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versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.54 n.1 Concepción 2009
J. Chil. Chem. Soc, 54, N° 1 (2009); págs: 89-92
A FACILE AND EFFICIENT SYNTHESIS OF HIGHLY FUNCTIONALIZED TERMINAL OLEFINES FROM α-ALKOXY-ß-HALIDES USING ZINC DUSTΨ
SAKKARAPALAYAM M. MAHALINGAMA, HEMA KRISHNANA AND HARI N. PATFB*
A Department of Chemistry, Iridian Institute of Technology Madras, Chennai 600 036, India
B Department of Chemistry, Sambalpur University, Jyoti Vihar 768 019, India *e-mail: email@example.com
A simple and efficient method of zinc dust/ammonium chloride system for the terminal olefination of α-alkoxy- ß-halides has been described. The reaction is carried out under mild conditions and yields of the corresponding terminal olefinic products are good. The significant feature of this method is the isolation of the pure product by simple work up in a short time.
Keywords: Terminal defines / zinc dust / olefination / dehydrohalogenation.
The terminal olefination or dehydrohalogenation of α-alkoxy-ß-halides is a useful chemicaltransformation inthe synthesis of numerous organic compounds and also desired during the synthesis of compounds which are key intermediates in the synthesis of many pharmacological important substances. These terminal olefinic compounds are very important key precursors to synthesis the natural products via the ring closing metathesis1 and radical cyclization.2
Earlier reports reveal that the terminal olefination or dehydrohalogenation of α-alkoxy-ß-halides has been achieved with systems like Cp2TiBH4 and [Cp2TinBu2]-MgCl+.3 However, these systems require reaction times as long as 5-10 hours at reflux, expensive catalysts and also offer very low yields. Therefore, there is need of an efficient and inexpensive non toxic reagent system for the terminal olefination of α-alkoxy-ß-halides.
In order to develop new organic transformation, we report herein that zinc dust/NH4Cl reagent system, might be a useful and inexpensive reagent for the reduction of α-alkoxy-ß-halides. Although, zinc has also been extensively used in the preparation of organometallic compounds" and as a reducing agent! in organic synthesis. The utility of zinc for the synthesis of ß, α-unsaturated ketones by a reaction of an acid chloride with allyl bromide6 and homoallylic alcohols7 has been demonstrated. Furthermore, the zinc mediated amide formation,8 Fridel-Crafts acylation9 and carbamate formation10 has also been reported. Inthis context, the use of zinc as non-toxic 'green' reagent in organic synthetic processes has gained considerable importance due to its ability to promote and catalyze organic transformations of commercial importance under ambient conditions, without the need for any added catalyst or ligand.
RESULTS AND DISCUSSION
We began our study with the α-alkoxy-ß-halide 1a.11 Compound containing a neighbouring alkoxy group (OBn) and a halo (iodo) group at p position and reaction with zinc dust in ammonium chloride could conceivably give rise to either the acylic ß-elimination product 2a or the simple reduced compound 3.
In this letter, we report a rapid and efficient reductive elimination of α-alkoxy-ß-halides to the corresponding terminal olefinic products using low
cost zinc dust and ammonium chloride at 60 °C in methanol as depicted in Scheme 1.
We found that the reductive elimination of 1a with zinc dust and ammonium chloride furnished only the ß-elimination product 2a and not the simple reduced compound 3. The reaction was very clean with satisfactory yield and there was no side products observed.
The possible mechanism is depicted in figure 1. Probably, the α-alkoxy -ß-halides reacl wilh zinc leads lo the formation of either the intermediate A or B. These reactions are obviously substitution reactions, but they cannot be classified as nucleophilic substitutions. The facile elimination of the POZnX from the intermediate A or B furnished the terminal olefin 2. From these observations the α-alkoxy-ß-halides are considered to be masked olefins (Figure 1).
