Preparation of aromatic geraniol analogues via a Cu(I)-mediated Grignard coupling

ARTICLE

Preparation of aromatic geraniol analogues via a Cu(I)-mediated Grignard coupling

J. Luis Paz; J. Augusto R. Rodrigues^*

Instituto de Química, Universidade Estadual de Campinas, 13083-862 Campinas - SP, Brazil

ABSTRACT

Difunctional allylic terpenes are important synthetic building blocks. Functionalization of protected geranyl derivatives by SeO₂/t-BuO₂H adsorbed on SiO₂ provides a convenient route to such compounds. The chosen protecting groups clearly influence the oxidation process. Also, an efficient synthesis of 2-geranylphenol derivatives via a Cu(I)-mediated Grignard coupling of 2-lithiophenols and geranyl substrates was developed.

Keywords: allylic oxidation, selenium dioxide, homoallylic alcohols, Grignard coupling, 2-geranylphenols

RESUMO

Terpenos alílicos difuncionais constituem-se em importantes blocos de construção sintéticos. A funcionalização de derivados geranílicos protegidos por SeO₂/t-BuO₂H adsorvido em SiO₂, propicia uma rota conveniente para tais compostos. Os grupos protetores escolhidos efetivamente influenciam o processo de oxidação. Também, desenvolveu-se uma eficiente síntese de derivados 2-geranilfenóis através de um acoplamento de Grignard mediado por Cu(I) entre derivados de 2-lítiofenóis e substratos geranílicos.

Introduction

A wide variety of phenolic natural products contain isoprenoid residues.¹ It has been suggested that the biosynthetic origin of phenolic isoprenoids involves initial C-alkylation of a preformed phenol or its poly-b-ketonic precursor by an active isoprenoid allylic alcohol derivative.² For example, C-geranyl compounds may arise by nucleophilic attack of a phenol on the resonance stabilized allylic cation or by an S_N2-type displacement of pyrophosphate from geranylpyrophosphate.

C-geranyl and C-farnesyl phenols have been prepared by reaction of geranyl or farnesyl bromide with the sodium salt of the phenol. Alkylation of sodium salts, however, invariably leads to complex mixtures containing both ring and oxygen substituted products.³C-alkylation has been obtained by acid-catalyzed condensation of geraniol or farnesol with phenols in aprotic solvents in the presence of Lewis acids,⁴ mineral acids such as p-toluenesulfonic acid,⁵ Friedel-Crafts alkylation,⁶ copper-induced isomerization to 2-alkenyl 2-lithiophenyl ethers,⁷ or Cu(I)-mediated Grignard coupling of THP ethers.⁸ As part of our continuing interest in phenolic oxidative coupling mediated for iodine hypervalent reagents,⁹ we describe the preparation of 2-geranylphenolic intermediates that will be necessary for future studies on synthesis of cyclic isoprenoids.

Results and Discussion

The Sharpless conditions for oxidation of geranyl acetate employs 0.5 equiv. of SeO₂ and 2 equiv. of t-BuO₂H, and the reaction was complete after 8 h¹⁰ with formation of colloidal selenium by-products in small amounts that were difficult to eliminate after column chromatography or distillation. Using lower molar quantities of SeO₂ should reduce selenium by-products, facilitating the purification of selenium-free products. Our efforts with 1-2 mol% of SeO₂, reproducing literature conditions of these reactions, failed to proceed to completion.¹¹ Very recently, improvement of the oxidation procedure using 5 mol% of SeO₂ and 3.6 equiv. of t-BuO₂H led to complete consumption of the geranyl acetate, giving allylic alcohol (43%) and aldehyde (9%) in 52% yield.¹² We were able to improve those conditions and obtain complete conversion of geranyl acetate after 24 h at room temperature into alcohol 2 and aldehyde 3 in 70% and 30% yield, respectively (Table 1, entry 1). By making certain changes in above methods and employing SeO₂/t-BuO₂H adsorbed on silica gel, Chhabra et al. found this to be a highly selective reagent for the oxidation of allylic methyl groups.¹³ In our hands, this methodology (SeO₂/SiO₂, 10% m/m, dichloromethane as solvent) gave after 24h at room temperature a 75% yield of 2 and a 25% yield of 3, but the reaction was cleaner without by-products of organo-selenium and the work up was carried out easier (entry 2). The crude mixture was reduced with NaBH₄/EtOH to give the alcohol 2 in 75% after the two steps.

