Base-Promoted Ring Contractionof Eight-Membered Cyclic Acyloins and their Ethers to C _s-Symmetric α-Ketols

Abstract

In the presence of potassium hexamethyldisilazide and 18-crown-6in THF at room temperature, 2-benzyloxycyclo­octanone andits Δ⁵-unsaturated congener undergo a two-stagere­arrangement to give symmetrical seven-membered ringproducts. The pathway consists of the sequential operation of anO → C 1,2-shift and an α-ketol rearrangement.The greater thermodynamic stability of the cycloheptane or cycloheptenegenerated in this fashion is substantiated by MM3 calculations.

Key words

ring contractions - ketones - rearrangements - carbocycles - isomerizations

References

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Several by-products are also generatedunder these longer-term conditions. These proved to be difficultto separate and have not been characterized.