Absolute Configuration Determination from Low ee Compounds by the Crystalline Sponge Method. Unusual Conglomerate Formation in a Pre-Determined Crystalline Lattice.

When chiral compounds with low enantiomeric excess (ee, R:S=m:n) were absorbed into the void of the crystalline sponge (CS), enantiomerically pure [(R)m (S)n ] chiral composites were formed, changing the centrosymmetric space group into non-centrosymmetric one. The absolute configuration of the analyte compounds was elucidated with a reasonable Flack (Parsons) parameter value. This phenomenon is characteristic to the "post-crystallization" in the pre-determined CS crystalline lattice, seldom found in common crystallization where the crystalline lattice is defined by an analyte itself. The results highlight the potential of the CS method for absolute configuration determination of low ee samples, an often encountered situation in asymmetric synthesis studies.

Stepwise helicity inversions by multisequential metal exchange.

Development of artificial helical molecules that can undergo responsive helicity inversion has been a challenging research target in functional molecular chemistry. However, most reported helicity inversions are based on a single-mode transition, i.e., the conversion between right- and left-handed states. We report here the first molecular system that allows stepwise multisequential helicity inversion utilizing metal exchange of helical complexes derived from a hexaoxime ligand, H6L(1). The ligand H6L(1) underwent a four-step conversion (H6L(1) → L(1)Zn3 → L(1)Zn5 → L(1)Zn3Ba → L(1)Zn3La) upon sequential metal addition (Zn(2+), Ba(2+), then La(3+)). Associated with the conversion, three-step helicity inversion took place (L(1)Zn3, right-handed → L(1)Zn5, left-handed → L(1)Zn3Ba, right-handed → L(1)Zn3La, left-handed). This is the first example of stepwise multimode helicity inversion of a discrete molecule, which could be useful as a platform for construction of dynamic regulation systems with multiple asymmetric functions.

Response speed control of helicity inversion based on a "regulatory enzyme"-like strategy.

In biological systems, there are many signal transduction cascades in which a chemical signal is transferred as a series of chemical events. Such successive reaction systems are advantageous because the efficiency of the functions can be finely controlled by regulatory enzymes at an earlier stage. However, most of artificial responsive molecules developed so far rely on single-step conversion, whose response speeds have been difficult to be controlled by external stimuli. In this context, developing artificial conversion systems that have a regulation step similar to the regulatory enzymes has been anticipated. Here we report a novel artificial two-step structural conversion system in which the response speed can be controlled based on a regulatory enzyme-like strategy. In this system, addition of fluoride ion caused desilylation of the siloxycarboxylate ion attached to a helical complex, resulting in the subsequent helicity inversion. The response speeds of the helicity inversion depended on the reactivity of the siloxycarboxylate ions; when a less-reactive siloxycarboxylate ion was used, the helicity inversion rate was governed by the desilylation rate. This is the first artificial responsive molecule in which the overall response speed can be controlled at the regulation step separated from the function step.

Enantioselective synthesis of tetrahydrocyclopenta[b]indole bearing a chiral quaternary carbon center via Pd(ii)-SPRIX-catalyzed C-H activation.

The highly enantioselective cyclization of 3-alkenylindole via C-H activation has been established using Pd(OCOCF3)2 in conjunction with the chiral spiro bis(isoxazoline) ligand (SPRIX). The presence of an N-allyl substituent on the substrate has a strong impact on both reactivity and selectivity, leading to tricyclic indole products (up to 96% ee) with a chiral quaternary carbon center.

Determination of the absolute configuration of compounds bearing chiral quaternary carbon centers using the crystalline sponge method.

Determination of the absolute configuration of chiral tetra-substituted carbon centers is one of the most taxing steps in the enantioselective construction of this structural motif in asymmetric synthesis. Here, we demonstrate that the crystalline sponge method provides an effective way to crystallographically determine the absolute configuration of organic compounds bearing chiral quaternary carbons (including tetra-substituted ones) that are synthesized by recently developed enantioselective catalytic reactions.

Lanthanide contraction for helicity fine-tuning and helix-winding control of single-helical metal complexes.

Lanthanide contraction was used for helicity fine-tuning and helix winding control of single-helical tetranuclear complexes LZn3Ln (Ln = La-Lu); heavier lanthanides formed a tighter helix with a higher M/P ratio and gave a more abundant partially-coiled structure, resulting in 14-step structural control of the helical complexes.