J. CHEM. SOC. PERKIN TRANS. I 1995 Cyclopenta[b]indoles. Part 1. Synthesis of cyclopenta[b]indoles by formal [3 + 21 addition of indolylmethyl cations to alkenes Carrie-Ann Harrison, Ralf Leineweber, Christopher J. Moody* and Jonathan M. J. Williams Department of Chemistry, Loughborough University of Technology, Loughborough, Leicestershire LEI 1 3TU, UK Treatment of indole-2- or 3-methanols with tin(1v) chloride as Lewis acid in the presence of styrenes or indene results in formal [3 + 23 addition of the indole stabilised cation to the alkene to give cyclopenta[b]indoles with a high degree of stereoselectivity; use of methylcyclohexene as the alkene component gave the cis-fused cyclopenta[b]indole 17, which was independently synthesised as its ( -)-enantiomer from the diketone 18.The cyclopenta[b]indole ring system occurs in a number of indole alkaloids, notably the structurally complex tremorgenic mycotoxins such as paxilline, paspaline, the lolitrems, penitrems and janthitrems, ’ and the monoterpenoid yueh- chukene 1.2We now report the details of a new approach to cyclopenta[b]indoles based on the formal [3 + 21 cyclo- addition of the stabilised cation derived from indole-2- or 3-methanols to alkene~.~ Results and discussion It is well known that on treatment with acids, 1H-indole-3- methanol 2 is readily converted into a stabilised cation which doH-QJ$N+ NH H 2 subsequently reacts further to give diindol-3-ylmethane. The same product is obtained by reaction of indole itself with formaldehyde, and further examples of ‘dimerisation’ reactions involving indole stabilised cations (derived from indolemethanols as above or by protonation of vinylindolesj are known.2a,2d,6 By analogy with the recently reported preparation of dihydroindenes by reaction of benzylic cations with styrenes, it seemed possible that, in the presence of a sufficiently reactive alkene, the ‘dimerisation’ of indole stabilised cations might be suppressed in favour of a formal [3 + 21 addition to the alkene to give cyclopenta[b]indoles as shown in Scheme 1. This indeed proved to be the case and the reaction provides a simple route to a range of cyclopenta[b]indoles. Thus, treatment of a mixture of 1 H-indole-3-methanol 2, or its N-methyl derivative 3, and prop-I-enylbenzene 4 with tin(1v) chloride at -78 “C gave, after aqueous work-up and chromatography, the desired cyclopenta[b]indoles 5 (55%) and 2 R‘ I.Scheme 1 6 (55%j, the trans-stereochemistry being proved by NOE difference spectroscopy. The use of alternative Lewis acids such as boron trifluoride-diethyl ether proved unsatisfactory. The substituted indole-3-methanol 7 reacts similarly with the styrene 4 to give the cyclopenta[b]indoles 8 and 9 (63% combined) as a 1: 10 mixture of diastereoisomers (Table 1). Thus, in common with the related reactions involving benzylic cation^,^ the formal [3 + 21 additions of indole stabilised cations with alkenes are highly stereoselective; not only is the original alkene stereochemistry preserved, but the new stereocentre at C-1 is also formed stereoselectively. Angle has speculated about the origin of the stereocentre in his related cationic