Diels-Alder Reaction of 3- Methylpenta-1,2,4-trienyl

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Alkatrienyl Sulfoxides and Sulfones, VIII: Diels-Alder Reaction of 3Methylpenta-1,2,4-trienyl Phenyl Sulfoxide with Maleic Anhydride Valerij C. Christova; Ivaylo K. Ivanova a Department of Chemistry, University of Shumen, Shumen, Bulgaria

To cite this Article Christov, Valerij C. and Ivanov, Ivaylo K.(2007) 'Alkatrienyl Sulfoxides and Sulfones, VIII: Diels-Alder

Reaction of 3-Methylpenta-1,2,4-trienyl Phenyl Sulfoxide with Maleic Anhydride', Synthetic Communications, 37: 18, 3201 — 3209 To link to this Article: DOI: 10.1080/00397910701547698 URL: http://dx.doi.org/10.1080/00397910701547698

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Synthetic Communicationsw, 37: 3201–3209, 2007 Copyright # Taylor & Francis Group, LLC ISSN 0039-7911 print/1532-2432 online DOI: 10.1080/00397910701547698

Alkatrienyl Sulfoxides and Sulfones, VIII: Diels – Alder Reaction of 3-Methylpenta-1,2,4-trienyl Phenyl Sulfoxide with Maleic Anhydride

Downloaded At: 16:27 8 January 2011

Valerij C. Christov and Ivaylo K. Ivanov Department of Chemistry, University of Shumen, Shumen, Bulgaria

Abstract: Regio- and stereo-selective Diels – Alder reaction of 3-methylpenta1,2,4-trienyl phenyl sulfoxide with maleic anhydride in the absence or presence of a Lewis acid as catalyst with formation of sulfinyl-substituted benzofuran or norbornene cycloadducts, respectively, is described. Keywords: 2-benzofuran-1,3-dione, Diels –Alder reaction, norbornene, 1-sulfinylcyclopenta-1,3-diene, vinylallenyl sulfoxide

Over the past three decades, the synthesis and use of allene derivatives have been expanded in synthetic organic chemistry. An impressive number of carbo- and heterocyclic systems has been prepared from allenic starting materials. Because it allows the selective formation of two carbon–carbon bonds and up to four centers of chirality in a single step, the Diels–Alder reaction is perhaps one of the most important transformations in organic chemistry and is probably the most valuable and most applied reaction in synthetic organic chemistry.[1] Among the many different dienes and dienophiles employed in Diels–Alder reactions, allenes have played only a minor role.[2] In most cases, they have been used as dienophiles and converted into cycloadducts by reactions with dienes.[3] If the allene bears another conjugated Received in the UK January 31, 2007 Address correspondence to Valerij C. Christov, Department of Chemistry, University of Shumen, 115, Universitetska str., BG-9700 Shumen, Bulgaria. E-mail: [email protected] 3201


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double bond, however, it can be used as the diene component, and the Diels– Alder reaction with dienophiles gives cycloadducts with an exocyclic double bond.[4] However, there are only a limited number of examples for the use of vinylallenes as dienes in inter-[5] and intra-molecular [4 þ 2] cycloadditions.[6] Compared to the application of ordinary conjugated dienes, the use of vinylallenes as diene components is advantageous from the viewpoints of both reactivity and stereoselectivity. The equilibrium between the s-trans and s-cis conformers is more on the side of the s-cis isomer for vinylallenes than it is for the 1,3-dienes.[7] Consequently, vinylallenes exibit a higher reactivity. The synthetic utility of vinylallenyl sulfoxides[8a – 8d] has been demostrated by Okamura and coworkers in a variety of preparations and interesting reactions, including the preparation of vinylallenes,[8] which are useful intermediates in organic synthesis in general,[8e] and natural polyenes, such as vitamins A and D, in particular.[8f] In our previous work concerning electrophile-induced cyclization reactions of alkatrienyl sulfoxides and sulfones,[9a – 9d] we were able to show that 1- and 3-vinylallenyl sulfoxides and sulfones are readily accessible by [2,3]-sigmatropic rearrangement of the corresponding 1- and 3-vinylpropargylic sufenates and sulfinates formed in the reactions of the corresponding a-alkynols with sulfenyl or sulfinyl chlorides. In view of the advantages of vinylallenes as the diene component in [4 þ 2] cycloaddition reactions[9g] and our results on the cycloaddition reactions of vinylallenyl sulfoxides and sulfones with dimethyl but-2-ynedioate[9e] and cheletropic addition of sulfur dioxide[9f] to them, we continued a study of the use of the 3-methylpenta1,2,4-trienyl sulfoxide[9a] in the Diels –Alder reaction with maleic anhydride. The results of this work are presented here. We initiated this study with the reaction of the 3-vinylallenyl sulfoxide 1 with maleic anhydride 2 in the absence of a catalyst. The reaction conditions have been optimized to obtain better yields. Thus, treatment of 1 with maleic anhydride resulted in a complete consumption of the starting materials within 6 h at reflux in toluene, and the (Z )-endo-cycloadduct 3 was obtained in 75% yield (Scheme 1). Note that at room temperature the two reactants still interacted (much more slowly) with the formation of the cyclic compound 3. It is

