Allene epoxide rearrangement

The thermal rearrangement of methyleneoxiranes (allene epoxides) and a related photochemical rearrangement lead to cyclopropanones. An example is the rearrangement of 1 to 2. ... 

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]. 

Common reactions of the ylide include (i) [2,3]-sigmatropic rearrangement of allylic, propargylic, and allenic ylides (ii) [l,2]-shift (Stevens rearrangement) (iii) 1,3-dipolar cycloaddition of the ylide generated from carbonyl compounds or imines with dipolarophiles, usually G=G or C=C bonds and (iv) nucleophilic addition/elimination, leading to the formation of epoxides or cyclopropanes (Figure 2). 

The reaction of linalool with boron trifluoride etherate has been re-examined no pinenes or camphene were obtained.146 Dehydrolinalool reacts with methyl iso-propenyl ether under acidic conditions by Claisen rearrangement to give the allene (58).147 Further papers in this section include reaction of monoterpenoid alcohols with paraformaldehyde-acetic anhydride-sodium acetate,148 rearrangement of the alcohol (47 X = OH) to the oxabicycloheptane (59) and the ketone (60),149 and the rearrangement of a typical monoterpenoid vicinal hydroxy-ester to an epoxide.150... 

The structure of 19 was unambiguously confirmed by an X-ray diffraction study. A mechanistic rationale is depicted in Scheme 11. After Co2(CO)6-complexed alkyne A is obtained, a Sn2 attack of the Co2(CO)6 fragment opens the epoxide moiety to afford intermediate B, which subsequently incorporates CO to give C. The latter rearranges into cobalt-stabilized cyclic allene species D. The net result is a [5 + 1] cyclization that creates the lactone group. Coordination of the tethered olefin leads to oxidative cyclization to give E. Finally, insertion of CO followed by reductive elimination affords the desired product 19. 

A number of useful enantioselective syntheses can be performed by attaching a chiral auxihary group to the selenium atom of an appropriate reagent. Examples of such chiral auxiliaries include (49-53). Most of the asymmetric selenium reactions reported to date have involved inter- or intramolecular electrophilic additions to alkenes (i.e. enantioselective variations of processes such as shown in equations (23) and (15), respectively) but others include the desymmefrization of epoxides by ringopening with chiral selenolates, asymmetric selenoxide eliminations to afford chiral allenes or cyclohexenes, and the enantioselective formation of allylic alcohols by [2,3]sigmafropic rearrangement of allylic selenoxides or related species. 

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). ... 

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. " ... 

Methanesulfonates. The most common use of methanesulfonyl chloride is for the synthesis of sulfonate esters from alcohols. This can be readily accomplished by treatment of an alcohol with mesyl chloride in the presence of a base (usually Triethy-lamine or Pyridine). The methanesulfonates formed are functional equivalents of halides. As such they are frequently employed as intermediates for reactions such as displacements, eliminations, reductions, and rearrangements. Selective mesylation of a vicinal diol is a common method of preparation of epoxides." Alkynyl mesylates can be used for the synthesis of trimethylsilyl allenes. Oxime mesylates undergo a Beckmann rearrangement upon treatment with a Lewis acid. Aromatic mesylates have been used as substrates for nucleophilic aromatic substitution. Mesylates are more reactive than tosylates toward nucleophilic substitution, but less reactive toward solvolysis. 

---