No abstract

We have unambiguously demonstrated the binding of the silver(I) ion in the central cavity of [7]-helicene, given that the counterion is weakly coordinating. In such a binding mode, the helicene is functioning as a chiral molecular tweezer.

A rapid route to diversely functionalized 1-aza[6]helicenes has been achieved via the development of a copper mediated cross-coupling reaction, followed by PtCl 4 -catalyzed cycloisomerization. Not only does this method allow access to these functionally important molecules on gram scale, but this strategy is also suitable for relaying the axial chirality of a key intermediate to the helicity of the product.Interest in the inherently chiral aromatics known as the helicenes continues to expand due to their fascinating helical, and therefore chiral, topology coupled with their fully conjugated aromatic structure. 1 These chiral aromatics have promise to have wide-ranging impact, with preliminary studies already reported in the fields of catalysis, 2-7 non-linear optics, 8 electrooptical switches 9 and molecular recognition 10-13 amongst others. Azahelicenes (such as 4) have been a particularly exciting target class of helicenes recently for chemical synthesis, [1][2][3]14 exhibiting fascinating coordination chemistry, [13][14][15][16] self-assembly potential 17 and interesting photophysics. 18 We have recently demonstrated the high potential of this class in materials science, using enantiopure dopant quantities of 1-aza[6]helicene (4) to induce circularly polarized (CP) electroluminescence from an achiral light emitting polymer, 19 and fabricating organic phototransistors based on 4 that can reversibly detect CP light. 20 Furthermore, there has been much interest in exploiting the chiral scaffold of azahelicenes in asymmetric organocatalysis (Figure 1). Takenaka and coworkers have reported 1-azahelicene derivatives 1 and 2 as helical organocatalysts 2a-d for enantioselective ring opening of meso-epoxides, the addition of dihydroindoles to nitroalkenes, and the propargylation of aldehydes with allenyltrichlorosilane. Starý, Stará, and co-workers also used 1-aza-(4) and 2-aza[6]helicene (5) as organocatalysts in the asymmetric acyl transfer reactions of rac-1-phenylethanol. 2e Similarly, kinetic resolution chemistry has been reported by Carbery and co-workers using a helicenoidal DMAP analog 3, with good to excellent levels of selectivity (S ≤ 116).

Starting from Dane’s diene and methylcyclopentenedione, (+)-estrone is synthesized along the Quinkert−Dane route in 24% total yield. The key step is an enantioselective Diels−Alder reaction promoted by an amidinium catalyst as efficiently as by a traditional Ti-TADDOLate Lewis acid.

Chiroptical spectroscopy, such as electronic circular dichroism (ECD) and vibrational circular dichroism (VCD) are highly sensitive techniques to probe molecular conformation, configuration, solvation and aggregation. Here we report the application of these techniques to study the fungal metabolite brevianamide B. Comparison of the experimental ECD and VCD spectra with the density functional theory (DFT) simulated counterparts establishes that VCD is the more reliable technique to assign absolute configuration due to the larger functional and dispersion dependence of computed ECD spectra. Despite a low amount of available material, and a relatively unusual example of using VCD carbonyl multiplets, the absolute configuration could be reliably predicted, strengthening the case for application of 2 VCD in the study of complex natural products. Spectral and crystallographic evidence for or against the formation of a dimeric aggregate is discussed; in solution the VCD spectra strongly suggest only monomeric species are present.