Stereochemical features of the anomerizations in the 5,6-dihydrothymine nucleoside series

Abstract

The transformation of (5S)-1-(3,5-anhydro-2-deoxy-β-(1) or α-(4)-D-threo-pentofuranosyl)5,6-dihydrothymine by sodium iodide in acetic acid–butan-2-one gave the same mixture of (5S)1-(2,5-dideoxy-5-iodo-β(2) and α-(3)-D-threo-pentofuranosyl)-5,6-dihydrothymine in a ratio of 1 : 2. The intramolecular cyclization of the anomers (2) and (3) by silver acetate in methanol afforded the respective (5S)-1-(3,5-anhydro-2-deoxy-β-(1) and α-(4)-D-threo-pentofuranosyl)5,6-dihydrothymine. The reductive dehalogenation of (2) and (3) led to (5S)-1-(2,5-dideoxy-β(5) and α-(6)-D-threo-pentofuranosyl)-5,6-dihydrothymine, respectively. Treatment of the β-3′,5′-anhydro structure (1) with the strong acidic cation-exchange resin gave mainly the α-anomer (4). Some correlations of the stereoelectronic features with the anomerizations in the 2′-deoxy-D-threo- and erythro-pentofuranosyl series are indicated.