1ST SYNTHESIS OF ALDOPENTONO-1,4-THIOLACTONES

A convenient synthesis of enantiomerically pure aldopentono-1,4-thiolactones is described. Thus, 4-thio-D-ribono-1,4-lactone (12) has been prepared from D-gluono-1,4-lactone (1), via its 2,3-O-isopropylidene derivative 3. The 5,6-glycol system of 3 was oxidized with NaIO4. Chemoselective reduction of the resulting aldehyde function with NaBH3CN led to 2,3-O-isopropylidene-L-lyxono-1,4-lactone (7). Tosylation of 7 and subsequent treatment of the tosylate 8 with sodium methoxide afforded methyl 4,5-epoxy-2,3-O-isopropylidene-L-lyxonate (9) as a key intermediate. Treatment of 9 with thiourea gave the 4,5-thiirane derivative having the D-ribo configuration (10). Regioselective opening of the thiirane ring and simultaneous thiolactonization took place by heating 10 with KOAc-HOAc-DMF. The resulting 5-O-acetyl-2,3-0-isopropylidene-4-thio-D-ribono-1,4-lactone (11) was readily converted, by acid removal (2 % HCI) of the protecting groups, into the crystalline thiolactone 12. A similar approach was employed for the synthesis of 4-thio-L-lyxono-1,4-lactone (19), starting from D-ribono-1,4-lactone (13).