Singh et al. observed a faster reaction when SeO₂ and t-BuO₂H were adsorbed on silicagel without solvents and exposed to microwave irradiation for the oxidation of allylic methyl groups.¹⁴ Using this condition for the oxidation of geranyl acetate, with 10 min of microwave irradiation (640 W), we obtained exclusively the aldehyde 3. The reduction of crude mixture gave the alcohol 2 in 75% yield (Table 1, entry 3).

Geranyl bromide 4 was necessary in the next steps and its preparation (although it is commercially available) was achieved after some attempts in order to optimize the conditions for getting high yield. Bromination of geraniol¹⁵ with phosphorus tribromide (0.5 equiv.) in hexane under argon at -30 °C followed by a slow increased of the temperature to -10 °C gave, after 45 min, pure geranyl bromide 4 in 98% yield.

Our target was to obtain 2-[(2E,6E)-3,7-dimethyl-8-hydroxy-2,6-octadienyl]phenol 9 by optimizing the coupling of geranyl acetate derivatives with more appropriate reagent O-protected 2-iodophenol. Scheme 2 shows the first attempt. Treating (2-iodophenyl)-2-tetrahydropyranyl ether 6 with butyllithium in THF-TMEDA at low temperature and then adding the geranyl bromide 4 gave the phenol derivative 7, which was isolated after the usual work up in 95% yield. 2-Geranylphenol 8 was obtained in 93% yield after deprotection of precursor 7 with pyridinium p-toluenesulfonate (PPTS). This compound was identified by comparison of its NMR, IR and MS spectra with literature data.¹⁶ Oxidation of the geranyl moiety with SeO₂/SiO₂, t-BuO₂H and then reduction with NaBH₄ of the intermediate aldehyde using the optimized protocol described above gave alcohol 9, which was isolated in low yield (47%).

To improve the low yield that was obtained for the oxidation of 8, we investigated a new route to reach 9. Protected geranyl phenol 7 was oxidized under our optimized selenium oxide/silica gel and t-butylhydroperoxide methodology, and allylic alcohol 10 was obtained in 58% yield, a yield superior when compared with the sequence 8 to 9, described earlier. After usual deprotection, phenol 9 was isolated in 92% yield (Scheme 3).

A new sequence was investigated with another substrate in an attempt to improve the yield of the desired phenol-alcohol 9 (Scheme 4). Using a modified Mechelke-Wiemer protocol,⁸ the transmetalation of the lithium derivative of the O-protected 2-iodophenol 11 with magnesium bromide was performed at low temperature to give the Grignard intermediate and then with CuI to afford the intermediate organocuprate which was alkylated with geranyl 2-tetrahydropyranyl ether 12 to give 13 in 88% yield. Oxidation of the geranyl moiety with SeO₂/SiO₂, t-BuO₂H and then with NaBH₄ reduction of the intermediate aldehyde using the optimized protocol described above gave the allylic alcohol 14 in 43% yield. This result was still not satisfactory so we tried a different approach. The O-protected 2-iodophenol 11 was treated with n-BuLi in THF at low temperature and then with MgBr₂. After allowing the reaction mixture to reach ambient temperature, CuI was added followed by the addition of allylic alcohol 2. The reaction mixture was then heated to 50 °C and after usual work-up, monoterpenylphenol 9 was isolated with significative improvement of the overall yield (83% yield).

