HETEROCYCLES

Paper | Regular issue | Vol. 81, No. 2, 2010, pp. 371-380
Received, 4th November, 2009, Accepted, 22nd December, 2009, Published online, 25th December, 2009.
DOI: 10.3987/COM-09-11868

■ Synthesis of a 6H-Chromeno[3,4-b]quinoline and a 6a,12a-Dehydro-7-azarotenoid

Andrew L. C. Morris and Yvette A. Jackson*

Department of Chemistry, Mona Campus, University of the West Indies, Mona, Kingston 7, Jamaica

Abstract

The preparation of a 6H-chromeno[3,4-b]quinoline via a palladium-mediated coupling reaction, and the first synthesis of a nitrogenous dehydro-analogue of the naturally occurring rotenoids are reported.

INTRODUCTION
The presence of planar aromatic and heteroaromatic polycyclic systems is usually a key feature of compounds possessing DNA intercalating properties, an important characteristic of antitumor drugs.1 Chromenoquinolines, in particular, are attractive molecules for drug discovery as many compounds of this type compounds have been shown to exhibit excellent biological activity.2 Steroidal natural products used as therapeutic agents also have undesirable side effects, and these have been averted by the use of synthetic chromenoquinoline analogues.3 For example, 5H-chromeno[3,4-f]quinolines (1) have been shown to mimic the effects of natural glucocorticoids such as cortisone and cortisol, used as anti-inflammatory agents,4 and are the first reported non-steroidal agents which posses these properties. 6H-Chromeno[4,3-b]quinolines (2) show inhibition of estrogens by binding to estrogen receptor beta (ERβ) sites.5 It has also been demonstrated that 5-benzylidene-1,2-dihydrochromeno[3,4-f]quinolines (3) behave as highly potent progestins6 and that the 5-oxo-5H-chromeno[2,3-b]quinolines (4) are highly potent inhibitors of the passive cutaneous anaphylaxis.7 The development of simple protocols for the preparation of functionalized quinoline derivatives, therefore, represents an interesting challenge for synthetic chemists.

There have been only few reports on the preparation of chromenoquinolines. These include (i) condensation of 2-aminobenzaldehyde with chroman-4-one8a,b and (ii) treatment of N-phenyl-2-(prop-2-ynyloxy)benzamide with POCl3.8c The synthesis of chromeno[3,4-b]quinolines have even less prevalent in the literature. The only reported preparation of chromeno[3,4-b]quinolines has been via the Friedlander-Kempter reaction of chroman-3-one with 2-aminobenzaldehyde or 2-aminoacetophenone. This, to the best of our knowledge, is the only reported synthesis of this unique heterocyclic system. Preliminary tests showed these compounds to be mild carcinogens.9

Our interest in the synthesis of polycyclic heteroatomic molecules with potential biological activity led us to explore methods for the preparation of the chromeno[3,4-b]quinoline (5) ring system. It seemed that success here could also give us access to the aza-rotenoids – nitrogen analogues of the insecticidal rotenoids of basic skeleton (6). The rotenoids, compounds bearing the 12-oxo-chromanochromene core (6), possess a wide range of biological activities including insecticidal, antifeedant, antimicrobial and piscicidal properties.10,11 Other beneficial biological effects associated with the rotenoids include inhibition of the formation of microtubules from tubulin, a prelude to anti-cancer activity. The biological activity associated with rotenoids has been attributed to the intact A/B/C/D core.12 Herein, we report the first palladium-mediated synthesis of a chromeno[3,4-b]quinoline (14) as well as a 12-oxo-dihydro-analogue (24), representing the first report of a 7-aza-analogue of the rotenoids.

