Silica epoxide rearrangments

Other useful p-menthane syntheses of no great novelty are of cis- and trans-piperitol from 2a,3o -epoxycarane (silica-catalysed rearrangement to ds-p-menth-2-en-l,8-diol is also reported), of ( )-dihydrocarvone, isopulegone, and p-menthofuran via /S-keto-sulphoxides, of p-mentha-l,4(8)-diene via a bromination-dehydrobromination sequence, and of trans-carveol by benzoyl peroxide-CuCl oxidation of a-pinene. Further details for the conversion of (-)-(142) into (+)-(142), via its epoxide, are reported (Vol. 5, p. 25 cf. Vol. 3, p. 44). " ... 

As already mentioned, epoxidation of a A -20-acetoxypregnane always produces the 17a-epoxide. If the epoxy acetate is allowed to stand on a silica gel column, or is heated, it rearranges to the 17l5-acetoxy-17a-pregnan-20-one ° this may be pictured as follows ... 

It is beneficial in terms of yield to convert an a,/3-unsaturated ketone into the cyclopropyl ketone using dimethyloxosulfonium methylide. The epoxide, formed as above, is rearranged during chromatography on silica gel. The corresponding pent-2-ene-l,5-diols are also formed, presumably through hydration of the cation, and these may be dehydrated to the pyran with p-toluenesulfonic acid (74JCS(P1)1674). 

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. 

The epoxy sulfones were prepared by exhaustive peracid oxidation of the corresponding alkene sulfides. These were generated by ring expansion of cyclic 1-methyl-2-vinyl sulfonium salts 1 via 2,3-sigmatropic rearrangement of the methanides. From the five-membered sulfonium ylide la, (Z)- and ( )-thiacyclooct-4-enes 2 were obtained as an 85 15 mixture. Their chromatographic separation turned out not to be feasible due to concomitant EjZ isomerization on the silica gel column. Since the separation of these epoxy sulfone mixtures obtained by exhaustive oxidation also proved unsuccessful, it was found expedient to first oxidize the mixture of sulfides to an alkene sulfone mixture ( )-3, which could be separated and eventually epoxidized into epoxides cis- and trans-4 and cis- and trans-5. 

Transformation of 945 into the thiohydroxamate ester 947 followed by visible-light photolysis in the presence of tributyltin hydride allows large scale preparation of the methylene cyclopentane 948. The overall yield of 948 from 945 after chromatographic purification on silica gel is 55—65%. Desilylation of 948 and subsequent epoxidation of the exocyclic olefin with freshly prepared dimethyldioxirane solution provides the epoxyalcohol 949 in high yield. A Dess—Martin oxidation to the epoxyketone followed by rearrangement on silica gel to the enone, a Luche reduction from the convex face, and a final isopropylidene ketal hydrolysis provides 950 as a single diastereomer [262] (Scheme 205). 

Carvone-derived 2,3-epoxy alcohol derivatives (317) have been found to rearrange with stereoselective formation of ring-contracted ketones (318) in a process where the stereochemical result seems to be independent of epoxide configuration. On the basis of this study, the authors concluded that the rearrangement of tetrasubstituted cyclic epoxy alcohol derivatives remains a transformation for which both regiose-lectivity and stereochemical outcome are difficult to predict. New solid acid catalysts based on silica-supported zinc triflate have been prepared for use in the rearrangement of a-pinene oxide to campholenic aldehyde. ... 

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