Rearrangement alcohol protection, allylic carbonates

A 1,2-hydride shift has been invoked399 to account for the formation of p-methoxyphenylbutyraldehyde derivatives (337) during the treatment of />methoxy-benzyl-protected allylic alcohols (336) with zeolites. A similar C-glycosidation procedure involving Lewis acid-catalysed anomeric oxygen to carbon rearrangement of tetrahydropyranyl ether derivatives has been reported400 (see Scheme 82). It has been... 

The [2,3]- or [3,3]-sigmatropic rearrangements (Scheme 24) provide a means to introduce either the protected amine or the carbon atom which will become the carboxylic acid, while also positioning the double bond in the correct position for the alkene isosteres. Moreover, when starting from homochiral allyl alcohols, a very effective chirality transfer assures the stereospecific construction of the R1 and R2 side-chain stereochemistries. 

The choice of methyl carbonate protection for the C-1 hydroxyl group was inspired by the expectation that upon cleavage of the acetonides, a cyclic carbonate would form spontaneously. Reacetonation of the vicinal glycol then affords the allylic alcohol (132) which is required for the Claisen rearrangement. For this rearrangement, the stereochemical outcome at C-12 of (133) was predictable on the basis of the chair-like transition state in which the bulky cyclic carbonate is equatorially oriented (55). 

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