RESULTS AND DISCUSSION
The preparation of compound 14 was achieved as outlined in Scheme 1. Heating commercially available p-anisidine (7) at reflux in EtOH for 4 hours in the presence of ethyl acetoacetate provided ester 8 in 87% yield. Thermally-induced cyclization of compound 8 in Ph2O at 250 °C then afforded quinolin-4H-one 9 (79%). With the palladium-mediated reaction in mind, the 3-bromo- derivative of compound 9 was prepared without incident following its treatment with NBS in a mixture of acetic acid/CH2Cl2. Stirring the mixture at room temperature for 30 minutes produced 3-bromoquinolin-4H-one (10) in quantitative yield. Initial treatment of compound 10 with MsCl in dichloromethane, DMAP and lutidine at ambient temperatures provided chloroquinoline 11 in only 34% yield. Starting material was, however, recovered in 49% yield. In an effort to improve the yield, quinolone 10 was subsequently treated with PCl5 in the presence of excess organic base for 4 hours in refluxing DMF. 4-Chloroquinoline 11 was thus obtained in 82% yield from compound 10. No starting material was recovered.

Light-promoted bromination of compound 11 resulted in dibromide 12 in 50% yield (see EXPERIMENTAL). Larger-scaled reactions (in excess of 600 mg starting material) resulted in lower yields. Reaction of 12 with phenol then afforded ether 13 (81%). 12-Chloro-10-methoxy-chromeno[3,4-b]quinoline (14) was then prepared in 58% yield when a mixture of 13, Pd(OAc)2, PPh3 and triethyl amine in DMF for 15 minutes was allowed to react under microwave conditions. Compound 14 was also obtained from 13 under thermal conditions with Pd(OAc)2, PPh3, K2CO3 in refluxing acetonitrile (45%).13
Having successfully obtained chromeno[3,4-b]quinoline 14, reduction of ring C and subsequent benzylic oxidation seemed to be a ready route to 7-azarotenoid 15. All attempts at reduction of ring C, however – NaBH4/AcOH, NaCNBH3/BF3·OEt2/MeOH, NaBH4/NiCl2/MeOH,14 ammonium formate/Pd-C/MeOH,15 hydrogenation in the presence of Pd/C and Raney Ni catalysts and LAH/Et2O16 – yielded either starting material or the product of hydrogenolysis (compound 16).

The synthesis of a azarotenoid 24 was achieved via a Dieckmann condensation reaction and makes use of an intermediate prepared, and an idea developed, by Crombie in the preparation of rotenoid 20 (Scheme 2).17 Crombie and his group treated keto ester 18 with phloroglucinol dehydrate and produce dehydrorotenoid 19 which was then reduced by DIBAL to produce rotenoid 20. We explored this pathway, anticipating that use of aniline in lieu of phloroglucinol, would give rise to the aza-compound 24. Success at this would lead to a novel heteroatomic analogue of the rotenoids. Thio-derivatives have been previously reported.18

Ester 22 was prepared from readily available 2'-hydroxyphenylacetic acid (21) under acidic conditions in excellent yield, ­c.a. 98% (Scheme 3). Subsequent alkylation of compound 22 with ethyl bromoacetate resulted in the diester 17 which was subjected to base-induced cyclisation with sodium methoxide. 1H NMR and 13C NMR data confirmed that cyclisation of diester 17 had produced enol 18b in 82% yield. With enol 18b in hand, it was subjected to condensation with aniline to produce amine 23 (68% yield), which was then cyclized in refluxing Ph2O to afford dehydro-7-azarotenoid 24 in 65% yield. Reduction of the unsaturated 6a,12a-bond of compound 24 with a view to producing the azarotenoid in its natural oxidation state has so far been unsuccessful. Reduction has been attempted with DIBAL in THF, at -78 °C and at room temperature, conditions which have been used successfully for reduction of the 4-chromone-type system in reported syntheses of rotenoids.17,19 Reduction using NaBH4 and NaCNBH3 in refluxing MeOH and EtOH has also been attempted without success.

