Synthesis of some fenchyl-substituted alkenes and enol-ethers containing 3-oxyphenyl substituents by the Barton-Kellogg reaction

ARTICLE

Synthesis of some fenchyl-substituted alkenes and enol-ethers containing 3-oxyphenyl substituents by the Barton-Kellogg reaction

Luiz Francisco M. L. Ciscato^I,III; Erick L. Bastos^II; Fernando H. Bartoloni^I; Wolfgang Günther^III; Dieter Weiss^III; Rainer Beckert^III; Wilhelm Josef BaaderI,^*

^IDepartamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, 05508-000 São Paulo-SP, Brazil

^IICentro de Ciências Naturais e Humanas, Universidade Federal do ABC, 09210-170 Santo André-SP, Brazil

^IIIInstitut für Organische Chemie und Makromolekulare Chemie, Friedrich-Schiller Universität Jena, 07737 Jena, Germany

ABSTRACT

The synthesis of one fenchyl-substituted alkene and two enol-ethers, containing 3-oxyphenyl substituents by the Barton-Kellogg reaction is described. The tri-substituted aromatic fenchyl-alkene 1a was prepared in 53% yield from thiofenchone and a diazoanisole; whereas enol-ethers 1b and 1c were obtained (95 and 75% yield, respectively) using an inverse approach based on diazofenchone and aromatic thionoesters. A mixture of Z and E isomers was obtained in all cases; isomer attribution and quantification has been carried out by analysis of NMR spectroscopic data assisted by theoretical calculations (E/Z ratio: 1a = 0.72, 1b = 2.2, 1c = 1.8). Reaction proceeds with low stereoselectivity leading to the preferential formation of diastereoisomeric olefins and enol-ethers where the aromatic substituent resides at the side of the two fenchyl methyl groups.

Keywords: Barton-Kellogg olefination, hindered alkenes, diazo compounds, thioketone, fenchone

RESUMO

Usando-se a reação de Barton-Kellogg, foram sintetizados um alceno e dois enóis-éteres fenchyl-substituídos, contendo grupos 3-oxifenila como substituintes. O fenchil-alceno aromático tri-substituído 1a foi preparado com rendimento de 53% a partir de tiofenchona e um diazoanisol. A abordagem inversa, baseada no acoplamento entre diazofenchona e tionoésteres aromáticos, foi utilizada para produzir os enóis-éteres 1b e 1c (rendimentos 95 e 75%, respectivamente). Em todos os casos foi obtida uma mistura dos isômeros E e Z; a atribuição e quantificação destes isômeros foram realizadas pela análise dos dados de espectroscopia de RMN assistida por cálculos teóricos (relação E/Z 1a = 0,72, 1b = 2,2, 1c = 1,8). A reação ocorre com baixa estereosseletividade, levando à formação preferencial dos diastereoisômeros das olefinas e enóis-éteres, nos quais o substituinte aromático está localizado ao lado dos dois grupos metila da porção fenchila.

Introduction

The chemistry of hindered olefins is a challenging topic for both synthetic and theoretical chemists.^1,2 Strained alkenes containing the fenchone-type skeleton are chiral elements in asymmetric synthesis^3-5 and model compounds in the study of structural deformations on double bonds.⁶ The Barton-Kellogg (BK) reaction, or Barton-Kellogg (BK) olefination,^7-9 is the method of choice for the efficient preparation of such alkenes. The synthesis of 2-diphenylmethylenefenchane, 2-cyclohexylidenefenchane and bis-fenchylidene, one of the most hindered olefin yet synthesized, were accomplished with this method.^6,10,11 The reaction consists of the 1,3-dipolar cycloaddition between a diazo and a thio or selenocarbonyl compound, furnishing the target alkene after two-fold extrusion of nitrogen and sulfur/selenium from Δ³-1,3,4-thia- or selenodiazolines.⁹ The synthesis of enol-ethers by the BK method has been scarcely investigated,^12,13 and derivatives bearing the fenchyl group have not yet been described.

The stabilization provided by the fenchyl group to these alkenes and enol-ethers, can bring new knowledge and possibilities to areas like organic synthesis and material science, as well as to chemiluminescence research, where they can be used as affordable starting materials for the synthesis of thermally stable 1,2-dioxetane derivatives.^14-16 Therefore, herein we present: (i) the synthesis of the tri-substituted alkene 1a by conventional BK reaction from thiofenchone (5a) and diazoalkane 4a, and (ii) the synthesis of enol-ethers 1b and 1c using the inverse approach for the BK reaction, i.e., from diazofenchane (4b) and thionoesters 5b and 5c (Schemes 1 and 2).

