Another opportunity for improvement in the NCB field is expansion of the chemical space. Within the realm of simple NCBs, described by Black et al. as structures which do not contain nucleotide components, there are vast possibilities for the R1 and R2 substituents (N-substituents and C3substituents, respectively). However, only a small number of NCBs have been synthesized and characterized in vitro. Tan et al. investigated chemical and physical properties of NCBs with a methyl R3 substituent (C5-substituent), pointing out the lack of research into modifications at the R3 position. They proposed adding an R3 methyl substituent to only two NCB structures of the eleven they report. Additionally, R2 variants have only been examined for NCBs with a benzyl R1 substituent. In our literature review, we found sixteen simple synthetic NCBs out of which eleven R1, four R2, and two R3 substituents are explored (Figure ). In this work, we expand the chemical space of NCB candidates through a combinatorial generation of simple NCBs from the previously evaluated substituents, resulting in a library of 132 NCB candidates (including the sixteen previously proposed) (Figure