An improved route to 1,2,4-trioxanes using tin(IV) as a hydrogen equivalent

摘要

J. CHEM. SOC. PERKIN TRANS. 1 1992 An Improved Route to 1,2,4-Trioxanes Using Tin(1v) as a Hydrogen Equivalent Jiaqiang Cai and Alwyn G. Davies" Chemistry Department, University College London, 20 Gordon Street, London WC7 H OAJ, UK Bloodworth's route to the 1,2,4-trioxanes has been duplicated using tin(iv) as a hydrogen equivalent throughout. Thus a tetraallyltin compound is treated with singlet oxygen to give a tetraallylperoxytin compound; this adds to an aldehyde to give the tin derivative of a peroxyhemiacetal, and this tin alkoxide undergoes ring-closing intramolecular addition to the olefinic group in the presence of mercury(ii) acetate to give the 1,2,4-trioxane. The antimalarial activity of the plant extract Qinghaosu 1 is associated with the presence of the 1,2,6trioxane ring 2,' and a substantial effort has been devoted to developing new synthetic routes to these cyclic peroxide^.^^^ M e y0 0, M e 0-p "H Lo Me 2 0 1 Bloodworth and his colleagues3 have formed the ring by preparing a peroxyhemiacetal from an allylic hydroperoxide and an aldehyde, followed by intramolecular oxymercuration to close the ring, then reductive demercuration (Scheme 1).RI amp;02H RCHO --HORcHbsol;OHORcHbsol;O R2 W R2{ 'R3 R R I I AcOHg R' R2 R3 R Me Me Me Me, Et, Pr, Pri,Bur,2-NOamp;H4, 4-CIC6H, H Ph H Me, Pri,But, C13C Scheme 1 The two key steps in building the ring involve reactions of OH nucleophiles. We have been interested in the principle that metal substituents often simulate the behaviour of the hydrogen which they replace: and that by a judicious choice of the metal and its ligands, metals may with advantage be used as hydrogen equivalent^.^ We report here such a modification of Blood-worth's methodology as shown in Scheme 2.Results Tetrapropenyltin 3a and tetra(2-methylprop-2-enyl)tin3b were prepared in greater than 80yield from tin tetrachloride and Me 3 4 7 6 s7a R1 = H, R2 = H, R3 = H b R1 = Me, Rz = H, R3 = H c R1 = H, R2 = Me, R3 = H 3-9 R' = H,R2,R3 = -(CH2)3-Scheme2 the appropriate Grignard reagent. The compound 3q6 which was derived from the Grignard reagent from crotyl chloride, might be expected to contain a mixture of E-and Z-but-2-enyl, and l-methylprop-2-enyl groups, but the NMR spectra showed that it contained only but-2-enyl groups, and this was confirmed by the spectra of the derived peroxides 4c, 5c and 6c (see below).Tetra(cyc1ohex-2-eny1)tin 3d was obtained in 80 yield by metallation of cyclohexene with butyllithium and potassium tert-butoxide in the presence of tetramethylethylenediamine,5b*7 then reaction with SnC1,. The reaction of tetra(prop-2-eny1)tin 3a with singlet oxygen, generated from triplet oxygen and tetraphenylporphine in the presence of sodium light, to give tetra(prop-2-enylperoxy)tin(1v) 4a, has already been reported.5cCompounds 3b-d