Results and Discussion

Treatment of (–)-fenchone (3a) with Lawesson's reagent in refluxing xylene afforded (–)-thiofenchone (5a) in 95% yield (Scheme 1).¹⁷ Alternatively, 5a was prepared by the reaction of 3a with a mixture of H₂S/HCl gas in trimethyl orthoformate (88% yield, orange crystals). The diazo compound 4a was prepared by the Bamford-Stevens route from the corresponding tosylhydrazone 2a, using potassium tert-butoxide and pyridine in THF.¹⁸ This compound is highly unstable and therefore it was not isolated or characterized. Reaction yield was estimated in 75% from the stoichiometry of the subsequent reaction with thiofenchone (Scheme 1). The reaction of 5a with the deep red diazo compound 4a occurred immediately at room temperature affording a diastereoisomeric mixture of Δ³-1,3,4-thiadiazolinic intermediates 6a, as indicated by discoloration of the mixture and TLC analysis (Scheme 1). Warming up of the reaction mixture leads to nitrogen liberation and formation of thiirane 7a as a colorless solid. A solution of 7a in toluene was treated with triphenylphosphine and refluxed for 24 h, resulting, after sulfur extrusion, in a mixture of E/Z isomers of the alkene 1a (overall yield: 53% from 5a). The products were isolated and characterized by spectroscopic analysis.

Enol-ethers 1b and 1c were prepared by the coupling of diazofenchane (4b) with thionoesters 3b and 3c (Scheme 2).^12,13 This inverse approach is required because the conventional BK reaction would require the difficult preparation of a methoxy-substituted diazomethane derivative.¹⁹ The Bamford-Stevens method was unsuccessful to convert the corresponding tosylhydrazone to diazofenchane (4b). Thus, 4b was prepared by oxidation of fenchylhydrazone (2b) with nickel peroxide and calcium oxide in dimethoxyethane at –78 ºC.^20,21 Again, due to the highly unstable nature of 4b, the yield of this transformation could only be estimated to be as high as 90% based on the stoichiometry of the subsequent reaction with the thionoesters. Thionoesters 5b and 5c were prepared from the corresponding esters with Lawesson's reagent in 84% (R = Me) and 45% (R = TBDMS) yields,^17,22 and stored at –20 ºC after purification by column chromatography, to avoid polymerization.²³ The reaction between diazofenchane (4b) and thionoesters 5b or 5c was carried out in a similar manner as described previously for diazo compound 4a, except that the solution of 6c in toluene was refluxed with copper-bronze for 6 h after nitrogen extrusion (Scheme 2). Enol-ethers 1b and 1c were obtained as mixtures of isomers, with overall yields of 95 and 75%, respectively.

The fenchyl-substituted alkene 1a and enol-ethers 1b and 1c were obtained in better yields with the BK method that by other classical methods of carbon-carbon double-bond formation. For example, the methylenation of fenchone by an optimized Wittig reaction was reported to afford a yield of 5%.²⁴ The same reaction using the very expensive Tebbe's reagent gives a yield of only 16%, showing that the preparation of fenchone-derived olefins generally occurs with low yields.²⁴ Our own attempts to prepare these compounds using Wittig and Horner-Wadsworth-Emmons (HWE) methods failed or resulted in very poor yields. (–)-Fenchone showed to be not reactive in HWE conditions, independent of the base used (NaH, n-BuLi, sec-BuLi, LDA, LiHMDS), with no product formation detected at all, even after a 24 h reaction time. Also, the intermolecular McMurry cross-coupling^15,25 of methyl 3-methoxybenzoate and (–)-fenchone, in the presence of either TiCl₃/LiAlH₄/TEA or TiCl₄/Zn, did not produce reproducible results.²⁶

The attribution of NMR signals and quantification of the E and Z isomers of 1a and 1b was performed as follows. The ¹H NMR spectra of 1a featured two singlet signals at 6.15 and 6.16 ppm, which clearly indicate the existence of two different olefinic hydrogen atoms. The HSQC experiment (see Figure S1) shows that there are two different sets of signals corresponding to three methyl groups each: one group with ¹H shifts of 1.27, 1.04 and 0.98 ppm, and another group at 1.15, 1.14 and 0.95 ppm (Figure S2 and Figure S3). The integration of the corresponding carbon peaks in the ¹³C spectrum showed a slight majority of the first set of methyl groups. The attribution of E and Z isomers could not be done by means of a straightforward 1D NOESY spectrum, because the irradiation intensity at the olefinic hydrogen had to be very high to produce any detectable nOe signal for the adjacent methyl groups, causing a broadening of the signal that prevented the selective excitation of just one hydrogen atom. The 2D NOESY experiments (see Figure S4) showed that the hydrogen atom at 6.15 ppm is located in the proximity of the region of the two methyl groups with signals at 1.15 and 1.14 ppm, an indication that these signals correspond to the E isomer. Contrarily, the hydrogen atom at 6.16 ppm is located in the proximity of only one methyl group at 1.27 ppm, therefore corresponding to the Z isomer (see spatial structures in Scheme 1). Consequently, the signals at 6.15 and 6.16 ppm were attributed to the olefinic hydrogen atoms of isomers E and Z, respectively (E/Z ratio = 0.72). NMR spectra (HMBC, HSQC, NOESY, HSQC/HMBC) are given in Supplementary Information.

