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Synlett 2009(18): 2987-2991  
DOI: 10.1055/s-0029-1218274
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Transition-Metal-Catalyzed Rearrangement of 1,1-(Oligomethylene)-4-aryl-2-butene-1,4-diols: Ring Expansion vs. Aryl Group Migration

Alex Lange, Wolfgang Heydenreuter, Helge Menz, Stefan F. Kirsch*
Department Chemie and Catalysis Research Center, Technische Universität München, Lichtenbergstr. 4, 85747 Garching, Germany
Fax: +49(89)13115; e-Mail: stefan.kirsch@ch.tum.de;
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Publication History

Received 13 July 2009
Publication Date:
09 October 2009 (online)

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Abstract

The transition-metal-catalyzed rearrangement of 1,1-(oligomethylene)-4-aryl-2-butene-1,4-diols was investigated. In the presence of PdCl2(MeCN)2 and Cu(OTf)2, a rapidly equilibrating 1,3-isomerization is followed by 1,2-migration to produce cyclopentanones or cyclohexanones through expansion of four- or five-membered ring systems. When employing larger ring systems or acyclic cores, aryl migration provides 2-aryl aldehydes.

Key words

rearrangements - alkenes - diols - transition metals - catalysis

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Exposure of six-membered ring substrate 1c to 5 mol% of PPTS at 23 ˚C in CH2Cl2 provided an inseparable mixture of 3c and 4c in low yields (e.g., 29% after 14 h).

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Reaction of 1b in the presence of MgBr2 (5 mol%) led to clean formation of 4b without traces of 2b (or 3b).

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Synthesis of ( E )-2-(4-Methoxystyryl)cyclohexanone (2b)
Cu(OTf)2 (1 mg, 0.003 mmol, 1 mol%) was added to a solution of (E)-1-(4-methoxyphenyl)-3-(1-(triethylsilyl-oxy)cyclopentyl)prop-2-en-1-ol (1c, 100 mg, 0.28 mmol) in CH2Cl2 (2.8 mL) and stirred at r.t. for 30 min (until TLC analysis indicated complete conversion). The reaction mixture was concentrated under reduced pressure, and the residue was purified by flash chromatography on silica (pentanes-Et2O = 95:5). Compound 2b was obtained as a colorless solid in 96% yield (62 mg, 0.27 mmol). ¹H NMR (360 MHz, CDCl3): δ = 1.69-1.83 (m, 3 H), 1.89-1.96 (m, 1 H), 2.01-2.09 (m, 1 H), 2.14-2.20 (m, 1 H), 2.31-2.40 (m, 1 H), 2.45-2.51 (m, 1 H), 3.17 (ddd, J = 0.9, 6.4, 11.3 Hz, 1 H), 3.80 (s, 3 H), 6.27 (dd, J = 5.9, 16.1 Hz, 1 H), 6.33 (d, J = 16.1 Hz, 1 H), 6.77-6.91 (m, 2 H), 7.29-7.33 (m, 2 H). ¹³C NMR (90.6 MHz, CDCl3): δ = 24.4 (t), 27.6 (t), 34.5 (t), 41.7 (t), 54.0 (d), 55.3 (d), 113.9 (d), 125.3 (d), 127.4 (d), 130.0 (s), 130.8 (d), 159.0 (s), 211.3 (s). LRMS (EI): m/z = 230 (100)[M+], 202 (26), 173 (25), 159 (21), 134 (38), 121 (37). HRMS: m/z calcd for C15H18O2 [M+]: 230.1307; found: 230.1307.

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Synthesis of 3-Cyclohexylidene-2-(4-methoxyphenyl)-propanal (3a)
Following the procedure to prepare 2b,¹8 allylic alcohol 1g (100 mg, 0.26 mmol) was converted into the corresponding aldehyde 3a in the presence of Cu(OTf)2 (1 mg, 1 mol%). The reaction mixture was concentrated under reduced pressure, and the residue was purified by flash chromatography on silica (pentanes-Et2O = 95:5). Compound 3a was obtained in 79% yield (50 mg, 0.20 mmol). ¹H NMR (360 MHz, CDCl3): δ = 1.45-1.59 (m, 6 H), 2.12-2.20 (m, 4 H), 3.80 (s, 3 H), 4.41 (dd, J = 2.6, 8.8 Hz, 1 H), 5.39-5.46 (m, 1 H), 6.88-6.94 (m, 2 H), 7.13-7.19 (m, 2 H), 9.55 (d, J = 2.6 Hz, 1 H). ¹³C NMR (90.6 MHz, CDCl3): δ = 26.6, 27.6, 28.5, 29.6, 37.3, 55.3, 56.3, 114.4, 115.1, 128.9, 129.4, 145.7, 158.8, 198.5. LRMS (EI): m/z = 244 (2) [M+], 215 (100), 147 (23), 121 (25). HRMS: m/z calcd for C16H20O2 [M+]: 244.1463; found: 244.1468.

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We also observed a high-yielding elimination when using 2-butene-1,4-diols that possess Bz-protected primary allylic alcohols. According to preliminary studies, this elimination appears to be quite general (Scheme  [7] ).

Scheme 7

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In seminal studies on related 1,3-isomerizations of phenylpropenyl carbinols, chirality transfer was reported:

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This correlates with the migratory aptitude observed in Wagner-Meerwein shifts.