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Indian Journal of Chemistry Vol. 44B, August 2005, pp. 1686-1692 Reaction of orthoesters with alcohols in the presence of acidic catalysts: A study H M Sampath Kumar*, Sipak Joyasawal, B V S Reddy, P Pawan Chakravarthy, A D Krishna, & J S Yadav Organic Chemistry Division I, Indian Institute of Chemical Technology, Hyderabad 500007, India Email: [email protected] Received 30 September 2003; accepted (revised) 9 April 2005 Allylic and benzylic alcohols are converted into corresponding unsymmetrical ethers when reacted with various orthoesters in the presence of montmorillonite KSF at ambient temperature. A detailed study has been undertaken to examine the mechanism and generality of these reactions with regard to various acidic catalysts, which reveal interesting competitive reactions mainly O-acetylation, together with trace amount of dimerized product. The type of the side product and their relative quantity depends upon the nature of the catalyst employed. Furthermore, the low yields of the Claisen rearrangement product obtained from allylic alcohols under heating is rationalized due to the formation of some of these products. Keywords: Allylic alcohols, benzylic alcohols, unsymmetrical ethers, orthoesters, montmorillonite KSF, O-acetylation, Claisen rearrangement IPC: Int.Cl.7 C 07 C 31/00, C 07 C 69/00 Claisen-Johnson orthoester rearrangement1 is an important C-C bond forming reaction, which yields vinylic esters. This reaction being heat promoted is generally low yielding. The poor conversion is usually attributed to the inactivation of the catalyst or decomposition of the orthoester and to compensate these losses, the reaction is generally conducted for long reaction time heating at high temperature (with excess of orthoester in order to improve the yield). Even though reaction of orthoesters with various carbonyl functions such as ketones, aldehydes, carboxylic as well as sulphonic acids were studied in detail in the past, 5, 6 this study pertaining to alcohols is only limited to Claisen-Johnson rearrangement. However, to our knowledge, there has not been any indepth study regarding the nature of the side products or the details of the competitive reaction associated with this important transformation. As a part of ongoing activity to prepare dimethyl, vinyl carboxylate derivatives required for the synthesis of various pyrethroid analogs, we have been screening a number of solid acid catalysts to evolve an eco-friendly protocol2 for the Claisen-Johnson rearrangement. During our pursuit, we found serendipitously that various primary (allylic and benzylic) alcohols undergo rapid O-alkylation in the presence of Montmorillonite KSF at room temperature.3 This reaction was found to be general with regard to various orthoesters and alcohols, even though it was found to be selective with regard to allylic and benzylic alcohols. Our strategy was successfully applied to the total synthesis of natural products by Mori et al4. which proved the utility of this protocol as an attractive alternative to the existing O-alkylation protocols. ⎯⎯⎯⎯⎯⎯ IICT Communication No 020402 All the reactions were conducted at ambient