Reactions Catalyzed by Titanium and Zirconium IV Complexes

This enantioselective preparation of allylic alcohols has been applied to the synthesis of the side chain of prostaglandins [297]. The addition to functionalized aldehydes, such as 483 allows the synthesis of C2-symmetrical 1,4-diols, such as 484 with excellent diastereoselectivity and enantioselectivity [93b, 298]. An extension of this method allows the synthesis of Cj-symmetrical diol [299]. Aldol-type products result from the catalytic enantioselective addition of functionalized dialkylzincs to 3-TIPSO-substituted aldehydes, such as 485, followed by a protec- 

Further applications to the preparation of chiral polyoxygenated molecules ]302] and to the synthesis of the natural product (-)-mucocin [303] have been reported. The enantioselective addition of the functionalized diorganozinc reagent 

491 to dodecanal in the presence of Ti(Oi-Pr)4 and the chiral ditriflamide 476 provides the desired chiral alcohol 492 with 99% of diastereomeric excess and 65% yield. The diorganozinc reagent was obtained by treating the alkyl iodide 493 with an excess of EtjZn in the presence of CuCN (3 mol%). The chiral building block 

492 was used in the total synthesis of cydoviracin B1 of interest for its selective antiviral activity [304]. An elegant synthesis of (R)-(-)-muscone has been reported by Oppolzer et al. [305]. The alkenylzinc reagent was prepared by hydroboration of the acetylenic aldehyde 494 with (c-Hex)2BH at 0 °C, followed by a B/Zn-exchange with EtjZn. The intramolecular addition in the presence of the chiral amino-alcohol 495 provides the allylic alcohol 496 with 72% yield and 92% ee [104c,305]. The enantioselective addition of dialkylzincs to aromatic ketones is espedally difficult. Yus et al. have developed a new chiral ligand 497 allowing the addition of dialkyl- 

Reactions of Zinc Organometallics Catalyzed by Cobalt, Iron or Manganese Complexes 

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