To the best of our knowledge, the only previous report of a nitrogen analogue of the rotenoids has been the synthesis of the 5-aza-compound 25 by Srimanarayana and Kumar.20 Our synthesis of compound 24 represents the first 7-aza-analogue of the rotenoids. This paves the way for structure-activity relationship (SAR) studies on the rotenoids, the previously reported 5-thio and now, the aza-derivatives.

ACKNOWLEDGEMENTS
Partial support of this work by the Royal Society of Chemistry is gratefully acknowledged.

EXPERIMENTAL
Melting points were determined in capillary tubes on a Thomas Hoover Melting Point Apparatus and are uncorrected. IR (KBr) spectra were recorded on a Bruker Vector 22 FTIR instrument. NMR spectra were recorded using a Bruker Avance 200 MHz or a Bruker Avance 500 MHz spectrometer and, unless stated otherwise, were determined in CDCl3 solution. Resonances are reported in δ units downfield from TMS; J-Values are given in Hz. Elemental analyses were carried out by MEDAC Ltd., Egham, Surrey, United Kingdom. A 150 W tungsten bulb was used as the source for light-promoted bromination. Chromatography was carried out using silica as support.

Ethyl (2Z)-3-[(4-methoxyphenyl)amino]-2-butenoate (8)
A mixture of p-anisidine 7 (1.76g, 13.5 mmol), ethyl acetoacetate (1.96 g, 14.9 mmol), glacial acetic acid (0.5 mL) and calcium sulfate (4 g) in dry EtOH (15 mL) was heated at reflux for 3 h. The reaction filtered and the residue washed with EtOH. The filtrate was concentrated under reduced pressure then diluted with CH2Cl2 (20 mL), washed with saturated aqueous NaHCO3 (2 x 10 mL) and H2O (2 x 10 mL), dried and concentrated en vacuo. The resulting yellow oil was purified via chromatography to give pure 8 (2.75 g, 87%); mp 41 - 42 °C (hexanes) (lit.,21 mp 43 - 44 °C); νmax/cm-1 3443, 2947, 1656, 1612; δH: 1.28 (3H, t, J = 7 Hz, -CH2CH3), 1.88 (3H, d, J = 0.5 Hz, -CH3), 3.80 (3H, s, -OCH3), 4.14 (2H, q, J = 7 Hz, -CH2CH3), 4.65 (1H, d, J = 0.5 Hz, vinylic H), 6.85 (2H, dd, J = 9, 2 Hz, 3- and 5-H), 7.02 (2H, dd, J = 9, 2 Hz, 2- and 6-H), 10.16 (1H, s, -NH-). δC: 14.6, 20.1, 55.4, 58.6, 84.7, 114.2, 126.8, 132.1, 157.4, 160.0, 170.5.

6-Methoxy-2-methylquinolin-4(1H)-one (9)
Ph2O (2 mL) was preheated under an atmosphere of nitrogen at 120 °C at which point ester 3 (496 mg, 2.11 mmol) was added as a solution in Ph2O (1 mL) and the temperature rapidly increased to 260 ºC and this temperature was maintained for 15-20 min with continuous stirring. Upon cooling, an off-white slurry was formed, hexanes (10 mL) was added and the mixture was filtered. The residue was then boiled in acetone to give the desired quinolone 9 as a off-white amorphous solid (314 mg, 79%), mp >300 ºC (decomp.); IR νmax/cm-1 3260, 2800, 1630, 1540; δH (DMSO): 2.33 (3H, s, -CH3), 3.82 (3H, s, -OCH3), 5.88 (1H, s, 3-H) 7.23 (1H, dd, J = 9, 3 Hz, 7-H), 7.43 (1H, d, J = 2 Hz, 5-H), 7.45 (1H, d, J = 4 Hz, 8-H), 11.54 (1H, s, NH). δC: 20.2, 56.2, 105.3, 108.3, 120.3, 122.5, 126.4, 135.6, 149.4, 156.1, 177.0. Anal. Calcd for C11H11NO2: C, 69.83; H, 5.86; N, 7.40%. Found: C, 69.81; H, 5.81; N, 7.23%.