Scheme 1. Diels – Alder reaction of 3-methylpenta-1,2,4-trienyl phenyl sulfoxide 1 with maleic anhydride 2 in the absence or presence of BF3 . OEt2.


Alkatrienyl Sulfoxides and Sulfones VIII

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necessary to carry out this reaction under an argon atmosphere because vinylallenyl sulfoxides and sulfones are sensitive to moisture.[9] The assignment of the (Z )-configuration to the exocyclic double bond and the determination of the relative configuration of the two centers of chirality of the benzofuran 3 are based on the NMR data. A coupling constant of 3JHH 8.5 Hz was found, which is typical of endo-Diels –Alder adducts of this type.[5e] Thus, the cycloaddition takes place with complete endo-selectivity via a transition state with the maximal distance between the sulfoxide group of the vinylallene 3 and the anhydride part of the dienophile. The (Z)-configuration of cycloadduct 3 is in accordance[5e] with the low chemical shift value (d 6.88 ppm) of the olefinic proton of the exocyclic double bond. On the other hand, the reaction of these reagents with boron trifluoride etherate as a catalyst for 4 days led to a dramatic change of the reaction course—after chromatographic purification, the norbornene derivative 4 was isolated with 58% yield (Scheme 1). The coupling constant 3J2,6 10.8 Hz was found, which is typical of endo-norbornene cycloadducts of this type.[5e] For an explanation of this result, we examined the treatment of vinylallene 1 with BF3 . Et2O under the same reaction conditions. In this case, five-membered cyclization occurred, and 1-sulfinyl-cyclopentadiene 5 was formed exclusively after 3 days at room temperature in 72% yield; no other products could be detected in the crude product. Although the exact mechanistic aspects of this transformation have not rigorously been elucidated, the following pathway is a probable chemical route on the basis of experimental results as illustrated in Scheme 2. The plausible mechanistic scenario starts[5e] with 1,2- or 2,3-attack of the Lewis acid at the central allenic carbon atom to give species A, which is then cyclized to intermediates B. Loss of the Lewis acid and hydrogen transfer lead to the cyclopentadiene 5, which should exist in five tautomeric forms (not shown in Scheme 3). Of these, the Diels – Alder reaction of the tautomer 5 with maleic anhydride should give the cycloadduct 4 (Scheme 4). According to this scenario, five tautomeric cyclopentadienes and therefore five isomeric Diels – Alder products are conceivable. The experimental observation of the norbornene cycloadduct 4 only might be attributed to a higher stability and/ or reactivity of the cyclopentadiene 5 with respect to the other tautomers.[5e]

Scheme 2. Cyclization reaction of the 3-methylpenta-1,2,4-trienyl phenyl sulfoxide 1 with BF3 . OEt2.

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Scheme 3. Plausible mechanism for the cyclization of 3-methylpenta-1,2,4-trienyl phenyl sulfoxide 1 in the presence of BF3 . OEt2.

