Oxiranes, rearrangements

Hi. Role of additive. There are some reports in the literature of the beneficial effect of powerful donor solvents such as DBU on the reactivity and enantioselectivity of HCLA-mediated oxirane rearrangements for both stoichiometric and catalytic processes. However, this effect is not general (see above) and the role of such additives is still unclear. In one study, the influence of the concentration of DBU on the relationship between the ee s of catalyst and the product for the enantioselective isomerization of cyclohexene oxide mediated by substoichiometric amount of HCLA 56a (20 mol%) in the presence of LDA (2 equiv) has been investigated. At high DBU concentration (6 equiv), the enantiomeric... 

The above general statements will now be illustrated with some examples. Alumina, modified alumina, and silica-gel-induced oxirane rearrangements have been examined in the cases of 114-1... 

Facile oxirane rearrangements in the presence of Cp2ZrCl2 and catalytic AgC104 have been observed <93JOC825>. 

In an analogous manner, 2-(l-substituted cyclopropyl)oxiranes rearrange to hydroxymethyl-substituted fluorocyclobutanes 12 and 13. ... 

Keywords SN2 and SN2 reactions Baldwin mles Oxirane opening Activation of a cyclopropane 1,2-rearrangement Ring contraction Ring expansion Solvolysis Neighboring group participation Oxirane rearrangement Classical and nonclassical carbocations 5-exo- vis-a-vis 5-endo cyclization Addition and elimination reactions... 

Oxirane formation can also occur via free radical mechanisms, as in the reaction of certain fluoroalkenes with oxygen. Under pressure at elevated temperatures, oxygen alone can suffice, but activation is frequently provided in the form of radical initiators (e.g., tribromofluoromethane) and ultraviolet light. Thermolysis of dioxole 5, comonomer from which DuPont s Teflon-AF is made, offers an unusual route to an oxirane. Rearrangement of the heterocycle presumably takes place via a biradical intermediate. ... 

With appropriately substituted oxetanes, aluminum-based initiators (321) impose a degree of microstmctural control on the substituted polyoxetane stmcture that is not obtainable with a pure cationic system. A polymer having largely the stmcture of poly(3-hydroxyoxetane) has been obtained from an anionic rearrangement polymerisation of glycidol or its trimethylsilyl ether, both oxirane monomers (322). Polymerisation-induced epitaxy can produce ultrathin films of highly oriented POX molecules on, for instance, graphite (323). Theoretical studies on the cationic polymerisation mechanism of oxetanes have been made (324—326). 

The isomerization of vinyl- or ethynyl-oxiranes provides a frequently exploited source of dihydrofurans or furans, but analogous conversions of vinylaziridines have not been applied so often. While most of the examples in Scheme 87 entail cleavage of the carbon-heteroatom bond of the original heterocycle, the last two cases exemplify a growing number of such rearrangements in which initial carbon-carbon bond cleavage occurs. 

These are discussed in (B-71MS4). Oxirane itself shows a strong molecular ion peak and a slightly stronger base peak at mje 29 (CHO ) due to isomerization to ethanal and loss of a methyl radical. Substituted oxiranes tend to show only weak molecular ion peaks, because of rearrangement and fragmentation. 

The most important oxirane syntheses are by addition of an oxygen atom to a carbon-carbon double bond, i.e. by the epoxidation of alkenes, and these are considered in Section 5.05.4.2.2. The closing, by nucleophilic attack of oxygen on carbon, of an OCCX moiety is dealt with in Section 5.05.4.2.1 (this approach often uses alkenes as starting materials). Finally, oxirane synthesis from heterocycles is considered in Section 5.05.4.3 one of these methods, thermal rearrangement of 1,4-peroxides (Section 5.05.4.3.2), has assumed some importance in recent years. The synthesis of oxiranes is reviewed in (B-73MI50500) and (64HC(19-1U). 

A -1,3,4-Oxadiazoline, fluoromethyl-rearrangement, 6, 437 Oxadiazolines alkylation, 6, 431 irradiation, 6, 437 mass spectra, 6, 380 oxirane synthesis from, 7, 117-118 ring cleavage... 

The biomimetic approach to total synthesis draws inspiration from the enzyme-catalyzed conversion of squalene oxide (2) to lanosterol (3) (through polyolefinic cyclization and subsequent rearrangement), a biosynthetic precursor of cholesterol, and the related conversion of squalene oxide (2) to the plant triterpenoid dammaradienol (4) (see Scheme la).3 The dramatic productivity of these enzyme-mediated transformations is obvious in one impressive step, squalene oxide (2), a molecule harboring only a single asymmetric carbon atom, is converted into a stereochemically complex polycyclic framework in a manner that is stereospecific. In both cases, four carbocyclic rings are created at the expense of a single oxirane ring. 

The rearrangement of 3 -benzylspiro[2//-1-benzothiopyran-3(4//),2 -oxirane]s 7, induced by Lewis or proton acid catalysts, gives the seven-membered ring dione systems 8. Compounds... 

The isomerization of acetylenic oxiranes cis- and trows-91 to allenic ketone 94 has recently been described (Scheme 5.18). It is proposed that the rearrangement proceeds via a dilithium ynenolate [33]. 

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