Ethers, vinyl epoxidation

If the leaving group of the substrate, like in allyl ethers, vinyl epoxides and vinyl cyclopropanes, is sufficiently basic to deprotonate the conjugate acid of the nucleophile then no additional base has to be added, thus broadening the scope and simplicity of this method. 

Allylic acetates are widely used. The oxidative addition of allylic acetates to Pd(0) is reversible, and their reaction must be carried out in the presence of bases. An important improvement in 7r-allylpalladium chemistry has been achieved by the introduction of allylic carbonates. Carbonates are highly reactive. More importantly, their reactions can be carried out under neutral con-ditions[13,14]. Also reactions of allylic carbamates[14], allyl aryl ethers[6,15], and vinyl epoxides[16,17] proceed under neutral conditions without addition of bases. 

The relative stability of l, 2 -epoxyvinylbital, and the great instability of its diol, can be gainfully compared with the fate of the corresponding metabolites of phenyl vinyl ether (10.69, R = H) and 4-nitrophenyl vinyl ether (10.69, R = N02), two mutagenic and tumorigenic compounds [144] [145]. Their primary metabolic route is by vinyl epoxidation to the corresponding... 

Vinyl epoxides represent another class of compounds known to participate in CM reactions mediated by ruthenium NHG catalysts. The reactivity of these catalysts is unique, as catalyst 2 is typically limited to allyl glycidyl ether ... 

Carboxylates behave as an O-nucleophile and are allylated. Reaction of AcONa with cyclopentadiene monoxide (139) proceeds with retention of stereochemistry as shown by 140, to give the 3,5-c -disubstituted cyclopentene 141 [74], Although alcohols are rather unreactive nucleophiles for the Pd-catalysed allylation, the alkoxide anions generated by the treatment of silyl ethers with TBSF are easily allylated. Desilylated alcohol from 142 reacts intramolecularly with the cis and trans vinyl epoxides to give the cis and trans pyrans 143 and 144 regio- and stereoselectively, and... 

Vinyl epoxides react with Pd(0) to yield electrophilic allyl complexes which can convert alcohols and phenols into allyl ethers (Scheme 4.80). These alkylations usually yield 2-alkoxy-2-vinylethanols, and if the Pd-mediated etherification is performed in the presence of a chiral, enantiomerically pure diphosphine, enantiomeri-cally enriched ethers may be obtained (Scheme 4.80) [356, 357]. 

The palladium-catalyzed [3 + 2] cycloaddition of vinylic oxirane 20a [42] and aziridine 20b [39] with the activated olefin 4a for the formation of five membered cyclic ether 21a and pyrrolidine derivative 21b has also been reported in our laboratories. The mechanistic issue is very much similar to that discussed in Scheme 9. Pd(0) catalyst added oxidatively to 20 to produce the 7r-allylpalladium complex 22. The Michael addition of a hetero nucleophile in 22 to the activated olefin 4a gives 23 which undergoes intramolecular nucleophilic attack on the inner 7r-allylic carbon atom to give the cy-clized products 21 and Pd(0) species is generated (Scheme 10). Similarly, the palladium-catalyzed [3 + 2] cycloaddition of vinylic oxirane 20a with the N-losylimincs 24 is also known (Scheme 11) [43]. Intermolecular cycloaddition of vinyl epoxides and aziridines with the heterocumulenes such as isocyanates, carbodiimides and isothiocyanates is also known [44,45]. Alper et al. reported the regio- and enatioselective formation of the thiaolidine, oxathiolane, and dithiolane derivatives by the palladium-catalyzed cyclization reaction of 2-vinylthiirane with heterocumulenes [46]. 

As the key precursor towards the realization of 1,7-epoxy cyclononanes and 1,8-epoxycyclodecanes, an 1 l-oxatricyclo[6.2.1.02,6]decane skeleton was constructed by using the Hoffmann [4+3] cycloaddition methodology <07EJO4383>. In Majetich s total synthesis of (-)-salviasperanol, the key step was the trifluoroacetic acid-promoted isomerization of a vinyl epoxide to 2,5-dihydrofuran, whose O-thiocarbamate was removed by radical reaction to produce salviasperanol dimethyl ether <07OL85>. The synthetic route is depicted below. 

In studies directed towaids the synthesis of the fungal isonitrile antibiotic isonitrin B (103), a cy-clopentanoid metabolite related to trichoviridine (104), various trialkylsilyl enol ethers were irradiated with propionaldehyde to produce equimolar amounts of stereoisomeric bicyclic oxetanes (105). The corresponding silyl dienol ethers, however, gave little oxetane product owing to preferential diene photopolymerization. The use of vinyl epoxide derivatives such as (106), however, offers a possible access to the epoxycyclopentanoid system with simultaneous control of the face selectivity of the photoaddition by the epoxide moiety. 

The 3-oxa-di- i -methane rearrangement is far less common. As a symmetrical system, the product should be a vinyl epoxide (equation 23). So far, no 3-oxa-di-T -methane products have been observed from the divinyl ethers depicted in the equation. It was shown that the solution photochemistry of (21a) derives from its singlet i , i -state and parallels that of the unsubstituted divinyl ether (21d) and of furan in the gas phase. All products could be rationalized in terms of the initial formation of a singlet vinyl-vi-nyloxy radical pair (equation 24). Triplet sensitization brings about cis-trans isomerization and consequent deactivation. Had the 3-oxa-di- ir-methane product, i.e. the vinyloxirane (22), been formed, it would have decomposed via Griffin fragmentation as confirmed by irradiation of an authentic sample. 

Allylic halides couple effectively with vinyltin reagents to provide the corresponding 1,4-dienes in high yields.2 Like nickel- and palladium-catalyzed Grignard reactions with allylic ethers, this reaction also proceeds with net inversion of configuration at the allylic center. Similarly, vinyl epoxides react... 

Ethylene oxide and simple primary and secondary epoxides 271 Tertiary epoxides 272 Acetals and epoxy ethers 272 Vinyl epoxides 273... 

Usually ,(5-vinylic epoxides undergo -elimination induced by lithium amides in nonpolar solvents, i.e. diethyl ether, hexane, benzene. In hexamethylphosphoric triamide, a total change of reactivity is possible y-elimination takes place and leads to the formation of cyclized products when the favorable epoxide configuration can be attained, otherwise /S-elimination is still observed. 

In the recent decay, there is a considerable interest in making green photopolymerizations using photosensitizers. One way to obtain nontoxic polymers through sensitization is to copolymerize compounds that can behave either as a photosensitizer or monomer with different monomers [113]. Another way is to polymerize these monomeric photosensitizers and afterward subject them to sensitize the polymerization of convenient monomers. In both ways, nontoxic and odorless polymers can be obtained after polymerization. Compounds introducing phenothiazine moiety and their polymeric analogs were found the display high efficiency in PCP of vinyl ethers and epoxides. 

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