Di- and tetrahydrofurans

The addition of active methylene compounds to 2,3-dihydrofuran was promoted by catalytic amounts of AuCl3-AgOTf, providing 2-substituted THF derivatives, as depicted in the scheme below 05OL673 . 2-Lithiated dihydrofurans were shown to lead to heterospiro cycloalkanones 05H(66)57 . 

A full account of the Ru(PyBox)-catalyzed enantioselective 13-dipolar cycloaddition between 2,3-dihydrofuran and diazopyruvates, first described in 2004, was reported 05HCA101Q . 2,3-Dihydrofurans having a 3-acetyl group, e.g. benzocycloalla[l,2-fojfurans and spiro[furan-2(37/),r-benzocycloalkane], underwent benzannulation via photo-induced cleavage of the dihydrofuran ring 05rL7303 . An example that produced a helicene-type compound is shown below. 

Related to the example described in 2004, another novel reaction of 23-dihydrofuran with zirconocene was reported. As shown in the scheme below, regioselective insertion of 

3-dihydrofuran into aryne-zirconocene complexes, generated from aryllithiums, and subsequent reaction with electrophiles, provided 1,2-disubstituted products that contain a (Z)-alkene 05SL2513 . 

Further studies of the regioselective tandem ring opening/cross metathesis of 2-emlo-substituted 7-oxanorbomenes with electron-rich olefins, a process first described in 2004, were reported. Reaction of the 2-exo isomer, like that of the 2-endo isomer, was also found to 

An interesting example of a triple electrophilic aromatic substitution between an oxonium ion, generated from a trisubstituted dihydrofuran, and phloroglucinol was exploited for the total synthesis of the C3-symmetric xyloketal A, as depicted in the scheme below C06OL1427 06JOC1620 . 

As exemplified below, an interesting rearrangement of fused 2,5-dihydrofurans to tricyclic lactones under strongly acidic conditions was observed 06JOC9544 . 

Upon treatment with Me3SiOTf, the 2,5-dihydrofuran-containing 14-membered marine cembranoid sarcophytoxide was converted to a 10-membered ring product as shown below 06OL2957 . Presumably, the Lewis acid promoted the cleavage of the dihydrofuran ring to provide a transient allylic cation, leading to a transannular cyclization. 

2-Alkylidenetetrahydrofurans underwent a cyclo-condensation with amidines to give 4-(3-hydroxyalkyl)pyrimidines, as can be seen below 06T5426 . 

3-Aryl furans were prepared in moderate yields by Rh-catalyzed regioselective hydroformylation of substituted propargylic alcohols followed by cyclization, and an example is shown below 06ASC545 . 

---

A rhodium-mediated carbonyl ylide approach to di- and tetrahydrofurans (Scheme... 

The synthesis of di- and tetrahydrofurans continues to be one of the most active areas in synthetic organic chemistry. In the construction of tetrahydrofurans, the Williamson cycloetherization has always been a practical approach. Lautens employed a three-step procedure to convert the alcohol shown below to a bis-tetrahydrofuran <02OL1879>. A similar cyclization route was also utilized in the total synthesis of muconin <02TL8661>. 

Reiss et have recently carried out an extensive theoretical study of the linear and nonlinear properties of pNA in the gas phase, of three solvents, cyclohexane (CH), 1,4-dioxane (DI) and tetrahydrofuran (THF) and of solutions of pNA in these solvents. The aim of the work is to provide a treatment in which the liquid phase molecules are treated discretely via a statistical simulation of the liquid and solution structures. The general strategy for attaining this goal can be summarized as follows -... 

It is well known that di- and tetrahydrofurans can be obtained by inter- and intramolecular 1,3-dipolar cycloaddition reactions of carbonyl ylides with alkynes and alkenes. This methodology has also been shown to be of value in the preparation of a furanophane <92TL57>. 

Five-Membered Ring Systems Furans and Benzofiirans S.3.2.2 Di- and Tetrahydrofurans... 

These furan endoperoxides are valuable building blocks for the synthesis of di- and tetrahydrofuran derivatives, e.g., bis-epoxides, epoxy lactones, ene diones, cis-diacyloxiranes, enol esters, butenolides, and others. Furthermore, the primary [4 + 2]-cycloadducts may rearrange into 1,2-dioxetanes or 1,2-dioxolenes. ... 

---