Unusual fragmentation of fulvene endoperoxides with phenyliodosyl bis(trifluoroacetate) (PIFA)

Article abstract of DOI:10.1002/jhet.5570400319

Reaction of unsaturated fulvene endoperoxides with dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate gave saturated fulvene endoperoxides containing the 1,2-dihydropyridazine ring. Treatment of dihydropyridazine endoperoxides with water followed by phenyliodosyl bis(trifluoroacetate) oxidation provided acrylic acid derivatives and dimethyl pyridazine-3,6-dicarboxylate.

Full text of DOI:10.1002/jhet.5570400319

Unusual Fragmentation of Fulvene Endoperoxides with
Phenyliodosyl bis(trifluoroacetate) (PIFA)
May-Jun 2003
529
Galip Özer [a], Nurullah Saraçog˘lu [a] and Metin Balci* [b]
[a] Department of Chemistry, Faculty of Art and Sciences, Atatürk University, 25240 Erzurum-Turkey
[b] Department of Chemistry, Middle East Technical University, 06531 Ankara-Turkey
Received November 25, 2002
Reaction of unsaturated fulvene endoperoxides with dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate gave
saturated fulvene endoperoxides containing the 1,2-dihydropyridazine ring. Treatment of dihydropyridazine
endoperoxides with water followed by phenyliodosyl bis(trifluoroacetate) oxidation provided acrylic acid
derivatives and dimethyl pyridazine-3,6-dicarboxylate.
J. Heterocyclic Chem., 40, 529 (2003).
Introduction.
Recently, we reported [5] the trapping of highly reactive
fulvene endoperoxides 2 [4], obtained by photooxygena-
tion of the appropriate fulvenes in the presence of
tetraphenyl porphyrine (TPP), with dimethyl 1,2,4,5-
tetrazine-3,6-dicarboxylate (5). The treatment of the trap-
ping products 3 with cobalt (II) tetraphenylporphyrine
gave alkylidene- and arylidenemalonaldehydes 4 [6]. In
this paper, we report a new fragmentation of the fulvene
Bicyclic endoperoxides obtained by [4+2] cycloaddition
of singlet oxygen to various cyclic 1,3-dienes have become
increasingly significant in a variety of chemical [1] and bio-
chemical transformations [2]. On the other hand, the low
chemical stability of the endoperoxides can be taken advan-
tage of to perform selective transformations giving impor-
tant compounds. Fulvenes 1 have been favorite substrates
for singlet oxygenation. Sensitized photooxygenation of 6-
substituted fulvenes has led to a number of interesting oxy-
genated derivatives [3]. Unstable fulvene endoperoxides 2
can be conveniently and selectively reduced to their respec-
tive stable saturated fulvene endoperoxides [4a]. Erden et
al. [3d,4] have shown that functionalized 1,5-dicarbonyl
compounds and tetrahydrofurans can be obtained by a rela-
tively simple protocol involving thermolysis of saturated
fulvene endoperoxides in the presence of acetic acid. Also,
saturated fulvene endoperoxides derived from 6-vinylful-
vene proved to be excellent precursors of 5-propylal-substi-
tuted 2-cyclopentenones [4].

530
G. Özer, N. Saraçog˘lu and M. Balci
Vol. 40
1
13
endoperoxides 3 with phenyliodosyl bis(trifluoroacetate)
(PIFA) [7] to give the correponding acrylic acid deriva-
tives. This methodology may serve as an alternative route
for the synthesis of substituted acrylic acid derivatives.
The structures of 6a-e were assigned by H- and C-
1
NMR. The most conspicuous feature in the H-NMR
spectra of 6a-e was an AB system, which corresponds to
vicinal protons in the 1,2-dihydropyridazine ring. The
A-part, as well as the B-part of the AB system in 6a
resonate as a doublet of doublets at 3.46 ppm (J = 9.3, 1.5
Hz) and 3.06 ppm (J = 9.3, 2.5 Hz), respectively. The
peroxide-bridgehead protons appear as a singlet at 4.68
ppm and a doublet at 5.08 ppm (J = 1.5 Hz). There was no
measurable coupling between protons in the dihydropyri-
dazine ring and the bridgehead protons at 4.68 ppm due to
a nearly 90° dihedral angle. Inspection of Drieding
models confirmed this angle. On the other hand, the
Results and Discussion.