reacted in the same way to give the corresponding peroxides 4b-d in quantitative yield (NMR). The tetra(ally1peroxy)tin compounds 4a-d reacted with acetaldehyde immediately on mixing at room temperature to give the corresponding stannylated peroxyhemiacetals 5a4, again in quantitative yield.The NMR spectrum showed that 5c consisted of two diastereoisomers in approximately equal amount, but 5d showed the presence of only one isomer. The peroxyhemiacetals 5a-c were then stirred with mercuric acetate in dichloromethane for 5-8 hours, when the Hg(OAc), dissolved to give the mercurated trioxanes 6a-c; no reaction occurred with the cyclohexene derivative 5d under these conditions. The mercurated trioxanes 6a-c were demercurated by J. CHEM. SOC. PERKIN TRANS. 1 1992 Bloodworth's method of reduction with sodium borohydride in sodium hydroxide and the trioxanes 7a-c were isolated by bulb-to-bulb distillation followed by column chromatography to remove the allylic alcohol which was also formed.The overall yields of the trioxanes 7a-c were 25-30 based on the allylic tin compounds 3a-c. As Bloodworth showed,," the carbonyl addition, ring closure, and reduction can be carried out in a one-pot procedure. The trioxane 7a contains two chiral centres, and the proton NMR spectrum showed the presence of two diastereoisomers in the relative concentrations 5:l. The major isomer shows 3J5+d.~10.2 Hz and thus has 5-Me equatorial. To confirm this, the organomercury precursor, which was normally reduced in situ, was isolated as its bromide, 6a'. Again two isomers were present in the ratio ca. 5: 1, with 3J5ax-H4a,.H 9.58 Hz.7a 6a' The trioxane 7c contains three chiral centres, and showed the presence of two, rather than four, isomers, in the ratios 42: 58. The minor isomer has 5-Me, 6-Me and 3-Me (see below) equatorial, with 3J5.,.H4,,.H 8.8 Hz, and the major isomer has 5-Me axial, with 3Jsc,-H4ax-H3.1 Hz. The configuration of the trioxanes at C-3 was determined by 13C NMR spectroscopy in a separate study in collaboration with Drs. J. E. Anderson and A. J. Bloodworth, and included further trioxanes prepared in Dr. Bloodworth's research group.3 By NOE experiments, we showed that the value of '.IC-, at the C-3 position in trioxanes lies in the range 166.8-169.3 Hz for axial protons, and 163.5-164.8 Hz for equatorial protons. These compounds therefore show a reversal of the usual Perlin effect.* By this criterion, all the trioxanes prepared here from acetaldehyde have the methyl group at C-3 in the equatorial position, as indicated in the formulae.This work has been published ~eparately.