Epoxidation of allene

Epoxidations of chiral allenamides lead to chiral nitrogen-stabilized oxyallyl catioins that undergo highly stereoselective (4 + 3) cycloaddition reactions with electron-rich dienes.6 These are the first examples of epoxidations of allenes, and the first examples of chiral nitrogen-stabilized oxyallyl cations. Further elaboration of the cycloadducts leads to interesting chiral amino alcohols that can be useful as ligands in asymmetric catalysis (Scheme 2). 

Epoxidation of allenes.1 The spirodioxides formed by epoxidation of allenes are unstable to acids, and only hindered ones have been obtained on epoxidation with peracids. They can be obtained, however, in 90-95% yield by epoxidation of allenes (even monosubstituted ones) with dimethyldioxirane in acetone buffered with solid K2C03. 

Use of a chiral auxiliary on the allene results in excellent control of product stereochemistry. For example, epoxidation of allene 179 in the presence of furan leads to 180 in 80% yield (Eq. 13.61). The ratio of endo to exo isomers was 96 4 [69]. 

Asymmetric epoxidation has been performed with optically active peracid. " and racemic oxiranes have been resolved on a glc column containing an optically active complex. The epoxidation of allenes has been examined, as have the reactive intermediates formed in the epoxidation of simpler allenes. Such intermediates have been isolated from sterically hindered allenes. PNPBA epoxidation of a series of vinylallenes results in a-allenoxiranes and, as the main products, conjugated cyclopentenones. ... 

An interesting route to the cyclopropanone system involves the rearrangement of allene oxides, usually generated by the epoxidation of allenes. Thus, 1,3-di-t-butylallene oxide (11) may be prepared by the reaction of 1,3-di-t-butylallene with m-chloroperbenzoic acid. Heating 11 to 100 °C leads to isomerization, forming truns-2,3-di-t-butylcyclopropanone (10) (Scheme 4) Similarly, 1,1-di-t-butylallene (15) yields 2,2-di-t-butylcyclopropanone with peracetic acid (equation 7) ... 

The details of the Favorskii rearrangement continue to attract attention and cyclopropanone intermediates in the peracid epoxidation of allenes have been noted. The fluoride-ion-promoted elimination of chlorotrimethylsilane from (375) leads to the allene oxide (376) which undergoes regiospecific ring-opening with nucleophiles. However, rearrangement of (376) to cyclopropanone (377) only occurs prior to nucleophilic capture when C-1 carries an aryl substituent (Scheme 45). ... 

Since cumulenes and alkynes are often easily interconvertible, many syntheses discussed above have allenic counterparts, especially base-catalyzed cyclizations of allenic alcohols.77 And, of course, several of the alkyne-based syntheses may well have allenic intermediates. There are, however, a few syntheses based specifically upon allene chemistry. In an important one, due to Stirling and his collaborators,78 an allenic sulfonium salt reacts with an enolate anion. Scheme 12 sketches the main features yields as high as 86% are recorded. Methoxyallene is easily metallated by butyllithium and so converted into an allenic epoxide that can be isomerized by fe/T-butoxide into a furan (Scheme 13) or an exocyclic equivalent similar to 15 clearly this method is particularly suited to the preparation of 3-methoxyfuran... 

Photo-addition of allene to the enone (90) yield adduct (91) in 75 % yield, which was subjected to ketalization in 77% yield. Epoxidation of (92) with perbenzoic acid followed by chromatography on alumina afforded two expoxides (93) and (94). Both (93) and (94) could be converted separately through (95) and (96) respectively which was the common intermediate leading to isoishwarane (98) and ishwarane following a deketalization-retroaldol-aldol process to furnish the keto-alcohol (97) (99) 30>. 

In 1974, Vermeer et al. described formation of allenic alcohols 61 by the reaction of alkynyl epoxides 60 with Grignard reagents in the presence of 10mol% of Cul (Scheme 3.33) [71]. In the absence of Cul, a complicated mixture of products was obtained. Furthermore, the Cu-catalyzed reactions exhibited higher yields and higher selectivity than analogous reactions of alkynyl epoxides with lithium dialkylcup-rates [72], This method was applied to a reaction of allylmagnesium bromide with an alkynyl epoxide [73]. 