Conclusion

In brief, we optimized a selective oxidation of allylic methyl groups in geraniol derivatives over a solid support to the corresponding trans-a,b-unsaturated alcohols and aldehydes, using selenium dioxide and t-butylhydroperoxide adsorbed on silica gel as oxidants. The chosen protecting groups clearly influence the oxidation process. Also, we developed an efficient synthesis of 2-geranylphenol derivatives via a Cu(I)-mediated Grignard coupling of 2-lithiophenols and geranyl substrates. Further studies on phenolic oxidation of these synthetic intermediates to achieve the synthesis of cyclic isoprenoids will be reported in due course.

Experimental

The IR spectra were recorded on a Hartmann & Braun BOMEM MB SERIES spectrometer. The ¹H NMR and ¹³C NMR were recorded on a VARIAN-INOVA spectrometer. Mass spectra were recorded on a SHIMADZU GC-MS QP 5000 gas chromatograph/mass spectrometer and with helium as carrier gas. A 30 m x 0.25 mm I.D. capillary column of fused silica, SUPELCO SIMPLICITY 1^TM, was used. An injector temperature of 230 °C and a detector temperature of 280 °C, with the column at 50 °C for 3 min; then using a rate of 20 °C min^-1 up to 280 °C, with a pressure of 100 kPa and gas flow of 80 mL min^-1, was used. HMRS were obtained on a Fison VG Autospec. Preparative column chromatography was carried using silica gel 60 (Merck 7734, 70-230 mesh). Completion of the reactions was established by TLC analysis. Geraniol was purchased from Aldrich Chem. Co. (purity >98%), othres reagents were analytical (Aldrich or Acros) and we employed 230-400 mesh silica gel for flash chromatography.

(2E,6E)-3,7-Dimethyl-8-hydroxy-2,6-octadienyl acetate (2)

A suspension of selenium oxide (0.44 g, 5 mmol) and t-butylhydroperoxide (2.75 mL, 70% m/m, 20 mmol) in anhydrous dichloromethane (30 mL, ethanol free) was stirred for 20 min at room temperature and then silica gel (230-400 mesh, 5.55 g) was added. After 30 min the temperature was decreased to 0 °C and geranyl acetate (10 mmol) was slowly added. The mixture was stirred for 48 h. The solvent was stripped off until a yellow powder was obtained that was transferred to a fibrous glass frit Büchner funnel equipped with a layer of celite and neutral alumina, and washed with ethyl acetate/hexane (2/3, v/v, 3x50 mL). The extracts were washed with a saturated aqueous solution of FeSO₄.2H₂O (3x30 mL), acidified with conc H₂SO₄, then washed with brine (2x30 mL) and dried with anhydrous MgSO₄. After the solvent was removed, the oily residue was dissolved in methanol/THF (1/20, v/v, 25 mL) while keeping the temperature at -10 °C (ice-water/NaCl bath 1:1, m/m) and then NaBH₄ was added (0.30 g, 80 mmol) in four portions. After 30 min, a cold saturated solution of NH₄Cl (30 mL) was added, and the mixture was extracted with CH₂Cl₂ (3x30 mL), washed with water and brine, and dried over MgSO₄. The solvent was removed and the residue was purified by flash chromatography eluting with AcOEt/hexane (3/7, v/v), to give pure 2 as an yellow oil (1.59 g, 75% yield); IR (film) n_max/cm^-1: 3411, 2961, 2924, 2861, 1740, 1670, 1444, 1383, 1367, 1234, 1022, 954, 853, 608; ¹H NMR (300 MHz, CDCl₃/TMS) d 5.30-5.40 (m, 2H), 4.58 (d, ³J 7.0 Hz, 2H), 3.98 (s, 2H), 2.05-2.25 (m, 4H), 2.06 (s, 3H), 1.99 (s, 1H), 1.71 (s, 3H), 1.66 (s, 3H); ¹³C NMR (75 MHz, CDCl₃/TMS) d 171.3, 141.8, 135.3, 125.2, 118.6, 68.6, 61.3, 38.9, 25.5, 20.8, 16.2, 13.4.