In conclusion, we have developed efficient regio- and stereo-selective synthesis of sulfinyl-substituted benzofuran and norbornene via Diels– Alder reaction of the 3-methylpenta-1,2,4-trienyl phenyl sulfoxide 1 with maleic anhydride as a dienophile. This study reveals the possibility for synthetic innovations as well as the potential of the Diels –Alder reaction of the vinylallenyl sulfoxides and sulfones to selectively construct exocyclic double bonds on sixmembered rings. Further expansion and applications of this methodology are in progress and will be reported in due course.

EXPERIMENTAL Method of Analysis 1

H and 13C NMR spectra were obtained on a Brucker DRX-250 spectrometer for solutions in CDCl3. Chemical shifts are in parts per million downfield from internal TMS. IR spectra were recorded with an IR-72 spectrophotometer (Carl Zeiss, Jena). Elemental analyses were carried out by the University of

Scheme 4. Reaction scheme for the synthesis of the norbornene cycloadduct 4 with the intermediate formation of the 1-sulfinylcyclopenta-1,3-diene 5.


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Shumen Microanalytical Service Laboratory. The melting points were measured in open capillary tubes and are uncorrected. The solvents were purified by standard methods. Reactions were carried out in oven-dried glassware under an argon atmosphere and exclusion of moisture. All compounds were checked for their purify on thin-layer chromatography (TLC) plates. Starting Materials The 3-methylpenta-1,2,4-trienyl phenyl sulfoxide 1 was synthesized according to the established procedure.[9a]


Diels – Alder Reaction of 3-Methylpenta-1,2,4-trienyl Phenyl Sulfoxide 1 with Maleic Anhydride 2 To a solution of vinylallene 1 (0.225 g, 1.1 mmol) in dry toluene (3 mL), a solution of maleic anhydride 2 (0.167 g, 1.7 mmol) was added in the same solvent (3 mL) under an argon atmosphere, and the mixture was stirred for 15 min. Then, the mixture was heated at reflux for 6 h. After the reaction was complete as monitored by TLC (eluent: ethyl acetate – heptane, 1:1), the solvent was evaporated under reduced pressure to give an oily residue, which was purified by column chromatography using silica gel (Kieselgel Merck 60 F254, 70 – 230 mesh ASTM, 0.063 – 0.200 mm) and ethyl acetate/ heptane as eluent to afford orange crystals of the cyclic product 3. 4-(Benzenesulfinyl)methylidene-5-methyl-3a,4,7,7a-tetrahydro2-benzofuran-1,3-dione (3) Yield: 0.249 g (75%), mp 143– 1448C, Rf (ethyl acetate –heptane ¼ 1:1) 0.73. Anal. calcd. for C16H14O4S: C, 63.56; H, 4.67; S, 10.61; found: C, 63.47; H, 4.63; S, 10.46. IR (nujol): 1058, 1434, 1474, 1577– 1615, 1778, 1842. 1H NMR (CDCl3, 250 MHz): d ¼ 2.18 (s, 3H, Me), 2.37, 2.49 (mm, 2H, CH2), 3.69 (dd, J ¼ 8.5, 7.3 Hz, 1H, C7a-H), 3.88 (d, J ¼ 8.5 Hz, 1H, C3a-H), 6.11 (m, 1H, 55C6-H), 6.88 (s, 1H, 55C-H), 7.49– 8.0 (m, 5H, Ph). 13C NMR (CDCl3, 50 MHz): d ¼ 20.37, 23.63, 39.12, 47.06, 124.73, 125.68, 128.91, 129.35, 130.24, 136.22, 139.52, 146.75, 170.5, 172.46.