Spino and Frechette reported the synthesis of non-racemic allenic alcohol 168 by a combination of Shi s asymmetric epoxidation of 166 and its organocopper-mediat-ed ring-opening reaction (Scheme 4.43) [74]. Reduction of the ethynyl epoxide 169 with DIBAL-H stereoselectively gave the allenic alcohol 170, which was converted to mimulaxanthin 171 (Scheme 4.44) [75] (cf. Section 18.2.2). The DIBAL-H reduction was also applied in the conversion of 173 to the allene 174, which was a synthetic intermediate for peridinine 175 (Scheme 4.45) [76], The SN2 reduction of ethynyl epoxide 176 with DIBAL-H gave 177 (Scheme 4.46) [77]. 

Not least for the syntheses of natural products, alkoxycarbonylations with formation of allenic esters, often starting from mesylates or carbonates of type 89, are of great importance [35, 137]. In the case of carbonates, the formation of the products 96 occurs by decarboxylation of 94 to give the intermediates 95 (Scheme 7.14). The mesylates 97 are preferred to the analogous carbonates for the alkoxycarbonylation of optically active propargylic compounds in order to decrease the loss of optical purity in the products 98 [15]. In addition to the simple propargylic compounds of type 89, cyclic carbonates or epoxides such as 99 can also be used [138]. The obtained products 100 contain an additional hydroxy function. 

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]. 

Epoxidation of amidoallenes with dimethyldioxirane leads to allene oxides as reactive intermediates which can be trapped with dienes in a [4+ 3]-cycloaddition reaction. Exposure of a mixture of amidoallene 177 with cydopentadiene to a small excess of dimethyldioxirane at -45 °C produced endo-bicydooctanone 178 in 60% yield (Eq. 13.60) [69]. The allene oxide is electrophilic, since no reaction takes place with methyl acrylate. 

The reaction of allenes with peracids and other oxygen transfer reagents such as dimethyldioxirane (DM DO) or hydrogen peroxide proceeds via allene oxide intermediates (Scheme 17.17). The allene oxide moiety is a versatile functionality. It encompasses the structural features of an epoxide, an olefin and an enol ether. These reactive intermediates may then isomerize to cyclopropanones, react with nucleophiles to give functionalized ketones or participate in a second epoxidation reaction to give spirodioxides, which can react further with a nucleophile to give hydroxy ketones. 

In situ epoxidation of allenyl alcohols [20], aldehydes [21], acids [22] and sulfonamides [23] followed by intramolecular ring opening of the intermediates was thoroughly investigated by Crandall and co-workers. They showed that products formed either from the allene oxide or the spirodioxide intermediate can be prepared selectively. Allenyl acids 56, for example, react first with DMDO on their more substituted double bond. When the concentration of the oxidant is low (DMDO is formed... 

Dioxiranes are three-membered cyclic ring peroxides that are expected to be very unstable owing to ring strain. They are effective oxygenating agents for epoxidations of olefins, allenes, polycyclic aromatic hydrocarbons, enols. and a, /i-unsaturated ketones for insertions of oxygen into X—H... 

Although the iron-catalyzed synthesis of allenes from propargylic halides was reported by Pasto and coworkers in 1978 [67], little progress was achieved in this field until recently [68]. In 2003, Fiirstner and coworkers discovered propargylic epoxides as valuable substrates for the reaction with Grignard reagents in presence of catalytic amounts of Fe(acac)3 to generate 2,3-allenol derivatives (Scheme 5.24) [69]. 

Other synthetically useful intramolecular epoxide ring openings have been reported. For example, the strained methylene epoxide 79, derived from DMD epoxidation of the corresponding allene 78, undergoes spontaneous isomerization to the lactone 81 via attack of the... 

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