Tetra hydrofuran ring

As matters would have it, the mercuricyclization of 36 can advance comparably on both surfaces of the it bond, with the result that 43 and 44 are formed competitively (Scheme 3-7). [31] These diastereomeric products diffracted well and provided the important structural information displayed in Figs. 3-3 and 3-4. [32] In 43, all six heteroatoms are projected equatorially. This is not so for 44 where the axial C-Hg and C-O bonds generated along the reaction coordinate are preserved. The latter phenomenon is made possible because the methylene groups from four tetra-hydrofuran rings are able to remain axially disposed. 

The radical reduction of ester-substituted alkyl iodides bearing a chiral tetrahydrofuranyl substituent proceeds with high selectivities1522. A bulky group in the 4-position of the tetra-hydrofuran ring has little influence on the diastereomeric excess, whereas an alkoxy substituent in the 3-position increases the synjanti ratio. 

The radical polymerization of diallyl ether with sulfur dioxide produced soluble copolymers whose composition was between 1 1 and 2 1 on SO2. Formation of rings made of diallyl ether and SO2 and 3,4-disubstituted tetra-hydrofuran rings by the cyclized diallyl ether units was proposed on the basis of H NMR data [61k]. 

As part of a general approach to the characterization and representation of conformational behaviour of furanoses, we would like to focus in this article on the conformational behaviour of the tetra-hydrofuran ring of a small series of voluntarily simple 2,5-anhydro-heptitols (C-furanosyl compounds). One issue in particular warrants more study in our opinion, and is a focus of this article what is the most appropriate representation of the conformation of a furanose ring ... 

Semmelhack MF, Zhang N. Stereoselective formation of tetra-hydrofuran rings via intramolecular alkoxycarbonylation of hydroxyalkenes. J. Org. Chem. 1989 54 4483-4485. 

The polymerisatibh of c-cstproliactarn is sbmetiiries known as a ring-, opening polymefisation, a, technique also. hsed with ethylene ioxide, tetra- hydrofuran and a numl r of other rhanoirters. 

Mori et al. have demonstrated the most dramatic uses of lithiated epoxides in natural product synthesis [62]. By employing the chemistry developed by Jackson, and subsequently performing a Lewis acid-catalyzed (BF3 OEt2) cyclisation, tetra-hydrofuran, tetrahydropyran, and oxepane rings are readily accessed this strategy is demonstrated by the synthesis of the marine epoxy lipid 173 (Scheme 5.40) [63]. 

Some of the more important monomers whose ring opening polymerisations have been induced by stable cation salts include, 1,4-epoxides, notably tetra-hydrofuran (20,112,113), 1,2-epoxides (114), 1,3-episulphides (thietans) (33,53), 1,2-episulphides (thiiranes) (53), azetidines (115,116), aziridines (117), the cyclic formals, 1,3-dioxolan (23,54, 118-120), and 1,3-dioxepan (118,119), trioxane (121,122) and more recently lactones (123). Aldehydes (124) may also be included since these molecules can be regarded as the smallest possible oxygen hetero-... 

Further difficulties arise in explaining the solvent dependence in opening the rings using the MejNBHj-AiClj system. The expected stronger solvation of AICI3 in tetra-hydrofuran, versus toluene or dichloromethane, would indicate preferential electrophilic attack at 0-6 in tetrahydrofuran, and the production of the 4-benzyl ether, contrary to what is observed. 

While the mercury compound shows localized N—Hg bonds, the TADB-system (which is isoelectronic with cyclopentadiene) forms a low-spin complex with iron. This was the first identified example of a sandwich-compounds of a boron-nitrogen ring. The complex is prepared by adding an etheric solution of TADBLi to a suspension of FeCl2 in tetra-hydrofuran. 

Photolysis of a-cyanostilbene oxides has been examined in cydoaddition reactions with the dipolarophile dimethyl fumarate. The stereochemistry of the tetra-hydrofuran adduct prepared from the ylides formed in the disrotatory electro-cyclic ring-opening predicted by Woodward and Hoffman points to cis-trans isomerization of the carbonyl-ylide, probably via conrotatory cyclization. 

On treatment of the l-azido-2-iodo-l-methylcyclobutane (1) with diborane in tetra-hydrofuran, 2-iodo-l-methylcyclobutylamine (2) was effectively isolated in 50% yield, then lithium aluminum hydride reduction gave the same product of ring contraction 3. ... 

It is relatively easy to ring-open ethylene oxide, more difficult to open oxetane, and tetra-hydrofuran is relatively unreactive. The ease of ring opening oxirane favors its use in macrocycle synthesis. In addition, though, the placement of the heteroatoms that results from its use is particularly favorable. 

The key reaetion to the pentaeyele 23, unstable towards aeids, is performed by refluxing 22 with trifluoroaeetie anhydride in 2,6-lutidine. After deproteetion of 23 to the seeondary alcohol 24 by lithium in ethanol, the CC double bond in conjugation with the furan ring is reduced by sodium in liquid ammonia. Work-up in tetra-hydrofuran (THF) and water predominantly yields the desired trans-isomer 25. 

The electrocyclic opening of cyclopropane derivatives containing internal nucleophilic groups provides a new route to vinyl lactones, tetrahydropyrans, and tetra-hydrofurans (Danheiser et al. 1981). The silver salt initiates the opening of the ring... 

N-methyltryptamine (dipterine) has been converted into its Grignard derivative and oxidized in situ with ethereal ferric chloric to yield dl- along with a trace of meso-chimonanthine. These were the expected products since they were produced under conditions where no possibility for hydrolytic ring opening was provided. In a different approach, N-methoxycarbonyl 3-2 -aminoethyloxindole was dimerized by means of iodine in tetra-hydrofuran to furnish two isomeric dimers which were reduced separately with lithium aluminium hydride which converted the — NHCOOMe to — NHMe and the oxindole to the carbinolamine. 

In 1995, Ando reported that a,p-unsaturated esters could be obtained with high Z selectivity from the HWE reaction of aldehydes with diphenylphosphonoacetates in the presence of benzyltrimethylammonium hydroxide (Triton B) or NaH in tetra-hydrofuran [182], Later, Ando [183-186], Motoyoshiya [187, 188], and Touchard [189, 190] further defined this protocol by modifying diphenylphosphonoacetates and found that the employment of bis(o-alkylphenyl)phosphonoacetates led to higher Z selectivity (Scheme 39). The combination of NaH and Nal has been identified as an effective base/additive system to improve the Z selectivity [191]. Moreover, the Ando modification has recently been applied to the ring closure of various macrolides with high Z selectivity, which is complementary to that of traditional intramolecular HWE reaction (Scheme 40) [192, 193]. 

Zuo and Louie also found that NHC-Ni complexes catalyzed the rearrangement of cyclopropylen-yne derivatives. However, the selectivity of the reaction was strongly dependent on the NHC-Ni catalyst as well as on the substrate substitution. With a SIPr/Ni 1 1 system, a hindered vinylcyclo-propylene-yne gave the isomerized seven-membered ring as the sole product while the corresponding methyl derivative afforded a di-substituted tetra-hydrofuran derivative. The authors then described a successful and general access to the latter by using the ItBu/Ni 1 1 system (Scheme 10.10). 

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