The evaluation of the ¹H NMR spectrum of 1b also suggests the presence of two isomers, because there are two different olefinic methoxy group signals at 3.09 and 3.08 ppm, but only one signal at 3.73 ppm, corresponding to the aromatic methoxy group of both isomers. The HSQC spectra (see Figure S5) also corroborate the presence of two isomers: methyl groups of the fenchyl moiety are observed at 1.45, 1.01 and 0.52 ppm for one isomer, and 1.23 (two signals together) and 0.56 ppm for the other isomer (Figure S2). With the methyl groups of different isomers localized, selective excitation 2D- and 1D-NOESY experiments were carried out to assert the attribution of E and Z isomers of 1b (see Figures S5 and S6). Selective excitation of the single methyl group at 1.45 resulted in the nOe enhancement of the methoxy group signal at 3.09 ppm, identifying it as belonging to the E isomer (Figure S7). Excitation of the two methyl groups at 1.23 ppm resulted in the NOE enhancement of the signal at 3.08 ppm, therefore belonging to the Z isomer (see structures in Scheme 2). The integration of the carbon peaks in the INVGATE ¹³C NMR spectrum²⁷ revealed an E/Z ratio of 2.2.

The spectrum of 1c showed to be very similar to that of 1b, so E/Z attribution could be performed in analogy to the former compound. Two sets of three methyl groups could also be observed in the ¹H NMR spectrum, one set with peaks at 1.50, 1.06 and 0.60 ppm, and another one with two peaks together at 1.29 and one at 0.63 ppm. For 1b, the two CH₃ signals together correspond to the Z isomer; therefore it is assumed that this is also the case for 1c. An E/Z ratio of 1.8 could be obtained from the integration of these ¹H signals.

A quantum chemistry study was carried out in order to provide further structural evidence on E/Z attribution of 1a and 1b isomers from spectroscopic data. The equilibrium geometries, vibrational frequencies and thermochemical data were obtained using the hybrid generalized gradient approximation functional PBE1PBE²⁸ with the 6-311+G(d,p) basis set (see Supplementary Information).²⁹ This functional was used because it produced excellent results in the calculation of bond angles in a dataset of 27 small organic molecules, when compared with other 37 density functionals, including the popular B3LYP.³⁰ Also, structural parameters of E/Z isomers of bis-fenchylidene obtained at the PBE1PBE/6-311+G(d,p) level are in good agreement with both X-ray data as well as theoretical calculations (see Supplementary Information).^31-35 The optimized structures of E/Z , 1a and 1b are depicted in Figure 1 and relevant geometry parameters are given in Table S1 (see Supplementary Information). The isotropic chemical shielding at H and C nuclei of 1a, 1b and TMS were calculated at the GIAO/IEFPCM/W04P/cc-pVDZ//pbe1pbe/6-311+G(d,p) level.³⁴ This approach allowed us to calculate ¹H and ¹³C chemical shifts of 1a and 1b, which were compared to the experimental data (Figure S8).³⁵ The adjusted coefficients of determination (adj-R² > 0.99) indicate that the predicted d values for both ¹H and ¹³C nuclei are in excellent agreement with experimental data.

Conclusions

Sterically-hindered fenchyl-substituted aromatic alkene 1a can be prepared using the Barton-Kellogg method from thiofenchone and a diazoanisole; whereas enol-ethers 1b and 1c can be obtained from diazofenchane and aromatic thionoesters. This approach results in product formation in better yields than those obtained by classical methods and can be useful for the synthesis of encumbered alkenes. Reaction proceeds with low stereoselectivity leading to the preferential formation of diastereoisomeric olefins and enol-ethers where the aromatic substituent resides at the side of the two fenchyl methyl groups.

Experimental

All reactants were purchased at highest commercial quality and used without purification, except otherwise mentioned. Solvents were purified and dried as described by Armarengo and Perrin.³⁶ Bulb-to-bulb distillations were made using a Kugelrohr apparatus (Büchi Glass Oven B-580). Melting points (ºC) were determined in a Kofler hot-stage apparatus and uncorrected. NMR spectra were acquired on a Bruker spectrometer AC-250, DRX-400, or DRX-500 (25 ºC, CDCl₃, tetramethylsilane (TMS) as internal reference). Mass spectra were obtained on a CG-MS Hewlett-Packard 5890/5988, MAZ 95 XL or Finnigan MAT SSQ 710.