temperature by stirring the alcohol with stoichio Results and Discussion In the context of our earlier observations, we studied various acids such as, silicon dioxide, clay, BF3, amberlyst-15 as an effort to evolve a high yielding and environmental friendly protocol and found that with the change in the catalyst, there has been a significant change in the yields as well as the nature of products formed (Scheme I). The formation of the side products in case of orthoester reaction at RT will be important and particularly interesting in case of Claisen-Johnson orthoester rearrangement at elevated temperature, as it would influence the yields and hence the results of our studies in this regard is presented here. First the outcome of this reaction of the orthoesters with alcohols at room temperature in the presence of various catalysts is presented in Table I. KUMAR et al.: REACTION OF ORTHOESTERS WITH ALCOHOLS IN THE PRESENCE OF ACIDIC CATALYSTS O OH R R'C ( O R ") 3 C atalyst R O R'' + R R O + R O 1 2 R = V in yl, A ryl o r n -B u tyl R'= H , C H 3 R "= C H 3 , C 2 H 5 , C 3 H 7 C atalyst: M o n tm o rillo n ite-K S F , K 1 0 , B F 3 .E t 2 O , S iO 2 , am b erlyst-1 5 3 Scheme I Table I ⎯ Reaction of orthoesters with alcohols in the presence of various acid catalysts at rt Conditiona Entry 1 2 Alcohol Time 1 2 3 12hr 12hr 12hr 12hr 12hr 4hr 80 83 65 13 35 75 5 15 20 5 5 4 5 45 4 1 MMKSF 8hr MMKSF 12hr MMKSF 8hr MMKSF 12hr MMK10 12hr 12hr SiO2 SiO2(μw)c 5min BF3.O(Et)2 6hr Amberlyst-15 12hr 90 72 93 51 76 15 5 5 2 10 4 15 3 35 82 71 41 6 4 15 5 7 Orthoester* Catalyst* TEOA MMKSF TMOA MMKSF TEOA MMK10 TEOA SiO2 TEOA Amberlyst-15 TEOA BF3.O(Et)2 C6H5CH2OH TEOA TEOF TMOA TPOF TEOA TEOA TEOF TEOA TEOA C6H5CH=CHCH2OH Product Yield (%)b 3 CH3(CH2)3CH2OH TEOA SiO2 12hr 10 10 35 4 CH2=CH-CH2OH TMOA MMKSF 8hr 86c 5 - 5 (CH3)2C=CH-CH2OH TMOA MMKSF 10hr 81c 4 - TMOA MMKSF 8hr 62 10 - TMOA MMK10 12h r 10 62 - TMOA MMKSF 12hr 92 - - TEOA MMKSF 12hr 90 - - TMOA MMKSF 12hr 84 - - TEOA SiO2 12hr 75 - 10 TEOA SiO2 12hr 78e - 6 6 OH O OH 7 8 C H C C H H2 O CH2OH O 9i CH2OH S OH j 10 Ph OH 11d MeO a. All reactions are carried out at room temperature. b. Yield of isolated product after chromatography. c. Based on GC analysis of the reaction mixture. c. Pulsed irradiation in domestic microwave oven (BPL BMO 700T) using Pyrex test tube. d. Optically pure (absolute configuration S), Optical rotation [α ]D25 -31.0 [c 1.5 CHCl3]. e. Optically pure (absolute configuration S), Optical rotation [α]D25 -4.5 [c 2.2 CHCl3]. * TEOA; Triethylorthoacetate, TMOA: Trimethylorthoacetate, TPOF: Tripropylorthoformate TEOF: Triethylorthoformate, MMKSF: Montmorillonite KSF, MMK10: Montmorillonite K10 1687 INDIAN J. CHEM., SEC B, AUGUST 2005 1688 Plausible mechanism HO EtO OEt OEt Alcohol, Catalyst H3C H3C EtOH OEt O + i H3C OEt O Ph + EtOH o Et2O O + O Et Et HO H3C + CH3CO2Et ii OEt Scheme II ⎯ Reaction of orthester withalcohols at rt in the presence of acidic catalysts metric quantity of orthoester together with appropriate solid catalysts under N2. The products were isolated and characterized after chromatography. We found that Montmorillonite catalysts (both KSF and K10) gave excellent yields of