Allene oxides rearrangement

Macomber has described the oxidation of 3-methyl-l,2-butadienephosphonic acid with 3-chloroperbenzoic acid to the corresponding cyclic allenic oxide, which undergoes rearrangement to produce the corresponding unstable 2,5-dihydro-1,2-oxaphosphole-2-oxide derivative (Scheme 25) [66],... 

Scheme 4.27 Asymmetric synthesis of allenes by oxidative rearrangement.

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]. 

The rearrangements of 3-methylbut-l-ene oxides" and l,2-epoxybut-3-ene on lithium phosphate have been studied, and a detailed theoretical study of the rearrangement of allene oxide (342) to cyclopropanone (344), which shows that the transformation proceeds via an intermediate oxyallyl (343), has been presented. It has been shown that aldehydes, ketones, and cyclic ethers are all produced... 

In connection with the above discussion, formation of 3,3-disubsti-tuted 2 (3 H)-oxepinones (73) in the dye-sensitized photooxygenation of 6,6-disubstituted fulvenes is of special interest. 57>58> The reaction may be pictured in terms of an allene oxide intermediate which, as shown in Scheme 11, isomerizes to a cyclopropanone, followed by intramolecular rearrangement. 

Propargyl phenyl selenide is a versatile multifunctional acrylate synthon, as shown in Scheme 12. The (Uanion is prepared and reacted successively with an alkylating agent (R— X) and an electrophile (E ). The oxidative rearrangement of the propargylic selenoxide (35) to an allenic selenenate (36), and thence to the a-phenylselenoenone (37), forms the keystone of this synthetic method, and ovendl yields firom propargyl phenyl selenide are in the range of 38-68%. Further elaboration of (37) is possible... 

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) ... 

An unusual example of the allene oxide-cyclopropanone isomerization is found in the results of singlet oxygen oxygenation of 6,6-disubstituted fulvenes (18) The production of 3,3-disubstituted 2(3//)-oxepinones (20) in this process can be rationalized by the sequence of transformations outlined in equation 8. The initially formed endoperoxide (19) undergoes a well-precedented rearrangement to the epoxy ketone (19a) which contains the allene oxide residue. Rearrangement to the cyclopropanone form is... 

In a similar study, formation of allene oxides has been inferred from the collisional activation (CA) mass spectra of the products from rearrangements of a-methoxy- or thiomethoxyketones in the gas phase <93JCS(P 1)2235). 

Cyclopropanones are involved in the Favorski rearrangement, and there has been considerable interest in the allene oxide-cyclopropanone system. Estimation of the relative energies of the various structures involved (Scheme 62) is notoriously... 

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). ... 

Epoxidation of f-butyldimethylsilyl-l-(ethoxyethoxy)allene (6) with /n-chloroperbenzoic acid leads to an a-keto acylsilane, presumably through an allene oxide intermediate (eq 6). Deprotonation of (6) at C-3 and trapping of the anion with selenium, followed by iodomethane, produces an aUenyl selenide. The reaction of this material with peracid follows a different course, leading to an acetylenic acylsilane, presumably via [2,3]-sigmatropic rearrangement of an allenyl selenide (eq 6) ... 

Reaction of exo-9-oxabicyclo[4.2.1]non-7-ene oxide with n-BuLi to exo-S-hydroxybicyclo[3.3.0]octan-2-one has been suggested to occur by elimination to a transient allene oxide that rearranges to a trans-tpoxide enolate before undergoing epoxide a-lithiation and transannular C-H insertion. " ... 

Tlie thioketone 5-oxide 85, generated from allene 86 and SOCk in situ, decomposes to give the alkene 87. A mechanism, involving the transient formation of the 1,2-dithietane 88 (by dimerization of 85 followed by rearrangement), was proposed (85AGE855). 

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