3-Bromo-6-methoxy-2-methylquinolin-4(1H)-one (10)
4-Quinolone 9 (2.04 g, 10.8 mmol) was dissolved in a mixture of HOAc:CH2Cl2 (40 mL, 3:7). N-Bromosuccinimide (1.97 g, 11.0 mmol) was then added in small portions over 5 min with continuous stirring at ambient temperatures. After stirring for a further 15 min the mixture was poured into H2O (30 mL) and neutralized with 3 M NaOH (8 mL). The mixture was then filtered, the residue boiled in EtOH and filtered to give pure 3-bromo-4-quinolone 10 (2.88 g, 100%) as a white powder: mp >300 ºC (decomp.); IR νmax/cm-1 3264, 2760, 1637; δH (CDCl3/TFA): 2.73 (3H, s, -CH3), 3.92 (3H, s, -OCH3), 7.38 (2H, m, 5- and 8-H), 7.72 (1H, dd, J = 9, 1 Hz, 7-H). δC: 21.9, 56.0, 101.4, 120.1, 121.0, 126.8, 133.1, 152.4, 159.0, 164.8, 178.6.

3-Bromo-4-chloro-6-methoxy-2-methylquinoline (11)
Compound 9 (3.74 g, 14.0 mmol) was heated in a mixture of 2,6-lutidine (4.50 g, 42.0 mmol) and DMAP (171 mg, 1.40 mmol) in DMF (20 mL) until dissolved. PCl5 (3.65 g, 17.5 mmol) was then added in small portions over 5 min and the resulting mixture heated at reflux for 2 h. After cooling, the mixture diluted with CH2Cl2 (50 mL) and washed with saturated aqueous NH4Cl (3 x 20 mL). The organic layer was then washed with H2O (5 x 15 mL), dried (Na2SO4) and concentrated en vacuo. The residue was then purified via column chromatography (hexanes: CH2Cl2, 7:3) to give the desired 4-chloroquinoline 11 (3.30 g, 82%) as yellow needles: mp 107 - 108 °C (acetone). IR νmax/cm-1 1620, 1561, 1489 δH: 2.86 (3H, s, -CH3), 3.97 (3H, s, -OCH3), 7.37 (2H, m, 5- and 8-H), 7.90 (1H, dd, J = 9, 3 Hz, 7-H); δC: 27.0, 55.7, 102.4, 120.2, 123.0, 127.0, 130.6, 140.4, 142.5 155.3, 158.7. Anal. Calcd for C11H9BrClNO: C, 46.11; H, 3.17; N, 4.89%. Found: C, 46.31; H, 3.05; N, 4.94%.

3-Bromo-2-(bromomethyl)-4-chloro-6-methoxyquinoline (12)
Compound 11 (250mg, 0.872 mmol) was dissolved in CCl4 (4 mL) and irradiated. NBS (217 mg, 1.22 mmol) added in small portions over 10 min. The mixture was then refluxed for 2-4 h while being irradiated. The mixture was then filtered and the filtrate concentrated, diluted with CH2Cl2 (15 mL) and washed with 30% aqueous NaHSO3 solution (3 x 10mL). The organic layer was dried over Na2SO4, concentrated and recrystallized to give dibromide 12 (159 mg, 50%) as off-white plates; mp 148 - 150 °C (acetone); νmax/cm-1 1618, 1561, 1488; δH: 3.99 (3H, s, -OCH­3), 4.90 (2H, s, -CH2Br), 7.40 (2H, m, 5-H and 7-H), 7.92 (1H, d, J = 8 Hz, 8-H); δC: 35.1, 55.8, 102.3, 118.9, 123.8, 128.1, 131.4, 141.5, 142.1, 152.6, 159.9.

