Asymmetric alkylation of the 3,4-dimethyl-5-phenyl-2,2'-biphospholyl anion with the (2R,4R)-(-)-pentaneditosylate leads to a new chirally flexible 2,2'-biphosphole ligand as a mixture of three diasteroisomers. By complexation with Pd(II), a chirality control occurs to afford enantiopure Pd complex.

No abstract

The stereochemically dynamic 2,2′-biphospholes 8, 9, and 10 have been synthesized by asymmetric alkylation of 2,2′-biphospholyl anion with enantiomerically pure diol ditosylates. A good carbon-to-phosphorus induction has been obtained in particular in the case of compound 8. The introduction of a chiral linker between the two phosphorus atoms resulted in partial chirality control of the central and axial chiralities of the 2,2′-biphosphole framework, as three diastereoisomers a, b, and c are obtained among the six expected. In solution at room temperature a, b, and c diastereoisomers exist as an equilibrium mixture; the interconversion occurred by phosphorus-inversion inducing atropoinversion. Reaction of the equilibrium mixture of 2,2′-biphosphole with [MCl 2 (CH 3 CN) 2 ] resulted in dynamic resolution leading to diastereo-and enantiopure Pd-and Pt-complexes. Palladium-catalyzed asymmetric allylic substitution can be achieved with the ligands 8 and 10 with high activities but still moderate enantioselectivities.

The structure of the Escherichia coli O-antigen 6 has been investigated using n.m.r. spectroscopy, methylation analysis, and various specific degradations. It is concluded that the O-antigen is composed of pentasaccharide repeating-units having the following structure. (Formula: see text)

No abstract

No abstract

No abstract

No abstract