1 A regiodivergent synthesis of ring A C-prenyl flavones Alberto Minassi, Anna Giana, Abdellah Ech-Chahad and Giovanni Appendino* Dipartimento di Scienze Chimiche, Alimentari, Farmaceutiche e Farmacologiche Università del Piemonte Orientale, Via Bovio 6, 28100 Novara, Italy E-mail: appendino@pharm.unipmn.it Supporting Information General Experimental Procedures: Gravity Column Chromatography (CC): Merck Silica Gel (70-230 mesh). IR: Shimadzu DR 8001 spectrophotometer. NMR: Jeol Eclipse (300 MHz and 75 MHz for 1 H and 13 C, respectively). Chemical shifts are reported in values downfield from TMS. Acetonitrile (HPLC-grade) was dried over 3Å molecular sieves, CH 2 Cl 2 and THF by filtration over alumina directly in the reaction flask, and pyridine by distillation from KOH. Petroleum ether refers to the fraction boiling between 40 and 60 C. Melting points (mp) are uncorrected. All reactions were conducted under nitrogen in dry solvents unless stated otherwise. Monitoring by TLC was done on Merck 60 F 254 (0.25 mm) plates, that were visualized by UV inspection and/or staining with 5% H 2 SO 4 in ethanol and heating. Organic phases were dried with Na 2 SO 4 before evaporation. Abbreviations: CC: Column Chromatography: DCM: Dichloromethane; DMAP: 4- Dimethylaminopyridine; DIAD: diisopropylazodicarboxylate; EtOAc: Ethyl acetate; MeOH: methanol; MOM: methoxymethyl: Piv: pivaloyl (= trimethylacetyl); TEA: triethylamine; TBAF:
2 tetrabutylammonium fluoride; TBDMS: tert-butyldimethylsilyl; TPP: triphenylphosphine; TPPO: Triphenylphosphine oxide. 2,4-Bis(tert-butyldmethylsilyl)phloracetophenone (8b): To a stirred suspension of phoracetophenone (8a, 2 g, 11.9 mmol) in dry CH 2 Cl 2 (20 ml ), TEA (5.48 ml, 3.97 g, 39.24 mmol, 3.3 mol. equiv.) was added, resulting in the formation of a clear yellowish solution. TBDMS-Cl (1M in CH 2 Cl 2, 26.16 ml, 26.16 mmol, 2.2 mol. equiv.) was then added, and the solution was stirred at room temperature for 90 min, and then worked up by dilution with CH 2 Cl 2 and addition of 2N H 2 SO 4. The organic phase was dried and evaporated, and the residue was purified by gravity CC on silica gel (50 g, petroleum ether as eluant) to afford 3.91 g (83%) 8b as a colorless oil. IR (KBr) max cm -1 : 3181, 1627, 1590, 1434, 1365, 1287, 1255, 1174, 1067, 840, 783; 1 H NMR (300 MHz, CDCl 3 ): 6.07 (d, J = 2.4 Hz, 1H), 5.83 (d, J = 2.4 Hz, 1H), 2.61 (s, 3H), 0.99 (s, 9H), 0.95 (s, 9H), 0.33 (s, 6H), 0.21 (s, 6H). 13 C NMR (CDCl 3, 75 MHz): 203.3 (s), 166.4 (s), 162.3 (s), 159.5 (s), 109.0 (s), 102.7 (d), 101.7 (d), 32.8 (q), 26.1 (q), 25.5 (q), 18.9 (q), 18.9 (s), 18.2 (s), -3.5 (q), -4.4 (q); CI-EIMS: m/z [M+Na] + 419 [C 20 H 36 O 4 Si 2 +Na] +. 