Crams Chiral Binaphthyl Systems

A large number of chiral crowns have been prepared by numerous groups. The reader is directed to the tables at the end of this chapter to obtain an overview of these structures. It would not be useful to try to recount the synthetic approaches used in the preparation of all of these compounds we have chosen rather to subdivide this mass of compounds into three principal groups. The groups are (1) Cram s chiral binaphthyl systems (2) chiral crowns based on the tartaric acid unit and (3) crowns incorporating sugar subunits. These are discussed in turn, below. 

Cram and his coworkers have pioneered the use of bis-binaphthyl crowns as chiral com-plexing agents for ammonium salts and amino acid salts. In these systems, the chiral binaphthyl unit provides a steric barrier within the macrocycle which allows discrimina- 

In Cram s first synthesis of a chiral bis-binaphthyl system, optically pure binaph-thol and diethylene glycol ditosylate were heated at reflux in tetrahydrofuran solution for 15 h with potassium f-butoxide, two products were obtained. The 1 + 1 product (mp 230—231°) was isolated in 5% and the 2 + 2 product (mp 123—126°) was obtained in 31% yield. The reaction is shown in Eq. (3.51). 

When phenolic nucleophiles were used, either potassium hydroxide or potassium f-butoxide was generally chosen as the base. When aliphatic hydroxyls constituted the nucleophiles, a stronger base was required and sodium hydride was generally chosen. 

An example of this is found in the synthesis of a crown containing a mefa-xylylene unit (see also Sect. 3.5) and a convergent methoxyl group. 4-Methyl-2,6-bishydroxymethyl-anisole was treated with pentaethylene glycol and sodium hydride in THF solution to afford the crown (mp 71—73°) shown in Eq. (3.52) in 59% yield. 

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The structural variations reported by Cram and coworkers relate to an appreciable extent to the various ancillary functions which have been appended to the binaphthyl units or elsewhere in the macrocyclic system. Enhancements of the chiral barrier or functionalization through arms has generally been effected at the 3-or 6-positions. These positions are adjacent to the hydroxyl group or directly across the second ring from it, respectively. 

If a bridged bis-crown is used instead of diaza-18-crown-6, the cryptand contains two macrorings facing each other (see Table 8.5). Note also that the 2,2 -binaphthyl unit has been used extensively by Cram and his coworkers to provide chirality to mono-cyclic systems as well (see Sect. 3.13). 

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