CAUTION: Although no accident occurred during the progress of this work, diazo compounds are known to decompose explosively.³⁷ Therefore, all caution should be taken.

(1R)-2-Thione-1,3,3-trimethylbicyclo[2.2.1]heptane ((–)-thiofenchone, 5a)

Method A: A mixture of (–)-fenchone 3a (1.5 g, 10 mmol) and Lawesson's Reagent (8.1 g, 20 mmol) was refluxed in xylene (20 mL) under inert atmosphere for 3 h and filtered over silica gel (5 cm, heptane). The resulting orange solution was evaporated in vacuo (0.3 mbar, rt) until 5a starts to distillate. Xylene was mostly removed by stirring the solution at temperatures between –50 ºC and room temperature at 0.3 mbar. The product was obtained in 95% yield (1.6 g, 0.95 mmol) containing small amounts of residual xylene.

Method B: A mixture of (–)-fenchone (4.0 g, 27 mmol) and Lawesson's reagent (6.5 g, 16 mmol) was refluxed under inert atmosphere for 1 h until it becomes a homogeneous solution. Thereafter, the mixture was allowed to reach room temperature and the resulting oil was purified by column chromatography (SiO₂, hexanes/ethyl acetate 9:1), resulting in 5a (3.1 g, 21 mmol, 78% yield).

Method C: A solution of (–)-fenchone (1.5 g, 10 mmol) and trimethylorthoformate (12 g, 94 mmol) in dry methanol (10 mL) was simultaneously bubbled at 0 ºC with gaseous HCl and H₂S for 2 h. Ethyl ether (25 mL) was added and the resulting orange solution washed with water until the aqueous phase became neutral. The organic layer was dried (MgSO₄) and the solvent evaporated in vacuo. The resulting orange oil was purified by column chromatography (SiO₂, hexanes), resulting in pure 5a (1.48 g, 8.8 mmol, 88%).

¹H NMR (250 MHz, CDCl₃, 25 ºC): δ 1.9-1.6 (m, 4H, fenchyl H), 1.4-1.2 (m, 3H, fenchyl H), 1.28 (s, 3H, 1-CH₃), 1.14 (s, 3H, 3-CH₃), 1.10 (s, 3H, 3-CH₃) ppm. ¹³C NMR (62.9 MHz, CDCl₃, 25 ºC): δ 279.8 (C=S), 66.2 (1-C), 57.7 (3-C), 46.9 (4-C), 43.7 (7-C), 35.4 (6-C), 28.6 (5-C), 26.4 (3-CH₃), 25.0 (3-CH₃), 19.2 (1-CH₃) ppm. mp 22-24 ºC (reported: 22-24 ºC)^7-9

(1R)-2-Diazo-1,3,3-trimethylbicyclo[2.2.1]heptane (diazofenchane, 4b)

A solution of (–)-fenchone 3a (3.0 g, 20 mmol), hydrazine hydrate 100% (4.5 g, 20 mmol), and acetic acid (1.2 mL) in ethanol (8 mL) was refluxed for 8 h.³⁸ After reaching room temperature, the solution was evaporated in vacuo and Et₂O (30 mL) was added to the residue. The solution was washed with aqueous solutions of NaOH (10%, 30 mL) and brine (30 mL). The organic layer was dried (MgSO₄) and evaporated in vacuo, resulting in fenchylhydrazone (3.0 g, 18 mmol, 89% yield), which was recrystallized from heptane at –20 ºC. A solution of fenchylhydrazone (2.0 g, 12 mmol) in dry dimethoxyethane (60 mL) was cooled to –78 ºC and a mixture of NiO₂ (4 g, 44 mmol) and CaO (1 g, 18 mmol) was added slowly. The resulting suspension was stirred for 30 min and the temperature was allowed to reach 0 ºC. The suspension containing 4b was filtered under inert atmosphere and used immediately.

1-(Diazomethyl)-3-methoxybenzene (4a)

A solution of 3-methoxybenzaldehyde (1.0 g, 7.3 mmol) and N-tosylhydrazide (1.4 g, 7.3 mmol) in ethanol (15 mL) was refluxed for 8 h. After this period, the resulting solution was evaporated and the residue was purified by column chromatography (SiO₂, hexanes/ethyl acetate 1:1) resulting in N'-(3-methoxybenzylidene)-4-methylbenzenesulfonohydrazide 2a (2.2 g, 7.3mmol, 100%). To a solution of 2a (2.2 g, 7.3 mmol) in pyridine (15 mL) was added t-BuOK (0.8 g, 7.5 mmol). The mixture was warmed to 60 ºC under stirring, resulting in a red solution, which was poured into water (100 mL). The mixture was extracted with Et₂O (3 × 30 mL) and the unified organic layers were dried (MgSO₄, 0 ºC, 15 min). After filtration, the deep red solution of 1-(diazomethyl)-3-methoxybenzene (4a) was used immediately.