3-Bromo-4-chloro-6-methoxy-2-(phenoxymethyl)quinoline (13)
Bromomethylquinoline 12 (300 mg, 0.821 mmol) was dissolved in acetone (5 mL), K2CO3 (204 mg, 1.48 mmol) added and the mixture was stirred at rt for 15 min. Phenol (93 mg, 0.985 mmol) was added as a solution in acetone (5 mL) and the mixture heated at reflux for 4 h. The mixture was then filtered to remove inorganic material, concentrated, diluted with CH2Cl2 (20 mL) and washed with 2 M NaOH (2 x 5 mL). The organic layer was dried over Na2SO4 and concentrated to give ether 13 (252 mg, 81%) as pale brown needles: mp 109 - 111 °C (EtOH); IR νmax/cm-1 1618, 1600, 1489, 1228; δH: 3.98 (3H, s, -OCH3), 5.43 (2H, s, -CH2O-), 6.99 (1H, tt, J = 7, 1 Hz, 4'-H), 7.08 (2H, dd, J = 9, 1 Hz, 2'- and 6'-H), 7.34 (4H, m, 3'-, 5'-, 5- and 7-H), 8.00 (1H, td, J = 10, 1 Hz, 8-H); δC: 55.8, 72.0, 102.2, 115.1, 121.3, 128.3, 128.2, 129.5, 131.6, 142.4, 151.8, 159.7. Anal. Calcd for C17H13BrClNO2: C, 53.92; H, 3.46; N, 3.70%. Found: C, 53.88; H, 3.44; N, 3.60%.

12-Chloro-10-methoxy-6H-chromeno[3,4-b]quinoline (14)
Method 1:
A mixture of quinoline 13 (190 mg, 0.502 mmol), Pd(OAc)2 (6 mg, 0.025 mmol), PPh3 (10 mg, 0.038 mmol) and Et3N (36 mg, 0.502 mmol) in DMF (1 mL) was irradiated (microwave, 70 W) for 15 min in a sealed tube. The crude mixture was then purified via flash chromatography (CH2Cl2: hexanes 2:1) to give compound 14 as a white crystalline solid (87 mg, 58%): mp 61 - 63 °C; IR νmax/cm-1 1618, 1600, 1489; δH: 4.00 (3H, s, -OCH­3), 5.20 (2H, s, –CH2O-), 7.15 (1H, dd, J = 8, 1 Hz, 9-H), 7.18 (1H, dt, J = 8, 1 Hz, 4-H), 7.38 (2H, m, 2 and 3-H), 7.60 (1H, d, J = 2 Hz, 11-H) 7.93 (1H, d, J = 2 Hz, 8-H), 8.58 (1H, dd, J = 8, 1 Hz, 1-H); δC: 55.8, 71.4, 102.8, 107.1, 113.5, 118.0, 122.0, 122.9, 128.6, 129.0, 130.6, 130.7, 141.8, 142.7, 156.7, 158.92. Anal. Calcd for C17H12ClNO2: C, 68.58; H, 4.06; N, 4.70%. Found: C, 68.68; H, 4.17; N, 4.59%.
Method 2:
A mixture of quinoline 13 (100 mg, 0.264 mmol), Pd(OAc)2 (6 mg, 0.025 mmol), PPh3 (14 mg, 0.053 mmol) and K2CO3 (73 mg, 0.528 mmol) in MeCN (3 mL) was stirred at reflux for 8 h under a steady stream of nitrogen. The mixture was filtered and the residue concentrated. The mixture was then purified via flash chromatography (CH2Cl2: hexanes 2:1) to give compound 14 as a white crystalline solid (41 mg, 45%).

