engleski

Chiroptical properties of tetrahydropyran-3, 4-diols and 2-hydroxymethyltetrahydropyran-3-ols derived from L-arabinose, D-galactose, D-glucose, and D-xylose, and enantioselectivity in reduction with their complexes

(3R, 4R)-(1a) and (3S, 4R)-tetrahydropyran-3, 4-diol (2a), and (2R, 3S)- (3a) and (2R, 3R)-2-hydroxymethyltetrahydropyran-3-ol (4a), derived from D-xylose, L-arabinose, D-glucose, and D-galactose, respectively, are structurally the simplest chiral carbohydrate-type precursors for bidentate ligands. The c.d. spectra of bidentate complexes between these diols and [Mo2(OAc)4], as well as of the benzoates (1b– 4b) and tosylates (1c– 4c), and the copper(I) complexes (1d– 4d) of the diphenylphosphinites (1e– 4e) are discussed. The enantioselective reduction of acetophenone to S(R)-1-phenylethanol with the complexes (10 and 11, respectively) of the trans compounds 1a and 3a with lithium aluminium hydride has been studied. Low enantiomeric excess (15%) was obtained, which was enhanced when an achiral modifier (ethanol, 2-propanol) was added to the complexes 10 and 11 prepared in situ. Enantioselective catalytic hydrogenation of Z-α -acetamidocinnamic acid was performed with the Rh(I)-norbornadiene complexes 2f and 4f, derived from the cis compounds 2a and 4a ; substantially lower enantiomeric excess (<0%) of S(R)-N-acetylphenylalanine was achieved than with the analogous complexes 1f and 3f (90%). The results of the enantioselective reductions are discussed in the light of the conformational properties of 1a– 4a and their congeners deduced from the c.d. spectra.

tetrahydropyran-3 ; 4-diols ; 2-hydroxymethyltetrahydropyran-3-ols

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