Method and cyclic carbonates for nucleotide analogues

A method for the preparation of the alkylated base of structure (VIII) ##STR9## wherein B is the residue of a purine or pyrimidine base and R.sup.1 is H;

• C1-C3 straight or branched alkyl;
• C2-C6 straight or branched alkenyl or alkynyl;
• or C1-C2 alkyl substituted with fluoro, azido, amino, OH, NHR.sup.2, N(R.sup.2).sub.2 or OR.sup.3 ;
• and R.sup.2 and R.sup.3 independently are protecting groups;
• comprising
• (a) reacting with a purine or pyrimidine base a cyclic carbonate of structure (I) ##STR10## in a solvent consisting essentially of an aprotic dipolar solvent selected from the group consisting of DMF, N-methylpyrrolidone and DMSO and
• (b) crystallizing the alkylated base of structure (VIII) by a method consisting essentially of cooling the aprotic dipolar solvent reaction product of step (a) to about 0.degree. C. and adding an organic solvent that is substantially nonpolar relative to DMF selected from the group consisting of toluene, ethylacetate, ether, ethanol, methanol or xylene such that the alkylated base of structure (VIII) is crystallized from the solvent mixture, and
• (c) recovering the crystalline alkylated base of structure (VIII).