Cleavage of epoxides

There is continuous interest in cleavage of epoxides as a general method lor the preparation of fluorohydrins Epoxides are effectively cleaved under mild conditions by a 70% hydrogen fluoride-pyridine complex (Olah s reagent). The... 

Reduction of epoxide 21 with lithium aluminium hydride gave a crystalline branched-chain methyl heptoside derivative 24. The NMR spectra of compounds 21 and 24 were very similar. In the spectrum of compound 24 the disappearance of the two sharp doublets at r 6.80 and 7.45 (2 protons) and the appearance of a singlet at r 8.65 (3 protons) is consistent with the reductive cleavage of epoxide 21 to give a substance 24 with a methyl substituent. The multiplet at r 7.40-8.50 ( 5 protons ) was assigned to the four protons of the two methylene groups and the hydroxylic proton. 

Cleavage of epoxides with hydrogen halides Addition of hypohalous acids to alkenes... 

The cleavage of epoxides by water is a classical reaction. Such epoxide cleavage can be catalyzed by both acids and bases in aqueous media. In the presence of other nucleophiles, the corresponding nucleophilic ringopening products are obtained with the nucleophiles being incorporated into the products.68 Examples include azides, iodides, and thiols in the presence or absence of metal salts in aqueous media. The pH of the reaction medium controls the reactivity and regioselectivity of the... 

Hydroxy stannanes can be prepared by cleavage of epoxides with BusSnLi or cleavage of epoxy stannanes with organocuprates (equation 33)72. The two methods are stere-ochemically complementary. The higher order cyanocuprate, Bu3Sn(Bu)Cu(CN)Li2, also affords /1-hydroxy stannanes by reaction with epoxides54. 

One of the methods by which replacement of secondary hydroxyl groups by fluorine in pentofuranoses and pento- and hexo-pyranoses has been achieved is by cleavage of epoxide rings with fluoride ion. The epoxide rings are always cleaved trans-diaxially, according to the Fiirst-Plattner rule. 

Nitratoalkyl perchlorates have been synthesized from the cleavage of epoxides with nitronium perchlorate. These materials are extremely dangerous with little value as practical... 

Uncatalysed cleavage of epoxides has been reported [e.g. tetrahydro-(39)- dihydro-(64 X = OH)]," and oxidation of tetrahydrogeranyl acetate with ozone on silica gel yields dihydro-(65 R = H) predominantly with some 8-acetoxy-6-methyloctan-2-one (cf. Vol. 7, p. 7)." ... 

Cleavage of epoxides (6, 16-17 8, 10-12). Cleavage of epoxides catalyzed by Woelm neutral alumina is particularly useful in the case of medium-ring epoxides, which are generally rather unreactive and which are liable to undergo transannular reactions. For example, rw-cyclooctene oxide supported on neutral alumina is converted to rran.r-2-acetoxycyclooctanol in 78% yield by reaction with acetic acid at... 

Cleavage of epoxides. Tris(phenylseleno)borane and tris(meChylseleno)borane2 react rapidly with terminal and a,/ -disubstitutcd epoxides to give /1-hydroxy selenidcs, or in some cases alkenes. In the latter case, there are marked differences between the cis- and frans-cpoxidcs the latter react very slowly to give mainly trans-alkenes. 

Cleavage of epoxides with alkylthiotriaikylsilanes. In the presence of Znl, these thiosilanes (R SSiR3) convert epoxides into /J-trimethylsilyloxy thioethers, which can be hydrolyzed to fi-hydroxy thioethers. The regioselectivity can be reversed by use of n-butyllithium as the catalyst. 

Although the above authors did not advance a detailed mechanism, it appears probable that nitrosyl cation can in effeot function us a proton, forming an oxonium-type intermediate (Eq. 940). Attack hv Cl ion is thereby facilitated and occurs with Walden inversion, us in the cleavage of epoxides with hydrogen chloride itself. 

Cleavage of epoxides with hydrogen sulfide has been exploited advantageously by Perveev and co-workers13 1 the syn-... 

Cleavage of epoxides The epoxide 1 is cleaved by N3Si(CH3)3 and a catalytic amount of ZnCl2 to the hydroxy azide 2 in 90% yield. The corresponding benzoate (3) on hydrogenation undergoes O — N benzoyl transfer to provide 4 in 95% ee. 

Contra-trans-diaxial cleavage of epoxides.2 The reaction of the a, p-epoxy alcohol 2 with NaSeC6H5 provides the 1,3-diol 3 in 90% yield. Use of 1 results in the opposite regioselectivity to provide 4 as the major product. 

Cleavage of epoxides andoxetanes.5 In the presence of diethylaluminum chloride as catalyst, cyanotrimethylsilane reacts with epoxides and oxetanes regioselectively to give open-chain products in which the CN group is attached to the less substituted a-carbon of the cyclic ether. 

Selective cleavage of epoxides. The a- and /f-benzyloxy epoxides 1 and 3 (either cis or trans) undergo almost exclusive /(-addition with trimethylaluminum catalyzed by n-butyllithium or lithium methoxide to give the alcohols 2 and 4, respectively. The regioisomeric alcohols are formed only in traces. CHjMgBr, CH3Li, or (CH3)2Mg are much less selective.2... 

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