cycli~ations,~but the exact mechanism remains unknown, although in the case of our indoles we assume that a spiro[cyclobutane- 1,3’-indolium] cation must be an inter- mediate (Scheme 1 j.8 The method was extended to other styrenes (the allylic ether 10 and indenej, and to the indole-2-methanols 13 and 14 to give the corresponding cyclopenta[b]indoles 11, 12, 15 and 16, stereospecifically albeit in poorer yield (Table I).Finally, the use of alkenes other than styrenes was investigated. Although no cyclopentaindoles could be isolated from reactions involving dihydropyran, 1-diethy laminocyclohexene or allyltriisopropyl- silane,7c*9 the use of 1 -methylcyclohexene gave the octahydro- indenoindole 17 in 18% yield (Scheme 2).The structure and cis-stereochemistry of 17 was confirmed by an independent synthesis of the (-j-enantiomer (-j-17, starting from the known cis-fused ketone 21,” prepared by reduction of the enantiomerically pure (R)-diketone 18 (Scheme 2). Thus, the (Rj-diketone 18, prepared by the (Dj- proline catalysed Robinson annelation of 2-methylcyclopent- ane- 1,3-dione with methyl vinyl ketone,12 was treated with sodium boranuide (NaBH,) under acidic conditions. ’‘9’ ’This resulted in complete reduction of the enone carbonyl, with 1128 J. CHEM. SOC. PERKIN TRANS. I 1995 Table 1 Synthesis of cyclopenta[b]indoles by reaction of indolemethanols with alkenes Indole Alkene Product Yield (%) ph * \ R 2 R=H 4 3 R=Me [E:Z=9:11 4doH 7 2 PhwOMe 10 2 moH' #RR 4 13 R=H 14 R=Me concomitant reduction of the 5-membered ring ketone to give the corresponding alcohol 19.Catalytic hydrogenation of the double bond resulted in formation of the cis-fused hexahydro- indane 20,accompanied by ca.4% of the trans-isomer; this was followed by pyridinium chlorochromate (PCC) oxidation to give the ketone 21. Finally, a Fischer indole reaction with phenylhydrazine gave the desired cyclopenta[b]indole 17 in 40% yield. Experimental For general experimental details, see ref. 14. Compounds characterised by high resolution mass spectrometry were chromatographically homogeneous.General procedure for cationic cyclisation Tin(1v) chloride (4 equiv.) was added slowly to a stirred solution of the indolemethanol (1 equiv.) and the alkene (2 equiv.; 5 equiv. in the case of methylcyclohexene) in dry dichloromethane under nitrogen at -78 "C. After the addition was complete, the mixture was stirred for 10 min at -78 OC, allowed to warm to room temperature, and stirred for a further 40 min (15 h for indolemethanol 13) when the solution was poured into saturated aqueous sodium hydrogen carbonate, and extracted 5 55 6 55 R 5 R=H 6 R=Me 8+9 63 (8:9= 1 :lo] 22 11 & 19 H 12 15 27 16 17 15 R=H 16 R=Me with dichloromethane.The extracts were washed with water, dried (MgSO,) and evaporated. The residue was purified by flash chromatography on silica gel to give the cyclopenta- [bl-indole. The following compounds were made by this method: 2-Methyl-3-phenyl-l,2,3,4-tetrahydrocyclopenta [b]indole 5. 