Novel stereoselective entry to 2'-beta-carbon-substituted 2'-deoxy-4'-thionucleosides from 4-thiofuranoid glycals.

2'-Beta-methyl- and 2'-beta-hydroxymethyl-2'-deoxy-4'-thionucleosides have been synthesized through PhSeCl-mediated electrophilic glycosidation using 4-thiofuranoid glycals having carbon substituents at the C2-position as a glycosyl donor. Preparation of these glycals were carried out by means of the C2 lithiation of 1-chloro-4-thiofuranoid glycal with LTMP followed by the Birch reduction of the chlorine atom. [reaction: see text]

Stereoselective synthesis of 2'-beta-carbon-substituted 2'-deoxy-4'-thioribonucleosides from 4-thiofuranoid glycal.

The 4-thiofuranoid glycal 6 was converted into 1-chlorinated derivative 7 through LDA lithiation. When 7 was treated with LTMP, successful C-2 lithiation occurred, and subsequent treatment of the lithiated species with MeI gave 8. Dechlorination of 8 with Na/liq.NH3 yielded the 2-methyl glycal 9. Glycosidation of 9 with silylated thymine was mediated with PhSeCl as an electrophile, and gave the beta-isomer 10 stereoselectively. Removal of the 2'-phenylselenenyl group of 10 was effected with tributyltin radical, and subsequent deprotection furnished the target 2'-beta-methyl 4'-thiothymidine (11). In a similar manner, the corresponding cytosine analogue 12 could also be synthesized.

Stereoselective synthesis of the beta-anomer of 4'-thionucleosides based on electrophilic glycosidation to 4-thiofuranoid glycals.

Three types of 4-thiofuranoid glycal with different 3,5-O-silyl protecting groups were prepared and their electrophilic glycosidation was investigated. The 3,5-bis-O-(tert-butyldimethylsilyl)-4-thiofuranoid glycal (5) was obtained through mesylation of 2-deoxy-4-thio-D-erythro-pentofuranose (4) and subsequent base-promoted elimination, while thermal elimination of sulfoxide derivatives was suitable for the preparation of 3,5-O-(tetraisopropyldisiloxane-1,3-diyl) (9) and 3,5-O-(di-tert-butylsilylene) (11) 4-thioglycals. The glycosidation reactions of these 4-thioglycals were carried out, in the presence of either PhSeCl or NIS, by using silylated derivatives of uracil, thymine, cytosine, and N(6)-benzoyladenine. Among the three 4-thioglycals, 11 was found to be an excellent glycosyl donor, forming the desired beta-anomer exclusively irrespective of the nucleobase employed.