Asymmetric Carbonyl Olefinations Without Usage of Optically Active Phosphorus Reagents

In an analogous manner to the route to the THF derivatives described above, the chiral THF derivatives 170 and 171 were successfully prepared from the meso-dialdehyde 69b by asymmetric HWE reaction followed by Pd-catalyzed intramolecular cyclization [98]. 

A change of the metal counterion to Sn + or Zn was quite effective to achieve both high diastereoselectivity and good chemical yield in these interesting asymmetric transformations. 

Asymmetric Carbonyl Olefinations Without Usage of Optically Active Phosphorus Reagents 

Recently, asymmetric induction mediated by external chiral ligands that are not covalently bonded to the reagent has attracted much attention, and it is believed that the information obtained from these studies will prove useful in developing a novel system for efficient catalytic asymmetric transformation. In order to explore the possibilities, a variety of reaction systems capable of effective asymmetric induction at a specific site in the course of a reaction have been devised, and several investigations have also been directed towards Wittig-type olefination. An early study using an optically active organic acid with stabilized ylides [22] was unfruitful, as discussed in the introductory section. 

When the achiral phosphonate 182 was reacted with the ketone la in the presence of Sn(II) triflate and N-ethylpiperidine, the chiral diamine 183 was shown to act as a good asymmetric inducer in generating the tetrasubstituted dissymmetric alkene 184 with a high level of enantiomeric excess [35j, 102]. In order to achieve high levels of asymmetric induction, stoichiometric amounts of ligands in relation to the external chiral source were required in all of the aforementioned asymmetric carbonyl olefinations. 

---