β‐Cleavage of Bis(homoallylic) Potassium Alkoxides. Two‐Step Preparation of Propenyl Ketones from Carboxylic Esters. Synthesis of ar ‐Turmerone, α‐Damascone, β‐Damascone, and β‐Damascenone

The transformation of 36 bis(homoallylic) alcohols VII to alkenones IX and X via β‐cleavage of their potassium alkoxides VIIa in HMPA has been investigated ( cf. Scheme 2 ). These studies have established an order of β‐cleavage for 2‐propenyl, 1‐methyl‐2propenyl, 2‐methyl‐2‐propenyl, 1,1‐dimethyl‐2propenyl, and benzyl groups in alkoxides 49a – 56a and have allowed a comparison between the β‐cleavege reaction and the oxy‐ Cope rearrangement in alkoxides 74a – 83a . As illustrative syntheti applications, a two‐step preparatio of propenyl ketones 15 – 42 from carboxylic esters is described, together with syntheses of ar ‐turmerone ( 48 ), α‐damascone (( E )‐ 71 ), β‐damascone (( E )‐ 109 ), and β‐damascenone (( E )‐ 111 ).