In the course of studying the trapping reactions of the
unsaturated fulvene endoperoxides 3 we noticed that water
adds to the 1,2-dihydropyridazine ring. Therefore, we
investigated the reaction of the trapping products 3a-e
with water. Two equivalents of water was added to a cold
solution (-40±5 °C) of trapping products 3a-e in CHCl .
3
Compounds 6a-e were formed by 1,2-addition of water as
the sole products (Scheme 1). However, in the case of
asymmetrical substitution at the fulvene ring, two isomeric
addition products of 6c and 6d were formed as expected
(Scheme 1).
13
14-line C-NMR spectrum of 6a supports strongly the
proposed asymmetrical structure. The stereochemistry of
-OH group was not determined.
Table 1
Acrylic Acids Obtained from some Unsaturated Fulvene Endoperoxides
Fulvene
Water Addition Product
Product
Yield, %
Endopeoxide
82 [a]
57 [b]
2a
2b
2c
2d
6a
7a
7b
67 [a]
13 [b]
6b
78 [a]
+
Z isomer 70 [c]
E isomer 30 [c]
6c
6c'
7c and 7c'
51 [a]
+
Z isomer 47 [c]
E isomer 53 [c]
6d
6d'
7d and 7d'
58 [a]
46 [b]
1
[a] Total crude crystalline yield; [b] Crystalline yield; [c] According to H-NMR.

May-Jun 2003
Unusual Fragmentation of Fulvene Endoperoxides
531
Addition of Water to Trapping Products.
To a stirred solution of the trapping product (1 mmol) prepared
The quantitative addition of water to the six-membered
ring at lower temperatures (-40 °C) indicates that the six-
membered ring must be highly strained due to the planarity
of ring and annelation of a strained five-membered ring.
This ring strain can be partially released by the addition of
water to the six-membered ring. Walls et al. have observed
similar results. They reported easy addition of methanol to
the dihydropyridazine ring [8].
as described above in 5 mL of CHCl cooled to -40±5 °C was
3
added 1 mmol of water during 1 min. After stirring for 1 day at
-40±5 °C, the solution was slowly warmed to room temperature.
The solvent was removed under reduced pressure. The water
addition products were obtained in quantitative yield and the
crude product was recrystallized from an appropriate solvent.
Dimethyl 6-Hydroxy-11-(1-methylethylidene)-9,10-dioxa-4,5-
Finally, the bicyclic endoperoxides 6a-e were treated
2,7
diazatricyclo[6.2.1.0 ]undec-5-ene-3,6-dicarboxylate (6a).
with phenyliodosyl bis(trifluoroacetate) in CHCl at -40 °C,
3
The compound was obtained as white crystals from
CH Cl /ether (80%), mp 110-112°; ir (potassium bromide): OH
3361, CO 1753, 1727 cm . H nmr (deuteriochloroform): δ 6.98
that resulted in the formation of the corresponding acrylic
acid derivatives 7a-e and diazine 8 (Table 1). The structures
were confirmed by means of spectroscopic methods. The
formation mechanism of the acrylic acid derivatives is
unclear. However, we assume that PIFA first attacks alcohol
group [9] and causes an oxidative fragmentation of the
fulvene endoperoxide ring. Consequently, the iodoso-
benzene which is probably formed under the given reaction
conditions acts as an oxygen-atom transfer agent and causes
an oxidative decarboxylation [10] to give the corresponding
acrylic acid derivatives 7a-e [11]. The bicyclic endoperox-
ides 4 have been treated directly with PIFA. However, the
expected aromatic compound 9 was not formed.
2
2
-1 1
(d, J = 2.5 Hz, 1H, NH), 5.08 (d, J = 1.5 Hz, 1H, H1), 4.68 (s, 1H,
H8), 4.23 (m, 1H, OH), 3.90 (s, 3H, OCH ), 3.82 (s, 3H, OCH ),
3
3
3.46 (dd, A-part of AB system, J = 9.3, 1.5 Hz, 1H, H2), 3.06 (dd,
B-part of AB system, J = 9.3, 2.5 Hz, 1H, H7), 1.68 (s, 3H, CH ),
3
13
1.63 (s, 3H, CH ); C nmr (deuteriochloroform): δ 171.5, 166.1,
3
140.9, 136.3, 128.9, 80.5, 79.6, 77.3, 55.4, 54.6, 51.9, 42.3, 24.0,
23.8.
