Vinylallene oxide

An ab initio study of the rearrangement of vinylallene oxide (181) to cyclopentenone (182), has suggested that either a concerted mechanism or an oxyallyl pathway (see Scheme 46) can be operative for the transformation. The transition states of suitable examples of intramolecular ene reactions with activating substituents have been investigated using a semiempirical quantum-mechanical method. The predicted... 

The Diels-Alder reaction outlined above is a typical example of the utilization of axially chiral allenes, accessible through 1,6-addition or other methods, to generate selectively new stereogenic centers. This transfer of chirality is also possible via in-termolecular Diels-Alder reactions of vinylallenes [57], aldol reactions of allenyl eno-lates [19f] and Ireland-Claisen rearrangements of silyl allenylketene acetals [58]. Furthermore, it has been utilized recently in the diastereoselective oxidation of titanium allenyl enolates (formed by deprotonation of /3-allenecarboxylates of type 65 and transmetalation with titanocene dichloride) with dimethyl dioxirane (DMDO) [25, 59] and in subsequent acid- or gold-catalyzed cycloisomerization reactions of a-hydroxyallenes into 2,5-dihydrofurans (cf. Chapter 15) [25, 59, 60],... 

Vinylallenes can also be cydized oxidatively. Vinylallene 61 when treated with vanadyl acetonacetonate and tert-butyl hydroperoxide in benzene produced cyclopen-tenone 63, the immediate precursor of methylenomytin B (cf. 5, Eq. 13.1), in 50%... 

A hydroxyalkyl substituent on the internal carbon atom of the allene can also be used to direct the epoxidation reaction (Eq. 13.22) [26]. In the case of vinylallene 70, hydroxyl-directed epoxidation, followed by cyclization of the allene oxide, leads to cyclopentenone 71 in 60% yield, along with 20% of epoxide 72. The greater reactivity of the allene ensures that the epoxidation step will be selective however, in this case the selectivity is not complete. 

The oxidative cyclization of vinylallenes need not be directed by a pendant hydroxyl group in order to succeed. The higher reactivity of the allene compared with the exocyclic methylene group in 73 (Eq. 13.23) with monoperphthalic acid leads primarily to the allene oxide which rearranges to cydopentenone 74 [27]. Inevitably some epoxidation of the alkene also takes place during the reaction. When m-CPBA is used as the oxidant, another side reaction is associated with m-chlorobenzoic add-mediated decomposition of the intermediate epoxide. It is possible to overcome this problem by performing the epoxidation in dichloromethane in a two-phase system with aqueous bicarbonate so as to buffer the add [28]. 

Mercuric acetate and thallic acetate have also been used for the oxidative cydiza-tion of vinylallenes (Eq. 13.24) [29]. Exposure of vinylallene 75 to stoichiometric mercuric acetate in acetic acid led to cydopentenone 76 in 75% yield. With thallium acetate as the oxidant, the yield of 76 was 60%. The presumed mechanism of the oxidative cyclization involves a Nazarov cyclization of acetoxymercury intermediate 77. 

In a different approach, Franck-Neumann et al. [24] utilized the manganese complex 14 (formed by deracemization) to obtain the enantiomerically pure target molecule 12 via Horner-Wadsworth-Emmons olefination and oxidative decomplexation of the intermediate vinylallene complex 15 (Scheme 18.6). 

In 1991, Thomas reported88a that the reaction between vinylketene complexes (221) and several phosphonoacetate anions generated vinylallene complexes (246), in some cases with extremely high stereoselectivity.88,89 This Wadsworth-Emmons type reaction occurs via attack by the phosphonoacetate carbanion at the ketene carbonyl carbon, and product ratios clearly depend on the steric bulk of the R and R substituents. The relative stereochemistry of the major isomers of 246 were determined by X-ray analysis. Upon oxidation of the vinylallene complexes with iron(III) chloride, a range of substituted furanones were isolated.8813,89... 

Since epoxidation at the vinyl double bond is unproductive, it is desirable to direct reaction on the al-lene moiety. This can be accomplished by taking advantage of the hydroxy-directed epoxidation of allylic alcohols using the t-butyl hydroperoxide/vanadium(V) system.The directing effects of both allylic and homoallylic type hydroxy groups have been examined at both positions of the vinylallene unit. " At the 1-position (64), primary, secondary and tertiary allylic tdcohols are effective, while only primary homoallylic alcohols have bran examined (equation 35). Presumably the directing effect of the hydroxy groups favors formation of the intermediate allene oxide (65). A sample of the compounds prepared by this route is shown in Scheme 32. ... 

Shimizu and Bartlett found that irradiation of an alkene in the presence of molecular oxygen and an a-diketone as sensitizer gives rise to oxiranes <76JA4193> (see also <81JA2049> and <82JA544 . This method is also applicable to deactivated alkenes. Under the same conditions, vinylallenes afford cyclopentenones, presumably via the allene oxide(s) (Scheme 64) <79JOC885>. 

Similar photo-oxidation procedures can be used to prepare 4-hydroxytropone and 3-methoxytropone. Sensitized photo-oxidation of the vinylallenes CH2=C==CRCR =CH2 [R = Bu, R = H R = (CH2)4Me, R = Me] and 3-(cyclohex-l-en-l-yl)penta-1,2-diene have been found to lead to the tetra-hydropyranones (34) and (35). 1,2-Dioxetans are reported to be products of the... 

Formation of Silylvinylallenes. Enynes couple with TMSCl in the presence of Li/ether or Mg/Hexamethylphosphoric Triamide to afford silyl-substituted vinylallenes. The vinylallene can be subsequently oxidized to give the silylated cyclopentanone (eq 28). ... 

Further work has been reported on the peracid oxidation of vinylallenes to cyclopentenones/ ... 

The palladium-catalysed oxidative acyloxylation/carbocyclization of allenynes (78) has been found to produce acyloxylated vinylallenes (82), where a new C-C bond, a new C-0 bond, and a new allene moiety are formed (Scheme 6). The reaction is believed to proceed via the initial Pd-coordination (79), followed by cyclization generating the 7 -complex (82) either via the Pd(II) (80a) or Pd(IV) (80b) complex. The reaction is completed by extrusion of the product (82) and re-oxidation of Pd(0) to Pd(II) by p-benzoquinone (p-BQ). Mechanistic evidence was gathered by isotopic labelling. An aerobic version was also realized using catalytic amounts of p-BQ together with a catalytic amount of cobalt salophen complex. ... 

The oxidative cyclization of vinylallenes to cyclopentenones has been extended, indicating the feasibility of the preparation of 4-vinylcyclopentanones from 1,2,4,6-tetraenes [equation (14)]. ° The results of detailed investigations of the... 

The oxidation of cyano-Gilman cuprates has enabled the synthesis of new cyclophanes. Moreover, while BuhCuLi-LiCN has been reported to effect vinylallene formation by the 1,5-substitution of enyne acetates, it has been revealed that the application of Bu"3P to the reaction system considerably enhances the enantioselectivity of the synthesis. " Recent kinetic isotope studies into the chlorotrimethylsilane-mediated addition of lithiocuprates to cyclo-hexenone have suggested that reaction proceeds via an intermediate rr-com-plex. ... 

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