(2E)-3,7-Dimethyl-2,6-octadienyl bromide (4)

To stirred solution of 1.54 g of geraniol (10 mmol) in 10 mL of hexane at - 30 °C under argon, was added 0.49 mL of phosphorus tribromide (5 mmol, recently destilled) was added over a period of 20 min. The resulting solution was stirred at - 30 °C for 35 min. The mixture was treated drop wise with cold metanol (1 mL) and extracted with hexane. The organic extracts were washed with NaHCO₃ solution 5%, dried over MgSO₄ and concentrated to give a crude geranyl bromide (2.12 g, 98%). This was used without any purification; IR (film) n_max/cm^-1: 2967, 2927, 2917, 2858, 1656, 1444, 1385, 1369, 1202, 1110, 840, 587.

(2-iodophenyl)-2-tetrahydropyranyl ether (6)

A solution of 2.2 g (10 mmol) of 2-iodophenol 5 in dichloromethane (10 mL) was stirred at room temperature under nitrogen and 23 mg (0.9 mmol%) of pyridinium p-toluenesulfonate was added, followed by drop wise addition of dihydropyran (1.13 mL, 12 mmol). The mixture was stirred at ambient temperature for 8 h and then diluted with 30 mL of ether. The organic layer was washed with two portions of brine then dried with anhydrous MgSO₄. The solvent was removed under vacuum and the residue was purified by flash chromatography, eluting with AcOEt/hexane (1/19, v/v), to give 2.89 g (95%) of the tetrahydropyranyl ether 6 as an oil; IR (film) n_max/cm^-1: 3060, 2943, 2872, 2851, 1582, 1471, 1439, 1356, 1275, 1240, 1202, 1123, 1036, 1018, 960, 920, 873, 749; ¹H NMR (300 MHz, CDCl₃/TMS) d 7.76 (dd, ³J 7,8 Hz, ⁴J 1.5 Hz, 1H), 7.22-7.29 (m, 1H), 7.07 (dd, ³J 8.2 Hz, ⁴J 1.4 Hz, 1H); 6.68-6.75 (m, 1H), 5.53 (t, J 2,8 Hz, 1H), 3.87 (td, ²J 11.3 Hz, ³J 11.3 Hz, ³J 2.8 Hz, 1H), 3.53-3.65 (m, 1H); 1.46-2.24 (m, 6H); ¹³C NMR (75 MHz, CDCl₃/TMS) d 155.8, 139.5, 129.6, 123.5, 115.4, 96.6, 87.6, 61.8, 30.2, 25.3, 18.3; HRMS calcd. for C₁₁H₁₃IO₂, 303.99603; found. 304.00748.

2-(2E)-3,7-Dimethyl-2,6-octadienyl]phenyl tetrahydro-2H-2-pyranyl ether (7)

To a two-necked 250 mL round bottom flask with 2-iodophenyl tetrahydropyranyl ether 6 (3.04 g, 10 mmol) in THF (100mL) under argon at -78 °C n-BuLi (2.28 mol L^-1, 4.60 mL, 10.5 mmol) was added with a syringe to form a yellowish solution. After 30 min, geranyl bromide (2.28 g, 10.5 mmol) was added and the mixture was stirred until the disappearance of the yellowish color. When the temperature has increased to -20 °C, methanol (2 mL) was added and the temperature was allowed to increase to room temperature. The solvent was removed under vacuum and the residue was purified by flash chromatography, eluting with AcOEt/hexane (3/7, v/v) to give 7 as an oil (2.98 g, 95%); IR (film) n_max/cm^-1: 3061, 3025, 2941, 2875, 2853, 1599, 1587, 1491, 1454, 1383, 1356, 1234, 1201, 1180, 1124, 1038, 1020, 970, 922, 818, 752; ¹H NMR (300 MHz, CDCl₃/TMS) d 7.06-7.17 (m, 3H), 6.91 (td, ³J 7.1 Hz, ⁴J 1.8 Hz, 1H), 5.44 (t, ³J 3.1 Hz, 1H), 5.34 (tq, ³J 7.3 Hz, ⁴J 1,3 Hz, 1H), 5.07-5.15 (m, 1H), 3.90 (ddd, ²J 11.4 Hz, ³J 9.7 Hz, ⁴J 3.5 Hz, 1H), 3.60 (dtd, ²J 11.4 Hz, ³J 3.5 Hz, ⁴J 1.5 Hz, 1H), 3.38 (d, J 7.3 Hz, 2H), 1.95-2.16 (m, 5H), 1.84-1.93 (m, 2H), 1.71 (s, 3H), 1.68 (s, 3H), 1.59 (s, 3H), 1.56-1.75 (m, 3H); ¹³C NMR (75 MHz, CDCl₃/TMS) d 154.7, 135.8, 131.4, 130.6, 129.4, 126.8, 124.4, 122.7, 121.3, 114.1, 96.5, 61.9, 39.8, 30.6, 28.5, 26.6, 25.7, 25.3, 18.9, 17.6, 16.1 ppm; HRMS calcd. for C₂₁H₃₀O₂, 314.22458; found. 314.21539.

