Enantioselective Synthesis of (−)-Terpestacin and Structural Revision of Siccanol Using Catalytic Stereoselective Fragment Couplings and Macrocyclizations

(−)-Terpestacin (1, naturally occurring enantiomer) and (+)-11-epi-terpestacin (2) were prepared using catalyst-controlled, stereoselective, intermolecular reductive coupling reactions of alkyne 9 and aldehyde 10, affording allylic alcohols 42 or 11-epi-42 in a 3:1 ratio (or 1:3 depending on the enantiomer of ligand 41a used). These stereoselective fragment couplings were instrumental in confirming that “siccanol” is not 11-epi-terpestacin but, in fact, is (−)-terpestacin itself. Several intramolecular alkyne−aldehyde reductive coupling approaches to 1 and 2 were also investigated and are discussed herein.