Anal. Calcd. for C H N O : C, 51.53; H, 5.56; N, 8.59.
14 18
2 7
Found: C, 51.17; H, 5.48; N, 8.78.
Dimethyl 6-Hydroxy-11-(diphenylmethylene)-9,10-dioxa-4,5-
2,7
diazatricyclo- [6.2.1.0 ]undec-5-ene-3,6-dicarboxylate (6b).
In conclusion, the unexpected reaction of phenyliodosyl
bis(trifluoroacetate) with fulvene endoperoxide is demon-
strated. The intriguing feature of this reagent is its
versatility and broad application to a large number of
compounds containing different functional groups.
Furthermore, this method may be applied to the synthesis
of highly functionalized acrylic acids.
The compound was obtained as white crystals from
CH Cl /ether (73%), mp 133-135°; ir (potassium bromide): OH
3390, CO 1759, 1738 cm . H nmr (deuteriochloroform): δ 7.35-
2
2
-1 1
7.17 (m, 10H, phenyl), 7.07 (d, J = 2,5 Hz, 1H, NH), 5.05 (d,
J = 1.7 Hz, 1H, H1), 4.85 (bs, 1H, H8), 3.70 (s, 3H, OCH ), 3.68
3
(s, 3H, OCH ), 3.58 (dd, A-part of AB system, J = 9.5, 1.7 Hz,
3
1H, H2), 3.23 (dd, B-part of AB system, J = 9.5 Hz, 2.5 Hz, 1H,
13
H7); C nmr (deuteriochloroform): δ 171.4, 165.8, 142.0, 141.8,
141.0, 140.7, 138.3, 131.2, 130.9, 130.5, 130.2, 130.1, 129.9,
82.0, 79.4, 78.6, 55.2, 54.4, 52.3, 42.4.
EXPERIMENTAL
Anal. Calcd. for C H N O : C, 63.99; H, 4.92; N, 6.22;
24 22
2 7
Found: C, 64.32; H, 5.08; N, 6.34.
General.
Dimethyl 6-Hydroxy-11-[(E) and (Z)-1-Phenylethylidene]-9,10-
dioxa-4,5-diaza- tricyclo[6.2.1.0 ]undec-5-ene-3,6-dicarboxyl-
ate (6c and 6c').
Solvents were concentrated at reduced pressure. Melting points
were determined on a Thomas-Hoover capillary melting appara-
tus. Infrared spectra were obtained from solution in 0.1 mm cells
or KBr pellets for solids on a Perkin-Elmer 337 infrared recording
2,7
The mixture was not stable toward column material. Yield
1
1
68%, white powder from CH Cl /ether. H nmr (deuteriochloro-
spectrophotometer. H nmr spectra were recorded on Varian
2
2
13
form) of the mixture; δ 7.31-7.03 (m, 12H, 2 phenyl and 2 NH),
60 and 200 MHz and C nmr spectra on a 50 MHz spectrometer
5.27 (d, J = 1.5 Hz, 1H), 4.84 (bs, 2H), 4.61 (s, 1H), 3.94, (s, 3H,
and are reported in δ units with SiMe as internal standard.
4
OCH ), 3.87 (s, 3H, OCH ), 3.59 (s, 6H, 2xOCH ), 3.56 (dd, J =
3
3
3
Photooxygenation of Fulvenes [4].
9.3, 1.5 Hz, 2H, H2), 3.15-3.05 (m, 2H, H7), 2.05 (s, 3H, CH ),
3
13
To a solution of fulvene (1 mmol) in 5 mL of CHCl was added
2.03 (s, 3H, CH ); C nmr (deuteriochloroform): δ 171.6, 171.2,
3
3
5-10 mg of TPP. The resulting mixture was irradiated with a pro-
jection lamp (150 W) while dry oxygen was being passed
through solution and the mixture was stirred for 1 h at -50±5 °C.
166.1, 165.6, 143.1, 143.0, 141.1, 140.6, 139.0 (2C), 133.3,
133.2, 130.2, 130.1 (2C), 129.5, 129.3, 129.2, 81.71, 80.7, 79.7,
79.3, 78.4, 77.6, 55.5, 55.1, 54.6, 54.2, 52.2, 51.8, 42.4, 42.3,
24.0, 23.6.
