A series of furyl alcohols and homofuryl alcohols was synthesized by reduction of furfurals or reaction of furyllithium compounds with epoxides and subsequent propargylation. The gold-catalyzed cycloisomerization of these products furnished dihydroisobenzofurans and isochromanes. Crystal structure analyses proved the sequence of the substituents for both classes of products. Unsaturated dicarbonyl compounds as side-products show the mechanistic relationship to the analogous platinum-catalyzed reactions. Neither ester groups, even on the 4-position of the furan ring, nor aryl bromides hinder the catalysis by gold. In the case of a substrate with an allyl ether in the side chain, a side-product, which provides evidence for a reaction of the alkyne with an inverse regioselectivity, was observed.
A new route to enantiomerically pure 8-hydroxytetrahydroisoquinolines is based on furan derivatives and amino acids from renewable resources and gold catalysis. The stereogenic centre from the amino acid remains unchanged in the product.
A series of seven symmetric difuryl-diynes were synthesized by a route delivering an anti-configurated product as the major product. The gold-catalyzed conversion of the individual antiand syn-diastereomers was investigated. The anti-diastereomer in a desymmetrization reaction chemoselectively delivered dihydroindenediols with a furyl and a 2-oxopropyl side chain. The syn-diastereomer was completely converted into oligomeric/polymeric material. Thus, it was not necessary to separate the minor amount of the syn-diastereomers from the product mixtures in order to isolate the dihydroindenediols.The exponential growth of homogeneous gold catalysis in the last decade 1 has led to many new synthetic methods and an increasing number of applications of these methods in total synthesis. 2In most examples, two identical functional groups in the substrates will react in an identical manner, as shown for many gold-catalyzed reactions and different substrates. 3Based on the gold-catalyzed furan-yne reaction, which usually delivers phenols as products, 4 we have recently investigated changes in the reaction pathway initiated by donors on the alkyne or different lengths of the tether between the furan and alkyne. 5Here we report the synthesis of symmetrical difuran-diyne substrates by bidirectional synthesis and their gold-catalyzed conversion with desymmetrization leading to products with only one, rather than the expected two, phenol groups.Recently, we published the organocatalytic conversion of furfurals 1 into the corresponding furoins 2, reduction to the 1,2-diol 3, propargylation to substrates 4, and subsequent gold-catalyzed transformation to give the bis-phenol derivatives 5. 6 In the gold-catalyzed step, exclusively cyclization to the five-membered ring product 5, but not the isomeric six-membered-ring product 6, was observed (Scheme 1).We have now prepared the corresponding symmetrical 1,2-dicarbonyl compounds 7 from the unsymmetrical furoins 2 by oxidation. Initially we assumed that we would need an oxidant, such as Dess-Martin periodinane, for this oxidation, but we found that the substrates 2 readily and selectively oxidize in air, only the formation of 2a was dependent on the oxidant Dess-Martin periodinane 7 (DMP). Thus the formation of the a-furils 7 from furfurals 1 overall is an efficient organocatalyzed dimerization/ autoxidation sequence. Table 1 lists the different substrates 7 prepared in the context of this investigation. The two alkyl derivatives 1c and 1d gave low yields of 7c and 7d; the same is true for 7e and 7f. Thus, these four substrates were not used for the next step. The aryl derivatives 7g-k were obtained in good yields (entries 7-11). Single crystals could be grown of 7b, 7j, and 7l and these were used for crystal structure analysis (Figure 1). They all show anti-conformation of the two carbonyl dipoles. In the case of 7b the crystal packing causes a slight tilt of the molecule, in 7j and 7l the molecules are planar.The substituted furfurals 1 were either commercially available or convenient...
Gold Catalysis: Dihydroisobenzofurans and Isochromans by the Intramolecular Furan/Alkyne Reaction. -The gold-catalyzed cycloisomerization of furans tethered with propargylic ethers, e.g. (I) or (III), affords the dihydroisobenzofurans (II) as well as isochromans (IV) in moderate to good yields. Analysis of the side products reveals that the reaction mechanism is analogous to that previously proposed for Pt-catalyzed cycloisomerizations. -(HASHMI*, A. S. K.; WOELFLE, M.; ATA, F.; HAMZIC, M.; SALATHE, R.; FREY, W.; Adv.
Gold Catalysis: Desymmetrization in the Furan-Yne Reaction. -The reaction of diketones (I) with Grignard reagent (II) affords a mixture of anti-and syn-homoproparyglic diols. The Au-catalyzed reaction of the anti-isomers results in formation of indene derivatives, whereas the syn-diastereomers are converted to oligomeric/polymeric material. -(HASHMI*, A. S. K.; WOELFLE, M.; ATA, F.; FREY, W.; ROMINGER, F.; Synthesis
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