~ For this ancillary study we needed to obtain a trioxane with both axial and equatorial protons on C-3, and to this end, tetra- (ally1peroxy)tin was added to formaldehyde, then the ring- closing oxymercuration was carried out in the usual way. By chromatography, both 5-bromomercurimethyl- 1,2,4-trioxane 8 and 4-bromomercurimethyl- 1,3-dioxolane 9 were isolated each in about 10 yield (Scheme 3). This partial reduction of the peroxide is not observed with acetaldehyde. To confirm the identity of 9, it was also prepared by acetoxymercurative ring closure of the hemiacetal formed between allyl alcohol and formaldehyde (Scheme 3).r 1 I I li + CHzO --/vAgBr 9 Scheme3 Reagents: i, Hg(OAc),; ii, NaBr Discussion A number of aspects of these syntheses illustrate the potential advantages of using metals as hydrogen equivalents. Bloodworth prepared the 1,1,2-trimethylprop-2-enylhydro-peroxide for Scheme 1 by treating 2,3-dimethylbut-2-ene with singlet ~xygen,~" but this process would not be convenient for the oxygenation of propene, 2-methylpropene, or but-2-ene which are gases. The allyltin compounds 3a-d on the other hand are easy to handle, are much more reactive than the hydrocarbons towards singlet oxygen,' and the allylperoxytin compounds which are formed appear to be safer to handle than the allyl hydroperoxides themselves, the lower members of which can be dangerously explosive.It is important that these tetraallyl compounds 3a-d react with singlet oxygen to show only the metalloene reaction, whereas tri(butylally1)tin compounds give also the products of the hydrogen-ene reaction, and of cycloaddition with shift of the metal. We have suggested 5c that this chemoselectivity may be the result of 7c-o conjugation between the W double bonds and the CH,-Sn bonds, which enhances the electropositivity of the tin, and this is conducive towards the metalloene reaction. After the reaction of the first of the allyl groups, the electro- negative allylperoxy ligand will further enhance the electroposi- tivity of the tin, and favour the metalloene process.Tin alkoxides are known to simulate the behaviour of alcohols in adding to carbonyl groups." Nothing appears to have been published on the comparison between the behaviour of tin peroxides and hydroperoxides towards carbonyl com- pounds, but from the little we have done here, the analogy appears to be close. The ring closure is the most interesting step in our synthesis, as it establishes that alkoxytin(1v) compounds simulate the behaviour of alcohols ' in the oxymercuration of alkenes. But whereas the addition of alcohols often needs an acid ~atalyst,~" our reactions proceed smoothly in the absence of a catalyst. This suggests that the scope of the hydroxy-,12 alkoxy-," hydroperoxy-' and alkylperoxy-mercuration l4 reactions, which are used in organic synthesisI5 should be widened considerably by the judicious use of a metallic derivative.Experimental 'H and 13C NMR spectra were recorded on CDCl, solutions on a Varian VXR-400 spectrometer unless otherwise stated; a Varian XL-200 instrument was used for those compounds which were thermally unstable and for which the spectra had to be recorded