The reaction on substrate 1a afforded compound 2a, which was conformed by 1H NMR and 13C NMR spectrum. The one proton from the methine (CH) unit of the olefinic residue appeared as multiplets from δ 5.77 to δ 5.88 ppm and the remaining two terminal olefinic protons (CH2) appeared as multiplets from δ 5.31 to δ 5.39 ppm. Firm evidence for the terminal olefination was obtained from peaks at δ 119.7 (methylene; CH2) and 135.3 ppm (methine; CH) in the 13C NMR spectrum. Also from the DEPT spectrum, one olefinic methylene carbon [at δ 119.7 ppm] and one olefinic methane carbon [at δ 135.3 ppm] were visible and it was clearly indicating the terminal olefinic product formation.
The reduction of α-alkoxy-ß-halides in the presence of zinc dust and ammonium chloride was completed within three to four hours (Table 1). The course of reaction was monitored by TLC. The work-up and isolation of the products were as usual. Thus, the α-alkoxy-ß-halides reduced by this system were obtained in good yields and no undesired side product was observed. Some of the results shown in table 1 clearly indicate the scope and generality of the reaction with respect to various α-alkoxy-ß-halides. All products were characterized by different spectroscopic techniques.
Compound 1b and 1c underwent reductive elimination, to give vinyl compounds 2b with good yields. We have also examined the four carbon a-benzyloxy-ß-halide 1d under the similar reaction conditions and as expected. we isolated the olefinic product 2c with 78% yield (Figure 2).
This similar kind of observation was noted by Hersant et. al. when using excess of Cp2TiCl as a reducing agent for α-acetoxy-ß-halides under photo irradiation conditions.3 Butthey observed amixture of products like p-eliminated product and simple reduced compound. In comparison with Cp2TiCl, zinc dust with ammonium chloride shows more reactivity, less reaction times and exclusive ß-elimination product with higher yields (Table 1).
The starting subslrale 1-deoxy-l-iodO-2,3-di-O-benzyl-4,5-O-isopropylidene-D-arabinilol (1a) was prepared from íhe corresponding standard lileralure method starting from D-(-)-arabinose.11 The substrates2-O-benzyl-1bromo-3,4:5,6-di-O-isopropylidene-D-glucitol (1b) and 2-O-methyl-1-bromo-3,4:5,6-di-(3-isopropylidene-D-glucitol (1c) were conveniently prepared starting from D-gluconolactone (Scheme 2). The procedure starts with 1,2:3,4:5,6-tri-O-isopropylidene-D-gluconate (4), conveniently prepared on a multi-gram scale by the procedure of Jarosz et. al, in a single step from D-gluconolactone.12 Lactone(4) on heating with morpholine in toluene at 90 °C furnished α-hydroxy amide 5 in an isolated yield of 95%. The α-hydroxy amide 5 was subjected to O-benzylation/O-methylation using sodium hydride as base in DMF at room temperature. Clean reaction was resulted in 30 minutes, giving the α-O-benzyl /α-O-methyl amide (6) in an isolated yield of 90% after the usual work-up and silica-gel chromatography. The amide 6 underwent clean reduction with sodium borohydride in ethanol at 60 °C, furnishing the 2-O-benzyl/2-O-methyl glucitol derivative 7. Bromination under modified Mitsunobu conditions using triphenyl phosphine and W-bromosuccinimide at 60 °C yielded the compound 1b and 1c (Scheme 2).
The four carbon ß-O-benzyl protected bromo compound 1d was prepared starting from D-(+)-tartaric acid (8). Stirring D-(+) tartaric acid with a catalytic amount of concentrated sulphuric acid in methanol at room temperature for 6 hours gave dimethyl tartrate (9) in 80% yield. Di-O-benzylation of 9 using silver oxide and benzyl bromide furnished compound 10, followed by sodium borohydride reduction of O-benzyl protected α-hydroxy methyl ester to afford the diol 11. For mono protection, the purified diol 11 was subjected to 1.0 equivalent of TBDPS-C1 in the presence of imidazole to afford the alcohol 12 in 72% yield. Bromination under modified Mitsunobu conditions using triphenyl phosphine and W-bromosuccinimide at 60 °C yielded compound Id in 78% (Scheme 3).