unsymmetrical ethers 1 and as stated before, this reaction is found to be general with regard to various orthoesters but selective with respect to various primary allylic and benzylic alcohols. However, when SiO2 is used as the catalyst, we could isolate corresponding O-acetylated compound 3 as major product (in case of primary alcohols) together with minor quantity of dimeric ethers 2 and unsymmetrical ethers 1 and this catalyst was effective in case of primary saturated (nonbenzylic and allylic) as well as secondary alcohols also. As O-acetylation with orthoester is not hitherto a known reaction, we were curious to explore this to find out whether such a transformation could be useful as a synthetic methodology and the results are summarized in Table I. Thus the reaction of alcohols and orthoester with various acid catalysts give following products at ambient temperature; unsymmetrical ether 1, Oacetylated compound 3, dimeric ethers 2. Formation of all the above products can be easily explained by a mechanism involving a cationic intermediate (i, ii) generated by the action of the acidic catalyst on the orthoester. The unsymmetrical ether formation is possible through a horizontal transfer of alkyl group to the alcohol-oxygen, whereas the O-acetylated product is possibly formed by the nucleophilic attack of the alcoholic oxygen on the positively charged carbon of the cationic intermediate (ii) as shown in Scheme II. This could be established by allowing the optically pure secondary alcohol (11, absolute configuration S) with orthoester in the presence of silicon dioxide which formed corresponding ether with retention of configuration as determined by chemical correlation (absolute configuration of the product was determined by chemical correlation with a literature precedent7 that compounds with close structural resemblance and having same sign of optical rotation should have same absolute configuration). However, in case of secondary alcohols, the acetylated product could be isolated in only trace amounts. As the relative abundance of these products mainly depends upon the nature of the catalyst used, the product formation can be correlated to the degree of stabilization of the positively charged intermediate on the solid acids. For instance, predominant formation of the unsymmetrical ether in case of Montmorillonite catalysts can only be explained due to the stabilization of the positively charged intermediate in the inter lamellar layer of the smectite clay, whereas the poor binding in case of silicon dioxide which allows the nucleophilic attack by the alcohol on the positively charged carbon center of the intermediate (i) followed by the loss of alkyl group either to the substrate (forms unsymmetrical ether) or ethanol (forms diethyl ether) may result in the formation of O-acetylated product as shown in Scheme II. When these reactions were conducted in the presence of acidic ion exchange resin i.e., amberlyst-15 or Lewis acid like BF3.Et2O, a mixture of unsymmetrical ether and dimeric ether were found to be the major components of the product mixture with only a trace amounts of the corresponding acetylated product (<5-7%). Formation of the dimeric ethers in