O-Methyl 3-methoxybenzothioate (5b)

A mixture of methyl 3-methoxybenzoate (3b, 1.0 g, 6.0 mmol) and Lawesson's reagent (3.3 g, 8.1 mmol) in dry xylene (10 mL) was refluxed for 12 hours. The suspension was filtered in silica gel (5 cm) and the solution evaporated in vacuo. The resulting yellow oil was purified by column chromatography (SiO₂, hexanes/ethyl acetate 5:1), yielding 5b (0.92 g, 5.0 mmol, 84%) as a bright yellow oil.

¹H NMR (250 MHz, CDCl₃, 25 ºC): δ 7.81 (dd, ³J_H,H 5 Hz, ⁴J_H,H 2.5 Hz, 1H, Ar, 4-H), 7.76 (t, ⁴J_H,H 2.5 Hz, 1H, Ar, 2-H), 7.31 (t, ³J_H,H 5 Hz, 1H, Ar, 5-H), 7.11 (dd, ³J_H,H 5 Hz, ⁴J_H,H 2.5 Hz, 1H, Ar, 6-H), 4.31 (s, 3H, C(S)OCH₃), 3.87 (s, 3H, ArOCH₃) . ¹³C NMR (62.9 MHz, CDCl₃, 25 ºC): δ 211.9 (C=S), 159.3 (Ar, 3-C), 139.5 (Ar, 1-C), 129.0 (Ar, 5-C), 121.2 (Ar, 6-C), 119.1 (Ar, 4-C), 113.6 (Ar, 2-C), 59.3 (ArOCH₃), 55.4 (C(S)OCH₃). Anal. calc. for C₉H₁₀O₂S: C 59.32, H 5.53, S 17.59, found: C 59.22, H 5.53, S 17.53%.

O-Methyl 3-(tert-butyldimethylsilyloxy)benzothioate (5c)

Methyl 3-(tert-butyldimethylsilyloxy)benzoate (3c) was prepared through microwave irradiation as described previously.³⁹ To a solution of 3c (3.0 g, 11 mmol) in dry xylene (50 mL), Lawesson's reagent (6.1 g, 15 mmol) was added and the resulting suspension was refluxed for 24 h. The suspension was filtered in silica gel (5 cm) and the solution evaporated in vacuo. The resulting yellow oil was purified by column chromatography (SiO₂, hexanes/ethyl acetate 9:1), yielding 5c (1.4 g, 5.9 mmol, 45%) as a bright yellow oil.

¹H NMR (250 MHz, CDCl₃, 25 ºC): δ 7.80 (dt, ³J_H,H 7.8 Hz, ⁴J_H,H 2.5 Hz, 1H, Ar, 4-H), 7.68 (t, ⁴J_H,H 2.0 Hz, 1H, Ar, 2-H), 7.26 (t, ³J_H,H 7.9 Hz, 1H, Ar, 5-H), 7.02 (ddd, ³J_H,H 8.0 Hz, ⁴J_H,H 2.5 Hz, ⁵J_H,H 1.0 Hz, 1H, Ar, 6-H), 4.29 (s, 3H, C(S)OCH₃), 1.01 (s, 9H, SiC(CH₃)₃), 0.24 (s, 6H, Si(CH₃)₂). ¹³C NMR (62.9 MHz, CDCl₃, 25 ºC): δ 211.9 (C=S), 155.4 (Ar, 3-C), 139.6 (Ar, 1-C), 128.5 (Ar, 5-C), 124.5 (Ar, 6-C), 121.8 (Ar, 4-C), 120.3 (Ar, 2-C), 59.3 (OCH₃), 25.4 (SiC(CH₃)₃), 18.2 (SiC(CH₃)₃), –5.0 (Si(CH₃)₂). MS (m/z): 282 (M⁺), 266, 251, 225, 209, 193, 177, 89. HRMS (Micro-ESI): m/z calc. for C₁₄H₂₃O₂SSi: 283.1188 (M-H)⁺; found: 283.1193.

3'-(3-Methoxyphenyl)-1,3,3-trimethylspiro[bicyclo[2.2.1]heptane-2,2'-thiirane] (7a)

Thiofenchone 5a (0.5 g, 3 mmol) was added to a solution of 1-diazomethyl-3-methoxybenzene (4a) (generated from 7.3 mmol of 2a) in diethyl ether. The reaction was refluxed for 5 h, the solvent evaporated in vacuo and the residue purified by column chromatography (SiO₂, heptane/ethyl acetate 9:1), yielding 7a (0.69 g, 2.4 mmol, 80%) as a clear oil containing a complex mixture of isomers.

In the attributions of the ¹H and ¹³C NMR spectra below, more than one chemical shift values have been assigned to several of the atoms, since a mixture of diastereoisomers has been obtained in the synthesis.