In general, the terminal olefination reactions (Table 1) are very clean, reasonably fast and high yielding compare to the other reported procedures. The present procedure using zinc dust with ammonium chloride provides an efficient route to highly functionalized terminal defines.
In conclusión, we have demonstrated a very simple, efficient and practical method for the terminal olefination using zinc dust with ammonium chloride. The important features of this method include: (a) operational simplicity, (b) no need for any other additive to promote the reaction, (c) shorter reaction time, (d) the use of cheap, commercially available, non toxic reagents, and (e) good to moderate yields of desired products . Moreover, 14 new compounds have been synthesized and íheir dala cited in this report for the first time.
1H NMR spectra were recorded on 400MHz Bruker AVANCE 400 spectrometer and 13C NMR spectra were recorded on 100MHz Bruker AVANCE 400 spectrometer, respectively, using CDCl3 as solvent and TMS as an internal standard. IR spectra were recorded on a Perkin-Elmer FT/IR 100 spectrometer. Mass spectra were recorded on Agilent-1100 mass spectrometer. Optical rotations were measured by a Rudolph Autopol V polarimeter. All the reactions were monitored by thin layer chromatography (TLC). TLC was performed on F254, 0.25 mm silica gel plates (Merck). Plates were eluted with appropriate solvent systems, and then stained with either alkali KMnO4 or Ceric ammonium molybdate solutions prepared in the laboratory. The developed plates were first analysed under UV 254nm then stained with appropriate reagent. Column chromatography was performed using silica gel with particle size 100-200 mesh.
General procedure for O-alkylation using NaH.
A solution of the hydroxy compound (1 mmol) in dry DMF (6 mi) was added to oil-free sodium hydride (1.1 mmol) in DMF (1 mi) at 0 °C under inert atmosphere. This was followed by addition of benzyl bromide/methyl iodide (1.1 mmol); the reaction mixture was allowed to stir at room temperature for 30 minutes. On completion of the reaction as monitored by thin layer chromatography, the product was extracted using ethyl acetate (20 mi) and the DMF was removed by washing with brine solution (2 x 50 mi). The organic layer was separated and evaporated under reduced pressure to yield the O-alkylated product. The crude product was subjected to silica gel column chromatography (ethyl acetate: hexane, 2: 8) to give a pure compound.
Morpholino- (3,4:5,6-di-O-isopropylidene) -D - gluconamide (5):
To a solution of the triacetonide 4 (5 g, 5.8 mmol) in toluene (16 mi), morpholine (5.5 mL, 63.3 mmol) was added and the reaction mixture was stirred at 90 °C for 18 hours. On completion of the reaction as monitored by thin layer chromatography, the reaction mixture was allowed to cool and toluene was evaporated under reduced pressure. Morpholine was removed by addition of toluene (2 x 10 mi) and distillation of the azeotropic mixture under reduced pressure. The crude product was subjected to silica gel column chromatography (ethyl acetate: hexane, 2: 8) to yield the pure compound 5 as a white solid. [α]D 27 : -15.8° (c 1, CHC13). IR (neat) vmaxcm-1: 3430, 1650, 1371, 1247, 1213, 1112, 1063, 847, 511. 1H NMR: δ 1.26 1.31, 1.34, 1.37(4 x s, 12H, 4 x CH3); 3.37-3.72 (m, 8H, morpholino); 3.76 (d, J = 9.2 Hz, 1H, -OH); 3.87-3.94 (m, 2H, CH2CH); 3.95-4.14 (m, 3H, 3 x CH); 4.51 (d, J = 9.2 Hz, 1H, CHCO); 13C NMR: δ 26.6, 27.2, (4x CH3); 42.9, 45.4 (2 x N-CH2); 66.3 (CH2CH); 66.7 (2 x 0-CH2); 68.0 (CH2CH); 77.1, 77.6 (2 x CH); 80.5 (CHCO); 109.6, 110.6, (2 x CMe2); 170.3 (CO) ; HRMS (TOF MS ES+) m/z [M+H]+ caled, for C16H27NO7 345.1866, found 345.1870.
