Abstract
1, 6-Addition of 2-methyl-1, 3-cyclopentanedione (8) to protoanemonin (7) gave 5-(2-methyl-1, 3-dioxo-2-cyclopentyl) methyl-2, 5H-furanone (6). The Grignard reaction of 6 with isopropenylmagnesium bromide-cuprous iodide provided two kinds of lactones, (10) and (11), in excellent yield through 1, 4-addition of the Grignard reagent to 6 and the subsequent internal aldol cyclization. These lactones, however, possessed the undesired stereostructures for the present synthesis and conversion of the lactone (10) into the desired lactone (4) was performed by retroaldol cleavage and subsequent internal aldol recyclization as shown in Chart 6.
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