2,4-Bis(tert-butyldmethylsilyl)-3-prenylphloracetophenone (9): To a cooled (ice bath) stirred solution of 8b (3g, 8.1 mmol ) in dry CH 2 Cl 2 (30 ml), prenyl alcohol (1.71 ml, 1.41 g, 16.3 mmol, 2 mol. equiv.), TPP (3.6 g, 13.8 mmol, 1.7 mol. equiv.) and DIAD (3.16 ml, 1.36 g, 32.6 mmol, 4 mol. equiv.) were sequentially added. The solution was let warm up to room temperature, stirred at 40 C for 2 h, and then worked up by evaporation. The residue, a yellowish paste, was dissolved in toluene (50 ml), cooled at 4 C overnight, and next filtered to remove the abundant precipitate of the TPPO-dihydroDIAD adduct. 1 The filtrate was evaporated and further purified by gravity CC on silica gel (50 g, petroleum ether as eluant) to afford 2.5 g (56%) of 6-prenyl-2,4-bis(tertbutyldimethylsilyl)phloracetophenone as a colorless oil. To a solution of the latter (2 g, 4.3 mmol) in toluene (20 ml), Eu(fod) 3 (893mg, 0.86 mmol, 0.2 mol. equiv.) was added, and the solution was
3 refluxed (oil bath temperature = 140 C) until TLC analysis (petroleum ether.etoac 95:5), or 1 H NMR control 2 evidenced the complete conversion to 9. After 30 min., the reaction was worked up by evaporation, and the residue was purified by gravity CC on silica gel (50 g, petroleum ether as eluant) to afford 1.6 g (81%) 9 as a pale-yellow amorphous powder. IR (KBr) max cm -1 : 3190, 1615, 1595, 15703, 1420, 1362, 1284, 1198, 1172, 840. 1 H NMR (300 MHz, CDCl 3 ): 5,83 (s, 1H), 5,13 (t, J = 1.5 Hz, 1H), 3,24 (d, J =6.6 Hz, 2H), 2,61 (s, 3H), 1,74 (s, 3H), 1,66 (s, 3H), 0,99 (s, 18H), 0,32 (s, 6H), 0,27 (s, 6H); 13 C NMR (CDCl 3, 75 MHz): 203.2 (s); 164.1 (s), 159.7 (s), 157.0 (s), 131.0 (s), 123.1 (d), 113.3 (s), 108.8 (s), 100.8 (d), 32.8 (q), 22.6 (q), 26.1 (q), 25.5 (q), 18.9 (q), 18.8 (s), 17.9 (s), -3.5 (q), -4.0 (q). CI-EIMS: m/z [M+Na] + 487 [C 25 H 44 O 4 Si 2 +Na] +. 1-[2,4-Bis(tert-butyldmethylsilyloxy)-6-hydroxy-3-prenylphenyl]-3-(4-pivaloyloxy-3- methoxyphenyl)-1,3-propandione (12): To a cooled (ice bath) sol. of 9 (400 mg, 0.86 mmol) in THF (4 ml), NaH (60%, 52 mg, 1.29 mmol, 1.5 mol. equiv.) was added. After stirring at 0 C for 7 min, the solution was added via cannula to a solution of pivaloyl vanilloyl chloride [prepared from 325 mg (1.29 mmol, 1.5 mol. equiv.) of pivaloylvanillic acid], 3, and the mixture was refluxed, monitoring the formation of the intermediate O-acyl derivative by TLC (petroleum ether-etoac 9:1, Rf change from 0.50 to 0.34). After 35 min., the acylation was complete, 4 and the reaction was cooled to room temperature Further NaH (72 mg, 1.8 mmol, 2 mol. equiv) was then added, and refluxing was continued, monitoring the course of the reaction by TLC (petroleum