Synthesis of some novel benzobisthiazoles

Article abstract of DOI:10.3987/COM-10-11943

The synthesis of angular and linear benzobisthiazoles via reaction of o-methoxythiobenzanilides with AIBN and basic ferricyanide is described. The Japan Institute of Heterocyclic Chemistry.

Full text of DOI:10.3987/COM-10-11943

HETEROCYCLES, Vol. 81, No. 7, 2010
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HETEROCYCLES, Vol. 81, No. 7, 2010, pp. 1641 - 1649. © The Japan Institute of Heterocyclic Chemistry
Received, 11th March, 2010, Accepted, 13th May, 2010, Published online, 13th May, 2010
DOI: 10.3987/COM-10-11943
SYNTHESIS OF SOME NOVEL BENZOBISTHIAZOLES
Oscene V. Barrett, Nadale K. Downer-Riley, and Yvette A. Jackson*
Department of Chemistry, University of the West Indies, Mona, Kingston 7, Jamaica,
West Indies
Abstract – The synthesis of angular and linear benzobisthiazoles via reaction of
o-methoxythiobenzanilides with AIBN and basic ferricyanide is described.
INTRODUCTION
The potent and selective antitumour activity of 2-arylbenzothiazoles has been well established.1 Although
benzobisthiazoles have been less researched, a few reports have indicated that these compounds
demonstrate potent anti-inflammatory, antibacterial, antifungal and antiprotozoal activity.2-4 In general,
linear benzobisthiazoles (type B) have been found to display higher biological activity than their angular
counterparts (type A).2, 3 We therefore sought to synthesize benzobisthiazoles in order to evaluate their
biological activity.
Y
N
N
N
N
Y
N
Y
X
N
X
Y
N
N
X
X
Linear types
Angular types
B: Linear Benzobisthiazoles: X = Y = S
A: Angular Benzobisthiazoles: X = Y = S
Figure 1. Examples of structural types
Benzobisthiazoles have previously been synthesized by oxidative cyclization of phenyldithiourea5 or
condensation of p-phenylenediaminedithiols with substituted nitriles, aldehydes or carboxylic acids.6 In
addition, we have demonstrated that a common ortho-methoxythiobenzanilide may serve as a precursor to
either of two benzothiazoles depending on the conditions used (Scheme 1).7 Subjecting o-
methoxythiobenzanilides to radical conditions may yield benzothiazoles with loss of the ortho methoxy
group unlike the more traditional methods such as the Jacobson synthesis (potassium ferricyanide and
base), which commonly yield benzothiazole from loss of the ortho hydrogen. In this paper, we report the
extension of our previous work to effect synthesis of both angular and linear benzobisthiazoles.

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HETEROCYCLES, Vol. 81, No. 7, 2010
OMe
OMe
OMe
NH
Ph
K3Fe(CN)6/NaOH
NH
Ph
N
S
N
S
Ph
Ph
OR
OR
S
O
R
R
R
R
1
4 (corresponding benzanilides)
3 (via loss of o-OMe)
2 (via loss of o-H)
AIBN
Scheme 1
RESULTS AND DISCUSSION
Compounds 1a-c were chosen as substrates and were obtained from commercially available 2,5-
dimethoxyaniline via the corresponding benzanilides as shown in Scheme 2. 4,7-Dimethoxy-5-nitro-2-
phenylbenzothiazole (6a) and 4,7-dimethoxy-6-nitro-2-phenylbenzothiazole (6b) were synthesized from
2,5-dimethoxyaniline (5) in four steps with an overall yield of 50%.8
OMe
OMe
OMe
OMe
N
S
NH
N
S
N
S
2
4 steps
i - ii
iii
Ph
NO
Ph
Ph
2
PhC(S)NH
PhC(O)NH
(ref. 8)
OMe
OMe
OMe
OMe
5
6a : R = 5-NO2
6b : R = 6-NO2
7a : R = 5-HNC(O)Ph (70%)
7b : R = 6-HNC(O)Ph (73%)
1a : R = 5-HNC(S)Ph (96%)
1b : R = 6-HNC(S)Ph (96%)
3 steps (ref 8)
OMe
NH
OMe
OMe
NH
Ph
Ph
NH
Ph
S
O
ii
iii
S
O
O
Ph
NH
H N
2
Ph
NH
OMe
8
OMe
OMe
9 (70%)
1c (90%)
Scheme 2. Reagents and conditions: (i) 10% Pd/C, MeOH, H2 atm; (ii) PhCOCl, toluene, pyridine, reflux
(iii) Lawesson’s reagent, toluene, N2, reflux.
Palladium catalyzed reduction of nitrophenylbenzothiazoles 6a and 6b in the presence of hydrogen gave
the corresponding aminophenylbenzothiazoles in almost quantitative yield. Upon treatment of
aminophenylbenzothiazoles with benzoyl chloride in pyridine followed by thionation of the resulting
benzanilides, thiobenzanilides 1a and 1b were obtained in 65% and 70% respectively over the 3 steps.
2,5-Dimethoxy-1,4-dithiobenzanilide 1c was also synthesized as shown in Scheme 2. 6-Amino-4,7-

