Syntheses and Reactions of 2,2‐Disubstituted Cyclopentane‐1,3‐diones, 6. Conversion of 2‐Methylcyclopentane‐1,3‐dione into 5‐Methylcycloheptane‐1,4‐dione via 3‐(1‐Methyl‐2,5‐dioxocyclopentyl)propanal

In contrast to earlier reported unsuccessful experiments the Michael addition of prop‐2‐enal to 2‐methylcyclopentane‐1,3‐dione (1) affords 3‐(1‐methyl‐2,5‐dioxocyclopentyl)propanal (3) in 90% yield, if the reaction is carried out in water without a basic catalyst. This aldehyde is converted by an intramolecular aldol reaction to (±)‐endo‐2‐ and (±)‐exo‐2‐hydroxy‐5‐methylbicyclo[3.2.1]octane‐6,8‐dione (rac‐4 and rac‐6), both affording (±)‐5‐methylcycloheptane‐1,4‐dione (rac‐8) after oxidation to (±)‐5‐methylbicyclo[3.2.1]octane‐2,6,8‐trione (rac‐2), hydrolytic ring‐opening and decarboxylation. This reaction sequence allows the transformation of the 2‐alkylated cyclopentane‐1,3‐dione 1 into the 5‐alkylated cycloheptane‐1,4‐dione rac‐8 in a good overall yield. The pathway of the ringopening process has been determined by an investigation of the reaction intermediates.