Rhodium-catalyzed synthesis furans

Gowravaram and Gallop (169) adapted the rhodium-catalyzed generation of isomiinchnones from diazo imides to the solid-phase synthesis of furans, following a 1,3-dipolar cycloaddition reaction with alkynes. A variety of furans were prepared in this fashion. With unsymmetrical electron-deficient alkynes (e.g., methyl... 

Intermolecular [3+2] 1,3-dipolar cycloaddition of a D-glyceraldehyde-derived nitrile oxide to the 4,5-double bond of 2-methylfuran gave a 60 40 diastereomeric ratio of the two furoisozaxoline isomers. This chemistry was employed in the synthesis of L-furanomycin <2005EJ03450>. As depicted in Scheme 43, an intramolecular cycloaddition of a furan with a carbonyl ylide dipole proceeded under rhodium-catalyzed microwave-promoted conditions to provide the cycloadduct in a modest yield <20040L3241>. 

Benzo[ ]furan-based diazobutenoates were used as a substrate to make a cyclopropane in 89% yield via a rhodium-catalyzed intramolecular process, as can be seen in Equation (74). Cyclopropane 88 was the key intermediate for the total synthesis of diazonamide A <20000L3521>. 

The synthesis of bicyclo[3.2.1]octadienes 86 has been accomplished by refluxing divinyl-cyclopropanes 85 in xylene. Subsequent research led to the development of conditions employing a rhodium-catalyzed step for the synthesis of bridged systems via the cyclopropanation of cyclopentadienes, furans and pyrroles (see section on transition-metal-mediated rearrangements). 

An efficient asymmetric synthesis of substituted methyl 2-aryl-dihydrobenzo[h]furan-3-carlx)xylate was achieved by a rhodium-catalyzed C-H bond activation route in an excellent yield, and the generated product was an intermediate applicable to the total synthesis of (+)-lithospermic acid <0 A13496>. A similar type of framework existing in ( )-E-viniferin was m e through a biomimetic transformation by a T1(N03)3-mediated oxidative dimerization of resveratrol <05T10285>. 

A mild intramolecular fluorocyclization reaction of benzylic alcohols and amines was developed, using commercially available Selectfluor to trigger electrophilic cyclizations to afford fluorinated dihydrobenzo[c]furans (14CC13928). A novel rhodium-catalyzed carbonylative [3 + 2 + 1] cycloaddition of alkylidenecyclopropanes was developed for the synthesis of phenols with benzo[c]furan species (140L4352). 

One of the earliest uses for rhodium(II)-catalyzed dipoles was demonstrated in Davies furan synthesis [22]. Isomiinchnones were also shown to produce substituted furans [115]. Additional furan syntheses have been described using silylacetates [116], unsaturated esters [117], and fluoroalkyl diazo acetates [118]. The synthesis of furofuranones and indenofuranones 35 from a-diazo ketones having pendant alkynes has also been reported (Eq. 6) [119]. Other fused heterocyclic systems include furo[3,4-c]furans [120, 121] furo[2,3-b]furans [122] as well as thiobenzofurans [123], and benzoxazoles[124] have also been synthesized with this methodology. 

Synthesis of Furans via Rhodium(II) Acetate Catalyzed Reaction of Acetylenes with a-Diazocarbonyls. 

In the total synthesis of (-)-conocarpan, the chiral 2,3-dihydrobenzo[b furan core was constructed by radical based intramolecular cyclization as shown below <07CC2151>. In addition, chiral 2-isopropeny 1-2,3-dihydrobenzo[6J furans and 2-amido-bcnzo b furans were be made by palladium-catalyzed reaction in the presence of Trost ligand, and the rhodium(I)-catalyzed cyclization, respectively <07JOC2857 07OL2361>. 

SYNTHESIS OF FURANS VIA RHODIUM(II) ACETATE-CATALYZED REACTION OF ACETYLENES WITH a-DIAZOCARBONYLS ETHYL 2-METHYL-5-PHENYL-3-FURANCARBOXYLATE (3-Furancarboxyllc acid, 2-methyl-5-phenyl>, ethyl ester)... 

A new strategy for the synthesis of highly substituted benzo[lr]furans from weta-substituted hydroxybenzenes and alkynes via a rhodium(III)-catalyzed activation of a sterically hindered C-H bond was demonstrated. A possible mechanism involving dual directing group-assisted ortho C—H bond activation was proposed (14OBC9105). 

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