ether-etoac 95:5, Rf change from 0.44 to 0.55). After 90 min, the Baker-Venkataraman rearrangement was over, and the reaction was worked up by cooling to room temperature, quenching with water, and extraction with EtOAc. The organic phase was dried and evaporated, and the residue was purified by gravity CC on silica gel (12.5 g, petroleum ether-ch 2 Cl 2 as eluant) to afford 500 mg (85%) 12 as a yellowish oil. IR (KBr) max cm -1 : 1758, 1577, 1508, 1479, 1258, 1103, 839; 1 H NMR (300 MHz, CDCl 3 ): 7.48 (d, J =3.0 Hz, 1H), 7.45 (dd, J = 9.2, 3.0 Hz, 1H), 7.11 (s, 1H), 7.06 (d, J = 9.0, 1H), 5.89 (s, 1H),
4 5.15 (t, J = 6.0 Hz, 1H), 3.86 (s, 3H), 3.28 (d, J = 6.0 Hz, 2H), 1.74 (s, 3H), 1.66 (s, 3H), 1.53 (s, 6H), 1.40 (s, 9H), 1.02 (s, 9H), 0.94 (s, 9H), 0.28 (s, 6H), 0,21 (s, 6H). 13 C NMR (CDCl 3, 75 MHz): 194.6 (s); 176.2 (s); 174.9 (s), 162.0 (d), 159.1 (s), 155.2 (s), 151.5 (s), 143.0 (s), 132.7 (s), 131.1 (s), 123.1 (d) 122.7 (d), 119.6 (d), 114.0 (s), 110.7 (d), 108.6 (s), 102.8 (d), 99.0 (d), 56.1 (q), 39.1 (t), 27.2 (q), 26.1 (q), 25.9 (q), 25.7 (q), 22.2 (q), 18.6 (s), 18.3 (q), 17.8 (s), -3.8 (q), -4.0 (q); CI- EIMS: m/z [M+Na] + 721 [C 38 H 58 O 8 Si 2 +Na] +. 7-tert-Butyldimethylsilyl-4 -pivaloylcannflavin B (13): To a soln of CuCl 2 (24 mg, 0.18 mmol, 0.5 mol. equiv.) in dry acetonitrile (24 ml), TMS-Cl (110 µl, 94.2 mg, 0.87 mmol, 2.4 mol. equivalents) was added slowly at room temperature, and the mixture was added via cannula to a solution of neat 12 (250 mg, 0.36 mmol) in a 50 ml round bottom flask. After stirring at room temperature for 90 min., the reaction was worked up by dilution with water and extraction with EtOAc. After drying and evaporation, the residue was purified by gravity CC on silica gel (5 g, petroleum ether-etoac 95:5 as eluant) to afford 85 mg 13 (60%) and 17 mg 15a. 13 was obtained as a yellowish foam, 1 H NMR (300 MHz, CDCl 3 ): 7.50 (dd, J = 8.4 Hz J = 2.1, 1H), 7.36 (d, J = 2.1 Hz, 1H), 7.13 (d, J =8.4 Hz, 1H), 7.06 (d, J =9.0 Hz, 1H), 6.62 (s, 1H), 6.41 (s, 1H), 5.18 (t, 1H), 3.90 (s, 3H), 3.35 (d, J=6.6 2 H), 1.77 (s, 3H), 1.67 (s, 3H), 1.38 (s, 9H), 1,03 (s, 9H), 0,34 (s, 6H); 13 C NMR (CDCl 3, 75 MHz): 233.1 (s);182.4 (s); 176.3 (s), 163.2 (d), 160.0 (s), 159.4 (s), 155.4 (s), 151.7 (s), 143.0 (s), 131.9 (s), 130.0 (s), 123.2 (d) 122.7 (d), 120.7 (d), 116.3 (s), 110.1 (q), 106.0 (q), 96.8 (d), 56.2 (q), 39.2 (s), 27.2 (q), 25.6 (q), 21.9 (q), 18.3 (q), 18.0 (s), -4.0 (q). Cannflavin B (1d): To a soln. of 13 (50 mg, 0.088 mmol) in dry THF (3 ml), TBAF (1M in THF, 130 µl, 0.13 mmol, 1.5 mol. equiv.) was added. After stirring at room temperature for 4 min., the reaction was worked up by dilution with water and extraction with EtOAc. After evaporation, the residue was re-dissolved in THF (2 ml), and, after cooling (ice bath) pyrrolidine (132 µl, 112.5
