Geertsema Edzard M, Schoevaars Anne Marie, Meetsma Auke, Feringa Ben L
Department of Organic and Molecular Inorganic Chemistry, Stratingh Institute, University of Groningen, Nijenborgh 4, 9747, AG Groningen, The Netherlands.
Org Biomol Chem. 2006 Nov 21;4(22):4101-12. doi: 10.1039/b609271c.
The concept of bisthioxanthylidene biscrown ethers as potential stereodivergent chiral ligands in asymmetric synthesis is introduced. Substituted bisthioxanthylidenes may be chiral and can exist as stable enantiomers due to their folded structure. As a result, both a right-handed helix (P) and left-handed helix (M) are present in this type of molecule. This offers the unique possibility to construct two crown ether moieties, attached to the same molecule, of which one exhibits (P)-helicity and the other (M)-helicity. When the crown ether moieties differ in size they can be complexed selectively with a base containing a cation of appropriate diameter. In this manner the (P)-helix and the (M)-helix can be activated selectively to serve as a chiral environment for base catalyzed asymmetric synthesis. Thus, we envisioned the new concept of a single chiral ligand to separately synthesize two enantiomers of a chiral product just by varying the added base. For this purpose, four new bisthioxanthylidene monocrown ethers and two new bisthioxanthylidene biscrown ethers were synthesized. Two biscrowns and two monocrowns were separated into their respective enantiomers (HPLC) and optical data (UV and CD) were collected to ensure stability of enantiomers at ambient temperatures. Ion complexation of one mono- and two biscrown ethers with potassium and sodium cations was investigated.
介绍了双噻吨叉双冠醚作为不对称合成中潜在的立体发散性手性配体的概念。取代的双噻吨叉可能是手性的，由于其折叠结构，可以作为稳定的对映体存在。因此，在这类分子中同时存在右手螺旋（P）和左手螺旋（M）。这提供了一种独特的可能性，即构建两个连接在同一分子上的冠醚部分，其中一个呈现（P）-螺旋性，另一个呈现（M）-螺旋性。当冠醚部分大小不同时，它们可以与含有适当直径阳离子的碱选择性地络合。通过这种方式，（P）-螺旋和（M）-螺旋可以被选择性地激活，作为碱催化不对称合成的手性环境。因此，我们设想了一种新的概念，即仅通过改变添加的碱，就可以用单一手性配体分别合成手性产物的两种对映体。为此，合成了四种新的双噻吨叉单冠醚和两种新的双噻吨叉双冠醚。通过高效液相色谱（HPLC）将两种双冠醚和两种单冠醚分离成各自的对映体，并收集光学数据（紫外和圆二色）以确保对映体在环境温度下的稳定性。研究了一种单冠醚和两种双冠醚与钾离子和钠离子的离子络合作用。
