Tetrahydrofuran ring

As mentioned previously, when Marko et al. [63] attempted to prepare the exomethylene tetrahydrofurans 175 by coupling allylsilane 171 with aldehyde 174, 

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Examine the electrostatic potential map of H3B THE (borane-tetrahydrofuran complex) on Learning By Modeling How does the electrostatic potential of the hydrogens bonded to boron dif fer from the potential of the hydrogens of the tetrahydrofuran ring" ... 

Tetrahydrofuran (3) is produced commercially from furfural by decarbonylation followed by hydrogenation it is also produced by several different methods from other raw materials. A complete discussion of tetrahydrofuran is found under Ethers. Polymers of tetrahydrofuran are covered under the general topic. Polyethers. Several other compounds containing the tetrahydrofuran ring, which are most readily produced from furfural, are discussed here. 

Therapeutics. Compounds containing the furan or tetrahydrofuran ring are biologically active and are present in a number of pharmaceutical products. Eurfurjdamine [617-89-0] is an intermediate in the diuretic, furosemide. Tetrahydrofurfurylamine [4795-29-3] may also have pharmaceutical applications. 5-(E)imethyiaininomethyi)furfuryi alcohol [15433-79-17 is an intermediate in the preparation of ranitidine, which is used for treating ulcers. 2-Acet5dfuran [1192-62-7] prepared from acetic anhydride and furan is an intermediate in the synthesis of cefuroxime, a penicillin derivative. 2-Euroic acid is prepared by the oxidation of furfural. Both furoic acid [88-14-2] and furoyl chloride [527-69-5] are used as pharmaceutical intermediates. 

The polymerizations of tetrahydrofuran [1693-74-9] (THF) and of oxetane [503-30-0] (OX) are classic examples of cationic ring-opening polymerizations. Under ideal conditions, the polymerization of the five-membered tetrahydrofuran ring is a reversible equiUbtium polymerization, whereas the polymerization of the strained four-membered oxetane ring is irreversible (1,2). 

Intermediate 8, the projected electrophile in a coupling reaction with intermediate 7, could conceivably be derived from iodolactone 16. In the synthetic direction, cleavage of the acetonide protecting group in 16 with concomitant intramolecular etherification could result in the formation of the functionalized tetrahydrofuran ring of... 

The furan ring can be made to polymerize through one or both of its double bonds and the polymers obtained will therefore have dihydro- and tetrahydrofuran rings in their backbone. This situation occurs when furan, the alkylfurans, benzofuran and some dihydrofurans are treated with suitable initiators and is discussed in the first section of this chapter. 

However, the 2 3-dihydrofuran units in these chains are readily susceptible to further attack by the acid or a growing species at the 4,5-unsaturation (see Section III-4) and this produces branching and the formation of tetrahydrofuran rings ... 

The spontaneous polymerization of furan adsorbed on carbon black with or without SnCl4 vapours35 has been explained by a similar cationic mechanism. Also, the polymerization of gaseous furan on liquid acidic surfaces35 has the same origin, but in these systems the polymers suffer an acid-catalyzed hydrolysis of their tetrahydrofuran rings which produces a considerable proportion of hydroxyl and carbonyl groups. 

Bei der Reduktion von prochiralen Ketonen werden die sterisch bevorzugten Alko-hol-Epimere erhalten (s. S. 333). In Gegenwart von Triathyl-boran spaltet es reduktiv den Tetrahydrofuran-Ring (s.S. 425). 

There are two possible ways for the ring opening of 35 in the polymerization The C1—O7 bond cleavage leads to the formation of a substituted tetrahydropyran ring 36 in the polymer chain, while the C1—O2 bond cleavage produces a substituted tetrahydrofuran ring 37. Product analysis of the acid-catalyzed hydrolysis of the... 

Although it is unlikely that the tetrahydrofuran ring would open under these conditions of polymerization, the polymer was hydrolyzed in 0.2 M sodium hydroxide solution, in order to confirm its structure. Clear colorless liquid was obtained after acidification followed by esterification with diazomethane. Its IR and NMR data compares exactly to that obtained from 59 which was prepared from the neutral hydrolysis of 57 and esterification of the resultant acid with diazomethane. Since the apparent sole product obtained from hydrolysis of the polymer was 59, they conclude that 58 represents the correct structure for this polymer. 

Cyclic hemiacetals or hemiketals of sugars with a five-membered (tetrahydrofuran) ring are called furanoses, those with a six-membered (tetrahydropyran) ring pyran-oses. For sugars with other ring sizes see 2-Carb-5. 