Ethyl 3-Anilino-2H-chromene-4-carboxylate (23)
A mixture of aniline (280 mg, 3.00 mmol), hydroxychromene 18b (600mg, 2.72 mmol), CaSO4 (1.5g), EtOH (10 mL) and acetic acid (200 mg) was heated at reflux for 4 h with continuous stirring. The mixture was then filtered and washed with EtOH, then concentrated. CH2Cl2 (30 mL) was added and washed with saturated aqueous NaHCO3 (2 x 10mL) then concentrated in vacuo to give 23, which crystallized as yellow needles (467 mg, 58%): mp 80 - 82 °C (EtOH); IR νmax/cm-1 2985, 1644, 1519, 1232; δH: 1.41 (3H, t, J = 7 Hz, -CH3), 4.36 (2H, q, J = 7 Hz, -CH2-), 4.65 (2H, s, -CH2O-), 6.95 (4H, m, 3'-, 5'-, 4'-, 8-H), 7.18 (2H, m, 2'-, 6'-H), 7.37 (2H, m, 6-, 8-H), 7.89 (1H, dd, J = 5, 3 Hz, 5-H), 11.05 (1H, s, NH); δC: 14.4, 60.1, 64.0, 92.6, 115.8, 122.2, 123.1, 124.2, 124.9, 125.6, 126.4, 129.4, 137.9, 151.9, 153.7, 168.8. Anal. Calcd for C18H17NO3: C, 73.20; H, 5.80; N, 4.74%. Found: C, 73.38; H, 5.91; N, 4.72%.

6H-Chromeno[3,4-b]quinolin-12H-one (24)
Ph2O (3 mL) was preheated under an atmosphere of nitrogen at 120 °C. Ester 23 (200 mg, 0.677 mmol) was then added as a solution in Ph2O (1 mL) and the temperature of the mixture rapidly increased to 260 ºC and maintained for 15-20 min with continuous stirring. Upon cooling, an off-white slurry was formed. Hexanes (5 mL) were added and the mixture was filtered. The residue was then boiled in acetone to give the desired compound 24 as a white amorphous solid (112 mg, 65%): mp >300 ºC (decomp.); IR νmax/cm-1 2919, 1640, 1559, 1510; δH (DMSO): 5.12 (2H, s, -CH2-), 6.94 (1H, d, J = 8 Hz, 4-H), 7.03 (1H, t, J = 8 Hz, 2-H), 7.14 (1H, td, J = 8, 2 Hz, 3-H), 7.37 (1H, t, J = 8 Hz, 10-H), 7.54 (1H, d, J = 8 Hz, 8-H), 7.67 (1H, t, J = 7 Hz, 9-H), 8.22 (1H, d, J = 8 Hz, 11-H), 8.93 (1H, dd, J = 8, 2 Hz, 1-H), 12.2 (1H, broad, NH); δC: 64.7, 108.0, 116.2, 118.7, 122.1, 122.3, 124.1, 126.0, 126.3, 126.7, 127.5, 132.2, 138.6, 144.4, 152.1, 174.0. Anal. Calcd for C16H11NO2: C, 77.10; H, 4.45; N, 5.62%. Found: C, 77.45; H, 4.34; N, 5.34%.

