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

Abstract

(3 R,4 R)- ( 1a) and (3 S,4 R)-tetrahydropyran-3,4-diol ( 2a), and (2 R,3 S)- ( 3a) and (2 R,3 R)-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 [Mo 2(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.