5 mg, 1.58 mmol, 18 mol. equivalents) was added. After refluxing for 2 h, the reaction was cooled, diluted with EtOAc, and quenched with 2N H 2 SO 4. The organic phase was washed with brine, dried and evaporated. The residue was crystallized from ether, to afford 19.5 mg (60%) 1d as a yellow powder. IR (KBr) max cm -1 : 3422, 2361, 2340, 1617, 1139, 1126, 441, 428, 420, 414, 401; 1 H NMR (300 MHz, acetone-d 6 ): 7.58 (dd, J = 3.9 Hz, 2H), 7.01 (d, J = 6.0 Hz, 1H), 6.69 (s, 1H), 6.65 (s, 1H), 5.29 (t, J = 7.0 Hz, 1H), 3.99 (s, 3H), 3.36 (d, J = 7.2 Hz, 2H), 1.79 (s, 3H), 1.66 (s, 3H); 13 C NMR (acetone-d 6, 75 MHz): 183.2 (s), 164.7 (s), 162.4 (s), 160.2 (s), 156.6 (s). 151.3 (s), 148.8 (s), 135.4 (s), 131.6 (s), 125.2 (d), 123.7 (s), 123.2 (d), 121.2 (d), 116.3 (d), 112.4 (s), 110.5 (d), 105.3 (s), 104.4 (d), 94.1 (d), 56.6 (q), 40.5 (t), 25.8 (q), 22.0 (t), 17.7 (q); CI-EIMS: m/z [M+1] + 369 [C 21 H 20 O 6 +1] +. 2,4-Bis(tert-butyldmethylsilyl)-6-pivaloyl-3-prenylphloracetophenone (10): To a solution of 9 (150 mg, 0.32 mmol) in dry pyridine (5 ml), pivaloyl chloride (158 µl, 155 mg, 1.3 mmol, 4 mol. equiv.) and DMAP (157 mg, 1.28 mmol, 4 mol. equiv.) were added. After stirring at 40 C for 15 min., the reaction was worked up by dilution with 2N H 2 SO 4 and extraction with EtOAc. The organic phase was washed with brine, dried and evaporated, to afford 10 (170 mg, quantitiative), that was directly used for the next step. 1 H NMR (300 MHz, CDCl 3 ): 6.19 (s, 1H), 4.97 (t, J = 1.5 Hz, 1H), 3.09 (d, J =5.8 Hz, 2H), 2.44 (s, 3H), 1.63 (s, 3H), 1.61 (s, 3H), 1.25 (s, 9H), 0.98 (s, 18H), 0.22 (s, 6H), 0.19 (s, 6H). 4-tert-butyldmethylsilyl-2-pivaloyl-5-prenylphloracetophenone (11): To a solution of 10 (170 mg, 0.32 mmol) in CH 2 Cl 2 (1.2 ml), TFA (1.2 ml) and water (1.2 ml) were added. The reaction was stirred at 30 C for 3 h, and then neutralized by addition of solid NaHCO 3, diluted with water and extracted with CH 2 Cl 2. After drying, the residue was purified by gravity CC on silica gel (5 g, petroleum ether as eluant) to afford 100 mg (72%) of a white powder. Mp 57 C; IR (KBr) max cm - 1 : 3854, 3807, 3751, 3676, 1752, 1560, 1477, 1362, 1263, 1082, 819, 687; 1 H NMR (300 MHz,