This compound was obtained as colourless crystals in 90% yield. [α]D 27 : +10.79° (c 1, CHCy. IR(neat)vmaxcm1: 1637, 1456, 1371, 1252, 1114, 1070, 847, 738, 699, 584. 1H NMR: δ 1/25, 1.29 (2 x s, 12H, 4 x CH3); 3.40-3.70 (m, 8H, morpholino); 3.78-3.88 (m, 2H, CH2CU); 3.97-4.20 (m, 3H, 3 x CH); 4.31 (d, J = 2.5 Hz, 1H, CHCO); 4.42 (d, J = 11.5 Hz, 1H, PhCHaHb); 4.59 (d, J = 11.5 Hz, 1H, PhCHaHb); 7.21-7.32 (m, 5H, Ar-H); 13C NMR: δ 25.2, 26.6, 27.2 (4x CH3); 43.2, 45.9 (2 x N-CH2); 66.9, 67.1 (2 x 0-CH2, morpholinyl); 67.6 (CH2CH); 72.9 (CH2CH); 76.94, 76.96 (2 x CH); 77.4 (PhCH2); 80.5 (CHCO); 109.7, 109.9 (2 x CMe2); 127.8, 128.1, 128.5, 136.9 (Aromatic); 168.5 (CO) ; HRMS (TOF MS ES+) m/z [M+H]+ caled, for C23H33NO7 436.2335, found 436.2340.
This compound was obtained as a syrupy liquid. [α]D 27 : +15.6° (c 1, CHCy. IR (neat) vmaxcm-1:1640, 1455, 1370, 1075. 1H NMR: δ 1.34, 1.36, 1.37, 1.41 (4 x s, 12H, 4 x CH3); 3.42 (s, 3H, H-OCH3), 3.60-3.92 (m, 8H, morpholino); 3.94-3.40 (m, 1H), 4.00-4.23 (m, 5H); 13C NMR: 8 25.1, 26.5, 26.6, 27.1 (4x CH3); 43.2, 45.6 (2 x N-CH2); 58.1 (OCH3); 67.0, 67.1 (2 x 0-CH2, morpholinyl); 67.6 (CH2CH); 76.6 (CH2CH); 77.0, 70.6 (2 x CH); 83.5 (CHCO); 109.6, 109.7 (2 x CMe2); 168.3 (CO) ; HRMS (TOF MS ES+) m/z [M+H]+ caled, for C17H29NO7 360.2022, found 360.2022.
General Procedure for NaBH4 reduction.
To a solution of the starting substrate (1 mmol) in 10 mi of ethanol, sodium borohydride (4 mmol) was added and the reaction mixture was stirred at 60 °C for 12 hours. On completion of the reaction as monitored by thin layer chromatography, the reaction mixture was allowed to cool and ethanol was evaporated under reduced pressure. The residue was dissolved in 25 mi of water and the solution was neutralized using acetic acid. The product was extracted using diethyl ether (2 x 30 mi) and the combined ether extract was evaporated under reduced pressure to give the crude product. The crude product was subjected to silica gel column chromatography (ethyl acetate: hexane, 2.5: 7.5) to give pure compound.
