Stereoselective syntheses of (+)-isoretronecanol and (+)-5-epi-tashiromine via addition of chiral titanium (IV) enolates to cyclic N-acyliminium ions

The stereoselective addition of the titanium (IV) enolates derived from (S)-4-isopropyl-N-4-chlorobutyryl-1,3-thiazolidine-2-thione (8) and from (S)-4-isopropyl-N-4-chloropentanoyl-1,3-thiazolidine-2-thione ( 9) to N-Boc-2-methoxypyrrolidine (5b) afforded the addition products (+)-10 and (+)-11 in 84% yield in both cases, as 8.6:1 and 10:1 diastereoisomeric mixtures, respectively. A threestep sequence allowed to convert these adducts to (+)- isoretronecanol (1) and (+)- 5-epi-tashiromine(2) in 43% and 49% overall yield, respectively.31477177