from allene to spirane

Diverse Chiral Scaffolds from Diethynylspiranes: All-Carbon Double Helices and Flexible Shape-Persistent Macrocycles. State-of-the-art chiroptical spectroscopies are valuable tools for structural elucidation. However, the potential of these spectroscopies for everyday applications has not been exploited to date partially due to the lack of sufficiently stable and efficient chiroptical systems. To this end, the development of suitable chiroptical structures is essential. Herein, we present the synthesis of spiro-compounds (P2)-1 and (P4)-2 as well as (M2)-1 and (M4)-2 exhibiting remarkable chiroptical responses. Theoretical simulations show that (P2)-1, constituted by two (P)-configured spiranic chiral axes, presents an all-carbon double helix structure with (M)-helicity. On the other hand, molecular dynamic simulations reveal (P4)-2 to have a single path for geometry-modification along its flat conformational space, certifying it as a chiral flexible shape-persistent macrocycle. Geometric quantification of chirality has been used to compare the spiranic derivatives presented herein.

Opening Access to New Chiral Macrocycles: from Allenes to Spiranes. Chiral macrocycles offer great potential and versatility regarding their applications. The have been emplyed in  asymmetric catalysts, as chiral sensors and chiral supramolecular frameworks. For these reasons they have been attracting increasing interest over the years. Despite all the work developed in this area, most of the reported chiral macrocycles are not conformationally stable and present weak chiroptical responses. Such features substantially limit the scope of applications for these compounds. On the other hand, we have shown that axially chiral allenes can be introduced into macrocycles, conferring conformational stability and outstanding chiroptical responses. However, these allenes photoisomerize when conjugated with electron-donating groups, hampering the possibility of synthesizing systems with tuned optical properties. To overcome all these limitations with a single structural motif, we propose the use of spiranes to construct new stable, conformationally rigid and chemically functionalizable macrocyclic structures with strong chiroptical responses. As a first step in this new direction, we theoretically predict the chiroptical responses for macrocycles bearing spiranes to be as strong as with their allenic counterparts. As a side product we also test the popular Minnesota functional, M06-2X, and compared it with cam-B3LYP, which has been previously analyzed with respect to experimental data in our laboratory. Thus, we hereby propose that spiranes are a good alternative to allenes for the construction of new chiral macrocycles.

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