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

Abstract

Abstract 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 ).