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 ).