Diels – Alder Reaction of 3-Methylpenta-1,2,4-trienyl Phenyl Sulfoxide 1 with Maleic Anhydride 2 in the Presence of BF3 . OEt2 A solution of vinylallene 1 (0.327 g, 1.6 mmol), maleic anhydride 2 (0.314 g, 3.2 mmol), and a small amount of hydroquinone in dry dichloromethane

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(5 mL) was treated at 2108C with a solution of BF3 . OEt2 (0.60 mL, 4.8 mmol) in the same solvent (1 mL) under an argon atmosphere, and the mixture was stirred for 15 min. Then, the mixture was stirred for 4 days at room temperature. After the reaction was complete as monitored by TLC (eluent: ethyl acetate –heptane, 1:1), it was then diluted with diethyl ether and washed with a saturated NaHCO3 solution. After drying with Na2SO4 and removal of the solvent under reduced pressure, the crude product was purified by column chromatography with silica gel (Kieselgel Merck 60 F254, 70– 230 mesh ASTM, 0.063 –0.200 mm) and ethyl acetate/heptane as eluent, providing yellow needle crystals of the norbornene product 4.


1-Benzenesulfinyl-8-methyl-4-oxatricyclo[5,2,1,02,6]-dec-8-ene3,5-dione (4) Yield: 0.280 g (58%), mp 123 –1248C, Rf (ethyl acetate –heptane ¼ 1:1) 0.58. Anal. calcd. for C16H14O4S: 63.56; H, 4.67; S, 10.61; found: C, 63.42; H, 4.79; S, 10.77. IR (nujol): 1062, 1427, 1469, 1581, 1773, 1839. 1H NMR (CDCl3, 250 MHz): d ¼ 1.37 – 1.52 (m, 2H, CH2), 1.74 (s, 3H, Me), 2.83 (m, 1H, C7-H), 3.51 (d, J ¼ 10.8 Hz, 1H, C2-H), 3.89 (dd, J ¼ 1.2, 10.8 Hz, 1H, C6H), 5.89 (s, 1H, 55C-H), 7.43 –7.71 (m, 5H, Ph). 13C NMR (CDCl3, 50 MHz): d ¼ 16.23, 31.12, 47.29, 49.11, 52.84, 58.27, 123.8, 126.77, 130.09, 135.92, 137.14, 144.72, 166.61, 168.1. Cyclization Reaction of 3-Methylpenta-1,2,4-trienyl Phenyl Sulfoxide 1 with BF3 . OEt2 A solution of vinylallene 1 (0.409 g, 2.0 mmol) and a small amount of hydroquinone in dry dichloromethane (3 mL) was treated at 2108C with a solution of BF3 . OEt2 (0.75 mL, 6.0 mmol) in the same solvent (2 mL) under an argon atmosphere, and the mixture was stirred for 15 min. Then, the mixture was stirred for 3 days at room temperature. After the reaction was complete as monitored by TLC (eluent: ethyl acetate –heptane, 1:1), it was then diluted with diethyl ether and washed with a saturated NaHCO3 solution. After drying with Na2SO4 and removal of the solvent under reduced pressure, the crude product was purified by column chromatography with silica gel (Kieselgel Merck 60 F254, 70 –230 mesh ASTM, 0.063 – 0.200 mm) and ethyl acetate/heptane as eluent, providing colorless crystals of the cyclic product 4.

3-Methyl-1-benzenesulfinylcyclopenta-1,3-diene (5) Yield: 0.294 g (72%), mp 84– 858C, Rf (ethyl acetate – heptane ¼ 1:2) 0.46. Anal. calcd. for C12H12OS: C, 70.55; H, 5.92; S, 15.70; found: C, 70.41; H,


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5.98; S, 15.76. IR (nujol), cm21: 1058, 1428, 1464, 1578 –1672. 1H NMR (CDCl3, 250 MHz): d ¼ 1.27 (s, 3H, Me), 2.84 –3.31 (dddd, J ¼ 0.9, 2.7, 16.1 Hz, 2H, CH2), 6.47 (t, J ¼ 2.7 Hz, 1H, 55C4-H), 7.22 (s, 1H, 55C2-H), 7.52– 8.14 (m, 5H, Ph). 13C NMR (CDCl3, 50 MHz): d ¼ 19.04, 37.12, 124.74, 125.8, 126.75, 128.66, 132.08, 137.24, 140.15, 144.43.

ACKNOWLEDGMENT Support from the Research Fund of the University of Shumen (Project Nos. 30/2005 and 17/2006) is acknowledged.


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