Allylic anions synthetic utility

Allyl, pentadienyl, and heptatrienyl anions can in principle undergo electrocyclic rearrangements (81). The thermal conversion of a pentadienyl into a cyclopentenyl anion is predicted to be a disrotatory process. The cyclooctadienyl anion cyclizes to the thermodynamically stable isomer of the bicyclo[3.3.0]octenyl ion having cis fused rings (52,82,83). The acyclic pentadienyl anions, however, do not normally cyclize. On the other hand, heptatrienyl anions cyclize readily at — 30°C by a favorable conrotatory thermal process (41,84). This reaction sets a limit upon the synthetic utility of such anions. 

Since the preparation of anionic species or organosilicon compounds is largely restricted by its reaction conditions, the synthetic utility of silyl anions for the synthesis of organosilicon compounds is limited. However silyllithiums possessing one or more phenyl groups on a silicon atom are readily prepared by the reductive metallation of the corresponding halosilanes with lithium. Allylsilanes are synthesized by the reaction of allyl acetates with the cuprate prepared from such a reagent (eq (47)) [43]. 

Deallylation of allylic compounds proceeds with liberation of rather stable carban-ions such as malonate anions. The deallylation can offer an opportunity to regard the allylic group as a protective group for acidic C-H bonds. Further synthetic utility of the deallylation is yet to be established. 

The electrophilic reactivity of the j3-carbon in an a/S-unsaturated aldehyde is reversed in l,3-bis(methylthio)allyl-lithium (151), derived by metallation of l,3-bis(methylthio)-2-methoxypropane with lithium di-isopropylamide this reagent functions as the synthetic equivalent of the unknown p-formylvinyl anion (152) in the preparation of a number of ay -unsaturated aldehydes, as exemplified in Scheme 72. Corey has utilized this reagent further in a total synthesis of prostaglandin p2 . 

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