Epoxide migration

On the other hand, the situation is more complex with flexible oxirane derivatives. This is clearly demonstrated by comparison of the equilibrium mixtures resulting after epoxide migration of different types of anhydro hexoses,278,286,304 their 6-deoxy derivatives 175, 176,520 branched-chain hexoses 177, 178,520,521 and anhydro hex-2-uloses.304 

The isomerization of alloepoxides 138 and 139 effected by sodium iodide in acetone may be described as a pseudo-epoxide migration, and yields a mixture of both in the 3 1 ratio, reportedly via the 3-deoxy-3-iodo intermediate 179.522 

It was shown that methyl 2,3-anhydro-/l-D-ribofuranoside (180) forms an equilibrium mixture with methyl 3,5- anhy dr o -/T i)- x y 1 oI uranoside (181), whereas the corresponding a anomer 182 gives in sodium methoxide solution the major product of epoxide-ring opening, namely methyl 2-0-methyl-a-D-arabinofuranoside (183).523,524 

Acid hydrolysis of 2,3-anhydrohexopyranosides yields free 3,6-anhydro-hexoses (for example, 184 185), which suggests that the hydrolysis of the glycoside bond precedes the formation of the oxolane ring.525,526 

2-Anhydroaldohexopyranoses show a tendency for conversion into 1,6-anhydrohexopyranoses if a free hydroxymethyl group is present at C-5 and the oxirane ring adopts the appropriate trans orientation, see Section II.l. 

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It is appropriate at this juncture to address some of the more useful transformations of 2,3-epoxy alcohols.913 A 2,3-epoxy alcohol such as compound 14 possesses two obvious electrophilic sites one at C-2, and the other at C-3. But in addition, C-l of a 2,3-epoxy alcohol also has latent electrophilic reactivity. For example, exposure of 14 to aqueous sodium hydroxide solution results in the formation of triol 19 in 79% yield (see Scheme 5). In this interesting transformation, hydroxide ion induces the establishment of an equilibrium between 2,3-epoxy-l-ol 14 and the isomeric 1,2-epoxy-3-ol 18. This reversible, base-induced epoxide migration reaction is a process known as the Payne rearrangement.14... 

The behavior of 7-bromo-2,3,7-trideoxy-D-amfe/ o-heptono- 1,4-lactone (205) and 7-bromo-2,7-dideoxy- (208) and 7-bromo-3,7-dideoxy-D-g/Mco-heptono-1,4-lactone (211) toward aqueous base has also been studied (229). The 6,7-epoxide is formed by potassium carbonate treatment of each bro-modeoxy heptonolactone. With potassium hydroxide, epoxide migration occurs giving mixtures of epoxides that undergo intramolecular attack by the... 

Base-promoted isomerization of 2,3-epoxy alcohols. Also known as epoxide migration. 

Figure 23. Treatment of diols with NaOH under cxmditions where epoxide migration can occur.

Sodium methoxide in methanol, often with chloroform as cosolvent, has customarily been the basic reagent employed. Less frequently, particularly with water-soluble esters, sodium or potassium hydroxide in aqueous solution has been used. Generally, an excess of the basic reagent is taken, except where the possibility of epoxide migration arises (see p. 127). In the latter situation, only a limited excess of reagent is used, at low temperature, or, alternatively, the... 

The possibility of occurrence of this kind of epoxide migration should be borne in mind in essaying the preparation and considering the reactions of compounds of this kind in basic media, particularly at elevated temperatures. 

When the methoxyl substituent at Ca is of the inverted configuration, epoxide migration appears to take place in the opposite wise. Thus, although it had been reported previously 89"117 that the... 

The phenomenon of epoxide migration is directly demonstrable with l,2-anhydro-oUo-inogitol,S4 which gives on treatment with dilute alkali at room temperature the isomeric substance ),2-anhydro eo-inositol (Eq, 250). 

Treatment of l,6 2,3-dianhydro- and l,6 3,4-dianhydro-/i-i>-hcxopyranoses with aqueous NaOH or Nal in acetone caused epoxide or pseudo-epoxide migration giving equilibrium mixtures. Experimental data were compared with DFT calculations and the role of chair-boat equilibration of l,6-anhydro-3-dcoxy-3-halo-/i-i>-glucopyranoses was analysed.21... 

In the tosylation of LXI, only the equatorial hydroxyl group reacts readily introduction of a second tosyl group (onto the axial oxygen atom) is very difficult. Detosylation is carried out with a strong-base ion-exchange resin to minimize epoxide migration (see p. 182) if alkali is used, rac-inositol is the main product. 

A final possibility was raised by the observation that (Cp Re0)2(jU.-0)2 deoxygenated epoxides. Migration of carbon not to rhenium but to oxygen would result in a Re(V)-epoxide complex ... 

Hanson, R. M. Epoxide migration (Payne rearrangement) and related reactions. Org. React. 2002, 60,1-156. 

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