Trapping of Fulvene Endoperoxides 2 with Dimethyl 1,2,4,5-
tetrazine-3,6-dicarboxylate [6].
Anal. Calcd. for C H N O (mixture): C, 59.31; H, 4.99; N,
19 20
2 7
7.01; Found: C, 58.76; H, 5.19; N, 7.21.
To a stirred solution of fulvene endoperoxide 2 (1 mmol)
Dimethyl 6-Hydroxy-11-[(E) and (Z)-1-Phenylmethylidene]-
9,10-dioxa-4,5-diaza- tricyclo[6.2.1.0 ]undec-5-ene-3,6-dicar-
boxylate (6d and 6d').
prepared as described above in 5 mL of CHCl was added
3
2,7
1 mmol of dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate [12] at
-50±5 °C. After complete addition (1-2 min), the mixture was
stirred for 1 day at -50±5 °C. The resulting mixture was used to
subsequent procedure.
1
The H NMR spectrum showed that the reaction mixture
consisted of a mixture of 6d/d' (1:1). Reaction mixture was

532
G. Özer, N. Saraçog˘lu and M. Balci
Vol. 40
1
(E)- and (Z)-3-Phenylbut-2-enoic Acids (7c and 7c').
unstable toward column materials. Yield 68%; H nmr (deuterio-
chloroform) of the mixture: δ 7.32-7.09 (m, 12H), 6.58 (s, 1H,
=CH), 6.56 (s, 1H, =CH), 5.48 (bs, 1H), 5.07 (s, 1H), 5.00 (s,
These compounds were obtained in a combined yield of 78%
1
[11a,15] (207 mg). Integration of H-NMR of the isomers
1H), 4.66 (s, 1H), 3.89 (s, 3H, OCH ), 3.84 (s, 3H, OCH ), 3.82
(s, 3H, OCH ), 3.74 (s, 3H, OCH ), 3.7-3.6 (m, 2H, H2), 3.21-
3.11 (m, 2H, H7). C nmr of the mixture (deuteriochloroform) δ
171.4, 171.3, 166.1, 166.0, 142.5, 141.8, 141.3, 139.9, 137.3,
137.2, 130.5, 130.4, 130.1, 130.0, 124.7, 124.5, 84.4, 81.0, 79.7,
79.6, 79.4, 76.9, 55.4, 55.3, 54.6, 54.4, 52.4, 51.3, 42.7, 42.5.
3
3
mixture showed that E- and Z-isomers were present in the ratio
3
3
1
30:70. 7c: H nmr (deuteriochloroform): δ 8.5-9.0 (br. s. 1H,
13
COOH), 7.6-7.2 (m, 5H, aromatic), 5.95 (q, J = 1.0 Hz, 1H,
1
=CH), 2.2 (d, J = 1.0 Hz, 3H, CH ). 7c': H nmr (deuteriochloro-
3
form): δ 8.5-9.0 (br. s. 1H, COOH), 7.5-7.2 (m, 5H, phenyl), 6.2
(q, J = 1.5 Hz, 1H, =CH), 2.62 (d, J = 1.5 Hz, 3H, CH ).
3
Anal. Calcd. for C H N O (mixture); C, 57.75; H, 4.85; N,
18 18
2 7
(E)- and (Z)-3-Phenylpropenoic Acids (7d and 7d').
7.48; Found: C, 58.81; H, 5.04; N, 7.31.
These compounds were obtained in a combined yield of 51%
[11a]: (117 mg). Integration of H-NMR of the isomers mixture
Dimethyl 6-Hydroxy-11-cyclohexylidene-9,10-dioxa-4,5-diaza-
tricyclo[6.2.1.0 ]- undec-5-ene-3,6-dicarboxylate (6e).
1
2,7
showed that E- and Z-isomers were present in a ratio of 53:47.