HETEROCYCLES, Vol. 81, No. 7, 2010
1643
dimethoxybenzanilide 8 was obtained from compound 5 in three steps.8 Treatment of compound 8 with
benzoyl chloride in pyridine followed by thionation of the resulting dibenzanilide 9 with Lawesson’s
reagent furnished dithiobenzanilide 1c.
The Jacobson reaction is known to induce cyclization of thiobenzamides with loss of the ortho hydrogen.9
We have reported, however, that under these conditions (potassium ferricyanide and base), cyclization of
highly activated o-methoxythiobenzanilides may proceed with loss of the ortho methoxy group.7
Subjecting compound 1a to Jacobson conditions yielded no linear benzobisthiazole from loss of the ortho
hydrogen, and 1b yielded only minor amounts of the corresponding 2b (Table 1). Not surprisingly,
benzobisthiazoles 3a and 3b, formed by loss of the ortho methoxy group, were the dominant products. As
was expected too,7 the reaction of 2-methoxythiobenzanilides 1a and 1b with 2,2′-azobisisobutyronitrile
(AIBN), proceeded with loss of methoxy radical to give benzobisthiazoles 3a and 3b as the only products.
Under these (Jacobson and AIBN) conditions, however, thiobenzanilide 1c did not produce linear
benzobisthiazole 3c by double ipso substitution of the methoxy groups. Under both sets of reaction
conditions, benzobisthiazole 3b was obtained showing cyclization with loss of ortho methoxy group at
one thiobenzanilide moiety and loss of ortho hydrogen at the other. Compound 1d, the product of
cyclization with loss of one methoxy group, was also isolated from these reactions (Table 2, entry 3).
Treatment of thiobenzanilide 1d with AIBN or basic K3Fe(CN)6 resulted in cyclization with loss of the
ortho hydrogen yielding 3b in 45% and 35% respectively, and indicating that 1d was an intermediate in
the conversion of 1c to 3b. Complete conversion of 1c to 3b was not observed even with extended
reaction time or addition of excess reagents.
We have thus successfully synthesized the novel benzobisthiazoles 2b, 3a, and 3b.We will continue work
in this area to gain a better understanding of the factors which give rise to the selectivity in the cyclization
reaction. Initial screening of these compounds against common bacterial pathogens such as B. subtilis, P.
aeruginosa, E. coli, and S. aureus indicated no antibacterial activity.

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Table 1. Cyclization of thiobenzanilides 1a-c using the basic ferricyanide and AIBN
OMe
OMe
OMe
NH
Ph
K3Fe(CN)6/NaOH
NH
Ph
N
S
N
S
Ph
Ph
OR
OR
S
O
or
R
R
R
R
AIBN
1
4 (corresponding benzanilides)
3 (via loss of o-OMe)
2 (via loss of o-H)
Entry
Thiobenzanilides
Basic ferricyanide
AIBN/PhNO2
Ph
Ph
OMe
S
S
Ph
NH
N
N
N
Ph
N
S
N
S
Ph
Ph
S
S
1
OMe
OMe
OMe
1a
3a (73%)
3a (86%)
OMe
Ph
OMe
S
N
S
N
S
S
N
Ph
S
Ph
Ph
2
S
Ph
Ph
NH
N
N
OMe
OMe
OMe
2b (10%)
1b
3b (74%)
3b (64%)
OMe
Ph
Ph
NH
Ph
S
S
S
N
N
S
S
S
3
Ph
NH
Ph
N
OMe
Ph
NH
1c
OMe
OMe
1d (23%)
3b (25%)
4d (23%)
3b (15%)
1d (23%)
S.M. (43%)
Ph
Ph
Ph
S
S
S
N
N
N
S
S
O
Ph
4
N
Ph
NH
Ph
NH
OMe
OMe
OMe
1d
3b (35%)
4d (27%)
4d (5%)
3b (45%)
S.M. (12%)

