Synthesis of 5'-C-Methyl-D-allo- & L-Talo-ribonucleoside 3'-O-Phosphoramidites & Their Incorporation into Hammerhead Ribozymes

5′-C-Methyl-D-allo & L-talo-ribonucleoside 3′-O-phosphoramidites were prepared from L-rhamnose in 13 and 15 steps respectively. Incorporation of L-talo residues in the hammerhead ribozyme and the resulting activity and stability of the modified ribozymes is described.