Prostaglandins and prostaglandin intermediates. Part 26. A novel route to PGF2? using triisopropoxy-hept-1-ynyl-titanium as precursor for the ?-side chain

The benzoylated Corey aldehyde 1a can be alkynylated with hept-1-ynyl-triisopropoxytitanium in good chemical yield and high diastereoselectivity to the (R)-alcohol 3c admixed with about 10% of the diastereomeric (S)-alcohol 2c. Without removal of this admixture the (R)-alcohol 3c was converted into PGF2α, which was easily separated from the accompanying 15-epi-PGF2α. The main steps of this reaction sequence involve the conversion of a propargylic alcohol into an (E)-allylic alcohol and the palladium (II)-catalyzed allylic rearrangement of its acetate. With hept-1-ynyllithium the trimethylsilyloxy aldehyde 1b affords the (S)-alcohol 2c as the predominating product admixed with about 20% of the diastereomeric (R)-alcohol 3c. Again, without removal of the admixture the (S)-alcohol 2c was converted into known PGF2α intermediates via the palladium(II)-catalyzed allylic rearrangement of the corresponding (Z)-allyl alcohol.