HETEROCYCLES, Vol. 81, No. 7, 2010
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EXPERIMENTAL
1. General
IR spectra were obtained on a Perkin-Elmer 735B FT-IR spectrometer as KBr discs. NMR spectra
(Bruker 200 and 500 MHz) were obtained in CDCl3 solution and the resonances are reported in δ units
1
downfield from TMS as an internal standard; J values are given in Hz. The H NMR spectroscopic data
for all known compounds were identical to that reported in the literature. Column chromatography was
carried out using silica gel as the adsorbent. All elemental analyses were done by MEDAC Ltd, Egham,
Surrey, UK.
1.1. General procedure for the benzoylation of anilines
To a solution of an aniline (6.0 g) in dry toluene (35 mL) and pyridine (25 mL), benzoyl chloride (1.2
molar equiv.) was added. The solution was heated at reflux for 2 - 6 h, cooled to room temperature and
poured into water (200 mL). The two layers were separated and the aqueous layer extracted with EtOAc
(3 x 25 mL). The organic fractions were combined, washed with 1M HCl (3 x 25 mL) followed by
saturated aqueous sodium hydrogen carbonate (3 x 25 mL) and then dried over magnesium sulfate. The
solution was filtered and the solvent removed in vacuo to yield the corresponding benzanilide, which was
recrystallized from MeOH or CH2Cl2 –hexanes.
1.1.1. N-(2-Phenyl-4,7-dimethoxybenzothiazol-5-yl)benzamide (7a).
o
Off-white crystals (70%); mp 185-186 C (MeOH); Anal. Calcd for C22H18N2O3S: C, 67.67; H, 4.65; N,
7.17%. Found: C, 67.97; H, 4.72; N, 7.23%; IR max/cm-1 3437, 2818, 1684, 1525; 1H NMR 3.95 (3H, s,
OCH3), 4.36 (3H, s, OCH3), 7.53 (6H, m, H-3′,4′,5′ and H-3″,4″,5″), 7.91 (2H, d, J = 7 Hz, H-2′, 6′), 8.12
13
(2H, d, J = 3 Hz, H-2″, 6″), 8.36 (1H, s, H-6), 8.87 (1H, s, N-H); C NMR δ 56.0, 56.6, 99.1, 99.3,
119.9, 127.5, 128.0, 128.9, 129.0, 131.0, 132.0, 133.6, 135.1, 136.2, 146.3, 149.2, 165.4, 168.1.
1.1.2. N-(2-Phenyl-4,7-dimethoxybenzothiazol-6-yl)benzamide (7b).
Green finely divided crystals (73%); mp 153-155 oC (MeOH); Anal. Calcd for C22H18N2O3S: C, 67.67; H,
4.65; N, 7.17%. Found: C, 67.48; H, 4.63; N, 7.36%; IR max/cm-1 3419, 2820, 1671, 1522; 1H NMR 4.07
(3H, s, OCH3), 4.18 (3H, s, OCH3), 7.53 (6H, m, H-3′,4′,5′ and H-3″,4″,5″), 7.91 (2H, dd, J = 8, 2 Hz, H-
2′, 6′), 8.12 (2H, m, H-2″, 6″), 8.32 (1H, s, H-6), 8.67 (1H, s, N-H); 13C NMR δ 56.5, 59.9, 100.9, 127.0,
127.3, 127.6, 128.7, 128.9, 129.0, 130.8, 132.1, 133.2, 134.8, 135.9, 141.4, 150.0, 165.4, 165.5.
1.1.3. N,N′-[2,5-Dimethoxylphenyl]-1,4-dibenzamide (9).
Light brown needles (70%); mp 218-220 oC (MeOH); Anal. Calcd for C22H20N2O4: C, 70.20; H, 5.36; N,

