Convenient Preparation of Allylic and Vinylic Ketones from Carboxylic Esters and Grignard Reagents. Synthesis of ar-Turmerone

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A method was established for the synthesis of long chain ketones by reactions of 3, 4-unsaturated carboxylic esters with alkyl and alkenyl Grignard reagents. Grignard reaction of ethyl (E) -3-hexenoate (1b) with n-pentylmagnesium bromide (2a) gave 3-dodecen-6-one (3b) and 6-n-pentyl-3-dodecen-6-ol (5b). Subsequent to reactions in the presence of Et3N at 253K, greater ketone selectivity was noted. Reactions of other 3, 4-unsaturated carboxylic esters (1b-d) and acids (1a'-c') with Grignard reagents (2a-c) gave ketones (3b, c, d, f) in good yields under the same conditions. Dialkylmonohomoallyl alcohols (5b, f-i), produced as by-products, readily underwent conversion to symmetrical ketones (6b, f-i) by thermolysis. With trihomoallyl alcohols (5q, r, t), diallylic ketones (3q, r, t) were obtained. The ketones quickly underwent isomerization to conjugated ketones (7q, t) by treatment with acid catalysts. By these reactions, various unsaturated ketones including ar-turmerone (8h) could be prepared selectively from 1, 4-addition reaction of conjugated ketones with Grignard reagents.