This compound was obtained as colourless syrup. [α]D 27: +26.38° (c 1, CHC13). IR (neat) vmaxcm-1 : 3481, 1217, 1071, 772, 698, 669. 1H NMR: δ 1.35, 1.39, 1.42 (3 x s, 12H, 4 x CH3); 3.60-4.20 (multiplets, 8H), 4.68 (d, J = 11.7 Hz, 1H, PhCHaHb); 4.78 (d, J = 11.7 Hz, 1H, PhCHaHb); 7.25-7.39 (m, 5H, Ar-H); 13C NMR: δ 25.2, 26.4, 26.7, 27.2 (4x CH3); 62.2 (CH2OH); 67.9 (C-O-CH2); 72.5 (C-O-CH2CH); 77.4, 77.9 (2 x CH); 78.1 (PhCH2); 81.9 (CHCH2OH); 109.77, 109.79 (2 x CMe2); 127.7, 127.8, 127.9,128.5, 138.3 (aromatic); HRMS (TOF MS ES+) m/z [M+Na]+ caled, for C19H28O6 375.1784, found 375.1789.
This compound was obtained as colourless syrup. [α]D 27: +23.40° (c 1, CHC13). IR (neat) vmaxcm-1: 1210, 1075, 772, 698, 668. 1H NMR: δ 1.36, 1.38, 1.42, 1.43 (4 x s, 12H, 4 x CH3); 3.38-3.42 (m, 1H), 3.51 (s, 3H, H-OCH3), 3.75-3.80 (m, 1H), 3.87-3.92 (m, 1H), 3.95-4.02 (m, 2H), 4.03-4.12 (m, 2H), 4.16-4.20 (m, 1H). 13C NMR: δ 25.2, 26.4, 26.7, 27.1 (4x CH3); 52.1, 58.4 (OCH3), 61.5 (CH2OH); 65.8, 68.0, 80.1, 81.9, 109.7, 109.9 (2 x CMe2).
General procedure for bromination using V-bromosuccinimide.
To a solution of the hydroxy compound (1 mmol) in dry DMF (10 mi) was added triphenyl phosphine (2 mmol) and V-bromosuccinimide (2 mmol). The reaction mixture was then heated at 60 °C for 3 hours. On completion of the reaction, water (20 mi) was added and the mixture was extracted with diethyl ether (3 x 10 mi). The combined organic layer was dried over anhydrous sodium sulphate, and evaporated under reduced pressure to afford the crude residue. The residue was purified by silica-gel column chromatography (8:2, hexane-ethyl acetate) to produce the pure bromo compound.
This compound was obtained as colourless syrup. IR (neat) vmaxcm-1: 1215, 1071, 772, 690. 1H NMR: δ 1.27, 1.30, 1.31, 1.33 (4 x s, 12H, 4TcH3); 3.47-3.51 (m, 2H), 3.70-3.76 (m, 1H), 3.78-3.84 (m, 1H), 3.87-3.93 (m, 1H), 3.96-4.03 (m, 1H), 4.04-4.10 (m, 1H), 4.15-4.20 (m, 1H), 4.55 (d, J= 11.7 Hz, 1H, PhCHaHb); 4.72 (d, J= 11.7 Hz, 1H, PhCHaHb); 7.21-7.30 (m, 5H, Ar-H); 13C NMR: δ 25.2, 26.6, 26.7, 27.2 (4x CH3); 30.17 (CH2Br); 67.9 (C-O-CH2); 73.5 (C-O-CH2CH); 77.1, 77.3 (2 x CH); 78.4 (PhCH2); 80.1 (CHCH2Br); 109.7, 109.8 (2 x CMe2); 127.8, 127.9, 128.2, 137.9 (Aromatic).
This compound was obtained as colourless syrup. IR (neat) vmaxcm-1: 1215, 1071, 772, 690. 1H NMR: δ 1.28, 1.29, 1.32, 1.35 (4 x s, 12°H, 4 x CH3); 3.45-3.50 (m, 6H), 3.82-3.90 (m, 2H), 3.95-4.03 (m, 1H), 4.07-4.17 (m, 2H); 13C NMR: δ 21.65, 25.63, 26.05 (4 x CH3), 29.48 (CH2Br), 58.38 (OCH3), 66.95 (C-O-CH2), 76.16, 78.81 (2 x CH), 79.09 (CHCH2Br), 108.54, 108.69 (2 x CMe2).