¹H NMR (250 MHz, CDCl₃, 25 ºC): δ 7.14-7.19 (m, 1H, Ar, 4-H), 7.01-7.03 (m, 1H, Ar, 2-H), 6.97-7.00 (m, 1H, Ar, 5-H), 6.73-6.77 (m, 1H, Ar, 6-H), 4.26, 4.08, 3.87 (s, 1H, thiirane H), 3.785, 3.783, 3.780, 3.776 (s, 3H, OCH₃), 1.88-1.69, 1.66-1.42 (m, 7H, fenchyl H), 1.318, 1.223, 1.172, 1.142, 1.132, 1.128, 1.097, 1.034, 1.022, 1.002, 0.998, 0.953 (s, 9H, fenchyl CH₃) . ¹³C NMR (125 MHz, CDCl₃, 25 ºC): δ 159.45, 159.32, 158.98 (Ar, 3-C), 140.06, 139.69, 138.79, 138.00 (Ar, 1-C), 128.81, 128.71, 128.68, 128.39 (Ar, 5-C), 122.49, 121.95, 121.54 (Ar, 6-C), 116.00, 115.06, 114.87, (Ar, 2-C), 112.56, 112.31, 112.21, 112.17 (Ar, 4-C), 75.82, 75.21, 74.91, 74.78 (fenchyl C-S), 43.66, 43.30, 42.92, 42.75 (HC-S), 52.31, 51.39, 51.06, 50.83, 50.46, 49.31, 49.21, 49.06, 48.28, 46.90, 45.79, 45.50, 43.27, 42.70, 42.08, 42.06, 35.43, 34.51, 33.77, 32.07, 29.43, 28.97, 28.63, 27.61 (fenchyl CH and CH₂), 26.89, 26.73, 26.41, 26.34, 26.18, 25.94, 25.76, 25.09, 21.06, 19.15, 18.26, 18.10 (fenchyl CH₃) . MS (m/z): 288 (M⁺), 256, 213, 173, 121.

(E,Z)-2-[1-(3-Methoxyphenyl)-methylyliden]-1,3,3-trimethylbicyclo[2.2.1]heptane (1a)

Triphenylphosphine (0.52 g, 2.0 mmol) was added to a solution of 7a (5.2 g, 1.8 mmol) in dry toluene (5 mL) and the resulting mixture refluxed for 24 h. The solution was cooled to room temperature and 1 mL of methyl iodide was added, and the resulting suspension kept at 50 ºC for 1 h, filtered and the solvent removed in vacuo. The oily liquid obtained was purified by column chromatography (Al₂O₃, heptane), yielding 1a (0.24 g, 53%) as a clear oil.

In the attributions of the ¹H and ¹³C NMR spectra below, two chemical shift values have been assigned to several of the atoms, since a mixture of two diastereoisomers has been obtained in the synthesis.

¹H NMR (400 MHz, CDCl₃, 25 ºC): δ 7.19-7.13 (m, 1H, Ar, 5-H), 6.79-6.69 (m, 3H, Ar, 2-H, 4-H, 6-H), 6.16 (Z), 6.15 (E) (s, 1H, HC=C), 3.79 (s, 3H, ArOCH₃), 1.88-1.63, 1.58-1.43, 1.38-1.18 (m, 7H, fenchyl H), 1.27, 1.15, 1.14, 1.04, 0.98, 0.95 (s, 9H, fenchyl CH₃) . ¹³C NMR (100 MHz, CDCl₃, 25 ºC): δ 160.57, 160.52 (Ar, 3-C), 158.96, 158.83 (olefin fenchyl C), 141.09, 140.78 (Ar, 1-C), 128.41, 128.20 (olefin CH), 122.11, 122.07 (Ar, 5-C), 117.00, 116.46 (Ar, 6-C), 115.11, 114.95 (Ar, 4-C), 111.42, 111.30 (Ar, 2-C), 55.14 (ArOCH₃), 55.14, 51.09, 50.64, 49.95, 46.88, 45.20, 43.97, 43.48, 36.40, 36.12, 29.58, 28.64 (fenchyl CH and CH₂), 27.23, 26.25, 25.74, 25.58, 20.32, 19.30 (fenchyl CH₃) . MS (m/z): 256 (M⁺), 213, 173, 159, 121, 115. Anal. Calc. for C₁₈H₂₄O: C 84.32, H 9.44; found: C 84.41, H 9.51%.