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7.44%. Found: C, 70.60; H, 5.56; N, 7.63%; IR max/cm-1 3326, 2824, 1660, 1540; 1H NMR 3.95 (6H, s, 2
x OCH3), 7.51 (6H, m, H-3′,4′,5′ and H-3″,4″,5″), 7.89 (4H, d, J = 8 Hz, H-2′, 6′ and H-2″, 6″), 8.43 (2H,
s, H-3, 6), 8.63 (2H, s, 2 x N-H); 13C NMR δ 56.5, 103.2, 123.4, 127.0, 128.9, 131.8, 135.1, 141.9, 165.1.
1.2. General procedure for the thionation of benzanilides
To a solution of the benzanilide (1.0 g) in dry toluene (40 mL) was added Lawesson’s reagent (0.6 molar
equiv.). The mixture was heated at reflux under an atmosphere of nitrogen for 2-6 h, after which it was
concentrated and purified by column chromatography or recrystallized from MeOH or EtOAc-hexanes to
give finely divided yellow crystals.
1.2.1. N-(2-Phenyl-4,7-dimethoxybenzothiazol-5-yl)thiobenzamide (1a).
o
(96% from 7a); mp. 160-161 C (MeOH); Anal. Calcd for C22H18N2O2S2: C, 65.00; H, 4.46; N, 6.89%.
1
Found: C, 65.17; H, 4.43, N, 6.78%; IR max/cm-1 3361, 2927, 1607, 1522, 1362; H NMR 4.02 (3H, s,
OCH3), 4.36 (3H, s, OCH3), 7.45 (6H, m, H-3′,4′,5′ and H-3″,4″,5″), 7.91 (2H, d, J = 8 Hz, H-2′, 6′), 8.15
(2H, m, H-2″, 6″), 8.96 (1H, s, H-6), 9.96 (1H, s, N-H); 13C NMR δ 55.5, 62.6, 100.3, 122.4, 126.8, 127.6,
128.8, 129.1, 129.7, 129.9, 131.2, 133.4, 138.4, 144.0, 146.2, 148.3, 168.2, 196.2.
1.2.2. N-(2-Phenyl-4,7-dimethoxybenzothiazol-6-yl)thiobenzamide (1b).
o
(96% from 7b); mp. 165-167 C (MeOH); Anal. Calcd for C22H18N2O2S2: C, 65.00; H, 4.46; N, 6.89%.
1
Found: C, 65.13; H, 4.49; N, 7.13%; IR max/cm-1 3369, 3005, 1592, 1341; H NMR 4.03 (3H, s, OCH3),
4.16 (3H, s, OCH3), 7.51 (6H, m, H-3′,4′,5′ and H-3″,4″,5″), 7.89 (2H, s, H-2′, 6’), 8.15 (2H, m, H-2″, 6″),
13
8.86 (1H, s, H-5), 9.71 (1H, s, N-H); C NMR δ 55.5, 62.6, 100.3, 122.4, 126.8, 127.6, 128.8, 129.7,
129.9, 131.2, 131.2, 133.4, 138.4, 144.0, 146.2, 148.3, 168.2, 196.2.
1.2.3. N,N′-(2,5-Dimethoxyphenyl)-1,4-dithiobenzamide (1c).
o
(90% from 9); mp. 223-225 C (MeOH); Anal. Calcd for C22H20N2O2S2: C, 64.68; H, 4.93; N, 6.86%.
1
Found: C, 64.82; H, 4.97; N, 7.00%; IR max/cm-1 3327, 2994, 1538, 1357; H NMR 3.95 (6H, s, 2 x
OCH3), 7.45 (6H, m, H-3′,4′,5′ and H-3″,4″,5″), 7.86 (4H, d, J = 8 Hz, H-2′, 6′ and H-2″, 6″), 9.43 (2H, s,
H-3, 6), 9.87 (2H, s, 2 x N-H); 13C NMR δ 56.8, 103.8, 126.1, 126.7, 128.8, 131.2, 142.6, 144.2, 195.0.
1.3. General procedure for the Jacobson Synthesis
To the thiobenzanilide (0.4 g) in EtOH (0.5 mL) was added 30% aqueous NaOH solution (8 molar equiv.).
The mixture was diluted with water to provide a final solution / suspension in 10% aqueous NaOH.