immediately. Chemical shifts were measured relative to the solvent using 6, 7.24 and 6, 77.00. Complete analysis of the 'H NMR spectra of the tin(rv) compounds was sometimes not possible, because the four ligands each contained a number of chiral centres, giving rise to a number of diastereoisomers with overlapping spectra.Coupling constants are in Hz. IR spectra were recorded on a Perkin-Elmer PE983 instruments, and mass spectra on a VG7070H spectrometer at 70 eV. Column chromatography was carried out on Merck silica gel 60 (70-230 mesh). Allylic Tin Compounds 3a-c. General Procecure.-A solution of allyl chloride or bromide, methallyl chloride, or crotyl bromide (20 mmol) in THF (20 cm3) was added dropwise to magnesium (20 mmol) in THF (100 cm3) under nitrogen, with stirring and ice-cooling to keep the temperature below the b.p. of THF. After all the allylic halide had been added, the solution was stirred for a further 2 h. A solution of SnC1, (3 mmol) in hexane (10 cm3) was then added dropwise at room temperature. After work-up, the crude product was chromatographed on silica gel using light petroleum (b.p.30-40 "C) as eluent. The yields and characteristics of the products were as follows. J. CHEM. SOC. PERKIN TRANS. 1 1992 Tetraprop-2-enyltin 3a.5c Yield 8595. Tetra(2-methylprop-2-enyl)tin3b.I6 Yield 85-95; 6, 1.69 (12 H, t, J 1.1, JSn11.7, Me), 1.9 (8 H, s, J1liSn 60.7, Jl19sn 63.2, CH,), 4.5 (4 H, s, J,, 21.5, olefinic) and 4.5 (4 H, s, Jsn21.5, olefinic); 6, 21.70 (.Isn254.8, C-1), 25.02 (.Isn10.0, C-4), 107.66 (Jsn 45.6, C-3) and 144.53 (Jsn 43.1, C-2). v,,,(neat)/cm-' 3069.4, 2967.7, 2925.9, 1622.7, 1437.3, 1371.5, 1275.8, 1111.3, 991.7, 973.8, 863.1, 824.3, 755.5 and 707.6; m/z 285 (M -methylpropenyl, 27), 230 (M -2 x methylpropenyl, 5), 175 (M -3 x methylpropenyl, 100), 120 (Sn, 20) and 55 (methylpropenyl, 32); this pattern of fragmentation has been observed before for allylic tin compounds.' ' Tetra(3-methylprop-2-enyl)tin 3c.'* Yield 35; 6, 1.57-1.65 (12 H, Me), 1.75-1.84 (8 H, CH,), 5.15-5.32 (4 H, olefinic) and 5.47-5.65 (4 H, olefinic); 6c 10.79, 11.03, 11.24, 12.47 (Me); 14.70, 14.95, 15.16, 17.87 (CH,); 118.84, 118.99, 119.14, 119.29 (olefinic); 121.07, 121.19, 121.31,121.43 (olefinic); 127.97, 128.03, 128.10 (olefinic); and 128.83, 128.90, 128.97 (olefinic); v,,,(neat)/cm-' 3009.6, 2949.8, 2919.9, 1652.6w, 1637.6, 1449.2, 1392.4, 1356.5, 1156.2, 1093.4, 1066.5, 991.7, 958.8 and 719.6; m/z 285 (M -methylpropenyl, 25), 230 (M -2 x methylpropenyl, 5) and 175 (M -3 x methylpropenyl, 100).Tetrahex-2-enyltin 3d.--Cyclohexene was metallated as described previou~ly,~~ then SnCl, (3 mmol) in hexane (10 cm3) was added dropwise with stirring. After work-up, the crude product was chromatographed on silica gel using light petroleum as eluent; yield 80; 6, 1.27-2.22 (24 H, m), 2.46 (4 H, m, CHSn), 5.46 (4 H, m, olefinic) and 5.84 (4 H, m, olefinic); 6, 23.67, 25.00, 26.85, 28.63 and 28.47 (CHSn) and 122.72, 130.97 and 131.05 (olefinic); m/z 363 (M -cyclohexenyl, 5), 282 (M -2 x cyclohexenyl, 2), 201 (M -3 x cyclohexenyl, 90) and 81 (cyclohexenyl, 100). Tetra(ally1peroxy)stannanes4a-d.