The stereoselective synthesis of 4′-β-thioribonucleosides via the Pummerer reaction

An efficient stereoselective synthesis of 4'-beta-thioribonucleosides 14, 15, 27, and 30 using the Pummerer reaction as the key step is described. The Pummerer reaction of 1,4-anhydro-2-0-(2,4-dimethoxybenzoyl)-3,5-O-( 1,1,3,3-tetraisopropyldisiloxane-1,3-diyl)-3-sulfinyl-D-ribitol (R-10:S-10 = 2.7:1) in the presence of silylated uracil afforded the desired p-anomer of the 4'-thiouridine derivative 11 in 66% yield without formation of its a-anomer. The reaction with R-10 gave 11 in 87% yield, while the one with S-10 resulted in a 27% decrease of the desired product 11 along with a 22% yield of 3,6-O-(1,1,3,3-tetraisopropyldisiloxane-1,3-diyl)-3-hydroxy-2-hydroxymethylthiophene (12). A likely explanation for the observed difference in the reaction of R-10 and S-10 is that the reaction proceeds via an E2 type pathway, which prefers anti elimination. Thus, R-10 would preferentially afford the alpha-thiocarbocation intermediate 21 via an E2 anti elimination under the reaction conditions. The resulting 21 would be expected to react with silylated uracil stereoselectively to give 11 in good yield. However, formation of the more stable tertiary alpha-thiocarbocation intermediate 23, which would prefer to give 12 and/or to decompose, would compete with the formation of the desired 21 in the reaction with S-10. Consequently, this argument would explain the low yields of the desired product 11 and the poor mass balance in the reaction with S-10. When the sulfoxide 10 (R-10:S-10 = >16:1) prepared by oxidation of 9 with ozone was used for the Pummerer reaction, the desired 11 was obtained in 80% yield. Compound 11 was converted to 4'-beta-thiouridine (14) by treatment of 11 with ammonium fluoride, followed by methanolic ammonia. Similarly, 4'-beta-thiocytidine (15) was prepared when silylated N-4-acetylcytosine was used in the Pummerer reaction. For the Pummerer reactions with purine bases, 6-chloropurine and 2-amino-6-chloropurine were found to be the most suitable. When the reactions were conducted in a mixture of acetonitrile and 1,2-dichloroethane at room temperature, followed by reflux, the desired products 25 and 28 were obtained in 65% and 56% yields, respectively. These compounds were then converted to 4'-beta-thioadenosine (27) and 4'-beta-thioguanosine (30) under the usual conditions. This is therefore the first time that the stereoselective synthesis of 4'-beta-thioribonucleosides has been performed using the neighboring group participation of the Pummerer reaction.