Attempts using Directed Syntheses

As seen above, the randomness of the Mobius strip approach and numerous difficult steps in Schill s directed approach were highly limiting factors in a trefoil knot synthesis. Both these major obstacles might be circumvented by the use of an unambiguous templated synthesis procedure. 

---

Interest in optically active polymers arose from analogy with macromolecules of biological origin. In addition, there was the hope to obtain new information to clarify the stereochemical features of synthetic polymers this, in fact, did come about. Attempts to direct the course of polymerization using chiral reagents had been made already prior to the discovery of stereospecific polymerization. It was only after the 1950s, however, that the problem of polymer chirality was tackled in a rational way. The topic has been reviewed by several authors (251-257). In this section I shall try to illustrate three distinct aspects the prediction of chirality in macromolecular systems, the problems regarding the synthesis of optically active polymers, and polymer behavior in solution. 

An analogous synthesis of acromelic acid B 6 was also attempted using enamide 185. C-2 methyl ester reduction and hydroxyl-directed enamide hydrogenation proceeded smoothly giving a 10 1 mixture of C-4 epimeric catechols, 197 and 198 (Scheme 74). 

Shapley (26) reported a direct synthesis of mono- and disubstituted phosphorus ligand derivatives of Ir (CO)j2. Jfe attempted to apply their method using the attached ligand P)-PPh2 instead of PPh3, keeping all the synthesis conditions constant. The desired formation of attached tetrairidium species did not occur. But when the methoxyethanol solvent was replaced by a mixture of... 

The synthesis of optically active cyclohex-2-enone oxide (55) from cyclohex-2-enone has been reported. The epoxidizing medium contained a chiral catalyst (quininium benzyl chloride) with Bu OOH and a small amount of solid NaOH in toluene. This heterogeneous mixture avoided the use of the strongly alkaline aqueous phase which may have been responsible for the failure of earlier attempts at direct, chiral synthesis of (55). Chemical yields of 60% were obtained, with an e.e. of 20 3%. 

---