Diastereoselective reductive ring expansion of spiroketal dihydropyranones to cis-fused bicyclic ethers.

A novel double cascade synthetic strategy was developed for the diastereoselective syntheses of cis-fused bicyclic ethers, featuring cascade Achmatowicz rearrangement/spiroketalization and cascade spiroketal reduction/oxa-Michael cyclization. Especially, the chemo-, regio-, and diastereoselective reduction of densely functionalized spiroketal dihydropyranones, followed by oxa-Michael cyclization in a one-pot fashion, was achieved.