Helical Chirality Using Achiral Building Blocks

Helical Chirality Using Achiral Building Blocks 

One of the simplest ways to access chiral macrocydes involves the use of achiral building blocks and takes advantage of molecular entanglements to bias the cyclic core such that a helical conformation is preferred. An early example of a carbon-rich macrocycle possessing helical chirality is racemic compound 1, prepared by Staab and coworkers in 1972 (Fig. 6.2) [6]. The synthesis of 1 was inspired by an interest in the intramolecular interactions of transatmular triple bonds. Crystallographic analysis later confirmed the hdical structure of 1 (Fig. 6.2) and the distance between the centers of the triple bonds was measured to be 2.851 A [7]. 

Youngs and coworkers reported the synthesis of a hexa-benzarmulene in the early 1990s [8]. Annulene 2 was prepared in low yield via the Castro-Stephens coupling of ortho-iodophenylacetylene (Fig. 6.3). X-ray crystallographic analysis confirmed the helical conformation of 2 in the solid-state and showed that two of the diphenylacetylene moieties are nearly edipsed in this somewhat strained, distorted-boat conformation. 

Using a similar synthetic protocol, Fallis and coworkers also prepared cydophane 25 (Fig. 6.5) [14]. Crystallographic analysis of this macrocyde confirmed that 25 adopted an essentially unstrained, helical conformation in the solid-state. 

The same authors also targeted the synthesis of cyclophanes 26-29 that, due to the presence of long alkyl side chains, were expected to possess liquid crystalline properties [13]. Macrocycles 26-29 were isolated in slightly higher yields than 24 and 25, presumably the result of the introduction of the solubilizing side chains. The helical racemic cyclophanes 26-29 were evaluated for liquid crystalline behavior, but these analyses were inconclusive due to the fact that all compounds were a mixture of regioisomers. 

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Helical Chirality Using Achiral Building Blocks... 

The observed chirality in helicates can arise from the inherent asymmetric spiral arrangement of the molecular components, even if the building blocks are achiral or alternatively, it may result from the use of chiral bridging agents so that the chirality is fiansferred to the coordinated... 

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