This reagent has been used in multistep syntheses to convert a tetrahydrofuran ring into a y-lactone. 

Jiang Z, Chen Y, Ruo-Yun CH, De-Quan Y. Mono-tetrahydrofuran ring acetogenins from Goniothalamus donnaiensis. Phytochemistry 1997 46 327-331. 

This procedure illustrates a fundamentally new method for constructing substituted tetrahydrofurans.5-10 This practical method assembles the tetrahydrofuran ring from allylic diol and carbonyl components and in the process forms three ring bonds C(2)-C(3), C(4)-C(5) and 0-C(5). Both aldehydes (eq 1) and ketones (illustrated in the present procedure) can be employed as the carbonyl component. Although it is often convenient to isolate the acetal intermediate, conversion to the 3-acyltstrahydrofuran can also be accomplished in many cases by the direct reaction of the diol and carbonyl components.8 High ds stereoselectivity (at least 20 1) is observed in the preparation of tetrahydrofurans that contain single side chains at carbons 2 and 5 (eq 1). The kinetically controlled product also has the cis relationship of these side chains and the 3-acyl substituent. 

The products arising from such condensations of the hexoses readily lose the elements of water in a way similar to the dehydration of 7-hydroxy acids, when heated in aqueous solution the hydroxyl groups corresponding to those at C3 and C6 of the original sugar are involved, and a substituted tetrahydrofuran ring results. 

The influence of certain rings and radicals on the stabilization of the dihydroxy tetrahydrofuran ring may also be mentioned, since rings of this kind are present in many natural substances. 

In Morimoto s total synthesis of (-t-)-eurylene and (+)-14-deacetyleurylene, the pivotal steps are the construction of trans- and cw-tetrahydrofuran rings via a hydroxy-directed syn oxidative cyclization of acyclic bishomoallylic alcohols promoted by Re(VII) and Cr(VI) oxides, respectively. As depicted below, the trans-THF is achieved by treatment of the triol with the oxorhenium(VII) complex, and the cis-THF is constructed by the use of PCC <00AG(E)4082>. 

A new entry to exocyclic dienes was reported by Sha who uncovered that a radical cyclization of the vinyl iodide 100 can lead to the formation of an exocyclic dienes fused with a tetrahydrofuran ring. The cyclization is proposed to proceed in a 5-(n-exo)-exo-dig fashion <00OL2011>. 3,4-Disubstituted tetrahydrofurans can also be constructed via the cyclization of O-stannyl ketyls and allylic 0-stannyI ketyls onto electron-rich or electron-poor alkenes <00TL8941>. 

Viewed in a much broader context, C-aldopentofuranosyl derivatives may also be regarded as highly functionalized, chiral derivatives of substituted tetrahydrofuran. As such, they may be useful intermediates in the synthesis of a variety of compounds that contain a chiral, substituted tetrahydrofuran ring. 

For the depiction of structural formulas of hexofuranoses, a combination of a three-dimensional, Haworth-perspective tetrahydrofuran ring with a Fischer projection of the C-5-C-6 side-chain is commonly used, as exemplified by formulas 3 and 6. With the formal closure of the second ring and formation of a 2,6-dioxabicyclo[3.3.0]octane system, however, the depiction of the C-6-C-3 ring, as in formula 7, also assumes three-dimensional geometry, and this does not correspond to the Fischer projection rule.11 Consequently, structural representations of such bicyclic molecules should be as close as possible to the actual steric situation, as shown by structures 4 and 8. 

In contrast to these compounds, the coupling constants obtained from n.m.r. measurements of 25 might, on first sight, appear to be consistent with the 3T2, as well as with the E2 and 3E conformations, respectively. However, the following considerations clearly favor the 3T2 conformation, with a more moderate puckering of the tetrahydrofuran ring. 

This rearrangement of allylic acetals can also be used for furan annelations, in which the formation of the new tetrahydrofuran ring is coupled with ring enlargement of the starting ring.3 The same dr-fused bicyclic tetrahydrofuran is formed from either one of the cis- or mwr-allylic diols used as starting materials. 

Cyclization of parts of the PAF molecular backbone produces the second family of PAF-related antagonists, those with a constrained framework. For example, insertion of a tetrahydrofuran ring has led to derivatives such as SRI 63-072 (8), SRI 63-441 (9a), SRI 63-675 (9b) and BN 55009 (10). The... 

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