Ring expansions 3.3 -sigmatropic rearrangements

Besides [2,3]-sigmatropic rearrangement and [l,2]-shift reactions, the oxonium ylide may undergo other reactions. The oxonium ylide intermediate can be trapped by a protic nucleophile. Oku and co-workers have developed a method for ring expansion of cyclic ethers through oxonium ylide formation. Bicyclic oxonium ylide... 

Eight-membered ring expansion. Treatment of the cyclohexane derivative 1 (trans/cis = 9 1) with KH at room temperature results in the 14-membered ring trienone 2 in 90% yield. The reaction may involve a [5.5]vinylogous oxy-Cope rearrangement or two consecutive [3.3]sigmatropic rearrangements as shown. 

Ring Expansion/Con traction via Sigmatropic Rearrangements - Cope Rearrangement... 

V. The Cope Rearrangement, the [1.3] Sigmatropic Shift, the Sommelet-Hauser Reaction, and Sulfur-Mediated Ring Expansions... 

V/101, and V/103, have been prepared in 80, 50, and 74 % yield, respectively, by [2.3] sigmatropic rearrangement of the ylides, see Scheme V/16. The ring expansion of a mixture of trans- and cis-l-ethyl-2-vinylthiolanium hexafluoro-phosphates (V/105) gave a mixture of three sulfides, (Z)-2-methyl-thiacyclooct-4-ene (V/106) and the two diastereoisomerically related ( )-(SR,RS)- and ( )-(RR,SS)-2-methylthiacyclooct-4-enes (V/107) and (V/108)3 Scheme V/17. The existence of the two (E) isomers is evidence for the structure of the molecule, holding two elements of chirality, a chiral center, and a plane. The dia-stereoisomers are stabile because of restricted conformational inversion around the chiral plane [45] [60]. 

Ketene, dichloroketene, and related compounds react with allylic ethers, sulfides, and selenides in a [3.3] sigmatropic rearrangement. This interesting reaction allows the synthesis of a number of medium and large ring compounds by expansion [72] [73] [74]. Dichloroketene has been prepared in situ by slow addi-... 

Vedejs has iso discussed the inefficiency of copper catalysts in ylidic 3,2-sigmatropic processes. Optimization of ring expansion reactions using diazomalonates were found to be unsatisfactory and analogous reactions using diazoketones were totally unsuccessful. Takano has taken advantage of a rhodium-promoted ylide formation followed by a 3,2-rearrangement, in a useful synthesis of y,8-unsaturated carbonyl compounds (Scheme 45). ... 

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