1-Spiro(1,3,3-trimetilbiciclo[2.2.1]heptyl)-2-methoxy-2-(3-methoxyphenyl)thiirane (7b)

A solution of diazofenchane 4b (obtained from 2.0 g fenchylhydrazone, 12 mmol) in DME (10 mL) was added in small portions at room temperature to O-methyl 3-methoxythiobenzoate 5b (1.1 g, 6.0 mmol). The reaction was stirred for 20 h and the consumption of the thionoester was monitored by TLC (Al₂O₃, heptane/EtAc 20:1). Afterwards, the solvent was evaporated in vacuo and the residue purified by column chromatography (Al₂O₃, heptane/ethyl acetate 20:1), furnishing 7b (1.8 g, 5.6 mmol, 94%) as a pale yellow oil.

In the attributions of the ¹H and ¹³C NMR spectra below, more than one chemical shift values have been assigned to several of the atoms, since a mixture of diastereoisomers has been obtained in the synthesis.

¹H NMR (250 MHz, CDCl₃, 25 ºC): δ 7.31-7.05 (m, 3H, Ar, 2-H, 4-H, 5-H), 6.88-6.82 (m, 1H, Ar, 6-H), 3.851, 3.842, 3.836, 3.823 (s, 3H, ArOCH₃), 3.177, 3.153, 3.080, 3.052 (s, 3H, S-C-OCH₃), 2.00-0.75 (m, 7H, fenchyl H), 1.48, 1.37, 1.21, 1.19, 1.08, 1.00, 0.99, 0.95, 0.94 (s, 9H, fenchyl CH₃) . ¹³C NMR (62.9 MHz, CDCl₃, 25 ºC): δ 159.44, 159.38, 159.23, 156.70 (Ar, 3-C), 141.24, 140.84, 139.28, 138.60 (Ar, 1-C), 128.96, 128.75, 128.61, 128.28 (Ar, 5-C), 124.00, 123.28, 121.98, 121.42 (Ar, 6-C), 117.28, 116.56, 114.90, 114.44 (Ar, 2-C), 113.52, 113.25, 113.18 (Ar, 4-C), 95.50, 94.96 (S-C-OCH₃), 81.22, 80.62, 80.32, 80.18 (fenchyl C-S), 55.30, 55.23, 55.03, 54.74 (ArOCH₃), 53.47, 51.98, 50.95, 50.39 (S-C-OCH₃), 45.64, 44.86, 44.44, 43.91, 43.26, 42.77, 41.31, 41.00, 37.48, 37.40, 32.77, 32.09, 32.01, 31.87, 31.72, 31.45, 31.41, 29.50, 28.97, 28.75, 28.52 (fenchyl CH and CH₂), 27.01, 26.60, 25.91, 25.77, 25.65, 24.97, 24.18, 23.01, 22.68, 21.93, 21.71, 20.70 (fenchyl CH₃). MS (m/z): 318 (M⁺), 303, 286, 271, 257, 237, 217.

(E,Z)-2-[1-Methoxy-(3-methoxyphenyl)-methylyliden]-1,3,3-trimethylbicyclo[2.2.1]heptane (1b)

To a solution of 7b (1.1 g, 6.0 mmol) in dry toluene (20 mL) was added copper powder (3.0 g) and this suspension was stirred and refluxed for 6 hours. After filtration and concentration, the oily liquid obtained was purified by column chromatography (SiO₂, heptane/ethyl acetate 5:1) yielding 1b (1.6 g, 5.7 mmol, 95%) as a clear oil, which was formed by a mixture of E and Z isomers.

In the attributions of the ¹H and ¹³C NMR spectra below, two chemical shift values have been assigned to several of the atoms, since a mixture of two diastereoisomers has been obtained in the synthesis.

¹H NMR (250 MHz, CDCl₃, 25 ºC): δ 7.15-7.09 (m, 1H, Ar, 5-H), 6.80-6.72 (m, 3H, Ar, 2-H, 4-H, 6-H), 3.73 (s, 3H, ArOCH₃), 3.09, 3.08 (s, 3H, C=COCH₃), 1.82-1.53, 1.52-1.27 (m, 7H, fenchyl H), 1.45, 1.23, 1.01, 0.56, 0.52 (s, 9H, fenchyl CH₃) . ¹³C NMR (62.9 MHz, CDCl₃, 25 ºC): δ 158.92, 158.86 (Ar, 3-C), 147.43, 146.13 (olefin COCH₃), 138.68, 138.54 (Ar, 1-C), 137.92, 137.79 (Ar, 5-C), 128.32, 128.27 (Ar, 6-C), 123.54 (Ar, 4-C), 116.18 (Ar, 2-C), 113.46 (olefin fenchyl C), 56.20, 55.89 (olefin OCH₃), 55.15 (ArOCH₃), 51.15, 50.40, 48.66, 48.48, 47.04, 45.70, 45.29, 43.18, 37.33, 36.16, 30.17, 26.53 (fenchyl CH and CH₂), 25.76, 25.72, 25.35, 25.08, 21.19, 20.92 (fenchyl CH₃) . MS (m/z): 286 (M⁺), 271, 257, 243, 217, 135. Anal. calc. for C₁₉H₂₆O₂: C 79.68, H 9.15; found: C 79.17, H 9.26%.

