Supports epoxide rearrangments

The rearrangement of mono terpenoid epoxides on alumina and silica gel ° surfaces has been studied. On the latter support, the rearrangements are typical of carbonium ions. 

For this reason heterogeneous catalysts such as Si02, AI2O3, ZnO, and WO, supported metals and a variety of precipitated phosphates have been tested for epoxide rearrangement reactions. Use of these conventional catalysts often results in the formation of aldol condensation products and mixtures of ketones and aldehydes as by-products. The heavier molecules formed by aldol condensation are the first step in the formation of coke thus limiting the lifetime of these catalysts. 

The oxygen rebound mechanism was supported by experimental evidence including (1) high kinetic isotope effects, (2) partial positional or stereochemical scrambling, and (3) allylic rearrangements. For instance, in the presence of [Fe(TPP)Cl] and PhIO, dx-stilbene was stereospecihcally epoxidized. In addition, it was found that cis-stilbene was 15 times more reactive than trans-stilbene in competitive epoxidations. (see Figure 7.20). " ... 

Interestingly, 1,2-disubstituted epoxides led to a,a-disubstituted propargyl alcohols, indicating that 1,2 shifts and therefore epoxide-to-aldehyde rearrangements occurred during the reactions (Scheme 10.12). Mechanistic studies by NMR validated this rearrangement and also supported the transmetallation-abstraction mechanism as already proposed (Scheme 10.13).8... 

In addition, its proton magnetic resonance spectrum unambiguously supported the 5-ketose structure (69). Rearrangements of epoxides to ketones when dicobalt octacarbonyl is used as the catalyst at temperatures above 100 , or when cobalt hydrocarbonyl is used at lower temperatures, are well known. By applying the technique of double irradiation to a sample of (68), the main component was shown to possess structure (68). Presumably, the free aldehyde group of the hydroformylation product immediately cyclized with the free hydroxyl group on C-3 to give the tricyclic structure (68). A third component (68a) (isolated in less than 5 % yield) was undoubtedly formed by subsequent reduction of the dialdose derivative (68). 

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