significant proportions may be attributed to the formation of allylic and benzylic cations by the action of strong Lewis acid or ion exchange resin which in turn undergoes nucleophilic attack by the other alcoholic oxygen to form dimeric ether. However, when TEOF was used, the transformation was relatively clean as no acetylated compound was KUMAR et al.: REACTION OF ORTHOESTERS WITH ALCOHOLS IN THE PRESENCE OF ACIDIC CATALYSTS formed. Further, this reaction could be conveniently accelerated at elevated temperature (conventional or microwave irradiation inw the presence of SiO2) which gave corresponding unsymmetrical ether in high yields. Since the above reactions viz., O-alkylation, Oacetylation and dimerization occur at ambient temperature conditions, this could pose a problem particularly when we attempt Claisen-Johnson orthoester rearrangement involving allylic alcohols employing any such catalysts. It has been disclosed by other groups that carboxylic and sulfonic acids form corresponding esters when refluxed with orthoesters.5 As we carry out Claisen rearrangement at higher temperature thresholds, utility of carboxylic acids (eg, butyric acid) as catalysts would be disadvantageous. Because these catalysts get inactivated (in the form of esters) rapidly and this in turn demands addition of aliquots of catalysts at regular intervals in order to take the reaction to forward direction. Even though use of phenol is found to be an attractive option in this regard, our continued interest in finding alternative catalysts to derive an eco-friendly experimental protocol for this transformation employing reusable solid catalysts such as Montmorillonites or silicon R' CH3C(OEt)3 R OH R' dioxide, prompted us to attempt Claisen-Johnson orthoester rearrangement of some representative allylic alcohols using these catalysts. The kind of products derived from such an effort is clearly evident as we could isolate mixture of products together with Claisen rearranged product, which is in conformity with our earlier observation (Scheme III). We could also isolate ethyl ether 3 together with the expected products, i.e., unsymmetrical terminal ether 4, dimeric ether 5, claisen ester 1 and acetylated product 2. Thus the complex mixture of the products formed clearly renders the above protocol unsuitable for this purpose (Table II). In conclusion, we have for the first time unravelled a series of reactions that are associated with the interaction of orthoesters with primary and secondary alcohols in the presence of various acid catalysts. OAcetylation is another important transformation observed for the first time by the interaction of the alcohols and orthoesters. These reactions, particularly O-alkylations and O-acetylation could be serious side reactions when Claisen Johnson orthoester rearrangement is attempted in the presence of various acid catalysts. O R R + EtO C Catalyst, reflux 2 2 R OEt R + O R' 4 R' 5 Scheme III Table II ⎯ Result of Claisen-Johnson orthoester rearrangement in the presence of various acid catalysts under conventional heating Entry R R' Alcohols OH R R' R' R R' + O 1 Catalyst= Montmorillonite-KSF and K10, SiO2 R,R'=H,CH3, Ph 1689 Conditions Product yield (%) Orthoester* Catalyst* Time 1 2 3 4 5 1 R=H,R'=C6H5 R=R'=CH3 TEOA TEOA MMKSF MMKSF 6hr 5hr 24 22 5 23 35 3 7 21 32 5 2 R=H, R'=C6H5 R=R'= CH3 TEOA TEOA MMK10 MMK10 8hr 6hr 16 13 8 27 34 4 5 24 34 6 3 R=H, R'=C6H5 R=R'=CH3 TEOA TEOA SiO2 SiO2 6hr 6hr *. TEOA: Triethylorthoacetate. MMKSF: Montmorillonite-KSF MMK10: Montmorillonite-K10 3 32 10 23 3 4 28 8 27 5 OEt 3 1690 INDIAN J. CHEM., SEC B, AUGUST 2005 Experimental Section Infrared spectra were recorded on a Perkin-Elmer Infrared 683 spectrophotometer with NaCl optics. Proton magnetic resonance spectra were recorded on a Varian GEMINI-200, AVANCE-300, Varian UNITY400 NMR spectrometer in CDCl3. In the 1H NMR spectra tetramethylsilane was used as an internal reference. Chemical shifts were reported in ppm downfield from TMS and were given on the δ scale. Mass measurements were carried out on a MicroMass VG70-70H mass spectrometer operating at 70 eV using direct inlet system and were given in the mass units (m/z). Unless otherwise stated, all non-aqueous reactions were performed under an atmosphere of nitrogen in flame dried glass equipped with stir bar and a rubber septum. Standard inert atmosphere techniques were used in handling all air and moister-sensitive reagents. Reactions were monitored by analytical thin layer chromatography (TLC) using 0.25-mm E.Merk precoated silicagel plate (60F254). The spots were detected using UV light (254 nm), blowing I2 or by dipping into anisaldehyde/sulfuric acid or βnaphthol/sulfuric acid solution followed by charring on a hot plate. Product purification by flash column chromatography was performed on silica gel (100-200 mesh). Solutions in organic solvents were dried over anhydrous sodium sulfate and solvents were stripped off on a Buchi rotary evaporator connected to water aspirator. Trace solvent was removed on a vacuum pump. TEOF, TEOA, TMOA, TPOF, Benzyl alcohol, cinnamyl alcohol, Silica Gel, MMK10, MMKSF etc. are commercially available and used after drying. (a) Reaction of orthoesters with alcohols at rt.; O-alkylation/acetylation of alcohols. General procedure. Mixture of alcohol (1 mole), orthoester 2 moles and catalyst (30% m/m) was stirred at rt under N2 for 4-12 hr. The reaction was monitored by TLC and after completion of reaction, the reaction mixture was filtered and catalyst was washed with DCM (10 mL), the filtrate was concentrated in vacuo and the products were separated and purified by column chromatography on silica gel to afford the products. Wherever, BF3.Et2O was used as the catalyst, the crude product was isolated by quenching the reaction mass with water (10 mL) followed by extraction with ether (2×10 mL) and evaporation of the solvent. Benzyl ethyl ether: 1H NMR (200 MHz, CDCl3): δ 1.25 (t, J= 8.0 Hz, 3H, OCH2CH3), 3.55 (q, J = 8.0 Hz, 2H, OCH2CH3), 4.50 (s, 2H, benzylic CH2) 7.207.40 (m, 5H, aromatic H); IR: 1230, 1087 cm –1; mass (m/z): 136 (M+). Dibenzyl ether: 1H NMR (200 MHz, CDCl3): δ 4.58 (s, 4H, benzylic CH2), 7.20-7.40 (m, 10H, aromatic H); IR: 1113 cm –1; mass (m/z): 198 (M+). Benzyl acetate: 1H NMR (400 MHz, CDCl3): δ 2.08 (s, 3H, COCH3), 5.10 (s, 2H, benzylic CH2), 7.20-7.40 (m, 5H, aromatic H); IR: 1741,1220, 1027 cm –1; mass (m/z): 150 (M+). Benzyl methyl ether: 1H NMR (200 MHz, CDCl3): δ 3.30 (s, 3H, OCH3), 4.55 (s, 2H, benzylic CH2), 7.20-7.40 (m, 5H, aromatic H); IR: 1225, 1077 cm –1; mass (m/z): 122 (M+). Ethyl 3-phenyl- (E)-2-propenyl ether: 1H NMR (300 MHz, CDCl3): δ 1.20 (t, J = 8.0 Hz, 3H, OCH2CH3), 3.50 (q, 8.0 Hz, 2H, OCH2CH3), 4.15 (d, J = 7.2 Hz, 2H, allylic CH2), 6.22-6.30 (m, 1H, olefinic H), 6.50 (d, J = 17.4 Hz, 1H, olefinic H), 7.20-7.40 (m, 5H, aromatic H); IR: 1625, 1203, 1070 cm.