Asymmetric furan

Yeo W-C, Chen S, Tan G-K, Leung P-H (2007) Synthesis of P-chiral phosphines via chiral metal template promoted asymmetric furan Diels-Alder reaction. J Organomet Chem 692 2539-2547... 

Fig. 6 AlBrs activated oxazaborolidine catalyst for asymmetric furan Diels-Alder reaction and [2+2] cycloaddition...

Scheme 50 Asymmetric furan cyclization and the formal total synthesis of (+)-cw-solamin by...

Asymmetric furans 79a and 79b were prepared as shown in Scheme 14.27 [70]. A one-pot sequence comprising monolithiation of furan (15) and alkylation of the resulting lithiofuran 78 with 1-bromohexane gave 2-hexylfuran (79a) in 43% yield. Then, a second lithiation of 79a followed by reaction of the resulting lithiofuran 80 with acetone and hydrogenolysis of alcohol 81 over Pd/C yielded 2-hexyl-5-isopropylfuran (79b) in 34% three-step yield from 79a. Reaction of 2,6-naphthodiyne equivalent 39 (0.5 equiv) with asymmetric furans 79 in the presence of KF and 18-crown-6 in THF at room temperature furnished a mixture of bis(furan)... 

Remote asymmetric induction using chiral sulfoxides, derivatives of furan, thiophene, and pyrrole 98YGK798. 

Although furan is usually a poor diene in the Diels-Alder reaction, the chiral copper reagent 24b promotes its asymmetric addition to acryloyloxazolidinone to afford the 7-oxabicyclo[2.2.1]hept-2-ene derivative in high optical purity (Scheme 1.40). Because a retro-Diels-Alder reaction occurs above -20 °C, the reaction must be performed at low temperature (-78 °C) to obtain a high optical yield. The bicy-... 

This article aims to review papers that were published in 2006 on reactions and syntheses of furans, benzofurans and their derivatives. Two reviews have summarized the syntheses of furans <06OBC1627> and tetrahydrofurans <06EJO1627>. Another review <06001613> records the progress of transition metal-catalyzed asymmetric ring opening of oxabenzonorbornadienes. 

Asymmetric synthesis of the rocaglamides was accomplished by employing [3+2] photo-cycloaddition mediated by functionalized TADDOL based chiral Brpnsted acids. The synthesis consisted of a [3+2] dipolar cycloaddition, a base-mediated a-ketol rearrangement and a hydroxyl-directed reaction <06JA7754>. Asymmetric synthesis of 1,2-dihydrobenzo /j]furans was achieved by adamantylglycine derived dirhodium tetracarboxylate catalyzed C-H insertion <06OL3437>. 

Zamojski and coworkers have explored the use of the furan-carbonyl photocycloaddition in asymmetric synthesis, with somewhat limited success88. Irradiation of chiral glyoxylate derivative 196 [R = (R)(—)-menthyl and (R)(—)-8-phenylmenthyl] afforded... 

A short five-step synthesis of a bifuran, namely ( )-2,2 -bis(diphenylphosphino)-3,3 -binaphtho[2,l-I>]furan (BINAPFu) from naphthofuranone via a low-valent titanium-mediated dimerization was reported. The newly developed resolution procedure for phosphines was utilized to provide the optically active bidentate phosphine ligands (BINAPFu), which consistently outperforms BINAP in the asymmetric Heck reaction between 2,3-dihydrofuran and phenyl triflate . Another way in which a benzofuranone can be converted into benzo[7 ]furan is by treatment of the former with 1-BU2AIH at -78°C followed by an acidic work up <00TL5803>. 

The asymmetric synthesis of a galanthamine alkaloid relies also on the intramolecular Heck reaction for the preparation of the benzo[h]furan-based key intermediate with a crucial chiral quaternary center, which eventually leads to the synthesis of (-)-galanthamine <00JA11262>. A similar approach towards the construction of galanthamine ring system via an intramolecular Heck reaction has also been investigated <00SL1163>. 

The first example of a nitrone reaction with pyrroles and furan in the presence of HC1 as an activating agent was recently reported (589). Depending on reaction conditions, these acid-catalyzed reactions make it possible to obtain both, 2-heteroaromatic N -benzylhydroxylamines and symmetric as well as asymmetric 2,2 -bis -(heteroaryl) alkanes (Scheme 2.155). 

The high levels of enantioselectivity obtained in the asymmetric catalytic carbomagnesa-tion reactions (Tables 6.1 and 6.2) imply an organized (ebthi)Zr—alkene complex interaction with the heterocyclic alkene substrates. When chiral unsaturated pyrans or furans are employed, the resident center of asymmetry may induce differential rates of reaction, such that after -50 % conversion one enantiomer of the chiral alkene can be recovered in high enantiomeric purity. As an example, molecular models indicate that with a 2-substituted pyran, as shown in Fig. 6.2, the mode of addition labeled as I should be significantly favored over II or III, where unfavorable steric interactions between the (ebthi)Zr complex and the olefmic substrate would lead to significant catalyst—substrate complex destabilization. 

One active field of research involving the Heck reaction is asymmetric Heck reactions (AHR). The objective is to achieve enantiomerically-enriched Heck products from racemic substrates using a catalytic amount of chiral ligands, making the process more practical and economical Although intermolecular Heck reactions that occurred onto carbocyclic arenes are rare, they readily take place onto many heterocycles including thiophenes, furans, thiazoles, oxazoles,... 

Schlessinger and Bergstrom146 reported some asymmetric Diels-Alder reactions of several polystyrene bound furans to which a chiral auxiliary had been attached with methyl acrylate. The adducts were obtained with de values of more than 99%, as was determined after cleavage of the adducts from the resin. 

The use of cyclic alkenes as substrates or the preparation of cyclic structures in the Heck reaction allows an asymmetric variation of the Heck reaction. An example of an intermolecular process is the addition of arenes to 1,2-dihydro furan using BINAP as the ligand, reported by Hayashi [23], Since the addition of palladium-aryl occurs in a syn fashion to a cyclic compound, the 13-hydride elimination cannot take place at the carbon that carries the phenyl group just added (carbon 1), and therefore it takes place at the carbon atom at the other side of palladium (carbon 3). The normal Heck products would not be chiral because an alkene is formed at the position where the aryl group is added. A side-reaction that occurs is the isomerisation of the alkene. Figure 13.20 illustrates this, omitting catalyst details and isomerisation products. 

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