Preliminary protecting group compatibility tests were performed on a pentamer alternating each individual test monomer with standard POM-dT-PSI (Figure ), followed by a mixed base sequence. For simplicity sake, this was performed on both isomers of the oligonucleotide but only the R-isomer is shown (see SI for S isomer). Each synthesized oligonucleotide was cartridge purified, followed by LC/MS analysis to determine if full-length product was observed A systematic screening of adenosine protecting groups via (RP)-15 revealed that Pya, dimethylacetamidine (dma), the non-protected (none) base are compatible with solidphase oligonucleotide synthesis (SPOS) with similar yields. In contrast, Benzoyl (Bz)-protecting groups were found to exhibit lower compatibility with P(V) SPOS synthesis. (Figure ). The instability of Bz protected adenosine monomers to DBU has also been reported recently. For the guanosine base, by synthesizing (RP)-16 it was found that the isobutyrl (iBu) protecting group is compatible with P(V) conditions. This offers the advantage of being a commercially available nucleoside and a more cost-effective starting material (DMTr-G-iBu is [$2/g]) compared to the Pya protecting group, which is not commercially available. Through a similar process, it was found that cytosine monomers with Acetyl (Ac) and Bz protecting groups can both be utilized to prepare (RP)-17 in nearly identical quality and yield compared to the Pya comparator. Lastly, it was found that (RP)-18 synthesized using thymidine monomers with and without protection with equivalent yield and purity, supporting the notion that the there is no need for protection on thymidine bases.