Stereodefined phosphorothioate oligonucleotides, related nucleoside monomers and methods of synthesis of both are described. The disclosed isobutyryl protected L-LNA-G monomers have following Formula 50 or 51 where R3 is independently CH2ODMTr or CH2OMMTr and R8, when present, is cyanoethyl. The monomers have improved solubility and stability characteristics, resulting in improved efficacy in oligonucleotide synthesis.