–1; mass (m/z): 162 (M+). Methyl 3-phenyl- (E)-2-propenyl ether: 1H NMR (200 MHz, CDCl3): δ 3.40 (s, 3H, OCH3), 4.15 (d, J = 7.2, 2H, allylic CH2), 6.20-6.40 (m, 1H, olefinic H), 6.60 (d, J = 17.4 Hz, 1H, olefinic H), 7.20-7.40 (m, 5H, aromatic H); IR: 1641, 1200, 1065 cm.–1; mass (m/z): 148 (M+). Propyl 3-phenyl- (E)-2-propenyl ether: 1H NMR (200 MHz, CDCl3): δ 1.0 (t, J = 7.6 Hz, 3H, OCH2CH2CH3), 1.60-1.80 (m, 2H, OCH2CH2CH3), 3.44 (t, J = 7.6 Hz, 2H, OCH2CH2CH3), 4.15 (d, J = 7.2 Hz, 2H, allylic CH2), 6.20-6.40 (m, 1H, olefinic H), 6.60 (d, J = 17.4 Hz, 1H, olefinic H), 7.20-7.40 (m, 5H, aromatic H); IR: 1635, 1203, 1070 cm.–1; mass (m/z): 176 (M+). 3-Phenyl- (E)-2-propenyl acetate: 1H NMR (300 MHz, CDCl3): δ 2.25 (s, 3H, COCH3), 4.10 (d, J = 7.2 Hz, 2H, allylic CH2), 6.22-6.30 (m, 1H, olefinic H), 6.60 (d, J = 17.4 Hz, 1H, olefinic H), 7.15-7.35 (m, 5H, aromatic H); IR: 1736, 1635, 1236, 1027 cm –1; mass (m/z): 176 (M+). Pentyl acetate: 1HNMR (200MHz, CDCl3): δ 0.90-1.40 (m, 9H, OCH2CH2CH2CH2CH3), 2.00 (s, 3H, COCH3), 4.05 (t, J = 7 Hz, 2H, OCH2); IR: 1740,1235,1050 cm.–1; mass (m/z): 130 (M+). Ethyl pentyl ether: 1H NMR (200 MHz, CDCl3): δ 0.90-1.50 (m, 9H, CH3(CH2)3CH2O), 1.20 (t, J = 8.0 Hz, 3H,OCH2CH3), 3.50 (t, J = 7.4 Hz, 2H, OCH2(CH2)3CH3), 3.60 (q, J = 8.0 Hz, 2H, OCH2CH3); IR: 1155, 1090 cm –1; mass (m/z): 116 (M+). Allyl methyl ether: 1H NMR (200 MHz, CDCl3): δ 3.30 (s, 3H, CH3O), 3.74 (d, J = 7 Hz, 2H, allylic CH2), 4.90-5.10 (m, 2H, olefinic H), 5.60-5.80 (m, KUMAR et al.: REACTION OF ORTHOESTERS WITH ALCOHOLS IN THE PRESENCE OF ACIDIC CATALYSTS 1H, olefinic H); IR: 1645, 1208, 1078 cm.–1; mass (m/z): 72 (M+). Methyl 3-methyl-2-butenyl ether: 1H NMR (200 MHz, CDCl3): δ 1.50 (s, 3H, vinylic CH3), 1.60 (s, 3H, vinylic CH3), 3.30 (s, 3H, OCH3), 3.80 (d, 8 Hz, 2H, allylic CH2), 5.10-5.20 (m, 1H, olefinic H); IR: 1675, 1215, 1090 cm.–1; mass (m/z): 100 (M+). 1-[4-Methoxy-(Z)-2-butenyloxymethyl] benzene: 1 H NMR (200 MHz, CDCl3): δ 3.30 (s, 3H, CH3O), 4.00 (dd, J= 8.2, 16.8 Hz, 4H, allylic CH2O), 4.45 (s, 2H, benzylic CH2), 5.70-5.90 (m, 2H, olefinic H), 7.20-7.40 (m, 5H, aromatic H); IR: 1640, 1210,1125, 1085, 1034 cm.–1; mass (m/z): 192 (M+). 3,7-Dimethyl-(2E)-2,6-octadienyl methyl ether: 1 H NMR (300 MHz, CDCl3): δ 1.60 (s, 9H, vinylic CH3), 2.00 (m, 4H, CH2CH2), 3.20 (s, 3H, CH3O), 3.80 (d, J = 8 Hz, 2H,allylic CH2), 5.00 (t, J = 7 Hz, 1H, olefinic H ), 5.25 (t, J = 8 Hz, 1H, olefinic H); IR: 1675, 1660, 1205, 1105 cm.–1; mass (m/z): 168 (M+). Benzo[d][1,3]dioxol-5-ylmethyl methyl ether: 1 H NMR (200 MHz, CDCl3): δ 3.30 (s,3H, CH3O), 4.40 (s, 2H, benzylic CH2), 5.90 (s, 2H, OCH2O), 6.72 (s, 2H, aromatic H), 6.82 (s,1H, aromatic H); IR: 1272, 1196, 1050 cm.