-The photoxidations were carried out in CH,Cl, or CHCl, in a temperature-controlled cell, using a 400 W sodium lamp, and tetraphenylporphine as the photosensitizer, on 2.5 mmol of the stannane, as describe previ~usly.~Reactions were also carried out on an analytical scale in CDCl, in an NMR tube (200 MHz).The yields of the peroxides 4a-d were quantitative (NMR). Tetra@rop-2-enylperoxy)tin 4a. 6, 4.49 (8 H, d, J 6.2, CH,), 5.30 (4 H, dd, J 1.4 and 5.6, 3-H), 5.37 (4 H, dd, J 1.4 and 12.8, 3'-H) and 5.97 (4 H, ddt, J 5.6, 12.8 and 6.2,2-H). Tetra(2-methylprop-2-enylperoxy)tin4b. 6, 1.81 (12 H, s, Me), 4.42 (8 H, s, CH,), 5.03 (4 H, s, 3-H) and 5.04 (4 H, s, 3'-H). Tetra(1-methylprop-2-enylperoxy)tin4c. 6, 1.25 and 1.26 (12 H, 2 d, J6.5, Me), 4.30 and 4.48 (4 H, 2 q, J6.5, 1-H), 5.02- 5.34 (8 H, m, 3-H and 3'-H) and 5.75-5.90 (4 H, m, 2-H).Tetra(cyc1ohex-2-enyIperoxy)tin4d.6, 1.10-2.20 (24 H, m), 4.46 (4 H, m, 1-H), 5.73 (4 H, m, olefinic) and 5.98 (4 H, m, olefinic). Peroxyhemiacetals 5a-d.-The peroxyhemiacetals were formed in quantitative yield (200 MHz NMR) when acetalde- hyde (3 equiv.) was added to the allylperoxytin compound. Tetrakisc 1 -@rop-2-enylperoxy)ethoxy tin 5a. 6, 1.24 (12 H, d, J 5.4, Me), 4.49 (8 H, dm, J 6.1, CH,), 5.22 (4 H, m, olefinic), 5.27 (4 H, m, olefinic), 5.36 (4 H, q, J 5.5, MeCH) and 5.93 (4 H, m, olefinic). TetrakisC1-(2-methylprop-2-enylperoxy)ethoxytin 5b. 6, 1.14 (12 H, d, J 5.5, MeCH), 1.66 (12 H, s, Me), 4.32 (8 H, s, CH,), 4.82 (4 H, s, olefinic), 4.86 (4 H, s, olefinic) and 5.28 (4 H, q, J 5.5, MeCH). Tetrakisc 1 -(1-methylallylperoxy)ethoxy tin 5c.6, 1.18-1.29 (24 H, m, Me) 4.56 (4 H, m, CH,CH), 4.99-5.42 (12 H, m, olefinic and MeCH) and 5.73-5.97 (4 H, m, olefinic). TetrakisC1-(cyclohex-2-enylperoxy)ethoxytin 5d. 6, 1.2 1 3385 (12 H, d, J 5.4, CH3CH) 1.10-2.20 (24 H, m), 4.50 (4 H, m, CHO,) 5.34 (4 H, q, J 5.4, CH3CH), 5.69 (4 H, m, olefinic) and 5.90 (4 H, m, olefinic). 1,2,4- Trioxanes 6a-c and 7a-c. General Procedure.-A mixture of mercury(I1) acetate (10 mmol) and a solution of the tetrakis( 1-ally1peroxyethyoxy)tin compound 5 (2.5 mmol) in CH,Cl, was stirred at room temperature. Dissolution of the mercury acetate was complete in 5-8 h. The solvent and the excess of acetaldehyde was removed under reduced pressure. The residue was treated with CH2C1, then with 2 mol dm-, NaOH solution (10 cm3) and with NaBH, in 2 mol dm-3 NaOH (30 cm3) at 0deg;C.After work-up, the crude product subjected to bulb-to-bulb distillation, then was chromato-graphed on silica gel with pentane-CH,Cl, (2:l) as eluent. The following trioxanes were obtained. 