Yield 81%, white crystals from CH Cl /ether, mp 90-91 °C; ir
1
2
2
7d: H nmr (deuteriochloroform): δ 10.0 (bs, 1H, COOH), 7.80
-1
1
(potassium bromide): OH 3420, CO 1760, 1740 cm . H nmr
(d, A-part of AB system, J = 16.0 Hz, 1H, H3) 7.30-764 (m, 5H,
phenyl), 6.46 (d, B-part of AB system, J = 16.0 Hz, 1H, H2). 7d':
H nmr (deuteriochloroform): δ 10.0 (bs, 1H, COOH), 7.30-7.64
(m, 5H, phenyl), 7.09 (d, A-part of AB sysyem, J = 12.8 Hz, 1H,
H3) 6.00 (d, B-part of AB system, J = 12.8 Hz, 1H, H2).
(deuteriochloroform): δ 7.04 (bd, J = 2.4 Hz, 1H, NH), 5.12 (d, J
=1.1 Hz, 1H, H1), 4.69 (s, 1H, H8), 3.89 (s, 3H, OCH ), 3.81 (s,
1
3
3H, OCH ), 3.43 (dd, A-part of AB system, J = 9.3, 1.1 Hz, 1 H,
3
H2), 3.05 (dd, B-part of AB system J =9.3, 2.4 Hz, 1H, H7), 2.07-
13
2.01 (m, 4H, =C(CH )), 1.57-1.34 (m, 6H (CH ) ); C nmr
2
2 3
2-Cyclohexylideneacetic Acid (7e).
(deuteriochloroform): δ 171.6, 166.1, 140.4, 136.0, 133.1, 80.1,
79.5, 76.7, 55.3, 54.5, 51.6, 42.2, 35.0 (2C), 29.0, 28.5, 28.0;
This compound was obtained in 58% yield (81 mg), and was
crystallized from ether/hexane, 64 mg (46%); mp 89-90°, (mp
[11c], 90-91.5°); H nmr (deuteriochloroform): δ 10.71 (bs, 1H,
Anal. Calcd. for C H N O ; C, 55.73; H, 6.05; N, 7.65;
17 22
2 7
Found: C, 55.45; H, 6.00; N, 7.89.
1
COOH), 5.62 (s, 1H, =CH), 2.82 (m, 2H, CH ), 2.24 (m, 2H,
2
Oxidation of Addition Products 6a-e with Phenyliodosyl Bis-
(trifluoroacetate).
13
CH ), 1.62 (m, 6H, CH ); C nmr (deuteriochloroform): δ
2
2
-1
174.5, 168.8, 114.5, 40.2, 32.1, 30.6, 29.8, 28.2; IR (KBr, cm )
3029, 2953, 2876, 1702, 1651, 1446, 1421, 1319, 1293, 1268,
1242, 1217, 1191, 1012, 961, 859.
To a solution of 6a-e (1 mmol) in 10 mL of CHCl , prepared as
3
described above was added phenyliodosyl bis(trifluoroacetate)
(Merck) (1 mmol) at -40±5 °C and the resulting mixture was
stirred for 1-2 min. The reaction mixture was allowed to warm-up
slowly to room temperature. The solvent was removed under
reduced pressure and 50 mL of ether was added. Addition of ether
precipitated the diazine 8 as yellow crystals (70-80%).
Acknowledgment.
The authors are indebted to the Department of Chemistry
(Atatürk University) and Turkish Academy of Sciences (TUBA)
and for financial support of this work.
Recrystallization from CH Cl /ether of residue gave pure diazine
2
2
8. The organic layer was washed with NaOH (1 N, 2x25 mL). The
aqueous layer was acidified to pH 1 with concentrated hydro-
chloric acid and extracted with ether (2x50 mL). The ether extract
REFERENCES AND NOTES
[1a] M. Balci, Chem. Rev., 81, 91 (1981); [b] K. J. McCullough
and M. Nojima, Curr. Org. Chem., 5, 601 (2001).
was dried (CaCl ) and evaporated at reduced pressure. The crude
product was the corresponding acrylic acid derivatives 7a-e.
2
[2] N. I. Krinsky, in Singlet Oxygen, H. H. Wassermann and
R. W. Murray, ed., Academic Press: New York, 1975, pp. 597 – 636.
[3a] W. Skorianetz, K. H. Schulte-Elte and G. Ohloff, Helv. Chim.