–1; mass (m/z): 166 (M+). Benzo[d][1,3]dioxol-5-ylmethyl ethyl ether : 1H NMR (200 MHz, CDCl3): δ 1.20 (t, J = 8.0 Hz, 3H, OCH2CH3), 3.48 (q, J = 8.0 Hz, 2H, OCH2CH3), 4.40 (s, 2H, benzylic CH2), 5.90 (s, 2H, OCH2O), 6.72 (s, 2H, aromatic H), 6.82 (s,1H, aromatic H); IR: 1247, 1189, 1099 cm.–1; mass (m/z): 180 (M+). Methyl 2-thienylmethyl ether: 1H NMR (200 MHz, CDCl3): δ 3.44 (s, 3H, CH3O), 4.40 (s, 2H, CH2O), 6.75 (m, 2H, aromatic H), 7.05 (m, 1H, aromatic H); IR: 1208, 109°cm. -1; mass (m/z): 128 (M+). Ethyl 1-phenylethyl ether: 1H NMR (200 MHz, CDCl3): δ 1.22 (t, J = 8.0 Hz, 3H, OCH2CH3), 1.50 (d, J = 7.2 Hz, 3H, CHCH3), 3.38(q, J = 8.0 Hz, 2H, OCH2CH3), 4.50 (q, J = 7.2 Hz, 1H, benzylic H), 7.20-7.30 (m, 5H,aromatic H); IR: 1225,1086 cm.–1; mass (m/z): 150 (M+). 1-Phenylethyl acetate: 1H NMR (200 MHz, CDCl3): δ 1.56 (d, J = 7.2 Hz, 3H, CHCH3), 2.11 (s, 3H, COCH3), 5.88 (q, J = 7.2 Hz, 1H, benzylic CH), 7.20-7.30 (m, 5H,aromatic H); IR: 1735, 1261, 1040 cm.–1; mass (m/z): 164 (M+). 2-[1-Ethoxy-(1S)-ethyl]-6-methoxynapthalene: 1 H NMR (200 MHz, CDCl3): δ 1.22 (t, J = 8.0 Hz, 3H, OCH2CH3), 1.50 (d, J = 8.2 Hz, 3H, benzylic CH3 ), 3.38 (q, J = 8.0 Hz, 2H, OCH2CH3), 3.68 (s, 3H, 1691 OCH3), 4.50 (q, J = 8.2 Hz, 1H, benzylic CH), 7.407.56 (m, 3H, aromatic H), 7.62-7.82 (m, 3H, aromatic H); IR: 1270, 1179, 1101, 1020 cm.–1; mass (m/z): 230 (M+); [α]D25 -4.5 (c= 2.2 CHCl3). b) Reaction of orthoesters with allylic alcohols at elevated temperature; Claisen-Johnson orthoester rearrangement. General procedure: Mixture of allylic alcohol (1 mole), orthoester (3 moles) and catalyst (30% m/m) was heated at 100°C under N2 for 4-8 hr. The reaction was followed by TLC and after completion of reaction, the reaction mixture was filtered and catalyst was washed with DCM (10 mL), the combined filtrate was concetrated in vacuo and the products were separated and purified by column chromatography on silica gel. Ethyl 3-phenyl-4-pentenoate: 1H NMR (200 MHz, CDCl3): δ 1.18 (t, J = 8.0 Hz, 3H, OCH2CH3), 2.65 (dd, J = 5.2, 9 Hz, 2H, CH2CO2Et), 3.84 (q, J = 9 Hz, 1H, benzylic CH), 4.05 (q, J = 8.0 Hz, 2H, OCH2CH3), 5.00-5.20 (m, 2H, olefinic H), 5.90-6.10 (m, 1H, olefinic H), 7.15-7.30 (m, 5H, aromatic H); IR: 1740, 1642, 1235, 1030 cm.–1; mass (m/z): 204 (M+). Ethyl 2-phenylallyl ether: 1H NMR (300 MHz, CDCl3): δ 1.22 (t, J = 8 Hz, 3H, OCH2CH3), 3.5 (q, J = 8 Hz, 2H, O CH2CH3), 4.7 (d, J = 8 Hz, 1H, benzylic CH), 5.05 (d, J = 18 Hz, 1H, olefinic H), 5.22 (d, J = 8 Hz, 1H, olefinic H), 5.80-6.00 (m, 1H, olefinic H), 7.25-7.35 (m, 5H, aromatic H); IR: 1650, 1200, 1057 cm.–1; mass (m/z): 162 (M+). Ethyl 3,3-dimethyl-4-pentenoate: 1H NMR (200 MHz, CDCl3): δ 1.10 (s, 6H, (CH3)2C), 1.18 (t, J = 8 Hz, 3H, OCH2CH3), 2.22 (s, 2H, CH2C=O), 4.05 (q, J = 8 Hz, 2H, OCH2CH3), 4.85 (d, 1H, J = 7.5 Hz, olefinic H), 4.95 (d, 1H, J = 15 Hz, olefinic H), 5.82 (dd, J = 7,16 Hz, 1H, olefinic H); IR:1742, 1645, 1220, 1034 cm.–1; mass (m/z): 156 (M+). Acknowledgement One of the authors (S J) thanks CSIR, New Delhi for the award of fellowship. References 1 (a) Johnson W S, Werthemann L, Bartlett W R, Brockson T J, Li T, Faulkner D J & Petersen M R, J Am Chem Soc, 92, 1970, 741. 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