Additions of Silyl Enol Ethers to β‐Formyl Esters – a Chelate‐controlled Synthesis of the Pheromone (+)‐Eldanolide

Chelate‐controlled Mukaiyama reaction of silyl enol ethers 2a, 2b, 2c, and 2d with β‐formylcarboxylate 1 in the presence of TiCl4 provided γ‐lactones 3a, 3b, 3c, and 3d in very good yields and with good to excellent trans selectivities. The use of BF3, SnCl4, and SnBr4 as Lewis acids afforded 3a with moderate cis selectivity thus excluding chelate control. Starting with (S)‐1 TiCl4‐promoted reaction with 2b, we obtained optically active γ‐lactone 3b which was efficiently converted into the pheromone (+)‐eldanolide in two steps. The enantiomeric excess of 87% proved that the sequence proceeds without detectable racemization.