Synthesis of (2R,3R)-dimethyl 2,3-bis(O-benzyl)tartrate (10):
A suspensión of (+)-dimethyl tartrate (9) (0.5 g, 2.8 mmol) in dichloromethane (10 ml) was treated with silver oxide (1.43 g, 6.2 mmol), and allowed to stir for 12 hours at room temperature. The reaction mixture was filtered through a celite bed, and washed with additional volumes of dichloromethane. The original fíltrate and the dichloromethane washings were combined and evaporated to dryness on the rotary evaporator. The residue was subjected to silica gel chromatography to yield (0.72 g; 75%) pure 10 as colourless syrup. 1H NMR: δ 3.65 (6H, s, H-(2 x CH3)), 4.38-4.44 (4H, m), 4.85-4.88 (2H, m), 7.25-7.36 (10H, m, H-Ph). 13C NMR: δ 52.1 (OCH3), 73.2 (CH2(benzylic)), 78.2 (CH), 126.9, 127.6, 128.0, 128.3, 136.8 (C-Ph), 169.5 (C=0).
This compound was obtained as colourless syrup in 77% yield. 1H NMR: δ 3.6 8-3.83 (6H, m), 4.63-4.67 (4H, m), 7.25-7.36 (10H, m). 13C NMR: δ 60.8 (CH2-OH), 72.6 (CH2(benzylic)), 78.9 (CH), 126.9, 127.9, 128.0, 128.5, 138.0 (C-Ph).
Synthesis of (2R,3R)-2,3-bis(benzyloxy)-4-(tert-butyldiphenylsilyloxy) butan-1-ol (12):
To a solution of the d1Hydroxy compound 11 (0.5 g, 1.65 mmol) in dry DMF (4 mi) was added imidazole (0.224 g, 3.3 mmol) followed by chloro terf-butyl diphenylsilane (0.5 g, 1.8 mmol) at room temperature. After 6 hours stirring, water (15 mi) was added and the reaction mixture was extracted with ether (3 x 10 mi). The combined organic layer was dried over Na2S04 and concentrated under reduced pressure. The residue thus obtained was purified by silica-gel column chromatography (8:2, hexane: ethyl acetate) to produce pure compound 12 as colourless syrup (0.48 g; 74%). 1H NMR: δ 1.05 (9H, s), 3.63-3.70 (2H, m), 3.70-3.80 (2H, m), 3.82-3.87 (2H, m), 4.47-4.70 (4H, m), 7.25-7.43 (16H, m), 7.66-7.70 (4H, m). 13C NMR: δ 20.33, 27.9, 62.9, 64.0, 66.5, 73.9, 74.1, 80.2, 81.1, 128.1, 128.8, 128.9, 128.9, 129.0, 129.1, 129.5, 129.5, 129.7, 130.9, 130.9, 134.2, 134.3, 136.7, 136.8, 139.4, 139.4. HRMS (TOFMS ES+) m/z [M+Na]+ caled, for C34H40O4Si 563.2594, found 563.2590.
((2R,3R)-2,3-Bis(benzyloxy)-4-bromobutoxy)(tert-butyl) diphenylsilane(l d):
This compound was obtained as colourless syrup in 72% yield. 1H NMR: δ 1.04 (6H, s), 1.57 (3H, s), 3.43-3.45 (1H, m), 3.50-3.58 (1H, m), 3.73-3.85 (2H, m), 3.87-4.0 (2H, m), 4.55-4.75 (4H, m), 7.25-7.37 (16H, m), 7.63-7.66 9 (4H, m). 13CNMR: δ 19.0, 26.8, 30.0, 31.3, 62.6, 73.3, 73.7, 78.9, 127.7, 127.7, 127.7, 128.0, 128.1, 128.3, 128.3, 128.5, 129.7, 129.8, 135.6. HRMS (TOFMS ES+) m/z [M+Na]+ caled, for C34H39O3SiBr 625.1750, found 625.1777.