3,5-Dimethyl- 1,2,4-trioxane 7a. Yield 25-30 (CH2C12 solvent), 3540 (CHCl, solvent). Major (cis) isomer: 6, 1.16 (3 H, d, J6.2,5-Me,,), 1.26 (3 H, d, J5.4, 3-Meeq), 3.85 (1 H, dd, J2.5 and 11.9, 6-He,), 3.98 (1 H, ddq, J 10.2,2.5, and 6.2, 5-H,,), 4.07 (1 H, dd, J 10.2 and 11.9, 6-H,,) and 5.37 (1 H, q, J 5.4, 3-H,,); 6, 16.47 (5-Me), 18.13 (3-Me), 69.96 (C-5), 76.46 (C-6) and 101.55 ('JC-, 169.0, C-3); v,,,(neat)/cm-' 2973.7, 2901.9, 1446.2, 1398.4, 1377.5, 1335.6, 1296.7, 1254.9, 1174.1, 1150.2, 1117.3, 1087.4, 994.7,949.9, 893.0, 869.1, 842.2, 806.3 and 665.8 (Found: C, 50.1; H, 8.6.C5H1003 requires C, 50.84; H, 8.53). For the minor (trans) isomer (15 of crude); 6, 1.22 (3 H, d, J 5.9, 3-Me), 1.41 (3 H, d, J 6.9, 5-Meax), 3.73 (1 H, dd, J 1.5 and 12.4, 6-He,), 4.59 (1 H, dd, J 3.2 and 12.4, 6-Ha,) and 5.62 (1 H, 9, J 5.4, 3-H,,). 3,5,5-Trimethyl-1,2,4-trioxane7b. Yield 25-30; 6, 1.20 (3 H, s, 5-Meeq), 1.22 (3 H, d, J5.3, 3-Mee,), 1.38 (3 H, s, 5-Meax), 3.65 (1 H, d, J 12.2, 6-He,), 4.18 (1 H, d, J 12.3, 6-Ha,) and 5.59 (1 H, q, J 5.3, 3-Ha,); 6, 18.37 (5-Mea,), 20.91 (5-Mea,), 25.86 (3-Me), 69.84 (C-5), 78.91 (C-6) and 96.04 ('J,-, 170, C-3); v,,,(neat)/cm-' 2973.7, 2907.9, 1470.2, 1440.3, 1389.4, 1368.5, 1239.9, 1216.0, 1186.1, 1144.2, 1114.3, 1093.4, 1024.6, 997.7, 985.7,952.9,908.0, 878.1, 854.2, 806.3, 776.4 and 677.7; m/z 117 (M -Me, 22) 101 (100) and 88 (M -MeCHO, 70) (Found: C, 54.1; H, 8.9.C6HI2O3 requires C, 54.53; H, 9.15). 356- Trimethyl- 1,2,4-trioxane 7c. Yield 25-30. Minor isomer (40); 6, 1.03 (3 H, d, J 6.5, 6-Me,,), 1.17 (3 H, d, J 6.4, 5-Me,,), 1.25 (3 H, d, J 5.4, 3-Meeq), 3.52 (1 H, dq, J6.4 and 8.8, 5-H,,), 4.02 (1 H, dq, J 6.4 and 8.8, 6-Ha,) and 5.34 (1 H, q, J 5.4, 3-Ha,); 6, 13.89 (Me), 16.46 (Me), 17.91 (Me), 72.30 (C-5), 80.73 (C-6) and 101.62 ('JC-, 168.6, C-3). Major isomer (60); 6, 1.09 (3 H, d, J 6.6, 6-Meeq), 1.25 (3 H,d, J5.4, 3-Meeq), 1.40(3 H,d, J6.5, 5-Me,,), 3.83 (1 H,dq, J 6.5 and 3.1, 5-He,), 4.09 (1 H, dq, J 6.6 and 3.1, 6-Ha,) and 5.33 (1 H, q, J 5.3, 3-H,,); 6, 11.87 (Me), 16.75 (Me), 18.03 (Me), 75.83 (c-5), 78.42 (C-6) and 101.59 ('J~~~168.4, C-3); v,,,(neat mixture of isomers)/cm-' 2979.7, 2931.8, 2884.0, 1443.3, 1398.4, 1377.5, 1332.6, 1186.1, 1144.2, 1117.3, 1093.4, 1057.5, 1006.7, 988.7, 967.8, 946.9, 884.1, 854.2, 830.3, 809.3, 776.4, 737.6 and 683.7 (Found: C, 53.7; H, 9.6.C,H,,O3 requires C, 54.53; H, 9.15). 5-Bromomercurimethyl-3-methyl-1,2,4-trioxane 6a'. After the dissolution of the mercuric acetate, a solution of KBr (20 mmol) in water (15 cm3) was added, and the mixture was stirred for 5 min. After a hydrolytic work-up, the product was chromato- graphed on silica gel with pentane-CH,Cl, as eluent, giving 6a' as a viscous oil; yield 50.This was further purified twice by chromatography and recrystallised from the chromatography solvent to give the major isomer as crystals, m.p. 92-93 "C; 6, 1.27(3H,d,J5.4,3-Me,,), 