6 CDCl 3 ): 6.30 (s, 1H), 4.93 (t, J = 1.5 Hz,1H), 3.07 (br d, 2H), 2.50 (s, 3H), 1.64 (s, 6H), 1.37 (s, 9H), 0.96 (s, 9H), 0.27 (s, 6H); 13 C NMR (CDCl 3, 75 MHz): 201.7 (s); 176.8 (s), 163.7 (s), 160.9 (s), 151.4 (s), 131.9 (s), 123.0 (d), 118.3 (s), 110.3 (s), 104.9 (d), 39.6 (s), 31,6 (q), 27.5 (q), 25,31 (q), 23,1 (t), 25.6 (q), 18,2 (q), 18.3 (s), -4.15 (q); CI-EIMS: m/z [M-1] 363 [C 24 H 38 O 5 Si-1]. 1-(4-tert-Butyldmethylsilyloxy-2-hydroxy-6-pivaloyloxy-3-prenylphenyl)-3-(4-pivaloyloxy-3- methoxyphenyl)-1,3-propandione (14): To a cooled (ice bath) sol. of 11 (270 mg, 0.41 mmol ) in THF (3 ml), NaH (60%, 52 mg, 0.83 mmol, 2 mol. equiv.) was added. After stirring at 0 C for 7 min, the solution was added via cannula to a solution of pivaloylvanilloyl chloride [prepared from 174 mg (0.69 mmol, 1.6 mol. equiv.) of pivaloyl vanillic acid], 3, and the mixture was refluxed, monitoring the formation of the intermediate O-acyl derivative by TLC (petroleum ether-etoac 9:1, Rf change from 0.50 to 0.34). After 35 min., the acylation was complete, 4 and the reaction was cooled to room temperature Further NaH (33 mg, 0.83 mmol, 2 mol. equiv) was then added, and refluxing was continued, monitoring the course of the reaction by TLC (petroleum ether-etoac 9:1, Rf change from 0.34 to 0.38). After 90 min, the Baker-Venkataraman rearrangement was over, and the reaction was worked up by cooling to room temperature, quenching with water, and extraction with EtOAc. The organic phase was dried and evaporated, and the residue was purified by gravity CC on silica gel (12.5 g, petroleum ether-etoac 95:5 as eluant) to afford 270 mg (66%) as a yellowish oil. IR (KBr) max cm -1 : 2960, 2929, 1758, 1577, 1508, 1479, 1258, 1103, 839, 783; 1 H NMR (300 MHz, CDCl 3 ): 7.5 (d, J =3.0 Hz, 1H), 7.43 (dd, J = 8.1, 3.0 Hz, 1H), 7.06 (d, J = 8.1, 1H), 6.02 (s, 1H), 5.15 (t, J = 6.9 Hz, 1H), 3.91 (s, 2H), 3.88 (s, 2H), 3.33 (d, J = 7.2 Hz, 2H), 1.89 (s, 3H), 1.84 (s, 3H), 1.40 (s, 9H), 1.02 (s, 9H), 0.94 (s, 9H), 0.28 (s, 6H), 0,21 (s, 6H). 7-(tert-Butyldimethylsilyl)-5,4 -Dipivaloylisocannflavin B (15b): To a soln of CuCl 2 (19 mg, 0.14 mmol, 0.5 mol. equivalents) in dry acetonitrile (19 ml), TMS-Cl (86 µl, 74 mg, 0.68 mmol,
7 2.4 mol. equiv.) was added slowly at room temperature, and the mixture was added via cannula to a solution of 14 (190 mg, 0.28 mmol) in dry acetonitrile (2 ml). After stirring at room temperature for 90 min., the reaction was worked up by dilution with water and extraction with EtOAc. After drying and evaporation, the residue was purified by gravity CC on silica gel (12.5 g, petroleum ether-etoac 95:5 as eluant) to afford 110 mg (60%) 15b as a yellowish foam. IR (KBr) max cm -1 : 3088, 2959, 1759, 1655, 1616, 1509, 1424, 1365, 1348, 1264, 1106, 890, 840, 811, 787, 539; 1 H NMR (300 MHz, CDCl 3 ): 7.5 (dd, J=9.9; 1H), 7.4 (br d, 1H), 7.1 (d, J=8.4, 1H), 6.6 (s, 1H), 6.3 (s, 1H), 5.2 (br t, 1H), 3.8 (s, 3H), 3.5 (d, J=6.3, 2H), 1.7 (s, 3H), 1.6 (s,3h), 1.3 (s, 9H), 1.02 (s, 9H), 0.3 (s, 6H). Isocannflavin