Homochiral macrocycles

Fig. 8. Self-assembly of H-bonded superstructures from enantiomeric components 12 A heterochiral strand B homochiral macrocycle C observed homochiral assembly...

Cyclodextrins (CDs) are naturally occurring homochiral macrocycles composed of a-l,4-linked o-glucopyranose in the Ci conformation, and as a class constitute one of the most widely used molecular components in supramolecular chemistry. The CDs were first reported by Villiers in 1891 and were produced from starch by the action of CD transferases produced by bacteria exemplified by Bacillus macerans and Bacillus circulans. Genetically engineered CD transferases of higher activity are now widely used in industrial CD production which amounts to thousands of tonnes annnally. The smallest of these native CDs are a-, fi-, and y-CD, which consist of 6, 7, and 8 a-l,4-linked o-glncopyranose units, respectively, and are the most... 

The amplification of two different macrocyclic receptors for similar diastereomeric compounds has been observed by Sanders and coworkers [13]. A dynamic system of macrocyclic polyhydrazones ranging from dimer to at least undecamer was prepared from the indicated homochiral hydra-zone. Templating with quinine caused the percentage of cyclic tetramer to increase from 63 % to 91 %. The same experiment with quinidine, a diastereomer of quinine, instead amplified the dimer from 9% to 45% (Fig. 5.13). These species were characterized by HPLC and ESI MS. Association... 

Heo J, Jeon Y-M, Mirkin CA. Reversible inteiconversion of homochiral triangular macrocycles and heUcal coordination polymers. J Am Chem Soc 2007 129 7712-7713. 

Fig. 22 a Skeletal representations of the four closed tetramers with alkyl substituents removed b space-filling models of the isopropyl-derived cube and the more open square-capped macrocycle both homochiral and obtained from DFT calculations... 

As previously noted, discrete assemblies based on 1-dimensional H-bonded motifs, also termed a-networks, may be obtained by covalently tethering the H-bonding recognition arrays or by their fusion to yield Janus molecules (Fig. 2). The information manifested in the relative orientation (120°) and juxtaposition of H-bonding donor/acceptor sites (DAD/ADA), in the specific case of 5 and 6, may generate three types of association a linear ribbon or tape, a H-bonded macrocycle and an intermediate wavy ribbon or crinkled tape [46]. These structures have been characterized in the solid state by X-ray crystallographic analysis. Homochiral crinkled tapes are formed in the course... 

In a continuation of the study on chiral tetraphenylenes, Wong reported a series of rod-like, D2-symmetric, enantiopure molecules and chiral hehcal macrocycles based on metal coordination with homochiral tetraphenylenes [48, 51]. From optically pure tetrahydroxytetraphenylene 98, two useful chiral budding blocks, namely, l,8,9,16-tetrakis(diphenylphosphino)tetraphenylene (104) and 1,8,9,16-tetraethynyltetraphenylene (105), were synthesized (Scheme 5.24). 

Rather than relying upon self-assembly processes, it is also possible to synthesize metaldirect formation of metal-carbon bonds. Tilley and Schafer have reported a unique, diastereoselective synthesis of zirconocene-containing metallamacrocycles [82]. As outlined in Scheme 6.30, treatment of racemic BINOL-based diyne 234 with Negishi-type zirconocene coupling conditions led to the diastereoselective formation of only two enantiomeric stereoisomers of metallamacrocycle (R,R)- and (S,S)-235 in an excellent 91% yield. Use of (R)-234 gave the expected homochiral enantiopure macrocycle (R,R)-235, confirming the diastereoselectivity of this process. 

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