(E,Z)-tert-Butyl-(3-[methoxy-[1,3,3-trimethylbicyclo[2.2.1]-yliden]-methyl]-phenoxy)dimethylsilane (1c)

In the attributions of the ¹H and ¹³C NMR spectra below, two chemical shift values have been assigned to several of the atoms, since a mixture of two diastereoisomers has been obtained in the synthesis.

¹H NMR (250 MHz, CDCl₃, 25 ºC): δ 7.18-7.14 (m, 1H, Ar, 5-H), 6.88-6.75 (m, 3H, Ar, 2-H, 4-H, 6-H), 3.155, 3.152 (s, 3H, OCH₃), 1.90-1.00 (m, 7H, fenchyl H), 1.50, 1.29, 1.06, 0.63, 0.60 (s, 9H, fenchyl CH₃), 0.99 (s, 9H, SiC(CH₃)₃), 0.19 (s, 6H, Si(CH₃)₂) . ¹³C NMR (62.9 MHz, CDCl₃, 25 ºC): δ 154.92 (Ar, 3-C), 147.31, 146.05 (olefin COCH₃), 138.37, 138.28 (Ar, 1-C), 137.97, 137.80 (Ar, 5-C), 128.27 (Ar, 4-C), 124.43, 124.17 (Ar, 6-C), 122.58 (Ar, 2-C), 119.70 (olefin fenchyl C), 56.12, 55.81 (OCH₃), 51.10, 50.37, 48.64, 48.47, 47.04, 45.71, 45.25, 43.16, 37.32, 36.14, 31.86, 30.13 (fenchyl CH and CH₂), 29.00 (SiC(CH₃)₃), 26.54, 25.34, 25.06, 21.18, 20.98, 18.18 (fenchyl CH₃), 25.67 (SiC(CH₃)₃), –5.0 (Si(CH₃)₂). MS (m/z): 386 (M⁺), 371, 317, 235, 73. Anal. calc. for C₂₄H₃₈O₂Si: C 74.55, H 9.91; found: C 74.70, H 9.97%.

Computational details

Gaussian03 was used for all calculations.⁴⁰ All geometries were optimized at the pbe1pbe/6-311+G(d,p) theoretical level.²⁸ To model the solvent effect (chloroform), CPCM-SCRF (self-consistent reaction field) procedure was employed.⁴¹ Stationary points were characterized as minima by vibrational analysis. All reported energies include zero-point energy (ZPE) as well as thermal corrections (T = 298.15 K) from frequency calculations. Gauge invariant atomic orbitals (GIAO) method was used to calculate isotropic chemical shielding.^42,43 Calculations were carried out using the WP04 functional, a version of the B3LYP functional explicitly reparameterized for the calculation of chemical shifts in chloroform,⁴⁴ with Dunning's cc-pVDZ basis.³⁴ WP04 functional was invoked by specifying the BLYP keyword and adding iop (3/76 = 1000001189, 3/77 = 0961409999, 3/78 = 0000109999) to the keyword line. The theoretical NMR chemical shifts were calculated as the difference isotropic shielding constants with respect to tetramethylsilane (GIAO/WP04/cc-pVDZ//pbe1pbe/6-311+G(d,p)) ¹H shielding = 31.79, ¹³C shielding = 188.78). Solvent effects in GIAO calculations were modeled by the IEFPCM-SCRF method.⁴⁵ The IEFPCM-SCRF method was used to model solvent (chloroform) effects, which are known to be of great importance in such calculations.³⁴ The IEFPCM was used because it has been successfully used in association with the W04P functional in the accurate prediction of ¹H and ¹³C chemical shifts in chloroform.³⁴

Supplementary Information

HSQC, HMBC and NOESY NMR spectra, theoretical coordinates for the equilibrium geometries and vibrational frequencies of 1a and 1b are available free of charge at http://jbcs.sbq.org.br, as PDF file.

Acknowledgments

We thank Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for research grants (W. J. B. 2006/03420-7, E. L. B. 2007/00684-6) and for a PhD fellowship (F. H. B. 2005/58320-4), Conselho Nacional de Pesquisa e Desenvolvimento (CNPq) for a PhD fellowship for L. F. M. L. C., as well as, Deutscher Akademischer Austauschdienst (DAAD) and Coordenadoria de Aperfeiçoamento de Pessoal de Ensino Superior (CAPES) for an international fellowship for L. F. M. L. C. (BEX 1132/04-0).

Submitted: April 14, 2010

Published online: June 23, 2010

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Publication Dates

• Publication in this collection
  01 Dec 2010
• Date of issue
  2010

History

• Received
  14 Apr 2010
• Accepted
  23 June 2010