Alkenyl furans

Wahrend sich 2-Alkyl- bzw. 2-Alkenyl-furane glatt zu Tetrahydrofuranen hydrieren lassen(Tab. 65, Nr. 2,3,4,6,24,25,26u.a.),werden2-Aryl-furanesowohlanRaney-Nik-kel, als auch an Palladium/Kohle oder Platindioxid nur hydrogenolytisch gespalten3,4. 

The characteristic features of the Kobayashi approach (Scheme 50) are the construction of the Cig alkenyl-furan by a Cf + Cn + C4 sequence, the Sharpless asymmetric dihydroxylation of the olefinic C=C bond, and the oxidative transformation, efficiently and under mild conditions, of the substituted furan ring into a 4-oxobut-2-enoic-acid moiety, which is diastereoselec-tively reduced by Zn(BH4)2. 

Intramolecular thermal and catalytic [4 + 2] cycloaddition in 2-alkenyl-furans 05UK707. 

A C-H insertion was also employed in the synthesis of 3-substituted furans. Coupling of 2,5-dimethylfuran with 4-methystyrene using palladium(II) acetate gave the tra/w-alkenyl furan selectively and in modest yield. ... 

Hashmi and coworkers found that intramolecular fi-alkenylated furan product, polyheterocycle 4,5,6,7-tetrahydrofuro[2,3-c]pyridine derivatives 127 rather than phenolic products [59], could be efficiently constructed via gold(I)-catalyzed aco-selective intramolecular hydroarylation of silylated y-alkynylfurans 126 (Scheme 12.55) [60]. Substrates bearing different silylated groups have a different reactivity. They also found that gold(I) catalyst efficiently serves as a hydroarylation catalyst for the silylated y-alkynylfurans, while the platinum(II)... 

Furans substituted at the 2-position efficiently undergo coupling to give 2-alkenylated furans in good yields (Scheme 2.20). Selected alkenes as the coupling partner are acrylates, acrylamides, methylvinylketone, acry-laldehyde, acrylonitrile, and styrenes. 

The formation of disubstituted alkynes by coupling of terminal alkynes, followed by intramolecular attack of an alcohol or amine, is used for the preparation of benzofurans and indoles. The benzo[il)]furan 356 can be prepared easily by the reaction of o-iodophenol with a terminal alkyne[262]. The 2-substituted indole 358 is prepared by the coupling of 2-ethynylaniline (357) with aryl and alkenyl halides or triflates, followed by Pd(ll)-catalyzed cycliza-tion[263]. 

There are reports of an increasing number of palladium-assisted reactions, in some of which the palladium has a catalytic function. Thus furan and thiophene undergo facile palladium-assisted alkenylation giving 2-substituted products. Benzo[6 Jfuran and TV- acetyl-indole yield cyclization products, dibenzofurans and carbazoles respectively, in addition to alkenylated products (8UOC851). The arylation of pyrroles can be effected by treatment with palladium acetate and an arene (Scheme 86) (81CC254). 

Furan, 2-alkenyl-3-hydroxytetrahydro-synthesis, 4, 677 Furan, alkyl-reactions, 4, 644 synthesis, 4, 710 Furan, 2-alkyl-mass spectra, 4, 21-22 synthesis, 4, 666 Furan, 3-alkyl-mass spectra, 4, 21-22 synthesis, 4, 665, 710 Furan, 5-alkyl-2-phenylthio-reactions, 4, 80 Furan, 2-alkyltetrahydro-synthesis, 4, 675, 711 Furan, 3-amido-synthesis, 4, 665 Furan, 2-amino-synthesis, 4, 74, 121, 661 tautomerism, 4, 38 Furan, 3-amino-tautomerism, 4, 38 Furan, 2-amino-3-cyano-synthesis, 4, 661, 689, 712 Furan, 3-amino-2-methyl-reaction, 4, 74 Furan, 2-aryl-reactions... 

Gcwdini, A. The Behaviour of Furan Derivatives in Polymerization Reactions. Vol. 25, pp. 47-96. Gandini, A. and Cheradamc, H. Cationic Polymerization. Initiation with Alkenyl Monomers. Vol. 34/35, pp. 1-289. 

An electron-withdrawing group on the alkenyl double bond has been shown to greatly facilitate the Diels-Alder cycloaddition with furan as depicted below < 00TL9387>. 

The reactions of heteroaromatic compounds such as furans, pyrroles, and indoles with alkynoates proceed under very mild conditions (in acetic acid or even in neutral solvents such as CH2C12 at room temperature). For example, the reaction of pyrrole with ethyl phenylpropiolate gives the 2-alkenylated pyrrole (Equation (44)).47c This reaction is applied to the direct synthesis of a /3-alkenylpyrrole, the pyrrole fragment of haemin (Equation (45)).47d The present reaction provides a very convenient method for functionalization of arenes and heteroarenes. 

Substituting the benzene ring with a double bond, Pd-catalyzed intramolecular alkoxylation of alkyne 122 also proceeded via an alkenyl palladium complex to form furan 123 instead of a benzofurans [99, 100]. In addition, 3-hydroxyalkylbenzo[fc]furans was prepared by Bishop et al via a Pd-catalyzed heteroannulation of silyl-protected alkynols with 2-iodophenol in a fashion akin to the Larock indole synthesis, [101]. 

In the same year, Hibino et al. reported a total synthesis of furostifoline (224) employing a new type of electrocyclic reaction (636). This cyclization proceeds through a 2-alkenyl-3-allenylindole intermediate, which is derived from 2-(fur-3-yl)-3-propargyUndole 1128. Compound 1128 was prepared starting from 2-chloroindole-3-carbaldehyde (891), furan-3-boronic acid (1124), and ethynylmagnesium bromide. 

Regio- and enantioselective Heck reactions of 2 3-dihydrofuran with aryl and alkenyl triflates in the presence of the chiral ligand (R)-BITIANP provides 2-substituted 23-dihydro-furans with complete regioselectivity, high enantioselectivity (86-96% ee) and good yields (76-93%) <99CC1811>. A catalytic oxyselenylation-deselenylation reaction of alkenes offers a stereoselective one-pot conversion of alkenes into 2 -dihydrofurans <99EF0797>. 

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