55%, mp 155 OC (Found: M +,247.1360; C, ,H, ,N requires M, 247.1360); v,,,(CHCl,)/cm~' 3065, 920, 800 and 725; 6,(400 MHz; CDCl,) 1.34 (3 H, d, J7, CH,), 2.55 (1 H, dd, J 14 and 7, CH,), 2.85 (1 H, m, CHCH,), 3.19 (1 H, dd, J 14 and 7, CH,), 3.92(1 H,d, J7,CHPh),7.10-7.14(2H,m),7.2&7.35(5H,m), 7.50-7.53 (2 H, m) avd 7.63 (1 H, br s, NH); 6,(100.6 MHz; CDCl,) 19.91 (CH,), 32.74 (CHCH,), 50.91 (CH,), 53.88 (CHPh), 111.46, 118.65, 119.49, 120.77, 124.61, 126.66, 127.70, 128.55, 129.40, 140.62, 143.55 and 144.1 1; m/z 247 (M', loo%), 232 (45) and 218 (65).2,4-Dimethyl-3-phenyl-l,2,3,4-tetrahydrocyclopenta[bJ indole 6.55%, mp 79 "C (Found: Mf, 261.1501; C,,H,,N requires M, 261.15 17); v,,,(CHCl,)/cm-' 3019, 1466, 1368, 792, 735 and 705; 6,(250 MHz; CDC1,) 1.36 (3 H, d, J 7, CHCH,), 2.55 (1 H, dd, J 14 and 7, CH,), 2.88 (1 H, m, CHCH,), 3.20 (I H, m, CH,), 3.33 (3 H, S, NCH,), 3.95 (1 H, d, J7, CHPh), 7.16-7.35 (8 H, m) and 7.54 (1 H, d, J 7); m/z 261 (M', loo%), 246 (38), 232 (35), 170 (28), 115 (27) and 69 (53). J. CHEM. SOC. PERKIN TRANS. I 1995 doH@H iNH 1 17 0Po 18I ii H 0gl21 iiia- HO' HO' ' 19 20 Scheme 2 Reagents and conditions: i, 1-methylcyclohexene, SnCl,, CH,CI,, -78 OC; ii, NaBH,, CF,CO,H; iii, H,, Pd-C, EtOH; iv, PCC, CH,CI,; v, PhNHNH,, H+ 1,2-Dimethyl-3-phenyl-1,2,3,4-tetrahydrocyclopenta[b]indole 9.63% (with 8), mp 118 "C (Found: M', 261.1495; C19H19N requires M, 26 1.1517); v,,,(CHCl,)/cm~' 30 19, 793 and 674; NMR data for major isomer: 6,(400 MHz; CDC1,) 1.18 (3 H, d, J7, CH,). 1.22 (3 H, d, J7, CH,), 2.87 (1 H, m, CHCH,), 3.42 (1 H,m,CHCH,),3.94(1 H,d, J7,CHPh),7.1&7.14(2H,m), 7.21-7.36 (6 H, m), 7.53-7.56 (1 H, m) and 7.59 (1 H, br s, NH); 6,(100.6 MHz; CDCI,) 14.08 (CH,), 15.54 (CH,), 35.67 (CHCH,), 52.02 (CHCH,), 53.86 (CHPh), 111.52, 118.37, 119.44, 120.71, 125.43, 126.66, 127.84, 127.92, 128.50, 140.40, 142.90and 143.20;m/z 261 (M', 8673,246 (loo), 217 (29), 170 (30) and 129 (28).2-Methoxymethyl-3-phenyl-l,2,3,4-tetrahydrocyclopenta[b]-indole 11. 22%, mp 56 "C (Found: M', 277.1452; C19H19N0 requires M, 277.1466); v,,,(CHCl,)/cm~' 3065, 794, 703 and 675; dH(25O MHz; CDCI,) 2.69 (1 H, dd, J 12 and 6, CH,), 3.06 3.21 (2 H, m, CH, and CHCH,O), 3.39 (3 H, s, OCH,), 3.60 (2 H, d, J6, CH,O), 4.25 (1 H, d, J6, CHPh), 7.10-7.31 (8 H, m), 7.49 (1 H, m) and 7.69 (1 H, br s, NH); m/z 277 (M+, 5973,244 (28), 232 (100) and 115 (33). 5a,6,7,11 c-Tetrahydro-5H-indeno [2',1';4,5]cyclopenta- [1,2-b]indole 12. 19%, mp 54°C (Found: M+, 245.1206; C1,HI 5N requires M, 245.1204); v,,,(CHCl,)/cm-' 3067. 793, 723 and 665;6,(400 MHz; CDCI,) 2.72 (1 H, dm, J 14.5,indole-CH,), 2.91 (1 H, dd, J 16.5 and 6, CH,Ph), 3.2 (1 H, dd, J 14.5 and 7.6, indole-CH,), 3.40 (1 H, dd, J 16.5 and 9.5, CH,Ph), 4.71 (1 H, d, J7.6, CH,PH), 3.85-3.90 (1 H, m, CH,CHCH,), 7.04-7.44 (8 H, m) and 7.90 (1 H, br s, NH); 6,(100.6 MHz; CDC1,) 3 1.8 1 (CH, CHCH,), 40.29, 48.17, 50.68 (indole-CHPh), 111.43, 118.64, 119.50, 120.71, 123.67, 124.93, 126.43, 126.46, 126.96, 142.29 and 143.49; m/z 245 (M+, loo%), 230 (18), 217 (21) and 130 (92).2-Methyl-1-phenyl-l,2,3,4-tetrahydrocyclopenta[b]indole 15. 