Rearrangements of oxiranes

The enantioselective base-promoted rearrangement of oxiranes was achieved by White-sell and Fehnan in 1980. Various homochiral lithium amides were used for the isomerization of cyclohexene oxide with an enantiomeric excess (ee) up to 36% with the employment of 50 in refluxing THF (Scheme 24). 

TABLE 5. Catalytic asymmetric rearrangement of oxiranes mediated by HCLA 56a and 56b... 

Unlike an A, A -disubstituted imidazoline-2-thione, the reaction of imidazolidine-2-thione 665 itself with 2-phenyloxirane in the presence of BF3 leads to the formation of l,3-bis[(i4)-2-phenylethen-l-yl]im>dazolidine-2-thione 666 (Scheme 161) <2005HCA3253>. This unusual mode of reaction is presumably due to a Lewis-acid-catalyzed rearrangement of oxirane to aldehyde followed by an ene-like reaction with the iminothiol. 

Studies have been made of the rearrangement of oxiranes containing an acetylenic side-chain, isoprene oxides, and 7,5-unsaturated oxiranes. ... 

The rearrangement of oxiranes with various structures in the presence of AICI3, ZnCl2, and other Lewis acids has been examined by taking into consideration the strengths of the Lewis acids, the molecular structure, and steric and electronic fac-The effects of Lewis acid and the solvent have been investigated in the... 

Japanese authors have made comprehensive investigations of the rearrangements of oxiranes in the presence of solid acids, bases, and salts.The model compounds employed were cyclohexene oxide and 1-methylcyclohexene oxide. The effects of the acidic and basic properties of the catalysts on the selectivity were interpreted on the basis of the products obtained. The main products are carbonyl compounds and allyl alcohol isomers. Rearrangements of limonene oxide over acids and bases were studied on five different types of Al203 similar research has been carried out on 2- and 3-carene oxides, cis- and trans-carvomenthene oxides and a-pinene oxide. ... 

The most important oxirane syntheses are by addition of an oxygen atom to a carbon-carbon double bond, i.e. by the epoxidation of alkenes, and these are considered in Section 5.05.4.2.2. The closing, by nucleophilic attack of oxygen on carbon, of an OCCX moiety is dealt with in Section 5.05.4.2.1 (this approach often uses alkenes as starting materials). Finally, oxirane synthesis from heterocycles is considered in Section 5.05.4.3 one of these methods, thermal rearrangement of 1,4-peroxides (Section 5.05.4.3.2), has assumed some importance in recent years. The synthesis of oxiranes is reviewed in (B-73MI50500) and (64HC(19-1U). 

The rearrangement of 3 -benzylspiro[2//-1-benzothiopyran-3(4//),2 -oxirane]s 7, induced by Lewis or proton acid catalysts, gives the seven-membered ring dione systems 8. Compounds... 

Other predicted reactions, not shown in Fig. 32, include rearrangements of the oxiranes to give a cyclohexanone, and various allyl alcohols. These predicted products are entirely consistent with the type of by-product to be expected under such reaction conditions. 

The l-oxa-2,4,5-cycloheptatrienes 602 and 603 were postulated to be intermediates in the rearrangement of certain (ethynylfuryl)oxiranes to furo[3,4-b]furans [251]. The replacement of the methylene groups of 1,2-cycloheptadiene (465) by SiMe2 groups and the introduction of substituents at the allene moiety allowed the preparation of isolable seven-membered ring allenes. Thus, Barton et ah [177] obtained 604 and Ando et al. [178] 605. A few reactions of these systems have also been studied [177, 252]. Both groups [178, 253] synthesized the [4.4]betweenallene 606 and determined its structure by X-ray diffraction. 

The oxiranes obtained from the reaction of chloromethylsulphones with aldehydes and ketones can be isolated [26, 27], but tend to be unstable in the basic media. Rearrangement of the toluenesulphonyloxiranes produces the sulphonyl aldehydes (Scheme 6.15) [26]. When chiral chloromethylsulphonamides are used, asymmetric... 

The reaction gave only the rearrangement products 333 and 334, and the side product 335, as expected from the reactivity of alkylidenecyclopropane derivatives (Scheme 49). Compound 333 might arise from the 0-0 bond cleavage followed by the rearrangement of a cyclopropyloxy cation to an oxoethyl cation (Scheme 49, path a). Spiro-hexanone 334 could arise from a different fragmentation of ozonide C-O bond and further cyclopropyloxy-cyclobutanone rearrangement (Scheme 49, path b). Oxirane 335 can eventually derive from the same path b or from other side processes [13b]. 

Methylenecyclohexane oxide has been prepared by the oxidation of methylenecyclohexane with benzonitrile-hydrogen peroxide or with peracetic acid by treatment of 1-chlorocyclo-hexylmethanol with aqueous potassium hydroxide and by the reaction of dimethylsulfonium methylide with cyclohexanone. This reaction illustrates a general method for the conversion of ketones and aldehydes into oxiranes using the methylene-transfer reagent dimethyloxosulfonium methylide. The yields of oxiranes are usually high, and the crude products, in most cases, are of sufficient purity to be used in subsequent reactions (e.g., rearrangement to aldehydes) without further purification. 

Nevertheless, whereas the base-promoted isomerization of simple linear oxiranes and cyclohexene oxide occurs via a -deprotonation mechanism, recent denterinm-labeUng experiments demonstrate that the LDA-mediated rearrangement of cyclopentene oxide in nonpolar solvents furnishes the corresponding cyclopentenol through an a-deprotonation route (Scheme 7) . 

The origin of the enantiodiscrimination appears to be strongly dependent on the structure of the HCLA employed. For HCLA bases of type A (53 to 56), stereoselectivity has been empirically deduced to arise [in the transition state (TS)] from the difference of energy between the two diastereoisomeric 1/1 HCLA/oxirane complexes TSl and TS2 (Scheme 27). Indeed, the steric repulsions between cyclohexene oxide and the pyrrolidinyl substituents in TS 1 favor TS 2, as proposed by Asami in 1990 for enantioselective rearrangement of cyclohexene oxide by HCLA 53 (Scheme 26) . ... 

The first example of such a process was reported in 1994 by Asami, who noticed that LDA was less reactive than HCLA 53 toward oxirane and thus proposed its use as a co-base in a catalytic cycle . Based upon this seminal result, the system has been extended to other HCLAs and various co-bases have been tested Selected results for the asymmetric rearrangement of cyclohexene oxide mediated by sub-stoichiometric quantities of HCLA are collected in Table 4. 

This result and the availability of homochiral base 71 in both enantiomeric forms has made this procedure particularly interesting. It has then been successfully applied to the rearrangement of unprotected oxirane 72a into 73a, with excellent stereoselectivity (95% ee Scheme 31) . ... 

TABLE 8. Asymmetric rearrangement of various oxiranes using base 74... 

It is interesting to note that such model, which is in good agreement with the enantio-and diastereoselectivity observed, is consistent with the model proposed for the enantioselective rearrangement of meso oxiranes mediated by this base (see Section in.B.l.a). 

When both positions a to the C—Li bond of the lithiooxirane have no hydrogen to shift, a 1,2-alkyl shift (Scheme 58) can occur. This rearrangement has been observed recently in the case of oxiranes 125 derived from cyclopentenols. This 1,2-alkyl shift can occur on both sides of the lithiooxirane (Scheme 58, paths a and b). The resulting lithium enolate then undergoes a -elimination process of Li20, leading to diversely substituted cyclopentenones 126 and 127. 

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