Oxiranes, ring opening reaction

The oxirane ring-opening reaction requires the presence of a basic catalyst. An acidic catalyst also works, but the polymerization of the oxirane limits its usehilness. In the case of 2-mercaptoethanol (eq. 8), the product has been found to be autocatalytic, ie, the product is a catalyst for the reaction. 

Some examples of ring opening reactions with carbanions leading to five-membered heterocyclic ring formation are shown in Scheme 85. Pyrrole syntheses from functionally substituted oxiranes and amines are often described and typical examples are shown in Scheme 86. 

The combination of alkali metal acid fluorides and porous aluminum fluoride IS a stable, solid, and efficient substitute for anhydrous hydrogen fluoride for promoting the ring-opening reactions of simple aliphatic oxiranes to give the fluorohydrins under sonication [/5] (equations 14 and 15)... 

The oxirane ring opening reaction with organoUthium reagents is an important reaction as it gives direct access to -hydroxyalkylated products. This reaction has been reviewedseveral times, and only general trends will be described here. 

Early work has shown that the reaction of simple organolithium reagents with oxiranes lead mainly to products from a- or -deprotonation and not from the desired ring opening reaction. However, some reactions of methyllithium , aUyllithium , aryllithiums" , and vinyllithium/potassium reagents " with oxiranes are reported (Scheme 36). 

The oxirane ring opening reaction with alkynyllithiums is of considerable synthetic value, as stereodefined homopropargylie alcohols can be obtained. The lithium acetylide ... 

The amount of EGA (the current efficiency of EGA generation) is measured in both acetone and methylene chloride-THF. Both Figs. 1 and 2 show that a sharp increase in the concentration of EGA is observed in the electrolysis of lithium, sodium, and magnesium perchlorate solution, being consistent with the result of the oxirane ring opening reaction to ketones (Table 2). 

In the oxirane ring opening reaction (Eq. (2)), it is possible to prepare the ketone 5, the acetonide 10, and the allylic alcohol 11 selectively by controlling the reactivity of the intermediate carbenium ion 12 under some specified reaction conditions. In order to prepare 5 preferentially, a high concentration of perchlorate ion must be employed in CH2CI2, ClCHjCH Cl, or THF. In this system, an incipient carbenium ion 12 would be associated with perchlorate ion and stabilized so that the deprotona-... 

Whereas the thermal ring-opening reaction of oxiranes and aziridines is frequently used for generation of carbonyl ylides and azomethine ylides, the analogous procedure starting with thiiranes does not produce the expected thiocarbonyl ylides (8). However, in the case of tetraaryl-substituted thiiranes, the photolytically mediated reaction with tetracyanoethylene (TCNE) is believed to occur via a single electron transfer (SET) mechanism, also involving a thiocarbonyl ylide as a likely intermediate (75,76) (Scheme 5.14). 

Few solid-phase syntheses of oxazoles have been reported (Table 15.17). The most general strategy is the dehydration of a-(acylamino) ketones (Entry 2, Table 15.17) or 2-(acylamino)phenols (Entry 1, Table 15.17). Oxazolidin-2-ones have been prepared by intramolecular nucleophilic cleavage of carbamates from insoluble supports (Entries 5 and 6, Table 15.17). Resin-bound 2-aminoethanols, which are accessible by nucleophilic ring-opening of oxiranes with amines, undergo cyclocondensation with aldehydes to yield oxazolidines [220,221]. These compounds are unstable towards acids, and can be released from the support only under neutral or basic reaction conditions. 

The ring-opening of oxiranes, leading to the formation of isomeric carbonyl compounds by the action of acid catalysts as a result of rearrangement, is of continuing interest (refs. 1-4). However, most of these studies focus mainly on the transformations of terpene oxides or oxiranes with other functional groups in the liquid phase, under homogeneous reaction conditions. 

The two mono-formates are formed in comparable amounts, suggesting little regioselectivity in the oxirane ring opening reaction. Propylene carbonate, 1,2-propanediol, and formic acid were also found as reaction prod-... 

Nucleophilic ring opening of oxiranes gives intermediates of the type 22, which readily ring-closed (Scheme 8) <2004JOC8780>. A similar reaction can be carried out in one step <2003GC436>. 

The mechanisms and the regio- and stereo-selectivity of epoxide reactions have been reviewed.39 The review also covers the role of epoxides in biologically important reactions. The achiral and chiral catalysts used in these reactions are discussed. A second review40 discusses the ring-opening reactions of oxiranes with carbon nucleophiles. 

In addition to the above described procedures, another alternative has to be mentioned, the smooth displacement of the silyl moiety by means of fluoride ion catalysis, leaving the oxirane ring intact79. This method enables phenyl-oxirane (109) to be synthesized from E-l-phenyl-2-TMS-oxirane (93) (Scheme 12). For further ring opening reactions see also80,81. ... 

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