27%, mp 89 "C (Found: M', 247.1375; C,,H,,N requires M, 247.1360); ~,,,(CHC~,)/C~-~3020, 1220, 775, 725 and 669; dH(250 MHz; CDCI,) 1.36 (3 H, d, J7, CH,), 2.62 (1 H, dd, J 15 and 7, CH,), 2.80 (1 H, m, CHCH,), 3.18 (1 H, dd, J 15 and 7, CH2),3.96(1H,d, J6.6,CHPh),6.9&7.40(9H,m)and7.90(1 H, br s, NH); m/z 247 (M+, loo%), 232 (35), 218 (30) and 170 (30).2,3,3-Trimethyl-1-phenyl-l,2,3,4-tetrahydrocyclopenta[b]-indole 16.17%, mp 166 "C (Found: M+, 275.1677; requires M, C20H21N 275.1674); v,,,(CHCl,)/cm~' 3019, 1220, 791, 724 and 665; 6,(400 MHz; CDCI,) 1.1 1 (3 H, d, J 6.7, CHCH,), 1.16 (3 H, s, CCH,), 1.38 (3 H, s, CCH,), 2.33 (1 H, m, CHCH3), 3.87 (1 H, d, J8.8, CHPh), 6.96 (1 H, t, J6), 7.08, (1 H,d,J6),7.09(1H,dt,J7and1.5),7.27(lH,m),7.32(5H,m) and 7.83 (1 H, br s, NH); 6,(100.6 MHz, CDCl,) 11.49, 22.41, 26.26, 41.48, 51.27, 61.10, 111.37, 117.59, 118.80, 119.41, 120.46, 124.37, 126.08, 127.93, 128.08, 140.02, 143.92 and 151.45;m/z 275 (M+,loo%), 260 (68) and 167 (27).4a-Methyl-1,2,3,4,4a,5,lO,lOa-octahydroindeno[1,241indoie 17.IS%, Identical (except for rotation) with the sample prepared by the alternative route; data given below. (-)-(l R,7aR)-7a-Methyl-5,6,7,7a-tetrahydroindan-1-01 19 Dry trifluoroacetic acid (28 an3,0.36 mol) was added dropwise with stirring to a suspension of sodium boranuide (NaBH,) (4.4 g, 0.12 mol) in dry acetonitrile (36 cm3) and dry dichloromethane (28 cm3) at -40 "C under an atmosphere of nitrogen so that the temperature did not exceed -10 "C. The solution was then re-cooled to -40 "C and a solution of the diketone 18,'* (5 g, 0.03 mol) in dry dichloromethane (44 cm3) was added dropwise to it with stirring. The mixture was stirred at -40 "C for 2 h, allowed to warm to ambient temperature, and the stirring was continued overnight.The mixture was then neutralised with aqueous sodium hydroxide (4 mol drn-,; ca. 80 cm3) and extracted with dichloromethane (3 x 50 cm3). The combined dichloromethane extracts were washed with aqueous sodium hydroxide (2 mol dm-,; 100 cm3), dried (MgS0,) and evaporated under reduced pressure to give the title compound 19 (3.9 g, 82%) as a colourless solid, mp 84-86 "C (from diethyl ether-light petroleum) [lit.,', mp 86-88 "C for (1S,7aS)-isomer], [a];* -65 (c 0.5, CHCI,) {lit.,', [XI?+75 (c 1.0, CHCl,) for (lS,7aS)-isomer); v,,,(CH,Cl,)/cm-' 3362, 2965, 2852 and 1676; 6,(250 MHz; CDCI,) 0.94 (3 H, s, CH,), 1.15 (1 H, m), 1.68 (4H, m), 1.77 (1 H, m), 1.97 (4 H, m), 2.43 (1 H, m), 3.65 [l H, dd, J7.8 and 1.9, CH(OH)] and 5.35 (1 H, d, J2.9, CXH); 6,(62.5 MHz; CDCl,) 16.76 (CH,), 18.70 (CH,), 25.08 (CH2),25.87(CH,),29.39(CH,), 34.46(CH2),42.91 (C),82.23 [CH(OH)], 119.47 (WH) and 144.13 (C, C=CH); m/z 152 (M+, 4373, 134 (28), 110 (25), 108 (52) and 93 (100).(1R,3aR,7aR)-7a-Methylhexahydroindan-1-01 20 Palladium-on-carbon (10%; 0.2 g) and a catalytic amount of hydrochloric acid (2 mol dm-,) were added to a solution of the alcohol 19 (2 g, 0.013 mol) in ethyl acetate (20 cm3). The system was stirred at room temperature under an atmosphere of hydrogen (45 psi) overnight t after which it was filtered through Celite and washed with water. The ethyl acetate layer was dried (MgSO,) and evaporated under reduced pressure. The residue was distilled at 80-85 "C at 0.5 mmHg to give the tide compound (1.7 g, 72%) as a colourless semi-solid, [ct];' -14 (c 0.44, CHCl,) {lit.," [a];4 +20 (c 1.0, CHC1,) for (1S,3aS,7aS)-isomer}; v,,,(film)/cm-' 3395, 2926 and 2864; 6,(250 MHz; CDCI,) 0.98 (3 H, s, CH,), 1.16 (1 H, m), 1.45 (10 H, m), 1.79 (2 H, m), 2.18 (1 H, m) and 3.83 [l H, dd, J3.7 and 3.2, CH(OH)]; 6,(62.5 MHz; CDCI,) 19.56 (CH,), 21.71 (CH,), 22.17 (CH,), 25.90 (CH,), 26.09 (CH,), 31.33 (CH,), 31.78 (CH,), 41.85 (CH), 44.01 (C) and 79.93 [CH(OH)]; m/z 154 (M', 64%), 110 (80)and 95 (100).The product was contaminated with ca. 4% of the trans-(lR,3aS,7aR)-isomer; 6,(250 MHz; CDCI,) 0.74 (3 H, s, CH,) and 3.65 [l H, M, CH(OH)]; other peaks obscured. (3aR,7aR)-7a-Methylhexahydroindan-l-one 21 A mixture of the alcohol 20 (1.3 g, 8.4 mmol) and pyridinium chlorochromate(2.7 g, 12.6mmol) in dichloromethane (30 cm3) was stirred at room temperature for 90 min.The mixture was t 1 psi = 6.89 x lo3Pa. filtered through Celite and washed with water. The dichloromethane layer was dried (MgS0,) and the solvent removed under reduced pressure to give a brown residue, which was subjected to chromatography (silica, dichloromethane- light petroleum, 2 :1) which gave the title compound 21 (1.2 g, 92%) as a pale yellow liquid, [a];' -53 (c 0.6, CHCl,) {lit.," [a]A4 -57 (c 1.O, CHCI,)); v,,x(film)/cm-l 2957,2859 and 1736; 6,(250 MHz; CDCl,) 1.04 (3 H, s, CH,), 1.18 (1 H, m), 1.41 (5 H, m), 1.66 (3 H, m), 1.91 (2 H, m) and 2.37 (2 H, m); 6,(62.5 MHz; CDCl,) 20.92 (CH,), 22.14 (CH,), 22.35 (CH,), 23.03 (CH,), 26.72 (CH,), 29.50 (CH,), 34.95 (CH,), 42.54 (CH), 48.30 (C) and 222.89 (CO); m/z 152 (M', 20%),110 (35), 96 (43) and 8 1 (100).The product was contaminated with ca. 4% of the trans-(3aS,7aR)-isomer; S,(250 MHz; CDCl,) 0.87 (3 H, s, CH,); other peaks obscured. (4aR, 1 OaR)4a-Methyl-l,2,3,4,4a,5,10,1Oa-octahydroindeno-[1,2-b]indole 17 A mixture of the ketone 21 (1.2 g, 8 mmol) and phenylhydrazine (0.8 cm3, 8 mmol) was refluxed overnight in dry toluene (10 cm3) using a Dean-Stark apparatus. After removal of the solvent, the resulting hydrazone was refluxed in ethane- 1,2-diol (13 cm3) for 5 h. The mixture was cooled, then diluted with water and extracted into diethyl ether. The combined diethyl ether extracts were washed with hydrochloric acid (2 mol drn-,) and water, dried (MgSO,) and evaporated under reduced pressure to give a brown residue, which was subjected to chromatography (silica, dichloromethane-light petroleum, 2: 1) to give the title compound (0.71 g, 40%) as colourless crystals, mp 95-96°C (from light petroleum), [a];' -41 (c 0.46, CHC1,) (Found: C, 85.4; H, 9.1; N, 6.4%; M', 225.1523; C,,H,,N requires C, 84.8; H, 9.0; N, 6.2%; M, 225.1517); v,,,(CH2C1,)/cm-' 3401, 3060, 2926 and 2851; 6,(250 MHz; CDC1,) 1.29 (3 H, s, CH,), 1.42 (4 H, m), 1.58 (4 H, m), 2.49 (1 H,m,CH),2.51(1H,m),2.85(1H,m),7.07(2H,m),7.26(1H, m), 7.44 (1 H, m) and 7.63 (1 H, br s, NH); 6,(62.5 MHz; CDCl,) 21.67 (CH,), 22.82 (CH,), 24.54 (CH,) 27.26 (CH,), 28.89 (CH,), 35.36 (CH,), 41.28 (C), 50.60 (CH), 111.49 (CH), 115.55 (C), 118.51 (CH), 119.45 (CH), 120.14 (CH), 125.51 (C), 139.75 (C) and 151.89 (C);m/z 225 (M', 77%) and 210 (100). The product was contaminated with ca.4% of the trans-(4aR,lOaS)-isomer; 6,(250 MHz; CDCl,) 0.98 (3 H, s, CH,); other peaks obscured. Acknowledgements We thank the SERC for their support of this work, Dr 0. Howarth at the SERC NMR Service at Warwick for 400 MHz NMR spectra, and Professor E. Winterfeldt, Dr G.W. Weaver J. CHEM. SOC. PERKIN TRANS. I 1995 and Dr G. Sauer for helpful information concerning the reduction of the diketone 18. References 1 For a review, see: P. S. Steyn and R. Vleggar, Fortschr. Chem. Org. Naturst., 1985,48, 1.2 (a)K.