A new route to diastereoselective synthesis of (±) trans cognac lactone in five steps is described. The key intermediate methyl 3 formylbutanoate(4) was prepared from methyl crotonate(1) in high yield. Addition of n amylmagnesium bromide/cuprous iodide to the formyl of(4), through formation of seven membered ring chelate which involved both carbonyl function groups of (4) and MgX or Cu as a central metal, afforded (±) trans cognac lactone(5) in 52% yield with high diastereoselectivity: ratio of trans to cis isomers was 973 by GC. The procedure had advantages of simplicity, high stereoselectivity of products and mild reaction conditions.
