3,3-Sigmatropic rearrangements homologations

Shi epoxidation 283 Shibasald 234, 236, 242 Shikimate pathway 407 [3,3]-sigmatropic rearrangement 51 silicon-lithium exchange 157 single-carbon homologation... 

Aminophenols from anilines, 35, 2 Anhydrides of aliphatic dibasic acids, Friedel-Crafts reaction with, 5, 5 Anion-assisted sigmatropic rearrangements, 43, 2 Anthracene homologs, synthesis of, 1, 6 Anti-Markownikoff hydration of alkenes, 13, 1... 

Methylidenation of allylic thioethers. Methylidenation of an allylic phenyl-thioether with methylene iodide-diethylzinc is accompanied by a 2,3-sigmatropic rearrangement to a homologous allylic phenylthioether. The rearrangement is also initiated by ethylidene iodide. Cyclopropanation is not observed. The Simmons-Smith reaction with allylic sulfides results only in formation of an insoluble polymer. 

Homologation of allyl sulfides. Sulfonium ylides formed at low temperature are liable to undergo a [2,3]sigmatropic rearrangement on treatment with a mild base (e.g., DBU). 

W. S. Johnson and coworkers developed this protocol in the course of endeavours towards the synthesis of all /ra 5 -squalene. Upon heating with ethyl orthoacetate, the bis-allylic alcohol 342 yielded di-ester 343 to accomplish a double homologation strategy. Johnson recognized the potential of this [3,3]-sigmatropic rearrangement since the operation simultaneously produced two -trisubstituted alkenes as a single diastereomer. Further elaboration of intermediate 343 resulted all ( )-squalene. 

Other Unsaturated Alcohols, y,5-Unsaturated (homoallylic) alcohols may be made by homologation of allyl alcohols with allylic transposition using a new method that involves [2,3] sigmatropic rearrangement of an a-allyloxy-organolithium (34) (Scheme 17) some comments on the observed stereochemistry have been made. 

A reaction which transforms allylic alcohols to homologous P.y-unsaturated amides by means of a 2,3-sigmatropic process was first described by BUchi. When an allyl alcohol is heated with A -di-methylformamide aceuil, a rearrangement occurs apparently involving a nucleophilic carbene as an intermediate, leading to a product with 95-100% ( )-geometry of the double bond (Scheme 47). 

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