Oxiranyl anion

The reaction of oxiranes with base can follow several paths, giving products of type (34-38 Scheme 27). (a) Formation of an oxiranyl anion (34) is rare (Section 5.05.3.5). (b) Nucleophilic ring opening to give (35) is common with unhindered bases (Section 5.05.3.4). (c) a-Elimination to give a carbene or carbenoid (36) is favored by alkyllithium bases and... 

Reactions of oxiranyl anions as nucleophilic epoxides 97YGK176. 

Scheme 8.12 Schematic representation of the oxiranyl anion strategy for the synthesis of trans-fused THPs.

Metalated epoxides are a special class of a-alkoxy organometallic reagent. Unstabilized oxiranyl anions, however, tend to undergo a-elimination. On the other hand, attempts to metalate simple unfunctionalized epoxides may lead to nucleophilic ring opening. The anion-stabilizing capability of a trimethylsilyl substituent overcomes these problems. Epoxysilanes 22 were... 

A comparison of the configuration of the substrates and reaction products shows that the oxiranyl anions arc configurationally stable under the reaction conditions. Only one example is known in which isomerization was observed. When the ci.v-tm-butyl-substituted epoxysilane27 was metalated and quenched with 2-cyclohexenone, addition product 27 was obtained under inversion of the anionic center. Presumably the strain created in forcing the ter/-butyl and the trimethylsilyl group cis on the oxirane ring facilitates the isomerization process13. 

The addition of aziridinyl anions to aldehydes gave the alcohols 31 in good yields15. As with the oxiranyl anions, the diastereoselectivity of the addition reaction could be substantially enhanced by transmetalation of the lithium reagent with chlorotris(dimethylamino)titanium. 

Another carbenoid-typical reaction of a-lithiated epoxides is the 1,2-hydrogen shift, illustrated in Scheme 14. Two mechanistic pathways offer an explanation for the formation of the lithium enolate 94 First, the route via the a-ring opening of the epoxide followed by an 1,2-hydride shift in the carbene 93, and second, the electrocyclic ring opening of an oxiranyl anion 95 to an enolate anion 94. Both mechanisms are in accordance with different experimental... 

Competitive with -deprotonation, a-deprotonation furnishes the carbenoid-type oxiranyl anion species 10. In selected cases anion formation has been established to be a reversible process by deuterium-labeling experiments. As opposed to -deprotonation which gives only allylic alcohols, a-deprotonation can give rise to a variety of products as summarized in Scheme 4. This behavior will be further discussed in Section V. Some... 

Since lithium amides (unlike alkylithiums) are unreactive toward most oxiranes at low temperature, they are widely used for /3-deprotonation reaction around room temperature whereas (bulky) alkyllithium bases are generally employed to generate oxiranyl anions at low temperature (—78 °C). 

The synthesis of (65,7S,9i ,10i )-6,9-epoxy-nonadec-18-ene-7,10-diol 28, a marine epoxy lipid isolated from the brown algae Notheia anomala, by an oxiranyl anion strategy, was reported. One main step is the constmction of the furan ring <99TL731>. 

The chemistry of oxiranyl anions (117) and aziridinyl anions (118) has been reviewed186 and tris-1,2,3-/ -nitrophenylcyclopropcnc has been found to resist conversion to the corresponding anti-aromatic cyclopropenyl anion by deprotonation even though it has been estimated to have a pAa of 32.187... 

Conducting reactions at 48 °C, the formation of oxirane occurred rapidly, indicating that the deprotonation of styrene oxide 83 is rapid however, this was accompanied by decomposition of the oxiranyl anion 87. Reducing the reactor temperature to —78 °C, the anion 87 was found to be stable for up to 25 s, enabling efficient reaction with a series of electrophiles to afford the respective substituted epoxide in high yield, as illustrated in Table 11. 

Similar ring-expansion reactions were observed for SCBs and oxiranyl anions bearing the silyl group <1994BCJ1694, 1990TL6059>. 

So far the reactivity of epoxides has involved their use as an electrophile. However, oxtranyl anions can serve as functionalized nucleophiles in their own right. Thus, the sulfonyl substituted epoxide 107 can be deprotonated with -butyllithium to provide a stabilized anion which engages in facile Sn2 reaction with triflate 108 <03JOC9050>. Other examples of such stabilized epoxide anions include those derived from oxazolinyloxiranes (e.g., 110), which react with nitrones to provide the spirotricyclic heterocycles of type 112, Hydrolysis provides the epoxy amino acids 113, in which the carboxylic acid moiety was provided by the oxazoline nucleus and the amine functionality was derived from the nitrone <03OL2723>. A recent report has demonstrated that oxiranyl anions can also be stabilized by the amide functionality <03H(59)137>. 

In the absence of well-positioned double bonds, the oxiranyl anions can undergo another well-known reaction of carbenes, namely a 1,2-hydride or 1,2-alkyl shift <1998JOC3808, 20040L3509>. 

Introduction. Although epoxides are widely recognized as extremely versatile synthetic intermediates in view of their electrophilic nature, the reaction of an epoxide as a nucleophile, i.e. an oxiranyl anion, is less common. Recently, cumulative studies on the chemistry of oxiranyl anions have appeared and some aspects of the anions have been discussed. - ... 

Lithiated epoxides are more commonly trapped by electrophiles, generating elaborated oxiranes. In this arena, Hodgson and co-workers <04OL4187> have optimized the lithiation of non-stabilized terminal epoxides with iec-hutylhthium assisted by diamine ligands, such as dibutylbispidine (DBB, 99) or (-)-sparteine 100. The oxiranyl anions thus formed engage in smooth nucleophilic addition onto aldehydes to form epoxyalcohols e.g., 101) the same conditions can be used for the stannylation of epoxides e.g., 84 102). Similarly,... 

Yoshida and co-workers [40] subsequently demonstrated the use of sec-butyl-lithium (63) in a continuous flow reactor, for the in situ preparation of an oxiranyl anion, derived from styrene oxide (64) and 63. As Table 6.5 illustrates, the authors constructed a library of substituted epoxides derived from the reaction of 1,2-epoxyethylphenyllithium (65) with a series of electrophiles. The generation of 65, from styrene oxide (64) (0.10 M) and 63 (0.75 M), was conducted in the first micro mixer and was followed by trapping, with Mel (66) (0.45 M), in the second micro... 

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