General procedure for the reductive elimination and debromination of α-alkoxy-ß-halides.
To a solution of the starting substrates 1a-d (1 mmol) in methanol (10 mi) was added ammonium chloride (0.5 mmol) and zinc dust (2 mmol). The mixture was stirred at 60 °C for 3-4 hours. After the completion of the reaction (monitored by TLC), the reaction mixture was filtered through celite. The fíltrate was evaporated under vacuum and the residue was taken into chloroform or ether, washed twice with saturated brine solution and finally with water. The organic layer was dried over anhydrous sodium sulphate, and evaporation of the organic layer was followed by purification by column chromatography to yield the desired product.
Spectroscopic data for final olefinic products (2a-c):
Compound 2a. Colorless oil, IR (neat) vmaxcm-1: 3064, 3032, 2986, 2932, 2874, 1655, 1630. 1H NMR: δ 1.34 (s, 3H), L40 (s, 3H), 3.71-3.76 (m, 1H), 3.84-3.90 (m, 1H), 4.00-4.15 (m, 2H), 4.39 (d, 1H,J= 11.8 Hz), 4.63 (d, 1H,J = 11.8,), 5.31-5.39 (m, 2H), 5.76-5.88 (m, 1H), 7.27-7.34 (m, 5H). 13C NMR: δ 25.3, 26.6, 66.8, 70.6, 77.7, 81.1, 109.5, 119.6, 127.7, 127.9, 128.4, 135.3, 138.2; HRMS Caled, for C15H20O3 ([M+Na]+) m/z 249.1413, found 249.1415.
Compound 2b. Colorless oil, IR (neat) vmaxcm-1: 3089, 2988, 2935, 2884, 1647, 1457. 1H NMR: δ 1.34 (s, 3H), 1.41 (s, 9H), 3.68-3.74 (m, 1H), 3.93-3.96 (m, 1H), 4.08-4.16 (m,2H), 4.34-4.37 (m, 1H), 5.21 (d, 1H,J= 11.9 Hz), 5.41 (d, 1H, J = 11.9 Hz), 5.87-6.00 (m, 1H). 13C NMR: δ 25.2, 26.7, 26.9, 27.0, 67.0, 76.7, 80.5, 81.2, 109.4, 109.6, 117.2, 135.9. HRMS Caled, for C12H20O4 ([M+Na]+) m/z 251.1259, found 251.1252.
Compound 2c. Colorless oil, IR (neat) vmaxcm-1: 3090, 2985, 2935, 2884. 1H NMR: δ 1.05 (9H, s), 3.90-3.94 (m, 1H, H-OCH2), 4.13-4.16 (m, 2H, H-OCH2 & H-olefinic), 4.38-4.39 (m, 1H, H-olefinic), 4.48-4.51(t, 1H, J = 6.7 Hz, H-OCH), 4.79-4.88 (dd, 2H, J = 11.9 Hz, H-CH2(benzylic)), 7.25-7.43 (11H, m), 7.66-7.70 (4H, m).13C NMR: δ 20.3 (C-Si), 27.9 (Si(CH3)2), 62.9, 68.3 (OCH2-benzylic), 116.5 (CH2-olefinic), 128.1, 128.8, 128.9, 128.9, 129.0, 129.1, 129.5, 129.5, 129.7, 130.9, 130.9, 134.7 (CH- olefinic), 134.3, 136.7, 136.8, 139.4, 139.4 (C-Ar).
The authors thank the Department of Chemistry, IIT-Madras and Sambalpur University for support to the research.
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ΨS Dedicated to Prof. V. S. Parmar, University of Delhi on his 60th birthday.
(Received 8 July 2008 - Accepted 30 September 2008)