2-[(2E)-3,7-Dimethyl-2,6-octadienyl]phenol (8)

Tetrahydro-2H-2-pyranyl ether 7 (3.14 g, 10 mmol) in methanol (30 mL) and p-toluenesulfonate pyridinium (PPTS, 0.9 mmol%) were heated at 50 °C for 6 h. The solvent was removed under vacuum and the residue was diluted in ether (50 mL) and water (30 mL), then extracted with ether (2x30 mL), dried over MgSO₄ and filtered, and then the solvent was removed under reduced pressure. The residue was purified by flash chromatography, eluting with AcOEt/hexane (3/7, v/v), to give 8 (2.14 g, 93%) as an oil; IR (film) n_max/cm^-1: 3451, 3067, 3032, 2966, 2925, 2916, 2856, 1591, 1489, 1454, 1376, 1330, 1219, 846, 752; ¹H NMR (300 MHz, CDCl₃/TMS) d 7.08-7.15 (m, 2H), 6.86 (td, ³J 7.5 Hz, ⁴J 1.1 Hz, 1H), 6.81 (dd, ³J 8.4 Hz, ⁴J 1.1 Hz, 1H), 5.32 (td, ³J 7.1 Hz, ⁴J 1.1 Hz, 1H), 5.13 (sl, 1H), 5.03-5.20 (m, 1H), 3.37 (d, ³J 7.1 Hz, 2H), 2.03-2.22 (m, 4H), 1.77 (s, 3H), 1.69 (s, 3H), 1.60 (s, 3H); ¹³C NMR (75 MHz, CDCl₃/TMS) d 154.4, 138.5, 132.0, 129.9, 127.5, 126.8, 123.8, 121.6, 120.7, 115.8, 39.7, 29.8, 26.4, 25.7, 17.7, 16.1; HRMS calcd. for C₁₆H₂₂O, 230.16706; found 230.2832.

(2E,6E)-2,6-Dimethyl-[2-(tetrahydro-2H-2-pyranyloxy)phenyl]-2,6-octadienyl-8-ol (10)

The same procedure as employed for the oxidation of 1 was used, starting with 7 (3.15 g, 10 mmol), followed by oxidation with SeO₂ and reduction with NaBH₄. The residue was purified by flash chromatography, eluting with AcOEt/hexane (3/7, v/v), to give 10 (1.92 g, 58%) as a pale yellow oil; IR (film) n_max/cm^-1: 3325, 2965, 2915, 2856, 1020, 758; ¹H NMR (300 MHz, CDCl₃/TMS) d 6.93-6.97 (m, 2H), 6.81-6.85 (m, 1H), 6.73-6.76 (m, 1H), 5.46 (t, ³J 3.3 Hz, 1H), 5.37 (m, 1H), 5.26 (t, ³J 7.2 Hz, 1H), 4.02 (s, 2H), 3.83-3.94 (m, 1H), 3.55-3.63 (m, 1H), 3.25 (d, ³J 7.2 Hz, 2H), 2.00-2.21 (m, 4H), 1.85 (sl, 1H), 1.69 (s, 3H), 1.58 (s, 3H), 1.54-2.12 (m, 6H); ¹³C NMR (75 MHz, CDCl₃/TMS) d 154.9, 137.5, 135.2, 131.9, 128.0, 127.4, 122.4, 122.1, 121.0, 112.1, 98.1, 69.0, 61.9, 39.0, 30.8, 27.0, 26.3, 25.7, 19.3, 15.9, 13.7; GC/EIMS (70 eV) m/z 330 (M⁺, 1%), 246 (35), 245 (20), 228 (22), 85 (100); HRMS calcd. for C₂₁H₃₀O₃, 330.21950; found 330.22005.

