Oxolanes preparation

Preparation Of Oxacycloalkanes. Unsaturated alcohols can be cyclized under superacid conditions to yield oxolane derivatives. Laali et al.685 have studied the protonation of homoallylic adamantylideneadamantyl alcohols. The pseudo-axial alcohol 157 was protonated in HSO3F-SCUOF to give the intermediate protonated cyclic ether observed by 1H and 13C NMR spectroscopy, which, upon quenching, furnished the corresponding ether [Eq. (5.243)]. 

Whittaker and co-workers805,806 have also prepared a number of terpenoid bicyclic ethers, such as isomeric 1,6-dihydrocarveols 199 (HSO3F—S02,78°C) from unsaturated alcohols or diols.805 806pura-Menth-l-en-9-ol and a related diol afforded the seven-membered ring systems 200. The oxolane moiety in compound 201 was generated from the corresponding unsaturated alcohol precursor in 2 equivalents of triflic acid.807... 

For the removal of this protecting group, tetrabutylammonium fluoride in oxolane is the most frequently used [388, 389, 409-411]. A much simpler reagent to prepare, potassium fluoride — crown ether, has been introduced for the same purpose [427]. Silyl group at 0-2 of nucleosides is cleaved more rapidly [411] than at 0-5. Acyl migrations occurred under the tetrabutylammonium fluoride-catalyzed desilyla-tion [432, 434, 443], Differencies between the primary and secondary position were also observed for acid- or base-catalyzed solvolysis [391, 409-412], 5 -0-(7ert-butyl-dimethylsilyl)nucleosides are much more labile towards acid than either 2 - or 3 -silyl ethers [391, 410-412], whereas the situation is reversed for base hydrolysis [411], /V-Bromosuccinimide in aqueous DMSO is another alternative for the removal of this type of silyl group [444]. 

Fluorinated oxetanes, oxolanes and oxanes were prepared by several authors [189-194],... 

As already mentioned, oxocarbenium salts are efficient initiators42,45 for the polymerization of oxolane. They can be prepared in situ by a metathetic reaction between the corresponding acyl halide and silver hexafluoroantimonate ... 

Analysis of Reagent Purity H NMR (CDCls) may contain 1-5%of (4R, tra s)-2,2-dimethyl-a,ot,ot, ot -tetraphenyl-l,3-di-oxolane-4,5-dimethanol (but does not affect its efficiency). Preparative Methods see Trichloro(cyclopentadienyl)titanium. Handling, Storage, and Precautions the dry solid must be stored under exclusion of moisture and UV at rt (brown, tightly sealed bottle). It can, however, be handled quickly in the open, e.g. for weighing. Reactions should be carried out in dry equipment and with absolute solvents under argon or N2. 

Oxolane anhydronucleosides were prepared recently from cyclic 5,6-sulfate 97 derived from diacetonoglucose. Under the basic conditions the hydroxyl group at the C-3 position was liberated and the anion thus formed attacked the C-6 position of the sugar providing the target anhydro derivative (O Fig. 24) [64]. 

When a pyranose is transformed into a 1,6-anhydropyranose, the axial hydroxyl groups become equatorial and vice versa. This is undoubtedly the most interesting property from a synthetic point of view because of the change in reactivity. But there are limits to the usefulness of the 1,6-anhydro the endo face between the oxolane and the oxane is not very accessible and nucleophiles cannot be easily introduced there, so much so that it can be difficult to substitute activated hydroxyl groups by the 8 2 reaction involving endo attack. Finally, it should be noted that the preparation of certain 1,4-anhydropyranoses is also known. 

TTie 3 -phenylselenoyl-2 .3 -ene (25) has been prepared by ring-opening of a 2, 3 -Iyxo-epoxide with PhSeU, followed elimination and oxidation at selenium.45 The applications of (25) in synthesis are mentioned in Sections 2, 5, and 7, The 5 -deo3 -5 -phenylselenonucleosides (72) and (73) have been obtained by ring-opening of an oxetan and an oxolane, respectively,... 

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