Sigmatropic acyclic substrates

The stereochemical course of this [2,3] sigmatropic rearrangement for different types of cyclic and acyclic substrates is summarized in Table 3. 

Oxonium ylides have only recently been described in the context of [2,3] sigmatropic reactions 106 10 8, and the small number of acyclic substrates examined precludes the identification of a general trend regarding the olefin selectivity for these systems, Ylide 10 has been postulated as an intermediate in the trimethylsilyl triflate catalyzed rearrangement of allyloxy acetate 9, which yields an excess of the Z-olefin 11109. 

Chirahty transmission can also be induced in thio-Claisen reactions by a chiral center adjacent to the allyl vinyl sulfide core. The group of Metz-ner examined the diastereoselectivity of the rearrangement of acyclic S-allyl ketene dithioacetals bearing a chiral center adjacent to carbon 6. hi these substrates, the sigmatropic shift proceeded smoothly and with modest syn anti diastereoselectivity [74]. In contrast, higher selectivities are foimd for thio-Claisen precursors bearing a chiral center adjacent to carbon 1 mainly due to steric effects and allylic strain [75]. In 1991, the group of Beslin examined the effect of a hydroxyl substituted chiral center attached to C-1 of Z and E S-allyl ketene dithioacetals [76,77] (Scheme 7). Under the reaction conditions, these... 

The results indicate that for both the ( )- and the (Z)-substrate a chairlike transition state is preferred. Obviously. [3,3] sigmatropic rearrangements of iminium ions occur preferentially in the same topographical sense as the Cope rearrangement of acyclic 1,5-dienes. 

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