tert-Butyldimethylsilyl 2-iodophenol ether (11)

To a solution of 2-iodophenol (2.20 g, 10 mmol) in dry acetonitrile (20 mL) were added under an argon atmosphere, tert-butyldimethylsilyl chloride (TBDMSCl, 1.88 g, 12.5 mmol), imidazole (0.85 g, 12.5 mmol) and finally, tetrabutylammonium iodide (TBAI, 4.62 g, 12.5 mmol) were added. After stirring for 15 min at room temperature, cold pentane (25 mL) was added, and the organic layer was washed with cold water, extracted with pentane (2x25 mL) and then washed with brine. The organic layer was then dried over MgSO₄, filtered and the solvent was removed under reduced pressure. The crude residue was purified by column chromatographic with silica gel using AcOEt/hexane as eluent to give 11 as a colorless oil (3.21 g, 96%); IR (film) n_max/cm^-1: 3061, 2958, 2930, 2858, 1728, 1597, 1582, 1491, 1472, 1437, 1288, 1261, 917, 839, 781, 752; ¹H NMR (300 MHz, CDCl₃/TMS) d 7.77 (dd, ³J 7.9 Hz, ⁴J 1.6 Hz, 1H), 7.18-7.25 (m, 1H), 6.85 (dd, ³J 8.2 Hz, ⁴J 1.3 Hz, 1H), 6.70 (td, ³J 7.9 Hz, ³J 7.9 Hz, ⁴J 1.3 Hz, 1H), 1.06 (s, 9H), 0.28 (s, 6H); ¹³C NMR (75 MHz, CDCl₃/TMS) d 155.2, 139.6, 129.2, 122.7, 118.5, 90.5, 25.8, 18.3, -4.0; HRMS calcd. for C₁₂H₁₉ IOSi, 334.02500; found 334.02632.

2-[(2E)-3,7-Dimethyl-2,6-octadienyl]phenyl tert-butyldimethylsilyl ether (13)

In a three necked round bottom flask, tert-butyldimethylsilyl 2-iodophenol ether 11 (3.34 g, 10 mmol) and THF (20 mL) was added under an argon atmosphere at -78 °C. Then BuLi in hexane (2.28 mol L^-1, 10.5 mmol, 4.60 mL) and a solution of MgBr₂ (0.52 mol L^-1, 10.5 mmol) in THF (20 mL) were added. After the temperature reached room temperature, CuI (1.99 g, 10.5 mmol) was added in four portions with stirring. Then a solution of (2E)-3,7-dimethyl-2,6-octadienyl tetrahydro-2H-2-pyranyl ether 12 (10.5 mmol) in THF was added and the mixture was refluxed for 6 h. The reaction was cooled, a saturated solution of NH₄Cl was added, the solutions was extracted with ether (3x50 mL), dried over MgSO₄, filtered and the solvent was removed under reduced pressure by rotatory evaporation. The crude residue was purified by column chromatography with silica gel using AcOEt/hexane (1/8, v/v) as eluent to give 13 (3.03 g, 88%) as an oil; IR (film) n_max/cm^-1: 3065, 3032, 2958, 2929, 2858, 1599, 1581, 1489, 1451, 1254, 928, 837, 779, 754; ¹H NMR (300 MHz, CDCl₃/TMS) d 7.12 (d, ³J 7.3 Hz, 1H), 7.05 (td, ³J 7.7 Hz, ⁴J 1.8 Hz, 1H), 6.87 (t, ³J 7.3 Hz, 1H), 6.77 (d, ³J 7.7 Hz, 1H), 5.33 (t, ³J 7.0 Hz, 1H), 5.07-5.17 (m, 1H), 3.32 (d, ³J 7.0 Hz, 2H), 1.96-2.18 (m, 4H), 1.68 (s, 6H), 1.60 (s, 3H), 1.02 (s, 9H), 0.23 (s, 6H); ¹³C NMR (75 MHz, CDCl₃/TMS) d 153.4, 136.1, 132.2, 131.9, 129.6, 126.5, 124.4, 122.6, 121.0, 118.3, 39.7, 28.4, 26.6, 25.8, 25.7, 18.3, 17.7, 16.1, -4.1; GC/EIMS (70 eV) m/z 344 (M^+•, 2%), 287 (3), 275 (7), 231 (2), 165 (17), 163 (50), 135 (9), 123 (19), 121 (2), 109 (2), 91 (6), 77 (3), 73 (75), 70 (6), 69 (100), 59 (13), 45 (8), 43 (8), 41 (85); HRMS calcd. for C₂₂H₃₆OSi, 344.25354; found 344.27659.

