Oxyallyl intermediates

Methylideneoxiranes (allene oxides) (80T2269), e.g. (9) and (23), are also highly reactive, and undergo facile thermal isomerization to cyclopropanones, possibly via an oxyallyl intermediate (24 Scheme 20). 

The analogous calculations for the ring-opening of cyclopropanone to form the oxyallyl intermediate (11) give more pronounced minima in both the ground and excited configuration surfaces, and the A and S levels do not cross (see Fig. 2b). This requires, on orbital symmetry grounds, a disrotatory reclosure for oxyallyl. 

Semiempirical calculations have been used to study the mechanism of the ring opening of cyclopropanone and substituted analogues in a range of solvents of varying polarity. Transition states and oxyallyl intermediates have been characterized, as have the effects of solvents on their stability. The results are also compared with kinetic data in the literature. 

The formation of exs-di-Z-butylcyclopropanone (250) from the acyclic precursor (248) on treatment with Cr(CO)4NO has provided evidence both for the existence of oxyallyl intermediates (249) in the mechanism of this reaction and for the integrity with which oxyallyls ring close to cyclopropanones by a disrotatory route.291... 

Reactions of acyclic a,a- and c a -dihalo- and a,a,a- and o, a, a /-trihalo-ketones, with sodium 2,2,2-trifluoroethoxide in 2,2,2-trifluoroethanol in the presence of furan have been investigated with the aim of obtaining 4 + 3-cycloadducts of the oxyallyl intermediates. 4 + 3-Cycloaddition was favoured only for the l,l,3-trihalobutan-2-one (18).23... 

The unsymmetrically substituted oxyallyl intermediates (7i) and (7j), generated from the corresponding dibromo ketones, are tnqjped by styrene to give a regioisomeric mixture of 3-(dienylcyclopentanones (56a b) and (57a b), respectively. The results are summarized in Scheme 14. 

Allylic alcohols are isomerized via direct interaction of the ruthenium atom with alcohol. /3-Elimination of ruthenium hydride from metal alkoxide yields a ruthe-nium-enone species C which undergoes insertion of the olefinic moiety into the Ru-H to form an oxyallylic intermediate D. As a result, the hydrogen atom shifts from the a- to y-position of the allylalcohol. Protonolysis of the oxyallylic species leads to a saturated carbonyl compound and cationic unsaturated species, [CpRu(PPh3)2] A. 

Giese, S., West, F. G. Ionic hydrogenation of oxyallyl intermediates the reductive Nazarov cyclization. Tetrahedron 2000, 56, 10221-10228. 

Numerous more recent experiments (reviewed in ref. 12) have confirmed the general accuracy of the mechanism shown in Scheme 4, and the actual intermediate involved depends upon the relative stabilities of the zwitterions/oxyallyl cations and the corresponding cyclopropanones. These will be affected by the choice of solvent and structural features of the starting ketone, such as the degree of substitution and ring strain (in cyclic halo ketones). A recent example in which an oxyallyl intermediate and a cyclopropanone intermediate were both intercepted is shown in Scheme 8. ... 

Other researchers have also investigated the involvement of zwitterions in the cyclization of the dienone (162) to give bicyclic products in the presence of electron-rich ethenes. The key reaction is the c/Xtran -isomerism of the enone to afford the highly reactive dienone (163) which cyclises to an oxyallyl intermediate that reacts with the ethenes e.g. vinyl ethers). Cyclization within the resultant intermediate, possibly a zwitterion e.g. 164), can account for the formation of both the tricyclic ether (165) or the bridged ketone (166). The yields obtained are shown for the appropriate structures. The reaction also takes place with alkenes and, for example, using 2-methylpropene the adduct (167) is formed. 

The isomerization reaction of alkylidene oxiranes, i.e. allene epoxides, to cyclopropanones has stimulated many investigations. Several experimental and theoretical studies suggest that the reaction proceeds through an oxyallyl intermediate. 

Matlin, A. R., Photocycloaddition/Trapping Reactions of Cross conjugated Cyclic Dienones Capture of Oxyallyl Intermediates. In Horspool, W. M., Lenci, F. (eds), CRC Handbook of Organic Photochemistry and Photobiology, 2nd edn, CRC Press LLC, Boca Raton, FL, 2004, Chapter 81, PP- 1 12. 

Cyclization. The oxyallyl intermediate derived from treatment of a cross-conjugated dienone with FeClj is trapped by an intramolecular [4 + 3]cycloaddition if one of the a-positions of the dienone is connected to a diene. Interesting tricyclic systems can be produced in this Nazarov cyclization. ... 

The synthesis of seven-membered rings from furans and a,a -dibromoketones via oxyallyl intermediates is well known. It was reported that this reaction occurs quite easily when water is used as solvent <97TL8031>. In a study on the influence of steric and electronic effects on a function attached at C-2 of furans in the yield and diastereoselectivity of [4 + 3] cycloaddition reactions with oxyallyl cations it was found that in almost all cases a c/x-diastereospecificity and a high endo diastereoselectivity is obtained <97T11669>. A tandem Peterson olefination-[4 + 3] cycloaddition reaction with furans has been reported <97JOC1578 97TL386l>. For an intramolecular [4 -t- 3] cycloaddition see <97JOC6051>. The first asymmetric [4 + 3]... 

Reaction of a,a -dibromo ketones with iron carbonyls generates a reactive three-carbon unit. The first step of the reaction is the formation of the iron enolate (CLin) via a two-electron reduction of the dibromo ketone or through an oxidative addition of the carbon-bromine bond onto iron(0). The second step is an 8, 1 or Fe ion-assisted elimination of bromide ion to form the oxyallyl intermediate (CLIV). Evidence for the production of... 

Cycloaddition of the oxyallyl intermediates with open-chain or cyclic 1,3-dienes leads to 4-cycloheptenones (CLXI) in fair to good yields (Noyori et al., 1971a, 1973b). The seven-membered ring ketones thus obtained can be... 

Lubineau, A. Bouchain, G. (1997) Water-promoted reactivities generation of oxyallyl intermediates and their [4+3] cycloadditions with furan and cyclopentadiene. Facile access to bridged cycloheptenones. Tetrahedron Lett., in press. 

In 1998, the first deliberate attempt to trap the oxyallyl cation with a nucleophile during a thermal Nazarov reaction was reported by West and coworkers [23]. Coining the term interrupted Nazarov reaction, they described the interception of the oxyallyl intermediate of a Nazarov reaction by a tethered alkene (Scheme 3.20). Upon submission of divinyl ketones 87 to the LA, a conrotatory ring closure generates the oxyallyl cation intermediate 89. The alkene moiety adds selectively to the oxyallyl cation intermediate to form bicycle 90. The stabilized tertiary carbocation is then trapped by the enolate oxygen to give intermediate 91, which, under acidic aqueous conditions, is converted to the final hemiketal product 88. The reaction provides an efficient and stereoselective method to prepare tricychc compounds 88 in yields ranging from 42 to 89% [24]. 

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