Steric Control Based on Alkyl Substituents in the (2,3) Sigmatropic Rearrangement of Nine‐Membered Allylsulfonium Ylides. A New Entry to the Stereoselective Synthesis of Elemane‐Type Sesquiterpenoids.

The rhodium(II)-catalysed cyclisation of acyclic α-diazomalonate 1b and α-diazo-β-keto esters 1c, d give stereoselectively the highly substituted δ-lactone 3b and cyclohexanones 3c, d, respectively, by [2,3]sigmatropic rearrangement via the stereocontrolled nine-membered allylsulfonium ylides 2b–d.