(2E,6E)-2,6-Dimethyl-8-[2-(tert-butyldimethyl silyloxyphenyl]-2,6-octadienyl alcohol (14)

Method A. From 13. The same procedure as employed for allylic oxidation of 1 was used; compound 13 (3.45 g, 10 mmol) was oxidized with SeO₂ and then reduced with NaBH₄. The crude reaction product residue was purified by flash chromatography, eluting with AcOEt/hexane (1/3, v/v), to give 14 (2.99 g, 43%) as a pale yellow oil; IR (film) n_max/cm^-1: 3342, 2956, 2865, 1669, 1747, 1235, 1025, 1009, 836, 753; ¹H NMR (300 MHz, CDCl₃/TMS) d 6.92-6.96 (m, 2H), 6.81-6.84 (m, 1H), 6.72-6.76 (m, 1H), 5.36 (m, 1H), 5.26 (t, ³J 7.2 Hz, 1H), 4.01 (s, 2H), 3.27 (d, ³J 7.2 Hz, 2H), 2.02-2.21 (m, 4H), 1.98 (s, 1H), 1.71 (s, 3H), 1.58 (s, 3H), 0.95 (s, 9H), 0.06 (s, 6H); ¹³C NMR (75 MHz, CDCl₃/TMS) d 157.0, 137.5, 135.2, 132.7, 132.0, 126.4, 122.4, 121.0, 120.5, 119.9, 69.0, 39.0, 27.2, 27.0, 23.5, 20.4, 15.9, 13.7, -4.9; GC/EIMS (70 eV) m/z 360 (2%), 75 (100); HRMS calcd. for C₂₂H₃₆O₂Si 360.24846; found 360.24970.

Method B. From 11. The same procedure as employed for the coupling of 11 and 12 was used, starting with tert-butyldimethylsilyl 2-iodophenol ether 11 (3.34 g, 10 mmol) in THF (20 mL) under an argon atmosphere at -78 °C. Then BuLi in hexane (2.28 mol L^-1, 10.5 mmol, 4.60 mL) and a solution of MgBr₂ (0.52 mol L^-1, 10.5 mmol) in THF (20 mL) were added. After the temperature reached room temperature, CuI (1.99 g, 10.5 mmol) was added in four portions with stirring, followed by a solution of (2E,6E)-8-hydroxy-3,7-dimethyl-2,6-octadienyl acetate 2 (10.5 mmol) in THF and the mixture was refluxed for 6 h. The reaction was cooled, a saturated solution of NH₄Cl was added, the solutions was extracted with ether (3x50 mL), dried over MgSO₄, filtered and the solvent was removed under reduced pressure by rotatory evaporation. The crude residue was purified by column chromatography with silica gel using AcOEt/hexane (1/3, v/v) as eluent to give 14 (3.25 g, 90%) as an oil.

