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Synlett 2006(18): 3110-3114  
DOI: 10.1055/s-2006-951513
LETTER
© Georg Thieme Verlag Stuttgart · New York

Palladium(II)-Catalyzed Synthesis of 2-Alkoxytetrahydrofurans from Allylic Alcohols and Vinyl Ethers: Stereospecificity and Catalysis

Yasufumi Kawamura, Takuro Imai, Takahiro Hosokawa*
Department of Environmental Systems Engineering, Kochi University of Technology, 185 Miyanokuchi, Kami City, Tosayamada, Kochi 782-8502, Japan
Fax: +81(887)572520; e-Mail: hosokawa.takahiro@env.kochi-tech.ac.jp;
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Received 13 April 2006
Publikationsdatum:
25. Oktober 2006 (online)

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Abstract

The reaction of (E)- or (Z)-cinnamyl alcohol and ethyl vinyl ether with a catalyst consisting of Pd(OAc)2, Cu(OAc)2, and catechol (1:1:2) under O2 gives a good yield of (Z)- or (E)-4-benzylidene-2-ethoxytetrahydrofuran, respectively. Similarly, 2-butoxy-4-exomethylenetetrahydrofuran was obtained from allyl alcohol and butyl vinyl ether. In the case of allyl alcohol, however, the catalysis does not proceed well with forming palladium black, if a relatively large amount of Pd(OAc)2, such as 0.25 mmol to 0.05 mmol, is used even in a lower palladium/substrate ratio, e.g. 1 mol% concentration of the catalyst.

Key words

palladium - oxypalladation - palladium hydride elimination - oxidation - heterocycles

6

Experimental Procedure for the Preparation of ( Z )-2 and ( E )-5. A suspension of 3-phenyl-2-propyne-1-ol (231 mg, 1.75 mmol) and 5 wt% palladium on CaCO3 poisoned with lead (Aldrich, 46.1 mg, 0.087 mmol) in toluene (5 mL) was stirred under H2 (balloon) at r.t. for 2 h, and the reaction mixture was filtered through a pad of cotton with Et2O (50 mL). Evaporation of solvent gave nearly pure (Z)-2 (202 mg, 86% by NMR) as an oil. 1H NMR (400 MHz, CDCl3): δ = 4.45 (d, J = 6 Hz, 2 H), 5.88 (dt, J = 6.4 Hz, 1 H), 6.58 (d, J = 12 Hz, 1 H), 7.19-7.45 (m, 5 H).
A 25-mL side-armed round-bottom flask was charged with Pd(OAc)2 (11.2 mg, 0.05 mmol), Cu(OAc)2 (9.1 mg, 0.05 mmol), and catechol (11.0 mg, 0.10 mmol) to which was added MeCN (0.5 mL) under O2 (balloon). The resulting solution was stirred for 0.5 h at r.t., and ethyl vinyl ether (288 mg, 4.0 mmol) was added to the flask, and subsequently a solution of (Z)-2 (134 mg, 1.0 mmol) in MeCN (0.5 ml) was added. After stirring was continued for 3 h at r.t., the reaction mixture was filtered through Florisil column (3 g, 10 mm × 80 mm, EtOAc-n-hexane, 1:20), and the solvent was evaporated under reduced pressure. The product (E)-5, nearly pure by NMR, was obtained in 77% yield (157 mg, 0.77 mmol) as an oil. Further purification was made by TLC on SiO2. (E)-5; R f = 0.45 (EtOAc-n-hexane, 1:20). 1H NMR (400 MHz, CDCl3): δ = 1.20 (t, J = 7.2 Hz, 3 H), 2.84 (d, J = 16.8 Hz, 1 H), 2.91-2.98 (dm, 1 H), 3.50 (dq, J = 7.2 Hz, 1 H), 3.78 (dq, J = 7.2 Hz, 1 H), 4.54 (d, J = 12.8 Hz, 1 H), 4.63 (d, J = 12.8 Hz, 1 H), 5.36 (d, J = 5.2 Hz, 1 H), 6.38 (s, 1 H), 7.18-7.36 (m, 5 H).

7

NOE correlation of vinyl proton (δ = 6.42 ppm) in (Z)-4 was observed with one of C-3 protons (δ = 2.70 ppm), suggesting that these protons are proximal to each other, but not with C-5 protons. On the other hand, vinyl proton (δ = 6.38 ppm) in (E)-5 correlates with one of C-5 protons (δ = 4.54 ppm), but not with C-3 protons (Figure [3] ).

13

The reaction of 3,5-di-tert-butylcatechol (11, 67.0 mg, 0.3 mmol) with Pd(OAc)2 (33.7 mg, 0.15 mmol) and Cu(OAc)2 (27.2 mg, 0.15 mmol) in MeCN (1.5 mL) for 0.5 h under O2 (balloon) at r.t. gave quinone 10 in ca. 98% yield (GC). For the survey of these points, the use of catechol itself, instead of 11, was not successful, because the analysis of o-quinone was difficult because of its instability.

16

Experimental Procedure for the Preparation of ( E )-4 in a Relatively Large Scale. In a 25-mL side-armed round-bottomed flask, Pd(OAc)2 (65.1 mg, 0.25 mmol), Cu(OAc)2 (45.4 mg, 0.25 mmol), and catechol (55.0 mg, 0.50 mmol) were dissolved in MeCN (2.5 mL) under O2 (balloon). After the mixture was stirred for 0.5 h at r.t. (aging time of catalyst), ethyl vinyl ether (1442 mg, 20 mmol) was added to the flask, and a solution of (E)-3-phenyl-2-propen-1-ol (670 mg, 5.0 mmol) in MeCN (2.5 mL) was then added. The reaction mixture was stirred for 7 h at r.t. In order to determine GC yield of 4, tetraethylene glycol dimethyl ether (63.43 mg) was added to the flask as an internal standard. The GC analysis showed that 82% yield of 4 was formed. This mixture was subjected to purification on a Florisil column (3 g, 10 mm × 80 mm) with elution of EtOAc-n-hexane (1:20, 100 mL), and evaporation of solvent gave 4 (990 mg, 78%) as nearly pure state.

17

Isolation of 2-ethoxy-4-exomethylenetetrahydrofuran derived from 3a was rather difficult because of its lower boiling point, and thus butyl vinyl ether (3b) was used in these experiments.