Golden Opportunities in Stereoselective Catalysis: Optimization of Chirality Transfer and Catalyst Efficiency in the Gold-Catalyzed Cycloisomerization of α-Hydroxyallenes to 2,5-Dihydrofurans

Abstract

Whereas the cycloisomerization of phenyl-substituted α-hydroxyallene 1 to 2,5-dihydrofuran 2 catalyzed by gold(I) or gold(III) chloride takes place with considerable epimerization, high levels of axis-to-center chirality transfer can be reached in the ­presence of additives (e.g. 2,2′-bipyridine), by using a moderately coordinating solvent (e.g. THF), or by lowering the reaction temperature. The increased stability and selectivity of the catalysts thus formed allows their application to the cycloisomerization of various functionalized α- and β-hydroxyallenes, as well as a decrease of the catalyst loading to 0.1%.

allenes - chirality transfer - cycloisomerization - gold ­catalysis - heterocycles

References and Notes

<A NAME="RY00607ST-1">1</A>

Present address: Institute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, 55128 Mainz, Germany.

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In the presence of gold catalysts, γ- or δ-heterosubstituted allenes also provide five- or six-membered heterocycles (by exo-cyclization):

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No reaction takes place if substrate 1 is treated with small amounts of concd HCl alone.

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Mori, S.; Röder, J.; Krause, N. unpublished results.

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<A NAME="RY00607ST-24">24</A>

Representative Procedure and Spectroscopic Data - Synthesis of cis -2-( tert -Butyldimethylsilyloxy)-3-methyl-5-phenyl-2,5-dihydrofuran (2)
To a solution of 1-(tert-butyldimethylsilyloxy)-3-methyl-5-phenylpenta-3,4-dien-2-ol (1, 80 mg, 0.26 mmol) and 2,2′-bipyridine (2.0 mg, 12.8 µmol) in 3 mL of dry CH₂Cl₂ was added AuCl₃ (2.0 mg, 6.6 µmol). After stirring for 1 h at r.t., the solvent was evaporated, and the residue was purified by column chromatography (SiO₂, EtOAc-cyclohexane, 1:10) furnishing 56 mg (70%) of the 2,5-dihydrofuran 2 as a slightly yellow oil (dr > 97:3 according to NMR analysis). ¹H NMR (400 MHz, C₆D₆): δ = 7.50 (d, ³ J _HH = 7.5 Hz, 2 H), 7.29 (m, 2 H), 7.18 (m, 1 H), 5.79 (m, 1 H), 5.34 (m, 1 H), 4.77 (m, 1 H), 3.86 (dd, ² J _HH = 10.6 Hz, ³ J _HH = 4.3 Hz, 1 H), 3.76 (dd, ² J _HH = 10.6 Hz, ³ J _HH = 4.3 Hz, 1 H), 1.67 (s, 3 H), 1.01 (s, 9 H), 0.09 (2 s, 6 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 142.3, 137.4, 128.1, 127.5, 126.9, 125.4, 88.5, 86.7, 65.2, 25.8, 18.3, 12.6, -5.5, -5.6 ppm.