Acta, 54, 1913 (1971); [b] W. Skorianetz, K. H. Schulte-Elte and
G. Ohloff, Angew. Chem., 84, 3111 (1972); [c] N. Harada, H. Uda, H.
Ueno and S. Utsumi, Chem. Lett., 1173 (1973); [d] I. Erden, F. P. Xu,
J. Drummond and R. Alstad, J. Org. Chem., 58, 3611 (1993); [e] I. Erden,
J. Drummond, R. Alstad and F. P. Xu, Tetrahedron Lett., 34, 2291 (1993).
[4] For the synthesis and reactions of fulvene endoperoxides see:
[a] W. Adam and I. Erden, Angew. Chem. Int. Ed. Engl., 28, 210 (1978);
[b] I. Erden and A.M. Amputchs, Tetrahedron Lett., 28, 3779 (1987);
[c] I. Erden, J. Drummond, R. Alstad and F. Xu, Tetrahedron Lett., 34,
1255 (1993).
[5] M. Balci, N. Saraçog˘lu and A, Menzek, Tetrahedron Lett., 37,
921 (1996).
[6] G. Özer, N. Saraçog˘lu and M. Balci, Heterocycles, 53, 761
(2000).
[7a] Y. Tamura, T. Yakura, H. Tohma, K. Kihuchi and Y. Kita,
Synthesis, 126 (1989); [b] A.Varvoglis, Tetrahedron, 53, 1179 (1997).
[8] W. Friedrichsen and H. V. Walls, Tetrahedron, 34, 2509
(1978).
Dimethyl Pyridazine-3,6-dicarboxylate (8) [13].
This compound has mp 195-196°; ir (potassium bromide): CO
-1
1
1770, 1746 cm ; H nmr (deuteriochloroform): δ 8.31 (s, 2H,
13
aromatic), 4.06 (s, 6H, OCH ); C nmr (CDCl ): δ 165.9, 154.7,
3
3
130.23, 55.5.
3-Methylbut-2-enoic Acid (7a) [14].
The crude product (155 mg, 82%) was crystallized from
1
CCl /hexane,108 mg (57%); mp 63-65 °C; H nmr (deuteriochlo-
4
roform): δ 10.18 (bs, 1H, COOH), 5.69 (h, J =1.2 Hz, 1H, =CH),
13
2.17 (d, J =1.2 Hz, 3H, CH ), 1.93 (d, J =1.2 Hz, 3H, CH );
C
3
3
nmr (deuteriochloroform): δ 174.2, 162.0, 117,6, 29.6, 22.5.
3,3-Diphenylacrylic Acid (7b).
The crude product (150 mg, 67%) was crystallized from
1
CH Cl /hexane, mp 150-152° (mp [11c], 158-160°); H nmr
2
2
(deuteriochloroform): δ 7.39-7.19 (m, 10H, phenyl), 6.33
13
(s, 1H, =CH); C nmr (deuteriochloroform): δ 172.4, 160.8,
142.8, 140.4, 131.7, 131.2, 130.5, 130.4 (2C), 129.9, 118.4.

May-Jun 2003
Unusual Fragmentation of Fulvene Endoperoxides
533
[9] For the reaction of tert. alcohols with PIFA see: M. Kirihara, S.
[13a] S. Sueur, M. Lagrenee, F. Abraham and C. Bremard,
J. Heterocyclic Chem., 24, 1285 (1987); [b] J. Sauer, A. Mielert, D. Lang
and D. Peter, Chem. Ber., 98, 1435 (1965).
Yokoyama, H. Kakuda and T. Momese, Tetrahedron Lett., 36, 6907 (1995).
[10] Z. Wei-Shan, B. Jiang and X. F. Pan, J. Chem. Soc., Chem.
Commun., 2105 (1995).
[11] For synthesis of acrylic acids see: [a] J. R. Johnson, Org.
React., 1, 210 (1942); [b] G. Jones, Org. React., 15, 203 (1967);
[c] D. R. Brittelli, J. Org. Chem., 46, 2514 (1981).
[14a] H. Bretschneider and H. Haas, Monatsh. Chem., 81, 945
(1950); [b] G. F. Caplus J. L. O'Brien, J. Am. Chem. Soc., 71, 2933
(1949).
[15] M. Julia and J. M. Surzur, Bull Soc. Chim. France, 1626,
(1956).
[12] D. L. Boger, R. S. Coleman, J. S. Panek, F. X. Huber and
J. Sauer, J. Org. Chem., 50, 5377 (1985).