Introduction of a New Class of Ligands for the Metal‐Catalyzed Enantioselective Synthesis

Abstract

Protected thiosugars were prepared as Iigdnds for the metal-catalyzed enantioselective synthesis. The protecting groups in these ligands were varied to test a proposed new concept for the metal-catalyzed enantioselective synthesis. This new concept centres on the use of a stair-like ligand with a large substituent on one side and a small substituent on the other rather than the commonly employed ligands which have C, symmetry (see Fig.3). In such a ligand, both substituents should have a major influence on the coordination of a prochiral substrate. To test this proposal, 3-thio-a -u-glucofuranose derivatives with the following substituents were synthesized: 1,2-O-isopropylidene-5,6-0-methylidene (see 24), 1,2:5,6-di-O-isopropylidene (see 2), 5,6-0-cyclohexylidene-l,2-0-isopropylidene (see 23), 1,2-0 -cyclohexylidene-5,6-O-isopropylidene (see 14), 1,2:5,6-di-O-cyclohexylidene (see 13), 5,6-0-(adamantan-2-y1idene)-1.2-0-isopropylidene (see 21), and 1,2:5,6-di-O-(adamantan-2-ylidene) (see 25, Table 2). As a representative of the allofuranoses, 1,2:5,6-di-O-isopropylidene-3-thio-a -D-allofuranose (6) was chosen. The following derivatives of 1,2-0 -isopropylidene-cc -D-XylOfUranOSe were also synthesized: 1,2-O-isopropylidene-5deoxy-3-thio-a-D-XylOfUranose (29), 1,2-0-isopropylidene-3-thio-a-D-xylofuranose (28) and 5-O-[(tert-butyl)diphenylsilyl]-1,2-O-isopropylidene-3-thio-a -D-xylofuranose (15, see Table 2). The proposed concept was tested using the copper-catalyzed I ,4-addition of BuMgCl to cyclohex-2-en-1-one. The enantioselectivity was very dependent on the ligand used and was up to 58 %.

Conformation of chiral alkoxyallenes by proton NMR spectroscopy

“…1,2-O-Isopropylidene-5-O-trityl--D-xylofuranose (4) was obtained according to the known procedure (72%). 16 5-Deoxy-1,2-O-isopropylidene-α-D-xylofuranose (5) was obtained from 1,2-O-isopropylidene-5-O-tosyl-α-D-xylofuranose 17 according to the literature procedure (78%). 18…”

Ligands Derived from Carbohydrates for Asymmetric Catalysis

“…The best ee's were obtained with glucofuranoside ligand 59. 36 In 1995, Collman and co-workers developed threitol-strapped prophyrins 63 for the Mn-catalyzed epoxidation of unfunctionalized alkenes. 37…”

Iridium complexes containing the first sugar dithioether ligands. Application as catalyst precursors in asymmetric hydrogenation

“…Moreover, sulfur ligands have been studied less than phosphorus ligands despite their advantages of lower cost, toxicity and oxidation. 4 To our knowledge, only Spescha 5 and recently our group 6 have reported the use of sugar-derivative ligands containing sulfur as a unique donor atom in asymmetric catalysis.…”