Furan ring tetrahydro— from

The hydrolysis (or alcoholysis) of furans involves nucleophilic addition of water (or an alcohol) to an initially formed cation, giving rise to open-chain 1,4-dicarbonyl-compounds or derivatives thereof. This is in effect the reverse of one of the general methods for the construction of furan rings (18.13.1.1). Suc-cindialdehyde cannot be obtained from furan itself, presumably because this dialdehyde is too reactive under conditions for hydrolysis, but some alkyl-furans can be converted into 1,4-dicarbonyl products quite efficiently, and this can be viewed as a good method for their synthesis, and of cyclopentenones derived from them. ° Other routes from furans to 1,4-dicarbonyl compounds are the hydrolysis of 2,5-dialkoxy-tetrahydro-furans (18.1.1.4) and by various oxidative procedures (18.2). 

The 17a-ethynyl compound (59) has been prepared in 88% yield from estr-4-ene-3,17-dione (58) and acetylene, at 2-3 atm pressure in tetrahydro-furan in the presence of potassium t-butoxide. Presumably the A-ring enone system is protected as the enolate anion during the course of the reaction. 

Indolizines with a saturated six-membered ring may be obtained from pyrrole or furan derivatives.225,226 The reaction of 3,4-dimethylpyrryl-magnesium bromide with y-chlorobutyronitrile followed by dilute acid gave 5,6,7,8-tetrahydro-l,2-dimethyl-5-oxoindolizine. 5,6,7,8-Tetrahy-droindolizine can be obtained by the cyclodehydration of 3-(2-furyl)-propylamine over alumina at 400°C. The same product is obtained by the treatment of 3-(2-pyrryl)propyl cyanide with hydrogen chloride in the presence of boron trifluoride etherate catalyst, followed by Wolff-Kishner reduction of the intermediate ketone. 

Selective removal of the reW-butyldimethylsilyl protecting group from the enone rac-5 and subsequent cyclization was achieved by treatment with dilute hydrochloric acid in tetrahydro-furan at ambient temperature48. Whereas the ring closure was m-selective, a 6 1 mixture of epimers at C-7, rac-6 was obtained. 

On the action of BF3, a steroid oxirane undergoes rearrangement to a tetrahydro-furan. 7-Lactones are formed from oxirane by nucleophilic ring-opening (Eqs. 192-194). ° ... 

The difference between the dipole moments of the aromatic compound and the corresponding tetrahydro derivative is called the mesomeric moment and represents a measure of the -electron delocalization. The mesomeric dipole moment is directed, in all the congener systems, from the heteroatom toward the ring. The values obtained (furan 1.03 D, thiophene 1.35 D, selenophene 1.29 D, tellurophene 1.17 D) are in excellent quantitative agreement with other aromaticity indices based on structural and magnetic properties12 (see discussion in Section II,C). 

The term carbocyclic nucleoside [181] is used to describe a group of compounds structurally related to nucleosides in which the furanose ring has been replaced by a cyclopentane ring. A consequence of this substitution is an enhancement of the metabolic stability of the carbocyclic nucleosides, which are not subjected to the action of nucleoside phosphorylases and hydrolases that cleave normal nucleosides. However, from the conformational point of view the tetrahydro-furan and cyclopentane rings are similar. Thus, carbocyclic nucleosides may act as substrates or inhibitors of the enzymes that activate (kinases) and transform nucleosides and nucleotides in living cells and incorporate them into DNA. Most approaches to the synthesis of carbocyclic nucleosides begin with the construction of the nucleic acid base from a functionalized cyclopentylamine, which with some exceptions is obtained as a racemic mixture. The medicinal chemistry of carbocyclic nucleosides, as well as the synthesis of the intermediate cyclopentylamines have been reviewed [181]. This section deals only with work published after that review, directly related with anti-AIDS research. 

Octafluoro[2.2]paracyclophane is prepared in refluxing tetrahydro-furan (THF) and hexamethylphosphoramide or DMSO solution from 1,4-bis(bromodifluoromethyl)benzene. Trimethylsilyltributyllin is used as a reducing agent. Cesium fluoride as a catalyst gives superior yields (40%) in comparison to potassium fluoride. To favor the formation of rings, the reaction must be accomplished in highly diluted systems. 

An unsaturated alcohol where the double bond is conjugated to an aromatic ring was hydroformylated by Nahum [970]. From coniferyl alcohol he obtained a 25% yield of 3-(3-methoxy-4-hydroxyphenyl)-tetrahydro-furan the formation of which is explained by ring closure of the primary... 

Later, the liquid state measuring membrane was changed by an elastic plastic disk. This made the electrode form mechanically more stable. In the early times, well working potassium [16] and ammonium [17] ISEs were made with a silicon rubber matrix. Later, however, plasticized polyvinyl chloride (PVC) matrices became the most popular in ISE fabrication. Their preparation procedure was well worked out and clearly described by Shatkay [18] and Thomas [19], The high molecular weight PVC used in membrane preparation can be dissolved in tetrahydrofurane. In this solution, the plasticizer, the selective ionophore, and other ingredients like special lipophilic salt are dissolved. The solution is transferred into a ring-shaped flat bottomed mold. Care is taken to let the tetrahydro furane (THE) slowly evaporate. In a few days, a circular, flexible membrane is obtained. A disk with appropriate diameter is cut from the membrane and pasted on the tubular electrode body. 

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