References

1. a) W. A. Denny, B. F. Cain, G. J. Atwell, C. Hansch, A. Panthananickal, and A. Leo, J. Med. Chem., 1982, 25, 276; CrossRef b) H. Fujiwara and I. Okabayashi, Heterocycles, 1994, 38, 541. CrossRef
2. a) Y. J. Chen, K. C. Fang, J.-Y. Sheu, S. L. Hsu, and C. C. Tzeng, J. Med. Chem., 2001, 44, 2374; CrossRef b) S. Catoen-Chackal, M. Facompre, R. Houssin, N. Pommery, J.-F. Goossens, P. Colson, C. Bailly, and J.-P. Hénichart, J. Med. Chem., 2004, 47, 3665. CrossRef
3. J. P. Edwards, J. S. West, K. B. Karschke, D. E. Mais, M. M. Gottardis, and T. K. Jones, J. Med. Chem., 1998, 41, 303. CrossRef
4. M. J. Coghlan, P. R. Kym, S. W. Elmore, A. X. Wang, J. R. Luly, D. Wilcox, M. Stashko, C.-W. Lin, J. Miner, C. Tyree, M. Nakane, P. Jacobson, and B. C. Lane, J. Med. Chem., 2001, 44, 2879. CrossRef
5. A. T. Vu, A. N. Campbell, H. A. Harris, R. J. Unwalla, E. S. Manas, and R. E. Mewshaw, Bioorg. Med. Chem. Lett., 2007, 17, 4053. CrossRef
6. L. Zhi, C. M. Tegley, B. Pio, J. P. Edwards, M. Motamedi, T. K. Jones, K. B. Marschke, D. E. Mais, B. Risek, and W. T. Schrader, J. Med. Chem., 2003, 46, 4104. CrossRef
7. A. Nohara, T. Ishigura, and U. Kuyoshi, J. Med. Chem., 1985, 28, 559. CrossRef
8. a) G. Sabitha and B. V. S. Reddy, Synth. Commun., 1999, 29, 4403; CrossRef b) P. Jacquignon and N. P. Goisy, J. Chem. Soc., Perkin Trans. 1, 1972, 4266; c) E. Rougeot, H. Moskowitz, and M. Miocque, Tetrahedron Lett., 1983, 24, 2379. CrossRef
9. G. A. Bistochi, G. De Me, A. Ricci, A. Croisy, and P. Jacquignon, Heterocycles, 1978, 9, 247. CrossRef
10. B. LaForge, H. L. Haller, and L. E. Smith, Chem. Rev., 1933, 12, 181. CrossRef
11. O. Koul, ‘Insect Antifeedant,’ CRC Press, New York, 2005, p. 63.
12. L. Crombie and D. A. Whiting, Phytochemistry, 1998, 49, 1479. CrossRef
13. The aryl-aryl coupling is believed to have proceeded via electrophilic substitution by oxidatively added palladium. For analogous situations, see: a) H. Nishioka, C. Nagura, H. Abe, Y. Takeuchi, and T. Harayama, Heterocycles, 2006, 70, 549; CrossRef b) D. D. Hennings, S. Iwasa, and V. H. Rawal, J. Org. Chem., 1997, 62, 2; CrossRef c) S. Wiegand and H. J. Schäfer, Tetrahedron, 1995, 51, 5341; CrossRef d) D. E. Ames and A. Opalko, Tetrahedron, 1984, 40, 1919. CrossRef
14. A. Nose and T. Kudo, Chem. Pharm. Bull., 1988, 36, 1529.
15. P. Balczewski and J. A. Joule, Synth. Commun., 1990, 20, 2815. CrossRef
16. A. M. Jones, C. A. Russell, and O. Meth-Cohn, J. Chem. Soc. C, 1960, 3249.
17. L. Crombie and J. L. Josephs, J. Chem. Soc., Perkin Trans. 1, 1993, 2591. CrossRef
18. a) C. Gabbutt, J. D. Hepworth, and B. M. Heron, J. Chem. Soc., Perkin Trans. 1, 1992, 3015; CrossRef b) L. Crombie and J. L. Josephs, J. Chem. Soc., Perkin Trans. 1, 1993, 2599. CrossRef
19. a) M. J. Begley, L. Crombie, A. H. bin A. Hadi, and J. L. Josephs, J. Chem. Soc., Perkin Trans. 1, 1989, 204; CrossRef b) M. J. Begley, L. Crombie, A. H. bin A. Hadi, and J. L. Josephs, J. Chem. Soc., Perkin Trans. 1, 1993, 2605. CrossRef
20. G. Srimanarayana and K. A. Kumar, Indian J. Chem., 1981, 20B, 604.
21. Z.-H. Zhang and L.-M. Song, J. Chem. Res., 2005, 12, 817. CrossRef