-F. Cheng, Y.-C. Kong and T.-Y. Chan, J. Chem. Soc., Chem. Commun., 1985, 48; (b)E. Wenkert, P. D. R. Moeller, S. R. Piettre and A. T. McPhail, J. Org. Chem., 1987, 52, 3404; (c) J. P. Kutney, F. P. Lopez, S.-P. Huang, H. Kurobe, R. Flogans, K. Piotrowska and S. J. Rettig, Can. J. Chem., 1991, 69, 949; (d) J.-H. Sheu, Y.-K. Chen and Y.-L. V. Hong, Tetrahedron Lett., 1991, 32, 1045; (e) J. Bergman and L. Venemalm, Tetrahedron, 1992, 48, 759; (f) K. J. Henry and P. A. Grieco, J. Chem. Soc., Chem. Commun., 1993, 510; (8) K.-F. Cheng, G.-A. Cao, Y.-W. Yu and Y.-C. Kong, Synth. Commun., 1994,24,65. 3 For other approaches to cyclopentaindoles, see: J. Bergman and J. E. Backvall, Tetrahedron, 1975, 31, 2063; J. Bergman, L.Venemalm and A. Gogoll, Tetrahedron, 1990,46,6067. 4 Preliminary communication: C.-A. Harrison, R. Leineweber, C. J. Moody and J. M. J. Williams, Tetrahedron Lett., 1993, 34, 8527. 5 R. J. Sundberg, The Chemistry of Indoles, Academic Press, New York, 1970. 6 For example, see: J. Bergman, S. Hogberg and J.-0. Lindstrom, Tetrahedron, 1970,26, 3347; L. Pfeuffer and U. Pindur, Helv. Chim. Acta, 1988, 71, 467; J. Bergman, P.-0. Norrby, U. Tilstam and L. Venemalm, Tetrahedron, 1989,45, 5549; M. Eitel and U. Pindur, J. Org. Chern., 1990, 55, 5368; D. StC. Black, D. C. Craig and N. Kumar, Tetrahedron Lett., 1991,32, 1587. 7 (a) S. R. Angle and D. 0. Arnaiz, J. Org. Chem., 1992, 57, 5937; (b) S. R. Angle and R. P. Frutos, J. Org. Chem., 1993, 58, 5135; (c) S.R. Angle and J. P. Boyce, Tetrahedron Lett., 1994,35, 6461. 8 The intermediacy of a spiro[cyclobutane- 1,3'-indolium] cation in the cyclisation reaction of an indole-3-propanol derivative using methanesulfonyl chloride and triethylamine has been demonstrated recently; A. Ganesan and C. H. Heathcock, Tetrahedron Lett., 1993, 34,439. 9 For uses of allylsilanes in cyclopentannulation reactions, see: R. L. Danheiser, B. R. Dixon and R. W. Gleason, J. Org. Chem., 1992,57, 6094; J. S. Panek and N. F. Jain, J. Org. Chem., 1993,58,2345. 10 G. Demailly and G. Solladie, Bull. SOC.Chim. Fr., 1975,2128. 11 For another use of sodium boranuide-trifluoroacetic acid in the reduction of ketones, see: G. W. Gribble, W. J. Kelly and S. E. Emery, Synthesis, 1978, 763. 12 Z. G. Hajos and D. R. Parrish, Org. Synth. Coll. Vol. 7,363; U. Eder, G. Sauer and R. Wiechert, Angew. Chem., Int. Ed. Engl., 1971, 10, 496; S. D. Rychnovsky and D. E. Mickus, J. Org. Chem., 1992,57, 2732. 13 E. Winterfeldt and G. W. Weaver, personal communication. 14 C. J. Moody and E. Swann, J. Chem. Soc., Perkin Trans. I, 1993, 2561. Paper 4/07036D Received 17th November 1994 Accepted 15th December 1994
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