The thiopyran route to polypropionates : sequential enantiotopic group selective enolization of meso 1,9-diketones

Meso 1,9-diketones (six to seven stereocenters)are readily obtained by stepwise or simultaneous two-directional aldol reactions of tetrahydro-4H-thiopyran-4-one with a thiopyran-derived aldehyde. Enantioselective enolizations of these diketones with the lithium amide from (R,R)-bis(1-phenylethyl)amine occur with simultaneous kinetic resolution to give the mono-TMS enol ethers in >90% yields based on recovered starting material (BORSM) and >90% ee. The developed methodology was applied in synthetic studies towards the asymmetric synthesis of denticulatin A