Synthesis of the spirosesquiterpene (−)-acorenone and related cyclopentanoid monoterpenes

The first total synthesis of the spirosesquiterpene (−)-acorenone (1) is described. The critical step involves a spiroannelation of the enamine of aldehyde 11 with 1-methoxy-3-buten-2-one (16) in the presence of acetic acid to give ketone 17. Reaction of 17 with methylmagnesium iodide followed by treatment of the resultant alcohol 18 with p-toluenesulfonic acid in refluxing benzene leads directly to 1 in good overall yield. As the starting material for the preparation of the cyclopentanoid aldehyde 11 was (+)-limonene the final product is optically active and thus the absolute configuration of naturally occurring (−)-acorenone (1) is established. The preparation of a number of other optically active cyclopentanoid monoterpenes (4 and 13) is also described.