Oxiranes from carbonyl ylides

Oxiranes from carbonyl compounds with CH2 fragments (ylides)... 

Oxiranes from Carbonyl Compounds with CH2-equivalents (CH2N2, LiCH2X, S, Se, and As Ylides) 129... 

Azomethine ylides are also frequently obtained by ring opening of aziridines, and the analogous carbonyl ylides from oxiranes. These aspects are dealt with in Section 3.03.5.1. A variety of five-membered heterocycles can also function as masked 1,3-dipoles and this aspect is considered in Section 3.03.5.2. 

Imine ylides 7 and carbonyl ylides 8 are not stable but may be generated in situ by pyrolysis of suitably substituted aziridines and oxiranes. The energy of the HOMO, and therefore the nucleophilicity of the parent 18-electron dipoles, decreases from very high to very low across the series 7-18. In the same series, the electrophilicity increases from moderate to high, being consistently higher when the central atom is oxygen. 

The 877-electrocyclization of carbonyl ylides of type B, e.g., the ylide derived from oxirane 145, was less favored than the 6n electrocyclization to yield a mixture of three compounds 146 and isomers 147 (in a ratio of 3 7). The annulated vinylfuran derivative 147 formed as mixture of isomers (cis-trans 4 3) while the stereochemistry of the single isomer of 146 has been established by X-ray analysis of the Diels-Alder adduct (Scheme 44) [81TLC194953 85CB4035]. 

In the case of type C—the carbonyl ylides derived from oxiranes 148 (n = 1-4)—the effect of annulated cycloalkyl and aromatic rings has also... 

Transition-metal mediated carbene transfer from 205 to benzaldehyde generates carbonyl ylides 211 which are transformed into oxiranes 216 by 1,3-cyclization, into tetrahydrofurans 212, 213 or dihydrofurans 214 by [3 + 2] cycloaddition with electron-deficient alkenes or alkynes, and 1,3-dioxolanes 215 by [3 + 2] cycloaddition with excess carbonyl compound120 (equation 67). Related carbonyl ylide reactions have been performed with crotonaldehyde, acetone and cyclohexanone (equation 68). However, the ylide generated from cyclohexanone could not be trapped with dimethyl fumarate. Rather, the enol ether 217, probably formed by 1,4-proton shift in the ylide intermediate, was isolated in low yield120. In this respect, the carbene transfer reaction with 205 is not different from that with ethyl diazoacetate121, whereas a close analogy to diazomalonates is observed for the other carbonyl ylide reactions. 

Various bicyclic phosphaalkenes 21-24 were synthesized from oxirane 118. Acceptor-substituted oxiranes underwent a ring opening under thermal stress to give the unstable carbonyl ylides. It was shown that upon heating 2,3-diphenylindenone oxide 118 underwent conversion into ylide 119, which can react with various phosphaalkynes in [3+2] cycloaddition reaction to give 21-24 <2000T6259>. 

Carbonyl ylides, often generated from a carbene and an aldehyde or ketone, are in general reactive intermediates that can isomerize to stable products (oxiranes, enolethers) or can be tn ped by [3+2] cycloaddition reactions [18]. As Scheme 8 shows, silyldiazoacetic esters can also be used to generate carbonyl ylides [19]. 

Oxiranes can be prepared in excellent yield from carbonyl compounds by alkylidene transfer with sulfonium ylides. The reaction is generally carried out with dimethylsulfonium methylide 77, dimethylsulfoxonium methylide 78, or related compounds such as anionoid species originating from sulfylimines 79 and sulfox-imines 80 that can undergo addition to the electrophilic carbonyl carbon. 

Cu(hfacac)2-catalyzed elimination of N2 from aroyl(a-diazoacyl)hydra-zines 653 yields 2//-l,3,4-oxadiazin-2-ones 654 (70%). The reaction occurs consecutively via intramolecular carbonyl ylide formation, 1,3-cyclization into oxirane 655, and ring opening by the carbon-oxygen bond followed by a C- 0 shift with ring expansion (88CB887). 

Krief et al. have shown that selenium ylides behave as their sulfur analogues and convert a variety of carbonyl compounds to oxiranes <89H(28)1203>. The latter compounds can be directly obtained by using R2Se=CHR /i-hydroxyalkylselenides (available from carbonyl compounds by addition of RSeCH2Li) may serve as suitable precursors as well, either in a two-step protocol, via the selenonium salt by alkylation with magic methyl (MeS03F), or directly by treatment with thallous ethoxide in chloroform. Oxidation of the /t-hydroxyalkylselenides with peracid, followed by treatment of the resulting selenone with base, results in oxirane formation (Scheme 60). 

It has been shown that at least part of the carbene originates from the carbonyl ylide 1, which is formed by the UV initiated cleavage of the oxirane. Further fragmentation of 1 is accomplished by simultaneous irradiation with visible light. Otherwise, a [3 + 2] cycloaddition reaction between 1 and the added alkene takes place to give a 5-phenyltetrahydrofuran-2,2-dicarbonitrile 2. ... 

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