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Aliquots (0.5 mL) of this mixture were added at 1 minute intervals to a stirred solution of K3Fe(CN)6 (4
o
molar equiv.) in water at 90 C. The reaction mixture was then heated at reflux for a further 2 h and
allowed to cool to room temperature. The product was collected by filtration, washed with water and
recrystallized from MeOH or CH2Cl2-hexanes to give the corresponding benzothiazoles. Column
chromatography was done when necessary using CH2Cl2: hexanes – 3:1 as eluent.
1.3.1. 5-Methoxy-2,7-diphenylbenzo[1,2-d; 3,4-d]bisthiazole (3a).
Off-white crystals (73% from 1a); mp. 235-236 oC (MeOH); Anal. Calcd for C21H14N2OS2: C, 67.35; H,
1
3.77; N, 7.48%. Found: C, 67.54; H, 3.93; N, 7.43%; IR max/cm-1 3006, 1556, 1329; H NMR 4.03 (3H,
13
s, OCH3), 7.51 (7H, m, H-6, H-3′,4′,5′ and H-3″,4″,5″), 8.15 (4H, m, H-2′, 6′ and H-2″, 6″); C NMR δ
56.3, 100.2, 121.2, 122.6, 127.3, 127.5, 129.1, 130.8, 131.2, 133.5, 133.8, 147.8, 153.6, 154.6, 168.3,
170.1.
1.3.2. 4-Methoxy-2,7-diphenylbenzo[1,2-d; 4,3-d]bisthiazole (3b).
o
Yellow-green needles (64% from 1b and 15% from 1c); mp. 225-227 C (EtOH); Anal. Calcd for
C21H14N2OS2: C, 67.35; H, 3.77; N, 7.48%. Found: C, 67.32; H, 4.05; N, 7.30%; IR max/cm-1 3000, 1554,
1346; 1H NMR 4.16 (3H, s, OCH3), 7.48 (m, 6H, H-3′,4′,5′ and H-3″,4″,5″), 7.57 (1H, s, H-5), 8.14 (4H,
m, H-2′, 6′ and H-2″, 6″); 13C NMR δ 56.4, 101.8, 119.3, 127.3, 127.6, 129.0, 129.1, 131.0, 133.2, 133.4,
143.6, 153.0, 153.1, 166.4, 167.8.
1.3.3. 4,8-Dimethoxy-2,6-diphenylbenzo[1,2-d; 4,5-d]bisthiazole (2b).
o
Yellow crystals (10% from 1b) mp. 274-276 C (MeOH); Anal. Calcd for C22H16N2O2S2·½H2O: C,
63.92; H, 4.12; N, 6.78%. Found: C, 64.24; H, 3.93; N, 6.73%. IR νmax/cm-1 2820, 1481, 1349, 1261; 1H
NMR 4.51 (6H, s, OCH3), 7.51 (6H, m, H-3′,4′,5′ and H-3″,4″,5″), 8.14 (4H, dd, J = 7, 3 Hz, H-2′, 6′ and
H-2″, 6″); 13C NMR δ 61.7, 126.7, 127.6, 129.0, 130.1, 133.7, 142.4, 142.9, 166.9.
1.3.4. N-(5-Methoxy-2-phenylbenzothiazol-6-yl)thiobenzamide (1d)
o
Yellow needles (23% from 1c); mp. 137-140 C (EtOH); Anal. Calcd for C21H16N2OS2: C, 66.99; H,
4.28; N, 7.44%. Found: C, 67.27; H, 4.01; N, 7.41%; IR νmax/cm-1 3350, 2944, 1573, 1510; 1H NMR 4.04
(3H, s, OCH3), 7.50 (7H, m, H-4, H-3′4′,5′ and H-3″,4″,5″), 7.88 (2H, dd, J = 8, 2 Hz, H-2′, 6′), 8.07 (2H,
m, H-2″, 6″), 9.93 (1H, s, H-7), 10.06 (1H, s, N-H); 13C NMR δ 56.4, 103.8, 112.6, 115.4, 126.7, 127.3,
127.7, 128.8, 129.1, 130.9, 131.1, 133.6, 144.1, 149.4, 151.8, 169.5, 195.6.
1.3.5. N-(5-Methoxy-2-phenylbenzothiazol-6-yl)benzamide (4d)