1.97(1H,dd,JS.land 12.2,CH2Hg), 2.15 (1 H, dd, J 5.0 and 12.2, CH,Hg), 3.96 (1 H, dd, J 3.2 and 12.3, 6-He,), 4.03 (1 H, dd, J9.6 and 12.2, 6-H,,), 4.25 (1 H, m, 5-H,,) and 5.40 (1 H, q, J 5.3, 3-H,,); 6, 18.17 (3-Me), 35.49 (Jl99~~1557, CH,Hg), 72.47 (C-5), 77.32 (C-6) and 101.51 ('JC-, 169.6, C-3); vmax(Nujo1)/cm-' 2919.9, 2854.1, 1455.2, 1407.4, 1389.4, 1374.5, 1329.6, 1305.7, 1117.3, 1090.4, 1045.5, 979.8, 920.0, 869.1, 857.2, 800.3 and 740.5 (Found: C, 14.85; H, 2.05. C5H9BrHg03requires C, 15.10; H, 2.28).For the minor isomer (in mixture), 6, 1.31 (3 H, d, J 5.4, 3-Meeq),2.25 (1 H, dd, J 5.9 and 12.3, CH2Hga,), 2.59 (1 H, dd, J 9.8 and 12.3, CH,Hga,), 3.81 (1 H, dd, J2.4 and 12.6, 6-H,,), 4.31 (1 H, m, 5-H,,), 4.57 (1 H, dd, J3.2 and 12.6,6-H) and 5.68 (1 H, q, J5.4, 3-Ha,); 6, 18.10 (3-Me), 36.30 (CH,Hg), 68.35 (C-5), 75.42 (C-6) and 96.07 (.Ic-,167.8, C-3). 5-Bromomercurimethyl- 1,2,4-trioxane 8 and 4-Bromomercuri- methyl- 1,3-dioxoZane 9.-Formaldehyde (from 5 g paraform- aldehyde) was passed through a solution of tetra(ally1peroxy)tin 4a (2 mmol) in dichloromethane at room temperature. This solution was treated with mercury(r1) acetate, and worked up as described above for 6a', yielding the trioxane 8 and the dioxolane 9 with the following properties.8: Yield 10. M.p. 90-93 "C; 6, 1.95 (1 H, dd, J 7.7 and 12.2, JHg 202, CH,Hg), 2.16 (1 H, dd, J 4.8 and 12.1, J 216, CH,Hg), 3.97 (1 H, dd, J3.1 and 9.5, 6-H,,), 4.18 (1 H, t, J9.7, 6-Ha,), 4.21 (1 H, m, Ha,), 5.21 (1 H, dd, J2.9 and 8.7, 'JCPH 164.8, 3-Heq) and 5.45 (1 H, d, J 8.7, 'JC-, 168.6, 3-H,,); 6, 35.42 (CHg), 71.97 (C-5),78.84 (C-6) and 96.30 (' JC-,168.6and 164.8, C-3); v,,,(Nujol)/cm-' 2918.5, 2845.1, 1454.5, 1374.4, 1130.7, 1104.0, 1054.0,973.9,953.8,847.0,766.9,736.9and 720.2 (Found: C, 12.5; H, 1.75. C,H,BrHgO, requires C, 12.52; H, 1.84). 9: Yield 10. Viscous oil; 6, 2.13 (1 H, dd, J 6.0 and 11.9, JHg 204, CH,Hg), 2.27 (1 H, dd, J5.6 and 11.9,JHg 205, CH,Hg), 3.39 (1 H, dd, J6.1 and 8.1, 5-H),3.98 (1 H, dd, J6.3 and 8.1, 5-H), 4.47 (1 H, quin, J 6.0, JHg 232,4-H), 4.80 (1 H, s, 2-H) and 5.07 (1 H, S, 2-H); 6, 38.59 (JHg 1553, CHZHg), 71.93 (JHg 158, C-4), 74.80 (JHg 108, C-5) and 94.83 (C-2); v,,,(neat)/cm-' 2925.2, 2854.4, 1464.5, 1147.4, 1080.7, 1000.6, 930.5, 860.4 and 756.9 (Found: C, 13.1; H, 2.0.C,H,BrHgO, requires C, 13.07; H, 1.92). The same product was obtained in 70 yield when the reaction was carried out in the same way but with ally1 alcohol in place of the allylperoxytin compound. Acknowledgements We thank the Royal Society and the Chinese Academy of Sciences for the award of a Royal Fellowship to J. Cai, and Dr. A. J. Bloodworth for many helpful discussions. J. CHEM. SOC. PERKIN TRANS. 1 1992 References 1 D.Klayman, Science, 1985, 228, 1049; A. R. Butler and Y.-L. Wu, Chem. SOC.Rev., 1992,21,85. 2 G. Schmid and W. 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