B (1e): To a soln. of 15b (110 mg, 0.17 mmol) in THF (2 ml), TBAF (1M in THF, 250 µl, 0.25 mmol, 1.5 mol. equivalents) was added. After stirring at room temperature for 4 min., the reaction was worked up by dilution with water and extraction with EtOAc. After evaporation, the residue was re-dissolved in THF (1 ml), and, after cooling (ice bath), pyrrolidine (201 µl, 171 mg, 2.41 mmol, 18 mol. equivalents) was added. After refluxing for 2 h, the reaction was cooled, diluted with EtOAc, and quenched with 2N H 2 SO 4. The organic phase was washed with brine, dried and evaporated. The residue was purified by gravity CC on silica gel (10 g, petroleum ether-etoac 2:8 as eluant) to afford 45 mg (74%) 1e as a yellow powder. Mp 299 o C; IR (KBr) max cm -1 : 3853, 3676, 1560, 1508, 1297, 1181, 1095, 1030; 1 H NMR (300 MHz, methanol-d 4 ): 13.0 (s, 1H), 7.52 (dd, J =9.0, 2.5 Hz, 1H), 7.48 (d, J = 2.5 Hz, 1H), 6.93 (d, J = 9.0 Hz, 1H), 6.60 (s, 1H), 6.25 (s, 1H), 5.27 (t, J = 5.7 Hz, 1H), 3.93 (s, 3H), 3.52 (d, J = 6.9 Hz, 2H), 1.80 (s, 3H), 1.67 (s, 3H). 13 C- NMR (CDCl 3, 75 MHz): 182.6 (s), 164.4 (s), 161.3 (s), 160.1 (s), 155.2 (s), 150.6 (s), 148.1 (s), 131.5 (s), 123.1 (s), 122.7 (d), 120.5 (d), 115.6 (d), 111.7 (s), 109.7 (d), 105.0 (s), 103.4 (d), 98.5 (d), 55.7 (q), 24.9(q), 21.5 (t) 17.3 (q). CI-EIMS: m/z [M+1] 369 [C 21 H 20 O 6 +1], References
8 1. Anderson, N. G.; Lust, D. A.; Colapret, K. A.; Simpson, J. H.; Malley, M. E.; Gougoutas, J. Z. J. Org. Chem. 1996, 61, 7955-7958. 2. To monitor the reaction course by 1 H NMR spectroscopy, a few drops of the reaction mixture were filtered (Pasteur pipette) over a short pad of silica gel, evaporated and re-dissolved in CDCl 3. 3. A cooled (ice bath) solution of pivaloylvanillic acid in CH 2 Cl 2 (ca. 10 ml/g) was treated with SO 2 Cl 2 (4 mol. equiv), and cat. DMF. After stirring 1 h at 0 C, the cooling bath was removed, and stirring was continued at room temperature for 2 h. The solution was then evaporated and used directly. 4. If the reaction was quenched at this stage, the O-acyl derivative could be obtained and characterized. 1 H-NMR (300 MHz, CDCl 3 ): 7.76 (d, J=9, 1H), 7.68 (s, 1H), 7.07(d, J =9.0 Hz, 1H), 6.27 (s, 1H), 5.03 (t, J = 6.5 Hz, 1H), 3.86 (s, 3H), 3.17 (d, J = 6.3 Hz, 2H), 2.46 (s, 3H), 1.52 (s, 3H), 1.42 (s, 3H), 1.37 (s, 9H), 0.99 (s, 9H), 0.96 (s, 9H), 0.26 (s, 6H), 0.23(s, 6H). 13 C-NMR (CDCl 3, 75 MHz): 200.9 (s), 176.3 (s), 164.2 (s), 155.8 (s), 152.0 (s), 151.3 (s), 147.4 (s), 144.8 (s), 131.5 (s), 129.0 (s), 123.5 (d), 122.8 (d), 122 (s), 121.0 (d), 119.4 (s), 114.0 (d), 107.9 (d), 55.9 (q), 41.4 (s), 27.7 (q), 25.7 (q), 25.5 (q), 23.4 (q), 22.6 (t), 20.5 (q), 17.9 (s), -4.0 (q).