2-[(2E,6E)-3,7-Dimethyl-8-hydroxy-2,6-octadienyl]phenol (9)

Method A. From 8. The same procedure employed for the allylic oxidation of 1 was used; compound 8 (2.30 g, 10 mmol) was treated with SeO₂ (0.55 g, 5 mmol), t-butylhydroperoxide (2.75 mL, 20 mmol), SiO₂ (5.55 g) and then with NaBH₄ (0.30 g, 8 mmol). The crude reaction product residue was purified by flash chromatography, eluting with AcOEt/hexane (1/3, v/v), to give 9 as a pale yellow oil (1.16 g, 47%); IR (film) n_max/cm^-1: 3641, 3355, 3067, 3036, 2958, 2921, 2871, 1593, 1483, 1456, 1431, 1363, 1232, 1156, 1097, 1041, 1002, 862, 752; ¹H NMR (300 MHz, CDCl₃/TMS) d 7.06-7.16 (m, 2H), 6.86 (td, ³J 7.3 Hz, ⁴J 1.1 Hz, 1H), 6.81 (dd, ³J 8.1 Hz, ⁴J 1.1 Hz, 1H), 5.58 (s, 1H), 5.25-5.38 (m, 2H), 3.98 (s, 2H), 3.37 (d, J 7.0 Hz, 2H), 2.12-2.30 (m, 4H), 1.77 (s, 3H), 1.66 (s, 3H), 1.64 (s, 1H); ¹³C NMR (75 MHz, CDCl₃/TMS) d 154.4, 138.4, 136.1, 130.1, 127.7, 126.5, 124.9, 122.6, 120.8, 116.0, 68.9, 39.1, 30.4, 25.1, 15.7, 13.7; GC/EIMS (70 eV) m/z 246 (M^+•; 0.32), 228 (2), 213 (6), 185 (14), 161 (15), 146 (22), 145 (46), 133 (15), 131 (10), 128 (8), 121 (22), 119 (14), 115 (8), 108 (10), 107 (65), 105 (10), 93 (12), 91 (27), 81 (12), 79 (14), 77 (24), 69 (7), 67 (13), 65 (10), 55 (26), 53 (15), 51 (10), 43 (100), 41 (57); HRMS calcd. for C₂₁H₃₀O₃, 330.21950; found 330.22005.

Method B. From 10. To a solution of 10 (3.30g, 10 mmol) in methanol (30 mL) pyridinium p-toluenesulfonate (PPTS, 0.025g, 0.1 mmol) was added and then heated to 50 °C for 6 h. The solvent was removed under reduced pressure and the residue was diluted with ether (50 mL) and water (30 mL) and extracted with ether (2x50 mL). The ether phase was washed with water (30 mL), then with brine (2x30 mL), dried over MgSO₄, filtered and the solvent was removed under reduced pressure by rotatory evaporation. The residue was purified by flash chromatography, eluting with AcOEt/hexane (3/7 v/v) to give 9 (2.27 g, 92%).

Method C. From 14. To a stirred solution of 14 (3.61 g, 10 mmol) in anhydrous THF (20 mL) under argon atmosphere, a solution of tetrabutylammonium fluoride (TBAF, 5.23 g, 20 mmol) in THF (20 mL) was added. After the disappearance of the starting material, water (30 mL) was added and the mixture was extracted with dichloromethane (2x30 mL). The extracts were dried over MgSO₄, filtered and the solvent was removed under reduced pressure by rotatory evaporation. The crude residue was purified by column chromatography with silica gel using AcOEt/hexane to give pure 9 (2.29 g, 93%) as a yellow oil.

Acknowledgements

We thank FAPESP for financial support and CNPq for a scholarship for J.L.P.J.

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Received: February 4, 2003

Published on the web: November 28, 2003

FAPESP helped in meeting the publication costs of this article

This paper is dedicated to Prof. Albert J. Kascheres on occasion of his 60^th birthday

Publication Dates

• Publication in this collection
  11 Feb 2004
• Date of issue
  Dec 2003

History

• Received
  04 Feb 2003