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HETEROCYCLES, Vol. 81, No. 7, 2010
Finely divided yellow crystals (23% from 1c); mp. 182-184 oC (MeOH); Anal. Calcd for C21H16N2O2S: C,
69.98; H, 4.47; N, 7.77%. Found: C, 69.77; H, 4.38; N, 7.69; IR νmax/cm-1 3452, 2949, 2848, 1668, 1571;
1H NMR 4.04 (3H, s, OCH3), 7.50 (7H, m, H-4, H-3′,4′,5′ and H-3″,4″,5″), 7.88 (2H, dd, J = 8, 2 Hz, H-2′,
13
6′), 8.07 (2H, m, H-2″, 6″), 8.76 (1H, s, H-7), 9.22 (1H, s, N-H); C NMR δ 56.3, 103.6, 111.3, 126.7,
127.0, 127.2, 127.7, 128.8, 129.0, 130.7, 131.9, 133.7, 135.0, 148.3, 150.3, 165.3, 168.2.
1.4. General procedure for the AIBN-induced Cyclizaton of thiobenzanilides
To a solution of the thiobenzanilide (0.1 g) in nitrobenzene (5 mL) was added AIBN (1.2 molar equiv.)
and the mixture stirred at reflux for 30 min. The crude product was eluted through a silica column, first
with hexanes to remove the nitrobenzene followed by EtOAc. The EtOAc fraction was concentrated and
recrystallized from EtOAc-hexanes or EtOH.
1.4.1. 8-Methoxy-2,7-diphenylbenzo[2,3-d: 4,5-d]bisthiazole (3a).
Off-white crystals (86% from 1a).
1.4.2. 4-Methoxy-2,7-diphenylbenzo[2,3-d: 4,5-d]bisthiazole (3b).
Yellow-green needles (74 % from 1b, 25% from 1c; and 45% from 1d).
1.4.3. 5-Methoxy-6-thiobenzamidobenzothiazole (1d)
Yellow needles (23% from 1c).
ACKNOWLEDGEMENTS
Financial support of ND-R by a University of the West Indies (UWI) Research Fellowship is gratefully
acknowledged.
REFERENCES
1. E. Kashiyama, I. Hutchinson, M. Chua, S. F. Stinson, L. R. Phillips, G. Kaur, E. A. Sausville, T. D.
Bradshaw, A. D. Westwell, and M. F. G. Stevens, J. Med. Chem., 1999, 42, 4172; I. Hutchinson, M.
Chua, H. L. Browne, V. Trapani, T. D. Bradshaw, A. D. Westwell, and M. F. G. Stevens, J. Med.
Chem., 2001, 44, 1446; I. Hutchinson, S. A. Jennings, B. R. Vishnuvajjala, A. D. Westwell, and M. F.
G. Stevens, J. Med. Chem., 2002, 45, 744; C. A. Mathis, Y. Wang, D. P. Holt, G. Huang, M. L.
Debnath, and W. E. Klunk, J. Med. Chem., 2003, 46, 2740.
2. F. Stephens and D. Wibberley, J. Chem. Soc., 1950, 3336.
3. E. Cullen, R. Becker, K. Freter, T. LeClerq, G. Possanza, and H. Wong, J. Med. Chem., 1992, 35, 350.
4. D. Meurer, R. Hutzler, J. Fitch, and P. Cassidy, Macromolecules, 1994, 27, 1303.
5. H. Hugerscoft, Chem. Ber., 1901, 34, 3130; H. Hugerscoft, Chem. Ber., 1903, 36, 3121; A. F. Jordan,

HETEROCYCLES, Vol. 81, No. 7, 2010
1649
C. Luo, and A. B. Reitz, J. Org. Chem., 2003, 68, 8693.
6. S. Mukai, J. Polymer Science and Technology, 1997, 10, 83; A. G. Green and A. G. Perkin, J. Chem.
Soc., 1903, 83, 1201; S. L. Solar, R. J. Cox, and N. Clecak, J. Org. Chem., 1968, 33, 2132.
7. N. K. Downer and Y. A. Jackson, Org. Biomol. Chem., 2004, 2, 3039.
8. Y. A. Jackson, M. A. Lyon, N. Townsend, K. Bellabe, and F. Soltanik, J. Chem. Soc., Perkin Trans. 1,
2000, 205.
9. P. Jacobson, Chem. Ber., 1886, 19, 1067.