun. 1998, 28, 1913.
13. Ehrenfreund, J.; Zbiral, E. Tetrahedron 1972, 28, 1697.
14. Reddy, M. V. R.; Kumareswaran, R.; Vankar, Y. D.
Tetrahedron Lett. 1995, 36, 6751.
15. Nair, V.; Mathew, J.; Nair, L. G. Synth. Commun. 1997, 76,
3053.
16. Brimeyer, M. O.; Mehrota, A.; Quici, S.; Nigam, A.; Regen,
S. L. J. Org. Chem. 1980, 45, 4254.
17. Lewars, E. G. In Comprehensive Heterocyclic Chemistry,
Katritzky, A. R., Rees, C. W., Lwowski, W., Eds.; Pergamon:
Oxford, 1984; Vol. 7, p 178.
20. Majo, V. J.; Perumal, P. T. J. Org. Chem. 1998, 63, 7136.
21. For reviews see: (a) Wood, J. L. Organic Reactions; Adams,
R., Ed.; Wiley: New York, 1946; Vol. 3, Chapter 6, p 240.
(b) Harusawa, S.; Shioiri, T. Yuki Gosei Kagaku Kyokaishi 1981,
39, 741.
22. Toste, F. D.; LaRonde, F.; Still, I. W. J. Tetrahedron Lett.
1995, 36, 2949.
23. Shahidi, F. In Sulfur Compounds in Foods, Mussinan, C. J.,
Keelan, M. E., Eds.; American Chemical Society: Washington,
DC, 1994; Vol. 9, p 106.
24. Mehta, R. G.; Liu, J.; Constantinou, A.; Tomas, C. F.;
Hawthorne, M.; You, M.; Gerhaccuser, C.; Pezzuto, J. M.;
Moon, R. C.; Moriarty, R. M. Carcinogenesis 1995, 16, 399.
25. (a) Atkins, E. F.; Debbs, S.; Guy, R. G.; Mohomed, A. A.;
Mountford, P. Tetrahedron 1994, 50, 7253. (b) Tanabe, Y.; Makita,
T.; Mori, K. Chem. Lett. 1994, 2275.
26. Erian, A. W.; Sherif, S. M. Tetrahedron 1999, 55, 7957.
27. Dains, F. B.; Krober, O. A. J. Am. Chem. Soc. 1939, 61, 1830.
28. Dhami, K. S.; Arora, S. S.; Narang, K. S. J. Sci. Ind. Res. 1958,
18B, 392.
18. Takeuchi, H.; Yanagida, S-i.; Ozaki, T.; Hagiwara, S.; Eguchi,
S. J. Org. Chem. 1989, 54, 431.
29. Bretschneider, H. Monatsch Chem. 1953, 84, 1021.
30. Batanero, B.; Escudero, J.; Barba, F. Synthesis 1999, 1809.