Short path to a complex ring Nucleotide analogs are valuable tools and therapeutics because of their ability to interfere with processes such as DNA synthesis, which are vital to rapidly dividing cells and replicating viruses. These molecules are challenging to synthesize chemically. Meanwell et al. developed a “ribose last” synthetic strategy in which a fluorinated acyclic nucleic acid is formed by an l - or d -proline–catalyzed aldol reaction (see the Perspective by Miller). This intermediate can then be cyclized to yield the nucleic acid analog in one pot with control of anomeric conformation based on cyclization conditions. Nucleotide analogs accessible by this strategy include those with modifications at C2′ and C4′, purines and pyrimidines, and locked and protected products. Science , this issue p. 725 ; see also p. 623